Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if                            July 11,                          August 17, 2016
           approved)
Updates: 4492, 6066, 6961 (if approved)
Intended status: Standards Track
Expires: January 12, February 18, 2017

        The Transport Layer Security (TLS) Protocol Version 1.3
                        draft-ietf-tls-tls13-14
                        draft-ietf-tls-tls13-15

Abstract

   This document specifies version 1.3 of the Transport Layer Security
   (TLS) protocol.  TLS 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
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   This Internet-Draft will expire on January 12, February 18, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   5
     1.2.  Major Differences from TLS 1.2  . . . . . . . . . . . . .   6
     1.3.  Updates Affecting TLS 1.2 . . . . . . . . . . . . . . . .  11
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  10  11
     2.1.  Incorrect DHE Share . . . . . . . . . . . . . . . . . . .  14
     2.2.  Resumption and Pre-Shared Key (PSK) . . . . . . . . . . .  15
     2.3.  Zero-RTT Data . . . . . . . . . . . . . . . . . . . . . .  17
   3.  Presentation Language . . . . . . . . . . . . . . . . . . . .  18
     3.1.  Basic Block Size  . . . . . . . . . . . . . . . . . . . .  18
     3.2.  Miscellaneous . . . . . . . . . . . . . . . . . . . . . .  19
     3.3.  Vectors . . . . . . . . . . . . . . . . . . . . . . . . .  19
     3.4.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  20
     3.5.  Enumerateds . . . . . . . . . . . . . . . . . . . . . . .  20
     3.6.  Constructed Types . . . . . . . . . . . . . . . . . . . .  21
       3.6.1.  Variants  . . . . . . . . . . . . . . . . . . . . . .  21
     3.7.  Constants . . . . . . . . . . . . . . . . . . . . . . . .  23
   4.  Handshake Protocol  . . . . . . . . . . . . . . . . . . . . .  23
     4.1.  Key Exchange Messages . . . . . . . . . . . . . . . . . .  24
       4.1.1.  Cryptographic Negotiation . . . . . . . . . . . . . .  25
       4.1.2.  Client Hello  . . . . . . . . . . . . . . . . . . . .  25
       4.1.2.  26
       4.1.3.  Server Hello  . . . . . . . . . . . . . . . . . . . .  27
       4.1.3.  28
       4.1.4.  Hello Retry Request . . . . . . . . . . . . . . . . .  29
     4.2.  Hello Extensions  . . . . . . . . . . . . . . . . . . . .  30  31
       4.2.1.  Cookie  . . . . . . . . . . . . . . . . . . . . . . .  31  32
       4.2.2.  Signature Algorithms  . . . . . . . . . . . . . . . .  32  33
       4.2.3.  Negotiated Groups . . . . . . . . . . . . . . . . . .  35  36
       4.2.4.  Key Share . . . . . . . . . . . . . . . . . . . . . .  36  37
       4.2.5.  Pre-Shared Key Extension  . . . . . . . . . . . . . .  39
       4.2.6.  Early Data Indication . . . . . . . . . . . . . . . .  40  41
       4.2.7.  OCSP Status Extensions  . . . . . . . . . . . . . . .  43  44
       4.2.8.  Encrypted Extensions  . . . . . . . . . . . . . . . .  44  45
       4.2.9.  Certificate Request . . . . . . . . . . . . . . . . .  44  45
     4.3.  Authentication Messages . . . . . . . . . . . . . . . . .  46  47
       4.3.1.  Certificate . . . . . . . . . . . . . . . . . . . . .  47  49
       4.3.2.  Certificate Verify  . . . . . . . . . . . . . . . . .  51  52
       4.3.3.  Finished  . . . . . . . . . . . . . . . . . . . . . .  53  54
     4.4.  Post-Handshake Messages . . . . . . . . . . . . . . . . .  54  55
       4.4.1.  New Session Ticket Message  . . . . . . . . . . . . .  54  56
       4.4.2.  Post-Handshake Authentication . . . . . . . . . . . .  56  57
       4.4.3.  Key and IV Update . . . . . . . . . . . . . . . . . .  57  58
     4.5.  Handshake Layer and Key Changes . . . . . . . . . . . . .  59
   5.  Record Protocol . . . . . . . . . . . . . . . . . . . . . . .  58  59
     5.1.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  58  59
     5.2.  Record Payload Protection . . . . . . . . . . . . . . . .  59  61
     5.3.  Per-Record Nonce  . . . . . . . . . . . . . . . . . . . .  61  63
     5.4.  Record Padding  . . . . . . . . . . . . . . . . . . . . .  62  63
     5.5.  Limits on Key Usage . . . . . . . . . . . . . . . . . . .  63  64
   6.  Alert Protocol  . . . . . . . . . . . . . . . . . . . . . . .  63  65
     6.1.  Closure Alerts  . . . . . . . . . . . . . . . . . . . . .  65  66
     6.2.  Error Alerts  . . . . . . . . . . . . . . . . . . . . . .  66  67
   7.  Cryptographic Computations  . . . . . . . . . . . . . . . . .  69  70
     7.1.  Key Schedule  . . . . . . . . . . . . . . . . . . . . . .  69  70
     7.2.  Updating Traffic Keys and IVs . . . . . . . . . . . . . .  72  73
     7.3.  Traffic Key Calculation . . . . . . . . . . . . . . . . .  72  73
       7.3.1.  Diffie-Hellman  . . . . . . . . . . . . . . . . . . .  73  74
       7.3.2.  Elliptic Curve Diffie-Hellman . . . . . . . . . . . .  74  75
       7.3.3.  Exporters . . . . . . . . . . . . . . . . . . . . . .  74  75
   8.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  74  75
     8.1.  MTI Cipher Suites . . . . . . . . . . . . . . . . . . . .  75  76
     8.2.  MTI Extensions  . . . . . . . . . . . . . . . . . . . . .  75  76
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  76  77
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  76  77
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  79  80
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  79  80
     11.2.  Informative References . . . . . . . . . . . . . . . . .  82  83
   Appendix A.  Protocol Data Structures and Constant Values . . . .  89  90
     A.1.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  89  90
     A.2.  Alert Messages  . . . . . . . . . . . . . . . . . . . . .  89  90
     A.3.  Handshake Protocol  . . . . . . . . . . . . . . . . . . .  91  92
       A.3.1.  Key Exchange Messages . . . . . . . . . . . . . . . .  91  92
       A.3.2.  Server Parameters Messages  . . . . . . . . . . . . .  95  96
       A.3.3.  Authentication Messages . . . . . . . . . . . . . . .  96  97
       A.3.4.  Ticket Establishment  . . . . . . . . . . . . . . . .  96  97
     A.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  97  98
       A.4.1.  Unauthenticated Operation . . . . . . . . . . . . . . 102  99
   Appendix B.  Implementation Notes . . . . . . . . . . . . . . . . 102 100
     B.1.  API considerations for 0-RTT  . . . . . . . . . . . . . . 100
     B.2.  Random Number Generation and Seeding  . . . . . . . . . . 102
     B.2. 100
     B.3.  Certificates and Authentication . . . . . . . . . . . . . 103
     B.3. 100
     B.4.  Cipher Suite Support  . . . . . . . . . . . . . . . . . . 103
     B.4. 100
     B.5.  Implementation Pitfalls . . . . . . . . . . . . . . . . . 103
     B.5. 101
     B.6.  Client Tracking Prevention  . . . . . . . . . . . . . . . 105 102
   Appendix C.  Backward Compatibility . . . . . . . . . . . . . . . 105 102
     C.1.  Negotiating with an older server  . . . . . . . . . . . . 106 103
     C.2.  Negotiating with an older client  . . . . . . . . . . . . 106 104
     C.3.  Zero-RTT backwards compatibility  . . . . . . . . . . . . 107 104
     C.4.  Backwards Compatibility Security Restrictions . . . . . . 107 105
   Appendix D.  Overview of Security Properties  . . . . . . . . . . 108 106
     D.1.  Handshake . . . . . . . . . . . . . . . . . . . . . . . . 108 106
     D.2.  Record Layer  . . . . . . . . . . . . . . . . . . . . . . 110 108
   Appendix E.  Working Group Information  . . . . . . . . . . . . . 112 109
   Appendix F.  Contributors . . . . . . . . . . . . . . . . . . . . 112 109
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 116 113

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 TLS is to provide a secure channel between two
   communicating peers.  Specifically, the channel should provide the
   following properties.

   -  Authentication: The server side of the channel is always
      authenticated; the client side is optionally authenticated.
      Authentication can happen via asymmetric cryptography (e.g., RSA
      [RSA], ECDSA [ECDSA]) or a pre-shared symmetric key.

   -  Confidentiality: Data sent over the channel is not visible to
      attackers.

   -  Integrity: Data sent over the channel cannot be modified by
      attackers.

   These properties should be true even in the face of an attacker who
   has complete control of the network, as described in [RFC3552].  See
   Appendix D for a more complete statement of the 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 handshake
      protocol is designed to resist tampering; an active attacker
      should not be able to force the 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 records, each of which is independently protected
      using the traffic keys.

   TLS is application protocol independent; higher-level protocols can
   layer on top of TLS 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

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

   draft-15

   -  Allow cookies  New negotiation syntax as discussed in Berlin (*)

   -  Require CertificateRequest.context to be longer empty during handshake
      (*)

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

   -  Remove 0-RTT EncryptedExtensions and replace  Forbid application data messages in between post-handshake
      messages from the ticket_age
      extension with an obfuscated version.  Also necessitates a change
      to NewSessionTicket (*). same flight (*)

   -  Move the downgrade sentinel to the end  Clean up alert guidance (*)

   -  Clearer guidance on what is needed for TLS 1.2.

   -  Guidance on 0-RTT time windows.

   -  Rename a bunch of fields.

   -  Remove old PRNG text.

   -  Explicitly require checking that handshake records not span key
      changes.

   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 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.

1.3.  Updates Affecting TLS 1.2

   This document defines several changes that optionally affect
   implementations of TLS 1.2:

   -  A version downgrade protection mechanism is described in
      Section 4.1.3.

   -  RSASSA-PSS signature schemes are defined in Section 4.2.2.

   An implementation of TLS 1.3 that also supports TLS 1.2 might need to
   include changes to support these changes even when TLS 1.3 is not in
   use.  See the referenced sections for more details.

2.  Protocol Overview

   The cryptographic parameters of the session state are produced by the
   TLS handshake protocol.  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.  Once the handshake is complete, the peers
   use the established keys to protect application layer traffic.

   TLS supports three basic key exchange modes:

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

   -  A pre-shared symmetric key (PSK)

   -  A combination of a symmetric key and Diffie-Hellman

   Which mode is used depends on the negotiated cipher suite.
   Conceptually, the handshake establishes three secrets which are used
   to derive all the keys.

   Figure 1 below shows 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 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

   The handshake can be thought of as having three phases, indicated in
   the diagram above.

   -  Key Exchange: Establish shared keying material and select the
      cryptographic parameters.  Everything after this phase is
      encrypted.

   -  Server Parameters: Establish other handshake parameters.  (whether
      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 4.1.1) 4.1.2) message, which contains a random nonce
   (ClientHello.random), its offered protocol version, cipher suite, and
   extensions, and in general either one or more a list of
   symmetric cipher/HKDF hash pairs, some set of Diffie-Hellman key
   shares (in the "key_share" extension Section 4.2.4), one or more pre-
   shared key labels (in the "pre_shared_key" extension Section 4.2.5),
   or both. both, and potentially some other extensions.

   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 4.1.2]. 4.1.3].  The combination of the ClientHello and
   the ServerHello determines the shared keys.  If either a pure (EC)DHE
   or (EC)DHE-PSK cipher suite key
   establishment is in use, then the ServerHello will contain a
   "key_share" extension with the server's ephemeral Diffie-
   Hellman Diffie-Hellman
   share which MUST be in the same group as one of the client's shares.
   If a pure PSK or an (EC)DHE-PSK cipher suite key establishment is negotiated, in use, then the ServerHello will contain
   a "pre_shared_key" extension indicating which of the client's offered
   PSKs was selected.  Note that implementations can use (EC)DHE and PSK
   together, in which case both extensions will be supplied.

   The server then sends two messages to establish the Server
   Parameters:

   EncryptedExtensions.  responses to any extensions which are not
      required in order to determine the cryptographic parameters.
      [Section 4.2.8]

   CertificateRequest.  if certificate-based client authentication is
      desired, the desired parameters for that certificate.  This
      message will be omitted if client authentication is not desired.

   Finally, the client and server exchange Authentication messages.  TLS
   uses the same set of messages every time that authentication is
   needed.  Specifically:

   Certificate.  the certificate of the endpoint.  This message is
      omitted if the server is not authenticating with a certificate
      (i.e., with PSK or (EC)DHE-PSK cipher suites). certificate.
      Note that if raw public keys [RFC7250] or the cached information
      extension
      [I-D.ietf-tls-cached-info] [RFC7924] are in use, then this message will not contain
      a certificate but rather some other value corresponding to the
      server's long-term key.  [Section 4.3.1]

   CertificateVerify.  a signature over the 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 (EC)DHE-PSK cipher suites). certificate.
      [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 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
   sent prior to sending the Finished message.  Note that while the
   server may send application data prior to receiving the client's
   Authentication messages, any data sent at that point is, of course,
   being sent to an unauthenticated peer.

2.1.  Incorrect DHE Share

   If the client has not provided a sufficient "key_share" extension
   (e.g. it includes only DHE or ECDHE groups unacceptable or
   unsupported by the server), the server corrects the mismatch with a
   HelloRetryRequest and the client will need 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).

            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 initial ClientHello/
   HelloRetryRequest exchange; it is not reset with the new ClientHello.

   TLS also allows several optimized variants of the basic handshake, as
   described in the following sections.

2.2.  Resumption and Pre-Shared Key (PSK)

   Although TLS PSKs can be established out of band, PSKs can also be
   established in a previous session and then reused ("session
   resumption").  Once a handshake has completed, the server can send
   the client a PSK identity which corresponds to a key derived from the
   initial handshake (See Section 4.4.1).  The client can then use that
   PSK identity in future handshakes to negotiate use of the PSK.  If
   the server accepts it, then the security context of the new
   connection is tied to the 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

   PSKs can either use PSK in combination be used with an (EC)DHE exchange in order to provide forward
   secrecy in combination with shared keys, or can use PSKs be used alone, at the
   cost of losing forward secrecy.

   Figure 3 shows a pair of handshakes in which the first establishes a
   PSK and 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
            + 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 PSK

   As the server is authenticating via a 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 server
   as well to allow the server to decline resumption and fall back to a
   full handshake, if needed.  A "key_share"  The server responds with a
   "pre_shared_key" extension MUST also be sent
   if the client is attempting to negotiate an (EC)DHE-PSK cipher suite. use of PSK key establishment
   and can (as shown here) respond with a "key_share" extension to do
   (EC)DHE key establishment, thus providing forward secrecy.

2.3.  Zero-RTT Data

   When resuming via a PSK with 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 first offered PSK as the static secret.  As
   shown in Figure 4, the Zero-RTT data is just added to the 1-RTT
   handshake in the first flight.  The rest of 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 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 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: The security properties for 0-RTT data (regardless of
   the cipher suite) 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 are no guarantees of non-replay between connections.
       Unless the server takes special measures outside those provided
       by TLS, the server has no guarantee that the same 0-RTT data was
       not transmitted on multiple 0-RTT connections (See
       Section 4.2.6.2 for more details).  This is especially relevant
       if the data is authenticated either with TLS client
       authentication 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 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.)

   The remainder of this document provides a detailed description of
   TLS.

3.  Presentation Language

   This document deals with the formatting of data in an external
   representation.  The following very basic and somewhat casually
   defined presentation syntax will be used.  The syntax draws from
   several sources in its structure.  Although it resembles the
   programming language "C" in its syntax and XDR [RFC4506] in both its
   syntax and intent, it would be risky to draw too many parallels.  The
   purpose of this presentation language is to document TLS only; it has
   no general application beyond that particular goal.

3.1.  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 the byte stream, a multi-byte item (a numeric 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 the commonplace network
   byte order or big-endian format.

3.2.  Miscellaneous

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

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

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

3.3.  Vectors

   A vector (single-dimensioned array) is a stream of homogeneous data
   elements.  The size of the vector may be specified at documentation
   time or left unspecified until runtime.  In either case, the length
   declares the number of bytes, not the number of elements, in the
   vector.  The syntax for specifying a new type, T', that is a fixed-
   length vector of type T is

      T T'[n];

   Here, T' occupies n bytes in the data stream, where n is a multiple
   of the size of T.  The length of the vector is not included in the
   encoded stream.

   In the following example, Datum is defined to be three consecutive
   bytes that the protocol does not interpret, while Data 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 the byte stream.  The length will be in the form of a number
   consuming as many bytes as required to hold the vector's specified
   maximum (ceiling) length.  A variable-length vector with an actual
   length field of zero is referred to as an empty vector.

      T T'<floor..ceiling>;

   In the 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 represent the value 400 (see Section 3.4).  On the
   other hand, longer can represent up to 800 bytes of data, or 400
   uint16 elements, and it may be 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 length 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 to 400 16-bit unsigned integers */

3.4.  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 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 specification, are stored in
   network byte (big-endian) order; the uint32 represented by the hex
   bytes 01 02 03 04 is equivalent to 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 not used even if the most significant bit is set).

3.5.  Enumerateds

   An additional sparse data type is available called enum.  A field of
   type enum can only assume the values declared in the definition.
   Each definition is a different type.  Only enumerateds of the same
   type may be assigned or compared.  Every element of an enumerated
   must be assigned a value, as demonstrated in the following example.
   Since the elements 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 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 { red(3), blue(5), white(7) } Color;

   One may optionally specify a value without its associated tag to
   force the width definition without defining a superfluous element.

   In the following example, Taste will consume two bytes in the data
   stream but 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 the first example, a fully qualified reference to
   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;

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 type's name,
   with a syntax much like that available for enumerateds.  For example,
   T.f2 refers to the second field of the previous declaration.
   Structure definitions may be embedded.

3.6.1.  Variants

   Defined structures may have variants based on some knowledge that is
   available within the environment.  The selector must be an enumerated
   type that defines the possible variants the structure defines.  There
   must be a case arm for every element of the enumeration declared in
   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 the example below, "orange"
   and "banana" both contain V2.  Note that this is a new piece of
   syntax in TLS 1.2.

