Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if                             May 22,                            July 11, 2016
           approved)
Updates: 4492, 6066, 6961 (if approved)
Intended status: Standards Track
Expires: November 23, 2016 January 12, 2017

        The Transport Layer Security (TLS) Protocol Version 1.3
                        draft-ietf-tls-tls13-13
                        draft-ietf-tls-tls13-14

Abstract

   This document specifies Version version 1.3 of the Transport Layer Security
   (TLS) protocol.  The  TLS protocol allows client/server applications to communicate
   over the Internet in a way that is designed to prevent eavesdropping,
   tampering, and message forgery.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 23, 2016. January 12, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   5
     1.2.  Major Differences from TLS 1.2  . . . . . . . . . . . . .   6
   2.  Goals  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  Incorrect DHE Share . . . . . .  10
   3.  Goals of This Document . . . . . . . . . . . . .  14
     2.2.  Resumption and Pre-Shared Key (PSK) . . . . . .  10
   4.  Presentation Language . . . . .  15
     2.3.  Zero-RTT Data . . . . . . . . . . . . . . .  11
     4.1.  Basic Block Size . . . . . . .  17
   3.  Presentation Language . . . . . . . . . . . . .  11
     4.2.  Miscellaneous . . . . . . .  18
     3.1.  Basic Block Size  . . . . . . . . . . . . . . .  11
     4.3.  Vectors . . . . .  18
     3.2.  Miscellaneous . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Numbers . .  19
     3.3.  Vectors . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Enumerateds . .  19
     3.4.  Numbers . . . . . . . . . . . . . . . . . . . . .  13
     4.6.  Constructed Types . . . .  20
     3.5.  Enumerateds . . . . . . . . . . . . . . . .  14
       4.6.1.  Variants . . . . . . .  20
     3.6.  Constructed Types . . . . . . . . . . . . . . .  14
     4.7.  Constants . . . . .  21
       3.6.1.  Variants  . . . . . . . . . . . . . . . . . . .  15
     4.8.  Cryptographic Attributes . . .  21
     3.7.  Constants . . . . . . . . . . . . .  15
       4.8.1.  Digital Signing . . . . . . . . . . .  23
   4.  Handshake Protocol  . . . . . . . .  16
       4.8.2.  Authenticated Encryption with Additional Data (AEAD)   17
   5.  The TLS Record Protocol . . . . . . . . . . . . .  23
     4.1.  Key Exchange Messages . . . . . .  17
     5.1.  Connection States . . . . . . . . . . . .  24
       4.1.1.  Client Hello  . . . . . . . .  18
     5.2.  Record Layer . . . . . . . . . . . .  25
       4.1.2.  Server Hello  . . . . . . . . . .  20
       5.2.1.  Fragmentation . . . . . . . . . .  27
       4.1.3.  Hello Retry Request . . . . . . . . . .  20
       5.2.2.  Record Payload Protection . . . . . . .  29
     4.2.  Hello Extensions  . . . . . . .  22
       5.2.3.  Record Padding . . . . . . . . . . . . .  30
       4.2.1.  Cookie  . . . . . .  24
   6.  The TLS Handshaking Protocols . . . . . . . . . . . . . . . .  25
     6.1.  Alert Protocol .  31
       4.2.2.  Signature Algorithms  . . . . . . . . . . . . . . . .  32
       4.2.3.  Negotiated Groups . . . .  26
       6.1.1.  Closure Alerts . . . . . . . . . . . . . .  35
       4.2.4.  Key Share . . . . .  27
       6.1.2.  Error Alerts . . . . . . . . . . . . . . . . .  36
       4.2.5.  Pre-Shared Key Extension  . . .  29
     6.2.  Handshake Protocol Overview . . . . . . . . . . .  39
       4.2.6.  Early Data Indication . . . .  32
       6.2.1.  Incorrect DHE Share . . . . . . . . . . . .  40
       4.2.7.  OCSP Status Extensions  . . . . .  35
       6.2.2.  Resumption and Pre-Shared Key (PSK) . . . . . . . . .  36
       6.2.3.  Zero-RTT Data .  43
       4.2.8.  Encrypted Extensions  . . . . . . . . . . . . . . . .  44
       4.2.9.  Certificate Request . . .  38

     6.3.  Handshake Protocol . . . . . . . . . . . . . .  44

     4.3.  Authentication Messages . . . . .  39
       6.3.1.  Key Exchange Messages . . . . . . . . . . . .  46
       4.3.1.  Certificate . . . .  40
       6.3.2.  Hello Extensions . . . . . . . . . . . . . . . . .  47
       4.3.2.  Certificate Verify  .  46
       6.3.3.  Server Parameters . . . . . . . . . . . . . . . .  51
       4.3.3.  Finished  . .  60
       6.3.4.  Authentication Messages . . . . . . . . . . . . . . .  63
       6.3.5. . . . . .  53
     4.4.  Post-Handshake Messages . . . . . . . . . . . . . . .  71
   7.  Cryptographic Computations . .  54
       4.4.1.  New Session Ticket Message  . . . . . . . . . . . . .  54
       4.4.2.  Post-Handshake Authentication . .  74
     7.1.  Key Schedule . . . . . . . . . .  56
       4.4.3.  Key and IV Update . . . . . . . . . . . .  74
     7.2.  Updating Traffic Keys and IVs . . . . . .  57
   5.  Record Protocol . . . . . . . .  76
     7.3.  Traffic Key Calculation . . . . . . . . . . . . . . .  58
     5.1.  Record Layer  . .  76
       7.3.1.  Diffie-Hellman . . . . . . . . . . . . . . . . . . .  77
       7.3.2.  Elliptic Curve Diffie-Hellman . .  58
     5.2.  Record Payload Protection . . . . . . . . . .  78
       7.3.3.  Exporters . . . . . .  59
     5.3.  Per-Record Nonce  . . . . . . . . . . . . . . . .  78
   8.  Mandatory Algorithms . . . .  61
     5.4.  Record Padding  . . . . . . . . . . . . . . . .  79
     8.1.  MTI Cipher Suites . . . . .  62
     5.5.  Limits on Key Usage . . . . . . . . . . . . . . .  79
     8.2.  MTI Extensions . . . .  63
   6.  Alert Protocol  . . . . . . . . . . . . . . . . .  79
   9.  Application Data Protocol . . . . . .  63
     6.1.  Closure Alerts  . . . . . . . . . . . .  80
   10. Security Considerations . . . . . . . . .  65
     6.2.  Error Alerts  . . . . . . . . . .  80
   11. IANA Considerations . . . . . . . . . . . .  66
   7.  Cryptographic Computations  . . . . . . . . .  80
   12. References . . . . . . . .  69
     7.1.  Key Schedule  . . . . . . . . . . . . . . . . .  83
     12.1.  Normative References . . . . .  69
     7.2.  Updating Traffic Keys and IVs . . . . . . . . . . . . .  83
     12.2.  Informative References .  72
     7.3.  Traffic Key Calculation . . . . . . . . . . . . . . . .  86
   Appendix A.  Protocol Data Structures and Constant Values .  72
       7.3.1.  Diffie-Hellman  . . .  92
     A.1.  Record Layer . . . . . . . . . . . . . . . .  73
       7.3.2.  Elliptic Curve Diffie-Hellman . . . . . .  92
     A.2.  Alert Messages . . . . . .  74
       7.3.3.  Exporters . . . . . . . . . . . . . . .  92
     A.3.  Handshake Protocol . . . . . . .  74
   8.  Compliance Requirements . . . . . . . . . . . .  94
       A.3.1.  Key Exchange Messages . . . . . . .  74
     8.1.  MTI Cipher Suites . . . . . . . . .  94
       A.3.2.  Server Parameters Messages . . . . . . . . . . .  75
     8.2.  MTI Extensions  . .  98
       A.3.3.  Authentication Messages . . . . . . . . . . . . . . .  99
       A.3.4.  Ticket Establishment . . . .  75
   9.  Security Considerations . . . . . . . . . . . .  99
     A.4.  Cipher Suites . . . . . . .  76
   10. IANA Considerations . . . . . . . . . . . . . . . 100
       A.4.1.  Unauthenticated Operation . . . . . .  76
   11. References  . . . . . . . . 105
     A.5.  The Security Parameters . . . . . . . . . . . . . . . . . 105
     A.6.  Changes to RFC 4492  79
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  79
     11.2.  Informative References . 106
   Appendix B.  Implementation Notes . . . . . . . . . . . . . . . . 107
     B.1.  Random Number Generation  82
   Appendix A.  Protocol Data Structures and Seeding Constant Values . . . .  89
     A.1.  Record Layer  . . . . . . 107
     B.2.  Certificates and Authentication . . . . . . . . . . . . . 107
     B.3.  Cipher Suite Support . . .  89
     A.2.  Alert Messages  . . . . . . . . . . . . . . . 107
     B.4.  Implementation Pitfalls . . . . . .  89
     A.3.  Handshake Protocol  . . . . . . . . . . . 107
     B.5.  Client Tracking Prevention . . . . . . . .  91
       A.3.1.  Key Exchange Messages . . . . . . . 109
   Appendix C.  Backward Compatibility . . . . . . . . .  91
       A.3.2.  Server Parameters Messages  . . . . . . 109
     C.1.  Negotiating with an older server . . . . . . .  95
       A.3.3.  Authentication Messages . . . . . 110
     C.2.  Negotiating with an older client . . . . . . . . . .  96
       A.3.4.  Ticket Establishment  . . 111
     C.3.  Zero-RTT backwards compatibility . . . . . . . . . . . . 111
     C.4.  Backwards Compatibility Security Restrictions . .  96
     A.4.  Cipher Suites . . . . 111
   Appendix D.  Security Analysis . . . . . . . . . . . . . . . . . 112
     D.1.  Handshake Protocol .  97
       A.4.1.  Unauthenticated Operation . . . . . . . . . . . . . . 102
   Appendix B.  Implementation Notes . . . . 113
       D.1.1.  Authentication . . . . . . . . . . . . 102
     B.1.  Random Number Generation and Key Exchange Seeding  . . . . . . . . . . 102
     B.2.  Certificates and Authentication . 113
       D.1.2.  Version Rollback Attacks . . . . . . . . . . . . 103
     B.3.  Cipher Suite Support  . . . . . . . . . . . . . . . . . . 103
     B.4.  Implementation Pitfalls . . . . . . . . . . . . . . . . . 103
     B.5.  Client Tracking Prevention  . . . . . . . . . . . . . . . 105

   Appendix C.  Backward Compatibility . . . . . . . . . . . . . . . 105
     C.1.  Negotiating with an older server  . . . . . . . . . . . . 106
     C.2.  Negotiating with an older client  . . . . . . . 114
       D.1.3.  Detecting Attacks Against the Handshake Protocol . . 114
     D.2.  Protecting Application Data . . . 106
     C.3.  Zero-RTT backwards compatibility  . . . . . . . . . . . . 107
     C.4.  Backwards Compatibility Security Restrictions . . . . . . 114
     D.3.  Denial 107
   Appendix D.  Overview of Service Security Properties  . . . . . . . . . . 108
     D.1.  Handshake . . . . . . . . . . . . . . . . 115
     D.4.  Final Notes . . . . . . . . 108
     D.2.  Record Layer  . . . . . . . . . . . . . . . . . . . . 115 . . 110
   Appendix E.  Working Group Information  . . . . . . . . . . . . . 115 112
   Appendix F.  Contributors . . . . . . . . . . . . . . . . . . . . 115 112
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 119 116

1.  Introduction

   DISCLAIMER: This is a WIP draft of TLS 1.3 and has not yet seen
   significant security analysis.

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this
   draft is maintained in GitHub.  Suggested changes should be submitted
   as pull requests at https://github.com/tlswg/tls13-spec.
   Instructions are on that page as well.  Editorial changes can be
   managed in GitHub, but any substantive change should be discussed on
   the TLS mailing list.

   The primary goal of the TLS protocol is to provide privacy and data
   integrity a secure channel between two
   communicating peers.  The TLS protocol is
   composed of two layers: the TLS Record Protocol and  Specifically, the TLS Handshake
   Protocol.  At channel should provide the lowest level, layered on top
   following properties.

   -  Authentication: The server side of some reliable
   transport protocol (e.g., TCP [RFC0793]), the channel is always
      authenticated; the TLS Record Protocol.
   The TLS Record Protocol provides connection security that has two
   basic properties:

   -  The connection client side is private.  Symmetric cryptography is used for
      data encryption (e.g., AES [AES]).  The keys for this symmetric
      encryption are generated uniquely for each connection and are
      based on a secret negotiated by another the TLS Handshake
      Protocol.

   -  The connection is reliable.  Messages include an authentication
      tag which protects them against modification.

   Note: The TLS Record Protocol can operate in an insecure mode but is
   generally only used in this mode while another protocol is using the
   TLS Record Protocol as a transport for negotiating security
   parameters.

   The TLS Record Protocol is used for encapsulation of various higher-
   level protocols.  One such encapsulated protocol, the TLS Handshake
   Protocol, allows the server and client to authenticate each other and
   to negotiate an encryption algorithm and cryptographic keys before
   the application protocol transmits or receives its first byte of
   data.  The TLS Handshake Protocol provides connection security that
   has three basic properties:

   -  The peer's identity can be authenticated using asymmetric (public
      key) optionally authenticated.
      Authentication can happen via asymmetric cryptography (e.g., RSA
      [RSA], ECDSA [ECDSA]) or a pre-shared symmetric key.  The TLS server is always authenticated; client
      authentication is optional.

   -  The negotiation of a shared secret is secure:  Confidentiality: Data sent over the negotiated
      secret channel is unavailable not visible to eavesdroppers, and for any authenticated
      connection
      attackers.

   -  Integrity: Data sent over the secret channel cannot be obtained, even modified by
      attackers.

   These properties should be true even in the face of an attacker who
      can place himself in
   has complete control of the middle network, as described in [RFC3552].  See
   Appendix D for a more complete statement of the connection. relevant security
   properties.

   TLS consists of two primary components:

   -  A handshake protocol (Section 4) which authenticates the
      communicating parties, negotiates cryptographic modes and
      parameters, and establishes shared keying material.  The negotiation handshake
      protocol is reliable: no designed to resist tampering; an active attacker can modify the
      negotiation communication without being detected by the parties
      should not be able to force the communication.

   One advantage peers to negotiate different
      parameters than they would if the connection were not under
      attack.

   -  A record protocol (Section 5) which uses the parameters
      established by the handshake protocol to protect traffic between
      the communicating peers.  The record protocol divides traffic up
      into a series of TLS records, each of which is that it independently protected
      using the traffic keys.

   TLS is application protocol independent.
   Higher-level independent; higher-level protocols can
   layer on top of the TLS protocol transparently.  The TLS standard, however, does
   not specify how protocols add security with TLS; the decisions on how
   to initiate TLS handshaking and how to interpret the authentication
   certificates exchanged are left to the judgment of the designers and
   implementors of protocols that run on top of TLS.

   This document defines TLS version 1.3.  While TLS 1.3 is not directly
   compatible with previous versions, all versions of TLS incorporate a
   versioning mechanism which allows clients and servers to
   interoperably negotiate a common version if one is supported.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

   The following terms are used:

   client: The endpoint initiating the TLS connection.

   connection: A transport-layer connection between two endpoints.

   endpoint: Either the client or server of the connection.

   handshake: An initial negotiation between client and server that
   establishes the parameters of their transactions.

   peer: An endpoint.  When discussing a particular endpoint, "peer"
   refers to the endpoint that is remote to the primary subject of
   discussion.

   receiver: An endpoint that is receiving records.

   sender: An endpoint that is transmitting records.

   session: An association between a client and a server resulting from
   a handshake.

   server: The endpoint which did not initiate the TLS connection.

1.2.  Major Differences from TLS 1.2

   draft-14

   -  Allow cookies to be longer (*)

   -  Remove the "context" from EarlyDataIndication as it was undefined
      and nobody used it (*)

   -  Remove 0-RTT EncryptedExtensions and replace the ticket_age
      extension with an obfuscated version.  Also necessitates a change
      to NewSessionTicket (*).

   -  Move the downgrade sentinel to the end of ServerHello.Random to
      accomodate tlsdate (*).

   -  Define ecdsa_sha1 (*).

   -  Allow resumption even after fatal alerts.  This matches current
      practice.

   -  Remove non-closure warning alerts.  Require treating unknown
      alerts as fatal.

   -  Make the rules for accepting 0-RTT less restrictive.

   -  Clarify 0-RTT backward-compatibility rules.

   -  Clarify how 0-RTT and PSK identities interact.

   -  Add a section describing the data limits for each cipher.

   -  Major editorial restructuring.

   -  Replace the Security Analysis section with a WIP draft.

   (*) indicates changes to the wire protocol which may require
   implementations to update.

   draft-13
   -  Allow server to send SupportedGroups.

   -  Remove 0-RTT client authentication

   -  Remove (EC)DHE 0-RTT.

   -  Flesh out 0-RTT PSK mode and shrink EarlyDataIndiation EarlyDataIndication

   -  Turn PSK-resumption response into an index to save room

   -  Move CertificateStatus to an extension

   -  Extra fields in NewSessionTicket.

   -  Restructure key schedule and add a resumption_context value.

   -  Require DH public keys and secrets to be zero-padded to the size
      of the group.

   -  Remove the redundant length fields in KeyShareEntry.

   -  Define a cookie field for HRR.

   draft-12

   -  Provide a list of the PSK cipher suites.

   -  Remove the ability for the ServerHello to have no extensions (this
      aligns the syntax with the text).

   -  Clarify that the server can send application data after its first
      flight (0.5 RTT data)

   -  Revise signature algorithm negotiation to group hash, signature
      algorithm, and curve together.  This is backwards compatible.

   -  Make ticket lifetime mandatory and limit it to a week.

   -  Make the purpose strings lower-case.  This matches how people are
      implementing for interop.

   -  Define exporters.

   -  Editorial cleanup

   draft-11

   -  Port the CFRG curves & signatures work from RFC4492bis.

   -  Remove sequence number and version from additional_data, which is
      now empty.

   -  Reorder values in HkdfLabel.

   -  Add support for version anti-downgrade mechanism.

   -  Update IANA considerations section and relax some of the policies.

   -  Unify authentication modes.  Add post-handshake client
      authentication.

   -  Remove early_handshake content type.  Terminate 0-RTT data with an
      alert.

   -  Reset sequence number upon key change (as proposed by Fournet et
      al.)

   draft-10

   -  Remove ClientCertificateTypes field from CertificateRequest and
      add extensions.

   -  Merge client and server key shares into a single extension.

   draft-09

   -  Change to RSA-PSS signatures for handshake messages.

   -  Remove support for DSA.

   -  Update key schedule per suggestions by Hugo, Hoeteck, and Bjoern
      Tackmann.

   -  Add support for per-record padding.

   -  Switch to encrypted record ContentType.

   -  Change HKDF labeling to include protocol version and value
      lengths.

   -  Shift the final decision to abort a handshake due to incompatible
      certificates to the client rather than having servers abort early.

   -  Deprecate SHA-1 with signatures.

   -  Add MTI algorithms.

   draft-08

   -  Remove support for weak and lesser used named curves.

   -  Remove support for MD5 and SHA-224 hashes with signatures.

   -  Update lists of available AEAD cipher suites and error alerts.

   -  Reduce maximum permitted record expansion for AEAD from 2048 to
      256 octets.

   -  Require digital signatures even when a previous configuration is
      used.

   -  Merge EarlyDataIndication and KnownConfiguration.

   -  Change code point for server_configuration to avoid collision with
      server_hello_done.

   -  Relax certificate_list ordering requirement to match current
      practice.

   draft-07

   -  Integration of semi-ephemeral DH proposal.

   -  Add initial 0-RTT support.

   -  Remove resumption and replace with PSK + tickets.

   -  Move ClientKeyShare into an extension.

   -  Move to HKDF.

   draft-06

   -  Prohibit RC4 negotiation for backwards compatibility.

   -  Freeze & deprecate record layer version field.

   -  Update format of signatures with context.

   -  Remove explicit IV.

   draft-05

   -  Prohibit SSL negotiation for backwards compatibility.

   -  Fix which MS is used for exporters.

   draft-04

   -  Modify key computations to include session hash.

   -  Remove ChangeCipherSpec.

   -  Renumber the new handshake messages to be somewhat more consistent
      with existing convention and to remove a duplicate registration.

   -  Remove renegotiation.

   -  Remove point format negotiation.

   draft-03

   -  Remove GMT time.

   -  Merge in support for ECC from RFC 4492 but without explicit
      curves.

   -  Remove the unnecessary length field from the AD input to AEAD
      ciphers.

   -  Rename {Client,Server}KeyExchange to {Client,Server}KeyShare.

   -  Add an explicit HelloRetryRequest to reject the client's.

   draft-02

   -  Increment version number.

   -  Rework handshake to provide 1-RTT mode.

   -  Remove custom DHE groups.

   -  Remove support for compression.

   -  Remove support for static RSA and DH key exchange.

   -  Remove support for non-AEAD ciphers.

2.  Goals  Protocol Overview

   The goals cryptographic parameters of the TLS protocol, in order of priority, session state are as follows:

   1.  Cryptographic security: produced by the
   TLS should be used to establish handshake protocol.  When a secure
       connection between two parties.

   2.  Interoperability: Independent programmers should be able to
       develop applications utilizing TLS that can successfully exchange
       cryptographic parameters without knowledge of one another's code.

   3.  Extensibility: TLS seeks to provide a framework into which new
       public key client and record protection methods can be incorporated as
       necessary.  This will also accomplish two sub-goals: preventing
       the need to create server first start
   communicating, they agree on a new protocol (and risking the introduction
       of possible new weaknesses) version, select cryptographic
   algorithms, optionally authenticate each other, and avoiding establish shared
   secret keying material.  Once the need to implement an
       entire new security library.

   4.  Relative efficiency: Cryptographic operations tend to be highly
       CPU intensive, particularly public key operations.  For this
       reason, handshake is complete, the TLS protocol has incorporated an optional session
       caching scheme to reduce peers
   use the number of connections that need to
       be established from scratch.  Additionally, care has been taken keys to reduce network activity.

3.  Goals of This Document

   This document and the protect application layer traffic.

   TLS protocol itself have evolved from supports three basic key exchange modes:

   -  Diffie-Hellman (of both the SSL
   3.0 Protocol Specification as published by Netscape.  The differences
   between this version finite field and previous versions are significant enough
   that the various versions elliptic curve
      varieties).

   -  A pre-shared symmetric key (PSK)

   -  A combination of TLS and SSL 3.0 do not interoperate
   (although each protocol incorporates a mechanism by which an
   implementation can back down to prior versions).  This document is
   intended primarily for readers who will be implementing the protocol
   and for those doing cryptographic analysis of it.  The specification
   has been written with this in mind, symmetric key and it Diffie-Hellman

   Which mode is intended to reflect used depends on the
   needs of those two groups.  For that reason, many of negotiated cipher suite.
   Conceptually, the algorithm-
   dependent data structures and rules handshake establishes three secrets which are included in used
   to derive all the body of keys.

   Figure 1 below shows the
   text (as opposed to basic full TLS handshake:

       Client                                               Server

Key  ^ ClientHello
Exch | + key_share*
     v + pre_shared_key*         -------->
                                                       ServerHello  ^ Key
                                                      + key_share*  | Exch
                                                 + pre_shared_key*  v
                                             {EncryptedExtensions}  ^  Server
                                             {CertificateRequest*}  v  Params
                                                    {Certificate*}  ^
                                              {CertificateVerify*}  | Auth
                                                        {Finished}  v
                                 <--------     [Application Data*]
     ^ {Certificate*}
Auth | {CertificateVerify*}
     v {Finished}                -------->
       [Application Data]        <------->      [Application Data]

              +  Indicates extensions sent in an appendix), providing easier access to them.

   This document is not intended to supply any details of service
   definition the
                 previously noted message.

              *  Indicates optional or of interface definition, although it does cover select
   areas of policy as they situation-dependent
                 messages that are required not always sent.

              {} Indicates messages protected using keys
                 derived from handshake_traffic_secret.

              [] Indicates messages protected using keys
                 derived from traffic_secret_N

               Figure 1: Message flow for the maintenance of solid
   security.

4.  Presentation Language

   This document deals with the formatting of data in an external
   representation. full TLS Handshake

   The following very basic and somewhat casually
   defined presentation syntax will handshake can be used.  The syntax draws from
   several sources thought of as having three phases, indicated in its structure.  Although it resembles
   the
   programming language "C" in its syntax and XDR [RFC4506] in both its
   syntax diagram above.

   -  Key Exchange: Establish shared keying material and intent, it would be risky to draw too many parallels.  The
   purpose of select the
      cryptographic parameters.  Everything after this presentation language phase is to document TLS only; it has
   no general
      encrypted.

   -  Server Parameters: Establish other handshake parameters.  (whether
      the client is authenticated, application beyond that particular goal.

4.1.  Basic Block Size

   The representation of all data items is explicitly specified.  The
   basic data block size is one byte (i.e., 8 bits).  Multiple byte data
   items are concatenations of bytes, from left to right, from top to
   bottom.  From layer protocol support,
      etc.)

   -  Authentication: Authenticate the byte stream, server (and optionally the
      client) and provide key confirmation and handshake integrity.

   In the Key Exchange phase, the client sends the ClientHello
   (Section 4.1.1) message, which contains a multi-byte item (a numeric random nonce
   (ClientHello.random), its offered protocol version, cipher suite, and
   extensions, and in general either one or more Diffie-Hellman key
   shares (in the
   example) is formed (using C notation) by:

      value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
              ... | byte[n-1];

   This byte ordering for multi-byte values is "key_share" extension Section 4.2.4), one or more pre-
   shared key labels (in the commonplace network
   byte order "pre_shared_key" extension Section 4.2.5),
   or big-endian format.

4.2.  Miscellaneous

   Comments begin with "/*" both.

   The server processes the ClientHello and end determines the appropriate
   cryptographic parameters for the connection.  It then responds with "*/".

   Optional components are denoted by enclosing them in "[[ ]]" double
   brackets.

   Single-byte entities containing uninterpreted data are of type
   opaque.

4.3.  Vectors

   A vector (single-dimensioned array) is a stream of homogeneous data
   elements.
   its own ServerHello which indicates the negotiated connection
   parameters.  [Section 4.1.2].  The size combination of the vector may be specified at documentation
   time or left unspecified until runtime.  In either case, the length
   declares ClientHello and
   the number of bytes, not ServerHello determines the number of elements, shared keys.  If either a pure (EC)DHE
   or (EC)DHE-PSK cipher suite is in use, then the
   vector.  The syntax for specifying a new type, T', that is ServerHello will
   contain a fixed-
   length vector of type T is

      T T'[n];

   Here, T' occupies n bytes "key_share" extension with the server's ephemeral Diffie-
   Hellman share which MUST be in the data stream, where n same group as one of the client's
   shares.  If a pure PSK or an (EC)DHE-PSK cipher suite is negotiated,
   then the ServerHello will contain a multiple "pre_shared_key" extension
   indicating which of the size of T. client's offered PSKs was selected.

   The length of server then sends two messages to establish the vector is Server
   Parameters:

   EncryptedExtensions.  responses to any extensions which are not included
      required in the
   encoded stream.

   In the following example, Datum is defined order to be three consecutive
   bytes that determine the protocol does not interpret, while Data cryptographic parameters.
      [Section 4.2.8]

   CertificateRequest.  if certificate-based client authentication is three
   consecutive Datum, consuming a total of nine bytes.

      opaque Datum[3];      /* three uninterpreted bytes */
      Datum Data[9];        /* 3 consecutive 3 byte vectors */

   Variable-length vectors are defined by specifying a subrange of legal
   lengths, inclusively, using the notation <floor..ceiling>.  When
   these are encoded, the actual length precedes the vector's contents
   in
      desired, the byte stream.  The length desired parameters for that certificate.  This
      message will be in omitted if client authentication is not desired.

   Finally, the form of a number
   consuming as many bytes as required to hold client and server exchange Authentication messages.  TLS
   uses the vector's specified
   maximum (ceiling) length.  A variable-length vector with an actual
   length field same set of zero messages every time that authentication is referred to as an empty vector.

      T T'<floor..ceiling>;

   In
   needed.  Specifically:

   Certificate.  the following example, mandatory certificate of the endpoint.  This message is
      omitted if the server is not authenticating with a vector certificate
      (i.e., with PSK or (EC)DHE-PSK cipher suites).  Note that must if raw
      public keys [RFC7250] or the cached information extension
      [I-D.ietf-tls-cached-info] are in use, then this message will not
      contain
   between 300 and 400 bytes of type opaque.  It can never be empty.
   The actual length field consumes two bytes, a uint16, which is
   sufficient certificate but rather some other value corresponding to represent
      the value 400 (see Section 4.4).  On server's long-term key.  [Section 4.3.1]

   CertificateVerify.  a signature over the
   other hand, longer can represent up to 800 bytes of data, entire handshake using the
      public key in the Certificate message.  This message is omitted if
      the server is not authenticating via a certificate (i.e., with PSK
      or 400
   uint16 elements, (EC)DHE-PSK cipher suites).  [Section 4.3.2]

   Finished.  a MAC (Message Authentication Code) over the entire
      handshake.  This message provides key confirmation, binds the
      endpoint's identity to the exchanged keys, and it in PSK mode also
      authenticates the handshake.  [Section 4.3.3]

   Upon receiving the server's messages, the client responds with its
   Authentication messages, namely Certificate and CertificateVerify (if
   requested), and Finished.

   At this point, the handshake is complete, and the client and server
   may exchange application layer data.  Application data MUST NOT be empty.  Its encoding will include a
   two-byte actual length field prepended
   sent prior to sending the vector.  The length of
   an encoded vector must be an even multiple of Finished message.  Note that while the length
   server may send application data prior to receiving the client's
   Authentication messages, any data sent at that point is, of a single
   element (for example, a 17-byte vector of uint16 would be illegal).

      opaque mandatory<300..400>;
            /* length field is 2 bytes, cannot be empty */
      uint16 longer<0..800>;
            /* zero course,
   being sent to 400 16-bit unsigned integers */

4.4.  Numbers

   The basic numeric data type is an unsigned byte (uint8).  All larger
   numeric data types are formed from fixed-length series of bytes
   concatenated as described in Section 4.1 and are also unsigned.  The
   following numeric types are predefined.

      uint8 uint16[2];
      uint8 uint24[3];
      uint8 uint32[4];
      uint8 uint64[8];

   All values, here and elsewhere in the specification, are stored in
   network byte (big-endian) order; the uint32 represented by the hex
   bytes 01 02 03 04 is equivalent to unauthenticated peer.

2.1.  Incorrect DHE Share

   If the decimal value 16909060.

   Note that in some cases (e.g., DH parameters) it is necessary to
   represent integers as opaque vectors.  In such cases, they are
   represented as unsigned integers (i.e., additional leading zero
   octets are client has not used even if the most significant bit is set).

4.5.  Enumerateds

   An additional sparse data type is available called enum.  A field of
   type enum can provided a sufficient "key_share" extension
   (e.g. it includes only assume DHE or ECDHE groups unacceptable or
   unsupported by the values declared in server), the definition.
   Each definition is server corrects the mismatch with a different type.  Only enumerateds of
   HelloRetryRequest and the same
   type may be assigned or compared.  Every element of client will need to restart the handshake
   with an enumerated
   must be assigned a value, appropriate "key_share" extension, as demonstrated shown in Figure 2.  If
   no common cryptographic parameters can be negotiated, the following example.
   Since server will
   send a "handshake_failure" or "insufficient_security" fatal alert
   (see Section 6).

            Client                                               Server

            ClientHello
              + key_share             -------->
                                      <--------       HelloRetryRequest

            ClientHello
              + key_share             -------->
                                                            ServerHello
                                                            + key_share
                                                  {EncryptedExtensions}
                                                  {CertificateRequest*}
                                                         {Certificate*}
                                                   {CertificateVerify*}
                                                             {Finished}
                                      <--------     [Application Data*]
            {Certificate*}
            {CertificateVerify*}
            {Finished}                -------->
            [Application Data]        <------->     [Application Data]

        Figure 2: Message flow for a full handshake with mismatched
                                parameters

   Note: The handshake transcript includes the elements initial ClientHello/
   HelloRetryRequest exchange; it is not reset with the new ClientHello.

   TLS also allows several optimized variants of the enumerated are not ordered, they can be
   assigned any unique value, in any order.

      enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;

   An enumerated occupies basic handshake, as much space
   described in the byte stream as would its
   maximal defined ordinal value.  The following definition would cause
   one byte to sections.

2.2.  Resumption and Pre-Shared Key (PSK)

   Although TLS PSKs can be used to carry fields established out of type Color.

      enum { red(3), blue(5), white(7) } Color;

   One may optionally specify band, PSKs can also be
   established in a value without its associated tag to
   force the width definition without defining previous session and then reused ("session
   resumption").  Once a superfluous element.

   In the following example, Taste will consume two bytes in handshake has completed, the data
   stream but server can only assume the values 1, 2, or 4.

      enum { sweet(1), sour(2), bitter(4), (32000) } Taste;

   The names of the elements of an enumeration are scoped within the
   defined type.  In send
   the first example, client a fully qualified reference PSK identity which corresponds to a key derived from the second element of the enumeration would be Color.blue.  Such
   qualification is not required if the target of the assignment is well
   specified.

      Color color = Color.blue;     /* overspecified, legal */
      Color color = blue;           /* correct, type implicit */

   For enumerateds that are never converted to external representation,
   the numerical information may be omitted.

      enum { low, medium, high } Amount;

4.6.  Constructed Types

   Structure types may be constructed from primitive types for
   convenience.  Each specification declares a new, unique type.
   initial handshake (See Section 4.4.1).  The
   syntax for definition is much like client can then use that
   PSK identity in future handshakes to negotiate use of C.

      struct {
          T1 f1;
          T2 f2;
          ...
          Tn fn;
      } [[T]];

   The fields within a structure may be qualified using the type's name,
   with a syntax much like that available for enumerateds.  For example,
   T.f2 refers to PSK.  If
   the second field server accepts it, then the security context of the previous declaration.
   Structure definitions may be embedded.

4.6.1.  Variants

   Defined structures may have variants based on some knowledge that new
   connection is
   available within tied to the environment.  The selector must be original connection.  In TLS 1.2 and below,
   this functionality was provided by "session IDs" and "session
   tickets" [RFC5077].  Both mechanisms are obsoleted in TLS 1.3.

   PSK cipher suites can either use PSK in combination with an enumerated
   type that defines the possible variants (EC)DHE
   exchange in order to provide forward secrecy in combination with
   shared keys, or can use PSKs alone, at the structure defines.  There
   must be cost of losing forward
   secrecy.

   Figure 3 shows a case arm for every element pair of the enumeration declared handshakes in which the select.  Case arms have limited fall-through: if two case arms
   follow in immediate succession with no fields in between, then they
   both contain the same fields.  Thus, in first establishes a
   PSK and the example below, "orange" second uses it:

          Client                                               Server

   Initial Handshake:
          ClientHello
           + key_share              -------->
                                                          ServerHello
                                                          + key_share
                                                {EncryptedExtensions}
                                                {CertificateRequest*}
                                                       {Certificate*}
                                                 {CertificateVerify*}
                                                           {Finished}
                                    <--------     [Application Data*]
          {Certificate*}
          {CertificateVerify*}
          {Finished}                -------->
                                    <--------      [NewSessionTicket]
          [Application Data]        <------->      [Application Data]

   Subsequent Handshake:
          ClientHello
            + pre_shared_key
            + key_share*            -------->
                                                          ServerHello
                                                     + pre_shared_key
                                                         + key_share*
                                                {EncryptedExtensions}
                                                           {Finished}
                                    <--------     [Application Data*]
          {Finished}                -------->
          [Application Data]        <------->      [Application Data]

               Figure 3: Message flow for resumption and "banana" both contain V2.  Note that this PSK

   As the server is authenticating via a new piece of
   syntax in TLS 1.2.

   The body of PSK, it does not send a
   Certificate or a CertificateVerify.  When a client offers resumption
   via PSK it SHOULD also supply a "key_share" extension to the variant structure may server
   as well to allow the server to decline resumption and fall back to a
   full handshake, if needed.  A "key_share" extension MUST also be given sent
   if the client is attempting to negotiate an (EC)DHE-PSK cipher suite.

2.3.  Zero-RTT Data

   When resuming via a label for reference.
   The mechanism by which PSK with an appropriate ticket (i.e., one with
   the variant "allow_early_data" flag), clients can also send data on their
   first flight ("early data").  This data is selected at runtime encrypted solely under
   keys derived using the first offered PSK as the static secret.  As
   shown in Figure 4, the Zero-RTT data is not
   prescribed by just added to the presentation language.

      struct {
          T1 f1;
          T2 f2;
          ....
          Tn fn;
           select (E) {
               case e1: Te1;
               case e2: Te2;
               case e3: case e4: Te3;
               ....
               case en: Ten;
           } [[fv]];
      } [[Tv]];

   For example:

      enum { apple, orange, banana } VariantTag;

      struct {
          uint16 number;
          opaque string<0..10>; /* variable length */
      } V1;

      struct {
          uint32 number;
          opaque string[10];    /* fixed length */
      } V2;

      struct {
          select (VariantTag) { /* value 1-RTT
   handshake in the first flight.  The rest of selector is implicit */
              case apple:
                V1;   /* VariantBody, tag = apple */
              case orange:
              case banana:
                V2;   /* VariantBody, tag = orange or banana */
          } variant_body;       /* the handshake uses the
   same messages.