   The body of the variant structure may be given a label for reference.
   The mechanism by which the variant is selected at runtime is not
   prescribed by 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 of selector is 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 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 be assigned values.  No fields
   of 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.  Handshake Protocol

   The handshake protocol is used to negotiate the secure attributes of
   a session.  Handshake messages are supplied to the TLS record layer,
   where they are encapsulated within one or more TLSPlaintext or
   TLSCiphertext structures, which are processed and transmitted as
   specified by the 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 be sent in the 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 described in
   Section 10.

4.1.  Key Exchange Messages

   The key exchange messages are used to exchange security capabilities
   between the client and server and to establish the traffic keys used
   to protect the handshake and data.

4.1.1.  Client Hello

   When this message will be sent:

      When a client first connects to a server, it is required to send  Cryptographic Negotiation

   TLS cryptographic negotiation proceeds by the ClientHello as its first message.  The client will also send a
      ClientHello when offering the server has responded to
   following four sets of options in its ClientHello with
      a ServerHello that selects cryptographic parameters that don't
      match ClientHello.

   -  A list of cipher suites which indicates the client's "key_share" extension.  In that case, AEAD cipher/HKDF hash
      pairs which the client MUST send the same ClientHello (without modification)
      except: supports

   -  Including a new KeyShareEntry as the lowest priority share (i.e.,
      appended to  A "supported_group" (Section 4.2.3) extension which indicates the list of shares in
      (EC)DHE groups which the client supports and a "key_share" extension).
      (Section 4.2.4) extension which contains (EC)DHE shares for some
      or all of these groups

   -  Removing  A "signature_algorithms" (Section 4.2.2) extension which indicates
      the EarlyDataIndication Section 4.2.6 signature algorithms which the client can accept.

   -  A "pre_shared_key" (Section 4.2.5) extension if one
      was present.  Early data is not permitted after HelloRetryRequest. which contains the
      identities of symmetric keys known to the client and the key
      exchange modes which each PSK supports.

   If a the server receives does not select a ClientHello at any other time, it MUST send a
   fatal "unexpected_message" alert and close PSK, then the connection.

   Structure first three of 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 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 these
   options are entirely orthogonal: the compression_methods field in server independently selects a
   cipher suite, an
   extensions block.  The presence (EC)DHE group and key share for key establishment,
   and a signature algorithm/certificate pair to authenticate itself to
   the client.  If any of extensions can be detected by
   determining whether these parameters has no overlap between the
   client and server parameters, then the handshake will fail.  If there are bytes following
   is overlap in the compression_methods
   at "supported_group" extension but the end of client did not
   offer a compatible "key_share" extension, then the ClientHello.  Note that this method of detecting
   optional data differs from server will
   respond with a HelloRetryRequest (Section 4.1.4) message.

   If the normal TLS method of having server selects a
   variable-length field, but PSK, then the PSK will indicate which key
   establishment modes it is can be used for compatibility with TLS
   before extensions were defined.  As of TLS 1.3, all clients (PSK alone or with (EC)DHE)
   and
   servers will send at least one extension (at least "key_share" which authentication modes it can be used with (PSK alone or
   "pre_shared_key").

   client_version PSK
   with signatures).  The latest (highest valued) version of server can then select those key establishment
   and authentication parameters to be consistent both with the TLS
      protocol offered PSK and
   the other extensions supplied by the client.  This SHOULD be the same as the
      latest version supported.  For this version of the specification,  Note that if the version will PSK
   can be { 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 used without (EC)DHE or without signatures, then non-overlap
   in
      this version (see Section 2.2).  This field MUST either of these parameters need not be ignored by a fatal.

   The server negotiating TLS 1.3 and SHOULD be set indicates its selected parameters in the ServerHello as a zero length
      vector (i.e., 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
   follows: If PSK is a list of being used then the cryptographic options supported
      by server will send a
   "pre_shared_key" extension indicating the client, with selected key.  If PSK is
   not being used, then (EC)DHE and certificate-based authentication are
   always used.  When (EC)DHE is in use, the client's first preference first.  Each
      cipher suite defines a key exchange algorithm, server will also provide a record protection
      algorithm (including secret key length) and
   "key_share" extension.  When authenticating via a hash to be used with
      HKDF.  The certificate, the
   server will select send an empty "signature_algorithnms" extension in the
   ServerHello and will subsequently send Certificate (Section 4.3.1)
   and CertificateVerify (Section 4.3.2) messages.

   If the server is unable to negotiate a cipher suite or, if no acceptable
      choices are presented, supported set of parameters,
   it MUST return a "handshake_failure" alert and close the connection.  If the list contains cipher suites

4.1.2.  Client Hello

   When this message will be sent:

      When a client first connects to a server, it is required to send
      the ClientHello as its first message.  The client will also send a
      ClientHello when the server does not recognize, support, or wish has responded to use, its ClientHello with
      a HelloRetryRequest that selects cryptographic parameters that
      don't match the server client's "key_share" extension.  In that case, the
      client MUST ignore those cipher suites, and process send the remaining ones same ClientHello (without modification)
      except:

   -  Including a new KeyShareEntry as
      usual.  Values are defined in Appendix A.4.

   legacy_compression_methods  Versions of TLS before 1.3 supported
      compression with the lowest priority share (i.e.,
      appended to the list of supported compression methods being
      sent shares in this field.  For every TLS 1.3 ClientHello, this vector
      MUST contain exactly one byte set to zero, which corresponds to the "null" compression method in prior versions of TLS. "key_share" extension).

   -  Removing the EarlyDataIndication Section 4.2.6 extension if one
      was present.  Early data is not permitted after HelloRetryRequest.

   If a TLS
      1.3 server receives a ClientHello is received with at any other value in this field,
      the server time, it MUST generate send a
   fatal "illegal_parameter" alert.  Note
      that TLS 1.3 servers might receive TLS 1.2 or prior ClientHellos
      which contain other compression methods "unexpected_message" alert and MUST follow the
      procedures for close the appropriate prior version connection.

   Structure of TLS.

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

   In the event that a client requests additional functionality using
   extensions, and this functionality is not supplied message:

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

   struct {
       opaque random_bytes[32];
   } Random;

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

   struct {
       ProtocolVersion max_supported_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 field in an
   extensions block.  The presence of extensions can be detected by
   determining whether there are bytes following the server, compression_methods
   at the
   client MAY abort end of the handshake. ClientHello.  Note that this method of detecting
   optional data differs from the normal TLS 1.3 ClientHello
   messages MUST always contain extensions, and method of having a
   variable-length field, but it is used for compatibility with TLS 1.3 server MUST
   respond to any TLS 1.3 ClientHello without
   before extensions with a fatal
   "decode_error" alert. were defined.  As of TLS 1.3 1.3, all clients and
   servers may receive TLS 1.2
   ClientHello messages without extensions.  If negotiating TLS 1.2, a
   server MUST check that the amount of data in the message precisely
   matches will send at least one extension (at least "key_share" or
   "pre_shared_key").

   max_supported_version  The latest (highest valued) version of these formats; if not, then it MUST send a fatal
   "decode_error" alert.

   After sending the ClientHello message, TLS
      protocol offered by the client waits for a
   ServerHello or HelloRetryRequest message.

4.1.2.  Server Hello

   When this message will client.  This SHOULD be sent:

      The server will send the same as the
      latest version supported.  For this message in response to a ClientHello
      message when it was able to find an acceptable set version of algorithms
      and the client's "key_share" extension was acceptable.  If the
      client proposed groups are not acceptable by the 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 version of TLS negotiated for
      this session.  Servers MUST select the lower of the highest
      supported server version and the version offered by the client in
      the ClientHello.  In particular, servers MUST accept ClientHello
      messages with versions higher than those supported and negotiate
      the highest mutually supported version.  For this version of the
      specification, specification,
      the version is will be { 3, 4 }. (See Appendix C for details about
      backward compatibility.)

   random  This structure is  32 bytes generated by the 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 be ignored by a
      server negotiating TLS 1.3 and MUST SHOULD be
      generated independently set as a zero length
      vector (i.e., 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 is a list of the ClientHello.random.

   cipher_suite  The single symmetric cipher suite selected options
      supported by the server from client, specifically the
      list record protection
      algorithm (including secret key length) and a hash to be used with
      HKDF, in ClientHello.cipher_suites.  For resumed sessions, this
      field is the value from the state descending order of client preference.  If the session being resumed.
      [[TODO: interaction with PSK.]]

   extensions  A list of extensions.  Note that only extensions offered
      by the client can appear in
      contains cipher suites the server's list.  In TLS 1.3, as
      opposed server does not recognize, support, or
      wish to previous versions of TLS, the server's extensions are
      split between the ServerHello and use, the EncryptedExtensions
      Section 4.2.8 message.  The ServerHello server MUST only include
      extensions which are required to establish the cryptographic
      context.  Currently the only such extensions are "key_share",
      "pre_shared_key", ignore those cipher suites, and "early_data".  Clients MUST check the
      ServerHello for
      process the presence of any forbidden extensions and if
      any remaining ones as usual.  Values are found MUST terminate the handshake with a
      "illegal_parameter" alert.  In prior versions of TLS, the
      extensions field could be omitted entirely if not needed, similar
      to ClientHello.  As defined in
      Appendix A.4.

   legacy_compression_methods  Versions of TLS 1.3, all clients and servers will send
      at least one extension (at least "key_share" or "pre_shared_key").

   TLS before 1.3 has a downgrade protection mechanism embedded in supported
      compression with the server's
   random value. list of supported compression methods being
      sent in this field.  For every TLS 1.3 server implementations ClientHello, this vector
      MUST contain exactly one byte set to zero, which respond corresponds to
      the "null" compression method in prior versions of TLS.  If a TLS
      1.3 ClientHello is received with any other value in this field,
      the server MUST generate a client_version indicating fatal "illegal_parameter" alert.  Note
      that TLS 1.3 servers might receive TLS 1.2 or below prior ClientHellos
      which contain other compression methods and MUST
   set follow the last eight bytes
      procedures for the appropriate prior version of their Random value to TLS.

   extensions  Clients request extended functionality from servers by
      sending data in the bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to a extensions field.  The actual "Extension"
      format is defined in Section 4.2.

   In the event that a client requests additional functionality using
   extensions, and this functionality is not supplied by the server, the
   client MAY abort the handshake.  Note that TLS 1.3 ClientHello with
   messages MUST always contain extensions, and a
   client_version indicating TLS 1.1 or below SHOULD set the last eight
   bytes of their Random value 1.3 server MUST
   respond to the bytes:

     44 4F 57 4E 47 52 44 00 any TLS 1.3 clients receiving ClientHello without extensions with a fatal
   "decode_error" alert.  TLS 1.3 servers may receive TLS 1.2 or below ServerHello
   ClientHello messages without extensions.  If negotiating TLS 1.2, a
   server MUST check that the last eight octets are not equal to either amount of data in the message precisely
   matches one of these values.
   TLS 1.2 clients SHOULD also perform this check formats; if the ServerHello
   indicates TLS 1.1 or below.  If a match is found, the client not, then it MUST
   abort the handshake with send a fatal "illegal_parameter"
   "decode_error" alert.  This
   mechanism provides limited protection against downgrade attacks over
   and above that provided by

   After sending the Finished exchange: because ClientHello message, the
   ServerKeyExchange includes a signature over both random values, it is
   not possible client waits for an active attacker to modify the randoms without
   detection as long as ephemeral ciphers are used.  It does not provide
   downgrade protection when static RSA is used.

   Note: This is an update to TLS 1.2 so in practice many TLS 1.2
   clients and servers will not behave as specified above. a
   ServerHello or HelloRetryRequest message.

4.1.3.  Server Hello Retry Request

   When this message will be sent:

      Servers

      The server will send this message in response to a ClientHello
      message if
      they were able when it was able to find an acceptable set of algorithms
      and groups
      that are mutually supported, but the client's KeyShare did not
      contain an acceptable offer. "key_share" extension was acceptable.  If it cannot is
      not able to find such a match, it an acceptable set of parameters, the server will
      respond with a fatal "handshake_failure" fatal alert.

   Structure of this message:

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

   selected_group  The mutually supported group ServerHello;

   version  This field contains the server intends to
      negotiate and is requesting a retried ClientHello/KeyShare for.

   The server_version, cipher_suite, and extensions fields have version of TLS negotiated for this
      session.  Servers MUST select the same
   meanings as their corresponding values in lower of the ServerHello.  [[NOTE:
   cipher_suite may disappear. https://github.com/tlswg/tls13-spec/
   issues/528]] The highest supported
      server SHOULD send only the extensions necessary for version and the 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 first version offered by the client in
   its ClientHello.

   Upon receipt of a HelloRetryRequest, the client
      ClientHello.  In particular, servers MUST first verify
   that accept ClientHello
      messages with versions higher than those supported and negotiate
      the selected_group field corresponds to a group which was
   provided in highest mutually supported version.  For this version of the "supported_groups" extension in
      specification, the original
   ClientHello.  It version is { 3, 4 }.  (See Appendix C for
      details about backward compatibility.)

   random  This structure is generated by the server and MUST then verify that be
      generated independently of the selected_group field does
   not correspond to a group which was provided ClientHello.random.

   cipher_suite  The single cipher suite selected by the server from the
      list in ClientHello.cipher_suites.

   extensions  A list of extensions.  Note that only extensions offered
      by the "key_share"
   extension client can appear in the original ClientHello.  If either of these checks
   fails, then the client MUST abort the handshake with a fatal
   "handshake_failure" alert.  Clients SHOULD also abort with
   "handshake_failure" in response server's list.  In TLS 1.3, as
      opposed to any second HelloRetryRequest which
   was sent in the same connection (i.e., where previous versions of TLS, the ClientHello was
   itself in response to a HelloRetryRequest).

   Otherwise, server's extensions are
      split between the client MUST send a ClientHello with an updated
   KeyShare extension to ServerHello and the server. EncryptedExtensions
      Section 4.2.8 message.  The client ServerHello MUST append a new
   KeyShareEntry for the group indicated in the selected_group field only include
      extensions which are required to establish the groups in its original KeyShare.

   Upon re-sending cryptographic
      context.  Currently the ClientHello only such extensions are "key_share",
      "pre_shared_key", and receiving the server's
   ServerHello/KeyShare, the client "early_data".  Clients MUST verify that check the selected
   CipherSuite and NamedGroup match that supplied in
      ServerHello for the
   HelloRetryRequest.  If either presence of these values differ, the client any forbidden extensions and if
      any are found MUST
   abort terminate the connection handshake with a fatal "handshake_failure"
      "illegal_parameter" 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 particular extension type.

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

   The initial set  In prior versions of TLS, the
      extensions is defined in [RFC6066].  The list 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 types is maintained by IANA as described in Section 10.

   An extension type MUST NOT appear in the ServerHello (at least "key_share" or
   HelloRetryRequest unless the same extension type appeared "pre_shared_key").

   TLS 1.3 has a downgrade protection mechanism embedded in the
   corresponding ClientHello.  If server's
   random value.  TLS 1.3 server implementations which respond to a client receives an extension type in
   ServerHello
   ClientHello with a max_supported_version indicating TLS 1.2 or HelloRetryRequest that it did not request in the
   associated ClientHello, it below
   MUST abort set the handshake with an
   "unsupported_extension" fatal alert.

   Nonetheless, "server-oriented" extensions may be provided within this
   framework.  Such an extension (say, last eight bytes of type x) would require their Random value to the
   client bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to first send an extension of type x in a ClientHello with
   empty extension_data a
   max_supported_version indicating TLS 1.1 or below SHOULD set the last
   eight bytes of their Random value to indicate the bytes:

     44 4F 57 4E 47 52 44 00

   TLS 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
   that it supports the extension type.
   In last eight octets are not equal to either of these values.
   TLS 1.2 clients SHOULD also perform this case, check if the client ServerHello
   indicates TLS 1.1 or below.  If a match is offering found, the capability to understand client MUST
   abort the
   extension type, handshake with a fatal "illegal_parameter" alert.  This
   mechanism provides limited protection against downgrade attacks over
   and above that provided by the server Finished exchange: because the
   ServerKeyExchange includes a signature over both random values, it is taking
   not possible for an active attacker to modify the client up on its offer.

   When multiple extensions of different types randoms without
   detection as long as ephemeral ciphers are present used.  It does not provide
   downgrade protection when static RSA is used.

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

4.1.4.  Hello Retry Request

   When this message will be sent:

      Servers send this message in response to a ClientHello or ServerHello messages, message if
      they were able to find an acceptable set of algorithms and groups
      that are mutually supported, but the extensions MAY appear in any
   order.  There MUST NOT be more than one extension client's KeyShare did not
      contain an acceptable offer.  If it cannot find such a match, it
      will respond with a fatal "handshake_failure" alert.

   Structure of this message:

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

   selected_group  The mutually supported group the same type.

   Finally, note that extensions can be sent both when starting server intends to
      negotiate and is requesting a new
   session retried ClientHello/KeyShare for.

   The version and when in resumption-PSK mode.  A client that requests
   session resumption does not extensions fields have the same meanings as their
   corresponding values in general know whether the ServerHello.  The server will
   accept this request, and therefore it SHOULD send only
   the same extensions
   as it would send normally.

   In general, necessary for the specification of each extension type needs client to
   describe the effect of the extension both during full handshake and
   session resumption.  Most current TLS generate a correct
   ClientHello pair (currently no such extensions are relevant only
   when exist).  As with
   ServerHello, a session is initiated: when an older session is resumed, the
   server does not process these HelloRetryRequest MUST NOT contain any extensions in ClientHello, and does that
   were not
   include them in ServerHello.  However, some extensions may specify
   different behavior during session resumption.  [[TODO: update this
   and first offered by the previous paragraph to cover PSK-based resumption.]]

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

   -  Some cases where its ClientHello.

   Upon receipt of 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 for HelloRetryRequest, the former,
      and a client MUST first verify
   that the selected_group field corresponds to a group which was
   provided in the server "supported_groups" extension response for in the latter.

   -  Extensions should, as far as possible, be designed to prevent any
      attack original
   ClientHello.  It MUST then verify that forces use (or non-use) of a particular feature by
      manipulation of handshake messages.  This principle should be
      followed regardless of whether the feature is believed selected_group field does
   not correspond to cause a
      security problem.  Often the fact that the extension fields are
      included group which was provided in the inputs to the Finished message hashes will be
      sufficient, but extreme care is needed when the "key_share"
   extension changes
      the meaning of messages sent in the handshake phase.  Designers
      and implementors should be aware original ClientHello.  If either of these checks
   fails, then the fact that until client MUST abort the handshake has been authenticated, active attackers can modify
      messages and insert, remove, or replace extensions.

4.2.1.  Cookie

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

   Cookies serve two primary purposes:

   -  Allowing the server with a fatal
   "illegal_parameter" alert.  Clients SHOULD also abort with
   "unexpected_message" in response to force any second HelloRetryRequest
   which was sent in the client to demonstrate
      reachability at their apparent network address (thus providing a
      measure of DoS protection).  This is primarily useful for non-
      connection-oriented transports (see [RFC6347] for an example of
      this).

   -  Allowing same connection (i.e., where the server to offload state ClientHello
   was itself in response to a HelloRetryRequest).

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

   When sending a HelloRetryRequest, the server MAY provide a "cookie" an updated
   KeyShare extension to the server.  The client (this is an exception to MUST append a new
   KeyShareEntry for the usual rule that group indicated in the only extensions that may be sent are those that appear selected_group field to
   the groups in its original KeyShare.

   Upon re-sending the
   ClientHello).  When sending ClientHello and receiving the new ClientHello, server's
   ServerHello/KeyShare, the client MUST echo verify that the value of selected
   NamedGroup matches that supplied in the extension.  Clients HelloRetryRequest and MUST NOT use cookies in
   subsequent connections.

4.2.2.  Signature Algorithms

   The client uses
   abort the "signature_algorithms" connection with a fatal "illegal_parameter" alert if it
   does not.

4.2.  Hello Extensions

   The extension to indicate to
   the server which signature algorithms may be used in digital
   signatures.

   Clients which offer one or more cipher suites which use certificate
   authentication (i.e., any non-PSK cipher suite) 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 particular extension type.

   -  "extension_data" contains information specific 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 10.

   An extension type MUST send NOT appear in the
   "signature_algorithms" extension. ServerHello or
   HelloRetryRequest unless the same extension type appeared in the
   corresponding ClientHello.  If this a client receives an extension is type in
   ServerHello or HelloRetryRequest that it did not provided
   and no alternative cipher suite is available, request in the server
   associated ClientHello, it MUST close abort the connection handshake with a an
   "unsupported_extension" fatal "missing_extension" alert.  (see
   Section 8.2)

   The "extension_data" field of

   Nonetheless, "server-oriented" extensions may be provided within this
   framework.  Such an 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" (say, of type in TLS 1.2, which this replaces.  We use
   the term "signature algorithm" throughout the text.

   Each SignatureScheme value lists a single signature algorithm that x) would require the
   client is willing to verify.  The values are indicated in
   descending order of preference.  Note that a signature algorithm
   takes as input first send an arbitrary-length message, rather than a digest.
   Algorithms which traditionally act on a digest should be defined extension of type x in
   TLS to first hash the input with a specified hash function and then
   proceed as usual.  The code point groups listed above have the
   following meanings:

   RSASSA-PKCS1-v1_5 algorithms  Indicates a signature algorithm using
      RSASSA-PKCS1-v1_5 [RFC3447] ClientHello with the corresponding hash algorithm
      as defined in [SHS].  These values refer solely
   empty extension_data to signatures
      which appear in certificates (see Section 4.3.1.1) and are not
      defined for use in signed TLS handshake messages.

   ECDSA algorithms  Indicates a signature algorithm using ECDSA
      [ECDSA], indicate that it supports the corresponding curve as defined in ANSI X9.62 [X962]
      and FIPS 186-4 [DSS], and extension type.
   In this case, the corresponding hash algorithm as
      defined in [SHS].  The signature client is 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 offering the mask generation
      function capability to understand the
   extension type, and the digest being signed are both server is taking the corresponding
      hash algorithm as defined in [SHS]. client up on its offer.

   When used multiple extensions of different types are present in signed TLS
      handshake messages, the length of
   ClientHello or ServerHello messages, the salt extensions MAY appear in any
   order.  There MUST NOT be equal to the
      length more than one extension of the digest output.

   EdDSA algorithms  Indicates a signature algorithm using EdDSA as
      defined in [I-D.irtf-cfrg-eddsa] or its successors.  Note same type.

   Finally, note that
      these correspond to the "PureEdDSA" algorithms extensions can be sent both when starting a new
   session and when in resumption-PSK mode.  A client that requests
   session resumption does not in general know whether the
      "prehash" variants.

   The semantics of server will
   accept this extension are somewhat complicated because the
   cipher suite adds additional constraints on signature algorithms.
   Section 4.3.1.1 describes the appropriate rules.

   rsa_pkcs1_sha1, dsa_sha1, request, and ecdsa_sha1 therefore it SHOULD NOT be offered.
   Clients offering these values for backwards compatibility MUST list
   them send the same extensions
   as it would send normally.

   In general, the lowest priority (listed after all other algorithms in specification of each extension type needs to
   describe the
   supported_signature_algorithms vector). effect of the extension both during full handshake and
   session resumption.  Most current TLS 1.3 servers MUST NOT
   offer a SHA-1 signed certificate 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 certificates
   that are trust anchors extensions are not validated since they begin a
   certification path (see [RFC5280], Section 3.2).  A certificate that
   begins a certification path MAY use relevant only
   when a signature algorithm that session is initiated: when an older session is resumed, the
   server does not
   advertised as being supported process these extensions in the "signature_algorithms"
   extension.

   Note that TLS 1.2 defines this extension differently.  TLS 1.3
   implementations willing to negotiate TLS 1.2 MUST behave ClientHello, and does not
   include them in
   accordance with the requirements of [RFC5246] when negotiating that
   version.  In particular:

   -  TLS 1.2 ClientHellos ServerHello.  However, some extensions may omit specify
   different behavior during session resumption.  [[TODO: update this extension.

   -  In TLS 1.2,
   and the extension contained hash/signature pairs.  The
      pairs previous paragraph to cover PSK-based resumption.]]

   There are encoded in two octets, subtle (and not so SignatureScheme values have
      been allocated to align with TLS 1.2's encoding. subtle) interactions that may occur in
   this protocol between new features and 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 legacy
      pairs cases where a server does not agree to an extension are left unallocated.  These algorithms error
      conditions, and some are deprecated as of
      TLS 1.3.  They MUST NOT be offered or negotiated by any
      implementation. simply refusals to support particular
      features.  In particular, MD5 [SLOTH] and SHA-224 MUST NOT general, error alerts should be used.

   -  ecdsa_secp256r1_sha256, etc., align with TLS 1.2's ECDSA hash/
      signature pairs.  However, the old semantics did not constrain the
      signing curve.

4.2.3.  Negotiated Groups

   When sent by used for the client, former,
      and a field in the "supported_groups" server extension indicates
   the named groups which the client supports response for key exchange, ordered
   from most preferred the latter.

   -  Extensions should, as far as possible, be designed to least preferred.

   Note: In versions prevent any
      attack that forces use (or non-use) 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]. a particular feature by
      manipulation of handshake messages.  This extension was
   also used principle should be
      followed regardless of whether the feature is believed to negotiate ECDSA curves.  Signature algorithms cause a
      security problem.  Often the fact that the extension fields 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
      included in the inputs to the Finished message hashes will be
      sufficient, but extreme care is needed when the extension changes
      the meaning of messages sent in the handshake phase.  Designers
      and servers MUST NOT negotiate
   any implementors should be aware of these cipher suites unless a supported value was provided.  If
   this extension is not provided the fact that until the
      handshake has been authenticated, active attackers can modify
      messages and no alternative cipher suite is
   available, insert, remove, or replace extensions.

4.2.1.  Cookie

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

   Cookies serve two primary purposes:

   -  Allowing the server MUST close to force the connection with client to demonstrate
      reachability at their apparent network address (thus providing a fatal
   "missing_extension" alert.
      measure of DoS protection).  This is primarily useful for non-
      connection-oriented transports (see Section 8.2) If [RFC6347] for an example of
      this).

   -  Allowing the extension is
   provided, but no compatible group is offered, server to offload state to the client, thus allowing
      it to send a HelloRetryRequest without storing any state.  The
      server MUST NOT
   negotiate does this by pickling that post-ClientHello hash state into
      the cookie (protected with some suitable integrity algorithm).

   When sending a cipher suite of HelloRetryRequest, the relevant type.  For instance, if server MAY provide a "cookie"
   extension to the client supplies (this is an exception to the usual rule that
   the only ECDHE groups, extensions that may be sent are those that appear in the server
   ClientHello).  When sending the new ClientHello, the client MUST NOT negotiate
   finite field Diffie-Hellman.  If no acceptable group can echo
   the value of the extension.  Clients MUST NOT use cookies in
   subsequent connections.

4.2.2.  Signature Algorithms

   The client uses the "signature_algorithms" extension to indicate to
   the server which signature algorithms may be selected
   across all cipher suites, used in digital
   signatures.  Clients which desire the server to authenticate via a
   certificate MUST send this extension.  If a server is authenticating
   via a certificate and the client has not sent a
   "signature_algorithms" extension then the server MUST generate close the
   connection with a fatal
   "handshake_failure" alert. "missing_extension" alert (see Section 8.2).

   Servers which are authenticating via a certificate MUST indicate so
   by sending the client an empty "signature_algorithms" extension.

   The "extension_data" field of this extension contains a
   "NamedGroupList"
   "supported_signature_algorithms" value:

      enum {
          /* Elliptic Curve Groups (ECDHE) RSASSA-PKCS1-v1_5 algorithms */
          secp256r1 (23), secp384r1 (24), secp521r1 (25),
          x25519 (29), x448 (30),
          rsa_pkcs1_sha1 (0x0201),
          rsa_pkcs1_sha256 (0x0401),
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

          /* Finite Field Groups (DHE) ECDSA algorithms */
          ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
          ffdhe6144 (259), ffdhe8192 (260),
          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 */
          ffdhe_private_use (0x01FC..0x01FF),
          ecdhe_private_use (0xFE00..0xFEFF),
          private_use (0xFE00..0xFFFF),
          (0xFFFF)
      } NamedGroup;

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

   Elliptic Curve Groups (ECDHE)  Indicates support of the corresponding SignatureScheme;

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

   Note: This enum is named curve.  Note "SignatureScheme" because there is already a
   "SignatureAlgorithm" type in TLS 1.2, which this replaces.  We use
   the term "signature algorithm" throughout the text.

   Each SignatureScheme value lists a single signature algorithm that some curves
   the client is willing to verify.  The values are also recommended indicated in ANSI
      X9.62 [X962] and FIPS 186-4 [DSS].  Others are recommended
   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 in
      [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for private
      use.

   Finite Field Groups (DHE)
   TLS to first hash the input with a specified hash function and then
   proceed as usual.  The code point groups listed above have the
   following meanings:

   RSASSA-PKCS1-v1_5 algorithms  Indicates support of a signature algorithm using
      RSASSA-PKCS1-v1_5 [RFC3447] with the corresponding
      finite field group, hash algorithm
      as defined in [I-D.ietf-tls-negotiated-ff-dhe].
      Values 0x01FC through 0x01FF [SHS].  These values refer solely to signatures
      which appear in certificates (see Section 4.3.1.1) and are reserved not
      defined for private use.

   Items use in named_group_list are ordered according to the client's
   preferences (most preferred choice first).

   As of signed TLS 1.3, servers are permitted to send the "supported_groups"
   extension to handshake messages.

   ECDSA algorithms  Indicates a signature algorithm using ECDSA
      [ECDSA], the client.  If corresponding curve as defined in ANSI X9.62 [X962]
      and FIPS 186-4 [DSS], and the server has corresponding hash algorithm as
      defined in [SHS].  The signature is represented as a group it prefers to 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
   ones mask generation
      function and the digest being signed are both the corresponding
      hash algorithm as defined in [SHS].  When used in signed TLS
      handshake messages, the "key_share" extension but is still willing to accept length of the
   ClientHello, it SHOULD send "supported_groups" salt MUST be equal to update the client's
   view of its preferences.  Clients MUST NOT act upon any information
   found in "supported_groups" prior to successful completion
      length of the
   handshake, but MAY digest output.  This codepoint is defined for use the information learned from a successfully
   completed handshake to change what groups they offer to a
      with TLS 1.2 as well as TLS 1.3.  A server in
   subsequent connections.

4.2.4.  Key Share

   The "key_share" extension contains the 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
   extension and SHOULD send at least one supported KeyShareEntry value.
   Servers MUST NOT negotiate any of these uses RSASSA-PSS
      signatures with RSA cipher suites unless suites.

   EdDSA algorithms  Indicates a
   supported value was provided.  If this extension is not provided signature algorithm using EdDSA as
      defined in a
   ServerHello [I-D.irtf-cfrg-eddsa] or ClientHello, its successors.  Note that
      these correspond to the "PureEdDSA" algorithms and not the peer is offering (EC)DHE
      "prehash" variants.  A server uses EdDSA signatures with ECDSA
      cipher
   suites, then the endpoint suites.

   rsa_pkcs1_sha1, dsa_sha1, and ecdsa_sha1 SHOULD NOT be offered.
   Clients offering these values for backwards compatibility MUST close list
   them as the connection with a fatal
   "missing_extension" alert.  (see Section 8.2) Clients MAY send an
   empty client_shares vector lowest priority (listed after all other algorithms in order to request group selection from the server at the cost of an additional round trip.
   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 4.1.3)

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

   group 4.3.1.1).

   The named group for the key being exchanged.  Finite Field
      Diffie-Hellman [DH] parameters signatures on certificates that are described in Section 4.2.4.1;
      Elliptic Curve Diffie-Hellman parameters self-signed or certificates
   that are described in
      Section 4.2.4.2.

   key_exchange  Key exchange information.  The contents of this field trust anchors are determined 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 not validated since they begin a "KeyShare"
   value:

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

              case server:
                  KeyShareEntry server_share;
          }
      } KeyShare;

   client_shares
   certification path (see [RFC5280], Section 3.2).  A list of offered KeyShareEntry values in descending
      order of client preference.  This vector certificate that
   begins a certification path MAY be empty if the
      client is requesting use a HelloRetryRequest.  The ordering of values
      here SHOULD match signature algorithm that of the ordering of offered support is not
   advertised as being supported in the
      "supported_groups" "signature_algorithms"
   extension.

   server_share  A single KeyShareEntry value 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.  The key_exchange values for each KeyShareEntry MUST by
   generated independently.  Clients MUST NOT offer multiple
   KeyShareEntry values for the same group.  Clients and

   Note that TLS 1.2 defines this extension differently.  TLS 1.3
   implementations willing to negotiate TLS 1.2 MUST NOT offer
   any KeyShareEntry values for groups not listed behave in
   accordance with the client's
   "supported_groups" requirements of [RFC5246] when negotiating that
   version.  In particular:

   -  TLS 1.2 ClientHellos MAY omit this extension.

   Servers offer exactly one KeyShareEntry value, which corresponds to
   the key exchange used for

   -  In TLS 1.2, the negotiated cipher suite.  Servers extension contained hash/signature pairs.  The
      pairs are encoded in two octets, 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 offer a KeyShareEntry value for a group not be offered by the client
   in its corresponding KeyShare or "supported_groups" extension.

   Implementations MAY check for violations of these rules and negotiated by any
      implementation.  In particular, MD5 [SLOTH] and MAY
   abort the connection SHA-224 MUST NOT
      be used.

   -  ECDSA signature schemes align with a fatal "illegal_parameter" alert if one is
   violated.

   If TLS 1.2's ECDSA hash/signature
      pairs.  However, the server selects an (EC)DHE cipher suite and no mutually
   supported group is available between old semantics did not constrain the two endpoints' KeyShare
   offers, yet there signing
      curve.  If TLS 1.2 is negotiated, implementations MUST be prepared
      to accept a mutually supported group signature that can be found via uses any curve that they advertised in
      the "supported_groups" extension, then the server extension.

   -  Implementations that advertise support for RSASSA-PSS (which is
      mandatory in TLS 1.3), MUST reply with be prepared to accept a
   HelloRetryRequest.  If there signature
      using that scheme even when TLS 1.2 is no mutually supported group at all, negotiated.

4.2.3.  Negotiated Groups

   When sent by the server MUST NOT negotiate an (EC)DHE cipher suite.

   [[TODO: Recommendation about what client, the client offers.  Presumably
   which integer DH "supported_groups" extension indicates
   the named groups and which curves.]]

4.2.4.1.  Diffie-Hellman Parameters

   Diffie-Hellman [DH] parameters for both clients and servers are
   encoded in the opaque key_exchange field of a KeyShareEntry in a
   KeyShare structure.  The opaque value contains the Diffie-Hellman
   public value (Y = g^X mod p), encoded as a big-endian integer, padded
   with zeros client supports for key exchange, ordered
   from most preferred to the size of p in bytes. least preferred.

   Note: For a given Diffie-Hellman group, the padding results in all
   public keys having the same length.

   Peers SHOULD validate each other's public key Y by ensuring that 1 <
   Y < p-1.  This check ensures that the remote peer is properly behaved In versions of TLS prior to TLS 1.3, this extension was named
   "elliptic_curves" and isn't forcing the local system into a small subgroup.

4.2.4.2.  ECDHE Parameters

   ECDHE parameters for both clients only contained elliptic curve groups.  See
   [RFC4492] and servers [I-D.ietf-tls-negotiated-ff-dhe].  This extension was
   also used to negotiate ECDSA curves.  Signature algorithms are encoded in the the
   opaque key_exchange now
   negotiated independently (see Section 4.2.2).

   The "extension_data" field of this extension contains a KeyShareEntry in a KeyShare structure.

   For secp256r1,
   "NamedGroupList" value:

      enum {
          /* Elliptic Curve Groups (ECDHE) */
          secp256r1 (23), secp384r1 and secp521r1, the contents are the byte
   string representation (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 an elliptic curve public value following the
   conversion routine corresponding
      named curve.  Note that some curves are also recommended 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 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 [X962] and x448, the contents FIPS 186-4 [DSS].  Others are the byte string inputs and
   outputs recommended in
      [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for private
      use.