            Client                                               Server

            ClientHello
              + early_data
              + pre_shared_key
              + key_share*
            (Finished)
            (Application Data*)
            (end_of_early_data)       -------->
                                                            ServerHello
                                                           + early_data
                                                       + pre_shared_key
                                                           + key_share*
                                                  {EncryptedExtensions}
                                                  {CertificateRequest*}
                                                             {Finished}
                                      <--------     [Application Data*]
            {Certificate*}
            {CertificateVerify*}
            {Finished}                -------->

            [Application Data]        <------->      [Application Data]

                  *  Indicates optional label on variant */
      } VariantRecord;

4.7.  Constants

   Typed constants can be defined or situation-dependent
                     messages that are not always sent.

                  () Indicates messages protected using keys
                     derived from early_traffic_secret.

                  {} Indicates messages protected using keys
                     derived from handshake_traffic_secret.

                  [] Indicates messages protected using keys
                     derived from traffic_secret_N

          Figure 4: Message flow for purposes of specification by
   declaring a symbol of the desired type and assigning values to it.

   Under-specified types (opaque, variable-length vectors, and
   structures that contain opaque) cannot zero round trip handshake

   [[OPEN ISSUE: Should it be assigned values.  No fields
   of possible to combine 0-RTT with the server
   authenticating via a multi-element structure or vector may be elided.

   For example:

      struct {
          uint8 f1;
          uint8 f2;
      } Example1;

      Example1 ex1 = {1, 4};  /* assigns f1 = 1, f2 = 4 */

4.8.  Cryptographic Attributes signature https://github.com/tlswg/tls13-spec/
   issues/443]]

   IMPORTANT NOTE: The two cryptographic operations -- digital signing, and
   authenticated encryption with additional security properties for 0-RTT data (AEAD) -- (regardless of
   the cipher suite) are
   designated digitally-signed, and aead-ciphered, respectively.  A
   field's cryptographic processing weaker than those for other kinds of TLS data.
   Specifically:

   1.  This data is specified by prepending an
   appropriate key word designation before not forward secret, because it is encrypted solely
       with the field's type
   specification.  Cryptographic keys PSK.

   2.  There are implied no guarantees of non-replay between connections.
       Unless the server takes special measures outside those provided
       by TLS, the current session
   state (see Section 5.1).

4.8.1.  Digital Signing

   A digitally-signed element is encoded as a struct DigitallySigned:

      struct {
         SignatureScheme algorithm;
         opaque signature<0..2^16-1>;
      } DigitallySigned;

   The algorithm field specifies server has no guarantee that the algorithm used (see same 0-RTT data was
       not transmitted on multiple 0-RTT connections (See
       Section 6.3.2.2 4.2.6.2 for more details).  This is especially relevant
       if the definition of this field).  The signature data is authenticated either with TLS client
       authentication or inside the application layer protocol.
       However, 0-RTT data cannot be duplicated within a digital
   signature using those algorithms over connection
       (i.e., the contents of server will not process the element.
   The contents themselves do same data twice for the
       same connection) and an attacker will not be able to make 0-RTT
       data appear on the wire but are simply
   calculated. to be 1-RTT data (because it is protected with
       different keys.)

   The length remainder of the signature is specified by the signing
   algorithm and key.

   In previous versions this document provides a detailed description of TLS, the ServerKeyExchange format meant that
   attackers can obtain a signature of a message
   TLS.

3.  Presentation Language

   This document deals with a chosen, 32-byte
   prefix.  Because TLS 1.3 servers are likely to also implement prior
   versions, the contents of the element always start with 64 bytes formatting of
   octet 32 data in order to clear that chosen-prefix.

   Following that padding is a context string used to disambiguate
   signatures for different purposes. an external
   representation.  The context string following very basic and somewhat casually
   defined presentation syntax will be
   specified whenever a digitally-signed element is used.  A single 0
   byte is appended to the context to act as a separator.

   Finally, the specified contents of the digitally-signed structure
   follow the 0 byte after the context string.  (See the example at the
   end of this section.)  The combined input is then fed into the corresponding signature
   algorithm to produce the signature value on the wire.  See
   Section 6.3.2.2 for algorithms defined syntax draws from
   several sources in this specification.

   In the following example
      struct {
          uint8 field1;
          uint8 field2;
          digitally-signed opaque {
            uint8 field3<0..255>;
            uint8 field4;
          };
      } UserType;

   Assume that the context string for the signature was specified as
   "Example".  The input for the signature/hash algorithm would be:

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      4578616d706c6500

   followed by the encoding of its structure.  Although it resembles the inner struct (field3
   programming language "C" in its syntax and field4).

   The length of the structure, XDR [RFC4506] in bytes, both its
   syntax and intent, it would be equal risky to two bytes
   for field1 and field2, plus two bytes for the signature algorithm,
   plus two bytes for the length of the signature, plus the length of
   the output of the signing algorithm. draw too many parallels.  The length
   purpose of the signature this presentation language is
   known because the algorithm and key used for the signing are known
   prior to encoding or decoding this structure.

4.8.2.  Authenticated Encryption with Additional Data (AEAD)

   In AEAD encryption, the plaintext is simultaneously encrypted and
   integrity protected. document TLS only; it has
   no general application beyond that particular goal.

3.1.  Basic Block Size

   The input may be representation of any length, and aead-
   ciphered output all data items is generally larger than the input in order to
   accommodate the integrity check value.

5.  The TLS Record Protocol explicitly specified.  The TLS Record Protocol takes messages to be transmitted, fragments
   the data into manageable blocks, protects the records, and transmits
   the result.  Received
   basic data block size is decrypted and verified, reassembled,
   and then delivered one byte (i.e., 8 bits).  Multiple byte data
   items are concatenations of bytes, from left to higher-level clients.

   Three protocols that use right, from top to
   bottom.  From the TLS Record Protocol are described byte stream, a multi-byte item (a numeric in
   this document: the TLS Handshake Protocol,
   example) is formed (using C notation) by:

      value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
              ... | byte[n-1];

   This byte ordering for multi-byte values is the Alert Protocol, commonplace network
   byte order or big-endian format.

3.2.  Miscellaneous

   Comments begin with "/*" and
   the application end with "*/".

   Optional components are denoted by enclosing them in "[[ ]]" double
   brackets.

   Single-byte entities containing uninterpreted data protocol.  In order to allow extension are of type
   opaque.

3.3.  Vectors

   A vector (single-dimensioned array) is a stream of homogeneous data
   elements.  The size of the
   TLS protocol, additional record content types can vector may be supported by specified at documentation
   time or left unspecified until runtime.  In either case, the
   TLS Record Protocol.  New record content type values are assigned by
   IANA in length
   declares the TLS Content Type Registry as described in Section 11.

   Implementations MUST NOT send record types number of bytes, not defined the number of elements, in this
   document unless negotiated by some extension.  If a TLS
   implementation receives an unexpected record type, it MUST send an
   "unexpected_message" alert.

   Any protocol designed the
   vector.  The syntax for use over TLS must be carefully designed to
   deal with all possible attacks against it.  As specifying a practical matter,
   this means new type, T', that the protocol designer must be aware is a fixed-
   length vector of what security
   properties TLS does and does not provide and cannot safely rely on
   the latter.

   Note type T is

      T T'[n];

   Here, T' occupies n bytes in particular that the length of data stream, where n is a record or absence multiple
   of traffic
   itself the size of T.  The length of the vector is not protected by encryption unless included in the sender uses
   encoded stream.

   In the
   supplied padding mechanism - see Section 5.2.3 for more details.

5.1.  Connection States

   [[TODO: I plan to totally rewrite or remove this.  IT seems like just
   cruft.]]

   A TLS connection state following example, Datum is defined to be three consecutive
   bytes that the operating environment protocol does not interpret, while Data is three
   consecutive Datum, consuming a total of the TLS Record
   Protocol.  It specifies nine bytes.

      opaque Datum[3];      /* three uninterpreted bytes */
      Datum Data[9];        /* 3 consecutive 3 byte vectors */

   Variable-length vectors are defined by specifying a record protection algorithm and its
   parameters as well as the record protection keys and IVs for the
   connection in both subrange of legal
   lengths, inclusively, using the read and the write directions.  The security
   parameters notation <floor..ceiling>.  When
   these are set by encoded, the TLS Handshake Protocol, which also
   determines when new cryptographic keys are installed and used for
   record protection.  The initial current state always specifies that
   records are not protected. actual length precedes the vector's contents
   in the byte stream.  The security parameters for length will be in the form of a TLS Connection read and write state are
   set by providing number
   consuming as many bytes as required to hold the following values:

   connection end
      Whether this entity vector's specified
   maximum (ceiling) length.  A variable-length vector with an actual
   length field of zero is considered the "client" or referred to as an empty vector.

      T T'<floor..ceiling>;

   In the "server" in
      this connection.

   Hash algorithm
      An algorithm used following example, mandatory is a vector that must contain
   between 300 and 400 bytes of type opaque.  It can never be empty.
   The actual length field consumes two bytes, a uint16, which is
   sufficient to generate keys from represent the appropriate secret value 400 (see Section 7.1 and Section 7.3).

   record protection algorithm
      The algorithm 3.4).  On the
   other hand, longer can represent up to 800 bytes of data, or 400
   uint16 elements, and it may be used for record protection.  This algorithm empty.  Its encoding will include a
   two-byte actual length field prepended to the vector.  The length of
   an encoded vector must be an even multiple of the AEAD type and thus provides integrity and
      confidentiality as length of a single primitive.  This specification
      includes the key size of this algorithm and
   element (for example, a 17-byte vector of the nonce for the
      AEAD algorithm.

   master secret
      A 48-byte secret shared between the two peers in the connection
      and used to generate keys for protecting data.

   client random
      A 32-byte value provided by the client.

   server random
      A 32-byte value provided by the server.

   These parameters are defined in the presentation language as:

      enum { server, client } ConnectionEnd;

      enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

      enum { aes_gcm } RecordProtAlgorithm; uint16 would be illegal).

      opaque mandatory<300..400>;
            /* The algorithms specified in KDFAlgorithm and
         RecordProtAlgorithm may length field is 2 bytes, cannot be added to. empty */

      struct {
          ConnectionEnd          entity;
          KDFAlgorithm           kdf_algorithm;
          RecordProtAlgorithm    record_prot_algorithm;
          uint8                  enc_key_length;
          uint8                  iv_length;
          opaque                 hs_master_secret[48];
          opaque                 master_secret[48];
          opaque                 client_random[32];
          opaque                 server_random[32];
      } SecurityParameters;

   [TODO: update this to handle new key hierarchy.]

   The connection state will use the security parameters
      uint16 longer<0..800>;
            /* zero to generate the
   following four items:

      client write key
      server write key
      client write iv
      server write iv 400 16-bit unsigned integers */

3.4.  Numbers

   The client write parameters basic numeric data type is an unsigned byte (uint8).  All larger
   numeric data types are used by the server when receiving and
   processing records and vice versa.  The algorithm used for generating
   these items formed from the security parameters is fixed-length series of bytes
   concatenated as described in Section 7.3.

   Once the security parameters have been set 3.1 and are also unsigned.  The
   following numeric types are predefined.

      uint8 uint16[2];
      uint8 uint24[3];
      uint8 uint32[4];
      uint8 uint64[8];

   All values, here and elsewhere in the keys have been
   generated, specification, are stored in
   network byte (big-endian) order; the connection states can be instantiated uint32 represented by making them
   the current states.  These current states MUST be updated for each
   record processed.  Each connection state includes the following
   elements:

   cipher state
      The current state of the encryption algorithm.  This will consist
      of hex
   bytes 01 02 03 04 is equivalent to the scheduled key for decimal value 16909060.

   Note that connection.

   sequence number
      Each connection state contains a sequence number, which is
      maintained separately for read and write states.  The sequence
      number in some cases (e.g., DH parameters) it is set necessary to
   represent integers as opaque vectors.  In such cases, they are
   represented as unsigned integers (i.e., additional leading zero at the beginning of a connection, and
      whenever
   octets are not used even if the key most significant bit is changed.  The sequence number set).

3.5.  Enumerateds

   An additional sparse data type is incremented
      after each record: specifically, the first record transmitted
      under a particular connection state and record key MUST use
      sequence number 0.  Sequence numbers are available called enum.  A field of
   type uint64 and MUST
      NOT exceed 2^64-1.  Sequence numbers do not wrap.  If a TLS
      implementation would need to wrap a sequence number, it MUST
      either rekey (Section 6.3.5.3) or terminate enum can only assume the connection.

5.2.  Record Layer

   The TLS record layer receives uninterpreted data from higher layers
   in non-empty blocks of arbitrary size.

5.2.1.  Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Message
   boundaries are not preserved values declared in the record layer (i.e., multiple
   messages definition.
   Each definition is a different type.  Only enumerateds of the same ContentType MAY
   type may be coalesced into a single
   TLSPlaintext record, assigned or a single message MAY compared.  Every element of an enumerated
   must be fragmented across
   several records).  Alert messages (Section 6.1) MUST NOT assigned a value, as demonstrated in the following example.
   Since the elements of the enumerated are not ordered, they can be
   fragmented across records.

      struct
   assigned any unique value, in any order.

      enum {
          uint8 major;
          uint8 minor; e1(v1), e2(v2), ... , en(vn) [[, (n)]] } ProtocolVersion; Te;

   An enumerated occupies as much space in the byte stream as would its
   maximal defined ordinal value.  The following definition would cause
   one byte to be used to carry fields of type Color.

      enum {
          alert(21),
          handshake(22),
          application_data(23)
          (255)
      } ContentType;

      struct {
          ContentType type;
          ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
          uint16 length;
          opaque fragment[TLSPlaintext.length]; red(3), blue(5), white(7) } TLSPlaintext;

   type
      The higher-level protocol used Color;

   One may optionally specify a value without its associated tag to process
   force the enclosed fragment.

   record_version
      The protocol version width definition without defining a superfluous element.

   In the current record is compatible with.  This
      value MUST be set to following example, Taste will consume two bytes in the data
   stream but can only assume the values 1, 2, or 4.

      enum { 3, 1 sweet(1), sour(2), bitter(4), (32000) } for all records.  This field is
      deprecated and MUST be ignored for all purposes.

   length Taste;

   The length (in bytes) names of the following TLSPlaintext.fragment.  The
      length MUST NOT exceed 2^14.

   fragment
      The application data.  This data is transparent and treated as elements of an
      independent block to be dealt with by enumeration are scoped within the higher-level protocol
      specified by
   defined type.  In the type field.

   This document describes TLS Version 1.3, which uses first example, a fully qualified reference to
   the version { 3,
   4 }.  The version value 3.4 second element of the enumeration would be Color.blue.  Such
   qualification is historical, deriving from not required if the use target of {
   3, 1 } for TLS 1.0 and { 3, 0 } for SSL 3.0.  In order the assignment is well
   specified.

      Color color = Color.blue;     /* overspecified, legal */
      Color color = blue;           /* correct, type implicit */

   For enumerateds that are never converted to maximize
   backwards compatibility, external representation,
   the record layer version identifies as
   simply TLS 1.0.  Endpoints supporting other versions negotiate numerical information may be omitted.

      enum { low, medium, high } Amount;

3.6.  Constructed Types

   Structure types may be constructed from primitive types for
   convenience.  Each specification declares a new, unique type.  The
   syntax for definition is much like that of C.

      struct {
          T1 f1;
          T2 f2;
          ...
          Tn fn;
      } [[T]];

   The fields within a structure may be qualified using the
   version type's name,
   with a syntax much like that available for enumerateds.  For example,
   T.f2 refers to use by following the procedure and requirements in
   Appendix C.

   Implementations MUST NOT send zero-length fragments of Handshake or
   Alert types, even if those fragments contain padding.  Zero-length
   fragments second field of Application data MAY the previous declaration.
   Structure definitions may be sent as they are potentially
   useful as a traffic analysis countermeasure.

   When record protection has not yet been engaged, TLSPlaintext embedded.

3.6.1.  Variants

   Defined structures are written directly onto the wire.  Once record
   protection has started, TLSPlaintext records are protected and sent
   as described in may have variants based on some knowledge that is
   available within the following section.

5.2.2.  Record Payload Protection environment.  The record protection functions translate a TLSPlaintext selector must be an enumerated
   type that defines the possible variants the structure
   into defines.  There
   must be a TLSCiphertext.  The deprotection functions reverse the
   process.  In TLS 1.3 as opposed to previous versions case arm for every element of TLS, all
   ciphers are modeled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide a unified encryption
   and authentication operation which turns plaintext into authenticated
   ciphertext and back again.

   AEAD ciphers take as input a single key, a nonce, a plaintext, and
   "additional data" to be included the enumeration declared in
   the authentication check, as
   described select.  Case arms have limited fall-through: if two case arms
   follow in Section 2.1 of [RFC5116].  The key is either immediate succession with no fields in between, then they
   both contain the
   client_write_key or same fields.  Thus, in the server_write_key example below, "orange"
   and "banana" both contain V2.  Note that this is a new piece of
   syntax in TLS 1.3 1.2.

   The body of the
   additional data input variant structure may be given a label for reference.
   The mechanism by which the variant is empty (zero length). selected at runtime is not
   prescribed by the presentation language.

      struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version =
          T1 f1;
          T2 f2;
          ....
          Tn fn;
           select (E) { 3, 1 };
               case e1: Te1;
               case e2: Te2;
               case e3: case e4: Te3;
               ....
               case en: Ten;
           } [[fv]];
      } [[Tv]];

   For example:

      enum { apple, orange, banana } VariantTag;

      struct {
          uint16 number;
          opaque string<0..10>; /* TLS v1.x variable length */
       uint16 length;
       aead-ciphered
      } V1;

      struct {
          uint32 number;
          opaque content[TLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
       } fragment; string[10];    /* fixed length */
      } TLSCiphertext;

   opaque_type
      The outer opaque_type field V2;

      struct {
          select (VariantTag) { /* value of a TLSCiphertext record selector is always
      set to the value 23 (application_data) implicit */
              case apple:
                V1;   /* VariantBody, tag = apple */
              case orange:
              case banana:
                V2;   /* VariantBody, tag = orange or banana */
          } variant_body;       /* optional label on variant */
      } VariantRecord;

3.7.  Constants

   Typed constants can be defined for outward compatibility
      with middleboxes accustomed to parsing previous versions purposes of TLS.
      The actual content type specification by
   declaring a symbol of the record is found in fragment.type
      after decryption.

   record_version
      The record_version field is identical desired type and assigning values to
      TLSPlaintext.record_version it.

   Under-specified types (opaque, variable-length vectors, and is always { 3, 1 }.  Note
   structures that the
      handshake protocol including the ClientHello and ServerHello
      messages authenticates the protocol version, so this value is
      redundant.

   length
      The length (in bytes) contain opaque) cannot be assigned values.  No fields
   of the following TLSCiphertext.fragment.
      The length MUST NOT exceed 2^14 + 256.  An endpoint that receives
      a record that exceeds this length MUST generate a fatal
      "record_overflow" alert.

   fragment.content
      The cleartext of TLSPlaintext.fragment.

   fragment.type multi-element structure or vector may be elided.

   For example:

      struct {
          uint8 f1;
          uint8 f2;
      } Example1;

      Example1 ex1 = {1, 4};  /* assigns f1 = 1, f2 = 4 */

4.  Handshake Protocol

   The actual content type of handshake protocol is used to negotiate the record.

   fragment.zeros
      An arbitrary-length run secure attributes of zero-valued bytes may appear in the
      cleartext after the type field.  This provides an opportunity for
      senders
   a session.  Handshake messages are supplied to pad any the TLS record by a chosen amount as long as the
      total stays layer,
   where they are encapsulated within record size limits.  See Section 5.2.3 for one or more
      details.

   fragment
      The AEAD encrypted form of TLSPlaintext.fragment +
      TLSPlaintext.type + zeros, where "+" denotes concatenation.

   The length of the per-record nonce (iv_length) is set to max(8 bytes,
   N_MIN) for TLSPlaintext or
   TLSCiphertext structures, which are processed and transmitted as
   specified by the AEAD algorithm (see [RFC5116] Section 4).  An AEAD
   algorithm where N_MAX is less than 8 current active session state.

      enum {
          client_hello(1),
          server_hello(2),
          new_session_ticket(4),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          certificate_request(13),
          certificate_verify(15),
          finished(20),
          key_update(24),
          (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case hello_retry_request:   HelloRetryRequest;
              case encrypted_extensions:  EncryptedExtensions;
              case certificate_request:   CertificateRequest;
              case certificate:           Certificate;
              case certificate_verify:    CertificateVerify;
              case finished:              Finished;
              case new_session_ticket:    NewSessionTicket;
              case key_update:            KeyUpdate;
          } body;
      } Handshake;

   Protocol messages MUST NOT be used with TLS.
   The per-record nonce for sent in the AEAD construction is formed order defined below (and shown
   in the diagrams in Section 2).  Sending handshake messages in an
   unexpected order results in an "unexpected_message" fatal error.
   Unneeded handshake messages are omitted, however.

   New handshake message types are assigned by IANA as follows:

   1. described in
   Section 10.

4.1.  Key Exchange Messages

   The 64-bit record sequence number is padded key exchange messages are used to exchange security capabilities
   between the left with
       zeroes client and server and to iv_length.

   2.  The padded sequence number is XORed with the static
       client_write_iv or server_write_iv, depending on establish the role.

   The resulting quantity (of length iv_length) is traffic keys used as
   to protect the per-
   record nonce.

   Note: This is a different construction from that in TLS 1.2, which
   specified a partially explicit nonce.

   The plaintext is the concatenation of TLSPlaintext.fragment handshake and
   TLSPlaintext.type.

   The AEAD output consists of the ciphertext output by the AEAD
   encryption operation.  The length of the plaintext is greater than
   TLSPlaintext.length due data.

4.1.1.  Client Hello

   When this message will be sent:

      When a client first connects to the inclusion of TLSPlaintext.type and
   however much padding a server, it is supplied by required to send
      the sender. ClientHello as its first message.  The length of
   aead_output client will generally be larger than also send a
      ClientHello when the plaintext, but by an
   amount that varies server has responded to its ClientHello with
      a ServerHello that selects cryptographic parameters that don't
      match the AEAD cipher.  Since the ciphers might
   incorporate padding, the amount of overhead could vary with different
   lengths of plaintext.  Symbolically,

      AEADEncrypted =
          AEAD-Encrypt(write_key, nonce, plaintext of fragment) client's "key_share" extension.  In order to decrypt and verify, that case, the cipher takes
      client MUST send the same ClientHello (without modification)
      except:

   -  Including a new KeyShareEntry as input the key,
   nonce, and lowest priority share (i.e.,
      appended to the AEADEncrypted value.  The output is either list of shares in the
   plaintext or an error indicating that "key_share" extension).

   -  Removing the decryption failed.  There EarlyDataIndication Section 4.2.6 extension if one
      was present.  Early data is no separate integrity check.  That is:

      plaintext of fragment =
          AEAD-Decrypt(write_key, nonce, AEADEncrypted) not permitted after HelloRetryRequest.

   If the decryption fails, a server receives a ClientHello at any other time, it MUST send a
   fatal "bad_record_mac" "unexpected_message" alert MUST be
   generated.

   An AEAD cipher MUST NOT produce an expansion and close the connection.

   Structure of greater than 255
   bytes.  An endpoint that receives a record from its peer with
   TLSCipherText.length larger than 2^14 + 256 octets MUST generate a
   fatal "record_overflow" alert.  This limit is derived from the
   maximum TLSPlaintext length of 2^14 octets + 1 octet for ContentType
   + the maximum AEAD expansion of 255 octets.

5.2.3.  Record Padding

   All encrypted this message:

      struct {
          uint8 major;
          uint8 minor;
      } ProtocolVersion;

      struct {
          opaque random_bytes[32];
      } Random;

      uint8 CipherSuite[2];    /* Cryptographic suite selector */

      struct {
          ProtocolVersion client_version = { 3, 4 };    /* TLS records can be padded v1.3 */
          Random random;
          opaque legacy_session_id<0..32>;
          CipherSuite cipher_suites<2..2^16-2>;
          opaque legacy_compression_methods<1..2^8-1>;
          Extension extensions<0..2^16-1>;
      } ClientHello;

   TLS allows extensions to inflate follow the size compression_methods field in an
   extensions block.  The presence of extensions can be detected by
   determining whether there are bytes following the
   TLSCipherText.  This allows the sender to hide compression_methods
   at the size end of the
   traffic ClientHello.  Note that this method of detecting
   optional data differs from an observer.

   When generating the normal TLS method of having a TLSCiphertext record, implementations MAY choose to
   pad.  An unpadded record
   variable-length field, but it is just a record used for compatibility with a padding length TLS
   before extensions were defined.  As of
   zero.  Padding is a string TLS 1.3, all clients and
   servers will send at least one extension (at least "key_share" or
   "pre_shared_key").

   client_version  The latest (highest valued) version of zero-valued bytes appended to the
   ContentType field before encryption.  Implementations MUST set the
   padding octets to all zeros before encrypting.

   Application Data records may contain a zero-length fragment.content
   if TLS
      protocol offered by the sender desires. client.  This permits generation of plausibly-sized
   cover traffic in contexts where SHOULD be the presence or absence same as the
      latest version supported.  For this version of activity
   may the specification,
      the version will be sensitive.  Implementations { 3, 4 }. (See Appendix C for details about
      backward compatibility.)

   random  32 bytes generated by a secure random number generator.  See
      Appendix B for additional information.

   legacy_session_id  Versions of TLS before TLS 1.3 supported a session
      resumption feature which has been merged with Pre-Shared Keys in
      this version (see Section 2.2).  This field MUST NOT send Handshake or Alert
   records that be ignored by a
      server negotiating TLS 1.3 and SHOULD be set as a zero length
      vector (i.e., a single zero byte length field) by clients which do
      not have a zero-length fragment.content.

   The padding sent cached session ID set by a pre-TLS 1.3 server.

   cipher_suites  This is automatically verified a list of the cryptographic options supported
      by the client, with the client's first preference first.  Each
      cipher suite defines a key exchange algorithm, a record protection
   mechanism: Upon successful decryption of
      algorithm (including secret key length) and a TLSCiphertext.fragment, hash to be used with
      HKDF.  The server will select a cipher suite or, if no acceptable
      choices are presented, return a "handshake_failure" alert and
      close the receiving implementation scans connection.  If the field from list contains cipher suites the end toward
      server does not recognize, support, or wish to use, the
   beginning until it finds a non-zero octet.  This non-zero octet is server
      MUST ignore those cipher suites, and process the content type remaining ones as
      usual.  Values are defined in Appendix A.4.

   legacy_compression_methods  Versions of TLS before 1.3 supported
      compression with the message.  This padding scheme was selected
   because it allows padding list of any encrypted supported compression methods being
      sent in this field.  For every TLS record by an arbitrary
   size (from zero up 1.3 ClientHello, this vector
      MUST contain exactly one byte set to TLS record size limits) without introducing new
   content types.  The design also enforces all-zero padding octets, zero, which allows for quick detection of padding errors.

   Implementations MUST limit their scanning corresponds to
      the cleartext returned
   from the AEAD decryption. "null" compression method in prior versions of TLS.  If a receiving implementation does not
   find a non-zero octet TLS
      1.3 ClientHello is received with any other value in this field,
      the cleartext, it should treat the record as
   having an unexpected ContentType, sending an "unexpected_message" server MUST generate a fatal "illegal_parameter" alert.

   The presence  Note
      that TLS 1.3 servers might receive TLS 1.2 or prior ClientHellos
      which contain other compression methods and MUST follow the
      procedures for the appropriate prior version of padding does not change TLS.

   extensions  Clients request extended functionality from servers by
      sending data in the overall record size
   limitations - extensions field.  The actual "Extension"
      format is defined in Section 4.2.

   In the full fragment plaintext may not exceed 2^14 octets.

   Selecting a padding policy event that suggests when and how much to pad is a complex topic, client requests additional functionality using
   extensions, and this functionality is beyond not supplied by the scope of this specification.  If server, the application layer protocol atop TLS permits padding, it may be
   preferable to pad application_data TLS records within
   client MAY abort the application
   layer.  Padding for encrypted handshake and alert handshake.  Note that TLS records must
   still be handled at the TLS layer, though.  Later documents may
   define padding selection algorithms, or define 1.3 ClientHello
   messages MUST always contain extensions, and a padding policy
   request mechanism through TLS extensions or some other means.

6.  The 1.3 server MUST
   respond to any TLS Handshaking Protocols 1.3 ClientHello without extensions with a fatal
   "decode_error" alert.  TLS has two subprotocols that are used to allow peers to agree upon
   security parameters for the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to report error
   conditions to each other.

   The 1.3 servers may receive TLS Handshake Protocol is responsible for 1.2
   ClientHello messages without extensions.  If negotiating TLS 1.2, a session,
   which consists of
   server MUST check that the following items:

   peer certificate
      X509v3 [RFC5280] certificate amount of data in the peer.  This element message precisely
   matches one of these formats; if not, then it MUST send a fatal
   "decode_error" alert.

   After sending the
      state may be null.

   cipher spec
      Specifies the authentication and key establishment algorithms, the
      hash for use with HKDF to generate keying material, and ClientHello message, the record
      protection algorithm (See Appendix A.5 client waits for formal definition.)

   resumption master secret a secret shared between the client and server that can
   ServerHello or HelloRetryRequest message.

4.1.2.  Server Hello

   When this message will be used as
      a pre-shared symmetric key (PSK) sent:

      The server will send this message in future connections.

   These items are then used response to create security parameters for use by
   the record layer when protecting application data.  Many connections
   can be instantiated using the same session using a PSK established in ClientHello
      message when it was able to find an initial handshake.

6.1.  Alert Protocol

   One acceptable set of algorithms
      and the content types supported client's "key_share" extension was acceptable.  If the
      client proposed groups are not acceptable by the TLS record layer is server, it will
      respond with a "handshake_failure" fatal alert.

   Structure of this message:

      struct {
          ProtocolVersion server_version;
          Random random;
          CipherSuite cipher_suite;
          Extension extensions<0..2^16-1>;
      } ServerHello;

   server_version  This field contains the
   alert type.  Alert messages convey version of TLS negotiated for
      this session.  Servers MUST select the severity lower of the message
   (warning or fatal) highest
      supported server version and a description of the alert.  Alert version offered by the client in
      the ClientHello.  In particular, servers MUST accept ClientHello
      messages with a level of fatal result in versions higher than those supported and negotiate
      the immediate termination highest mutually supported version.  For this version of the
   connection.  In this case, other connections corresponding to
      specification, the
   session may continue, but version is { 3, 4 }.  (See Appendix C for
      details about backward compatibility.)

   random  This structure is generated by the session identifier server and MUST be invalidated,
   preventing
      generated independently of the failed session ClientHello.random.

   cipher_suite  The single cipher suite selected by the server from the
      list in ClientHello.cipher_suites.  For resumed sessions, this
      field is the value from the state of the session being used resumed.
      [[TODO: interaction with PSK.]]

   extensions  A list of extensions.  Note that only extensions offered
      by the client can appear in the server's list.  In TLS 1.3, as
      opposed to establish new
   connections.  Like other messages, alert messages previous versions of TLS, the server's extensions are encrypted as
   specified by
      split between the current connection state.

      enum { warning(1), fatal(2), (255) } AlertLevel;

      enum {
          close_notify(0),
          end_of_early_data(1),
          unexpected_message(10),               /* fatal */
          bad_record_mac(20),                   /* fatal */
          record_overflow(22),                  /* fatal */
          handshake_failure(40),                /* fatal */
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),                /* fatal */
          unknown_ca(48),                       /* fatal */
          access_denied(49),                    /* fatal */
          decode_error(50),                     /* fatal */
          decrypt_error(51),                    /* fatal */
          protocol_version(70),                 /* fatal */
          insufficient_security(71),            /* fatal */
          internal_error(80),                   /* fatal */
          inappropriate_fallback(86),           /* fatal */
          user_canceled(90),
          missing_extension(109),               /* fatal */
          unsupported_extension(110),           /* fatal */
          certificate_unobtainable(111),
          unrecognized_name(112),
          bad_certificate_status_response(113), /* fatal */
          bad_certificate_hash_value(114),      /* fatal */
          unknown_psk_identity(115),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

6.1.1.  Closure Alerts

   The client ServerHello and the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.  Failure EncryptedExtensions
      Section 4.2.8 message.  The ServerHello MUST only include
      extensions which are required to properly
   close a connection does not prohibit a session from being resumed.

   close_notify
      This alert notifies establish the recipient that cryptographic
      context.  Currently the sender will not send
      any more messages on this connection.  Any data received after a
      closure only such extensions are "key_share",
      "pre_shared_key", and "early_data".  Clients MUST be ignored.

   end_of_early_data
      This alert is sent by check the client to indicate that all 0-RTT
      application_data messages have been transmitted (or none will be
      sent at all) and that this is
      ServerHello for the end presence of the flight.  This alert
      MUST be at the warning level.  Servers MUST NOT send this alert any forbidden extensions and clients receiving it if
      any are found MUST terminate the connection handshake with an
      "unexpected_message" a
      "illegal_parameter" alert.

   user_canceled
      This alert notifies the recipient that the sender is canceling  In prior versions of TLS, the
      handshake for some reason unrelated to a protocol failure.  If a
      user cancels an operation after the handshake is complete, just
      closing the connection by sending a "close_notify" is more
      appropriate.  This alert SHOULD
      extensions field could be followed by a "close_notify".
      This alert is generally a warning.

   Either party MAY initiate a close by sending a "close_notify" alert.
   Any data received after a closure alert is ignored.  If a transport-
   level close is received prior omitted entirely if not needed, similar
      to a "close_notify", the receiver
   cannot know that ClientHello.  As of TLS 1.3, all the data that was sent has been received.

   Each party MUST clients and servers will send
      at least one extension (at least "key_share" or "pre_shared_key").

   TLS 1.3 has a "close_notify" alert before closing the write
   side of downgrade protection mechanism embedded in the connection, unless some other fatal alert has been
   transmitted.  The other party MUST server's
   random value.  TLS 1.3 server implementations which respond to a
   ClientHello with a "close_notify"
   alert of its own and close down client_version indicating TLS 1.2 or below MUST
   set the connection immediately,
   discarding any pending writes.  The initiator last eight bytes of their Random value to the close need not
   wait for the responding "close_notify" alert before closing bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to a ClientHello with a
   client_version indicating TLS 1.1 or below SHOULD set the read
   side last eight
   bytes of their Random value to the connection.

   If the application protocol using bytes:

     44 4F 57 4E 47 52 44 00

   TLS provides 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
   that any data may be
   carried over the underlying transport after the last eight octets are not equal to either of these values.
   TLS connection is
   closed, 1.2 clients SHOULD also perform this check if the ServerHello
   indicates TLS implementation must receive 1.1 or below.  If a match is found, the responding
   "close_notify" alert before indicating to client MUST
   abort the application layer handshake with a fatal "illegal_parameter" alert.  This
   mechanism provides limited protection against downgrade attacks over
   and above that provided by the TLS connection has ended.  If Finished exchange: because the application protocol will
   ServerKeyExchange includes a signature over both random values, it is
   not
   transfer any additional data, but will only close the underlying
   transport connection, then the implementation MAY choose to close the
   transport without waiting possible for the responding "close_notify".  No part
   of this standard should be taken an active attacker to dictate modify the manner in which a
   usage profile for TLS manages its data transport, including when
   connections randoms without
   detection as long as ephemeral ciphers are opened or closed.

   Note: used.  It does not provide
   downgrade protection when static RSA is assumed that closing a connection reliably delivers
   pending data before destroying the transport.

6.1.2.  Error Alerts

   Error handling used.

   Note: This is an update to TLS 1.2 so in the practice many TLS Handshake Protocol is very simple. 1.2
   clients and servers will not behave as specified above.

4.1.3.  Hello Retry Request

   When an
   error is detected, the detecting party sends this message will be sent:

      Servers send this message in response to a ClientHello message if
      they were able to its peer.
   Upon transmission or receipt find an acceptable set of a fatal alert message, both parties
   immediately close the connection.  Servers and clients MUST forget
   any session-identifiers, keys, algorithms and secrets associated with groups
      that are mutually supported, but the client's KeyShare did not
      contain an acceptable offer.  If it cannot find such a failed
   connection.  Thus, any connection terminated match, it
      will respond with a fatal alert MUST
   NOT be resumed.

   Whenever an implementation encounters a condition which "handshake_failure" alert.

   Structure of this message:

      struct {
          ProtocolVersion server_version;
          CipherSuite cipher_suite;
          NamedGroup selected_group;
          Extension extensions<0..2^16-1>;
      } HelloRetryRequest;

   selected_group  The mutually supported group the server intends to
      negotiate and is defined as requesting a fatal alert, it MUST retried ClientHello/KeyShare for.

   The server_version, cipher_suite, and extensions fields have the same
   meanings as their corresponding values in the ServerHello.  [[NOTE:
   cipher_suite may disappear. https://github.com/tlswg/tls13-spec/
   issues/528]] The server SHOULD send only the appropriate alert prior to closing extensions necessary for
   the connection.  For all errors where an alert level is client to generate a correct ClientHello pair (currently no such
   extensions exist).  As with ServerHello, a HelloRetryRequest MUST NOT
   contain any extensions that were not
   explicitly specified, first offered by the sending party MAY determine at client in
   its
   discretion whether to treat this as ClientHello.

   Upon receipt of a fatal error or not.  If the
   implementation chooses to send an alert but intends to close HelloRetryRequest, the
   connection immediately afterwards, it client MUST send first verify
   that alert at the
   fatal alert level.

   If an alert with selected_group field corresponds to a level of warning is sent and received, generally group which was
   provided in the connection can continue normally.  If "supported_groups" extension in the receiving party decides original
   ClientHello.  It MUST then verify that the selected_group field does
   not correspond to proceed with the connection (e.g., after having received a
   "user_canceled" alert that it is not willing to accept), it SHOULD
   send a fatal alert to terminate the connection.  Given this, group which was provided in the
   sending peer cannot, "key_share"
   extension in general, know how the receiving party will
   behave.  Therefore, warning alerts are not very useful when original ClientHello.  If either of these checks
   fails, then the
   sending party wants to continue client MUST abort the connection, and thus are
   sometimes omitted.  For example, if a party decides to accept an
   expired certificate (perhaps after confirming this handshake with the user) and
   wants to continue the connection, it would not generally send a
   "certificate_expired" alert.

   The following error alerts are defined:

   unexpected_message
      An inappropriate message was received.  This alert is always fatal
      and should never be observed
   "handshake_failure" alert.  Clients SHOULD also abort with
   "handshake_failure" in communication between proper
      implementations.

   bad_record_mac
      This alert is returned if a record is received response to any second HelloRetryRequest which cannot be
      deprotected.  Because AEAD algorithms combine decryption and
      verification, this alert is used for all deprotection failures.
      This alert is always fatal and should never be observed in
      communication between proper implementations (except when messages
      were corrupted
   was sent in the network).

   record_overflow
      A TLSCiphertext record same connection (i.e., where the ClientHello was received that had
   itself in response to a length more than
      2^14 + 256 bytes, or HelloRetryRequest).

   Otherwise, the client MUST send a record decrypted ClientHello with an updated
   KeyShare extension to the server.  The client MUST append a TLSPlaintext record
      with more than 2^14 bytes.  This alert is always fatal and should
      never be observed in communication between proper implementations
      (except when messages were corrupted new
   KeyShareEntry for the group indicated in the network).

   handshake_failure
      Reception selected_group field to
   the groups in its original KeyShare.

   Upon re-sending the ClientHello and receiving the server's
   ServerHello/KeyShare, the client MUST verify that the selected
   CipherSuite and NamedGroup match that supplied in the
   HelloRetryRequest.  If either of these values differ, the client MUST
   abort the connection with a fatal "handshake_failure" alert message indicates that alert.

4.2.  Hello Extensions

   The extension format is:

      struct {
          ExtensionType extension_type;
          opaque extension_data<0..2^16-1>;
      } Extension;

      enum {
          supported_groups(10),
          signature_algorithms(13),
          key_share(40),
          pre_shared_key(41),
          early_data(42),
          cookie(44),
          (65535)
      } ExtensionType;

   Here:

   -  "extension_type" identifies the sender was unable particular extension type.

   -  "extension_data" contains information specific to negotiate an acceptable the particular
      extension type.

   The initial set of security
      parameters given the options available.  This alert extensions is always
      fatal.

   bad_certificate
      A certificate was corrupt, contained signatures that did not
      verify correctly, etc.

   unsupported_certificate
      A certificate was defined in [RFC6066].  The list of an unsupported type.

   certificate_revoked
      A certificate was revoked
   extension types is maintained by its signer.

   certificate_expired
      A certificate has expired IANA as described in Section 10.

   An extension type MUST NOT appear in the ServerHello or is not currently valid.

   certificate_unknown
      Some other (unspecified) issue arose
   HelloRetryRequest unless the same extension type appeared in processing the
      certificate, rendering
   corresponding ClientHello.  If a client receives an extension type in
   ServerHello or HelloRetryRequest that it unacceptable.

   illegal_parameter
      A field did not request in the
   associated ClientHello, it MUST abort the handshake was out with an
   "unsupported_extension" fatal alert.

   Nonetheless, "server-oriented" extensions may be provided within this
   framework.  Such an extension (say, of range or inconsistent type x) would require the
   client to first send an extension of type x in a ClientHello with
      other fields.  This alert is always fatal.

   unknown_ca
      A valid certificate chain or partial chain was received, but
   empty extension_data to indicate that it supports the
      certificate was not accepted because extension type.
   In this case, the CA certificate could not
      be located or couldn't be matched with a known, trusted CA.  This
      alert client is always fatal.

   access_denied
      A valid certificate or PSK was received, but when access control
      was applied, offering the sender decided not capability to proceed with negotiation.
      This alert understand the
   extension type, and the server is always fatal.

   decode_error
      A message could not be decoded because some field was out taking the client up on its offer.

   When multiple extensions of different types are present in the
      specified range
   ClientHello or ServerHello messages, the length extensions MAY appear in any
   order.  There MUST NOT be more than one extension of the message was incorrect.  This
      alert is always fatal and should never same type.

   Finally, note that extensions can be observed in
      communication between proper implementations (except sent both when starting a new
   session and when messages
      were corrupted in the network).

   decrypt_error resumption-PSK mode.  A handshake cryptographic operation failed, including being unable
      to correctly verify a signature or validate a Finished message.
      This alert is always fatal.

   protocol_version
      The protocol version the peer has attempted to negotiate is
      recognized but client that requests
   session resumption does not supported.  (For example, old protocol versions
      might be avoided for security reasons.)  This alert is always
      fatal.

   insufficient_security
      Returned instead of "handshake_failure" when a negotiation has
      failed specifically because in general know whether the server requires ciphers more
      secure than those supported by the client.  This alert is always
      fatal.

   internal_error
      An internal error unrelated to will
   accept this request, and therefore it SHOULD send the peer or same extensions
   as it would send normally.

   In general, the correctness specification of each extension type needs to
   describe the effect of the
      protocol (such as extension both during full handshake and
   session resumption.  Most current TLS extensions are relevant only
   when a memory allocation failure) makes it impossible
      to continue.  This alert session is always fatal.

   inappropriate_fallback
      Sent by a initiated: when an older session is resumed, the
   server does not process these extensions in response ClientHello, and does not
   include them in ServerHello.  However, some extensions may specify
   different behavior during session resumption.  [[TODO: update this
   and the previous paragraph to an invalid connection retry
      attempt from a client. (see [RFC7507]) This alert is always fatal.

   missing_extension
      Sent by endpoints cover PSK-based resumption.]]

   There are subtle (and not so subtle) interactions that receive may occur in
   this protocol between new features and existing features which may
   result in a hello message significant reduction in overall security.  The following
   considerations should be taken into account when designing new
   extensions:

   -  Some cases where a server does not containing an
      extension that is mandatory agree to send for the offered TLS version.
      This message is always fatal.  [[TODO: IANA Considerations.]]

   unsupported_extension
      Sent by endpoints receiving any hello message containing an extension known are error
      conditions, and some are simply refusals to support particular
      features.  In general, error alerts should be prohibited used for inclusion in the given hello
      message, including any extensions in former,
      and a ServerHello not first
      offered field in the corresponding ClientHello.  This alert is always
      fatal.

   certificate_unobtainable
      Sent by servers when unable server extension response for the latter.

   -  Extensions should, as far as possible, be designed to obtain a certificate from prevent any
      attack that forces use (or non-use) of a URL
      provided particular feature by
      manipulation of handshake messages.  This principle should be
      followed regardless of whether the client via feature is believed to cause a
      security problem.  Often the fact that the "client_certificate_url" extension
      [RFC6066].

   unrecognized_name
      Sent by servers when no server exists identified by fields are
      included in the name
      provided by inputs to the client via Finished message hashes will be
      sufficient, but extreme care is needed when the "server_name" extension [RFC6066].

   bad_certificate_status_response
      Sent by clients when an invalid changes
      the meaning of messages sent in the handshake phase.  Designers
      and implementors should be aware of the fact that until the
      handshake has been authenticated, active attackers can modify
      messages and insert, remove, or unacceptable OCSP response is
      provided by replace extensions.

4.2.1.  Cookie

      struct {
          opaque cookie<0..2^16-1>;
      } Cookie;

   Cookies serve two primary purposes:

   -  Allowing the server via to force the "status_request" extension
      [RFC6066]. client to demonstrate
      reachability at their apparent network address (thus providing a
      measure of DoS protection).  This alert is always fatal.

   bad_certificate_hash_value
      Sent by servers when primarily useful for non-
      connection-oriented transports (see [RFC6347] for an example of
      this).

   -  Allowing the server to offload state to the client, thus allowing
      it to send a retrieved object HelloRetryRequest without storing any state.  The
      server does not have the correct
      hash provided this by pickling that post-ClientHello hash state into
      the client via the "client_certificate_url"
      extension [RFC6066].  This alert is always fatal.

   unknown_psk_identity
      Sent by servers when cookie (protected with some suitable integrity algorithm).

   When sending a PSK cipher suite is selected but no
      acceptable PSK identity is provided by HelloRetryRequest, the client.  Sending this
      alert is OPTIONAL; servers server MAY instead choose to send provide a
      "decrypt_error" alert "cookie"
   extension to merely indicate the client (this is an invalid PSK identity.

   New Alert values exception to the usual rule that
   the only extensions that may be sent are assigned by IANA as described those that appear in Section 11.

6.2.  Handshake Protocol Overview

   The cryptographic parameters of the session state are produced by
   ClientHello).  When sending the
   TLS Handshake Protocol, which operates on top of new ClientHello, the TLS record
   layer.  When a TLS client and server first start communicating, they
   agree on a protocol version, select cryptographic algorithms,
   optionally authenticate each other, and establish shared secret
   keying material.

   TLS supports three basic key exchange modes:

   -  Diffie-Hellman (of both MUST echo
   the finite field and elliptic curve
      varieties).

   -  A pre-shared symmetric key (PSK)

   -  A combination value of a symmetric key and Diffie-Hellman

   Which mode is used depends on the negotiated cipher suite.
   Conceptually, extension.  Clients MUST NOT use cookies in
   subsequent connections.

4.2.2.  Signature Algorithms

   The client uses the handshake establishes three secrets "signature_algorithms" extension to indicate to
   the server which are signature algorithms may be used
   to derive all in digital
   signatures.

   Clients which offer one or more cipher suites which use certificate
   authentication (i.e., any non-PSK cipher suite) MUST send the keys.

   Figure 1 below shows
   "signature_algorithms" extension.  If this extension is not provided
   and no alternative cipher suite is available, the basic full TLS handshake.

       Client                                               Server

Key  ^ ClientHello
Exch | + key_share*
     v + pre_shared_key*        -------->
                                                       ServerHello  ^ Key
                                                      + key_share*  | Exch
                                                 + pre_shared_key*  v
                                             {EncryptedExtensions}  ^  Server
                                             {CertificateRequest*}  v  Params
                                                    {Certificate*}  ^
                                              {CertificateVerify*}  | Auth
                                                        {Finished}  v
                                 <--------     [Application Data*]
     ^ {Certificate*}
Auth | {CertificateVerify*}
     v {Finished}                -------->
       [Application Data]        <------->      [Application Data]

              +  Indicates extensions sent in server MUST close
   the
                 previously noted message.

              *  Indicates optional or situation-dependent
                 messages that are not always sent.

              {} Indicates messages protected using keys
                 derived from handshake_traffic_secret.

              [] Indicates messages protected using keys
                 derived from traffic_secret_N

               Figure 1: Message flow for full TLS Handshake connection with a fatal "missing_extension" alert.  (see
   Section 8.2)

   The handshake can be thought "extension_data" field of as having three phases, indicated this extension contains a
   "supported_signature_algorithms" value:

      enum {
          /* RSASSA-PKCS1-v1_5 algorithms */
          rsa_pkcs1_sha1 (0x0201),
          rsa_pkcs1_sha256 (0x0401),
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

          /* ECDSA algorithms */
          ecdsa_secp256r1_sha256 (0x0403),
          ecdsa_secp384r1_sha384 (0x0503),
          ecdsa_secp521r1_sha512 (0x0603),

          /* RSASSA-PSS algorithms */
          rsa_pss_sha256 (0x0700),
          rsa_pss_sha384 (0x0701),
          rsa_pss_sha512 (0x0702),

          /* EdDSA algorithms */
          ed25519 (0x0703),
          ed448 (0x0704),

          /* Reserved Code Points */
          private_use (0xFE00..0xFFFF),
          (0xFFFF)
      } SignatureScheme;

      SignatureScheme supported_signature_algorithms<2..2^16-2>;

   Note: This enum is named "SignatureScheme" because there is already a
   "SignatureAlgorithm" type in TLS 1.2, which this replaces.  We use
   the diagram above.

   Key Exchange: establish shared keying material and select term "signature algorithm" throughout the
   cryptographic parameters.  Everything after this phase is encrypted.

   Server Parameters: establish other handshake parameters (whether text.

   Each SignatureScheme value lists a single signature algorithm that
   the client is authenticated, application layer protocol support, etc.)

   Authentication: authenticate the server (and optionally the client)
   and provide key confirmation and handshake integrity.

   In the Key Exchange phase, the client sends the ClientHello
   (Section 6.3.1.1) message, which contains a random nonce
   (ClientHello.random), its offered protocol version, cipher suite, and
   extensions, and in general either one or more Diffie-Hellman key
   shares (in the "key_share" extension Section 6.3.2.4), one or more
   pre-shared key labels (in the "pre_shared_key" extension
   Section 6.3.2.5), or both.

   The server processes the ClientHello and determines the appropriate
   cryptographic parameters for the connection.  It then responds with
   its own ServerHello which indicates the negotiated connection
   parameters.  [Section 6.3.1.2]. willing to verify.  The combination of the ClientHello
   and the ServerHello determines the values are indicated in
   descending order of ES and SS, as described
   above.  If either preference.  Note that a pure (EC)DHE or (EC)DHE-PSK cipher suite is in
   use, then the ServerHello will contain signature algorithm
   takes as input an arbitrary-length message, rather than a "key_share" extension with
   the server's ephemeral Diffie-Hellman share digest.
   Algorithms which MUST traditionally act on a digest should be defined in
   TLS to first hash the same
   group.  If input with a pure PSK or an (EC)DHE-PSK cipher suite is negotiated, specified hash function and then
   proceed as usual.  The code point groups listed above have the ServerHello will contain
   following meanings:

   RSASSA-PKCS1-v1_5 algorithms  Indicates a "pre_shared_key" extension
   indicating which if the client's offered PSKs was selected.

   The server then sends two messages to establish signature algorithm using
      RSASSA-PKCS1-v1_5 [RFC3447] with the Server
   Parameters:

   EncryptedExtensions  responses corresponding hash algorithm
      as defined in [SHS].  These values refer solely to any extensions signatures
      which appear in certificates (see Section 4.3.1.1) and are not
      required
      defined for use in order to determine the cryptographic parameters.
      [Section 6.3.3.1]

   CertificateRequest  if certificate-based client authentication is
      desired, signed TLS handshake messages.

   ECDSA algorithms  Indicates a signature algorithm using ECDSA
      [ECDSA], the desired parameters for that certificate.  This
      message will be omitted if client authentication corresponding curve as defined in ANSI X9.62 [X962]
      and FIPS 186-4 [DSS], and the corresponding hash algorithm as
      defined in [SHS].  The signature is not desired.

   Finally, represented as a DER-encoded
      [X690] ECDSA-Sig-Value structure.

   RSASSA-PSS algorithms  Indicates a signature algorithm using RSASSA-
      PSS [RFC3447] with MGF1.  The digest used in the client mask generation
      function and server exchange Authentication messages. the digest being signed are both the corresponding
      hash algorithm as defined in [SHS].  When used in signed TLS
   uses
      handshake messages, the same set length of messages every time that authentication is
   needed.  Specifically:

   Certificate the certificate of salt MUST be equal to the endpoint.  This message is omitted if
      length of the
      server is not authenticating with digest output.

   EdDSA algorithms  Indicates a certificate (i.e., with PSK signature algorithm using EdDSA as
      defined in [I-D.irtf-cfrg-eddsa] or
      (EC)DHE-PSK cipher suites). its successors.  Note that if raw public keys
      [RFC7250] or
      these correspond to the cached information "PureEdDSA" algorithms and not the
      "prehash" variants.

   The semantics of this extension
      [I-D.ietf-tls-cached-info] are in use, then this message will not
      contain a certificate but rather some other value corresponding to somewhat complicated because the server's long-term key.  [Section 6.3.4.1]

   CertificateVerify
      a
   cipher suite adds additional constraints on signature over algorithms.
   Section 4.3.1.1 describes the entire handshake using appropriate rules.

   rsa_pkcs1_sha1, dsa_sha1, and ecdsa_sha1 SHOULD NOT be offered.
   Clients offering these values for backwards compatibility MUST list
   them as the public key lowest priority (listed after all other algorithms in the
      Certificate message.  This message is omitted if the server is not
      authenticating via
   supported_signature_algorithms vector).  TLS 1.3 servers MUST NOT
   offer a SHA-1 signed certificate (i.e., with PSK unless no valid certificate chain
   can be produced without it (see Section 4.3.1.1).

   The signatures on certificates that are self-signed or (EC)DHE-PSK
      cipher suites).  [Section 6.3.4.2]

   Finished certificates
   that are trust anchors are not validated since they begin a MAC over the entire handshake.  This message provides key
      confirmation, binds the endpoint's identity to the exchanged keys,
      and in PSK mode also authenticates the handshake.
      [Section 6.3.4.3]

   Upon receiving the server's messages, the client responds with its
   Authentication messages, namely Certificate and CertificateVerify (if
   requested), and Finished.

   At this point, the handshake
   certification path (see [RFC5280], Section 3.2).  A certificate that
   begins a certification path MAY use a signature algorithm that is complete, and the client and server
   may exchange application layer data.  Application data MUST NOT be
   sent prior to sending not
   advertised as being supported in the Finished message. "signature_algorithms"
   extension.

   Note that while the
   server may send application data prior TLS 1.2 defines this extension differently.  TLS 1.3
   implementations willing to receiving negotiate TLS 1.2 MUST behave in
   accordance with the client's
   Authentication messages, any data sent at that point is, requirements of course,
   being sent to an unauthenticated peer.

   [[TODO: Move this elsewhere?  Note [RFC5246] when negotiating that higher layers should not be
   overly reliant on whether
   version.  In particular:

   -  TLS always negotiates 1.2 ClientHellos may omit this extension.

   -  In TLS 1.2, the strongest
   possible connection between two endpoints.  There extension contained hash/signature pairs.  The
      pairs are a number of
   ways encoded in which a man-in-the-middle attacker can attempt to make two
   entities drop down to the least secure method they support (i.e.,
   perform a downgrade attack).  The TLS protocol has octets, so SignatureScheme values have
      been designed allocated to
   minimize this risk, but there align with TLS 1.2's encoding.  Some legacy
      pairs are still attacks available: for
   example, an attacker could block access to the port a secure service
   runs on, or attempt to get the peers to negotiate an unauthenticated
   connection.  The fundamental rule is that higher levels must be
   cognizant of what their security requirements left unallocated.  These algorithms are and never transmit
   information over a channel less secure than what they require.  The deprecated as of
      TLS protocol is secure in that 1.3.  They MUST NOT be offered or negotiated by any cipher suite offers its promised
   level of security: if you negotiate AES-GCM [GCM] with a 255-bit
   ECDHE key exchange with a host whose certificate chain you have
   verified, you can expect that to
      implementation.  In particular, MD5 [SLOTH] and SHA-224 MUST NOT
      be reasonably "secure" against
   algorithmic attacks, at least in the year 2015.]]

6.2.1.  Incorrect DHE Share

   If used.

   -  ecdsa_secp256r1_sha256, etc., align with TLS 1.2's ECDSA hash/
      signature pairs.  However, the client has old semantics did not provided an appropriate "key_share" extension
   (e.g. it includes only DHE or ECDHE groups unacceptable or
   unsupported constrain the
      signing curve.

4.2.3.  Negotiated Groups

   When sent by the server), client, the server corrects "supported_groups" extension indicates
   the mismatch with a
   HelloRetryRequest and named groups which the client will need supports for key exchange, ordered
   from most preferred to restart the handshake
   with an appropriate "key_share" extension, as shown in Figure 2.  If
   no common cryptographic parameters can be negotiated, the server will
   send a "handshake_failure" or "insufficient_security" fatal alert
   (see Section 6.1).

            Client                                               Server

            ClientHello
              + key_share             -------->
                                      <--------       HelloRetryRequest

            ClientHello
              + key_share             -------->
                                                            ServerHello
                                                            + key_share
                                                  {EncryptedExtensions}
                                                  {CertificateRequest*}
                                                         {Certificate*}
                                                   {CertificateVerify*}
                                                             {Finished}
                                      <--------     [Application Data*]
            {Certificate*}
            {CertificateVerify*}
            {Finished}                -------->
            [Application Data]        <------->     [Application Data]

        Figure 2: Message flow for a full handshake with mismatched
                                parameters least preferred.

   Note: the handshake transcript includes the initial ClientHello/
   HelloRetryRequest exchange.  It is not reset with the new
   ClientHello. In versions of TLS prior to TLS 1.3, this extension was named
   "elliptic_curves" and only contained elliptic curve groups.  See
   [RFC4492] and [I-D.ietf-tls-negotiated-ff-dhe].  This extension was
   also allows several optimized variants of the basic handshake, as
   described below.

6.2.2.  Resumption used to negotiate ECDSA curves.  Signature algorithms are now
   negotiated independently (see Section 4.2.2).

   Clients which offer one or more (EC)DHE cipher suites MUST send at
   least one supported NamedGroup value and Pre-Shared Key (PSK)

   Although TLS PSKs can be established out servers MUST NOT negotiate
   any of band, PSKs can also be
   established in these cipher suites unless a previous session supported value was provided.  If
   this extension is not provided and then reused ("session
   resumption").  Once a handshake has completed, no alternative cipher suite is
   available, the server can send MUST close the client a PSK identity which corresponds to connection with a key derived from the
   initial handshake (See fatal
   "missing_extension" alert.  (see Section 6.3.5.1).  The client can then use
   that PSK identity in future handshakes to negotiate use of 8.2) If the PSK;
   if extension is
   provided, but no compatible group is offered, the server accepts it, then the security context MUST NOT
   negotiate a cipher suite of the original
   connection is tied to relevant type.  For instance, if a
   client supplies only ECDHE groups, the new connection.  In TLS 1.2 and below, server MUST NOT negotiate
   finite field Diffie-Hellman.  If no acceptable group can be selected
   across all cipher suites, then the server MUST generate a fatal
   "handshake_failure" alert.

   The "extension_data" field of this
   functionality was provided by "session resumption" extension contains a
   "NamedGroupList" value:

      enum {
          /* Elliptic Curve Groups (ECDHE) */
          secp256r1 (23), secp384r1 (24), secp521r1 (25),
          x25519 (29), x448 (30),

          /* Finite Field Groups (DHE) */
          ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
          ffdhe6144 (259), ffdhe8192 (260),

          /* Reserved Code Points */
          ffdhe_private_use (0x01FC..0x01FF),
          ecdhe_private_use (0xFE00..0xFEFF),
          (0xFFFF)
      } NamedGroup;

      struct {
          NamedGroup named_group_list<1..2^16-1>;
      } NamedGroupList;

   Elliptic Curve Groups (ECDHE)  Indicates support of the corresponding
      named curve.  Note that some curves are also recommended in ANSI
      X9.62 [X962] and "session
   tickets" [RFC5077].  Both mechanisms FIPS 186-4 [DSS].  Others are obsoleted recommended in TLS 1.3.

   PSK cipher suites can either use PSK
      [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for private
      use.

   Finite Field Groups (DHE)  Indicates support of the corresponding
      finite field group, defined in combination with an (EC)DHE
   exchange [I-D.ietf-tls-negotiated-ff-dhe].
      Values 0x01FC through 0x01FF are reserved for private use.

   Items in order named_group_list are ordered according to provide forward secrecy in combination with
   shared keys, or can use PSKs alone, at the cost client's
   preferences (most preferred choice first).

   As of losing forward
   secrecy.

   Figure 3 shows TLS 1.3, servers are permitted to send the "supported_groups"
   extension to the client.  If the server has a pair of handshakes group it prefers to the
   ones in which the first establishes a
   PSK and "key_share" extension but is still willing to accept the second uses it:

          Client                                               Server

   Initial Handshake:
          ClientHello
           + key_share              -------->
                                                          ServerHello
                                                          + key_share
                                                {EncryptedExtensions}
                                                {CertificateRequest*}
                                                       {Certificate*}
                                                 {CertificateVerify*}
                                                           {Finished}
                                    <--------     [Application Data*]
          {Certificate*}
          {CertificateVerify*}
          {Finished}                -------->
                                    <--------      [NewSessionTicket]
          [Application Data]        <------->      [Application Data]

   Subsequent Handshake:
          ClientHello
            + key_share
            + pre_shared_key        -------->
                                                          ServerHello
                                                     + pre_shared_key
                                                         + key_share*
                                                {EncryptedExtensions}
                                                           {Finished}
                                    <--------     [Application Data*]
          {Finished}                -------->
          [Application Data]        <------->      [Application Data]

               Figure 3: Message flow for resumption and PSK

   As the server is authenticating via a PSK,
   ClientHello, it does not SHOULD send "supported_groups" to update the client's
   view of its preferences.  Clients MUST NOT act upon any information
   found in "supported_groups" prior to successful completion of the
   handshake, but MAY use the information learned from a
   Certificate or a CertificateVerify.  When a client offers resumption
   via PSK it SHOULD also supply successfully
   completed handshake to change what groups they offer to a server in
   subsequent connections.

4.2.4.  Key Share

   The "key_share" extension to contains the server
   as well; endpoint's cryptographic
   parameters for non-PSK key establishment methods (currently DHE or
   ECDHE).

   Clients which offer one or more (EC)DHE cipher suites MUST send this allows server to decline resumption
   extension and fall back to SHOULD send at least one supported KeyShareEntry value.
   Servers MUST NOT negotiate any of these cipher suites unless a
   full handshake.  A "key_share"
   supported value was provided.  If this extension MUST also be sent if is not provided in a
   ServerHello or ClientHello, and the
   client peer is attempting to negotiate an (EC)DHE-PSK offering (EC)DHE cipher suite.

6.2.3.  Zero-RTT Data

   When resuming via a PSK
   suites, then the endpoint MUST close the connection with a fatal
   "missing_extension" alert.  (see Section 8.2) Clients MAY send an appropriate ticket (i.e., one with
   the "allow_early_data" flag), clients can also send data on their
   first flight ("early data").  This data is encrypted solely under
   keys derived using the PSK as the static secret.  As shown
   empty client_shares vector in
   Figure 4, the Zero-RTT data is just added order to request group selection from
   the 1-RTT handshake in
   the first flight, server at the rest cost of the handshake uses the same messages.

            Client                                               Server

            ClientHello
              + early_data
              + key_share*
            (EncryptedExtensions)
            (Finished)
            (Application Data*)
            (end_of_early_data)        -------->
                                                            ServerHello
                                                           + early_data
                                                            + key_share
                                                  {EncryptedExtensions}
                                                  {CertificateRequest*}
                                                             {Finished}
                                      <--------     [Application Data*]
            {Certificate*}
            {CertificateVerify*}
            {Finished}                -------->

            [Application Data]        <------->      [Application Data]

                  *  Indicates optional or situation-dependent
                     messages that are not always sent.

                  () Indicates messages protected using keys
                     derived from early_traffic_secret.

                  {} Indicates messages protected using keys
                     derived from handshake_traffic_secret.

                  [] Indicates messages protected using keys
                     derived from traffic_secret_N

          Figure 4: Message flow for a zero an additional round trip handshake

   [[OPEN ISSUE: Should it be possible to combine 0-RTT with the server
   authenticating via a signature https://github.com/tlswg/tls13-spec/
   issues/443]]
   IMPORTANT NOTE: trip.  (see
   Section 4.1.3)

      struct {
          NamedGroup group;
          opaque key_exchange<1..2^16-1>;
      } KeyShareEntry;

   group  The security properties named group for 0-RTT data (regardless of the cipher suite) key being exchanged.  Finite Field
      Diffie-Hellman [DH] parameters are weaker than those for other kinds of TLS data.
   Specifically:

   1.  This data is not forward secret, because it is encrypted solely
       with the PSK.

   2.  There described in Section 4.2.4.1;
      Elliptic Curve Diffie-Hellman parameters are no guarantees described in
      Section 4.2.4.2.

   key_exchange  Key exchange information.  The contents of non-replay between connections.
       Unless this field
      are determined by the server takes special measures outside those provided
       by TLS (See Section 6.3.2.7.2), the server has no guarantee that
       the same 0-RTT data was not transmitted on multiple 0-RTT
       connections.  This is especially relevant if the data is
       authenticated either with TLS client authentication specified group and its corresponding
      definition.  Endpoints MUST NOT send empty or inside the
       application layer protocol.  However, 0-RTT data cannot be
       duplicated within a connection (i.e., the server will not process
       the same data twice otherwise invalid
      key_exchange values for the same connection) and an attacker will
       not be able to make 0-RTT data appear to be 1-RTT data (because
       it is protected with different keys.) any reason.

   The contents and significance "extension_data" field of each message will be presented in
   detail this extension contains a "KeyShare"
   value:

      struct {
          select (role) {
              case client:
                  KeyShareEntry client_shares<0..2^16-1>;

              case server:
                  KeyShareEntry server_share;
          }
      } KeyShare;

   client_shares  A list of offered KeyShareEntry values in descending
      order of client preference.  This vector MAY be empty if the following sections.

6.3.  Handshake Protocol

   The TLS Handshake Protocol
      client is one requesting a HelloRetryRequest.  The ordering of values
      here SHOULD match that of the defined higher-level clients ordering of offered support in the TLS Record Protocol.  This protocol is used to negotiate
      "supported_groups" extension.

   server_share  A single KeyShareEntry value for the
   secure attributes negotiated cipher
      suite.

   Clients offer an arbitrary number of KeyShareEntry values, each
   representing a session.  Handshake messages are supplied to
   the TLS record layer, where they are encapsulated within one or more
   TLSPlaintext single set of key exchange parameters.  For instance,
   a client might offer shares for several elliptic curves or TLSCiphertext structures, which are processed and
   transmitted as specified by the current active session state.

      enum {
          client_hello(1),
          server_hello(2),
          session_ticket(4),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          certificate_request(13),
          certificate_verify(15),
          finished(20),
          key_update(24),
          (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case hello_retry_request:   HelloRetryRequest;
              case encrypted_extensions:  EncryptedExtensions;
              case certificate_request:   CertificateRequest;
              case certificate:           Certificate;
              case certificate_verify:    CertificateVerify;
              case finished:              Finished;
              case session_ticket:        NewSessionTicket;
              case key_update:            KeyUpdate;
          } body;
      } Handshake; multiple
   FFDHE groups.  The TLS Handshake Protocol messages are presented below in the order
   they key_exchange values for each KeyShareEntry MUST be sent; sending handshake messages in an unexpected order
   results in an "unexpected_message" fatal error.  Unneeded handshake
   messages can be omitted, however.

   New handshake message types are assigned by IANA as described in
   Section 11.

6.3.1.  Key Exchange Messages

   The key exchange messages are used to exchange security capabilities
   between
   generated independently.  Clients MUST NOT offer multiple
   KeyShareEntry values for the client and server same group.  Clients and to establish MUST NOT offer
   any KeyShareEntry values for groups not listed in the traffic keys used client's
   "supported_groups" extension.

   Servers offer exactly one KeyShareEntry value, which corresponds to protect
   the handshake and key exchange used for the data.

6.3.1.1.  Client Hello

   When this message will be sent:

      When negotiated cipher suite.  Servers MUST
   NOT offer a client first connects to KeyShareEntry value for a server, it is required to send group not offered by the ClientHello as its first message.  The client will also send a
      ClientHello when the server has responded to
   in its ClientHello corresponding KeyShare or "supported_groups" extension.

   Implementations MAY check for violations of these rules and and MAY
   abort the connection with a ServerHello that selects cryptographic parameters that don't
      match the client's "key_share" extension.  In that case, fatal "illegal_parameter" alert if one is
   violated.

   If the
      client MUST send server selects an (EC)DHE cipher suite and no mutually
   supported group is available between the same ClientHello (without modification)
      except including two endpoints' KeyShare
   offers, yet there is a new KeyShareEntry as the lowest priority share
      (i.e., appended to mutually supported group that can be found via
   the list of shares in "supported_groups" extension, then the "key_share"
      extension).  If a server receives MUST reply with a ClientHello
   HelloRetryRequest.  If there is no mutually supported group at any other time,
      it all,
   the server MUST send a fatal "unexpected_message" alert NOT negotiate an (EC)DHE cipher suite.

   [[TODO: Recommendation about what the client offers.  Presumably
   which integer DH groups and close which curves.]]

4.2.4.1.  Diffie-Hellman Parameters

   Diffie-Hellman [DH] parameters for both clients and servers are
   encoded in the
      connection.

   Structure opaque key_exchange field of this message:

      The ClientHello message includes a random structure, which is used
      later KeyShareEntry in the protocol. a
   KeyShare structure.  The cipher suite list, passed from opaque value contains the client Diffie-Hellman
   public value (Y = g^X mod p), encoded as a big-endian integer, padded
   with zeros to the server size of p in bytes.

   Note: For a given Diffie-Hellman group, the
   ClientHello message, contains padding results in all
   public keys having the combinations of cryptographic
   algorithms supported same length.

   Peers SHOULD validate each other's public key Y by ensuring that 1 <
   Y < p-1.  This check ensures that the client in order of remote peer is properly behaved
   and isn't forcing the client's
   preference (favorite choice first).  Each cipher suite defines local system into a key
   exchange algorithm, a record protection algorithm (including secret
   key length) small subgroup.

4.2.4.2.  ECDHE Parameters

   ECDHE parameters for both clients and a hash to be used with HKDF.  The server will select
   a cipher suite or, if no acceptable choices servers are presented, return a
   "handshake_failure" alert and close the connection.  If the list
   contains cipher suites the server does not recognize, support, or
   wish to use, encoded in the server MUST ignore those cipher suites, and process the remaining ones as usual.

      struct {
          opaque random_bytes[32];
      } Random;

      uint8 CipherSuite[2];    /* Cryptographic suite selector */

      struct {
          ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
          Random random;
          opaque legacy_session_id<0..32>;
          CipherSuite cipher_suites<2..2^16-2>;
   opaque legacy_compression_methods<1..2^8-1>;
          Extension extensions<0..2^16-1>;
      } ClientHello;

   TLS allows extensions to follow the compression_methods key_exchange field in an
   extensions block.  The presence of extensions can be detected by
   determining whether there are bytes following the compression_methods
   at a KeyShareEntry in a KeyShare structure.

   For secp256r1, secp384r1 and secp521r1, the end of contents are the ClientHello.  Note that this method byte
   string representation of detecting
   optional data differs from an elliptic curve public value following the normal TLS method
   conversion routine in Section 4.3.6 of having ANSI X9.62 [X962].

   Although X9.62 supports multiple point formats, any given curve MUST
   specify only a
   variable-length field, but it is single point format.  All curves currently specified
   in this document MUST only be used for compatibility with TLS
   before extensions were defined.  As of TLS 1.3, the uncompressed point format
   (the format for all clients ECDH functions is considered uncompressed).

   For x25519 and
   servers will send at least one extension (at least "key_share" or
   "pre_shared_key").

   client_version
      The version of the TLS protocol by which the client wishes to
      communicate during this session.  This SHOULD be x448, the latest
      (highest valued) version supported by contents are the client.  For this
      version byte string inputs and
   outputs of the specification, the version will be { 3, 4 }. (See
      Appendix C for details about backward compatibility.)

   random corresponding functions defined in [RFC7748], 32 bytes generated by a secure random number generator.  See
      Appendix B
   for additional information.

   legacy_session_id x25519 and 56 bytes for x448.

   Note: Versions of TLS before prior to 1.3 permitted point negotiation; TLS
   1.3 supported a session resumption removes this feature which has been merged with Pre-Shared Keys in this version
      (see Section 6.2.2).  This field MUST be ignored by a server
      negotiating TLS 1.3 and SHOULD be set as a zero length vector
      (i.e., favor of a single zero byte length field) by clients which do not
      have a cached session ID set by a pre-TLS 1.3 server.

   cipher_suites
      This point format for each
   curve.

4.2.5.  Pre-Shared Key Extension

   The "pre_shared_key" extension is a list of used to indicate the cryptographic options supported by identity of
   the
      client, pre-shared key to be used with the client's first preference first.  Values are
      defined a given handshake in Appendix A.4.

   legacy_compression_methods
      Versions of TLS before 1.3 association
   with a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for
   background).

   Clients which offer one or more PSK cipher suites MUST send at least
   one supported compression psk_identity value and the list servers MUST NOT negotiate any
   of
      compression methods these cipher suites unless a supported value was supplied in this field.  For any TLS 1.3
      ClientHello, provided.  If
   this vector MUST contain exactly one byte set to
      zero, which corresponds to the "null" compression method in prior
      versions of TLS.  If a TLS 1.3 ClientHello extension is received with any
      other value in this field, not provided and no alternative cipher suite is
   available, the server MUST generate close the connection with a fatal
      "illegal_parameter"
   "missing_extension" alert.  Note  (see Section 8.2)

   The "extension_data" field of this extension contains a
   "PreSharedKeyExtension" value:

      opaque psk_identity<0..2^16-1>;

      struct {
          select (Role) {
              case client:
                  psk_identity identities<2..2^16-1>;

              case server:
                  uint16 selected_identity;
          }
      } PreSharedKeyExtension;

   identities  A list of the identities (labels for keys) that TLS 1.3 servers might
      receive TLS 1.2 or prior ClientHellos which contain other
      compression methods and MUST follow the procedures
      client is willing to negotiate with the server.  If sent alongside
      the "early_data" extension (see Section 4.2.6), the first identity
      is the one used for 0-RTT data.

   selected_identity  The server's chosen identity expressed as a
      (0-based) index into the
      appropriate prior version of TLS.

   extensions
      Clients request extended functionality from servers by sending
      data identies in the extensions field.  The actual "Extension" format client's list.

   If no suitable identity is
      defined in Section 6.3.2.