   Finite Field Groups (DHE)  Indicates support of the corresponding functions
      finite field group, defined in [RFC7748], 32 bytes
   for x25519 and 56 bytes [I-D.ietf-tls-negotiated-ff-dhe].
      Values 0x01FC through 0x01FF are reserved for x448.

   Note: Versions private use.

   Items in named_group_list are ordered according to the client's
   preferences (most preferred choice first).

   As of TLS prior to 1.3 1.3, servers are permitted point negotiation; TLS
   1.3 removes this feature in favor of a single point format for each
   curve.

4.2.5.  Pre-Shared Key Extension

   The "pre_shared_key" to send the "supported_groups"
   extension is used to indicate the identity of client.  If the pre-shared key to be used with server has a given handshake group it prefers to the
   ones in association
   with a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for
   background).

   Clients which offer one or more PSK cipher suites MUST the "key_share" extension but is still willing to accept the
   ClientHello, it SHOULD send at least
   one supported psk_identity value and servers "supported_groups" to update the client's
   view of its preferences.  Clients MUST NOT negotiate act upon any information
   found in "supported_groups" prior to successful completion of these cipher suites unless a supported value was provided.  If
   this extension is not provided and no alternative cipher suite is
   available, the server MUST close
   handshake, but MAY use the connection with information learned from a fatal
   "missing_extension" alert. successfully
   completed handshake to change what groups they offer to a server in
   subsequent connections.

4.2.4.  Key Share

   The "key_share" extension contains the endpoint's cryptographic
   parameters.

   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 8.2) 4.1.4)

      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 4.2.4.1;
      Elliptic Curve Diffie-Hellman parameters are described in
      Section 4.2.4.2.

   key_exchange  Key exchange information.  The contents of this field
      are determined 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
   "PreSharedKeyExtension" "KeyShare"
   value:

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

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

              case server:
                  uint16 selected_identity;
                  KeyShareEntry server_share;
          }
      } PreSharedKeyExtension;

   identities KeyShare;

   client_shares  A list of the identities (labels for keys) that offered KeyShareEntry values in descending
      order of client preference.  This vector MAY be empty if the
      client is willing to negotiate with the server.  If sent alongside requesting a HelloRetryRequest.  The ordering of values
      here SHOULD match that of the "early_data" extension (see Section 4.2.6), ordering of offered support in the first identity
      "supported_groups" extension.

   server_share  A single KeyShareEntry value that is in the one used for 0-RTT data.

   selected_identity  The server's chosen identity expressed same group
      as a
      (0-based) index into the identies in one of the client's list.

   If no suitable identity is provided, the server MUST NOT negotiate a
   PSK cipher suite and MAY respond with shares.

   Clients offer an "unknown_psk_identity" alert
   message.  Sending this alert is OPTIONAL; servers MAY instead choose
   to send a "decrypt_error" alert to merely indicate an invalid PSK
   identity or instead negotiate use arbitrary number of KeyShareEntry values, each
   representing a non-PSK cipher suite, if
   available.

   If the server selects a PSK cipher suite, it MUST send single set of key exchange parameters.  For instance,
   a
   "pre_shared_key" extension with the identity that it selected.  The client might offer shares for several elliptic curves or multiple
   FFDHE groups.  The key_exchange values for each KeyShareEntry MUST verify that by
   generated independently.  Clients MUST NOT offer multiple
   KeyShareEntry values for the server's selected_identity is within same group.  Clients MUST NOT offer any
   KeyShareEntry values for groups not listed in the
   range supplied by client's
   "supported_groups" extension.  Servers MAY check for violations of
   these rules and and MAY abort the client. connection with a fatal
   "illegal_parameter" alert if one is violated.

   If using (EC)DHE key establishment, servers offer exactly one
   KeyShareEntry.  This value MUST correspond to the server supplies an "early_data"
   extension, KeyShareEntry value
   offered by the client MUST verify that the server has selected for the first
   offered identity.  If any other value is returned, the client negotiated
   key exchange.  Servers MUST
   generate NOT send a fatal "unknown_psk_identity" alert and close the
   connection.

   Note that although 0-RTT data is encrypted with KeyShareEntry for any group
   not indicated in the first PSK
   identity, "supported_groups" extension.

   [[TODO: Recommendation about what the server MAY fall back to 1-RTT client offers.  Presumably
   which integer DH groups and select a different
   PSK identity if multiple identities which curves.]]

4.2.4.1.  Diffie-Hellman Parameters

   Diffie-Hellman [DH] parameters for both clients and servers are offered.

4.2.6.  Early Data Indication

   When PSK resumption is used, the client can send application data
   encoded in
   its first flight of messages.  If the client opts to do so, it MUST
   supply an "early_data" extension as well as the "pre_shared_key"
   extension.

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

      struct {
          select (Role) {
              case client:
                  uint32 obfuscated_ticket_age;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

   obfuscated_ticket_age  The time since the client learned about the
      server configuration that it is using, a KeyShareEntry in milliseconds.  This a
   KeyShare structure.  The opaque value is added modulo 2^32 to with contains the "ticket_age_add" Diffie-Hellman
   public value that
      was included (Y = g^X mod p), encoded as a big-endian integer, padded
   with zeros to the ticket, see Section 4.4.1.  This addition
      prevents passive observers from correlating sessions unless
      tickets are reused. size of p in bytes.

   Note: because ticket lifetimes are restricted
      to For a week, 32 bits is enough to represent any plausible age, even given Diffie-Hellman group, the padding results in milliseconds.

   A server MUST all
   public keys having the same length.

   Peers SHOULD validate each other's public key Y by ensuring that 1 <
   Y < p-1.  This check ensures that the ticket_age remote peer is within properly behaved
   and isn't forcing the local system into a small
   tolerance of the time since the ticket was issued (see
   Section 4.2.6.2).

   The subgroup.

4.2.4.2.  ECDHE Parameters

   ECDHE parameters for the 0-RTT data (symmetric cipher suite, ALPN,
   etc.) both clients and servers are the same as those which were negotiated encoded in the connection
   which established the PSK.  The PSK used to encrypt the early data
   MUST be the first PSK listed
   opaque key_exchange field of a KeyShareEntry in the client's "pre_shared_key"
   extension.

   0-RTT messages sent in the first flight have the same content types
   as their corresponding messages sent in other flights (handshake,
   application_data, a KeyShare structure.

   For secp256r1, secp384r1 and alert respectively) but secp521r1, the contents are protected under
   different keys.  After all the 0-RTT application data messages (if
   any) have been sent, an "end_of_early_data" alert byte
   string representation of type "warning"
   is sent to indicate an elliptic curve public value following the end
   conversion routine in Section 4.3.6 of the flight.  0-RTT ANSI X9.62 [X962].

   Although X9.62 supports multiple point formats, any given curve MUST always be
   followed by an "end_of_early_data" alert.

   A server which receives an "early_data" extension can behave
   specify only a single point format.  All curves currently specified
   in one
   of two ways:

   -  Ignore this document MUST only be used with the extension uncompressed point format
   (the format for all ECDH functions is considered uncompressed).

   For x25519 and return no response.  This indicates that x448, the server has ignored any early data contents are the byte string inputs and an ordinary 1-RTT
      handshake is required.

   -  Return an empty extension, indicating that it intends to process
   outputs of the early data.  It is not possible corresponding functions defined in [RFC7748], 32 bytes
   for the server x25519 and 56 bytes for x448.

   Note: Versions of TLS prior to accept only 1.3 permitted point negotiation; TLS
   1.3 removes this feature in favor of a subset single point format for each
   curve.

4.2.5.  Pre-Shared Key Extension

   The "pre_shared_key" extension is used to indicate the identity of
   the early data messages.

   In order pre-shared key to accept early data, the server server MUST have accepted be used with a given handshake in association
   with PSK cipher suite and selected the the first key offered in establishment (see [RFC4279] for background).

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

   enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeModes;
   enum { psk_auth(0), psk_sign_auth(1), (255) } PskAuthenticationModes;

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

   struct {
       PskKeMode ke_modes<1..255>;
       PskAuthMode auth_modes<1..255>;
       opaque identity<0..2^16-1>;
   } PskIdentity;

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

           case server:
               uint16 selected_identity;
       }
   } PreSharedKeyExtension;

   identities  A list of the
   client's "pre_shared_key" extension.  In addition, it MUST verify identities (labels for keys) that the following values are consistent
      client is willing to negotiate with those negotiated in the
   connection during which server.  If sent alongside
      the ticket was established.

   -  The TLS version number, symmetric ciphersuite, and "early_data" extension (see Section 4.2.6), the hash first identity
      is the one used for
      HKDF.

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

   - 0-RTT data.

   selected_identity  The server_name [RFC6066] value provided server's chosen identity expressed as a
      (0-based) index into the identies in the client's list.

   Each PSK offered by the client, if any.

   Future extensions MUST define their interaction client also indicates the authentication and
   key exchange modes with 0-RTT.

   If any of these checks fail, which the server MUST NOT respond can use it, with each list
   being in the
   extension and must discard all order of the remaining first flight data (thus
   falling back to 1-RTT).  If client's preference, with most preferred
   first.

   PskKeyExchangeModes have the client attempts a 0-RTT handshake but following meanings:

   psk_ke  PSK-only key establishment.  In this mode, the server rejects it, it will generally MUST
      not have supply a "key_share" value.

   psk_dhe_ke  PSK key establishment with (EC)DHE key establishment.  In
      this mode, the 0-RTT record
   protection keys client and must instead trial decrypt each record with the
   1-RTT handshake keys until it finds one that decrypts properly, and
   then pick up servers MUST supply "key_share" values
      as described in Section 4.2.4.

   PskAuthenticationModes have the handshake from that point.

   If following meanings:

   psk_auth  PSK-only authentication.  In this mode, the server chooses MUST NOT
      supply either a Certificate or CertificateVerify message.  [TODO:
      Add a signing mode.]

   In order to accept PSK key establishment, the "early_data" extension, then it
   MUST comply server sends a
   "pre_shared_key" extension with the same error handling requirements specified for
   all records when processing early data records.  Specifically,
   decryption failure of any 0-RTT record following an accepted
   "early_data" extension selected identity.  Clients MUST produce a fatal "bad_record_mac" alert as
   per Section 5.2.

   If
   verify that the server rejects server's selected_identity is within the "early_data" extension, range
   supplied by the client
   application MAY opt to retransmit and that the data once "key_share" and
   "signature_algorithms" extensions are consistent with the handshake has
   been completed.  TLS stacks SHOULD indicated
   ke_modes and auth_modes values.  If these values are not do this automatically consistent,
   the client MUST generate an "illegal_parameter" alert and close the
   connection.

   If the server supplies an "early_data" extension, the client applications MUST take care
   verify that the negotiated parameters are
   consistent with those it expected.  For example, if server selected the ALPN first offered identity.  If any
   other value
   has changed, it is likely unsafe to retransmit returned, the original
   application layer data.

4.2.6.1.  Processing Order

   Clients are permitted to "stream" client MUST generate a fatal
   "unknown_psk_identity" alert and close the connection.

   Note that although 0-RTT data until they receive is encrypted with the
   server's Finished, only then sending first PSK
   identity, the "end_of_early_data" alert.
   In order server MAY fall back to avoid deadlock, when accepting "early_data", servers MUST
   process the client's Finished 1-RTT and then immediately send the
   ServerHello, rather than waiting for the client's "end_of_early_data"
   alert.

4.2.6.2.  Replay Properties

   As noted in Section 2.3, TLS provides select a limited mechanism for replay
   protection for data sent by different
   PSK identity if multiple identities are offered.

4.2.6.  Early Data Indication

   When PSK resumption is used, the client can send application data in the
   its first flight.

   The "obfuscated_ticket_age" parameter in flight of messages.  If the client's client opts to do so, it MUST
   supply an "early_data" extension SHOULD be used by servers to limit as well as the "pre_shared_key"
   extension.

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

      struct {
          select (Role) {
              case client:
                  uint32 obfuscated_ticket_age;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

   obfuscated_ticket_age  The time over which since the
   first flight might be replayed.  A server can store client learned about the time at which
      server configuration that it sends a session ticket to the client, or encode the time is using, in the
   ticket.  Then, each time it receives an "early_data" extension, it
   can subtract the base milliseconds.  This
      value and check is added modulo 2^32 to see if with the "ticket_age_add" value used by that
      was included with the
   client matches its expectations.

   The ticket, see Section 4.4.1.  This addition
      prevents passive observers from correlating sessions unless
      tickets are reused.  Note: because ticket age (the value with "ticket_age_add" subtracted) provided
   by lifetimes are restricted
      to a week, 32 bits is enough to represent any plausible age, even
      in milliseconds.

   A server MUST validate that the client will be shorter than ticket_age is within a small
   tolerance of the actual time elapsed on since the
   server by a single round trip time.  This difference is comprised of ticket was issued (see
   Section 4.2.6.2).

   The parameters for the delay in sending 0-RTT data (symmetric cipher suite, ALPN,
   etc.) are the NewSessionTicket message to same as those which were negotiated in the client, plus connection
   which established the time taken PSK.  The PSK used to send encrypt the ClientHello to early data
   MUST be the server.  For this
   reason, a server SHOULD measure first PSK listed in the round trip time prior to sending client's "pre_shared_key"
   extension.

   0-RTT messages sent in the NewSessionTicket message and account for that first flight have the same content types
   as their corresponding messages sent in other flights (handshake,
   application_data, and alert respectively) but are protected under
   different keys.  After all the value it
   saves.

   To properly validate 0-RTT application data messages (if
   any) have been sent, an "end_of_early_data" alert of type "warning"
   is sent to indicate the ticket age, a end of the flight.  0-RTT MUST always be
   followed by an "end_of_early_data" alert.

   A server needs to save at least which receives an "early_data" extension can behave in one
   of two items: ways:

   -  The time  Ignore the extension and return no response.  This indicates that
      the server generated the session ticket has ignored any early data and the
      estimated round trip time can be added together to form a baseline
      time. an ordinary 1-RTT
      handshake is required.

   -  The "ticket_age_add" parameter from  Return an empty extension, indicating that it intends to process
      the NewSessionTicket early data.  It is needed
      to recover not possible for the ticket age from server to accept only
      a subset of the "obfuscated_ticket_age"
      parameter.

   There are several potential sources of error that make an exact
   measurement of time difficult.  Variations in client and server
   clocks are likely early data messages.

   In order to be minimal, outside of gross time corrections.
   Network propagation delays are most likely causes of accept early data, the server server MUST have accepted a mismatch
   PSK cipher suite and selected the the first key offered in
   legitimate the
   client's "pre_shared_key" extension.  In addition, it MUST verify
   that the following values for elapsed time.  Both are consistent with those negotiated in the NewSessionTicket and
   ClientHello messages might be retransmitted and therefore delayed,
   connection during which might be hidden by TCP.

   A small allowance for errors in clocks the ticket was established.

   -  The TLS version number, AEAD algorithm, and variations in measurements
   is advisable.  However, any allowance also increases the opportunity hash for replay.  In this case, it is better to reject early data than to
   risk greater exposure to replay attacks.

4.2.7.  OCSP Status Extensions HKDF.

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

   -  The server_name [RFC6066] and [RFC6961] provide value provided by the client, if any.

   Future extensions to negotiate MUST define their interaction with 0-RTT.

   If any of these checks fail, the server
   sending OCSP responses to MUST NOT respond with the client.  In TLS 1.2
   extension and below, must discard all the
   server sends an empty extension remaining first flight data (thus
   falling back to indicate negotiation of this
   extension and 1-RTT).  If the OCSP information is carried in client attempts a CertificateStatus
   message.  In TLS 1.3, 0-RTT handshake but
   the server's OCSP information is carried in an
   extension in EncryptedExtensions.  Specifically: The body of server rejects it, it will generally not have the
   "status_request" or "status_request_v2" extension from 0-RTT record
   protection keys and must instead trial decrypt each record with the server
   MUST be a CertificateStatus structure as defined in [RFC6066]
   1-RTT handshake keys until it finds one that decrypts properly, and
   [RFC6961] respectively.

   Note: This means
   then pick up the handshake from that point.

   If the certificate status appears prior server chooses to accept the
   certificates "early_data" extension, then it applies to.  This is slightly anomalous but matches
   MUST comply with the existing behavior same error handling requirements specified for SignedCertificateTimestamps [RFC6962], and
   is more easily extensible in the handshake state machine.

4.2.8.  Encrypted Extensions

   When this message will be sent:

      In
   all handshakes, the server records when processing early data records.  Specifically,
   decryption failure of any 0-RTT record following an accepted
   "early_data" extension MUST send produce a fatal "bad_record_mac" alert as
   per Section 5.2.

   If the EncryptedExtensions
      message immediately after server rejects the ServerHello message.  This is "early_data" extension, the
      first message that is encrypted under keys derived from
      handshake_traffic_secret.

   Meaning of this message:

      The EncryptedExtensions message contains any extensions which
      should be protected, i.e., any which are not needed client
   application MAY opt to establish retransmit the cryptographic context.

   The same extension types MUST NOT appear in both data once the ServerHello handshake has
   been completed.  TLS stacks SHOULD not do this automatically and
   EncryptedExtensions.  If the same extension appears in both
   locations, the
   client applications MUST rely only on the value in take care that the
   EncryptedExtensions block.  All server-sent extensions other than negotiated parameters are
   consistent with those explicitly listed in Section 4.1.2 or designated in it expected.  For example, if the IANA
   registry MUST only appear in EncryptedExtensions.  Extensions which
   are designated ALPN value
   has changed, it is likely unsafe to appear in ServerHello MUST NOT appear in
   EncryptedExtensions. retransmit the original
   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 "end_of_early_data" alert.
   In order to avoid deadlock, when accepting "early_data", servers MUST check EncryptedExtensions for
   process the
   presence of any forbidden extensions client's Finished and if any are found MUST
   terminate then immediately send the handshake with an "illegal_parameter"
   ServerHello, rather than waiting for the client's "end_of_early_data"
   alert.

   Structure of 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 sent:

      A non-anonymous server can optionally request

4.2.6.2.  Replay Properties

   As noted in Section 2.3, TLS provides a certificate from
      the client, if appropriate limited mechanism for replay
   protection for data sent by the selected cipher suite.  This
      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 client in the
      certificate request and which will be echoed first flight.

   The "obfuscated_ticket_age" parameter in the client's
      Certificate message.  The certificate_request_context MUST "early_data"
   extension SHOULD be
      unique within the scope of this connection (thus preventing replay
      of client CertificateVerify messages).

   supported_signature_algorithms  A list of used by servers to limit the signature algorithms
      that time over which the
   first flight might be replayed.  A server is able to verify, listed can store the time at which
   it sends a session ticket to the client, or encode the time in descending order of
      preference.  Any certificates the
   ticket.  Then, each time it receives an "early_data" extension, it
   can subtract the base value and check to see if the value used by the
   client matches its expectations.

   The ticket age (the value with "ticket_age_add" subtracted) provided
   by the client MUST will be
      signed using shorter than the actual time elapsed on the
   server by a signature algorithm found in
      supported_signature_algorithms.

   certificate_authorities  A list single round trip time.  This difference is comprised of
   the distinguished names [X501] of
      acceptable certificate_authorities, represented delay 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 sending the NewSessionTicket message can be used to describe known roots as well as a
      desired authorization space.  If the certificate_authorities list
      is empty, then client, plus
   the client MAY time taken to send any certificate that meets the
      rest of ClientHello to the selection criteria in server.  For this
   reason, a server SHOULD measure the CertificateRequest, unless
      there is some external arrangement round trip time prior to sending
   the contrary.

   certificate_extensions  A list of certificate extension OIDs
      [RFC5280] with their allowed values, represented NewSessionTicket message and account for that 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 MUST
      contain all of value it
   saves.

   To properly validate the specified extension OIDs ticket age, a server needs to save at least
   two items:

   -  The time that the client
      recognizes.  For each extension OID recognized by server generated the client, all
      of session ticket and the specified values MUST
      estimated round trip time can be present in added together to form a baseline
      time.

   -  The "ticket_age_add" parameter from the client certificate
      (but NewSessionTicket is needed
      to recover the certificate MAY have other values as well).  However, ticket age from the "obfuscated_ticket_age"
      parameter.

   There are several potential sources of error that make an exact
   measurement of time difficult.  Variations in client MUST ignore and skip any unrecognized certificate extension
      OIDs.  If the client has ignored some server
   clocks are likely to be minimal, outside of gross time corrections.
   Network propagation delays are most likely causes of a mismatch in
   legitimate values for elapsed time.  Both the required certificate
      extension OIDs, NewSessionTicket and supplied
   ClientHello messages might be retransmitted and therefore 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 opportunity
   for replay.  In this case, it is better to reject early data and fall
   back to a certificate that does full 1-RTT handshake than to risk greater exposure to
   replay attacks.  In common network topologies for browser clients,
   small allowances on the order of ten seconds are reasonable.  Clock
   skew distributions are not satisfy symmetric, so the request, optimal tradeoff may
   involve an asymmetric replay window.

4.2.7.  OCSP Status Extensions

   [RFC6066] and [RFC6961] provide extensions to negotiate the server MAY at its discretion either continue the
      session without client authentication, or terminate
   sending OCSP responses to the session
      with a fatal unsupported_certificate alert.  PKIX RFCs define a
      variety of certificate extension OIDs client.  In TLS 1.2 and their corresponding
      value types.  Depending on below, the type, matching certificate
   server sends an empty 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  The Key Usage indicate negotiation of this
   extension and the OCSP information is carried in a certificate matches CertificateStatus
   message.  In TLS 1.3, the request
         when all key usage bits asserted server's OCSP information is carried in the request are also
         asserted an
   extension in the Key Usage certificate extension.

      o EncryptedExtensions.  Specifically: The Extended Key Usage body of the
   "status_request" or "status_request_v2" extension in from the server
   MUST be a CertificateStatus structure as defined in [RFC6066] and
   [RFC6961] respectively.

   Note: This means that the certificate status appears prior to the
   certificates it applies to.  This is slightly anomalous but matches
   the
         request when all key purpose OIDs present existing behavior for SignedCertificateTimestamps [RFC6962], and
   is more easily extensible in the request handshake state machine.

4.2.8.  Encrypted Extensions

   When this message will be sent:

      In all handshakes, the server MUST send the EncryptedExtensions
      message immediately after the ServerHello message.  This is the
      first message that is encrypted under keys derived from
      handshake_traffic_secret.

   Meaning of this message:

      The EncryptedExtensions message contains any extensions which
      should be protected, i.e., any which are
         also found in not needed to establish
      the Extended Key Usage certificate extension. cryptographic context.

   The special anyExtendedKeyUsage OID same extension types MUST NOT be used appear in both the ServerHello and
   EncryptedExtensions.  If the same extension appears in both
   locations, the
         request.

      Separate specifications may define matching rules for other
      certificate extensions.

   Note: It is a fatal "handshake_failure" alert for an anonymous server
   to request client authentication.

4.3.  Authentication Messages

   As discussed MUST rely only on the value in the
   EncryptedExtensions block.  All server-sent extensions other than
   those explicitly listed in Section 2, TLS uses a common set of messages 4.1.3 or designated in the IANA
   registry MUST only appear in EncryptedExtensions.  Extensions which
   are designated to appear in ServerHello MUST NOT appear in
   EncryptedExtensions.  Clients MUST check EncryptedExtensions for
   authentication, key confirmation, and handshake integrity:
   Certificate, CertificateVerify, the
   presence of any forbidden extensions and Finished.  These messages if any are
   always sent as found MUST
   terminate the last messages in their handshake flight.  The with an "illegal_parameter" alert.

   Structure of this message:

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

   extensions  A list of extensions.

4.2.9.  Certificate and CertificateVerify messages are only sent under
   certain circumstances, as defined below.  The Finished Request

   When this message will be sent:

      A server which is
   always sent as part of the Authentication block.

   The computations for authenticating with a certificate can optionally
      request a certificate from the Authentication messages all uniformly take client.  This 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 following inputs:

   -  The
      certificate request and signing key to which will be used.

   -  A Handshake Context based on the hash of echoed in the handshake messages

   -  A base key to client's
      Certificate message.  The certificate_request_context MUST be used to compute a MAC key.

   Based on these inputs, the messages then contain:

   Certificate  The certificate to be used for authentication and any
      supporting certificates in
      unique within the chain.  Note that certificate-based scope of this connection (thus preventing replay
      of client authentication is not available in the 0-RTT case. CertificateVerify messages).  Within the handshake, this
      field MUST be empty.

   supported_signature_algorithms  A signature over list of the value Hash(Handshake Context
      + Certificate) + Hash(resumption_context) See Section 4.4.1 for signature algorithms
      that the definition server is able to verify, listed in descending order of resumption_context.

   Finished  A MAC over
      preference.  Any certificates provided by the value Hash(Handshake Context + Certificate +
      CertificateVerify) + Hash(resumption_context) client MUST be
      signed using a MAC key
      derived from the base key.

   Because the CertificateVerify signs the Handshake Context +
   Certificate and the Finished MACs signature algorithm found in
      supported_signature_algorithms.

   certificate_authorities  A list of the Handshake Context + Certificate
   + CertificateVerify, 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 is mostly equivalent message can be used to keeping describe known roots as well as a running
   hash of
      desired authorization space.  If the handshake messages (exactly so in certificate_authorities list
      is empty, then the pure 1-RTT cases).
   Note, however, client MAY send any certificate that subsequent post-handshake authentications do not
   include each other, just the messages through meets the end
      rest of the main
   handshake.

   The following table defines selection criteria in 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 CertificateRequest, unless
      there is used.

4.3.1.  Certificate

   When this message will be sent:

      The server MUST send a Certificate message whenever some external arrangement to the agreed-
      upon key exchange method uses certificates for authentication
      (this includes all key exchange methods defined contrary.

   certificate_extensions  A list of certificate extension OIDs
      [RFC5280] with their allowed values, represented in this document
      except PSK).

      The client MUST send a Certificate message if and only if DER-encoded
      [X690] format.  Some certificate extension OIDs allow multiple
      values (e.g.  Extended Key Usage).  If the server has requested client authentication via included a CertificateRequest
      message (Section 4.2.9).  If
      non-empty certificate_extensions list, the server requests client
      authentication but no suitable certificate is available, MUST
      contain all of the specified extension OIDs that the client MUST send a Certificate message containing no certificates
      (i.e., with
      recognizes.  For each extension OID recognized by the "certificate_list" field having length 0).

   Meaning client, all
      of this message:

      This message conveys the endpoint's specified values MUST be present in the client certificate chain to
      (but the peer.

      The certificate MUST be appropriate for MAY have other values as well).  However, the negotiated cipher
      suite's authentication algorithm
      client MUST ignore and skip any negotiated extensions.

   Structure of this message:

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

      struct {
          opaque certificate_request_context<0..2^8-1>;
          ASN1Cert certificate_list<0..2^24-1>;
      } Certificate;

   certificate_request_context unrecognized certificate extension
      OIDs.  If this message is in response to a
      CertificateRequest, the value client has ignored some of certificate_request_context in the required certificate
      extension OIDs, and supplied a certificate that message.  Otherwise, in does not satisfy
      the request, the case of server authentication
      this field SHALL be zero length.

   certificate_list  This is MAY at its discretion either continue the
      session without client authentication, or terminate the session
      with a sequence (chain) fatal unsupported_certificate alert.  PKIX RFCs define a
      variety of certificates.  The
      sender's certificate MUST come first in extension OIDs and their corresponding
      value types.  Depending on the list.  Each following
      certificate SHOULD directly certify one preceding it.  Because type, matching certificate validation requires
      extension values are not necessarily bitwise-equal.  It is
      expected that trust anchors be distributed
      independently, a TLS implementations will rely on their PKI libraries
      to perform certificate that specifies 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 trust anchor MAY be
      omitted from certificate matches the chain, provided that supported peers request
         when all key usage bits asserted in the request are known to
      possess any omitted certificates.

   Note: Prior to TLS 1.3, "certificate_list" ordering required each also
         asserted in the Key Usage certificate to certify extension.

      o  The Extended Key Usage extension in a certificate matches the one immediately preceding it, however some
   implementations allowed some flexibility.  Servers sometimes send
   both a current and deprecated intermediate for transitional purposes,
   and others are simply configured incorrectly, but these cases can
   nonetheless be validated properly.  For maximum compatibility,
         request when all
   implementations SHOULD be prepared to handle potentially extraneous
   certificates and arbitrary orderings from any TLS version, with key purpose OIDs present in the
   exception of request are
         also found in the end-entity Extended Key Usage certificate which MUST be first. extension.
         The server's certificate list special anyExtendedKeyUsage OID MUST always NOT be non-empty.  A client
   will send an empty used in the
         request.

      Separate specifications may define matching rules for other
      certificate list if it does not have extensions.

   Note: It is a fatal "unexpected_message" alert for an
   appropriate certificate anonymous
   server to send request client authentication.

4.3.  Authentication Messages

   As discussed in response to Section 2, TLS uses a common set of messages for
   authentication, key confirmation, and handshake integrity:
   Certificate, CertificateVerify, and Finished.  These messages are
   always sent as the server's
   authentication request.

4.3.1.1.  Server last messages in their handshake flight.  The
   Certificate Selection and CertificateVerify messages are only sent under
   certain circumstances, as defined below.  The following rules apply to the certificates Finished message is
   always sent by as part of the server:

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

   - Authentication block.

   The server's end-entity certificate's public key (and associated
      restrictions) MUST be compatible with computations for the selected authentication
      algorithm (currently RSA or ECDSA). Authentication messages all uniformly take
   the following inputs:

   -  The certificate MUST allow the and signing 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. used.

   -  The "server_name" and "trusted_ca_keys" extensions [RFC6066] are
      used to guide certificate selection.  As servers MAY require  A Handshake Context based on the
      presence hash of the "server_name" extension, clients SHOULD send this
      extension, when applicable.

   All certificates provided by the server MUST handshake messages

   -  A base key to be signed by a signature
   algorithm that appears in the "signature_algorithms" extension
   provided by the client, if they are able used to provide such compute a chain (see
   Section 4.2.2).  Certificates that are self-signed or certificates
   that are expected MAC key.

   Based on these inputs, the messages then contain:

   Certificate  The certificate to be trust anchors are not validated as part of
   the chain used for authentication and therefore MAY be signed with any algorithm.

   If
      supporting certificates in the server cannot produce a certificate chain chain.  Note that certificate-based
      client authentication is signed only
   via not available in the indicated supported algorithms, then it SHOULD continue 0-RTT case.

   CertificateVerify  A signature over the
   handshake by sending value Hash(Handshake Context
      + Certificate) + Hash(resumption_context) See Section 4.4.1 for
      the client a certificate chain definition of its choice
   that may include algorithms that are not known to be supported by resumption_context.

   Finished  A MAC over the
   client.  This fallback chain MAY use value Hash(Handshake Context + Certificate +
      CertificateVerify) + Hash(resumption_context) using a MAC key
      derived from the deprecated SHA-1 hash
   algorithm only if base key.

   Because the "signature_algorithms" extension provided by CertificateVerify signs the client permits it.  If Handshake Context +
   Certificate and the client cannot construct an acceptable
   chain using the provided certificates and decides to abort the
   handshake, then it MUST send an "unsupported_certificate" alert
   message and close the connection.

   If the server has multiple certificates, it chooses one 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 methods are specified
   for the TLS protocol, they will imply the certificate format and Finished MACs the
   required encoded keying information.

4.3.1.2.  Client Handshake Context + Certificate Selection

   The following rules apply
   + CertificateVerify, this is mostly equivalent to certificates sent by the client:

   In particular:

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

   -  If keeping a running
   hash of the certificate_authorities list handshake messages (exactly so in the certificate request
      message was non-empty, one of pure 1-RTT cases).
   Note, however, that subsequent post-handshake authentications do not
   include each other, just the certificates in messages through the certificate
      chain SHOULD be issued by one end of the listed CAs.

   - main
   handshake.

   The certificates MUST be signed using an acceptable signature
      algorithm, as described in Section 4.2.9.  Note that this relaxes following table defines the constraints on certificate-signing algorithms found in prior
      versions 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 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 the client, as described in
      Section 4.2.9.

   Note that, as with the server certificate, there are certificates
   that use algorithm combinations that cannot be currently used with
   TLS.

4.3.1.3.  Receiving a Certificate Message

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

   If the last three rows does not include
   any 0-RTT handshake messages, regardless of whether 0-RTT is used.

4.3.1.  Certificate

   When this message will be sent:

      The server supplies an empty MUST send a Certificate message, message whenever the agreed-
      upon key exchange method uses certificates for authentication
      (this includes all key exchange methods defined in this document
      except PSK).

      The client MUST
   terminate the handshake with send a fatal "decode_error" alert.

   If the Certificate message if and only if server
      has requested client does not send any certificates, authentication via a CertificateRequest
      message (Section 4.2.9).  If the server MAY at its
   discretion either continue requests client
      authentication but no suitable certificate is available, the handshake without
      client
   authentication, or respond with MUST send a fatal "handshake_failure" alert.
   Also, if some aspect of Certificate message containing no certificates
      (i.e., with the certificate chain was unacceptable (e.g.,
   it was not signed by a known, trusted CA), the server MAY at its
   discretion either continue the handshake (considering "certificate_list" field having length 0).

   Meaning of this message:

      This message conveys the client
   unauthenticated) or send a fatal alert.

   Any endpoint receiving any endpoint's certificate signed using any signature
   algorithm using an MD5 hash MUST send a "bad_certificate" alert
   message and close chain to the connection.  SHA-1 peer.

   Structure of this message:

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

      struct {
          opaque certificate_request_context<0..2^8-1>;
          ASN1Cert certificate_list<0..2^24-1>;
      } Certificate;

   certificate_request_context  If this message is deprecated and therefore
   NOT RECOMMENDED.  All endpoints are RECOMMENDED to transition to
   SHA-256 or better as soon as possible to maintain interoperability
   with implementations currently in response to a
      CertificateRequest, the process value of phasing out SHA-1
   support.

   Note certificate_request_context in
      that message.  Otherwise, in the case of server authentication
      this field SHALL be zero length.

   certificate_list  This is a sequence (chain) of certificates.  The
      sender's certificate containing a key for MUST come first in the list.  Each following
      certificate SHOULD directly certify one signature algorithm
   MAY preceding it.  Because
      certificate validation requires that trust anchors be signed using distributed
      independently, a different signature algorithm (for instance, an
   RSA key signed with an ECDSA key).

   Endpoints certificate that reject certification paths due to use of a deprecated
   hash MUST send specifies a fatal "bad_certificate" alert message before closing
   the connection.

4.3.2.  Certificate Verify

   When this message will trust anchor 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
      omitted from the handshake up chain, provided that supported peers are known to this point.
      possess any omitted certificates.

   Note: Prior to TLS 1.3, "certificate_list" ordering required each
   certificate to certify the one immediately preceding it, however some
   implementations allowed some flexibility.  Servers MUST sometimes send this message when using
   both a cipher suite current and deprecated intermediate for transitional purposes,
   and others are simply configured incorrectly, but these cases can
   nonetheless be validated properly.  For maximum compatibility, all
   implementations SHOULD be prepared to handle potentially extraneous
   certificates and arbitrary orderings from any TLS version, with the
   exception of the end-entity certificate which is
      authenticated via a certificate.  Clients MUST send this message
      whenever authenticating via a Certificate (i.e., when the
      Certificate message is non-empty).  When sent, this message be first.

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

4.3.1.1.  Server Certificate Message and immediately
      prior Selection

   The following rules apply to the Finished message.

   Structure of this message:

      struct {
           SignatureScheme algorithm;
           opaque signature<0..2^16-1>;
      } CertificateVerify; certificates sent by the server:

   -  The algorithm field specifies 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 the signature selected authentication
      algorithm (currently RSA or ECDSA).

   -  The certificate MUST allow the key to be used (see
   Section 4.2.2 for signing (i.e.,
      the definition of this field).  The signature digitalSignature bit MUST be set if the Key Usage extension is
      present) with a
   digital signature using that algorithm that covers scheme indicated in the hash output
   described in Section 4.3 namely:

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

   In TLS 1.3, the digital signature process takes as input:

   -  A signing key

   -  A context string client's
      "signature_algorithms" extension.