   In provided, the event that server MUST NOT negotiate a client requests additional functionality using
   extensions,
   PSK cipher suite and MAY respond with an "unknown_psk_identity" alert
   message.  Sending this functionality alert is not supplied by the server, the
   client OPTIONAL; servers MAY abort the handshake.  Note: TLS 1.3 ClientHello messages
   MUST always contain extensions, and instead choose
   to send a TLS 1.3 server MUST respond "decrypt_error" alert to
   any TLS 1.3 ClientHello without extensions with merely indicate an invalid PSK
   identity or instead negotiate use of a fatal
   "decode_error" alert.  TLS 1.3 servers may receive TLS 1.2
   ClientHello messages without extensions. non-PSK cipher suite, if
   available.

   If negotiating TLS 1.2, the server selects a PSK cipher suite, it MUST send a
   "pre_shared_key" extension with the identity that it selected.  The
   client MUST verify that the server's selected_identity is within the
   range supplied by the client.  If the server supplies an "early_data"
   extension, the client MUST check verify that the amount of data in server selected the message precisely
   matches one of these formats; if not, then it first
   offered identity.  If any other value is returned, the client MUST send
   generate a fatal
   "decode_error" alert.

   After sending "unknown_psk_identity" alert and close the ClientHello message,
   connection.

   Note that although 0-RTT data is encrypted with the client waits for first PSK
   identity, the server MAY fall back to 1-RTT and select a
   ServerHello or HelloRetryRequest message.

6.3.1.2.  Server Hello different
   PSK identity if multiple identities are offered.

4.2.6.  Early Data Indication

   When this message will be sent:

      The server will PSK resumption is used, the client can send this message application data in response to a ClientHello
      message when it was able to find an acceptable set
   its first flight of algorithms
      and the client's "key_share" extension was acceptable. messages.  If the client proposed groups are not acceptable by the server, opts to do so, it will
      respond with a "handshake_failure" fatal alert.

   Structure MUST
   supply an "early_data" extension as well as the "pre_shared_key"
   extension.

   The "extension_data" field of this message: extension contains an
   "EarlyDataIndication" value:

      struct {
          ProtocolVersion server_version;
          Random random;
          CipherSuite cipher_suite;
          Extension extensions<0..2^16-1>;
          select (Role) {
              case client:
                  uint32 obfuscated_ticket_age;

              case server:
                 struct {};
          } ServerHello;

   In prior versions of TLS, the extensions field could be omitted
   entirely if not needed, similar to ClientHello.  As of TLS 1.3, all
   clients and servers will send at least one extension (at least
   "key_share" or "pre_shared_key").

   server_version
      This field will contain the lower of that suggested by
      } EarlyDataIndication;

   obfuscated_ticket_age  The time since the client
      in the ClientHello and the highest supported by the server.  For
      this version of the specification, learned about the version
      server configuration that it is { 3, 4 }.  (See
      Appendix C for details about backward compatibility.)

   random using, in milliseconds.  This structure
      value is generated by added modulo 2^32 to with the "ticket_age_add" value that
      was included with the ticket, see Section 4.4.1.  This addition
      prevents passive observers from correlating sessions unless
      tickets are reused.  Note: because ticket lifetimes are restricted
      to a week, 32 bits is enough to represent any plausible age, even
      in milliseconds.

   A server and MUST be generated
      independently validate that the ticket_age is within a small
   tolerance of the ClientHello.random.

   cipher_suite time since the ticket was issued (see
   Section 4.2.6.2).

   The single cipher suite selected by parameters for the server from 0-RTT data (symmetric cipher suite, ALPN,
   etc.) are the list same as those which were negotiated in
      ClientHello.cipher_suites.  For resumed sessions, this field is the value from connection
   which established the state of PSK.  The PSK used to encrypt the session being resumed.  [[TODO:
      interaction with PSK.]]

   extensions
      A list of extensions.  Note that only extensions offered by early data
   MUST be the
      client can appear first PSK listed in the server's list.  In TLS 1.3 as opposed to
      previous versions of TLS, the server's extensions are split
      between client's "pre_shared_key"
   extension.

   0-RTT messages sent in the ServerHello and first flight have the EncryptedExtensions
      Section 6.3.3.1 message.  The ServerHello MUST only include
      extensions which same content types
   as their corresponding messages sent in other flights (handshake,
   application_data, and alert respectively) but are required protected under
   different keys.  After all the 0-RTT application data messages (if
   any) have been sent, an "end_of_early_data" alert of type "warning"
   is sent to establish indicate the cryptographic
      context.  Currently end of the only such extensions are "key_share",
      "pre_shared_key", and "early_data".  Clients flight.  0-RTT MUST check always be
   followed by an "end_of_early_data" alert.

   A server which receives an "early_data" extension can behave in one
   of two ways:

   -  Ignore the
      ServerHello for extension and return no response.  This indicates that
      the presence of server has ignored any forbidden extensions early data and if
      any are found MUST terminate the an ordinary 1-RTT
      handshake with a
      "illegal_parameter" alert.

   TLS 1.3 is required.

   -  Return an empty extension, indicating that it intends to process
      the early data.  It is not possible for the server implementations which respond to accept only
      a ClientHello with a
   client_version indicating TLS 1.2 or below MUST set the first eight
   bytes subset of their Random value the early data messages.

   In order to accept early data, the bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to a ClientHello with server MUST have accepted a
   client_version indicating TLS 1.1 or below SHOULD set
   PSK cipher suite and selected the the first eight
   bytes of their Random value to key offered in the bytes:

     44 4F 57 4E 47 52 44 00

   TLS 1.3 clients receiving a TLS 1.2 or below ServerHello
   client's "pre_shared_key" extension.  In addition, it MUST check verify
   that the top eight octets following values are not equal to either of these values. consistent with those negotiated in the
   connection during which the ticket was established.

   -  The TLS 1.2 clients SHOULD also perform this check version number, symmetric ciphersuite, and the hash for
      HKDF.

   -  The selected ALPN [RFC7443] value, if any.

   -  The server_name [RFC6066] value provided by the ServerHello
   indicates TLS 1.1 or below. client, if any.

   Future extensions MUST define their interaction with 0-RTT.

   If a match is found, any of these checks fail, the client server MUST
   abort the handshake NOT respond with a fatal "illegal_parameter" alert.  This
   mechanism provides limited protection against downgrade attacks over the
   extension and above that provided by must discard all the Finished exchange: because remaining first flight data (thus
   falling back to 1-RTT).  If the
   ServerKeyExchange includes client attempts a signature over both random values, 0-RTT handshake but
   the server rejects it, it is will generally not possible for an active attacker to modify have the randoms without
   detection as long as ephemeral ciphers are used.  It does not provide
   downgrade 0-RTT record
   protection when static RSA is used.

   Note: This is an update to TLS 1.2 so in practice many TLS 1.2
   clients keys and servers will not behave as specified above.

   Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of must instead trial decrypt each record with the
   Random value to encode
   1-RTT handshake keys until it finds one that decrypts properly, and
   then pick up the time since handshake from that point.

   If the UNIX epoch.  The sentinel
   value above was selected server chooses to avoid conflicting accept the "early_data" extension, then it
   MUST comply with any valid TLS 1.2
   Random value and to have a low (2^{-64}) probability of colliding
   with randomly selected Random values.

6.3.1.3.  Hello Retry Request

   When this message will be sent:

      Servers send this message in response to a ClientHello message the same error handling requirements specified for
   all records when it was able to find an acceptable set processing early data records.  Specifically,
   decryption failure of algorithms and
      groups that are mutually supported, but the client's KeyShare did
      not contain any 0-RTT record following an acceptable offer.  If it cannot find such a match,
      it will respond with accepted
   "early_data" extension MUST produce a fatal "handshake_failure" alert.

   Structure of this message:

      struct {
          ProtocolVersion server_version;
          CipherSuite cipher_suite;
          NamedGroup selected_group;
          Extension extensions<0..2^16-1>;
      } HelloRetryRequest;

   selected_group
      The mutually supported group the server intends to negotiate and
      is requesting a retried ClientHello/KeyShare for.

   The server_version, cipher_suite, and extensions fields have the same
   meanings "bad_record_mac" alert as their corresponding values in
   per Section 5.2.

   If the ServerHello.  The server SHOULD send only rejects the extensions necessary for "early_data" extension, the client
   application MAY opt to
   generate a correct ClientHello pair.  As with ServerHello, a
   HelloRetryRequest MUST NOT contain any extensions that were not first
   offered by retransmit the client in its ClientHello.

   Upon receipt of a HelloRetryRequest, data once the handshake has
   been completed.  TLS stacks SHOULD not do this automatically and
   client applications MUST first verify take care that the selected_group field corresponds to a group which was
   provided in the "supported_groups" extension in the original
   ClientHello.  It MUST then verify that negotiated parameters are
   consistent with those it expected.  For example, if the selected_group field does
   not correspond ALPN value
   has changed, it is likely unsafe to a group which was provided in the "key_share"
   extension in retransmit the original ClientHello.  If either of these checks
   fails,
   application layer data.

4.2.6.1.  Processing Order

   Clients are permitted to "stream" 0-RTT data until they receive the
   server's Finished, only then sending the client "end_of_early_data" alert.
   In order to avoid deadlock, when accepting "early_data", servers MUST abort
   process the handshake with a fatal
   "handshake_failure" client's Finished and then immediately send the
   ServerHello, rather than waiting for the client's "end_of_early_data"
   alert.  Clients SHOULD also abort with
   "handshake_failure"

4.2.6.2.  Replay Properties

   As noted in response to any second HelloRetryRequest which
   was Section 2.3, TLS provides a limited mechanism for replay
   protection for data sent in by the same connection (i.e., where client in the ClientHello was
   itself first flight.

   The "obfuscated_ticket_age" parameter in response to a HelloRetryRequest).

   Otherwise, the client MUST send a ClientHello with an updated
   KeyShare client's "early_data"
   extension SHOULD be used by servers to limit the server.  The client MUST append a new
   KeyShareEntry for time over which the group indicated in
   first flight might be replayed.  A server can store the selected_group field time at which
   it sends a session ticket to the groups client, or encode the time in its original KeyShare.

   Upon re-sending the ClientHello
   ticket.  Then, each time it receives an "early_data" extension, it
   can subtract the base value and receiving check to see if the server's
   ServerHello/KeyShare, value used by the
   client MUST verify that matches its expectations.

   The ticket age (the value with "ticket_age_add" subtracted) provided
   by the selected
   CipherSuite and NamedGroup match that supplied in client will be shorter than the
   HelloRetryRequest.  If either actual time elapsed on the
   server by a single round trip time.  This difference is comprised of these values differ,
   the client MUST
   abort delay in sending the connection with a fatal "handshake_failure" alert.

6.3.2.  Hello Extensions

   The extension format is:

      struct {
          ExtensionType extension_type;
          opaque extension_data<0..2^16-1>;
      } Extension;

      enum {
          supported_groups(10),
          signature_algorithms(13),
          key_share(40),
          pre_shared_key(41),
          early_data(42),
          ticket_age(43),
          cookie (44),
          (65535)
      } ExtensionType;

   Here:

   -  "extension_type" identifies the particular extension type.

   -  "extension_data" contains information specific NewSessionTicket message to the particular
      extension type.

   The initial set of extensions is defined in [RFC6066].  The list of
   extension types is maintained by IANA as described in Section 11.

   An extension type MUST NOT appear in client, plus
   the ServerHello or
   HelloRetryRequest unless time taken to send the same extension type appeared in ClientHello to the
   corresponding ClientHello.  If server.  For this
   reason, a client receives an extension type in
   ServerHello or HelloRetryRequest that it did not request in server SHOULD measure the
   associated ClientHello, it MUST abort round trip time prior to sending
   the handshake with an
   "unsupported_extension" fatal alert.

   Nonetheless, "server-oriented" extensions may be provided NewSessionTicket message and account for that in the
   future within this framework.  Such an extension (say, of type x)
   would require value it
   saves.

   To properly validate the client to first send an extension of type x in ticket age, a
   ClientHello with empty extension_data server needs to indicate save at least
   two items:

   -  The time that it supports the extension type.  In this case, server generated the client is offering session ticket and the
   capability
      estimated round trip time can be added together to understand the extension type, and form a baseline
      time.

   -  The "ticket_age_add" parameter from the server NewSessionTicket is taking
   the client up on its offer.

   When multiple extensions of different types are present in needed
      to recover the
   ClientHello or ServerHello messages, ticket age from the extensions MAY appear in any
   order. "obfuscated_ticket_age"
      parameter.

   There MUST NOT be more than one extension are several potential sources of the same type.

   Finally, note error that extensions can be sent both when starting a new
   session make an exact
   measurement of time difficult.  Variations in client and when requesting session resumption or 0-RTT mode.
   Indeed, server
   clocks are likely to be minimal, outside of gross time corrections.
   Network propagation delays are most likely causes of a client that requests session resumption does not mismatch in general
   know whether
   legitimate values for elapsed time.  Both the server will accept this request, NewSessionTicket and
   ClientHello messages might be retransmitted and therefore it
   SHOULD send delayed,
   which might be hidden by TCP.

   A small allowance for errors in clocks and variations in measurements
   is advisable.  However, any allowance also increases the same extensions as it would send if opportunity
   for replay.  In this case, it were not
   attempting resumption. is better to reject early data than to
   risk greater exposure to replay attacks.

4.2.7.  OCSP Status Extensions

   [RFC6066] and [RFC6961] provide extensions to negotiate the server
   sending OCSP responses to the client.  In general, TLS 1.2 and below, the specification of each
   server sends an empty extension type needs to
   describe the effect indicate negotiation of the this
   extension both during full handshake and
   session resumption.  Most current TLS extensions are relevant only
   when the OCSP information is carried in a session CertificateStatus
   message.  In TLS 1.3, the server's OCSP information is initiated: when carried in an older session is resumed,
   extension in EncryptedExtensions.  Specifically: The body of the
   "status_request" or "status_request_v2" extension from the server does not process these extensions in ClientHello, and does not
   include them
   MUST be a CertificateStatus structure as defined in ServerHello.  However, some extensions may specify
   different behavior during session resumption.  [[TODO: update this [RFC6066] and
   [RFC6961] respectively.

   Note: This means that the previous paragraph certificate status appears prior to cover PSK-based resumption.]]

   There are subtle (and not so subtle) interactions that may occur in
   this protocol between new features and the
   certificates it applies to.  This is slightly anomalous but matches
   the existing features which may
   result in a significant reduction in overall security.  The following
   considerations should be taken into account when designing new
   extensions:

   -  Some cases where a server does not agree to an extension are error
      conditions, and some are simply refusals to support particular
      features.  In general, error alerts should be used behavior for the former, SignedCertificateTimestamps [RFC6962], and a field
   is more easily extensible in the server extension response for the latter.

   - handshake state machine.

4.2.8.  Encrypted Extensions should, as far as possible,

   When this message will be designed to prevent any
      attack sent:

      In all handshakes, the server MUST send the EncryptedExtensions
      message immediately after the ServerHello message.  This is the
      first message that forces use (or non-use) of a particular feature by
      manipulation is encrypted under keys derived from
      handshake_traffic_secret.

   Meaning of handshake messages.  This principle this message:

      The EncryptedExtensions message contains any extensions which
      should be
      followed regardless of whether the feature is believed protected, i.e., any which are not needed to cause a
      security problem.  Often the fact that establish
      the cryptographic context.

   The same extension fields are
      included types MUST NOT appear in both the inputs to the Finished message hashes will be
      sufficient, but extreme care is needed when ServerHello and
   EncryptedExtensions.  If the same extension changes
      the meaning of messages sent appears in both
   locations, the handshake phase.  Designers
      and implementors should be aware of client MUST rely only on the fact that until value in the
      handshake has been authenticated, active attackers can modify
      messages and insert, remove, or replace extensions.

   -  It would be technically possible to use
   EncryptedExtensions block.  All server-sent extensions other than
   those explicitly listed in Section 4.1.2 or designated in the IANA
   registry MUST only appear in EncryptedExtensions.  Extensions which
   are designated to change major
      aspects of appear in ServerHello MUST NOT appear in
   EncryptedExtensions.  Clients MUST check EncryptedExtensions for the design
   presence of TLS; for example, any forbidden extensions and if any are found MUST
   terminate the design handshake with an "illegal_parameter" alert.

   Structure of cipher
      suite negotiation.  This is not recommended; it would this message:

      struct {
          Extension extensions<0..2^16-1>;
      } EncryptedExtensions;

   extensions  A list of extensions.

4.2.9.  Certificate Request

   When this message will be more
      appropriate to define sent:

      A non-anonymous server can optionally request a new version of TLS -- particularly since
      the TLS handshake algorithms have specific protection against
      version rollback attacks based on certificate from
      the version number, and client, if appropriate for the
      possibility selected cipher suite.  This
      message, if sent, will follow EncryptedExtensions.

   Structure of version rollback should be a significant
      consideration in any major design change.

6.3.2.1.  Cookie this message:

      opaque DistinguishedName<1..2^16-1>;

      struct {
          opaque cookie<0..255>; certificate_extension_oid<1..2^8-1>;
          opaque certificate_extension_values<0..2^16-1>;
      } Cookie;

   Cookies serve two primary purposes:

   -  Allowing the server to force CertificateExtension;

      struct {
          opaque certificate_request_context<0..2^8-1>;
          SignatureScheme
            supported_signature_algorithms<2..2^16-2>;
          DistinguishedName certificate_authorities<0..2^16-1>;
          CertificateExtension certificate_extensions<0..2^16-1>;
      } CertificateRequest;

   certificate_request_context  An opaque string which identifies the client to demonstrate
      reachability at their apparent network address
      certificate request and which will be echoed in the client's
      Certificate message.  The certificate_request_context MUST be
      unique within the scope of this connection (thus providing a
      measure preventing replay
      of DoS protection).  This is primarily useful for non-
      connection-oriented transports (see [RFC6347] for an example client CertificateVerify messages).

   supported_signature_algorithms  A list of
      this).

   -  Allowing the server to offload state to the client, thus allowing
      it to send a HelloRetryRequest without storing any state.  The
      server does this by pickling signature algorithms
      that post-ClientHello hash state into
      the cookie (protected with some suitable integrity algorithm).

   When sending a HelloRetryRequest, the server MAY provide a "cookie"
   extension to the client (this is an exception able to verify, listed in descending order of
      preference.  Any certificates provided by the usual rule that
   the only extensions that may client MUST be sent are those that appear
      signed using a signature algorithm found in
      supported_signature_algorithms.

   certificate_authorities  A list of the
   ClientHello).  When sending distinguished names [X501] of
      acceptable certificate_authorities, represented in DER-encoded
      [X690] format.  These distinguished names may specify a desired
      distinguished name for a root CA or for a subordinate CA; thus,
      this message can be used to describe known roots as well as a
      desired authorization space.  If the new ClientHello, certificate_authorities list
      is empty, then the client MUST echo MAY send any certificate that meets the value
      rest of the extension.  Clients MUST NOT use cookies selection criteria in
   subsequent connections.

6.3.2.2.  Signature Algorithms

   The client uses the "signature_algorithms" extension to indicate CertificateRequest, unless
      there is some external arrangement to the server which signature algorithms may be used in digital
   signatures.

   Clients which offer one or more cipher suites which use contrary.

   certificate_extensions  A list of certificate extension OIDs
      [RFC5280] with their allowed values, represented in DER-encoded
      [X690] format.  Some certificate extension OIDs allow multiple
      values (e.g.  Extended Key Usage).  If the server has included a
      non-empty certificate_extensions list, the client certificate
   authentication (i.e., any non-PSK cipher suite) MUST send
      contain all of the
   "signature_algorithms" extension. specified extension OIDs that the client
      recognizes.  For each extension OID recognized by the client, all
      of the specified values MUST be present in the client certificate
      (but the certificate MAY have other values as well).  However, the
      client MUST ignore and skip any unrecognized certificate extension
      OIDs.  If this the client has ignored some of the required certificate
      extension is not provided OIDs, and no alternative cipher suite is available, supplied a certificate that does not satisfy
      the request, the server MUST close MAY at its discretion either continue the connection
      session without client authentication, or terminate the session
      with a fatal "missing_extension" unsupported_certificate alert.  (see
   Section 8.2)

   The "extension_data" field  PKIX RFCs define a
      variety of this certificate extension contains a
   "supported_signature_algorithms" value:

      enum {
          /* RSASSA-PKCS-v1_5 algorithms */
          rsa_pkcs1_sha1 (0x0201),
          rsa_pkcs1_sha256 (0x0401),
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

          /* ECDSA algorithms */
          ecdsa_secp256r1_sha256 (0x0403),
          ecdsa_secp384r1_sha384 (0x0503),
          ecdsa_secp521r1_sha512 (0x0603),

          /* RSASSA-PSS algorithms */
          rsa_pss_sha256 (0x0700),
          rsa_pss_sha384 (0x0701),
          rsa_pss_sha512 (0x0702),

          /* EdDSA algorithms */
          ed25519 (0x0703),
          ed448 (0x0704),

          /* Reserved Code Points */
          private_use (0xFE00..0xFFFF),
          (0xFFFF)
      } SignatureScheme;

      SignatureScheme supported_signature_algorithms<2..2^16-2>;

   Note: This production is named "SignatureScheme" because there OIDs and their corresponding
      value types.  Depending on the type, matching certificate
      extension values are not necessarily bitwise-equal.  It is
   already a SignatureAlgorithm type in
      expected that TLS 1.2.  We use implementations will rely on their PKI libraries
      to perform certificate selection using certificate extension OIDs.
      This document defines matching rules for two standard certificate
      extensions defined in [RFC5280]:

      o  The Key Usage extension in a certificate matches the term
   "signature algorithm" throughout request
         when all key usage bits asserted in the text.

   Each SignatureScheme value lists a single signature algorithm that request are also
         asserted in the client is willing to verify. Key Usage certificate extension.

      o  The values are indicated Extended Key Usage extension in
   descending order of preference.  Note that a signature algorithm
   takes as input an arbitrary-length message, rather than a digest.
   Algorithms which traditionally act on a digest should be defined certificate matches the
         request when all key purpose OIDs present in
   TLS to first hash the input with a specified hash function and then
   proceed as usual. request are
         also found in the Extended Key Usage certificate extension.
         The code point groups listed above have special anyExtendedKeyUsage OID MUST NOT be used in the
   following meanings:

   RSASSA-PKCS-v1_5 algorithms
      Indicates
         request.

      Separate specifications may define matching rules for other
      certificate extensions.

   Note: It is a signature algorithm using RSASSA-PKCS1-v1_5 [RFC3447]
      with the corresponding hash algorithm as defined in [SHS].  These
      values refer solely fatal "handshake_failure" alert for an anonymous server
   to signatures which appear request client authentication.

4.3.  Authentication Messages

   As discussed in certificates
      (see Section 6.3.4.1.1) 2, TLS uses a common set of messages for
   authentication, key confirmation, and handshake integrity:
   Certificate, CertificateVerify, and Finished.  These messages are not defined for use
   always sent as the last messages in signed TLS their handshake flight.  The
   Certificate and CertificateVerify messages (see Section 4.8.1).

   ECDSA algorithms
      Indicates a signature algorithm using ECDSA [ECDSA], the
      corresponding curve as defined in ANSI X9.62 [X962] and FIPS 186-4
      [DSS], and the corresponding hash algorithm are only sent under
   certain circumstances, as defined in [SHS]. below.  The signature Finished message is represented
   always sent as a DER-encoded [X690] ECDSA-Sig-
      Value structure.

   RSASSA-PSS algorithms
      Indicates a signature algorithm using RSASSA-PSS [RFC3447] with
      MGF1. part of the Authentication block.

   The digest used in computations for the mask generation function and Authentication messages all uniformly take
   the
      digest being signed are both following inputs:

   -  The certificate and signing key to be used.

   -  A Handshake Context based on the corresponding hash algorithm as
      defined in [SHS].  When used in signed TLS handshake messages (see
      Section 4.8.1), the length of the salt MUST handshake messages

   -  A base key to be equal used to the length
      of the digest output.

   EdDSA algorithms
      Indicates compute a signature algorithm using EdDSA as defined in
      [I-D.irtf-cfrg-eddsa] or its successors.  Note that MAC key.

   Based on these
      correspond to inputs, the "PureEdDSA" algorithms messages then contain:

   Certificate  The certificate to be used for authentication and not any
      supporting certificates in the "prehash"
      variants.

   The semantics of this extension are somewhat complicated because chain.  Note that certificate-based
      client authentication is not available in the
   cipher suite adds additional constraints on 0-RTT case.

   CertificateVerify  A signature algorithms.
   Section 6.3.4.1.1 describes over the appropriate rules.

   rsa_pkcs1_sha1 and dsa_sha1 SHOULD NOT be offered.  Clients offering
   these values value Hash(Handshake Context
      + Certificate) + Hash(resumption_context) See Section 4.4.1 for backwards compatibility MUST list them as
      the lowest
   priority (listed after all other algorithms in definition of resumption_context.

   Finished  A MAC over the
   supported_signature_algorithms vector).  TLS 1.3 servers MUST NOT
   offer a SHA-1 signed certificate unless no valid certificate chain
   can be produced without it (see Section 6.3.4.1.1).

   The signatures on certificates that are self-signed or certificates
   that are trust anchors are not validated since they begin a
   certification path (see [RFC5280], Section 3.2).  A certificate that
   begins a certification path MAY use value Hash(Handshake Context + Certificate +
      CertificateVerify) + Hash(resumption_context) using a signature algorithm that is not
   advertised as being supported in MAC key
      derived from the "signature_algorithms"
   extension.

   Note that TLS 1.2 defines base key.

   Because the CertificateVerify signs the Handshake Context +
   Certificate and the Finished MACs the Handshake Context + Certificate
   + CertificateVerify, this extension differently.  TLS 1.3
   implementations willing is mostly equivalent to negotiate TLS 1.2 MUST behave in
   accordance with the requirements keeping a running
   hash of [RFC5246] when negotiating that
   version.  In particular:

   -  TLS 1.2 ClientHellos may omit this extension.

   -  In TLS 1.2, the extension contained hash/signature pairs.  The
      pairs are encoded in two octets, handshake messages (exactly so SignatureScheme values have
      been allocated to align with TLS 1.2's encoding.  Some legacy
      pairs are left unallocated.  These algorithms are deprecated as of
      TLS 1.3.  They MUST NOT be offered or negotiated by any
      implementation.  In particular, MD5 [SLOTH] and SHA-224 MUST NOT
      be used.

   -  ecdsa_secp256r1_sha256, etc., align with TLS 1.2's ECDSA hash/
      signature pairs.  However, in the old semantics did pure 1-RTT cases).
   Note, however, that subsequent post-handshake authentications do not constrain the
      signing curve.

6.3.2.3.  Negotiated Groups

   When sent by
   include each other, just the client, messages through the "supported_groups" extension indicates end of the named groups which main
   handshake.

   The following table defines the client supports, ordered from most
   preferred to least preferred. Handshake Context and MAC Base Key
   for each scenario:

   +------------+--------------------------------+---------------------+
   | Mode       | Handshake Context              | Base Key            |
   +------------+--------------------------------+---------------------+
   | 0-RTT      | ClientHello                    | early_traffic_secre |
   |            |                                | t                   |
   |            |                                |                     |
   | 1-RTT      | ClientHello ... later of Encry | handshake_traffic_s |
   | (Server)   | ptedExtensions/CertificateRequ | ecret               |
   |            | est                            |                     |
   |            |                                |                     |
   | 1-RTT      | ClientHello ... ServerFinished | handshake_traffic_s |
   | (Client)   |                                | ecret               |
   |            |                                |                     |
   | Post-      | ClientHello ... ClientFinished | traffic_secret_0    |
   | Handshake  | + CertificateRequest           |                     |
   +------------+--------------------------------+---------------------+

   Note: In versions The Handshake Context for the last three rows does not include
   any 0-RTT handshake messages, regardless of TLS prior to TLS 1.3, whether 0-RTT is used.

4.3.1.  Certificate

   When this extension was named
   "elliptic_curves" and only contained elliptic curve groups.  See
   [RFC4492] and [I-D.ietf-tls-negotiated-ff-dhe].  This extension was
   also used to negotiate ECDSA curves.  Signature algorithms are now
   negotiated independently (see Section 6.3.2.2).

   Clients which offer one or more (EC)DHE cipher suites message will be sent:

      The server MUST send at
   least one supported NamedGroup value and servers MUST NOT negotiate
   any of these cipher suites unless a supported value was provided.  If
   this extension is not provided and no alternative cipher suite is
   available, Certificate message whenever the server agreed-
      upon key exchange method uses certificates for authentication
      (this includes all key exchange methods defined in this document
      except PSK).

      The client MUST close the connection with send a fatal
   "missing_extension" alert.  (see Section 8.2) Certificate message if and only if server
      has requested client authentication via a CertificateRequest
      message (Section 4.2.9).  If the extension is
   provided, server requests client
      authentication but no compatible group suitable certificate is offered, available, the server
      client MUST NOT
   negotiate send a cipher suite Certificate message containing no certificates
      (i.e., with the "certificate_list" field having length 0).

   Meaning of this message:

      This message conveys the relevant type.  For instance, if a
   client supplies only ECDHE groups, endpoint's certificate chain to the server peer.

      The certificate MUST NOT negotiate
   finite field Diffie-Hellman.  If no acceptable group can be selected
   across all cipher suites, then appropriate for the server MUST generate a fatal
   "handshake_failure" alert.

   The "extension_data" field negotiated cipher
      suite's authentication algorithm and any negotiated extensions.

   Structure of this extension contains a
   "NamedGroupList" value:

      enum {
          /* Elliptic Curve Groups (ECDHE) */
          secp256r1 (23), secp384r1 (24), secp521r1 (25),
          x25519 (29), x448 (30),

          /* Finite Field Groups (DHE) */
          ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
          ffdhe6144 (259), ffdhe8192 (260),

          /* Reserved Code Points */
          ffdhe_private_use (0x01FC..0x01FF),
          ecdhe_private_use (0xFE00..0xFEFF),
          (0xFFFF)
      } NamedGroup; message:

      opaque ASN1Cert<1..2^24-1>;

      struct {
          NamedGroup named_group_list<1..2^16-1>;
          opaque certificate_request_context<0..2^8-1>;
          ASN1Cert certificate_list<0..2^24-1>;
      } NamedGroupList;

   Elliptic Curve Groups (ECDHE)
      Indicates support of Certificate;

   certificate_request_context  If this message is in response to a
      CertificateRequest, the corresponding named curve.  Note that
      some curves are also recommended value of certificate_request_context in ANSI X9.62 [X962] and FIPS
      186-4 [DSS].  Others are recommended
      that message.  Otherwise, in [RFC7748].  Values 0xFE00
      through 0xFEFF are reserved for private use.

   Finite Field Groups (DHE)
      Indicates support of the corresponding finite case of server authentication
      this field group, defined
      in [I-D.ietf-tls-negotiated-ff-dhe].  Values 0x01FC through 0x01FF
      are reserved for private use.

   Items SHALL be zero length.

   certificate_list  This is a sequence (chain) of certificates.  The
      sender's certificate MUST come first in named_group_list the list.  Each following
      certificate SHOULD directly certify one preceding it.  Because
      certificate validation requires that trust anchors be distributed
      independently, a certificate that specifies a trust anchor MAY be
      omitted from the chain, provided that supported peers are ordered according known to
      possess any omitted certificates.

   Note: Prior to the client's
   preferences (most preferred choice first).

   As of TLS 1.3, servers are permitted to send the "supported_groups"
   extension "certificate_list" ordering required each
   certificate to certify the client.  If the server has one immediately preceding it, however some
   implementations allowed some flexibility.  Servers sometimes send
   both a group it prefers to the
   ones in the "key_share" extension current and deprecated intermediate for transitional purposes,
   and others are simply configured incorrectly, but is still willing to accept the
   ClientHello, it these cases can
   nonetheless be validated properly.  For maximum compatibility, all
   implementations SHOULD send "supported_groups" be prepared to update handle potentially extraneous
   certificates and arbitrary orderings from any TLS version, with the client's
   view
   exception of its preferences.  Clients the end-entity certificate which MUST NOT act upon any information
   found be first.

   The server's certificate list MUST always be non-empty.  A client
   will send an empty certificate list if it does not have an
   appropriate certificate to send in "supported_groups" prior response to successful completion of the
   handshake, but MAY use the information learned from a successfully
   completed handshake to change what groups they offer server's
   authentication request.

4.3.1.1.  Server Certificate Selection

   The following rules apply to a server in
   subsequent connections.

   [[TODO: IANA Considerations.]]

6.3.2.4.  Key Share

   The "key_share" extension contains the endpoint's cryptographic
   parameters for non-PSK certificates sent by the server:

   -  The certificate type MUST be X.509v3 [RFC5280], unless explicitly
      negotiated otherwise (e.g., [RFC5081]).

   -  The server's end-entity certificate's public key establishment methods (and associated
      restrictions) MUST be compatible with the selected authentication
      algorithm (currently DHE or
   ECDHE).

   Clients which offer one RSA or more (EC)DHE cipher suites ECDSA).

   -  The certificate MUST send this allow the key to be used for signing (i.e.,
      the digitalSignature bit MUST be set if the Key Usage extension is
      present) with a signature scheme indicated in the client's
      "signature_algorithms" extension.

   -  The "server_name" and "trusted_ca_keys" extensions [RFC6066] are
      used to guide certificate selection.  As servers MAY require the
      presence of the "server_name" extension, clients SHOULD send at least one supported KeyShareEntry value.
   Servers this
      extension, when applicable.

   All certificates provided by the server MUST NOT negotiate any of these cipher suites unless be signed by a
   supported value was provided.  If this signature
   algorithm that appears in the "signature_algorithms" extension is not
   provided in by the client, if they are able to provide such a
   ServerHello chain (see
   Section 4.2.2).  Certificates that are self-signed or ClientHello, certificates
   that are expected to be trust anchors are not validated as part of
   the chain and therefore MAY be signed with any algorithm.

   If the peer server cannot produce a certificate chain that is offering (EC)DHE cipher
   suites, signed only
   via the indicated supported algorithms, then it SHOULD continue the endpoint MUST close
   handshake by sending the connection with client a fatal
   "missing_extension" alert.  (see Section 8.2) Clients MAY send an
   empty client_shares vector in order to request group selection from
   the server at the cost of an additional round trip.  (see
   Section 6.3.1.3)

      struct {
          NamedGroup group;
          opaque key_exchange<1..2^16-1>;
      } KeyShareEntry;

   group
      The named group for the key being exchanged.  Finite Field Diffie-
      Hellman [DH] parameters are described in Section 6.3.2.4.1;
      Elliptic Curve Diffie-Hellman parameters are described in
      Section 6.3.2.4.2.

   key_exchange
      Key exchange information.  The contents certificate chain of this field its choice
   that may include algorithms that are
      determined not known to be supported by the specified group and its corresponding
      definition.  Endpoints MUST NOT send empty or otherwise invalid
      key_exchange values for any reason.

   The "extension_data" field of this extension contains a "KeyShare"
   value:

      struct {
          select (role) {
              case client:
                  KeyShareEntry client_shares<0..2^16-1>;

              case server:
                  KeyShareEntry server_share;
          }
      } KeyShare;

   client_shares
      A list of offered KeyShareEntry values in descending order of
      client preference.
   client.  This vector fallback chain MAY be empty use the deprecated SHA-1 hash
   algorithm only if the "signature_algorithms" extension provided by
   the client is
      requesting a HelloRetryRequest.  The ordering of values here
      SHOULD match that of permits it.  If the ordering of offered support in client cannot construct an acceptable
   chain using the
      "supported_groups" extension.

   server_share
      A single KeyShareEntry value for provided certificates and decides to abort the negotiated cipher suite.

   Servers offer exactly
   handshake, then it MUST send an "unsupported_certificate" alert
   message and close the connection.

   If the server has multiple certificates, it chooses one KeyShareEntry value, which corresponds to of them based
   on the above-mentioned criteria (in addition to other criteria, such
   as transport layer endpoint, local configuration and preferences).

   As cipher suites that specify new key exchange used methods are specified
   for the negotiated cipher suite.

   Clients offer an arbitrary number of KeyShareEntry values, each
   representing a single set of key exchange parameters.  For instance,
   a client might offer shares for several elliptic curves or multiple
   FFDHE groups. TLS protocol, they will imply the certificate format and the
   required encoded keying information.

4.3.1.2.  Client Certificate Selection

   The key_exchange values for each KeyShareEntry MUST following rules apply to certificates sent by
   generated independently.  Clients the client:

   In particular:

   -  The certificate type MUST NOT offer multiple
   KeyShareEntry values for be X.509v3 [RFC5280], unless explicitly
      negotiated otherwise (e.g., [RFC5081]).

   -  If the same group and servers receiving
   multiple KeyShareEntry values for certificate_authorities list in the same group MUST abort certificate request
      message was non-empty, one of the certificates in the certificate
      chain SHOULD be issued by one of the
   connection with a fatal "illegal_parameter" alert.  Clients and
   servers MUST NOT offer or accept any KeyShareEntry values for groups
   not listed CAs.

   -  The certificates MUST be signed using an acceptable signature
      algorithm, as described in Section 4.2.9.  Note that this relaxes
      the client's "supported_groups" extension.  Servers constraints on certificate-signing algorithms found in prior
      versions of TLS.

   -  If the certificate_extensions list in the certificate request
      message was non-empty, the end-entity certificate MUST NOT offer a KeyShareEntry value for a group not offered match the
      extension OIDs recognized by the
   client client, as described in its corresponding KeyShare.

   If
      Section 4.2.9.