   -  The actual content "server_name" and "trusted_ca_keys" extensions [RFC6066] are
      used to be signed

   The digital signature is then computed using the signing key over the
   concatenation of:

   -  64 bytes of octet 32

   -  The context string

   -  A single 0 byte which guide certificate selection.  As servers as the separator

   -  The content to be signed

   This structure is intended to prevent an attack on previous versions
   of previous versions of TLS in which MAY require the ServerKeyExchange format
   meant that attackers could obtain a signature of a message with a
   chosen, 32-byte prefix.  The initial 64 byte pad clears that prefix.

   The context string for a server signature is "TLS 1.3, server
   CertificateVerify" and for a client signature is "TLS 1.3, client
   CertificateVerify".

   For example, if Hash(Handshake Context + Certificate) was 32 bytes of
   01 and Hash(resumption_context) was 32 bytes
      presence of 02 (these lengths
   would make sense for SHA-256, the input to "server_name" extension, clients SHOULD send this
      extension, when applicable.

   All certificates provided by the final signing process
   for a server CertificateVerify would be:

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      544c5320312e332c207365727665722043657274696669636174655665726966
      79
      00
      0101010101010101010101010101010101010101010101010101010101010101
      0202020202020202020202020202020202020202020202020202020202020202

   If sent MUST be signed by a server, the signature
   algorithm MUST be one offered that appears in the client's "signature_algorithms" extension unless no valid
   certificate
   provided by the client, if they are able to provide such a chain can be produced without unsupported algorithms (see
   Section 4.2.2).  Note  Certificates that there is a possibility for inconsistencies
   here.  For instance, the client might offer an ECDHE_ECDSA cipher
   suite but omit any ECDSA and EdDSA values from its
   "signature_algorithms" extension.  In order are self-signed or certificates
   that are expected to negotiate correctly, be trust anchors are not validated as part of
   the server MUST check chain and therefore MAY be signed with any candidate cipher suites against algorithm.

   If the
   "signature_algorithms" extension before selecting them.  This is
   somewhat inelegant but is server cannot produce a compromise designed to minimize changes
   to certificate chain that is signed only
   via the original cipher suite design.

   If sent indicated supported algorithms, then it SHOULD continue the
   handshake by sending the client a client, certificate chain of its choice
   that may include algorithms that are not known to be supported by the signature
   client.  This fallback chain MAY use the deprecated SHA-1 hash
   algorithm used in only if the signature "signature_algorithms" extension provided by
   the client permits it.  If the client cannot construct an acceptable
   chain using the provided certificates and decides to abort the
   handshake, then it MUST be send an "unsupported_certificate" alert
   message and close the connection.

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

4.3.1.2.  Client Certificate Selection

   The following rules apply to certificates sent by the CertificateRequest message. client:

   In addition, the signature algorithm particular:

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

   -  If the key certificate_authorities list in the sender's end-entity certificate.  RSA signatures MUST use an
   RSASSA-PSS algorithm, regardless certificate request
      message was non-empty, one of whether RSASSA-PKCS1-v1_5
   algorithms appear the certificates in "signature_algorithms".  SHA-1 the certificate
      chain SHOULD be issued by one of the listed CAs.

   -  The certificates MUST NOT be used
   in any signatures in CertificateVerify.  All SHA-1 signed using an acceptable signature
   algorithms
      algorithm, as described in Section 4.2.9.  Note that this specification relaxes
      the 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 match the
      extension OIDs recognized by the client, as described in
      Section 4.2.9.

   Note that, as with the server certificate, there are defined solely for certificates
   that use in legacy
   certificates, and algorithm combinations that cannot be currently used with
   TLS.

4.3.1.3.  Receiving a Certificate Message

   In general, detailed certificate validation procedures are not valid out of
   scope for CertificateVerify signatures.

   Note: When used TLS (see [RFC5280]).  This section provides TLS-specific
   requirements.

   If the server supplies an empty Certificate message, the client MUST
   terminate the handshake with non-certificate-based handshakes (e.g., PSK), a fatal "decode_error" alert.

   If the client's signature client does not cover send any certificates, the server's certificate
   directly, although it does cover server MAY at its
   discretion either continue the server's Finished message, which
   transitively includes handshake without client
   authentication, or respond with a fatal "handshake_failure" alert.
   Also, if some aspect of the server's certificate when the PSK derives
   from a certificate-authenticated handshake.  [PSK-FINISHED] describes chain was unacceptable (e.g.,
   it was not signed by a concrete attack on this mode if known, trusted CA), the Finished is omitted from server MAY at its
   discretion either continue the
   signature.  It is unsafe to use certificate-based client
   authentication when handshake (considering the client might potentially share
   unauthenticated) or send a fatal alert.

   Any endpoint receiving any certificate signed using any signature
   algorithm using an MD5 hash MUST send a "bad_certificate" alert
   message and close the same PSK/
   key-id pair connection.  SHA-1 is deprecated and therefore
   NOT RECOMMENDED.  All endpoints are RECOMMENDED to transition to
   SHA-256 or better as soon as possible to maintain interoperability
   with two implementations currently in the process of phasing out SHA-1
   support.

   Note that a certificate containing a key for one signature algorithm
   MAY be signed using a different endpoints.  In order signature algorithm (for instance, an
   RSA key signed with an ECDSA key).

   Endpoints that reject certification paths due to ensure this,
   implementations use of a deprecated
   hash MUST NOT mix certificate-based client authentication
   with pure PSK modes (i.e., those where the PSK was not derived from send a
   previous non-PSK handshake).

4.3.3.  Finished fatal "bad_certificate" alert message before closing
   the connection.

4.3.2.  Certificate Verify

   When this message will be sent:

      The Finished

      This message is used to provide explicit proof that an endpoint
      possesses the final message in the authentication
      block.  It is essential private key corresponding to its certificate and
      also provides integrity for providing authentication of the handshake and of the computed keys.

   Meaning of up to this message:

      Recipients of Finished messages point.
      Servers MUST verify that the contents are
      correct.  Once send this message when authenticating via a side has sent its Finished
      certificate.  Clients MUST send this message whenever
      authenticating via a Certificate (i.e., when the Certificate
      message is non-empty).  When sent, this message MUST appear
      immediately after the Certificate Message and received
      and validated immediately prior to
      the Finished message from its peer, it may begin to
      send and receive application data over the connection.

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

   Structure of this message:

      struct {
           SignatureScheme algorithm;
           opaque verify_data[Hash.length]; signature<0..2^16-1>;
      } Finished; CertificateVerify;

   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 algorithm field specifies the Hash signature algorithm used (see
   Section 4.2.2 for the handshake.  As
   noted above, the HMAC input can generally be implemented by definition of this field).  The signature is a running
   hash, i.e., just
   digital signature using that algorithm that covers the handshake hash at this point.

   In previous versions of TLS, the verify_data was always 12 octets
   long. output
   described in Section 4.3 namely:

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

   In TLS 1.3, the current version of TLS, it digital signature process takes as input:

   -  A signing key
   -  A context string

   -  The actual content to be signed

   The digital signature is then computed using the size of the HMAC
   output for the Hash used for signing key over the handshake.

   Note: Alerts and any other record types are not handshake messages
   and are not included in
   concatenation of:

   -  64 bytes of octet 32

   -  The context string

   -  A single 0 byte which servers as the hash computations.

4.4.  Post-Handshake Messages

   TLS also allows other messages separator

   -  The content to be sent after the 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 after the server has received signed

   This structure is intended to prevent an attack on previous versions
   of previous versions of TLS in which the client Finished
   message, it MAY send ServerKeyExchange format
   meant that attackers could obtain a signature of a NewSessionTicket message.  This message
   creates with a pre-shared key (PSK) binding between the ticket value
   chosen, 32-byte prefix.  The initial 64 byte pad clears that prefix.

   The context string for a server signature is "TLS 1.3, server
   CertificateVerify" and for a client signature is "TLS 1.3, client
   CertificateVerify".

   For example, if Hash(Handshake Context + Certificate) was 32 bytes of
   01 and Hash(resumption_context) was 32 bytes of 02 (these lengths
   would make sense for SHA-256, the following two values derived from input to 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 client MAY use this PSK final signing process
   for future handshakes a server CertificateVerify would be:

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      544c5320312e332c207365727665722043657274696669636174655665726966
      79
      00
      0101010101010101010101010101010101010101010101010101010101010101
      0202020202020202020202020202020202020202020202020202020202020202

   If sent by including a server, the
   ticket value signature algorithm MUST be one offered in
   the "pre_shared_key" client's "signature_algorithms" extension in its ClientHello
   (Section 4.2.5) and supplying a suitable PSK cipher suite.  Servers
   may send multiple tickets on a single connection, for instance after
   post-handshake authentication.  For handshakes that do not use unless no valid
   certificate chain can be produced without unsupported algorithms (see
   Section 4.2.2).

   If sent by a
   resumption_psk, client, the resumption_context is 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 ways signature algorithm used in which this ticket may the signature
   MUST be
      used (as a bitwise OR one of those present in the flags values).

   ticket_lifetime  Indicates supported_signature_algorithms
   field of the lifetime in seconds as a 32-bit
      unsigned integer CertificateRequest message.

   In addition, the signature algorithm MUST be compatible with the key
   in network byte order from the time of ticket
      issuance.  Servers sender's end-entity certificate.  RSA signatures MUST NOT use any value more than 604800 seconds
      (7 days).  The value an
   RSASSA-PSS algorithm, regardless of zero indicates that the ticket should be
      discarded immediately.  Clients whether RSASSA-PKCS1-v1_5
   algorithms appear in "signature_algorithms".  SHA-1 MUST NOT cache session tickets be used
   in any signatures in CertificateVerify.  All SHA-1 signature
   algorithms in this specification are defined solely for
      longer than 7 days, regardless of use in legacy
   certificates, and are not valid for CertificateVerify signatures.

   Note: When used with non-certificate-based handshakes (e.g., PSK),
   the ticket_lifetime.  It MAY
      delete client's signature does not cover the ticket earlier based on local policy.  A server MAY
      treat server's certificate
   directly, although it does cover the server's Finished message, which
   transitively includes the server's certificate when the PSK derives
   from a ticket as valid for certificate-authenticated handshake.  [PSK-FINISHED] describes
   a shorter period of time than what concrete attack on this mode if the Finished is
      stated in omitted from the ticket_lifetime.

   ticket_age_add  A randomly generated 32-bit value that
   signature.  It is used unsafe to
      obscure the age of the ticket that use certificate-based client
   authentication when the client includes in might potentially share the
      "early_data" extension.  The actual ticket age is added to this
      value modulo 2^32 same PSK/
   key-id pair with two different endpoints.  In order to obtain ensure this,
   implementations MUST NOT mix certificate-based client authentication
   with pure PSK modes (i.e., those where the value that PSK was not derived from a
   previous non-PSK handshake).

4.3.3.  Finished

   When this message will be sent:

      The Finished message is transmitted by the
      client.

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

   ticket  The value authentication
      block.  It is essential for providing authentication of the ticket to be used as the PSK identifier.
      The ticket itself is an opaque label.  It MAY either be a database
      lookup key or a self-encrypted
      handshake 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 flight (early data) encrypted under a key
      derived from computed keys.

   Meaning of 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
   offers/uses message:

      Recipients of Finished messages MUST have the same symmetric parameters (cipher/hash) as verify that the cipher suite negotiated for this connection.  If no flags contents are set
   that
      correct.  Once a side has sent its Finished message and received
      and validated the client recognizes, Finished message from its peer, it MUST ignore may begin to
      send and receive application data over the ticket.

4.4.2.  Post-Handshake Authentication connection.

   The server is permitted key used to request client authentication at any time
   after compute the handshake has completed finished message is computed from the
   Base key defined in Section 4.3 using HKDF (see Section 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 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 Hash algorithm for the handshake.  As
   noted above, the HMAC input can generally be implemented by sending a CertificateRequest
   message.  The client SHOULD respond with running
   hash, i.e., just the appropriate
   Authentication messages.  If handshake hash at this point.

   In previous versions of TLS, the client chooses to authenticate, it
   MUST send Certificate, CertificateVerify, and Finished.  If it
   declines, verify_data was always 12 octets
   long.  In the current version of TLS, it MUST send a Certificate message containing no
   certificates followed by Finished.

   Note: Because client authentication may require prompting is the user,
   servers MUST be prepared for some delay, including receiving an
   arbitrary number size of the HMAC
   output for the Hash used for the handshake.

   Note: Alerts and any other record types are not handshake messages between sending the
   CertificateRequest
   and receiving are not included in the hash computations.

   Any records following a response. 1-RTT Finished message MUST be encrypted
   under the application traffic key.  In addition, clients
   which receive multiple CertificateRequests particular, this includes any
   alerts sent by the server in close succession MAY
   respond response to them in a different order than they were received (the
   certificate_request_context value allows the server to disambiguate
   the responses).

4.4.3.  Key client Certificate and IV Update

    struct {} KeyUpdate;

   The KeyUpdate handshake message is used
   CertificateVerify messages.

4.4.  Post-Handshake Messages

   TLS also allows other messages to indicate that the sender
   is updating its sending cryptographic keys.  This message can be sent
   by the server after sending its first flight and the client after
   sending its second flight.  Implementations that receive a KeyUpdate
   message prior to receiving main handshake.
   These messages use a Finished message as part of the 1-RTT handshake MUST generate a fatal "unexpected_message" alert.  After
   sending a KeyUpdate message, content type and are encrypted under
   the sender SHALL send all its application traffic
   using the next generation of keys, computed as described in
   Section 7.2.  Upon receiving a KeyUpdate, key.

   Handshake messages sent after the receiver handshake MUST update
   their receiving keys and NOT be interleaved
   with other record types.  That is, if they have not already updated their
   sending state up to a message is split over two or past the then current receiving generation
   more handshake records, there MUST send their own KeyUpdate prior to sending NOT be 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 records between sending
   them.

4.4.1.  New Session Ticket Message

   At any time after the server has received the client Finished
   message, it MAY send a KeyUpdate NewSessionTicket message.  This message
   creates a pre-shared key (PSK) binding between the ticket value 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, 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 client MAY use this only results PSK for future handshakes by including the
   ticket value in an update of one
   generation; when each side receives the other side's update it just
   updates "pre_shared_key" extension in its receive keys and notes that ClientHello
   (Section 4.2.5).  Servers MAY send multiple tickets on a single
   connection, either immediately after each other or after specific
   events.  For instance, the generations match and
   thus no server might send update is needed.

   Note that a new ticket after post-
   handshake authentication in order to encapsulate the side which sends its KeyUpdate first needs additional
   client authentication state.  Clients SHOULD attempt to retain
   its receive traffic keys (though use each
   ticket no more than once, with more recent tickets being used first.
   For handshakes that do not use a resumption_psk, the traffic secret) for the
   previous generation
   resumption_context is a string of keys until it receives the KeyUpdate from the
   other side.

   Both sender and receiver MUST encrypt their KeyUpdate messages Hash.Length zeroes.  [[Note: this
   will not be safe if/when we add additional server signatures with
   the old keys.  Additionally, both sides
   PSK: OPEN ISSUE https://github.com/tlswg/tls13-spec/issues/558]]

   Any ticket MUST enforce that a KeyUpdate only be resumed with the old key a cipher suite that is received before accepting any messages encrypted
   with the new key.  Failure to do so may allow message truncation
   attacks.

5.  Record Protocol

   The TLS record protocol takes messages identical
   to be transmitted, fragments that negotiated connection where the data into manageable blocks, protects the records, and transmits
   the result.  Received data is decrypted and verified, reassembled,
   and then delivered to higher-level clients.

   TLS records are typed, ticket was established.

    enum { (65535) } TicketExtensionType;

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

    struct {
        uint32 ticket_lifetime;
        PskKeMode ke_modes<1..255>;
        PskAuthMode auth_modes<1..255>;
        opaque ticket<1..2^16-1>;
        TicketExtension extensions<0..2^16-2>;
    } NewSessionTicket;

   ke_modes  The key exchange modes with which allows multiple higher level protocols
   to this ticket can be multiplexed over the same record layer.  This document
   specifies three content types: handshake, application data, and
   alert.  Implementations MUST NOT send record types not defined used
      in descending order of server preference.

   auth_modes  The authentication modes with which this document unless negotiated by some extension.  If a TLS
   implementation receives an unexpected record type, it MUST send an
   "unexpected_message" alert.  New record content type values are
   assigned by IANA ticket can be
      used in descending order of server preference.

   ticket_lifetime  Indicates the TLS Content Type Registry as described lifetime in
   Section 10.

   Application data messages are carried by the record layer and are
   fragmented and encrypted as described below.  The messages are
   treated seconds as transparent data to the record layer.

5.1.  Record Layer

   The TLS record layer receives uninterpreted data from higher layers a 32-bit
      unsigned integer in non-empty blocks network byte order from the time of arbitrary size. ticket
      issuance.  Servers MUST NOT use any value more than 604800 seconds
      (7 days).  The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Message
   boundaries are not preserved in the record layer (i.e., multiple
   messages value of zero indicates that the same ContentType MAY ticket should be coalesced into a single
   TLSPlaintext record, or a single message MAY be fragmented across
   several records).  Alert messages (Section 6)
      discarded immediately.  Clients 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 higher-level protocol used to process cache session tickets for
      longer than 7 days, regardless of the enclosed
      fragment.

   record_version  The protocol version ticket_lifetime.  It MAY
      delete the current record is compatible
      with.  This value MUST be set to { 3, 1 } for all records.  This
      field is deprecated and MUST be ignored ticket earlier based on local policy.  A server MAY
      treat a ticket as valid for all purposes.

   length  The length (in bytes) a shorter period of time than what is
      stated in the following TLSPlaintext.fragment.
      The length MUST NOT exceed 2^14.

   fragment ticket_lifetime.

   ticket  The data being transmitted.  This value transparent and
      treated as an independent block of the ticket to be dealt with by the higher-
      level protocol specified by the type field.