   Note that, as with the server selects an (EC)DHE cipher suite and no mutually
   supported group is available between the two endpoints' KeyShare
   offers, yet certificate, there is a mutually supported group are certificates
   that can use algorithm combinations that cannot be found via
   the "supported_groups" extension, then the server MUST reply currently used with
   TLS.

4.3.1.3.  Receiving a
   HelloRetryRequest. Certificate Message

   In general, detailed certificate validation procedures are out of
   scope for TLS (see [RFC5280]).  This section provides TLS-specific
   requirements.

   If there is no mutually supported group at all, the server MUST NOT negotiate supplies an (EC)DHE cipher suite.

   [[TODO: Recommendation about what empty Certificate message, the client offers.  Presumably
   which integer DH groups and which curves.]]

6.3.2.4.1.  Diffie-Hellman Parameters

   Diffie-Hellman [DH] parameters for both clients and servers are
   encoded in MUST
   terminate the opaque key_exchange field of a KeyShareEntry in handshake with a
   KeyShare structure.  The opaque value contains fatal "decode_error" alert.

   If the Diffie-Hellman
   public value (Y = g^X mod p), encoded as a big-endian integer, padded
   with zeros to client does not send any certificates, the size of p.

   Note: For server MAY at its
   discretion either continue the handshake without client
   authentication, or respond with a given Diffie-Hellman group, fatal "handshake_failure" alert.
   Also, if some aspect of the padding results in all
   public keys having certificate chain was unacceptable (e.g.,
   it was not signed by a known, trusted CA), the same length.

6.3.2.4.2.  ECDHE Parameters

   ECDHE parameters for both clients and servers are encoded in server MAY at its
   discretion either continue the handshake (considering the
   opaque key_exchange field of client
   unauthenticated) or send a KeyShareEntry in fatal alert.

   Any endpoint receiving any certificate signed using any signature
   algorithm using an MD5 hash MUST send a KeyShare structure.

   For secp256r1, secp384r1 "bad_certificate" alert
   message and secp521r1, close the contents connection.  SHA-1 is deprecated and therefore
   NOT RECOMMENDED.  All endpoints are the byte
   string representation of an elliptic curve public value following the
   conversion routine in Section 4.3.6 of ANSI X9.62 [X962].

   Although X9.62 supports multiple point formats, any given curve MUST
   specify only a single point format.  All curves RECOMMENDED to transition to
   SHA-256 or better as soon as possible to maintain interoperability
   with implementations currently specified in this document MUST only be used with the uncompressed point format
   (the format for all ECDH functions is considered uncompressed).

   For x25519 and x448, the contents are the byte string inputs and
   outputs process of the corresponding functions defined in [RFC7748], 32 bytes
   for x25519 and 56 bytes phasing out SHA-1
   support.

   Note that a certificate containing a key for x448.

   Note: Versions of TLS prior one signature algorithm
   MAY be signed using a different signature algorithm (for instance, an
   RSA key signed with an ECDSA key).

   Endpoints that reject certification paths due to 1.3 permitted point negotiation; TLS
   1.3 removes this feature in favor use of a single point format for each
   curve.

6.3.2.5.  Pre-Shared Key Extension

   The "pre_shared_key" extension deprecated
   hash MUST send a fatal "bad_certificate" alert message before closing
   the connection.

4.3.2.  Certificate Verify

   When this message will be sent:

      This message is used to indicate the identity of provide explicit proof that an endpoint
      possesses the pre-shared private key corresponding to be used with a given its certificate and
      also provides integrity for the handshake in association
   with up to this point.
      Servers MUST send this message when using a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for
   background).

   Clients which offer one or more PSK cipher suites is
      authenticated via a certificate.  Clients MUST send at least
   one supported psk_identity value and servers MUST NOT negotiate any
   of these cipher suites unless a supported value was provided.  If this extension is not provided and no alternative cipher suite is
   available, message
      whenever authenticating via a Certificate (i.e., when the server
      Certificate message is non-empty).  When sent, this message MUST close
      appear immediately after the connection with a fatal
   "missing_extension" alert.  (see Section 8.2)

   The "extension_data" field Certificate Message and immediately
      prior to the Finished message.

   Structure of this extension contains a
   "PreSharedKeyExtension" value:

      opaque psk_identity<0..2^16-1>; message:

      struct {
          select (Role) {
              case client:
                  psk_identity identities<2..2^16-1>;

              case server:
                  uint16 selected_identity;
          }
           SignatureScheme algorithm;
           opaque signature<0..2^16-1>;
      } PreSharedKeyExtension;

   identities
      A list of CertificateVerify;

   The algorithm field specifies the identities (labels signature algorithm used (see
   Section 4.2.2 for keys) that the client is
      willing to negotiate with the server.

   selected_identity definition of this field).  The server's chosen identity expressed as signature is a (0-based) index into
   digital signature using that algorithm that covers the identies hash output
   described in Section 4.3 namely:

      Hash(Handshake Context + Certificate) + Hash(resumption_context)

   In TLS 1.3, the client's list.

   If no suitable identity digital signature process takes as input:

   -  A signing key

   -  A context string

   -  The actual content to be signed

   The digital signature is provided, then computed using the server MUST NOT negotiate a
   PSK cipher suite and MAY respond with an "unknown_psk_identity" alert
   message.  Sending this alert is OPTIONAL; signing key over the
   concatenation of:

   -  64 bytes of octet 32

   -  The context string

   -  A single 0 byte which servers MAY instead choose as the separator

   -  The content to send a "decrypt_error" alert be signed

   This structure is intended to merely indicate prevent an invalid PSK
   identity or instead negotiate use attack on previous versions
   of a non-PSK cipher suite, if
   available.

   If previous versions of TLS in which the server selects ServerKeyExchange format
   meant that attackers could obtain a PSK cipher suite, it MUST send signature of a
   "pre_shared_key" extension message with the identity that it selected. a
   chosen, 32-byte prefix.  The
   client MUST verify initial 64 byte pad clears that the server's selected_identity prefix.

   The context string for a server signature is within the
   range supplied by the client.  If any other value "TLS 1.3, server
   CertificateVerify" and for a client signature is returned, the "TLS 1.3, client MUST generate a fatal "unknown_psk_identity" alert
   CertificateVerify".

   For example, if Hash(Handshake Context + Certificate) was 32 bytes of
   01 and close Hash(resumption_context) was 32 bytes of 02 (these lengths
   would make sense for SHA-256, the connection.

6.3.2.6.  OCSP Status Extensions

   [RFC6066] and [RFC6961] provide extensions input to negotiate the final signing process
   for a server
   sending OCSP responses to CertificateVerify would be:

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      544c5320312e332c207365727665722043657274696669636174655665726966
      79
      00
      0101010101010101010101010101010101010101010101010101010101010101
      0202020202020202020202020202020202020202020202020202020202020202

   If sent by a server, the client.  In TLS 1.2 and below, signature algorithm MUST be one offered in
   the
   server sends an empty extension to indicate negotiation of this client's "signature_algorithms" extension and the OCSP information unless no valid
   certificate chain can be produced without unsupported algorithms (see
   Section 4.2.2).  Note that there is carried in a CertificateStatus
   message.  In TLS 1.3, possibility for inconsistencies
   here.  For instance, the server's OCSP information is carried in client might offer an
   extension in EncryptedExtensions.  Specifically: The body of the
   "status_request" or "status_request_v2" extension ECDHE_ECDSA cipher
   suite but omit any ECDSA and EdDSA values from its
   "signature_algorithms" extension.  In order to negotiate correctly,
   the server MUST be a CertificateStatus structure as defined in [RFC6066] and
   [RFC6961] respectively.

   Note: this means that the certificate status appears prior to check any candidate cipher suites against the
   certificates it applies to.
   "signature_algorithms" extension before selecting them.  This is slightly anomalous
   somewhat inelegant but matches
   the existing behavior for SignedCertificateTimestamps [RFC6962], and
   is more easily extensible in the handshake state machine.

6.3.2.7.  Early Data Indication

   When PSK resumption is used, a compromise designed to minimize changes
   to the client can send application data in
   its first flight of messages. original cipher suite design.

   If sent by a client, the client opts to do so, it MUST
   supply an "early_data" extension as well as the "pre_shared_key"
   extension.

   The "extension_data" field of this extension contains an
   "EarlyDataIndication" value:

      struct {
          select (Role) {
              case client:
                  opaque context<0..255>;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

   context
      An optional context value that can be signature algorithm used for anti-replay (see
      below).

   All of the parameters for in the 0-RTT data (symmetric cipher suite,
   ALPN, etc.) signature
   MUST be one of those which were negotiated present in the connection
   which established supported_signature_algorithms
   field of the PSK.  The PSK used to encrypt CertificateRequest message.

   In addition, the early data signature algorithm MUST be compatible with the first PSK listed key
   in the client's "pre_shared_key"
   extension.

   0-RTT messages sent sender's end-entity certificate.  RSA signatures MUST use an
   RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5
   algorithms appear in the first flight have the same content types
   as their corresponding messages sent "signature_algorithms".  SHA-1 MUST NOT be used
   in other flights (handshake,
   application_data, any signatures in CertificateVerify.  All SHA-1 signature
   algorithms in this specification are defined solely for use in legacy
   certificates, and alert respectively) but are protected under
   different keys.  After all not valid for CertificateVerify signatures.

   Note: When used with non-certificate-based handshakes (e.g., PSK),
   the 0-RTT application data messages (if
   any) have been sent, a "end_of_early_data" alert of type "warning" is
   sent to indicate client's signature does not cover the end of server's certificate
   directly, although it does cover the flight.  0-RTT MUST always be
   followed by an "end_of_early_data" alert.

   A server server's Finished message, which receives an "early_data" extension can behave in one
   of two ways:

   -  Ignore
   transitively includes the extension and return no response.  This indicates that server's certificate when the server has ignored any early data and an ordinary 1-RTT
      handshake PSK derives
   from a certificate-authenticated handshake.  [PSK-FINISHED] describes
   a concrete attack on this mode if the Finished is required.

   -  Return an empty extension, indicating that it intends to process omitted from the early data.
   signature.  It is not possible for the server unsafe to accept only
      a subset of use certificate-based client
   authentication when the early data messages.

   [[OPEN ISSUE: are client might potentially share the rules below correct? https://github.com/tlswg/
   tls13-spec/issues/451]] Prior same PSK/
   key-id pair with two different endpoints.  In order to accepting the "early_data"
   extension, the server ensure this,
   implementations MUST validate that the session ticket
   parameters are consistent NOT mix certificate-based client authentication
   with its current configuration.  It MUST
   also validate that the extensions negotiated in pure PSK modes (i.e., those where the PSK was not derived from a
   previous
   connection are identical to those being negotiated non-PSK handshake).

4.3.3.  Finished

   When this message will be sent:

      The Finished message is the final message in the
   ServerHello, with authentication
      block.  It is essential for providing authentication of the exception
      handshake and of the following extensions:

   -  The use computed keys.

   Meaning of "signed_certificate_timestamp" [RFC6962] MUST be
      identical but the server's SCT extension value may differ.

   -  The "padding" extension [RFC7685] MUST be ignored for this
      purpose.

   -  The values message:

      Recipients of "key_share", "pre_shared_key", and "early_data",
      which MUST be as defined in this document.

   In addition, it Finished messages MUST validate verify that the ticket_age is within contents are
      correct.  Once a small
   tolerance of side has sent its Finished message and received
      and validated the time since Finished message from its peer, it may begin to
      send and receive application data over the ticket was issued connection.

   The key used to compute the finished message is computed from the
   Base key defined in Section 4.3 using HKDF (see Section 6.3.2.7.2).

   If any 7.1).
   Specifically:

   client_finished_key =
       HKDF-Expand-Label(BaseKey, "client finished", "", Hash.Length)

   server_finished_key =
       HKDF-Expand-Label(BaseKey, "server finished", "", Hash.Length)

   Structure of these checks fail, the server MUST NOT respond with this message:

      struct {
          opaque verify_data[Hash.length];
      } Finished;

   The verify_data value is computed as follows:

      verify_data =
          HMAC(finished_key, Hash(
                                  Handshake Context +
                                  Certificate* +
                                  CertificateVerify*
                             ) +
                             Hash(resumption_context)
          )

      * Only included if present.

   Where HMAC [RFC2104] uses the
   extension and must discard all Hash algorithm for the remaining first flight data (thus
   falling back to 1-RTT).  If handshake.  As
   noted above, the client attempts HMAC input can generally be implemented by a 0-RTT running
   hash, i.e., just the handshake but hash at this point.

   In previous versions of TLS, the server rejects it, verify_data was always 12 octets
   long.  In the current version of TLS, it will generally not have is the 0-RTT record
   protection keys and must instead trial decrypt each record with size of the
   1-RTT handshake keys until it finds one that decrypts properly, and
   then pick up the handshake from that point.

   If the server chooses to accept the "early_data" extension, then it
   MUST comply with the same error handling requirements specified HMAC
   output for
   all records when processing early data records.  Specifically,
   decryption failure of any 0-RTT record following an accepted
   "early_data" extension MUST produce a fatal "bad_record_mac" alert as
   per Section 5.2.2.  Implementations SHOULD determine the security
   parameters Hash used for the 1-RTT phase of the connection entirely before
   processing the EncryptedExtensions handshake.

   Note: Alerts and Finished, using those values
   solely to determine whether to accept or reject 0-RTT data.

   [[TODO: How does the client behave if the indication is rejected.]]

6.3.2.7.1.  Processing Order

   Clients any other record types are permitted to "stream" 0-RTT data until they receive the
   server's Finished, only then sending the "end_of_early_data" alert.
   In order to avoid deadlock, when accepting "early_data", servers MUST
   process the client's Finished not handshake messages
   and then immediately send the
   ServerHello, rather than waiting for the client's "end_of_early_data"
   alert.

6.3.2.7.2.  Replay Properties

   As noted in Section 6.2.3, TLS provides only a limited inter-
   connection mechanism for replay protection for data sent by the
   client are not included in the first flight.

   The "ticket_age" extension hash computations.

4.4.  Post-Handshake Messages

   TLS also allows other messages to be sent by after the client SHOULD be used by
   servers to limit main handshake.
   These messages use a handshake content type and are encrypted under
   the application traffic key.

4.4.1.  New Session Ticket Message

   At any time over which after the first flight might be
   replayed.  A server can store has received the time at which client Finished
   message, it sends MAY send a server
   configuration to a client, or encode the time in NewSessionTicket message.  This message
   creates a ticket.  Then,
   each time it receives an early_data extension, it can check to see if pre-shared key (PSK) binding between the ticket value used by and
   the client matches its expectations. following two values derived from the resumption master secret:

      resumption_psk = HKDF-Expand-Label(
                           resumption_secret,
                           "resumption psk", "", Hash.Length)

      resumption_context = HKDF-Expand-Label(
                               resumption_secret,
                               "resumption context", "", Hash.Length)

   The "ticket_age" value provided client MAY use this PSK for future handshakes by including the client will be shorter than
   ticket value in the actual time elapsed "pre_shared_key" extension in its ClientHello
   (Section 4.2.5) and supplying a suitable PSK cipher suite.  Servers
   may send multiple tickets on the server by a single round trip time.
   This difference connection, for instance after
   post-handshake authentication.  For handshakes that do not use a
   resumption_psk, the resumption_context is comprised a string of Hash.Length
   zeroes.

    enum { (65535) } TicketExtensionType;

    struct {
        TicketExtensionType extension_type;
        opaque extension_data<1..2^16-1>;
    } TicketExtension;

    enum {
      allow_early_data(1),
      allow_dhe_resumption(2),
      allow_psk_resumption(4)
    } TicketFlags;

    struct {
        uint32 ticket_lifetime;
        uint32 flags;
        uint32 ticket_age_add;
        TicketExtension extensions<2..2^16-2>;
        opaque ticket<0..2^16-1>;
    } NewSessionTicket;

   flags  A 32-bit value indicating the delay ways in sending the
   NewSessionTicket message to the client, plus the time taken to send
   the ClientHello to the server.  For which this reason, ticket may be
      used (as a server SHOULD
   measure bitwise OR of the round trip time prior to sending flags values).

   ticket_lifetime  Indicates the NewSessionTicket
   message and account for that lifetime in seconds as a 32-bit
      unsigned integer in network byte order from the time of ticket
      issuance.  Servers MUST NOT use any value more than 604800 seconds
      (7 days).  The value it saves.

   There are several potential sources of error zero indicates that make an exact
   measurement of time difficult.  Variations in client and server
   clocks are likely to the ticket should be minimal, outside of gross time corrections.
   Network propagation delays are most likely causes of a mismatch in
   legitimate values
      discarded immediately.  Clients MUST NOT cache session tickets for elapsed time.  Both
      longer than 7 days, regardless of the NewSessionTicket and
   ClientHello messages might be retransmitted and therefore delayed,
   which might be hidden by TCP. ticket_lifetime.  It MAY
      delete the ticket earlier based on local policy.  A small allowance server MAY
      treat a ticket as valid for errors in clocks and variations in measurements a shorter period of time than what is advisable.  However, any allowance also increases
      stated in the opportunity
   for replay.  In this case, it ticket_lifetime.

   ticket_age_add  A randomly generated 32-bit value that is better to reject early data than to
   risk greater exposure used to replay attacks.

6.3.2.8.  Ticket Age

      struct {
          uint32 ticket_age;
      } TicketAge;

   When
      obscure the age of the ticket that the client sends includes in the
      "early_data" extension, it MUST also send a
   "ticket_age" extension in its EncryptedExtensions block.  This extension.  The actual ticket age is added to this
      value
   contains the time elapsed since the client learned about modulo 2^32 to obtain the server
   configuration value that it is using, in milliseconds.  This value can be
   used transmitted by the server to limit
      client.

   ticket_extensions  A placeholder for extensions in the ticket.
      Clients MUST ignore unrecognized extensions.

   ticket  The value of the time over which early data can be
   replayed.  Note: because ticket lifetimes are restricted to a week,
   32 bits is enough to represent any plausible age, even in
   milliseconds.

6.3.3.  Server Parameters

6.3.3.1.  Encrypted Extensions

   When this message will be sent:

      In all handshakes, used as the server MUST send the EncryptedExtensions
      message immediately after the ServerHello message.  This PSK identifier.
      The ticket itself is an opaque label.  It MAY either be a database
      lookup key or a self-encrypted and self-authenticated value.
      Section 4 of [RFC5077] describes a recommended ticket construction
      mechanism.

   The meanings of the flags are as follows:

   allow_early_data  When resuming with this ticket, the client MAY send
      data in its first message that is flight (early data) encrypted under keys a key
      derived from
      handshake_traffic_secret.  If this PSK.

   allow_dhe_resumption  This ticket MAY be used with (EC)DHE-PSK cipher
      suite.

   allow_psk_resumption  This ticket MAY be used with a pure PSK cipher
      suite.

   In all cases, the PSK or (EC)DHE-PSK cipher suites that the client indicates "early_data" in
      its ClientHello, it
   offers/uses MUST also send EncryptedExtensions immediately
      following have the ClientHello and immediately prior to same symmetric parameters (cipher/hash) as
   the Finished.

   Meaning of cipher suite negotiated for this message:

      The EncryptedExtensions message contains any extensions which
      should be protected, i.e., any which connection.  If no flags are not needed set
   that the client recognizes, it MUST ignore the ticket.

4.4.2.  Post-Handshake Authentication

   The server is permitted to establish request client authentication at any time
   after the cryptographic context. handshake has completed by sending a CertificateRequest
   message.  The same extension types MUST NOT appear in both client SHOULD respond with the ServerHello and
   EncryptedExtensions. appropriate
   Authentication messages.  If the same extension appears in both
   locations, the client chooses to authenticate, it
   MUST rely only on the value in the
   EncryptedExtensions block.  All server-sent extensions other than
   those explicitly listed in Section 6.3.1.2 or designated in the IANA
   registry send Certificate, CertificateVerify, and Finished.  If it
   declines, it MUST only appear in EncryptedExtensions.  Extensions which
   are designated to appear in ServerHello MUST NOT appear in
   EncryptedExtensions.  Clients MUST check EncryptedExtensions for send a Certificate message containing no
   certificates followed by Finished.

   Note: Because client authentication may require prompting the
   presence of any forbidden extensions and if any are found user,
   servers MUST
   terminate the handshake with be prepared for some delay, including receiving an "illegal_parameter" alert.

   The client's EncryptedExtensions apply only to
   arbitrary number of other messages between sending the early data with
   CertificateRequest and receiving a response.  In addition, clients
   which they appear.  Servers MUST NOT use them receive multiple CertificateRequests in close succession MAY
   respond to negotiate them in a different order than they were received (the
   certificate_request_context value allows the rest
   of server to disambiguate
   the handshake.  Only those extensions explicitly designated as
   being included in 0-RTT Encrypted Extensions in responses).

4.4.3.  Key and IV Update

    struct {} KeyUpdate;

   The KeyUpdate handshake message is used to indicate that the IANA registry sender
   is updating its sending cryptographic keys.  This message can be sent in
   by the client's EncryptedExtensions.

   Structure of this message:

      struct {
          Extension extensions<0..2^16-1>;
      } EncryptedExtensions;

   extensions
      A list of extensions.

6.3.3.2.  Certificate Request

   When this message will be sent:

      A non-anonymous server can optionally request a certificate from after sending its first flight and the client, if appropriate for client after
   sending its second flight.  Implementations that receive a KeyUpdate
   message prior to receiving a Finished message as part of the selected cipher suite.  This 1-RTT
   handshake MUST generate a fatal "unexpected_message" alert.  After
   sending a KeyUpdate message, if sent, will follow EncryptedExtensions.

   Structure of this message:

      opaque DistinguishedName<1..2^16-1>;

      struct {
          opaque certificate_extension_oid<1..2^8-1>;
          opaque certificate_extension_values<0..2^16-1>;
      } CertificateExtension;

      struct {
          opaque certificate_request_context<0..2^8-1>;
          SignatureScheme
            supported_signature_algorithms<2..2^16-2>;
          DistinguishedName certificate_authorities<0..2^16-1>;
          CertificateExtension certificate_extensions<0..2^16-1>;
      } CertificateRequest;

   certificate_request_context
      An opaque string which identifies the certificate request and
      which will be echoed sender SHALL send all its traffic
   using the next generation of keys, computed as described in
   Section 7.2.  Upon receiving a KeyUpdate, the client's Certificate message.  The
      certificate_request_context receiver MUST be unique within update
   their receiving keys and if they have not already updated their
   sending state up to or past the scope then current receiving generation
   MUST send their own KeyUpdate prior to sending any other messages.
   This mechanism allows either side to force an update to the entire
   connection.  Note that implementations may receive an arbitrary
   number of messages between sending a KeyUpdate and receiving the
   peer's KeyUpdate because those messages may already be in flight.

   Note that if implementations independently send their own KeyUpdates
   and they cross in flight, this connection (thus preventing replay of client
      CertificateVerify messages).

   supported_signature_algorithms
      A list only results in an update of one
   generation; when each side receives the signature algorithms other side's update it just
   updates its receive keys and notes that the server generations match and
   thus no send update is able needed.

   Note that the side which sends its KeyUpdate first needs to
      verify, listed in descending order of preference.  Any
      certificates provided by retain
   its receive traffic keys (though not the client MUST be signed using a
      signature algorithm found in supported_signature_algorithms.

   certificate_authorities
      A list of traffic secret) for the distinguished names [X501]
   previous generation of acceptable
      certificate_authorities, represented in DER-encoded [X690] format.

      These distinguished names may specify a desired distinguished name
      for a root CA or for a subordinate CA; thus, this message can be
      used to describe known roots as well as keys until it receives the KeyUpdate from the
   other side.

   Both sender and receiver MUST encrypt their KeyUpdate messages with
   the old keys.  Additionally, both sides MUST enforce that a desired authorization
      space.  If KeyUpdate
   with the certificate_authorities list old key is empty, then the
      client MAY send received before accepting any certificate that meets messages encrypted
   with the rest of new key.  Failure to do so may allow message truncation
   attacks.

5.  Record Protocol

   The TLS record protocol takes messages to be transmitted, fragments
   the
      selection criteria in data into manageable blocks, protects the CertificateRequest, unless there records, and transmits
   the result.  Received data is some
      external arrangement decrypted and verified, reassembled,
   and then delivered to higher-level clients.

   TLS records are typed, which allows multiple higher level protocols
   to be multiplexed over the contrary.

   certificate_extensions
      A list of certificate extension OIDs [RFC5280] with their allowed
      values, represented same record layer.  This document
   specifies three content types: handshake, application data, and
   alert.  Implementations MUST NOT send record types not defined in DER-encoded [X690] format.  Some
      certificate extension OIDs allow multiple values (e.g.  Extended
      Key Usage).
   this document unless negotiated by some extension.  If the server has included a non-empty
      certificate_extensions list, the client certificate TLS
   implementation receives an unexpected record type, it MUST contain
      all of the specified extension OIDs that the client recognizes.
      For each extension OID recognized send an
   "unexpected_message" alert.  New record content type values are
   assigned by IANA in the client, all of the
      specified values MUST be present in the client certificate (but
      the certificate MAY have other values TLS Content Type Registry as well).  However, the
      client MUST ignore and skip any unrecognized certificate extension
      OIDs.  If the client has ignored some of the required certificate
      extension OIDs, and supplied a certificate that does not satisfy
      the request, the server MAY at its discretion either continue the
      session without client authentication, or terminate described in
   Section 10.

   Application data messages are carried by the session
      with a fatal unsupported_certificate alert.  PKIX RFCs define a
      variety of certificate extension OIDs record layer and their corresponding
      value types.  Depending on the type, matching certificate
      extension values are not necessarily bitwise-equal.  It is
      expected that TLS implementations will rely on their PKI libraries
      to perform certificate selection using certificate extension OIDs.
      This document defines matching rules for two standard certificate
      extensions defined in [RFC5280]:

      o
   fragmented and encrypted as described below.  The Key Usage extension in a certificate matches the request
         when all key usage bits asserted in the request messages are also
         asserted in
   treated as transparent data to the Key Usage certificate extension.

      o record layer.

5.1.  Record Layer

   The Extended Key Usage extension TLS record layer receives uninterpreted data from higher layers
   in a certificate matches the
         request when all key purpose OIDs present non-empty blocks of arbitrary size.

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in the request chunks of 2^14 bytes or less.  Message
   boundaries are
         also found not preserved in the Extended Key Usage certificate extension.
         The special anyExtendedKeyUsage OID MUST NOT be used in record layer (i.e., multiple
   messages of the
         request.

      Separate specifications may define matching rules for other
      certificate extensions.

   Note: It is same ContentType MAY be coalesced into a fatal "handshake_failure" alert for an anonymous server
   to request client authentication.

6.3.4.  Authentication Messages

   As discussed in Section 6.2, TLS uses single
   TLSPlaintext record, or a common set of messages for
   authentication, key confirmation, and handshake integrity:
   Certificate, CertificateVerify, and Finished.  These messages are
   always sent as the last messages in their handshake flight.  The
   Certificate and CertificateVerify single message MAY be fragmented across
   several records).  Alert messages are only sent under
   certain circumstances, as defined below. (Section 6) MUST NOT be fragmented
   across records.

      enum {
          alert(21),
          handshake(22),
          application_data(23)
          (255)
      } ContentType;

      struct {
          ContentType type;
          ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
          uint16 length;
          opaque fragment[TLSPlaintext.length];
      } TLSPlaintext;

   type  The Finished message is
   always sent as part of higher-level protocol used to process the Authentication block. enclosed
      fragment.

   record_version  The computations for protocol version the Authentication messages current record is compatible
      with.  This value MUST be set to { 3, 1 } for all uniformly take records.  This
      field is deprecated and MUST be ignored for all purposes.

   length  The length (in bytes) of the following inputs:

   - TLSPlaintext.fragment.
      The certificate length MUST NOT exceed 2^14.

   fragment  The data being transmitted.  This value transparent and signing key
      treated as an independent block to be used.

   -  A Handshake Context based on dealt with by the hash of higher-
      level protocol specified by the handshake messages

   -  A base key to be used to compute a MAC key.

   Based on these inputs, type field.

   This document describes TLS Version 1.3, which uses the messages then contain:

   Certificate version { 3,
   4 }.  The certificate to be used for authentication and any supporting
      certificates in the chain.  Note that certificate-based client
      authentication is not available in the 0-RTT case.

   CertificateVerify
      A signature over the value Hash(Handshake Context + Certificate) +
      Hash(resumption_context) See Section 6.3.5.1 for the definition of
      resumption_context.

   Finished
      A MAC over the version value Hash(Handshake Context + Certificate +
      CertificateVerify) + Hash(resumption_context) using a MAC key
      derived 3.4 is historical, deriving from the base key.

   Because the CertificateVerify signs the Handshake Context +
   Certificate use of {
   3, 1 } for TLS 1.0 and { 3, 0 } for SSL 3.0.  In order to maximize
   backwards compatibility, the Finished MACs record layer version identifies as
   simply TLS 1.0.  Endpoints supporting other versions negotiate the Handshake Context + Certificate
   + CertificateVerify, this is mostly equivalent
   version to keeping a running
   hash of use by following the handshake messages (exactly so procedure and requirements in the pure 1-RTT cases).
   Note, however, that subsequent post-handshake authentications do
   Appendix C.

   Implementations MUST NOT send zero-length fragments of Handshake or
   Alert types, even if those fragments contain padding.  Zero-length
   fragments of Application data MAY be sent as they are potentially
   useful as a traffic analysis countermeasure.

   When record protection has not
   include each other, just the messages through yet been engaged, TLSPlaintext
   structures are written directly onto the end of wire.  Once record
   protection has started, TLSPlaintext records are protected and sent
   as described in the main
   handshake.

   The following table defines the Handshake Context and MAC Base Key
   for each scenario:

   +------------+--------------------------------+---------------------+
   | Mode       | Handshake Context              | Base Key            |
   +------------+--------------------------------+---------------------+
   | 0-RTT      | ClientHello                    | early_traffic_secre |
   |            |                                | t                   |
   |            |                                |                     |
   | 1-RTT      | ClientHello ... later of Encry | handshake_traffic_s |
   | (Server)   | ptedExtensions/CertificateRequ | ecret               |
   |            | est                            |                     |
   |            |                                |                     |
   | 1-RTT      | ClientHello ... ServerFinished | handshake_traffic_s |
   | (Client)   |                                | ecret               |
   |            |                                |                     |
   | Post-      | ClientHello ... ClientFinished | traffic_secret_0    |
   | Handshake  | + CertificateRequest           |                     |
   +------------+--------------------------------+---------------------+

   Note: The Handshake Context for the last three rows does not include
   any 0-RTT handshake messages, regardless of whether 0-RTT is used.

6.3.4.1.  Certificate

   When this message will be sent: section.

5.2.  Record Payload Protection

   The server MUST send record protection functions translate a Certificate message whenever TLSPlaintext structure
   into a TLSCiphertext.  The deprotection functions reverse the agreed-
      upon key exchange method uses certificates for authentication
      (this includes
   process.  In TLS 1.3 as opposed to previous versions of TLS, all key exchange methods defined in this document
      except PSK).

      The client MUST send
   ciphers are modeled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide a Certificate message if unified encryption
   and only if server
      has requested client authentication via operation which turns plaintext into authenticated
   ciphertext and back again.  Each encrypted record consists of a CertificateRequest
      message (Section 6.3.3.2).  If the server requests client
      authentication but no suitable certificate is available, the
      client MUST send
   plaintext header followed by an encrypted body, which itself contains
   a Certificate message containing no certificates
      (i.e., with the "certificate_list" field having length 0).

   Meaning of this message:

      This message conveys the endpoint's certificate chain to the peer.

      The certificate MUST be appropriate for the negotiated cipher
      suite's key exchange algorithm type and any negotiated extensions.

   Structure of this message: optional padding.

   struct {
      opaque ASN1Cert<1..2^24-1>; content[TLSPlaintext.length];
      ContentType type;
      uint8 zeros[length_of_padding];
   } TLSInnerPlaintext;

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque certificate_request_context<0..2^8-1>;
          ASN1Cert certificate_list<0..2^24-1>; encrypted_record[length];
   } Certificate;

   certificate_request_context:
      If this message is TLSCiphertext;

   content  The cleartext of TLSPlaintext.fragment.

   type  The content type of the record.

   zeros  An arbitrary-length run of zero-valued bytes may appear in response the
      cleartext after the type field.  This provides an opportunity for
      senders to pad any TLS record by a CertificateRequest, chosen amount as long as the
      total stays within record size limits.  See Section 5.4 for more
      details.

   opaque_type  The outer opaque_type field of a TLSCiphertext record is
      always set to the value 23 (application_data) for outward
      compatibility with middleboxes accustomed to parsing previous
      versions of certificate_request_context TLS.  The actual content type of the record is found
      in fragment.type after decryption.

   record_version  The record_version field is identical to
      TLSPlaintext.record_version and is always { 3, 1 }.  Note that message.  Otherwise, in the
      case of server authentication or client authentication in 0-RTT,
      handshake protocol including the ClientHello and ServerHello
      messages authenticates the protocol version, so this field SHALL be zero length.

   certificate_list
      This value is a sequence (chain) of certificates.
      redundant.

   length  The sender's
      certificate MUST come first in length (in bytes) of the list.  Each following
      certificate SHOULD directly certify one preceding it.  Because
      certificate validation requires
      TLSCiphertext.fragment, which is the sum of the lengths of the
      content and the padding, plus one for the inner content type.  The
      length MUST NOT exceed 2^14 + 256.  An endpoint that trust anchors be distributed
      independently, receives a certificate
      record that specifies exceeds this length MUST generate a trust anchor MAY be
      omitted from the chain, provided that supported peers are known to
      possess any omitted certificates.

   Note: Prior to TLS 1.3, "certificate_list" ordering required each
   certificate to certify fatal
      "record_overflow" alert.

   encrypted_record  The AEAD encrypted form of the one immediately preceding it, however some
   implementations allowed some flexibility.  Servers sometimes send
   both serialized
      TLSInnerPlaintext structure.

   AEAD ciphers take as input a current and deprecated intermediate for transitional purposes, single key, a nonce, a plaintext, and others are simply configured incorrectly, but these cases can
   nonetheless be validated properly.  For maximum compatibility, all
   implementations SHOULD be prepared
   "additional data" to handle potentially extraneous
   certificates and arbitrary orderings from any TLS version, with the
   exception of the end-entity certificate which MUST be first.

   The server's certificate list MUST always be non-empty.  A client
   will send an empty certificate list if it does not have an
   appropriate certificate to send included in response to the server's authentication request.

6.3.4.1.1.  Server Certificate Selection check, as
   described in Section 2.1 of [RFC5116].  The following rules apply to key is either the certificates sent by
   client_write_key or the server:

   -  The certificate type MUST be X.509v3 [RFC5280], unless explicitly
      negotiated otherwise (e.g., [RFC5081]).

   -  The server's end-entity certificate's public key (and associated
      restrictions) MUST be compatible with server_write_key, the selected key exchange
      algorithm.

           +----------------------+---------------------------+
           | Key Exchange Alg.    | Certificate Key Type      |
           +----------------------+---------------------------+
           | DHE_RSA or ECDHE_RSA | RSA public key            |
           |                      |                           |
           | ECDHE_ECDSA          | ECDSA or EdDSA public key |
           +----------------------+---------------------------+

   -  The certificate MUST allow nonce is derived from
   the key to be used for signing (i.e., sequence number (see Section 5.3) and the digitalSignature bit MUST be set if client_write_iv or
   server_write_iv, and the Key Usage extension additional data input is
      present) with a signature scheme indicated empty (zero
   length).  Derivation of traffic keys is defined in the client's
      "signature_algorithms" extension.

   - Section 7.3.

   The "server_name" and "trusted_ca_keys" extensions [RFC6066] are
      used to guide certificate selection.  As servers MAY require plaintext is the
      presence concatenation of TLSPlaintext.fragment,
   TLSPlaintext.type, and any padding bytes (zeros).

   The AEAD output consists of the "server_name" extension, clients SHOULD send this
      extension.

   All certificates provided ciphertext output by the server MUST be signed by a signature
   algorithm that appears in AEAD
   encryption operation.  The length of the "signature_algorithms" extension
   provided plaintext is greater than
   TLSPlaintext.length due to the inclusion of TLSPlaintext.type and
   however much padding is supplied by the client, if they are able to provide such a chain (see
   Section 6.3.2.2).  Certificates that are self-signed or certificates
   that are expected to be trust anchors are not validated as part sender.  The length of the chain and therefore MAY
   AEAD output will generally be signed with any algorithm.

   If larger than the server cannot produce a certificate chain plaintext, but by an
   amount that is signed only
   via varies with the indicated supported algorithms, then it SHOULD continue AEAD cipher.  Since the
   handshake by sending ciphers might
   incorporate padding, the client a certificate chain amount of its choice
   that may include algorithms that are not known to be supported by the
   client.  This fallback chain MAY use overhead could vary with different
   lengths of plaintext.  Symbolically,

      AEADEncrypted =
          AEAD-Encrypt(write_key, nonce, plaintext of fragment)

   In order to decrypt and verify, the deprecated SHA-1 hash
   algorithm only if cipher takes as input the "signature_algorithms" extension provided by key,
   nonce, and the client permits it.  If AEADEncrypted value.  The output is either the client cannot construct
   plaintext or an acceptable
   chain using error indicating that the provided certificates and decides to abort decryption failed.  There
   is no separate integrity check.  That is:

      plaintext of fragment =
          AEAD-Decrypt(write_key, nonce, AEADEncrypted)

   If the
   handshake, then it decryption fails, a fatal "bad_record_mac" alert MUST send be
   generated.

   An AEAD cipher MUST NOT produce an "unsupported_certificate" alert
   message and close the connection.