   This document describes TLS Version 1.3, which uses used as the version { 3,
   4 }. PSK identifier.
      The version value 3.4 ticket itself is historical, deriving from the use an opaque label.  It MAY either be a database
      lookup key or a self-encrypted and self-authenticated value.
      Section 4 of {
   3, 1 } [RFC5077] describes a recommended ticket construction
      mechanism.

   ticket_extensions  A set of extension values for TLS 1.0 and the ticket.  Clients
      MUST ignore unrecognized extensions.

   This document defines one ticket extension, "ticket_early_data_info"

      struct { 3, 0
          uint32 ticket_age_add;
      } for SSL 3.0.  In order to maximize
   backwards compatibility, the record layer version identifies as
   simply TLS 1.0.  Endpoints supporting other versions negotiate TicketEarlyDataInfo;

   This extension indicates that the
   version ticket may be used 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 of Application 0-RTT
   data MAY be sent as they are potentially
   useful as a traffic analysis countermeasure.

   When record protection has not yet been engaged, TLSPlaintext
   structures are written directly onto (Section 4.2.6)).  It contains one value:

   ticket_age_add  A randomly generated 32-bit value that is used to
      obscure the wire.  Once record
   protection has started, TLSPlaintext records are protected and sent
   as described age of the ticket that the client includes in the following section.

5.2.  Record Payload Protection
      "early_data" extension.  The record protection functions translate a TLSPlaintext structure
   into client-side ticket age is added to
      this value modulo 2^32 to obtain the value that is transmitted by
      the client.

4.4.2.  Post-Handshake Authentication

   The server is permitted to request client authentication at any time
   after the handshake has completed by sending a TLSCiphertext. CertificateRequest
   message.  The deprotection functions reverse client SHOULD respond with the
   process.  In TLS 1.3 as opposed appropriate
   Authentication messages.  If the client chooses to previous versions of TLS, all
   ciphers are modeled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide a unified encryption 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 operation which turns plaintext into authenticated
   ciphertext and back again.  Each encrypted record consists of a
   plaintext header followed by an encrypted body, which itself contains
   a type and optional padding.

   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;
       opaque encrypted_record[length];
   } 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 the
      cleartext after the type field.  This provides an opportunity for
      senders to pad any TLS record by a chosen amount as long as require prompting the
      total stays within record size limits.  See Section 5.4 user,
   servers MUST be prepared for more
      details.

   opaque_type  The outer opaque_type field some delay, including receiving an
   arbitrary number of other messages between sending the
   CertificateRequest and receiving a TLSCiphertext record is
      always set response.  In addition, clients
   which receive multiple CertificateRequests in close succession MAY
   respond to the them in a different order than they were received (the
   certificate_request_context value 23 (application_data) for outward
      compatibility with middleboxes accustomed allows the server to parsing previous
      versions of TLS.  The actual content type of disambiguate
   the record is found
      in fragment.type after decryption.

   record_version responses).

4.4.3.  Key and IV Update

    struct {} KeyUpdate;

   The record_version field KeyUpdate handshake message is identical used to
      TLSPlaintext.record_version and is always { 3, 1 }.  Note indicate that the
      handshake protocol including sender
   is updating its sending cryptographic keys.  This message can be sent
   by the ClientHello server after sending its first flight and ServerHello
      messages authenticates the protocol version, so this value is
      redundant.

   length  The length (in bytes) of the following
      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 client after
   sending its second flight.  Implementations that receives receive a
      record that exceeds this length KeyUpdate
   message prior to receiving a Finished message as part of the 1-RTT
   handshake MUST generate a fatal
      "record_overflow" "unexpected_message" alert.

   encrypted_record  The AEAD encrypted form of the serialized
      TLSInnerPlaintext structure.

   AEAD ciphers take as input a single key, a nonce,  After
   sending a plaintext, and
   "additional data" to be included in KeyUpdate message, the authentication check, sender SHALL send all its traffic
   using the next generation of keys, computed as described in
   Section 2.1 of [RFC5116].  The key is either 7.2.  Upon receiving a KeyUpdate, the
   client_write_key receiver MUST update
   their receiving keys and if they have not already updated their
   sending state up to or past the server_write_key, the nonce is derived from 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 sequence entire
   connection.  Note that implementations may receive an arbitrary
   number (see Section 5.3) and the client_write_iv or
   server_write_iv, of messages between sending a KeyUpdate and receiving the additional data input is empty (zero
   length).  Derivation of traffic keys is defined
   peer's KeyUpdate because those messages may already be in Section 7.3.

   The plaintext is the concatenation of TLSPlaintext.fragment,
   TLSPlaintext.type, flight.

   Note that if implementations independently send their own KeyUpdates
   and any padding bytes (zeros).

   The AEAD output consists they cross in flight, this only results in an update of one
   generation; when each side receives the ciphertext output by the AEAD
   encryption operation.  The length of other side's update it just
   updates its receive keys and notes that the plaintext generations match and
   thus no send update is greater than
   TLSPlaintext.length due needed.

   Note that the side which sends its KeyUpdate first needs to retain
   its receive traffic keys (though not the inclusion of TLSPlaintext.type and
   however much padding is supplied by traffic secret) for the sender.  The length
   previous generation of keys until it receives the
   AEAD output will generally be larger than KeyUpdate from the plaintext, but by an
   amount
   other side.

   Both sender and receiver MUST encrypt their KeyUpdate messages with
   the old keys.  Additionally, both sides MUST enforce that varies a KeyUpdate
   with the AEAD cipher.  Since old key is received before accepting any messages encrypted
   with the ciphers might
   incorporate padding, new key.  Failure to do so may allow message truncation
   attacks.

4.5.  Handshake Layer and Key Changes

   Handshake messages MUST NOT span key changes.  Because the amount
   ServerHello, Finished, and KeyUpdate messages signal a key change,
   upon receiving these messages a receiver MUST verify that the end of overhead could vary
   these messages aligns with different
   lengths of plaintext.  Symbolically,

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

   In order a record boundary; if not, then it MUST
   send a fatal "unexpected_message" alert.

5.  Record Protocol

   The TLS record protocol takes messages to decrypt and verify, be transmitted, fragments
   the cipher takes as input data into manageable blocks, protects the key,
   nonce, records, and transmits
   the AEADEncrypted value.  The output result.  Received data is either the
   plaintext or an error indicating that the decryption failed.  There
   is no separate integrity check.  That is:

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

   If the decryption fails, a fatal "bad_record_mac" alert MUST decrypted and verified, reassembled,
   and then delivered to higher-level clients.

   TLS records are typed, which allows multiple higher level protocols
   to be
   generated.

   An AEAD cipher multiplexed over the same record layer.  This document
   specifies three content types: handshake, application data, and
   alert.  Implementations MUST NOT produce an expansion of greater than 255
   bytes.  An endpoint that receives send record types not defined in
   this document unless negotiated by some extension.  If a TLS
   implementation receives an unexpected record from its peer with
   TLSCipherText.length larger than 2^14 + 256 octets type, it MUST generate a
   fatal "record_overflow" send an
   "unexpected_message" alert.  This limit is derived from  New record content type values are
   assigned by IANA in the
   maximum TLSPlaintext length of 2^14 octets + 1 octet for ContentType
   + TLS Content Type Registry as described in
   Section 10.

   Application data messages are carried by the maximum AEAD expansion of 255 octets.

5.3.  Per-Record Nonce

   A 64-bit sequence number is maintained separately for reading record layer and
   writing records.  Each sequence number is set to zero at the
   beginning of a connection are
   fragmented and whenever the key is changed. encrypted as described below.  The sequence number is incremented after reading or writing each
   record. messages are
   treated as transparent data to the record layer.

5.1.  Record Layer

   The first TLS record transmitted under a particular set layer receives uninterpreted data from higher layers
   in non-empty blocks of
   traffic keys arbitrary size.

   The record key MUST use sequence number 0.

   Sequence numbers do layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Message
   boundaries are not wrap.  If preserved in the record layer (i.e., multiple
   messages of the same ContentType MAY be coalesced into a TLS implementation would need to
   wrap single
   TLSPlaintext record, or a sequence number, it MUST either rekey single message MAY be fragmented across
   several records).  Alert messages (Section 4.4.3) or
   terminate the connection. 6) MUST NOT be fragmented
   across records.

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

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

   type  The length of higher-level protocol used to process the per-record nonce (iv_length) is enclosed
      fragment.

   legacy_record_version  This value MUST be set to max(8 bytes,
   N_MIN) { 3, 1 } for the AEAD algorithm (see [RFC5116] Section 4).  An AEAD
   algorithm where N_MAX all
      records.  This field is less than 8 bytes deprecated and MUST NOT be used with TLS.
   The per-record nonce ignored for the AEAD construction is formed as follows:

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

   2. all
      purposes.

   length  The padded sequence number is XORed with the static
       client_write_iv or server_write_iv, depending on length (in bytes) of the role. following TLSPlaintext.fragment.
      The resulting quantity (of length iv_length) is used MUST NOT exceed 2^14.

   fragment  The data being transmitted.  This value transparent and
      treated as an independent block to be dealt with by the per-
   record nonce.

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

5.4.  Record Padding

   All encrypted TLS records can be padded to inflate the size of by the
   TLSCipherText. type field.

   This allows the sender to hide the size of document describes TLS Version 1.3, which uses the
   traffic from an observer.

   When generating a TLSCiphertext record, implementations MAY choose to
   pad.  An unpadded record is just a record with a padding length of
   zero.  Padding version { 3,
   4 }.  The version value 3.4 is a string historical, deriving from the use of zero-valued bytes appended {
   3, 1 } for TLS 1.0 and { 3, 0 } for SSL 3.0.  In order to maximize
   backwards compatibility, the
   ContentType field before encryption.  Implementations MUST set record layer version identifies as
   simply TLS 1.0.  Endpoints supporting other versions negotiate the
   padding octets
   version to all zeros before encrypting.

   Application Data records may contain a zero-length fragment.content
   if use by following the sender desires.  This permits generation of plausibly-sized
   cover traffic procedure and requirements in contexts where the presence or absence of activity
   may be sensitive.
   Appendix C.

   Implementations MUST NOT send zero-length fragments of Handshake or
   Alert
   records that have a zero-length fragment.content.

   The padding types, even if those fragments contain padding.  Zero-length
   fragments of Application data MAY be sent is automatically verified by the as they are potentially
   useful as a traffic analysis countermeasure.

   When record protection
   mechanism: Upon successful decryption of a TLSCiphertext.fragment, has not yet been engaged, TLSPlaintext
   structures are written directly onto the receiving implementation scans wire.  Once record
   protection has started, TLSPlaintext records are protected and sent
   as described in the field from the end toward the
   beginning until it finds following section.

5.2.  Record Payload Protection

   The record protection functions translate a non-zero octet.  This non-zero octet is
   the content type of TLSPlaintext structure
   into a TLSCiphertext.  The deprotection functions reverse the message.  This padding scheme was selected
   because it allows padding
   process.  In TLS 1.3 as opposed to previous versions of any 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.  Each encrypted TLS record consists of a
   plaintext header followed by an arbitrary
   size (from zero up to encrypted body, which itself contains
   a type and optional padding.

   struct {
      opaque content[TLSPlaintext.length];
      ContentType type;
      uint8 zeros[length_of_padding];
   } TLSInnerPlaintext;

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion legacy_record_version = { 3, 1 };    /* TLS record size limits) without introducing new v1.x */
       uint16 length;
       opaque encrypted_record[length];
   } TLSCiphertext;

   content types.  The design also enforces all-zero padding octets,
   which allows for quick detection of padding errors.

   Implementations MUST limit their scanning to the cleartext returned
   from of TLSPlaintext.fragment.

   type  The content type of the AEAD decryption.  If a receiving implementation does not
   find a non-zero octet record.

   zeros  An arbitrary-length run of zero-valued bytes may appear in the cleartext, it should treat
      cleartext after the type field.  This provides an opportunity for
      senders to pad any TLS record by a chosen amount as long as
   having an unexpected ContentType, sending an "unexpected_message"
   alert.

   The presence of padding does not change the overall
      total stays within record size
   limitations - the full fragment plaintext may not exceed 2^14 octets.

   Selecting a padding policy that suggests when and how much to pad is limits.  See Section 5.4 for more
      details.

   opaque_type  The outer opaque_type field of a complex topic, and TLSCiphertext record is beyond
      always set to the scope value 23 (application_data) for outward
      compatibility with middleboxes accustomed to parsing previous
      versions of TLS.  The actual content type of this specification.  If the application layer protocol atop TLS has its own padding padding,
   it may be preferable record is found
      in fragment.type after decryption.

   legacy_record_version  The legacy_record_version field is identical
      to pad application_data TLS records within TLSPlaintext.legacy_record_version and is always { 3, 1 }.
      Note that the
   application layer.  Padding for encrypted handshake and alert TLS
   records must still be handled at protocol including the 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 cryptographic limits on ClientHello and
      ServerHello messages authenticates the amount protocol version, so this
      value is redundant.

   length  The length (in bytes) of plaintext the following
      TLSCiphertext.fragment, which can
   be safely encrypted under a given set is the sum of keys.  [AEAD-LIMITS]
   provides an analysis the lengths of these limits under the assumption that
      content and the
   underlying primitive (AES or ChaCha20) has no weaknesses.
   Implementations SHOULD do a 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 a
   given connection while keeping a safety margin of approximately 2^-57 padding, plus one for Authenticated Encryption (AE) security.  For ChaCha20/Poly1305,
   the record sequence number will wrap before the safety limit is
   reached.

6.  Alert Protocol

   One of the inner content types supported by the TLS record layer is the
   alert type.  Like other messages, alert messages are  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.

   encrypted_record  The AEAD encrypted as
   specified by form of the current connection state.

   Alert messages convey serialized
      TLSInnerPlaintext structure.

   AEAD ciphers take as input a single key, a nonce, a plaintext, and
   "additional data" to be included in the severity authentication check, as
   described in Section 2.1 of [RFC5116].  The key is either the message (warning
   client_write_key or fatal)
   and a description of the alert.  Warning-level messages are used to
   indicate orderly closure of server_write_key, the connection nonce is derived from
   the sequence number (see Section 6.1).  Upon
   receiving a warning-level alert, the TLS implementation SHOULD
   indicate end-of-data to 5.3) and the application and, if appropriate for client_write_iv or
   server_write_iv, and the
   alert type, send a closure alert in response.

   Fatal-level messages are used to indicate abortive closure additional data input is empty (zero
   length).  Derivation of the
   connection (See traffic keys is defined in Section 6.2).  Upon receiving a fatal-level alert,
   the TLS implementation SHOULD indicate an error to 7.3.

   The plaintext is the application concatenation of TLSPlaintext.fragment,
   TLSPlaintext.type, and MUST NOT allow any further data padding bytes (zeros).

   The AEAD output consists of the ciphertext output by the AEAD
   encryption operation.  The length of the plaintext is greater than
   TLSPlaintext.length due to be sent or received on the
   connection.  Servers and clients MUST forget keys inclusion of TLSPlaintext.type and secrets
   associated with a failed connection.  Stateful implementations
   however much padding is supplied by the sender.  The length of
   session tickets (as in many clients) SHOULD discard tickets
   associated with failed connections.

   All the alerts listed in Section 6.2 MUST
   AEAD output will generally be sent as fatal larger than the plaintext, but by an
   amount that varies with 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)

   In order to decrypt and MUST
   be treated verify, the cipher takes as fatal regardless of input the AlertLevel in key,
   nonce, and the message.
   Unknown alert types 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 AEADEncrypted value.  The client and output is either the server must share knowledge
   plaintext or an error indicating that the connection decryption failed.  There
   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 recipient that no separate integrity check.  That is:

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

   If the sender will
      not send any more messages on this connection.  Any data received
      after decryption fails, a closure fatal "bad_record_mac" alert MUST be ignored.

   end_of_early_data  This alert is sent by 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 the end of the flight.  This
      alert MUST be at the warning level.  Servers
   generated.

   An AEAD cipher MUST NOT send this
      alert and clients receiving it MUST terminate the connection with produce an "unexpected_message" expansion 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.

   user_canceled  This alert notifies the recipient that the sender limit is
      canceling derived from the handshake
   maximum TLSPlaintext length of 2^14 octets + 1 octet for some reason unrelated to a protocol
      failure.  If a user cancels an operation after ContentType
   + the handshake maximum AEAD expansion of 255 octets.

5.3.  Per-Record Nonce

   A 64-bit sequence number is
      complete, just closing maintained separately for reading and
   writing records.  Each sequence number is set to zero at the connection by sending
   beginning of a "close_notify" connection and whenever the key is more appropriate.  This alert SHOULD be followed by a
      "close_notify".  This alert changed.

   The sequence number is generally a warning.

   Either party MAY initiate a close by sending a "close_notify" alert.
   Any data received incremented after reading or writing each
   record.  The first record transmitted under a closure alert is ignored. particular set of
   traffic keys record key MUST use sequence number 0.

   Sequence numbers do not wrap.  If a transport-
   level close is received prior TLS implementation would need to
   wrap a "close_notify", sequence number, it MUST either rekey (Section 4.4.3) or
   terminate the receiver
   cannot know that all connection.

   The length of the data that was sent has been received.

   Each party MUST send a "close_notify" alert before closing the write
   side of per-record nonce (iv_length) is set to max(8 bytes,
   N_MIN) for the connection, unless some other fatal alert has been
   transmitted.  The other party AEAD algorithm (see [RFC5116] Section 4).  An AEAD
   algorithm where N_MAX is less than 8 bytes MUST respond NOT be used with a "close_notify"
   alert of its own and close down the connection immediately,
   discarding any pending writes. TLS.
   The initiator of the close need not
   wait per-record nonce for the responding "close_notify" alert before closing the read
   side of the connection.

   If AEAD construction is formed as follows:

   1.  The 64-bit record sequence number is padded to the application protocol using TLS provides that any data may be
   carried over left with
       zeroes to iv_length.

   2.  The padded sequence number is XORed with the underlying transport after static
       client_write_iv or server_write_iv, depending on the TLS connection role.

   The resulting quantity (of length iv_length) is
   closed, used as the per-
   record nonce.

   Note: This is a different construction from that in TLS implementation must receive the responding
   "close_notify" alert before indicating 1.2, which
   specified a partially explicit nonce.