   If the server has multiple certificates, it chooses one expansion of them based
   on the above-mentioned criteria (in addition to other criteria, such
   as transport layer endpoint, local configuration and preferences).

   As cipher suites greater than 255
   bytes.  An endpoint that specify new key exchange methods are specified receives a record from its peer with
   TLSCipherText.length larger than 2^14 + 256 octets MUST generate a
   fatal "record_overflow" alert.  This limit is derived from the
   maximum TLSPlaintext length of 2^14 octets + 1 octet for ContentType
   + the TLS protocol, they will imply maximum AEAD expansion of 255 octets.

5.3.  Per-Record Nonce

   A 64-bit sequence number is maintained separately for reading and
   writing records.  Each sequence number is set to zero at the certificate format
   beginning of a connection and whenever the
   required encoded keying information.

6.3.4.1.2.  Client Certificate Selection key is changed.

   The following rules apply to certificates sent by the client:

   In particular:

   - sequence number is incremented after reading or writing each
   record.  The certificate type first record transmitted under a particular set of
   traffic keys record key MUST be X.509v3 [RFC5280], unless explicitly
      negotiated otherwise (e.g., [RFC5081]).

   - use sequence number 0.

   Sequence numbers do not wrap.  If the certificate_authorities list in the certificate request
      message was non-empty, one of the certificates in the certificate
      chain SHOULD be issued by one of the listed CAs.

   -  The certificates a TLS implementation would need to
   wrap a sequence number, it MUST be signed using an acceptable hash/
      signature algorithm pair, as described in Section 6.3.3.2.  Note
      that this relaxes either rekey (Section 4.4.3) or
   terminate the constraints on certificate-signing
      algorithms found in prior versions connection.

   The length of TLS.

   -  If the certificate_extensions list in the certificate request
      message was non-empty, the end-entity certificate MUST match the
      extension OIDs recognized by per-record nonce (iv_length) is set to max(8 bytes,
   N_MIN) for the client, as described in AEAD algorithm (see [RFC5116] Section 6.3.3.2.

   Note that, as with the server certificate, there are certificates
   that use 4).  An AEAD
   algorithm combinations that cannot where N_MAX is less than 8 bytes MUST NOT be currently used with TLS.

6.3.4.1.3.  Receiving a Certificate Message

   In general, detailed certificate validation procedures are out of
   scope
   The per-record nonce for TLS (see [RFC5280]).  This section provides TLS-specific
   requirements.

   If the server supplies an empty Certificate message, the client MUST
   terminate AEAD construction is formed as follows:

   1.  The 64-bit record sequence number is padded to the handshake left with
       zeroes to iv_length.

   2.  The padded sequence number is XORed with a fatal "decode_error" alert.

   If the client does not send any certificates, static
       client_write_iv or server_write_iv, depending on the server MAY at its
   discretion either continue role.

   The resulting quantity (of length iv_length) is used as the handshake without client
   authentication, or respond with per-
   record nonce.

   Note: This is a fatal "handshake_failure" alert.
   Also, if some aspect different construction from that in TLS 1.2, which
   specified a partially explicit nonce.

5.4.  Record Padding

   All encrypted TLS records can be padded to inflate the size of the certificate chain was unacceptable (e.g.,
   it was not signed by a known, trusted CA),
   TLSCipherText.  This allows the server MAY at its
   discretion either continue sender to hide the handshake (considering size of the client
   unauthenticated) or send a fatal alert.

   Any endpoint receiving any certificate signed using any signature
   algorithm using
   traffic from an MD5 hash MUST send observer.

   When generating a "bad_certificate" alert
   message and close the connection.

   SHA-1 is deprecated and therefore NOT RECOMMENDED.  Endpoints that
   reject certification paths due TLSCiphertext record, implementations MAY choose to use of
   pad.  An unpadded record is just a deprecated hash MUST send record with a fatal "bad_certificate" alert message padding length of
   zero.  Padding is a string of zero-valued bytes appended to the
   ContentType field before closing encryption.  Implementations MUST set the
   connection.  All endpoints are RECOMMENDED to transition to SHA-256
   or better as soon as possible
   padding octets to maintain interoperability with
   implementations currently all zeros before encrypting.

   Application Data records may contain a zero-length fragment.content
   if the sender desires.  This permits generation of plausibly-sized
   cover traffic in contexts where the process presence or absence of phasing out SHA-1
   support.

   Note that a certificate containing a key for one signature algorithm
   MAY be signed using a different signature algorithm (for instance, an
   RSA key signed with an ECDSA key).

6.3.4.2.  Certificate Verify

   When this message will activity
   may be sent:

      This message is used to provide explicit proof that an endpoint
      possesses the private key corresponding to its certificate and
      also provides integrity for the handshake up to this point.
      Servers sensitive.  Implementations MUST NOT send this message when using Handshake or Alert
   records that have a cipher suite which zero-length fragment.content.

   The padding sent is
      authenticated via a certificate.  Clients MUST send this message
      whenever authenticating via a Certificate (i.e., when automatically verified by the
      Certificate message is non-empty).  When sent, this message MUST
      appear immediately after record protection
   mechanism: Upon successful decryption of a TLSCiphertext.fragment,
   the Certificate Message and immediately
      prior to receiving implementation scans the Finished message.

   Structure of this message:

      struct {
           digitally-signed struct {
              opaque hashed_data[hash_length];
           };
      } CertificateVerify;

      Where hashed_data is field from the hash output described in Section 6.3.4,
      namely Hash(Handshake Context + Certificate) +
      Hash(resumption_context).  For concreteness, this means that end toward the
      value that is signed is:

          padding + context_string + 00 + hashed_data

      The context string for a server signature is "TLS 1.3, server
      CertificateVerify" and for
   beginning until it finds a client signature non-zero octet.  This non-zero octet is "TLS 1.3, client
      CertificateVerify".  A hash of
   the handshake messages is signed
      rather than content type of the messages themselves message.  This padding scheme was selected
   because the digitally-signed
      format requires it allows padding and context bytes at the beginning of the
      input.  Thus, any encrypted TLS record by signing a digest of the messages, an
      implementation only needs to maintain a single running hash per
      hash type arbitrary
   size (from zero up to TLS record size limits) without introducing new
   content types.  The design also enforces all-zero padding octets,
   which allows for CertificateVerify, Finished and other messages. quick detection of padding errors.

   Implementations MUST limit their scanning to the cleartext returned
   from the AEAD decryption.  If sent by a server, the signature algorithm MUST be one offered receiving implementation does not
   find a non-zero octet in the client's "signature_algorithms" extension unless no valid
      certificate chain can be produced without unsupported algorithms
      (see Section 6.3.2.2).  Note that there is a possibility for
      inconsistencies here.  For instance, cleartext, it should treat the client might offer
      ECDHE_ECDSA key exchange but omit any ECDSA and EdDSA values from
      its "signature_algorithms" extension.  In order to negotiate
      correctly, record as
   having an unexpected ContentType, sending an "unexpected_message"
   alert.

   The presence of padding does not change the server MUST check any candidate cipher suites
      against overall record size
   limitations - the "signature_algorithms" extension before selecting
      them.  This is somewhat inelegant but is full fragment plaintext may not exceed 2^14 octets.

   Selecting a compromise designed to
      minimize changes padding policy that suggests when and how much to the original cipher suite design.

      If sent by pad is
   a client, the signature algorithm used in the signature
      MUST be one of those present in complex topic, and is beyond the supported_signature_algorithms
      field scope of this specification.  If
   the CertificateRequest message.

      In addition, the signature algorithm MUST application layer protocol atop TLS has its own padding padding,
   it may be compatible with the
      key in preferable to pad application_data TLS records within the sender's end-entity certificate.  RSA signatures MUST
      use an RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS-
      v1_5 algorithms appear in "signature_algorithms".  SHA-1 MUST NOT
      be used in any signatures in CertificateVerify.  (Note that
      rsa_pkcs1_sha1
   application layer.  Padding for encrypted handshake and dsa_sha1, alert TLS
   records must still be handled at the only defined SHA-1 signature TLS layer, though.  Later
   documents may define padding selection algorithms, or define a
   padding policy request mechanism through TLS extensions or some other
   means.

5.5.  Limits on Key Usage

   There are undefined for CertificateVerify signatures.)

   Note: When used with non-certificate-based handshakes (e.g., PSK),
   the client's signature does not cover the server's certificate
   directly, although it does cover cryptographic limits on the server's Finished message, amount of plaintext which
   transitively includes can
   be safely encrypted under a given set of keys.  [AEAD-LIMITS]
   provides an analysis of these limits under the server's certificate when assumption that the PSK derives
   from a certificate-authenticated handshake.  [PSK-FINISHED] describes
   underlying primitive (AES or ChaCha20) has no weaknesses.
   Implementations SHOULD do a concrete attack key update Section 4.4.3 prior to
   reaching these limits.

   For AES-GCM, up to 2^24.5 full-size records may be encrypted on this mode if a
   given connection while keeping a safety margin of approximately 2^-57
   for Authenticated Encryption (AE) security.  For ChaCha20/Poly1305,
   the Finished is omitted from record sequence number will wrap before the
   signature.  It safety limit is unsafe to use certificate-based client
   authentication when
   reached.

6.  Alert Protocol

   One of the client might potentially share content types supported by the same PSK/
   key-id pair with two different endpoints.  In order to ensure this,
   implementations MUST NOT mix certificate-based client authentication
   with pure PSK modes (i.e., those where TLS record layer is the PSK was not derived from a
   previous non-PSK handshake).

6.3.4.3.  Finished

   When this message will be sent:

      The Finished message is
   alert type.  Like other messages, alert messages are encrypted as
   specified by the final message in current connection state.

   Alert messages convey the authentication
      block.  It is essential for providing authentication severity of the
      handshake message (warning or fatal)
   and a description of the computed keys.

   Meaning of this message:

      Recipients of Finished alert.  Warning-level messages MUST verify that the contents are
      correct.  Once used to
   indicate orderly closure of the connection (see Section 6.1).  Upon
   receiving a side has sent its Finished message and received
      and validated warning-level alert, the Finished message from its peer, it may begin TLS implementation SHOULD
   indicate end-of-data to
      send and receive the application data over and, if appropriate for the connection.

   The key
   alert type, send a closure alert in response.

   Fatal-level messages are used to compute the finished message is computed from the
   Base key defined in Section 6.3.4 using HKDF (see Section 7.1).
   Specifically:

   client_finished_key =
       HKDF-Expand-Label(BaseKey, "client finished", "", L)

   server_finished_key =
       HKDF-Expand-Label(BaseKey, "server finished", "", L)

   Structure indicate abortive closure of this message:

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

   The verify_data value is computed as follows:

      verify_data =
          HMAC(finished_key, Hash(
                 Handshake Context + Certificate* + CertificateVerify*
              ) + Hash(resumption_context)
              )

      * Only included if present.

   Where HMAC [RFC2104] uses the Hash algorithm for
   connection (See Section 6.2).  Upon receiving a fatal-level alert,
   the handshake.  As
   noted above: TLS implementation SHOULD indicate an error to the HMAC input can generally application
   and MUST NOT allow any further data to be implemented by a running
   hash, i.e., just sent or received on the handshake hash at this point.

   In previous versions
   connection.  Servers and clients MUST forget keys and secrets
   associated with a failed connection.  Stateful implementations of TLS, the verify_data was always 12 octets
   long.  In the current version of TLS, it is
   session tickets (as in many clients) SHOULD discard tickets
   associated with failed connections.

   All the size alerts listed in Section 6.2 MUST be sent as fatal and MUST
   be treated as fatal regardless of the HMAC
   output for the Hash used for AlertLevel in the handshake.

   Note: Alerts and any other record message.
   Unknown alert types are not handshake messages MUST be treated as fatal.

      enum { warning(1), fatal(2), (255) } AlertLevel;

      enum {
          close_notify(0),
          end_of_early_data(1),
          unexpected_message(10),
          bad_record_mac(20),
          record_overflow(22),
          handshake_failure(40),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),
          unknown_ca(48),
          access_denied(49),
          decode_error(50),
          decrypt_error(51),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          inappropriate_fallback(86),
          user_canceled(90),
          missing_extension(109),
          unsupported_extension(110),
          certificate_unobtainable(111),
          unrecognized_name(112),
          bad_certificate_status_response(113),
          bad_certificate_hash_value(114),
          unknown_psk_identity(115),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

6.1.  Closure Alerts

   The client and are not included the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.  Failure to properly
   close a connection does not prohibit a session from being resumed.

   close_notify  This alert notifies the hash computations.

6.3.5.  Post-Handshake Messages

   TLS also allows other recipient that the sender will
      not send any more messages to on this connection.  Any data received
      after a closure MUST be ignored.

   end_of_early_data  This alert is sent after by the main handshake.
   These client to indicate that
      all 0-RTT application_data messages use a handshake content type have been transmitted (or none
      will be sent at all) and are encrypted under
   the application traffic key.

6.3.5.1.  New Session Ticket Message

   At any time after that this is the server has received end of the client Finished
   message, it MAY send a NewSessionTicket message. flight.  This message
   creates a pre-shared key (PSK) binding between
      alert MUST be at the ticket value warning level.  Servers MUST NOT send this
      alert and clients receiving it MUST terminate the following two values derived from connection with
      an "unexpected_message" alert.

   user_canceled  This alert notifies the resumption master secret:

     resumption_psk = HKDF-Expand-Label(resumption_secret,
                                        "resumption psk", "", L)

     resumption_context = HKDF-Expand-Label(resumption_secret,
                                            "resumption context", "", L)

   The client MAY use this PSK for future handshakes by including recipient that the
   ticket value in sender is
      canceling the "pre_shared_key" extension in its ClientHello
   (Section 6.3.2.5) and supplying a suitable PSK cipher suite.  Servers
   may send multiple tickets on a single connection, handshake for instance after
   post-handshake authentication.  For handshakes that do not use some reason unrelated to a
   resumption_psk, the resumption_context is protocol
      failure.  If a string of L zeroes.

    enum { (65535) } TicketExtensionType;

    struct {
        TicketExtensionType extension_type;
        opaque extension_data<0..2^16-1>;
    } TicketExtension;

    enum {
      allow_early_data(1)
      allow_dhe_resumption(2),
      allow_psk_resumption(4)
    } TicketFlags;

    struct {
        uint32 ticket_lifetime;
        uint32 flags;
        TicketExtension extensions<2..2^16-2>;
        opaque ticket<0..2^16-1>;
    } NewSessionTicket;

   flags
      A 32-bit value indicating the ways in which this ticket may be
      used (as user cancels an OR of operation after the flags values).

   ticket_lifetime
      Indicates handshake is
      complete, just closing the lifetime in seconds as connection by sending a 32-bit unsigned integer in
      network byte order from the time of ticket issuance.  Servers MUST
      NOT use any value "close_notify"
      is more than 604800 seconds (7 days).  The value of
      zero indicates that the ticket should appropriate.  This alert SHOULD be discarded immediately.
      Clients MUST NOT cache session tickets for longer than 7 days,
      regardless of the ticket_lifetime.  It followed by a
      "close_notify".  This alert is generally a warning.

   Either party MAY delete the ticket
      earlier based on local policy.  A server MAY treat initiate a ticket as
      valid for close by sending a shorter period of time than what "close_notify" alert.
   Any data received after a closure alert is stated in ignored.  If a transport-
   level close is received prior to a "close_notify", the
      ticket_lifetime.

   ticket_extensions
      A placeholder for extensions in receiver
   cannot know that all the ticket.  Clients data that was sent has been received.

   Each party MUST ignore
      unrecognized extensions.

   ticket
      The value of send a "close_notify" alert before closing the ticket to be used as write
   side of the PSK identifier. connection, unless some other fatal alert has been
   transmitted.  The
      ticket itself is an opaque label.  It MAY either be a database
      lookup key or other party MUST respond with a self-encrypted and self-authenticated value.
      Section 4 "close_notify"
   alert of [RFC5077] describes a recommended ticket construction
      mechanism. its own and close down the connection immediately,
   discarding any pending writes.  The meanings initiator of the flags are as follows:

   allow_early_data
      When resuming with this ticket, the client MAY send data in its
      first flight (early data) encrypted under a key derived from this
      PSK.

   allow_dhe_resumption
      This ticket MAY be used with (EC)DHE-PSK cipher suite

   allow_psk_resumption
      This ticket MAY be used with a pure PSK cipher suite.

   In all cases, the PSK or (EC)DHE-PSK cipher suites that close need not
   wait for the client
   offers/uses MUST have responding "close_notify" alert before closing the same symmetric parameters (cipher/hash) as read
   side of the cipher suite negotiated for this connection.

   If no flags are set the application protocol using TLS provides that any data may be
   carried over the client recognizes, it MUST ignore underlying transport after the ticket.

6.3.5.2.  Post-Handshake Authentication

   The server TLS connection is permitted
   closed, the TLS implementation must receive the responding
   "close_notify" alert before indicating to request client authentication at any time
   after the handshake has completed by sending a CertificateRequest
   message.  The client SHOULD respond with application layer that
   the appropriate
   Authentication messages. TLS connection has ended.  If the client chooses to authenticate, it
   MUST send Certificate, CertificateVerify, and Finished.  If it
   declines, it MUST send a Certificate message containing no
   certificates followed by Finished.

   Note: Because client authentication may require prompting application protocol will not
   transfer any additional data, but will only close the user,
   servers MUST be prepared for some delay, including receiving an
   arbitrary number of other messages between sending underlying
   transport connection, then the
   CertificateRequest and receiving a response.  In addition, clients
   which receive multiple CertificateRequests in close succession implementation MAY
   respond choose to them in a different order than they were received (the
   certificate_request_context value allows close the server
   transport without waiting for the responding "close_notify".  No part
   of this standard should be taken to disambiguate dictate the responses).

6.3.5.3.  Key and IV Update

   struct {} KeyUpdate;

   The KeyUpdate handshake message manner in which a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

   Note: It is used to indicate assumed that closing a connection reliably delivers
   pending data before destroying the sender transport.

6.2.  Error Alerts

   Error handling in the TLS Handshake Protocol is updating its sending cryptographic keys.  This message can be sent
   by very simple.  When an
   error is detected, the server after sending its first flight and the client after
   sending its second flight.  Implementations that receive detecting party sends a KeyUpdate message prior to receiving a Finished message as part its peer.
   Upon transmission or receipt of the 1-RTT
   handshake MUST generate a fatal "unexpected_message" alert.  After
   sending a KeyUpdate alert message, both parties
   immediately close the sender SHALL send all its traffic
   using the next generation of keys, computed connection.  Whenever an implementation
   encounters a condition which is defined as described in
   Section 7.2.  Upon receiving a KeyUpdate, the receiver MUST update
   their receiving keys and if they have not already updated their
   sending state up to or past the then current receiving generation fatal alert, it MUST
   send their own KeyUpdate the appropriate alert prior to sending any other messages.
   This mechanism allows either side to force an update to closing the entire connection.  Note that implementations may receive an arbitrary
   number  All
   alerts defined in this section below, as well as all unknown alerts
   are universally considered fatal as of messages between sending a KeyUpdate and receiving the
   peer's KeyUpdate because those messages may already TLS 1.3 (see Section 6).

   The following error alerts are defined:

   unexpected_message  An inappropriate message was received.  This
      alert should never be observed in flight.

   Note that communication between proper
      implementations.

   bad_record_mac  This alert is returned if implementations independently send their own KeyUpdates a record is received which
      cannot be deprotected.  Because AEAD algorithms combine decryption
      and they cross in flight, verification, this only results alert is used for all deprotection
      failures.  This alert should never be observed in an update of one
   generation; communication
      between proper implementations, except when each side receives messages were
      corrupted in the other side's update it just
   updates its receive keys and notes network.

   record_overflow  A TLSCiphertext record was received that had a
      length more than 2^14 + 256 bytes, or a record decrypted to a
      TLSPlaintext record with more than 2^14 bytes.  This alert should
      never be observed in communication between proper implementations,
      except when messages were corrupted in the generations match and
   thus no send update is needed.

   Note network.

   handshake_failure  Reception of a "handshake_failure" alert message
      indicates that the side which sends its KeyUpdate first needs sender was unable to retain negotiate an acceptable
      set of security parameters given the traffic keys (though options available.

   bad_certificate  A certificate was corrupt, contained signatures that
      did not the traffic secret) for the previous
   generation verify correctly, etc.

   unsupported_certificate  A certificate was of keys until it receives an unsupported type.

   certificate_revoked  A certificate was revoked by its signer.

   certificate_expired  A certificate has expired or is not currently
      valid.

   certificate_unknown  Some other (unspecified) issue arose in
      processing the KeyUpdate from certificate, rendering it unacceptable.

   illegal_parameter  A field in the other
   side.

   Both sender and receiver MUST encrypt their KeyUpdate messages handshake was out of range or
      inconsistent with other fields.

   unknown_ca  A valid certificate chain or partial chain was received,
      but the old keys.  Additionally, both sides MUST enforce that a KeyUpdate
   with certificate was not accepted because the old key is received before accepting any messages encrypted CA certificate
      could not be located or couldn't be matched with a known, trusted
      CA.

   access_denied  A valid certificate or PSK was received, but when
      access control was applied, the new key.  Failure sender decided not to do so may allow message truncation
   attacks.

7.  Cryptographic Computations

   In order to begin connection protection, proceed with
      negotiation.

   decode_error  A message could not be decoded because some field was
      out of the TLS Record Protocol
   requires specification specified range or the length of the message was
      incorrect.  This alert should never be observed in communication
      between proper implementations, except when messages were
      corrupted in the network.

   decrypt_error  A handshake cryptographic operation failed, including
      being unable to correctly verify a suite signature or validate a
      Finished message.

   protocol_version  The protocol version the peer has attempted to
      negotiate is recognized but not supported. (see Appendix C)

   insufficient_security  Returned instead of algorithms, "handshake_failure" when a master secret, and
      negotiation has failed specifically because the client and server random values.  The authentication, key
   exchange, and record protection algorithms are determined requires
      ciphers more secure than those supported by the
   cipher_suite selected by client.

   internal_error  An internal error unrelated to the server and revealed in peer or the ServerHello
   message.  The random values are exchanged in
      correctness of the protocol (such as a memory allocation failure)
      makes it impossible to continue.

   inappropriate_fallback  Sent by a server in response to an invalid
      connection retry attempt from a client. (see [RFC7507])

   missing_extension  Sent by endpoints that receive a hello messages.  All message not
      containing an extension that remains is mandatory to calculate send for the key schedule.

7.1.  Key Schedule

   The offered
      TLS handshake establishes one or more input secrets which are
   combined version.  [[TODO: IANA Considerations.]]

   unsupported_extension  Sent by endpoints receiving any hello message
      containing an extension known to create be prohibited for inclusion in
      the actual working keying material, as detailed
   below.  The key derivation process makes use of the following
   functions, based on HKDF [RFC5869]:

   HKDF-Extract(Salt, IKM) as defined given hello message, including any extensions in {{RFC5869}}.

   HKDF-Expand-Label(Secret, Label, Messages, Length) =
        HKDF-Expand(Secret, HkdfLabel, Length)

   Where HkdfLabel is specified as:

   struct HkdfLabel {
     uint16 length;
     opaque label<9..255>;
     opaque hash_value<0..255>;
   };

   - HkdfLabel.length is Length
   - HkdfLabel.label is "TLS 1.3, " + Label
   - HkdfLabel.hash_value is HashValue.

   Derive-Secret(Secret, Label, Messages) =
        HKDF-Expand-Label(Secret, Label,
                          Hash(Messages) + Hash(resumption_context), L))

   Given a set of n InputSecrets, ServerHello
      not first offered in the final "master secret" is computed corresponding ClientHello.

   certificate_unobtainable  Sent by iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc.  The initial secret is simply servers when unable to obtain a string of 0s as
   long as the size of the Hash that is the basis for
      certificate from a URL provided by the HKDF.
   Concretely, for client via the present version of TLS 1.3, secrets are added in
      "client_certificate_url" extension [RFC6066].

   unrecognized_name  Sent by servers when no server exists identified
      by the following order:

   -  PSK

   -  (EC)DHE shared secret

   This produces a full key derivation schedule shown in name provided by the diagram
   below.  In this diagram, client via the following formatting conventions apply:

   -  HKDF-Extract "server_name" extension
      [RFC6066].

   bad_certificate_status_response  Sent by clients when an invalid or
      unacceptable OCSP response is drawn as taking the Salt argument from the top and provided by the IKM argument from server via the left.

   -  Derive-Secret's Secret argument
      "status_request" extension [RFC6066].  This alert is indicated always fatal.

   bad_certificate_hash_value  Sent by servers when a retrieved object
      does not have the arrow coming
      in from correct hash provided by the left.  For instance, client via the Early Secret
      "client_certificate_url" extension [RFC6066].

   unknown_psk_identity  Sent by servers when a PSK cipher suite is the Secret
      for generating the early_traffic-secret.

                 0
                 |
                 v
      selected but no acceptable PSK ->  HKDF-Extract
                 |
                 v
           Early Secret  --> Derive-Secret(., "early traffic secret",
                 |                         ClientHello)
                 |                         = early_traffic_secret
                 v
(EC)DHE -> HKDF-Extract
                 |
                 v
              Handshake
               Secret -----> Derive-Secret(., "handshake traffic secret",
                 |                         ClientHello + ServerHello)
                 |                         = handshake_traffic_secret
                 v
      0 -> HKDF-Extract
                 |
                 v
            Master Secret
                 |
                 +---------> Derive-Secret(., "application traffic secret",
                 |                         ClientHello...Server Finished)
                 |                         = traffic_secret_0
                 |
                 +---------> Derive-Secret(., "exporter master secret",
                 |                         ClientHello...Client Finished)
                 |                         = exporter_secret
                 |
                 +---------> Derive-Secret(., "resumption master secret",
                                           ClientHello...Client Finished)
                                           = resumption_secret

   The general pattern here identity is that provided by the secrets shown down client.
      Sending this alert is OPTIONAL; servers MAY instead choose to send
      a "decrypt_error" alert to merely indicate an invalid PSK
      identity.

   New Alert values are assigned by IANA as described in Section 10.

7.  Cryptographic Computations

   In order to begin connection protection, the left side TLS Record Protocol
   requires specification of a suite of algorithms, a master secret, and
   the diagram client and server random values.  The authentication, key
   exchange, and record protection algorithms are just raw entropy without context, whereas determined by the
   secrets down
   cipher_suite selected by the right side include handshake context server and therefore
   can be used to derive working keys without additional context.  Note revealed in the ServerHello
   message.  The random values are exchanged in the hello messages.  All
   that remains is to calculate the different calls key schedule.

7.1.  Key Schedule

   The TLS handshake establishes one or more input secrets which are
   combined to Derive-Secret may take different Messages
   arguments, even with create the same secret.  In a 0-RTT exchange, Derive-
   Secret is called with four distinct transcripts; in a 1-RTT only
   exchange with three distinct transcripts.

   If a given secret is not available, then the 0-value consisting of a
   string actual working keying material, as detailed
   below.  The key derivation process makes use of L zeroes is used.

7.2.  Updating Traffic Keys the HKDF-Extract and IVs

   Once
   HKDF-Expand functions as defined for HKDF [RFC5869], as well as the handshake
   functions defined below:

       HKDF-Expand-Label(Secret, Label, HashValue, Length) =
            HKDF-Expand(Secret, HkdfLabel, Length)

       Where HkdfLabel is complete, it specified as:

       struct HkdfLabel
       {
         uint16 length = Length;
         opaque label<9..255> = "TLS 1.3, " + Label;
         opaque hash_value<0..255> = HashValue;
       };

       Derive-Secret(Secret, Label, Messages) =
            HKDF-Expand-Label(Secret, Label,
                              Hash(Messages) +
                              Hash(resumption_context), Hash.Length)

   The Hash function and the HKDF hash are the cipher suite hash
   function.  Hash.Length is possible for either side to
   update its sending traffic keys using the KeyUpdate handshake message
   Section 6.3.5.3.  The next generation output length.

   Given a set of traffic keys n InputSecrets, the final "master secret" is computed
   by
   generating traffic_secret_N+1 from traffic_secret_N iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc.  The initial secret is simply a string of zeroes
   as described in
   this section then re-deriving the traffic keys long as described in
   Section 7.3.

   The next-generation traffic_secret the size of the Hash that is computed as:

   traffic_secret_N+1 = HKDF-Expand-Label(traffic_secret_N, "application
   traffic secret", "", L)

   Once traffic_secret_N+1 and its associated traffic keys have been
   computed, implementations SHOULD delete traffic_secret_N.  Once the
   directional keys basis for the HKDF.
   Concretely, for the present version of TLS 1.3, secrets are no longer needed, they SHOULD be deleted as
   well.

7.3.  Traffic Key Calculation

   The traffic keying material is generated from added in
   the following input
   values: order:

   -  A secret value  PSK

   -  A phase value indicating the phase of  (EC)DHE shared secret

   This produces a full key derivation schedule shown in the protocol diagram
   below.  In this diagram, the keys are
      being generated for. following formatting conventions apply:

   -  A purpose value indicating  HKDF-Extract is drawn as taking the specific value being generated

   -  The length of Salt argument from the key.

   The keying material top and
      the IKM argument from the left.

   -  Derive-Secret's Secret argument is computed using:

      key = HKDF-Expand-Label(Secret,
                              phase + ", " + purpose, "",
                              key_length)

   The following table describes indicated by the inputs to arrow coming
      in from the key calculation left.  For instance, the Early Secret is the Secret
      for
   each class of traffic keys:

   +-------------+--------------------------+--------------------------+ generating the early_traffic-secret.

                 0
                 | Record Type
                 v
   PSK ->  HKDF-Extract
                 |
                 v
           Early Secret                   | Phase                    |
   +-------------+--------------------------+--------------------------+
   | 0-RTT       | early_traffic_secret     | ---> Derive-Secret(., "early handshake key     |
   | Handshake   |                          | expansion"               |
   |             |                          |                          | traffic secret",
                 | 0-RTT                         ClientHello)
                 |                         = early_traffic_secret
                 v
(EC)DHE -> HKDF-Extract
                 | "early application data  |
   | Application |                          | key expansion"           |
   |             |                          |                          |
   |
                 v
              Handshake   | handshake_traffic_secret |
               Secret -----> Derive-Secret(., "handshake key           |
   |             |                          | expansion"               |
   |             |                          | traffic secret",
                 |                         ClientHello + ServerHello)
                 | Application                         = handshake_traffic_secret
                 v
      0 -> HKDF-Extract
                 | traffic_secret_N
                 v
            Master Secret
                 |
                 +---------> Derive-Secret(., "application data key traffic secret",
                 |                         ClientHello...Server Finished)
                 | Data                         = traffic_secret_0
                 |
                 +---------> Derive-Secret(., "exporter master secret",
                 | expansion"                         ClientHello...Client Finished)
                 |
   +-------------+--------------------------+--------------------------+                         = exporter_secret
                 |
                 +---------> Derive-Secret(., "resumption master secret",
                                           ClientHello...Client Finished)
                                           = resumption_secret

   The following table indicates general pattern here is that the purpose values for each type secrets shown down the left side
   of
   key:

                 +------------------+--------------------+
                 | Key Type         | Purpose            |
                 +------------------+--------------------+
                 | Client Write Key | "client write key" |
                 |                  |                    |
                 | Server Write Key | "server write key" |
                 |                  |                    |
                 | Client Write IV  | "client write iv"  |
                 |                  |                    |
                 | Server Write IV  | "server write iv"  |
                 +------------------+--------------------+

   All the traffic keying material is recomputed whenever diagram are just raw entropy without context, whereas the underlying
   Secret changes (e.g., when changing from
   secrets down the right side include handshake context and therefore
   can be used to application
   data derive working keys or upon without additional context.  Note
   that the different calls to Derive-Secret may take different Messages
   arguments, even with the same secret.  In a key update).

7.3.1.  Diffie-Hellman

   A conventional Diffie-Hellman computation is performed.  The
   negotiated key (Z) 0-RTT exchange, Derive-
   Secret is converted to byte string by encoding called with four distinct transcripts; in big-
   endian, padded a 1-RTT only
   exchange with zeros up to three distinct transcripts.

   If a given secret is not available, then the size 0-value consisting of the prime.  This byte a
   string of Hash.length zeroes is used as the shared secret, and is used in the key schedule
   as specified above. used.  Note that this construction differs from previous versions of TLS
   which remove leading zeros.

7.3.2.  Elliptic Curve Diffie-Hellman

   For secp256r1, secp384r1 and secp521r1, ECDH calculations (including
   parameter does not mean
   skipping rounds, so if PSK is not in use Early Secret will still be
   HKDF-Extract(0, 0).

7.2.  Updating Traffic Keys and key generation as well as IVs

   Once the shared secret
   calculation) are performed according handshake is complete, it is possible for either side to [IEEE1363] using the ECKAS-
   DH1 scheme with the identity map as key derivation function (KDF), so
   that the shared secret is the x-coordinate of
   update its sending traffic keys using the ECDH shared secret
   elliptic curve point represented as an octet string.  Note that this
   octet string (Z KeyUpdate handshake message
   defined in IEEE 1363 terminology) as output Section 4.4.3.  The next generation of traffic keys is
   computed by FE2OSP, the
   Field Element to Octet String Conversion Primitive, has constant
   length for any given field; leading zeros found generating traffic_secret_N+1 from traffic_secret_N as
   described in this octet string
   MUST NOT be truncated.

   (Note that this use of section then re-deriving the identity KDF is a technicality. traffic keys as
   described in Section 7.3.

   The
   complete picture is that ECDH next-generation traffic_secret is employed with a non-trivial KDF
   because TLS does not directly use this secret for anything other than
   for computing other secrets.)

   ECDH functions computed as:

    traffic_secret_N+1 = HKDF-Expand-Label(
                             traffic_secret_N,
                             "application traffic secret", "", Hash.Length)

   Once traffic_secret_N+1 and its associated traffic keys have been
   computed, implementations SHOULD delete traffic_secret_N.  Once the
   directional keys are used no longer needed, they SHOULD be deleted as follows:

   -
   well.

7.3.  Traffic Key Calculation

   The public key to put into the KeyShareEntry.key_exchange
      structure traffic keying material is generated from the result following input
   values:

   -  A secret value

   -  A phase value indicating the phase of applying the ECDH function to protocol the
      secret key of appropriate length (into scalar input) and keys are
      being generated for

   -  A purpose value indicating the
      standard public basepoint (into u-coordinate point input). specific value being generated

   -  The ECDH shared secret is the result length of applying ECDH function to the secret key (into scalar input) and the peer's public key (into
      u-coordinate point input).

   The output is used raw, with no
      processing.

   For X25519 and X448, see [RFC7748].

7.3.3.  Exporters

   [RFC5705] defines keying material exporters for TLS in terms of the
   TLS PRF.  This document replaces the PRF with HKDF, thus requiring a
   new construction.  The exporter interface remains the same, however
   the value is computed as:

   HKDF-Expand-Label(exporter_secret,
                     label, context_value, using:

      key = HKDF-Expand-Label(Secret,
                              phase + ", " + purpose,
                              "",
                              key_length)

8.  Mandatory Algorithms

8.1.  MTI Cipher Suites

   In

   The following table describes the absence of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement inputs to the following
   cipher suites:

       TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   These cipher suites MUST support both digital signatures and key
   exchange with secp256r1 (NIST P-256) and SHOULD support calculation for
   each class of traffic keys:

   +-------------+--------------------------+--------------------------+
   | Record Type | Secret                   | Phase                    |
   +-------------+--------------------------+--------------------------+
   | 0-RTT       | early_traffic_secret     | "early handshake key exchange
   with X25519 [RFC7748].

   A TLS-compliant     |
   | Handshake   |                          | expansion"               |
   |             |                          |                          |
   | 0-RTT       | early_traffic_secret     | "early application SHOULD implement the data  |
   | Application |                          | key expansion"           |
   |             |                          |                          |
   | Handshake   | handshake_traffic_secret | "handshake key           |
   |             |                          | expansion"               |
   |             |                          |                          |
   | Application | traffic_secret_N         | "application data key    |
   | Data        |                          | expansion"               |
   +-------------+--------------------------+--------------------------+

   The following cipher
   suites:

       TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

8.2.  MTI Extensions

   In table indicates the absence purpose values for each type of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement the following
   TLS extensions:

   -  Signature Algorithms ("signature_algorithms"; Section 6.3.2.2)

   -  Negotiated Groups ("supported_groups"; Section 6.3.2.3)

   -  Key Share ("key_share"; Section 6.3.2.4)

   -  Pre-Shared
   key:

                 +------------------+--------------------+
                 | Key ("pre_shared_key"; Section 6.3.2.5)

   -  Server Name Indication ("server_name"; Section 3 of [RFC6066])

   -  Cookie ("cookie"; Section 6.3.2.1) Type         | Purpose            |
                 +------------------+--------------------+
                 | client_write_key | "client write key" |
                 |                  |                    |
                 | server_write_key | "server write key" |
                 |                  |                    |
                 | client_write_iv  | "client write iv"  |
                 |                  |                    |
                 | server_write_iv  | "server write iv"  |
                 +------------------+--------------------+

   All implementations MUST send and use these extensions when offering
   applicable cipher suites:

   -  "signature_algorithms" the traffic keying material is REQUIRED for certificate authenticated
      cipher suites

   -  "supported_groups" and "key_share" are REQUIRED for DHE recomputed whenever the underlying
   Secret changes (e.g., when changing from the handshake to application
   data keys or ECDHE
      cipher suites

   -  "pre_shared_key" upon a key update).

7.3.1.  Diffie-Hellman

   A conventional Diffie-Hellman computation is REQUIRED for PSK cipher suites

   -  "cookie" performed.  The
   negotiated key (Z) is REQUIRED for all cipher suites.

   When negotiating use converted to byte string by encoding in big-
   endian, padded with zeros up to the size of applicable cipher suites, endpoints MUST
   abort the connection with a "missing_extension" alert if prime.  This byte
   string is used as the required
   extension was not provided.  Any endpoint shared secret, and is used in the key schedule
   as specified above.

   Note that receives any invalid
   combination this construction differs from previous versions of cipher suites TLS
   which remove leading zeros.

7.3.2.  Elliptic Curve Diffie-Hellman

   For secp256r1, secp384r1 and extensions MAY abort secp521r1, ECDH calculations (including
   parameter and key generation as well as the connection
   with a "missing_extension" alert, regardless of negotiated
   parameters.

   Additionally, all implementations MUST support use of shared secret
   calculation) are performed according to [IEEE1363] using the
   "server_name" extension ECKAS-
   DH1 scheme with applications capable the identity map as key derivation function (KDF), so
   that the shared secret is the x-coordinate of using it.
   Servers MAY require clients to send a valid "server_name" extension.
   Servers requiring the ECDH shared secret
   elliptic curve point represented as an octet string.  Note that this extension SHOULD respond
   octet string (Z in IEEE 1363 terminology) as output by FE2OSP, the
   Field Element to a ClientHello
   lacking a "server_name" extension with a fatal "missing_extension"
   alert.

   Servers Octet String Conversion Primitive, has constant
   length for any given field; leading zeros found in this octet string
   MUST NOT send be truncated.

   (Note that this use of the "signature_algorithms" extension; if identity KDF is a
   client receives this extension it MUST respond technicality.  The
   complete picture is that ECDH is employed with a fatal
   "unsupported_extension" alert and close the connection.

9.  Application Data Protocol

   Application data messages non-trivial KDF
   because TLS does not directly use this secret for anything other than
   for computing other secrets.)

   ECDH functions are carried by used as follows:

   -  The public key to put into the record layer and are
   fragmented KeyShareEntry.key_exchange
      structure is the result of applying the ECDH function to the
      secret key of appropriate length (into scalar input) and encrypted based on the current connection state.
      standard public basepoint (into u-coordinate point input).

   -  The
   messages are treated as transparent data ECDH shared secret is the result of applying ECDH function to
      the record layer.

10.  Security Considerations

   Security issues are discussed throughout this memo, especially in
   Appendices B, C, secret key (into scalar input) and D.

11.  IANA Considerations

   This document uses several registries that were originally created in
   [RFC4346].  IANA has updated these to reference this document. the peer's public key (into
      u-coordinate point input).  The
   registries output is used raw, with no
      processing.

   For X25519 and their allocation policies are below:

   - X448, see [RFC7748].

7.3.3.  Exporters

   [RFC5705] defines keying material exporters for TLS Cipher Suite Registry: Values with the first byte in terms of the range
      0-254 (decimal) are assigned via Specification Required [RFC2434].
      Values
   TLS PRF.  This document replaces the PRF with HKDF, thus requiring a
   new construction.  The exporter interface remains the first byte 255 (decimal) are reserved for Private
      Use [RFC2434].  IANA [SHALL add/has added] same, however
   the value is computed as:

   HKDF-Expand-Label(exporter_secret,
                     label, context_value, key_length)

8.  Compliance Requirements
8.1.  MTI Cipher Suites

   In the absence of an application profile standard specifying
   otherwise, a "Recommended" column
      to TLS-compliant application MUST implement the following
   cipher suite registry.  All suites:

       TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   These cipher suites listed in
      Appendix A.4 are marked as "Yes".  All other MUST support both digital signatures and key
   exchange with secp256r1 (NIST P-256) and SHOULD support key exchange
   with X25519 [RFC7748].

   A TLS-compliant application SHOULD implement the following cipher suites are
      marked as "No".  IANA [SHALL add/has added] add
   suites:

       TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

8.2.  MTI Extensions

   In the absence of an application profile standard specifying
   otherwise, a note to this
      column reading:

         Cipher suites marked as "Yes" are those allocated via Standards
         Track RFCs.  Cipher suites marked as "No" are not; cipher
         suites marked "No" range from "good" to "bad" from a
         cryptographic standpoint.

   - TLS-compliant application MUST implement the following
   TLS ContentType Registry: Future values are allocated via
      Standards Action [RFC2434]. extensions:

   -  TLS Alert Registry: Future values are allocated via Standards
      Action [RFC2434].  Signature Algorithms ("signature_algorithms"; Section 4.2.2)

   -  TLS HandshakeType Registry: Future values are allocated via
      Standards Action [RFC2434].

   This document also uses a registry originally created in [RFC4366].
   IANA has updated it to reference this document.  The registry  Negotiated Groups ("supported_groups"; Section 4.2.3)

   -  Key Share ("key_share"; Section 4.2.4)

   -  Pre-Shared Key ("pre_shared_key"; Section 4.2.5)

   -  Server Name Indication ("server_name"; Section 3 of [RFC6066])

   -  Cookie ("cookie"; Section 4.2.1)

   All implementations MUST send and its
   allocation policy use these extensions when offering
   applicable cipher suites:

   -  "signature_algorithms" is listed below: REQUIRED for certificate authenticated
      cipher suites.

   -  TLS ExtensionType Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal)  "supported_groups" and "key_share" are reserved REQUIRED for Private Use [RFC2434].  IANA [SHALL update/has updated] this
      registry to include DHE or ECDHE
      cipher suites.

   -  "pre_shared_key" is REQUIRED for PSK cipher suites.

   -  "cookie" is REQUIRED for all cipher suites.

   When negotiating use of applicable cipher suites, endpoints MUST
   abort the "key_share", "pre_shared_key", and
      "early_data" extensions as defined in this document.

      IANA [shall update/has updated] this registry to include a "TLS
      1.3" column connection with a "missing_extension" alert if the following four values: "Client", indicating
      that the server shall required
   extension was not send them.  "Clear", indicating that
      they shall be in the ServerHello.  "Encrypted", indicating provided.  Any endpoint that
      they shall be in receives any invalid
   combination of cipher suites and extensions MAY abort the EncryptedExtensions block, "Early",
      indicating that they shall be only in connection
   with a "missing_extension" alert, regardless of negotiated
   parameters.

   Additionally, all implementations MUST support use of the client's 0-RTT
      EncryptedExtensions block,
   "server_name" extension with applications capable of using it.
   Servers MAY require clients to send a valid "server_name" extension.
   Servers requiring this extension SHOULD respond to a ClientHello
   lacking a "server_name" extension with a fatal "missing_extension"
   alert.

   Servers MUST NOT send the "signature_algorithms" extension; if a
   client receives this extension it MUST respond with a fatal
   "unsupported_extension" alert and "No" indicating that they close the connection.

9.  Security Considerations

   Security issues are not
      used discussed throughout this memo, especially in TLS 1.3.
   Appendices B, C, and D.

10.  IANA Considerations

   This column [shall be/has been] initially
      populated with the values document uses several registries that were originally created in this document.
   [RFC4346].  IANA [shall update/ has updated] this registry updated these to add a "Recommended" column.  IANA
      [shall/has] initially populated reference this column document.  The
   registries and their allocation policies are below:

   -  TLS Cipher Suite Registry: Values with the values first byte in the
      table below.  This table has been generated by marking Standards
      Track RFCs range
      0-254 (decimal) are assigned via Specification Required [RFC2434].
      Values with the first byte 255 (decimal) are reserved for Private
      Use [RFC2434].  IANA [SHALL add/has added] a "Recommended" column
      to the cipher suite registry.  All cipher suites listed in
      Appendix A.4 are marked as "Yes" and all others "Yes".  All other cipher suites are
      marked as "No".

   +-------------------------------+-----------+-----------------------+
   | Extension                     | Recommend |               TLS  IANA [SHALL add/has added] add a note to this
      column reading:

         Cipher suites marked as "Yes" are those allocated via Standards
         Track RFCs.  Cipher suites marked as "No" are not; cipher
         suites marked "No" range from "good" to "bad" from a
         cryptographic standpoint.

   -  TLS ContentType Registry: Future values are allocated via
      Standards Action [RFC2434].

   -  TLS Alert Registry: Future values are allocated via Standards
      Action [RFC2434].

   -  TLS HandshakeType Registry: Future values are allocated via
      Standards Action [RFC2434].  IANA [SHALL update/has updated] this
      registry to rename item 4 from "NewSessionTicket" to
      "new_session_ticket".

   This document also uses a registry originally created in [RFC4366].
   IANA has updated it to reference this document.  The registry and its
   allocation policy is listed below:

   -  TLS ExtensionType Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal) are reserved
      for Private Use [RFC2434].  IANA [SHALL update/has updated] this
      registry to include the "key_share", "pre_shared_key", and
      "early_data" extensions as defined in this document.

      IANA [shall update/has updated] this registry to include a "TLS
      1.3" column with the following four values: "Client", indicating
      that the server shall not send them.  "Clear", indicating that
      they shall be in the ServerHello.  "Encrypted", indicating that
      they shall be in the EncryptedExtensions block, and "No"
      indicating that they are not used in TLS 1.3.  This column [shall
      be/has been] initially populated with the values in this document.
      IANA [shall update/has updated] this registry to add a
      "Recommended" column.  IANA [shall/has] initially populated this
      column with the values in the table below.  This table has been
      generated by marking Standards Track RFCs as "Yes" and all others
      as "No".

   +-------------------------------+-----------+-----------------------+
   | Extension                     | Recommend |               TLS 1.3 |
   |                               |        ed |                       |
   +-------------------------------+-----------+-----------------------+
   | server_name [RFC6066]         |       Yes |             Encrypted |
   |                               |           |                       |
   | max_fragment_length [RFC6066] |       Yes |             Encrypted |
   |                               |           |                       |
   | client_certificate_url        |       Yes |             Encrypted |
   | [RFC6066]                     |           |                       |
   |                               |           |                       |
   | trusted_ca_keys [RFC6066]     |       Yes |             Encrypted |
   |                               |           |                       |
   | truncated_hmac [RFC6066]      |       Yes |                    No |
   |                               |           |                       |
   | status_request [RFC6066]      |       Yes |                    No |
   |                               |           |                       |
   | user_mapping [RFC4681]        |       Yes |             Encrypted |
   |                               |           |                       |
   | client_authz [RFC5878]        |        No |             Encrypted |
   |                               |           |                       |
   | server_authz [RFC5878]        |        No |             Encrypted |
   |                               |           |                       |
   | cert_type [RFC6091]           |       Yes |             Encrypted |
   |                               |           |                       |
   | supported_groups [RFC-ietf-   |       Yes |             Encrypted |
   | tls-negotiated-ff-dhe]        |           |                       |
   |                               |           |                       |
   | ec_point_formats [RFC4492]    |       Yes |                    No |
   |                               |           |                       |
   | srp [RFC5054]                 |        No |                    No |
   |                               |           |                       |
   | signature_algorithms          |       Yes |                Client |
   | [RFC5246]                     |           |                       |
   |                               |           |                       |
   | use_srtp [RFC5764]            |       Yes |             Encrypted |
   |                               |           |                       |
   | heartbeat [RFC6520]           |       Yes |             Encrypted |
   |                               |           |                       |
   | application_layer_protocol_ne |       Yes |             Encrypted |
   | gotiation [RFC7301]           |           |                       |
   |                               |           |                       |
   | status_request_v2 [RFC6961]   |       Yes |             Encrypted |
   |                               |           |                       |
   | signed_certificate_timestamp  |        No |             Encrypted |
   | [RFC6962]                     |           |                       |
   |                               |           |                       |
   | client_certificate_type       |       Yes |             Encrypted |
   | [RFC7250]                     |           |                       |
   |                               |           |                       |
   | server_certificate_type       |       Yes |             Encrypted |
   | [RFC7250]                     |           |                       |
   |                               |           |                       |
   | padding [RFC7685]             |       Yes |                Client |
   |                               |           |                       |
   | encrypt_then_mac [RFC7366]    |       Yes |                    No |
   |                               |           |                       |
   | extended_master_secret        |       Yes |                    No |
   | [RFC7627]                     |           |                       |
   |                               |           |                       |
   | SessionTicket TLS [RFC4507]   |       Yes |                    No |
   |                               |           |                       |
   | renegotiation_info [RFC5746]  |       Yes |                    No |
   |                               |           |                       |
   | key_share [[this document]]   |       Yes |                 Clear |
   |                               |           |                       |
   | pre_shared_key [[this         |       Yes |                 Clear |
   | document]]                    |           |                       |
   |                               |           |                       |
   | early_data [[this document]]  |       Yes |                 Clear |
   |                               |           |                       |
   | ticket_age [[this document]]  |       Yes |                 Early             Encrypted |
   |                               |           |                       |
   | cookie [[this document]]      |       Yes | Encrypted/HelloRetryR |
   |                               |           |                equest |
   +-------------------------------+-----------+-----------------------+

   In addition, this document defines two new registries to be
   maintained by IANA

   -  TLS SignatureScheme Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal) are reserved
      for Private Use [RFC2434].  This registry SHALL have a
      "Recommended" column.  The registry [shall be/ has been] initially
      populated with the values described in Section 6.3.2.2. 4.2.2.  The
      following values SHALL be marked as "Recommended":
      ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, rsa_pss_sha256,
      rsa_pss_sha384, rsa_pss_sha512, ed25519.

12.

11.  References

12.1.

11.1.  Normative References

   [AES]      National Institute of Standards and Technology,
              "Specification for the Advanced Encryption Standard
              (AES)", NIST FIPS 197, November 2001.

   [DH]       Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory,
              V.IT-22 n.6 , June 1977.

   [I-D.ietf-tls-chacha20-poly1305]
              Langley, A., Chang, W., Mavrogiannopoulos, N.,
              Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
              Cipher Suites for Transport Layer Security (TLS)", draft-
              ietf-tls-chacha20-poly1305-04 (work in progress), December
              2015.

   [I-D.irtf-cfrg-eddsa]
              Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-05
              (work in progress), March 2016.

   [I-D.mattsson-tls-ecdhe-psk-aead]
              Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and
              AES-CCM Cipher Suites for Transport Layer Security (TLS)",
              draft-mattsson-tls-ecdhe-psk-aead-05 (work in progress),
              April 2016.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              DOI 10.17487/RFC2434, October 1998,
              <http://www.rfc-editor.org/info/rfc2434>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              DOI 10.17487/RFC5288, August 2008,
              <http://www.rfc-editor.org/info/rfc5288>.

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,
              <http://www.rfc-editor.org/info/rfc5289>.

   [RFC5487]  Badra, M., "Pre-Shared Key Cipher Suites for TLS with SHA-
              256/384 and AES Galois Counter Mode", RFC 5487,
              DOI 10.17487/RFC5487, March 2009,
              <http://www.rfc-editor.org/info/rfc5487>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6209]  Kim, W., Lee, J., Park, J., and D. Kwon, "Addition of the
              ARIA Cipher Suites to Transport Layer Security (TLS)",
              RFC 6209, DOI 10.17487/RFC6209, April 2011,
              <http://www.rfc-editor.org/info/rfc6209>.

   [RFC6367]  Kanno, S. and M. Kanda, "Addition of the Camellia Cipher
              Suites to Transport Layer Security (TLS)", RFC 6367,
              DOI 10.17487/RFC6367, September 2011,
              <http://www.rfc-editor.org/info/rfc6367>.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655,
              DOI 10.17487/RFC6655, July 2012,
              <http://www.rfc-editor.org/info/rfc6655>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC6962]  Laurie, B., Langley, A.,

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and E. Kasper, "Certificate
              Transparency", Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6962, 6979, DOI 10.17487/RFC6962, June 10.17487/RFC6979, August
              2013,
              <http://www.rfc-editor.org/info/rfc6962>. <http://www.rfc-editor.org/info/rfc6979>.

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
              <http://www.rfc-editor.org/info/rfc7251>.

   [RFC7443]  Patil, P., Reddy, T., Salgueiro, G., and M. Petit-
              Huguenin, "Application-Layer Protocol Negotiation (ALPN)
              Labels for Session Traversal Utilities for NAT (STUN)
              Usages", RFC 7443, DOI 10.17487/RFC7443, January 2015,
              <http://www.rfc-editor.org/info/rfc7443>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

   [RFC7905]  Langley, A., Chang, W., Mavrogiannopoulos, N.,
              Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
              Cipher Suites for Transport Layer Security (TLS)",
              RFC 7905, DOI 10.17487/RFC7905, June 2016,
              <http://www.rfc-editor.org/info/rfc7905>.

   [SHS]      National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard", NIST FIPS
              PUB 180-4, March 2012.

   [X690]     ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2002, 2002.

   [X962]     ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 1998.

12.2.

11.2.  Informative References

   [AEAD-LIMITS]
              Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", 2016,
              <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.

   [BBFKZG16]
              Bhargavan, K., Brzuska, C., Fournet, C., Kohlweiss, M.,
              Zanella-Beguelin, S., and M. Green, "Downgrade Resilience
              in Key-Exchange Protocols", Proceedings of IEEE Symposium
              on Security and Privacy (Oakland) 2016 , 2016.

   [CHSV16]   Cremers, C., Horvat, M., Scott, S., and T. van der Merwe,
              "Automated Analysis and Verification of TLS 1.3: 0-RTT,
              Resumption and Delayed Authentication", Proceedings of
              IEEE Symposium on Security and Privacy (Oakland) 2016 ,
              2016.

   [CK01]     Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange
              Protocols and Their Use for Building Secure Channels",
              Proceedings of Eurocrypt 2001 , 2001.

   [DOW92]    Diffie, W., van Oorschot, P., and M. Wiener,
              ""Authentication and authenticated key exchanges"",
              Designs, Codes and Cryptography , n.d..

   [DSS]      National Institute of Standards and Technology, U.S.
              Department of Commerce, "Digital Signature Standard,
              version 4", NIST FIPS PUB 186-4, 2013.

   [ECDSA]    American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry: The
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI ANS X9.62-2005, November 2005.

   [FGSW16]   Fischlin, M., Guenther, F., Schmidt, B., and B. Warinschi,
              "Key Confirmation in Key Exchange: A Formal Treatment and
              Implications for TLS 1.3", Proceedings of IEEE Symposium
              on Security and Privacy (Oakland) 2016 , 2016.

   [FI06]     Finney, H., "Bleichenbacher's RSA signature forgery based
              on implementation error", August 2006,
              <https://www.ietf.org/mail-archive/web/openpgp/current/
              msg00999.html>.

   [GCM]      Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC",
              NIST Special Publication 800-38D, November 2007.

   [I-D.ietf-tls-cached-info]
              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-23 (work in progress), May 2016.

   [I-D.ietf-tls-negotiated-ff-dhe]
              Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
              ff-dhe-10 (work in progress), June 2015.

   [IEEE1363]
              IEEE, "Standard Specifications for Public Key
              Cryptography", IEEE 1363 , 2000.

   [LXZFH16]  Li, X., Xu, J., Feng, D., Zhang, Z., and H. Hu, "Multiple
              Handshakes Security of TLS 1.3 Candidates", Proceedings of
              IEEE Symposium on Security and Privacy (Oakland) 2016 ,
              2016.

   [PKCS6]    RSA Laboratories, "PKCS #6: RSA Extended Certificate
              Syntax Standard, version 1.5", November 1993.

   [PKCS7]    RSA Laboratories, "PKCS #7: RSA Cryptographic Message
              Syntax Standard, version 1.5", November 1993.

   [PSK-FINISHED]
              Cremers, C., Horvat, M., van der Merwe, T., and S. Scott,
              "Revision 10: possible attack if client authentication is
              allowed during PSK", 2015, <https://www.ietf.org/mail-
              archive/web/tls/current/msg18215.html>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, DOI 10.17487/RFC1948, May 1996,
              <http://www.rfc-editor.org/info/rfc1948>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <http://www.rfc-editor.org/info/rfc4279>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <http://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <http://www.rfc-editor.org/info/rfc4303>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <http://www.rfc-editor.org/info/rfc4346>.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
              <http://www.rfc-editor.org/info/rfc4366>.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <http://www.rfc-editor.org/info/rfc4506>.

   [RFC4507]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 4507, DOI 10.17487/RFC4507, May
              2006, <http://www.rfc-editor.org/info/rfc4507>.

   [RFC4681]  Santesson, S., Medvinsky, A., and J. Ball, "TLS User
              Mapping Extension", RFC 4681, DOI 10.17487/RFC4681,
              October 2006, <http://www.rfc-editor.org/info/rfc4681>.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, DOI 10.17487/RFC5054, November
              2007, <http://www.rfc-editor.org/info/rfc5054>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              DOI 10.17487/RFC5081, November 2007,
              <http://www.rfc-editor.org/info/rfc5081>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <http://www.rfc-editor.org/info/rfc5763>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC5878]  Brown, M. and R. Housley, "Transport Layer Security (TLS)
              Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
              May 2010, <http://www.rfc-editor.org/info/rfc5878>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <http://www.rfc-editor.org/info/rfc5929>.

   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication",
              RFC 6091, DOI 10.17487/RFC6091, February 2011,
              <http://www.rfc-editor.org/info/rfc6091>.

   [RFC6176]  Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
              (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
              2011, <http://www.rfc-editor.org/info/rfc6176>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,
              <http://www.rfc-editor.org/info/rfc6520>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <http://www.rfc-editor.org/info/rfc7366>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [RFC7568]  Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
              DOI 10.17487/RFC7568, June 2015,
              <http://www.rfc-editor.org/info/rfc7568>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

   [RFC7685]  Langley, A., "A Transport Layer Security (TLS) ClientHello
              Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
              October 2015, <http://www.rfc-editor.org/info/rfc7685>.

   [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the ACM v. 21, n. 2, pp.
              120-126., February 1978.

   [SIGMA]    Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' approach to
              authenticated Di e-Hellman and its use in the IKE
              protocols", Proceedings of CRYPTO 2003 , 2003.

   [SLOTH]    Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              Network and Distributed System Security Symposium (NDSS
              2016) , 2016.

   [SSL2]     Hickman, K., "The SSL Protocol", February 1995.

   [SSL3]     Freier, A., Karlton, P., and P. Kocher, "The SSL 3.0
              Protocol", November 1996.

   [TIMING]   Boneh, D. and D. Brumley, "Remote timing attacks are
              practical", USENIX Security Symposium, 2003.

   [X501]     "Information Technology - Open Systems Interconnection -
              The Directory: Models", ITU-T X.501, 1993.

12.3.

11.3.  URIs

   [1] mailto:tls@ietf.org

Appendix A.  Protocol Data Structures and Constant Values

   This section describes protocol types and constants.  Values listed
   as _RESERVED were used in previous versions of TLS and are listed
   here for completeness.  TLS 1.3 implementations MUST NOT send them
   but might receive them from older TLS implementations.

A.1.  Record Layer

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

   enum {
       invalid_RESERVED(0),
       change_cipher_spec_RESERVED(20),
       alert(21),
       handshake(22),
       application_data(23)
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque fragment[TLSPlaintext.length];
   } TLSPlaintext;

   struct {
      opaque content[TLSPlaintext.length];
      ContentType type;
      uint8 zeros[length_of_padding];
   } TLSInnerPlaintext;

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
       opaque content[TLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
       } fragment; encrypted_record[length];
   } TLSCiphertext;

A.2.  Alert Messages
      enum { warning(1), fatal(2), (255) } AlertLevel;

      enum {
          close_notify(0),
          end_of_early_data(1),
          unexpected_message(10),               /* fatal */
          bad_record_mac(20),                   /* fatal */
          decryption_failed_RESERVED(21),       /* fatal */
          record_overflow(22),                  /* fatal */
          decompression_failure_RESERVED(30),   /* fatal */
          handshake_failure(40),                /* fatal */
          no_certificate_RESERVED(41),          /* fatal */
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),                /* fatal */
          unknown_ca(48),                       /* fatal */
          access_denied(49),                    /* fatal */
          decode_error(50),                     /* fatal */
          decrypt_error(51),                    /* fatal */
          export_restriction_RESERVED(60),      /* fatal */
          protocol_version(70),                 /* fatal */
          insufficient_security(71),            /* fatal */
          internal_error(80),                   /* fatal */
          inappropriate_fallback(86),           /* fatal */
          user_canceled(90),
          no_renegotiation_RESERVED(100),       /* fatal */
          missing_extension(109),               /* fatal */
          unsupported_extension(110),           /* fatal */
          certificate_unobtainable(111),
          unrecognized_name(112),
          bad_certificate_status_response(113), /* fatal */
          bad_certificate_hash_value(114),      /* fatal */
          unknown_psk_identity(115),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

A.3.  Handshake Protocol

      enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          session_ticket(4),
          new_session_ticket(4),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          server_key_exchange_RESERVED(12),
          certificate_request(13),
          server_hello_done_RESERVED(14),
          certificate_verify(15),
          client_key_exchange_RESERVED(16),
          finished(20),
          key_update(24),
          (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case hello_retry_request:   HelloRetryRequest;
              case encrypted_extensions:  EncryptedExtensions;
              case certificate_request:   CertificateRequest;
              case certificate:           Certificate;
              case certificate_verify:    CertificateVerify;
              case finished:              Finished;
              case session_ticket: new_session_ticket:    NewSessionTicket;
              case key_update:            KeyUpdate;
          } body;
      } Handshake;

A.3.1.  Key Exchange Messages

      struct {
          uint8 major;
          uint8 minor;
      } ProtocolVersion;

      struct {
          opaque random_bytes[32];
      } Random;
      uint8 CipherSuite[2];    /* Cryptographic suite selector */

      struct {
          ProtocolVersion client_version = { 3, 4 };    /* TLS v1.3 */
          Random random;
          opaque legacy_session_id<0..32>;
          CipherSuite cipher_suites<2..2^16-2>;
          opaque legacy_compression_methods<1..2^8-1>;
          Extension extensions<0..2^16-1>;
      } ClientHello;

      struct {
          ProtocolVersion server_version;
          Random random;
          CipherSuite cipher_suite;
          Extension extensions<0..2^16-1>;
      } ServerHello;

      struct {
          ProtocolVersion server_version;
          CipherSuite cipher_suite;
          NamedGroup selected_group;
          Extension extensions<0..2^16-1>;
      } HelloRetryRequest;

      struct {
          ExtensionType extension_type;
          opaque extension_data<0..2^16-1>;
      } Extension;

      enum {
          supported_groups(10),
          signature_algorithms(13),
          key_share(40),
          pre_shared_key(41),
          early_data(42),
          ticket_age(43),
          cookie (44),
          cookie(44),
          (65535)
      } ExtensionType;

      struct {
          NamedGroup group;
          opaque key_exchange<1..2^16-1>;
      } KeyShareEntry;

      struct {
          select (role) {
              case client:

                  KeyShareEntry client_shares<0..2^16-1>;

              case server:
                  KeyShareEntry server_share;
          }
      } KeyShare;

      opaque psk_identity<0..2^16-1>;

      struct {
          select (Role) {
              case client:
                  psk_identity identities<2..2^16-1>;

              case server:
                  uint16 selected_identity;
          }
      } PreSharedKeyExtension;

      struct {
          select (Role) {
              case client:
                  opaque context<0..255>;
                  uint32 obfuscated_ticket_age;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

      struct {
          uint32 ticket_age;
      } TicketAge;

A.3.1.1.  Cookie Extension

      struct {
          opaque cookie<0..255>; cookie<0..2^16-1>;
      } Cookie;

A.3.1.2.  Signature Algorithm Extension
      enum {
          /* RSASSA-PKCS-v1_5 RSASSA-PKCS1-v1_5 algorithms */
          rsa_pkcs1_sha1 (0x0201),
          rsa_pkcs1_sha256 (0x0401),
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

          /* ECDSA algorithms */
          ecdsa_secp256r1_sha256 (0x0403),
          ecdsa_secp384r1_sha384 (0x0503),
          ecdsa_secp521r1_sha512 (0x0603),

          /* RSASSA-PSS algorithms */
          rsa_pss_sha256 (0x0700),
          rsa_pss_sha384 (0x0701),
          rsa_pss_sha512 (0x0702),

          /* EdDSA algorithms */
          ed25519 (0x0703),
          ed448 (0x0704),

          /* Reserved Code Points */
          dsa_sha1_RESERVED (0x0202),
          dsa_sha256_RESERVED (0x0402),
          dsa_sha384_RESERVED (0x0502),
          dsa_sha512_RESERVED (0x0602),
          ecdsa_sha1_RESERVED (0x0203),
          obsolete_RESERVED (0x0000..0x0200),
          obsolete_RESERVED (0x0203..0x0400), (0x0204..0x0400),
          obsolete_RESERVED (0x0404..0x0500),
          obsolete_RESERVED (0x0504..0x0600),
          obsolete_RESERVED (0x0604..0x06FF),
          private_use (0xFE00..0xFFFF),
          (0xFFFF)
      } SignatureScheme;

      SignatureScheme supported_signature_algorithms<2..2^16-2>;

A.3.1.3.  Named Group Extension
      enum {
          /* Elliptic Curve Groups (ECDHE) */
          obsolete_RESERVED (1..22),
          secp256r1 (23), secp384r1 (24), secp521r1 (25),
          obsolete_RESERVED (26..28),
          x25519 (29), x448 (30),

          /* Finite Field Groups (DHE) */
          ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
          ffdhe6144 (259), ffdhe8192 (260),

          /* Reserved Code Points */
          ffdhe_private_use (0x01FC..0x01FF),
          ecdhe_private_use (0xFE00..0xFEFF),
          obsolete_RESERVED (0xFF01..0xFF02),
          (0xFFFF)
      } NamedGroup;

      struct {
          NamedGroup named_group_list<1..2^16-1>;
      } NamedGroupList;

   Values within "obsolete_RESERVED" ranges were used in previous
   versions of TLS and MUST NOT be offered or negotiated by TLS 1.3
   implementations.  The obsolete curves have various known/theoretical
   weaknesses or have had very little usage, in some cases only due to
   unintentional server configuration issues.  They are no longer
   considered appropriate for general use and should be assumed to be
   potentially unsafe.  The set of curves specified here is sufficient
   for interoperability with all currently deployed and properly
   configured TLS implementations.

A.3.1.4.  Deprecated Extensions

   The following extensions are no longer applicable to TLS 1.3,
   although TLS 1.3 clients MAY send them if they are willing to
   negotiate them with prior versions of TLS.  TLS 1.3 servers MUST
   ignore these extensions if they are negotiating TLS 1.3:
   truncated_hmac [RFC6066], srp [RFC5054], encrypt_then_mac [RFC7366],
   extended_master_secret [RFC7627], SessionTicket [RFC5077], and
   renegotiation_info [RFC5746].

A.3.2.  Server Parameters Messages
      struct {
          Extension extensions<0..2^16-1>;
      } EncryptedExtensions;

      opaque DistinguishedName<1..2^16-1>;

      struct {
          opaque certificate_extension_oid<1..2^8-1>;
          opaque certificate_extension_values<0..2^16-1>;
      } CertificateExtension;

      struct {
          opaque certificate_request_context<0..2^8-1>;
          SignatureScheme
            supported_signature_algorithms<2..2^16-2>;
          DistinguishedName certificate_authorities<0..2^16-1>;
          CertificateExtension certificate_extensions<0..2^16-1>;
      } CertificateRequest;

A.3.3.  Authentication Messages

      opaque ASN1Cert<1..2^24-1>;

      struct {
          opaque certificate_request_context<0..2^8-1>;
          ASN1Cert certificate_list<0..2^24-1>;
      } Certificate;

      struct {
           digitally-signed struct {
           SignatureScheme algorithm;
           opaque hashed_data[hash_length];
           }; signature<0..2^16-1>;
      } CertificateVerify;

      struct {
          opaque verify_data[verify_data_length]; verify_data[Hash.length];
      } Finished;

A.3.4.  Ticket Establishment
    enum { (65535) } TicketExtensionType;

    struct {
        TicketExtensionType extension_type;
        opaque extension_data<0..2^16-1>; extension_data<1..2^16-1>;
    } TicketExtension;

    enum {
      allow_early_data(1)
      allow_early_data(1),
      allow_dhe_resumption(2),
      allow_psk_resumption(4)
    } TicketFlags;

    struct {
        uint32 ticket_lifetime;
        uint32 flags;
        TicketExtension extensions<2..2^16-2>;
        opaque ticket<0..2^16-1>;
    } NewSessionTicket;

A.4.  Cipher Suites

   A cipher suite defines a cipher specification supported in TLS and
   negotiated via hello messages in the TLS handshake.  Cipher suite
   names follow a general naming convention composed of a series of
   component algorithm names separated by underscores:

      CipherSuite TLS_KEA_AUTH_WITH_CIPHER_HASH = VALUE;

   +-----------+-------------------------------------------------------+
   | Component | Contents                                              |
   +-----------+-------------------------------------------------------+
   | TLS       | The string "TLS"                                      |
   |           |                                                       |
   | KEA       | The key exchange algorithm (e.g. ECDHE, DHE)          |
   |           |                                                       |
   | AUTH      | The authentication algorithm (e.g. certificates, PSK) |
   |           |                                                       |
   | WITH      | The string "WITH"                                     |
   |           |                                                       |
   | CIPHER    | The symmetric cipher used for record protection       |
   |           |                                                       |
   | HASH      | The hash algorithm used with HKDF                     |
   |           |                                                       |
   | VALUE     | The two byte ID assigned for this cipher suite        |
   +-----------+-------------------------------------------------------+
   The "CIPHER" component commonly has sub-components used to designate
   the cipher name, bits, and mode, if applicable.  For example,
   "AES_256_GCM" represents 256-bit AES in the GCM mode of operation.
   Cipher suite names that lack a "HASH" value that are defined for use
   with TLS 1.2 or later use the SHA-256 hash algorithm by default.

   The primary key exchange algorithm used ticket_lifetime;
        uint32 flags;
        uint32 ticket_age_add;
        TicketExtension extensions<2..2^16-2>;
        opaque ticket<0..2^16-1>;
    } NewSessionTicket;

A.4.  Cipher Suites

   A cipher suite defines a cipher specification supported in TLS is Ephemeral Diffie-
   Hellman [DH].  The finite field based version is denoted "DHE" and
   negotiated via hello messages in the elliptic curve based version is denoted "ECDHE".  Prior versions
   of TLS supported non-ephemeral key exchanges, however these are not
   supported by TLS 1.3.

   See the definitions of each cipher handshake.  Cipher suite in its specification
   document for the full details of each combination
   names follow a general naming convention composed of algorithms that
   is specified.

   The following is a list series of standards track server-authenticated (and
   optionally client-authenticated) cipher suites which are currently
   available in TLS 1.3:

   +-------------------------------+----------+------------------------+
   | Cipher Suite Name             | Value    | Specification          |
   +-------------------------------+----------+------------------------+
   | TLS_DHE_RSA_WITH_AES_128_GCM_ | {0x00,0x | [RFC5288]              |
   | SHA256                        | 9E}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_GCM_ | {0x00,0x | [RFC5288]              |
   | SHA384                        | 9F}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_ECDSA_WITH_AES_128_ | {0xC0,0x | [RFC5289]              |
   | GCM_SHA256                    | 2B}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_ECDSA_WITH_AES_256_ | {0xC0,0x | [RFC5289]              |
   | GCM_SHA384                    | 2C}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_RSA_WITH_AES_128_GC | {0xC0,0x | [RFC5289]              |
   | M_SHA256                      | 2F}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_RSA_WITH_AES_256_GC | {0xC0,0x | [RFC5289]              |
   | M_SHA384                      | 30}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_128_CCM  | {0xC0,0x | [RFC6655]              |
   |                               | 9E}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_CCM  | {0xC0,0x | [RFC6655]              |
   |                               | 9F}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_128_CCM_ | {0xC0,0x | [RFC6655]              |
   | 8                             | A2}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_CCM_ | {0xC0,0x | [RFC6655]
   component algorithm names separated by underscores:

      CipherSuite TLS_KEA_AUTH_WITH_CIPHER_HASH = VALUE;

   +-----------+-------------------------------------------------------+
   | Component | 8 Contents                                              | A3}
   +-----------+-------------------------------------------------------+
   | TLS       | The string "TLS"                                      |
   |           |                                                       |
   | TLS_ECDHE_RSA_WITH_CHACHA20_P KEA       | {0xCC,0x The key exchange algorithm (e.g. ECDHE, DHE)          | [I-D.ietf-tls-chacha20
   |           | OLY1305_SHA256                                                       | A8}
   | -poly1305] AUTH      | The authentication algorithm (e.g. certificates, PSK) |
   |           |                                                       |
   | TLS_ECDHE_ECDSA_WITH_CHACHA20 WITH      | {0xCC,0x The string "WITH"                                     | [I-D.ietf-tls-chacha20
   |           | _POLY1305_SHA256                                                       | A9}
   | -poly1305] CIPHER    | The symmetric cipher used for record protection       |
   |           |                                                       |
   | TLS_DHE_RSA_WITH_CHACHA20_POL HASH      | {0xCC,0x The hash algorithm used with HKDF                     | [I-D.ietf-tls-chacha20
   |           | Y1305_SHA256                                                       | AA}
   | -poly1305] VALUE     |
   +-------------------------------+----------+------------------------+

   Note: The values listed two byte ID assigned for ChaCha/Poly are preliminary but this cipher suite        |
   +-----------+-------------------------------------------------------+
   The "CIPHER" component commonly has sub-components used to designate
   the cipher name, bits, and mode, if applicable.  For example,
   "AES_256_GCM" represents 256-bit AES in the GCM mode of operation.
   Cipher suite names that lack a "HASH" value that are being defined for use
   with TLS 1.2 or will be later use the SHA-256 hash algorithm by default.

   The primary key exchange algorithm used for interop testing in TLS is Ephemeral Diffie-
   Hellman [DH].  The finite field based version is denoted "DHE" and therefore
   the elliptic curve based version is denoted "ECDHE".  Prior versions
   of TLS supported non-ephemeral key exchanges, however these are likely to be
   assigned.

   Note: ECDHE AES GCM was not yet standards track prior to
   supported by TLS 1.3.

   See the
   publication definitions of this specification.  This each cipher suite in its specification
   document promotes for the above-
   listed ciphers to standards track. full details of each combination of algorithms that
   is specified.

   The following is a list of standards track ephemeral pre-shared key server-authenticated (and
   optionally client-authenticated) cipher suites which are currently
   available in TLS 1.3:

   +------------------------------+----------+-------------------------+

   +----------------------------------------+-----------+--------------+
   | Cipher Suite Name                      | Value     | Specification Specificatio |
   +------------------------------+----------+-------------------------+
   | TLS_DHE_PSK_WITH_AES_128_GCM                                        | {0x00,0x           | [RFC5487] n            |
   +----------------------------------------+-----------+--------------+
   | _SHA256 TLS_DHE_RSA_WITH_AES_128_GCM_SHA256    | AA} {0x00,0x9 | [RFC5288]    |
   |                                        | E}        |              |
   | TLS_DHE_PSK_WITH_AES_256_GCM                                        | {0x00,0x           | [RFC5487]              |
   | _SHA384 TLS_DHE_RSA_WITH_AES_256_GCM_SHA384    | AB} {0x00,0x9 | [RFC5288]    |
   |                                        | F}        |              |
   |                                        | TLS_DHE_PSK_WITH_AES_128_CCM           | {0xC0,0x              | [RFC6655]
   | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA25 | {0xC0,0x2 | A6} [RFC5289]    |
   | 6                                      | B}        |              |
   |                                        | TLS_DHE_PSK_WITH_AES_256_CCM           | {0xC0,0x              | [RFC6655]
   | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA38 | {0xC0,0x2 | A7} [RFC5289]    |
   | 4                                      | C}        |              |
   |                                        |           |              |
   | TLS_PSK_DHE_WITH_AES_128_CCM TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256  | {0xC0,0x {0xC0,0x2 | [RFC6655] [RFC5289]    |
   | _8                                        | AA} F}        |              |
   |                                        |           |              |
   | TLS_PSK_DHE_WITH_AES_256_CCM TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384  | {0xC0,0x {0xC0,0x3 | [RFC6655] [RFC5289]    |
   | _8                                        | AB} 0}        |              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_AES_128_G TLS_DHE_RSA_WITH_AES_128_CCM           | {0xD0,0x {0xC0,0x9 | [I-D.mattsson-tls-ecdhe [RFC6655]    |
   | CM_SHA256                                        | 01} E}        | -psk-aead]              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_AES_256_G TLS_DHE_RSA_WITH_AES_256_CCM           | {0xD0,0x {0xC0,0x9 | [I-D.mattsson-tls-ecdhe [RFC6655]    |
   | CM_SHA384                                        | 02} F}        | -psk-aead]              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_AES_128_C TLS_DHE_RSA_WITH_AES_128_CCM_8         | {0xD0,0x {0xC0,0xA | [I-D.mattsson-tls-ecdhe [RFC6655]    |
   | CM_8_SHA256                                        | 03} 2}        | -psk-aead]              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_AES_128_C TLS_DHE_RSA_WITH_AES_256_CCM_8         | {0xD0,0x {0xC0,0xA | [I-D.mattsson-tls-ecdhe [RFC6655]    |
   | CM_SHA256                                        | 04} 3}        | -psk-aead]              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_AES_256_C TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_S | {0xD0,0x {0xCC,0xA | [I-D.mattsson-tls-ecdhe [RFC7905]    |
   | CM_SHA384 HA256                                  | 05} 8}        | -psk-aead]              |
   |                                        |           |              |
   | TLS_ECDHE_PSK_WITH_CHACHA20_ TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 | {0xCC,0x {0xCC,0xA | [I-D.ietf-tls-chacha20- [RFC7905]    |
   | POLY1305_SHA256 _SHA256                                | AC} 9}        | poly1305]              |
   |                                        |           |              |
   | TLS_DHE_PSK_WITH_CHACHA20_PO TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA | {0xCC,0x {0xCC,0xA | [I-D.ietf-tls-chacha20- [RFC7905]    |
   | LY1305_SHA256 256                                    | AD} A}        | poly1305]              |
   +------------------------------+----------+-------------------------+
   +----------------------------------------+-----------+--------------+

   Note: The values listed for ECDHE and ChaCha/Poly are preliminary but are being
   or will be used for interop testing and therefore are likely to be
   assigned.

   Note: [RFC6655] is inconsistent with respect to the ordering of
   components within PSK AES CCM cipher suite names.  The names above
   are as defined.

   All cipher suites in this section are specified for use with both TLS
   1.2 and TLS 1.3, as well as the corresponding versions of DTLS.  (see
   Appendix C)

   New cipher suite values are assigned by IANA as described in
   Section 11.

A.4.1.  Unauthenticated Operation

   Previous versions of TLS offered explicitly unauthenticated cipher
   suites based on anonymous Diffie-Hellman.  These cipher suites have
   been deprecated in TLS 1.3.  However, it is still possible to
   negotiate cipher suites that do not provide verifiable server
   authentication by several methods, including:

   -  Raw public keys [RFC7250].

   -  Using a public key contained in a certificate but without
      validation of the certificate chain or any of its contents.

   Either technique used alone is are vulnerable to man-in-the-middle
   attacks and therefore unsafe for general use.  However, it is also
   possible to bind such connections to an external authentication
   mechanism via out-of-band validation of the server's public key,
   trust on first use, or channel bindings [RFC5929].  [[NOTE: TLS 1.3
   needs a new channel binding definition that has ECDHE AES GCM was not yet been
   defined.]] If no such mechanism is used, then standards track prior to the connection has no
   protection against active man-in-the-middle attack; applications MUST
   NOT use TLS in such a way absent explicit configuration or
   publication of this specification.  This document promotes the above-
   listed ciphers to standards track.

   The following is a specific
   application profile.

A.5. list of standards track ephemeral pre-shared key
   cipher suites which are currently available in TLS 1.3:

   +------------------------------+----------+-------------------------+
   | Cipher Suite Name            | Value    | Specification           |
   +------------------------------+----------+-------------------------+
   | TLS_DHE_PSK_WITH_AES_128_GCM | {0x00,0x | [RFC5487]               |
   | _SHA256                      | AA}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_256_GCM | {0x00,0x | [RFC5487]               |
   | _SHA384                      | AB}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_128_CCM | {0xC0,0x | [RFC6655]               |
   |                              | A6}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_256_CCM | {0xC0,0x | [RFC6655]               |
   |                              | A7}      |                         |
   |                              |          |                         |
   | TLS_PSK_DHE_WITH_AES_128_CCM | {0xC0,0x | [RFC6655]               |
   | _8                           | AA}      |                         |
   |                              |          |                         |
   | TLS_PSK_DHE_WITH_AES_256_CCM | {0xC0,0x | [RFC6655]               |
   | _8                           | AB}      |                         |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA256                    | 01}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_256_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA384                    | 02}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_8_SHA256                  | 03}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA256                    | 04}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_256_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA384                    | 05}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_CHACHA20_ | {0xCC,0x | [RFC7905]               |
   | POLY1305_SHA256              | AC}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_CHACHA20_PO | {0xCC,0x | [RFC7905]               |
   | LY1305_SHA256                | AD}      |                         |
   +------------------------------+----------+-------------------------+

   Note: The Security Parameters

   These security parameters values listed for ECDHE and ChaCha/Poly are determined by the TLS Handshake
   Protocol preliminary but
   are being or will be used for interop testing and provided as parameters to the TLS record layer in order therefore are
   likely to initialize a connection state.  SecurityParameters includes:

      enum { server, client } ConnectionEnd;

      enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

      enum { aes_gcm } RecordProtAlgorithm;

      /* The algorithms specified in KDFAlgorithm and
         RecordProtAlgorithm may be added to. */

      struct {
          ConnectionEnd          entity;
          KDFAlgorithm           kdf_algorithm;
          RecordProtAlgorithm    record_prot_algorithm;
          uint8                  enc_key_length;
          uint8                  iv_length;
          opaque                 hs_master_secret[48];
          opaque                 master_secret[48];
          opaque                 client_random[32];
          opaque                 server_random[32];
      } SecurityParameters;

A.6.  Changes assigned.

   Note: [RFC6655] is inconsistent with respect to RFC 4492

   RFC 4492 [RFC4492] adds Elliptic Curve the ordering of
   components within PSK AES CCM cipher suite names.  The names above
   are as defined.

   All cipher suites to TLS.  This
   document changes some of the structures used in that document.  This this section details the required changes are specified for implementors of use with both RFC
   4492 and TLS 1.2.  Implementors of TLS
   1.2 who are not implementing
   RFC 4492 do not need to read this section.

   This document adds an "algorithm" field to the digitally-signed
   element in order to identify the signature and digest algorithms used
   to create a signature.  This change applies to digital signatures
   formed using ECDSA TLS 1.3, as well, thus allowing ECDSA signatures to be used
   with digest algorithms other than SHA-1, provided such use is
   compatible with the certificate and any restrictions imposed by
   future revisions well as the corresponding versions of [RFC5280].

   As DTLS.  (see
   Appendix C)

   New cipher suite values are assigned by IANA as described in
   Section 6.3.4.1.1, the restrictions 10.

A.4.1.  Unauthenticated Operation

   Previous versions of TLS offered explicitly unauthenticated cipher
   suites based on the signature
   algorithms used to sign certificates are no longer tied anonymous Diffie-Hellman.  These cipher suites have
   been deprecated in TLS 1.3.  However, it is still possible to the
   negotiate cipher
   suite.  Thus, the restrictions on suites that do not provide verifiable server
   authentication by several methods, including:

   -  Raw public keys [RFC7250].

   -  Using a public key contained in a certificate but without
      validation of the algorithm certificate chain or any of its contents.

   Either technique used alone is are vulnerable to sign
   certificates specified in Sections 2 man-in-the-middle
   attacks and 3 of RFC 4492 are therefore unsafe for general use.  However, it is also
   relaxed.  As in this document,
   possible to bind such connections to an external authentication
   mechanism via out-of-band validation of the restrictions server's public key,
   trust on first use, or channel bindings [RFC5929].  [[NOTE: TLS 1.3
   needs a new channel binding definition that has not yet been
   defined.]] If no such mechanism is used, then the keys connection has no
   protection against active man-in-the-middle attack; applications MUST
   NOT use TLS in the
   end-entity certificate remain. such a way absent explicit configuration or a specific
   application profile.

Appendix B.  Implementation Notes

   The TLS protocol cannot prevent many common security mistakes.  This
   section provides several recommendations to assist implementors.

B.1.  Random Number Generation and Seeding

   TLS requires a cryptographically secure pseudorandom number generator
   (PRNG).  Care must be taken in designing and seeding PRNGs.  PRNGs
   based on secure hash operations, most notably SHA-256, are
   acceptable, but cannot provide more security than the size of the
   random number generator state.

   To estimate the amount of seed material being produced, add the
   number of bits of unpredictable information in each seed byte.  For
   example, keystroke timing values taken from a PC compatible 18.2 Hz
   timer provide 1 or 2 secure bits each, even though the total size of
   the counter value is 16 bits or more.  Seeding a 128-bit PRNG would
   thus require approximately 100 such timer values.

   [RFC4086] provides guidance on the generation of random values.

B.2.  Certificates and Authentication

   Implementations are responsible for verifying the integrity of
   certificates and should generally support certificate revocation
   messages.  Certificates should always be verified to ensure proper
   signing by a trusted Certificate Authority (CA).  The selection and
   addition of trusted CAs should be done very carefully.  Users should
   be able to view information about the certificate and root CA.

B.3.  Cipher Suite Support

   TLS supports a range of key sizes and security levels, including some
   that provide no or minimal security.  A proper implementation will
   probably not support many cipher suites.  Applications SHOULD also
   enforce minimum and maximum key sizes.  For example, certification
   paths containing keys or signatures weaker than 2048-bit RSA or
   224-bit ECDSA are not appropriate for secure applications.  See also
   Appendix C.4.

B.4.  Implementation Pitfalls

   Implementation experience has shown that certain parts of earlier TLS
   specifications are not easy to understand, and have been a source of
   interoperability and security problems.  Many of these areas have
   been clarified in this document, but this appendix contains a short
   list of the most important things that require special attention from
   implementors.

   TLS protocol issues:

   -  Do you correctly handle handshake messages that are fragmented to
      multiple TLS records (see Section 5.2.1)? 5.1)?  Including corner cases
      like a ClientHello that is split to several small fragments?  Do
      you fragment handshake messages that exceed the maximum fragment
      size?  In particular, the certificate and certificate request
      handshake messages can be large enough to require fragmentation.

   -  Do you ignore the TLS record layer version number in all TLS
      records? (see Appendix C)

   -  Have you ensured that all support for SSL, RC4, EXPORT ciphers,
      and MD5 (via the "signature_algorithm" extension) is completely
      removed from all possible configurations that support TLS 1.3 or
      later, and that attempts to use these obsolete capabilities fail
      correctly? (see Appendix C)

   -  Do you handle TLS extensions in ClientHello correctly, including
      unknown extensions or omitting the extensions field completely?

   -  When the server has requested a client certificate, but no
      suitable certificate is available, do you correctly send an empty
      Certificate message, instead of omitting the whole message (see
      Section 6.3.4.1.2)? 4.3.1.2)?

   -  When processing the plaintext fragment produced by AEAD-Decrypt
      and scanning from the end for the ContentType, do you avoid
      scanning past the start of the cleartext in the event that the
      peer has sent a malformed plaintext of all-zeros?

   -  When processing a ClientHello containing a version of { 3, 5 } or
      higher, do you respond with the highest common version of TLS
      rather than requiring an exact match?  Have you ensured this
      continues to be true with arbitrarily higher version numbers?
      (e.g. { 4, 0 }, { 9, 9 }, { 255, 255 })

   -  Do you properly ignore unrecognized cipher suites (see Section 6.3.1.1), (Section 4.1.1),
      hello extensions (Section 4.2), named groups (see Section 6.3.2.3), (Section 4.2.3), and
      signature algorithms (see
      Section 6.3.2.2)? (Section 4.2.2)?

   Cryptographic details:

   -  What countermeasures do you use to prevent timing attacks against
      RSA signing operations [TIMING]?

   -  When verifying RSA signatures, do you accept both NULL and missing
      parameters (see Section 4.8)?
      parameters?  Do you verify that the RSA padding doesn't have
      additional data after the hash value?  [FI06]

   -  When using Diffie-Hellman key exchange, do you correctly preserve
      leading zero bytes in the negotiated key (see Section 7.3.1)?

   -  Does your TLS client check that the Diffie-Hellman parameters sent
      by the server are acceptable acceptable, (see Appendix D.1.1.1)? Section 4.2.4.1)?

   -  Do you use a strong and, most importantly, properly seeded random
      number generator (see Appendix B.1) when generating Diffie-Hellman
      private values, the ECDSA "k" parameter, and other security-critical security-
      critical values?  It is RECOMMENDED that implementations implement
      "deterministic ECDSA" as specified in [RFC6979].

   -  Do you zero-pad Diffie-Hellman public key values to the group size
      (see Section 6.3.2.4.1)? 4.2.4.1)?

B.5.  Client Tracking Prevention

   Clients SHOULD NOT reuse a session ticket for multiple connections.
   Reuse of a session ticket allows passive observers to correlate
   different connections.  Servers that issue session tickets SHOULD
   offer at least as many session tickets as the number of connections
   that a client might use; for example, a web browser using HTTP/1.1
   [RFC7230] might open six connections to a server.  Servers SHOULD
   issue new session tickets with every connection.  This ensures that
   clients are always able to use a new session ticket when creating a
   new connection.

Appendix C.  Backward Compatibility

   The TLS protocol provides a built-in mechanism for version
   negotiation between endpoints potentially supporting different
   versions of TLS.

   TLS 1.x and SSL 3.0 use compatible ClientHello messages.  Servers can
   also handle clients trying to use future versions of TLS as long as
   the ClientHello format remains compatible and the client supports the
   highest protocol version available in the server.

   Prior versions of TLS used the record layer version number for
   various purposes.  (TLSPlaintext.record_version &
   TLSCiphertext.record_version) As of TLS 1.3, this field is deprecated
   and its value MUST be ignored by all implementations.  Version
   negotiation is performed using only the handshake versions.
   (ClientHello.client_version & ServerHello.server_version) In order to
   maximize interoperability with older endpoints, implementations that
   negotiate the use of TLS 1.0-1.2 SHOULD set the record layer version
   number to the negotiated version for the ServerHello and all records
   thereafter.

   For maximum compatibility with previously non-standard behavior and
   misconfigured deployments, all implementations SHOULD support
   validation of certification paths based on the expectations in this
   document, even when handling prior TLS versions' handshakes. (see
   Section 6.3.4.1.1) 4.3.1.1)

   TLS 1.2 and prior supported an "Extended Master Secret" [RFC7627]
   extension which digested large parts of the handshake transcript into
   the master secret.  Because TLS 1.3 always hashes in the transcript
   up to the server CertificateVerify, implementations which support
   both TLS 1.3 and earlier versions SHOULD indicate the use of the
   Extended Master Secret extension in their APIs whenever TLS 1.3 is
   used.

C.1.  Negotiating with an older server

   A TLS 1.3 client who wishes to negotiate with such older servers will
   send a normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) in
   ClientHello.client_version.  If the server does not support this
   version it will respond with a ServerHello containing an older
   version number.  If the client agrees to use this version, the
   negotiation will proceed as appropriate for the negotiated protocol.
   A client resuming a session SHOULD initiate the connection using the
   version that was previously negotiated.

   Note that 0-RTT data is not compatible with older servers.  See
   Appendix C.3.

   If the version chosen by the server is not supported by the client
   (or not acceptable), the client MUST send a "protocol_version" alert
   message and close the connection.

   If a TLS server receives a ClientHello containing a version number
   greater than the highest version supported by the server, it MUST
   reply according to the highest version supported by the server.

   Some legacy server implementations are known to not implement the TLS
   specification properly and might abort connections upon encountering
   TLS extensions or versions which it is not aware of.
   Interoperability with buggy servers is a complex topic beyond the
   scope of this document.  Multiple connection attempts may be required
   in order to negotiate a backwards compatible connection, however this
   practice is vulnerable to downgrade attacks and is NOT RECOMMENDED.

C.2.  Negotiating with an older client

   A TLS server can also receive a ClientHello containing a version
   number smaller than the highest supported version.  If the server
   wishes to negotiate with old clients, it will proceed as appropriate
   for the highest version supported by the server that is not greater
   than ClientHello.client_version.  For example, if the server supports
   TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will
   proceed with a TLS 1.0 ServerHello.  If the server only supports
   versions greater than client_version, it MUST send a
   "protocol_version" alert message and close the connection.

   Note that earlier versions of TLS did not clearly specify the record
   layer version number value in all cases
   (TLSPlaintext.record_version).  Servers will receive various TLS 1.x
   versions in this field, however its value MUST always be ignored.

C.3.  Zero-RTT backwards compatibility

   0-RTT data is not compatible with older servers.  An older server
   will respond to the ClientHello with an older ServerHello, but it
   will not correctly skip the 0-RTT data and fail to complete the
   handshake.  This can cause issues when a client offers attempts to use
   0-RTT, particularly against multi-server deployments.  For example, a
   deployment may could deploy TLS 1.3 gradually with some servers
   implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3
   deployment may could be downgraded to TLS 1.2.

   If a

   A client accepts older versions of TLS and that attempts to send 0-RTT data MUST fail a connection if
   it receives an older
   ServerHello after sending a ClientHello ServerHello with 0-RTT data, it MAY retry
   the connection without 0-RTT.  It is NOT RECOMMENDED to retry the
   connection in response TLS 1.2 or older.  A client that
   attempts to repair this error SHOULD NOT send a more generic TLS 1.2 ClientHello,
   but instead send a TLS 1.3 ClientHello without 0-RTT data.

   To avoid this error or advertise lower
   versions of TLS.

   Multi-server condition, multi-server deployments are RECOMMENDED to SHOULD ensure
   a uniform and stable deployment of TLS 1.3 without 0-RTT prior to
   enabling 0-RTT.

C.4.  Backwards Compatibility Security Restrictions

   If an implementation negotiates use of TLS 1.2, then negotiation of
   cipher suites also supported by TLS 1.3 SHOULD be preferred, if
   available.

   The security of RC4 cipher suites is considered insufficient for the
   reasons cited in [RFC7465].  Implementations MUST NOT offer or
   negotiate RC4 cipher suites for any version of TLS for any reason.

   Old versions of TLS permitted the use of very low strength ciphers.
   Ciphers with a strength less than 112 bits MUST NOT be offered or
   negotiated for any version of TLS for any reason.

   The security of SSL 2.0 [SSL2] is considered insufficient for the
   reasons enumerated in [RFC6176], and MUST NOT be negotiated for any
   reason.

   Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-
   HELLO.  Implementations MUST NOT negotiate TLS 1.3 or later using an
   SSL version 2.0 compatible CLIENT-HELLO.  Implementations are NOT
   RECOMMENDED to accept an SSL version 2.0 compatible CLIENT-HELLO in
   order to negotiate older versions of TLS.

   Implementations MUST NOT send or accept any records with a version
   less than { 3, 0 }.

   The security of SSL 3.0 [SSL3] is considered insufficient for the
   reasons enumerated in [RFC7568], and MUST NOT be negotiated for any
   reason.

   Implementations MUST NOT send a ClientHello.client_version or
   ServerHello.server_version set to { 3, 0 } or less.  Any endpoint
   receiving a Hello message with ClientHello.client_version or
   ServerHello.server_version set to { 3, 0 } MUST respond with a
   "protocol_version" alert message and close the connection.

   Implementations MUST NOT use the Truncated HMAC extension, defined in
   Section 7 of [RFC6066], as it is not applicable to AEAD ciphers and
   has been shown to be insecure in some scenarios.

Appendix D.  Overview of Security Analysis Properties

   [[TODO: The entire security analysis This section is still a WIP and needs a rewrite.]] bunch more work.]]

   A complete security analysis of TLS is outside the scope of this
   document.  In this section, we provide an informal description the
   desired properties as well as references to more detailed work in the
   research literature which provides more formal definitions.

   We cover properties of the handshake separately from those of the
   record layer.

D.1.  Handshake

   The TLS handshake is an Authenticated Key Exchange (AKE) protocol
   which is designed intended to establish a secure connection between
   a client provide both one-way authenticated (server-only)
   and mutually authenticated (client and server) functionality.  At the
   completion of the handshake, each side outputs its view on the
   following values:

   -  A "session key" (the master secret) from which can be derived a server
      set of working keys.

   -  A set of cryptographic parameters (algorithms, etc.)

   -  The identities of the communicating over an insecure channel.  This
   document makes several traditional assumptions, including parties.

   We assume that
   attackers have substantial computational resources and cannot obtain
   secret information from sources outside the protocol.  Attackers attacker has complete control of the network in
   between the parties [RFC3552].  Even under these conditions, the
   handshake should provide the properties listed below.  Note that
   these properties are
   assumed not necessarily independent, but reflect the
   protocol consumers' needs.

   Establishing the same session key.  The handshake needs to have output the ability
      same session key on both sides of the handshake, provided that it
      completes successfully on each endpoint (See [CK01]; defn 1, part
      1).

   Secrecy of the session key.  The shared session key should be known
      only to capture, modify, delete, replay, and
   otherwise tamper with messages sent over the communication channel.
   This appendix outlines how TLS has been designed communicating parties, not to resist the attacker (See
      [CK01]; defn 1, part 2).  Note that in a variety
   of attacks.

D.1.  Handshake Protocol unilaterally
      authenticated connection, the attacker can establish its own
      session keys with the server, but those session keys are distinct
      from those established by the client.

   Peer Authentication.  The TLS Handshake Protocol client's view of the peer identity should
      reflect the server's identity.  If the client is responsible for selecting a cipher spec
   and generating a master secret, which together comprise authenticated,
      the primary server's view of the peer identity should match the client's
      identity.

   Uniqueness of the session key:  Any two distinct handshakes should
      produce distinct, unrelated session keys

   Downgrade protection.  The cryptographic parameters associated with a secure session.  The TLS
   Handshake Protocol can also optionally authenticate parties who have
   certificates signed by a trusted certificate authority.

D.1.1.  Authentication should be the
      same on both sides and Key Exchange

   TLS supports three authentication modes: authentication should be the same as if the peers had been
      communicating in the absence of both
   parties, server an attack (See [BBFKZG16]; defns 8
      and 9}).

   Forward secret  If the long-term keying material (in this case the
      signature keys in certificate-based authentication with an unauthenticated client, and
   total anonymity.  Whenever modes or the server is authenticated,
      PSK in PSK-(EC)DHE modes) are compromised after the channel handshake is secure
      complete, this does not compromise the security of the session key
      (See [DOW92]).

   Protection of endpoint identities.  The server's identity
      (certificate) should be protected against man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If passive attackers.  The
      client's identity should be protected against both passive and
      active attackers.

   Informally, the server is authenticated,
   its certificate message must signature-based modes of TLS 1.3 provide for the
   establishment of a valid certificate chain
   leading to unique, secret, shared, key established by an acceptable certificate authority.  Similarly,
   (EC)DHE key exchange and authenticated clients must supply an acceptable certificate by the server's signature over
   the handshake transcript, as well as tied to the
   server.  Each party is responsible for verifying that server's identity by
   a MAC.  If the other's
   certificate client is valid authenticated by a certificate, it also
   signs over the handshake transcript and has not expired or been revoked.

   [[TODO: Rewrite this because provides a MAC tied to both
   identities.  [SIGMA] describes the master_secret is not used analysis of this way
   any more after Hugo's changes.]] The general goal type of the key
   exchange
   process is to create protocol.  If fresh (EC)DHE keys are used for each
   connection, then the output keys are forward secret.

   The PSK and resumption-PSK modes bootstrap from a master_secret known to long-term shared
   secret into a unique per-connection short-term session key.  This
   secret may have been established in a previous handshake.  If
   PSK-(EC)DHE modes are used, this session key will also be forward
   secret.  The resumption-PSK mode has been designed so that the communicating
   parties
   resumption master secret computed by connection N and not needed to attackers (see Section 7.1).  The master_secret form
   connection N+1 is
   required to generate separate from the Finished messages and record protection traffic keys
   (see Section 6.3.4.3 and Section 7.3).  By sending a correct Finished
   message, parties used by connection
   N, thus prove that they know providing forward secrecy between the correct master_secret.

D.1.1.1.  Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is used, connections.

   For all handshake modes, the client and server use Finished MAC (and where present, the "key_share" extension to send temporary Diffie-Hellman
   parameters.  The signature
   signature), prevents downgrade attacks.  In addition, the use of
   certain bytes in the certificate verify message (if
   present) covers random nonces as described in Section 4.1.2
   allows the entire handshake up detection of downgrade to that point previous TLS versions.

   As soon as the client and thus
   attests the certificate holder's desire server have exchanged enough
   information to use the ephemeral DHE
   keys.

   Peers SHOULD validate each other's public key Y by ensuring that 1 <
   Y < p-1.  This simple check ensures that establish shared keys, the remote peer is properly
   behaved and isn't forcing remainder of the local system into a small subgroup.

   Additionally, using a fresh key for each handshake provides Perfect
   Forward Secrecy.  Implementations SHOULD generate a new X for each handshake when using DHE cipher suites.

D.1.2.  Version Rollback Attacks
   is encrypted, thus providing protection against passive attackers.
   Because TLS includes substantial improvements over SSL Version 2.0,
   attackers may try to make TLS-capable clients and servers fall back
   to Version 2.0.  This attack can occur if (and only if) two TLS-
   capable parties use an SSL 2.0 handshake.  (See also Appendix C.4.)

   Although the solution using non-random PKCS #1 block type 2 message
   padding is inelegant, server authenticates before the client, the client can
   ensure that it provides a reasonably secure way for Version
   3.0 servers only reveals its identity to detect an authenticated server.
   Note that implementations must use the attack.  This solution is not secure
   against attackers who can brute-force provided record padding
   mechanism during the key and substitute a new
   ENCRYPTED-KEY-DATA message containing handshake to avoid leaking information about the
   identities due to length.

   The 0-RTT mode of operation generally provides the same key (but security
   properties as 1-RTT data, with normal
   padding) before the application-specified wait threshold has expired.
   Altering two exceptions that the padding of 0-RTT
   encryption keys do not provide full forward secrecy and that the least-significant 8 bytes of the PKCS
   padding does
   server is not impact security able to guarantee full uniqueness of the handshake
   (non-replayability) without keeping potentially undue amounts of
   state.  See Section 4.2.6 for one mechanism to limit the size exposure to
   replay.

   The reader should refer to the following references for analysis of
   the signed hashes
   and RSA TLS handshake [CHSV16] [FGSW16] [LXZFH16].

D.2.  Record Layer

   The record layer depends on the handshake producing a strong session
   key lengths which can be used in the protocol, since this is essentially
   equivalent to increasing derive bidirectional traffic keys and
   nonces.  Assuming that is true, and the input block size by 8 bytes.

D.1.3.  Detecting Attacks Against keys are used for no more
   data than indicated in Section 5.5 then the Handshake Protocol record layer should
   provide the following guarantees:

   Confidentiality.  An attacker might try should not be able to influence determine the handshake exchange
      plaintext contents of a given record.

   Integrity.  An attacker should not be able to make the
   parties select craft a new record
      which is different encryption algorithms than they would
   normally choose.

   For this attack, from an existing record which will be accepted
      by the receiver.

   Order protection/non-replayability  An attacker must actively change one should not be able to
      cause the receiver to accept a record which it has already
      accepted or more
   handshake messages. cause the receiver to accept record N+1 without having
      first processed record N.  [[TODO: If we merge in DTLS to this occurs, the client and server will
   compute different values for the handshake message hashes.  As
      document, we will need to update this guarantee.]]

   Length concealment.  Given a record with a
   result, given external length, the parties will
      attacker should not accept each others' Finished messages.
   Without be able to determine the static secret, amount of the attacker cannot repair record
      that is content versus padding.

      Forward security after key change.  If the Finished
   messages, so traffic key update
      mechanism described in Section 4.4.3 has been used and the attack will
      previous generation key is deleted, an attacker who compromises
      the endpoint should not be discovered.

D.2.  Protecting Application Data

   The shared secrets are hashed able to decrypt traffic encrypted with
      the handshake transcript to
   produce unique record protection secrets for each connection.

   Outgoing data is protected using an AEAD algorithm before
   transmission.  The authentication data includes old key.

   Informally, TLS 1.3 provides these properties by AEAD-protecting the sequence number,
   message type, message length,
   plaintext with a strong key.  AEAD encryption [RFC5116] provides
   confidentiality and integrity for the message contents.  The message
   type field data.  Non-replayability is necessary to ensure that messages intended
   provided by using a separate nonce for one TLS each record, with the nonce
   being derived from the record layer client are not redirected to another.  The sequence number ensures that attempts to delete or reorder messages will be
   detected.  Since sequence numbers are 64 bits long, they should never
   overflow.  Messages from one party cannot be inserted into (Section 5.3), with the
   other's output, since they use independent keys.

D.3.  Denial of Service

   TLS is susceptible to a number of denial-of-service (DoS) attacks.
   In particular, an attacker who initiates a large
   sequence number being maintained independently at both sides thus
   records which are delivered out of TCP
   connections can cause a server to consume large amounts of CPU doing
   asymmetric crypto operations.  However, because TLS is generally used
   over TCP, it order result in AEAD deprotection
   failures.

   The plaintext protected by the AEAD function consists of content plus
   variable-length padding.  Because the padding is difficult for also encrypted, the
   attacker to hide their point cannot directly determine the length of
   origin if proper TCP SYN randomization is used [RFC1948] the padding, but may
   be able to measure it indirectly by the TCP
   stack.

   Because TLS runs over TCP, use of timing channels
   exposed during record processing (i.e., seeing how long it is also susceptible takes to
   process a number of DoS
   attacks on individual connections. record).  In particular, attackers can
   forge RSTs, thereby terminating connections, or forge partial TLS
   records, thereby causing the connection general, it is not known how to stall.  These attacks
   cannot in general be defended against by a TCP-using protocol.
   Implementors or users who are concerned with remove this class
   type of attack
   should use IPsec AH [RFC4302] or ESP [RFC4303].

D.4.  Final Notes

   For TLS to be able to provide channel because even a secure connection, both constant time padding removal function
   will then feed the client
   and server systems, keys, and applications must be secure.  In
   addition, content into data-dependent functions.

   Generation N+1 keys are derived from generation N keys via a key
   derivation function Section 7.2.  As long as this function is truly
   one way, it is not possible to compute the implementation must be free of previous keys after a key
   change (forward secrecy).  However, TLS does not provide security errors.

   The system for
   data which is only as strong as sent after the weakest traffic secret is compromised, even afer
   a key exchange and
   authentication algorithm supported, and only trustworthy
   cryptographic functions should be used.  Short public keys and
   anonymous servers should be used with great caution.  Implementations
   and users must be careful when deciding update (backward secrecy); systems which certificates want backward secrecy
   must do a fresh handshake and
   certificate authorities are acceptable; establish a dishonest certificate
   authority can do tremendous damage. new session key with an
   (EC)DHE exchange.

   The reader should refer to the following references for analysis of
   the TLS record layer.

Appendix E.  Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address tls@ietf.org [1].  Information on the group and
   information on how to subscribe to the list is at
   https://www1.ietf.org/mailman/listinfo/tls

   Archives of the list can be found at: https://www.ietf.org/mail-
   archive/web/tls/current/index.html

Appendix F.  Contributors

   -  Martin Abadi
      University of California, Santa Cruz
      abadi@cs.ucsc.edu

   -  Christopher Allen (co-editor of TLS 1.0)
      Alacrity Ventures
      ChristopherA@AlacrityManagement.com

   -  Steven M.  Bellovin
      Columbia University
      smb@cs.columbia.edu

   -  David Benjamin
      Google
      davidben@google.com

   -  Benjamin Beurdouche

   -  Karthikeyan Bhargavan (co-author of [RFC7627])
      INRIA
      karthikeyan.bhargavan@inria.fr

   -  Simon Blake-Wilson (co-author of [RFC4492])
      BCI
      sblakewilson@bcisse.com

   -  Nelson Bolyard (co-author of [RFC4492])
      Sun Microsystems, Inc.
      nelson@bolyard.com

   -  Ran Canetti
      IBM
      canetti@watson.ibm.com

   -  Pete Chown
      Skygate Technology Ltd
      pc@skygate.co.uk

   -  Antoine Delignat-Lavaud (co-author of [RFC7627])
      INRIA
      antoine.delignat-lavaud@inria.fr

   -  Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2)
      Independent
      tim@dierks.org

   -  Taher Elgamal
      Securify
      taher@securify.com

   -  Pasi Eronen
      Nokia
      pasi.eronen@nokia.com

   -  Cedric Fournet
      Microsoft
      fournet@microsoft.com

   -  Anil Gangolli
      anil@busybuddha.org

   -  David M.  Garrett

   -  Vipul Gupta (co-author of [RFC4492])
      Sun Microsystems Laboratories
      vipul.gupta@sun.com

   -  Chris Hawk (co-author of [RFC4492])
      Corriente Networks LLC
      chris@corriente.net

   -  Kipp Hickman

   -  Alfred Hoenes

   -  David Hopwood
      Independent Consultant
      david.hopwood@blueyonder.co.uk

   -  Subodh Iyengar
      Facebook
      subodh@fb.com

   -  Daniel Kahn Gillmor
      ACLU
      dkg@fifthhorseman.net

   -  Phil Karlton (co-author of SSL 3.0)

   -  Paul Kocher (co-author of SSL 3.0)
      Cryptography Research
      paul@cryptography.com

   -  Hugo Krawczyk
      IBM
      hugo@ee.technion.ac.il

   -  Adam Langley (co-author of [RFC7627])
      Google
      agl@google.com

   -  Ilari Liusvaara
      Independent
      ilariliusvaara@welho.com

   -  Jan Mikkelsen
      Transactionware
      janm@transactionware.com

   -  Bodo Moeller (co-author of [RFC4492])
      Google
      bodo@openssl.org

   -  Erik Nygren
      Akamai Technologies
      erik+ietf@nygren.org

   -  Magnus Nystrom
      RSA Security
      magnus@rsasecurity.com

   -  Alfredo Pironti (co-author of [RFC7627])
      INRIA
      alfredo.pironti@inria.fr

   -  Andrei Popov
      Microsoft
      andrei.popov@microsoft.com

   -  Marsh Ray (co-author of [RFC7627])
      Microsoft
      maray@microsoft.com

   -  Robert Relyea
      Netscape Communications
      relyea@netscape.com

   -  Kyle Rose
      Akamai Technologies
      krose@krose.org

   -  Jim Roskind
      Netscape Communications
      jar@netscape.com

   -  Michael Sabin

   -  Dan Simon
      Microsoft, Inc.
      dansimon@microsoft.com

   -  Nick Sullivan
      CloudFlare Inc.
      nick@cloudflare.com

   -  Bjoern Tackmann
      University of California, San Diego
      btackmann@eng.ucsd.edu

   -  Martin Thomson
      Mozilla
      mt@mozilla.com

   -  Filippo Valsorda
      CloudFlare Inc.
      filippo@cloudflare.com

   -  Tom Weinstein

   -  Hoeteck Wee
      Ecole Normale Superieure, Paris
      hoeteck@alum.mit.edu

   -  Tim Wright
      Vodafone
      timothy.wright@vodafone.com

Author's Address

   Eric Rescorla
   RTFM, Inc.

   EMail: ekr@rtfm.com