5.4.  Record Padding

   All encrypted TLS records can be padded to inflate the application layer that size of the TLS connection has ended.  If
   TLSCipherText.  This allows the application protocol will not
   transfer any additional data, but will only close sender to hide the underlying
   transport connection, then size of the implementation
   traffic from an observer.

   When generating a TLSCiphertext record, implementations MAY choose to close the
   transport without waiting for the responding "close_notify".  No part
   of this standard should be taken to dictate the manner in which
   pad.  An unpadded record is just a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

   Note: It record with a padding length of
   zero.  Padding is assumed that closing a connection reliably delivers
   pending data string of zero-valued bytes appended to the
   ContentType field before destroying encryption.  Implementations MUST set the transport.

6.2.  Error Alerts

   Error handling
   padding octets to 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 TLS presence or absence of activity
   may be sensitive.  Implementations MUST NOT send Handshake Protocol is very simple.  When an
   error or Alert
   records that have a zero-length fragment.content.

   The padding sent is detected, automatically verified by the detecting party sends a message to its peer. record protection
   mechanism: Upon transmission or receipt successful decryption of a fatal alert message, both parties
   immediately close TLSCiphertext.fragment,
   the connection.  Whenever an receiving implementation
   encounters scans the field from the end toward the
   beginning until it finds a condition which non-zero octet.  This non-zero octet is defined as a fatal alert, it MUST
   send
   the appropriate alert prior to closing content type of the connection.  All
   alerts defined in this section below, as well as all unknown alerts
   are universally considered fatal as message.  This padding scheme was selected
   because it allows padding of any encrypted TLS 1.3 (see Section 6). record by an arbitrary
   size (from zero up to TLS record size limits) without introducing new
   content types.  The following error alerts are defined:

   unexpected_message  An inappropriate message was received.  This
      alert should never be observed in communication between proper
      implementations.

   bad_record_mac  This alert is returned if a record is received design also enforces all-zero padding octets,
   which
      cannot be deprotected.  Because AEAD algorithms combine decryption
      and verification, this alert is used allows for all deprotection
      failures.  This alert should never be observed in communication
      between proper implementations, except when messages were
      corrupted in quick detection of padding errors.

   Implementations MUST limit their scanning to the network.

   record_overflow  A TLSCiphertext record was received that had a
      length more than 2^14 + 256 bytes, or cleartext returned
   from the AEAD decryption.  If a record decrypted to receiving implementation does not
   find 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 non-zero octet in the network.

   handshake_failure  Reception of a "handshake_failure" alert message
      indicates that cleartext, it should treat the sender was unable to negotiate record as
   having an acceptable
      set unexpected ContentType, sending an "unexpected_message"
   alert.

   The presence of security parameters given padding does not change the options available.

   bad_certificate  A certificate was corrupt, contained signatures that
      did overall record size
   limitations - the full fragment plaintext may not verify correctly, etc.

   unsupported_certificate  A certificate was of an unsupported type.

   certificate_revoked  A certificate was revoked by its signer.

   certificate_expired  A certificate has expired or exceed 2^14 octets.

   Selecting a padding policy that suggests when and how much to pad is not currently
      valid.

   certificate_unknown  Some other (unspecified) issue arose in
      processing
   a complex topic, and is beyond the certificate, rendering scope of this specification.  If
   the application layer protocol atop TLS has its own padding padding,
   it unacceptable.

   illegal_parameter  A field in may be preferable to pad application_data TLS records within the
   application layer.  Padding for encrypted handshake was out of range and alert TLS
   records must still be handled at the TLS layer, though.  Later
   documents may define padding selection algorithms, or
      inconsistent with other fields.

   unknown_ca  A valid certificate chain define a
   padding policy request mechanism through TLS extensions or partial chain was received,
      but the certificate was not accepted because some other
   means.

5.5.  Limits on Key Usage

   There are cryptographic limits on the CA certificate
      could not be located or couldn't amount of plaintext which can
   be matched with safely encrypted under a known, trusted
      CA.

   access_denied  A valid certificate or PSK was received, but when
      access control was applied, given set of keys.  [AEAD-LIMITS]
   provides an analysis of these limits under the sender decided not assumption that the
   underlying primitive (AES or ChaCha20) has no weaknesses.
   Implementations SHOULD do a key update Section 4.4.3 prior to proceed with
      negotiation.

   decode_error  A message could not
   reaching these limits.

   For AES-GCM, up to 2^24.5 full-size records may be decoded because some field was
      out encrypted on a
   given connection while keeping a safety margin of approximately 2^-57
   for Authenticated Encryption (AE) security.  For ChaCha20/Poly1305,
   the specified range or record sequence number will wrap before the length safety limit is
   reached.

6.  Alert Protocol

   One of the message was
      incorrect.  This content types supported by the TLS record layer is the
   alert type.  Like other messages, alert should never be observed in communication
      between proper implementations, except when messages were
      corrupted in are encrypted as
   specified by the network.

   decrypt_error  A handshake cryptographic operation failed, including
      being unable to correctly verify a signature current connection state.

   Alert messages convey the severity of the message (warning or validate fatal)
   and a
      Finished message.

   protocol_version  The protocol version description of the peer has attempted alert.  Warning-level messages are used to
      negotiate is recognized but not supported. (see Appendix C)

   insufficient_security  Returned instead
   indicate orderly closure of "handshake_failure" when a
      negotiation has failed specifically because the server requires
      ciphers more secure than those supported by connection (see Section 6.1).  Upon
   receiving a warning-level alert, the client.

   internal_error  An internal error unrelated TLS implementation SHOULD
   indicate end-of-data to the peer or the
      correctness of application and, if appropriate for the protocol (such as a memory allocation failure)
      makes it impossible to continue.

   inappropriate_fallback  Sent by
   alert type, send a server closure alert in response response.

   Fatal-level messages are used to an invalid indicate abortive closure of the
   connection retry attempt from a client. (see [RFC7507])

   missing_extension  Sent by endpoints that receive (See Section 6.2).  Upon receiving a hello message not
      containing fatal-level alert,
   the TLS implementation SHOULD indicate an extension that is mandatory error to send for the offered
      TLS version.  [[TODO: IANA Considerations.]]

   unsupported_extension  Sent by endpoints receiving application
   and MUST NOT allow any hello message
      containing an extension known further data to be prohibited for inclusion in sent or received on the given hello message, including any extensions in
   connection.  Servers and clients MUST forget keys and secrets
   associated with a ServerHello
      not first offered failed connection.  Stateful implementations of
   session tickets (as in many clients) SHOULD discard tickets
   associated with failed connections.

   All the corresponding ClientHello.

   certificate_unobtainable  Sent by servers when unable to obtain a
      certificate from a URL provided by the client via the
      "client_certificate_url" extension [RFC6066].

   unrecognized_name  Sent by servers when no server exists identified
      by alerts listed in Section 6.2 MUST be sent as fatal and MUST
   be treated as fatal regardless of the name provided by AlertLevel in the message.
   Unknown alert types 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 via the "server_name" extension
      [RFC6066].

   bad_certificate_status_response  Sent by clients when an invalid or
      unacceptable OCSP response is provided by and the server via must share knowledge that the
      "status_request" extension [RFC6066].  This alert connection is always fatal.

   bad_certificate_hash_value  Sent by servers when
   ending in order to avoid a retrieved object truncation attack.  Failure to properly
   close a connection does not have the correct hash provided by prohibit a session from being resumed.

   close_notify  This alert notifies the client via recipient that the
      "client_certificate_url" extension [RFC6066].

   unknown_psk_identity  Sent by servers when sender will
      not send any more messages on this connection.  Any data received
      after a PSK cipher suite is
      selected but no acceptable PSK identity closure MUST be ignored.

   end_of_early_data  This alert is provided sent by the client.
      Sending this alert is OPTIONAL; servers MAY instead choose client 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 TLS Record Protocol
   requires specification of a suite of algorithms, a master secret, that
      all 0-RTT application_data messages have been transmitted (or none
      will be sent at all) and that this is the client and server random values.  The authentication, key
   exchange, and record protection algorithms are determined by end of the
   cipher_suite selected by flight.  This
      alert MUST be at the server warning level.  Servers MUST NOT send this
      alert and revealed in clients receiving it MUST terminate the ServerHello
   message.  The random values are exchanged in connection with
      an "unexpected_message" alert.

   user_canceled  This alert notifies the hello messages.  All recipient that remains the sender is
      canceling the handshake for some reason unrelated to calculate a protocol
      failure.  If a user cancels an operation after the key schedule.

7.1.  Key Schedule

   The TLS handshake establishes one or is
      complete, just closing the connection by sending a "close_notify"
      is more input secrets which are
   combined appropriate.  This alert SHOULD 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 to create a "close_notify", the actual working keying material, as detailed
   below.  The key derivation process makes use receiver
   cannot know that all the data that was sent has been received.

   Each party MUST send a "close_notify" alert before closing the write
   side of the HKDF-Extract and
   HKDF-Expand functions as defined for HKDF [RFC5869], as well as the
   functions defined below:

       HKDF-Expand-Label(Secret, Label, HashValue, Length) =
            HKDF-Expand(Secret, HkdfLabel, Length)

       Where HkdfLabel is 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) connection, unless some other fatal alert has been
   transmitted.  The Hash function and the HKDF hash are the cipher suite hash
   function.  Hash.Length is its output length.

   Given other party MUST respond with a set "close_notify"
   alert of n InputSecrets, its own and close down the final "master secret" is computed
   by iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc. connection immediately,
   discarding any pending writes.  The initial secret is simply a string initiator of zeroes
   as long as the size close need not
   wait for the responding "close_notify" alert before closing the read
   side of the Hash connection.

   If the application protocol using TLS provides that is any data may be
   carried over the basis for underlying transport after the HKDF.
   Concretely, for TLS connection is
   closed, the present version of TLS 1.3, secrets are added in implementation must receive the following order:

   -  PSK

   -  (EC)DHE shared secret

   This produces a full key derivation schedule shown in responding
   "close_notify" alert before indicating to the diagram
   below.  In this diagram, application layer that
   the following formatting conventions apply:

   -  HKDF-Extract is drawn as taking TLS connection has ended.  If the Salt argument from application protocol will not
   transfer any additional data, but will only close the top and underlying
   transport connection, then the IKM argument from implementation MAY choose to close the left.

   -  Derive-Secret's Secret argument is indicated by the arrow coming
      in from
   transport without waiting for the left.  For instance, responding "close_notify".  No part
   of this standard should be taken to dictate the Early Secret manner in which a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

   Note: It is assumed that closing a connection reliably delivers
   pending data before destroying the Secret
      for generating transport.

6.2.  Error Alerts

   Error handling in the early_traffic-secret.

                 0
                 |
                 v
   PSK ->  HKDF-Extract
                 |
                 v
           Early Secret ---> Derive-Secret(., "early traffic secret",
                 |                         ClientHello)
                 |                         = early_traffic_secret
                 v
(EC)DHE -> HKDF-Extract
                 |
                 v TLS 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 Protocol is that the secrets shown down very simple.  When an
   error is detected, the left side detecting party sends a message to its peer.
   Upon transmission or receipt of a fatal alert message, both parties
   immediately close the diagram are just raw entropy without context, whereas the
   secrets down connection.  Whenever an implementation
   encounters a condition which is defined as a fatal alert, it MUST
   send the right side include handshake context and therefore
   can be used appropriate alert prior to derive working keys without additional context.  Note
   that closing the different calls to Derive-Secret may take different Messages
   arguments, even with the same secret.  In connection.  All
   alerts defined in this section below, as well as all unknown alerts
   are universally considered fatal as of 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 communication between proper
      implementations.

   bad_record_mac  This alert is returned if a 0-RTT exchange, Derive-
   Secret record is called with four distinct transcripts; received which
      cannot be deprotected.  Because AEAD algorithms combine decryption
      and verification, this alert is used for all deprotection
      failures.  This alert should never be observed in communication
      between proper implementations, except when messages were
      corrupted in the network.

   record_overflow  A TLSCiphertext record was received that had a 1-RTT only
   exchange with three distinct transcripts.

   If
      length more than 2^14 + 256 bytes, or a given secret is not available, then 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 0-value consisting network.

   handshake_failure  Reception of a
   string "handshake_failure" alert message
      indicates that the sender was unable to negotiate an acceptable
      set of Hash.length zeroes is used.  Note security parameters given the options available.

   bad_certificate  A certificate was corrupt, contained signatures that this does
      did not mean
   skipping rounds, so if PSK verify correctly, etc.

   unsupported_certificate  A certificate was of 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 use Early Secret will still be
   HKDF-Extract(0, 0).

7.2.  Updating Traffic Keys and IVs

   Once
      processing the handshake is complete, certificate, rendering it is possible for either side to
   update its sending traffic keys using unacceptable.

   illegal_parameter  A field in the KeyUpdate handshake message
   defined in Section 4.4.3.  The next generation was out of traffic keys is
   computed by generating traffic_secret_N+1 from traffic_secret_N as
   described in this section then re-deriving range or
      inconsistent with other fields.

   unknown_ca  A valid certificate chain or partial chain was received,
      but the traffic keys as
   described in Section 7.3.

   The next-generation traffic_secret is 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 certificate was not accepted because the
   directional keys are no longer needed, they SHOULD CA certificate
      could not be deleted as
   well.

7.3.  Traffic Key Calculation

   The traffic keying material is generated from the following input
   values:

   - located or couldn't be matched with a known, trusted
      CA.

   access_denied  A secret value

   - valid certificate or PSK was received, but when
      access control was applied, the sender decided not to proceed with
      negotiation.

   decode_error  A phase value indicating message could not be decoded because some field was
      out of the phase specified range or the length of the protocol message was
      incorrect.  This alert should never be observed in communication
      between proper implementations, except when messages were
      corrupted in the keys are
      being generated for

   - network.

   decrypt_error  A purpose value indicating the specific value handshake cryptographic operation failed, including
      being generated

   - unable to correctly verify a signature or validate a
      Finished message.

   protocol_version  The length of protocol version the key

   The keying material peer has attempted to
      negotiate is computed using:

      key = HKDF-Expand-Label(Secret,
                              phase + ", " + purpose,
                              "",
                              key_length)

   The following table describes recognized but not supported. (see Appendix C)

   insufficient_security  Returned instead of "handshake_failure" when a
      negotiation has failed specifically because the inputs server requires
      ciphers more secure than those supported by the client.

   internal_error  An internal error unrelated to the key calculation for
   each class of traffic keys:

   +-------------+--------------------------+--------------------------+
   | Record Type | Secret                   | Phase                    |
   +-------------+--------------------------+--------------------------+
   | 0-RTT       | early_traffic_secret     | "early handshake key     |
   | Handshake   |                          | expansion"               |
   |             |                          |                          |
   | 0-RTT       | early_traffic_secret     | "early application data  |
   | Application |                          | key expansion"           |
   |             |                          |                          |
   | Handshake   | handshake_traffic_secret | "handshake key           |
   |             |                          | expansion"               |
   |             |                          |                          |
   | Application | traffic_secret_N         | "application data key    |
   | Data        |                          | expansion"               |
   +-------------+--------------------------+--------------------------+

   The following table indicates peer or the purpose values for each type
      correctness 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 the underlying
   Secret changes (e.g., when changing from the handshake protocol (such as a memory allocation failure)
      makes it impossible to application
   data keys or upon continue.

   inappropriate_fallback  Sent by a key update).

7.3.1.  Diffie-Hellman

   A conventional Diffie-Hellman computation is performed.  The
   negotiated key (Z) is converted to byte string by encoding server in big-
   endian, padded with zeros up response to the size of the prime.  This byte
   string an invalid
      connection retry attempt from a client. (see [RFC7507])

   missing_extension  Sent by endpoints that receive a hello message not
      containing an extension that is used as mandatory to send for the shared secret, and is used offered
      TLS version.  [[TODO: IANA Considerations.]]

   unsupported_extension  Sent by endpoints receiving any hello message
      containing an extension known to be prohibited for inclusion in
      the key schedule
   as specified above.

   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 and key generation as well as given hello message, including any extensions in a ServerHello
      not first offered in the shared secret
   calculation) are performed according corresponding ClientHello.

   certificate_unobtainable  Sent by servers when unable to [IEEE1363] using obtain a
      certificate from a URL provided by the ECKAS-
   DH1 scheme with client via the identity map as key derivation function (KDF), so
   that
      "client_certificate_url" extension [RFC6066].

   unrecognized_name  Sent by servers when no server exists identified
      by the shared secret is name provided by the x-coordinate of client via the ECDH shared secret
   elliptic curve point represented as "server_name" extension
      [RFC6066].

   bad_certificate_status_response  Sent by clients when an octet string.  Note that this
   octet string (Z in IEEE 1363 terminology) as output invalid or
      unacceptable OCSP response is provided by FE2OSP, the
   Field Element to Octet String Conversion Primitive, has constant
   length for any given field; leading zeros found in this octet string
   MUST NOT be truncated.

   (Note that this use of server via the identity KDF is a technicality.  The
   complete picture is that ECDH
      "status_request" extension [RFC6066].  This alert is employed with always fatal.

   bad_certificate_hash_value  Sent by servers when a non-trivial KDF
   because TLS retrieved object
      does not directly use have the correct hash provided by the client via the
      "client_certificate_url" extension [RFC6066].

   unknown_psk_identity  Sent by servers when PSK key establishment is
      desired but no acceptable PSK identity is provided by the client.
      Sending this secret for anything other than
   for computing other secrets.)

   ECDH functions alert is OPTIONAL; servers MAY instead choose to send
      a "decrypt_error" alert to merely indicate an invalid PSK
      identity.

   New Alert values are used assigned by IANA as follows:

   -  The public key described in Section 10.

7.  Cryptographic Computations

   In order to put into the KeyShareEntry.key_exchange
      structure is begin connection protection, the result TLS Record Protocol
   requires specification of applying the ECDH function to the
      secret key a suite of appropriate length (into scalar input) algorithms, a master secret, and
   the
      standard public basepoint (into u-coordinate point input).

   - client and server random values.

7.1.  Key Schedule

   The ECDH shared secret is the result of applying ECDH function TLS handshake establishes one or more input secrets which are
   combined to create the secret actual working keying material, as detailed
   below.  The key (into scalar input) and derivation process makes use of the peer's public key (into
      u-coordinate point input).  The output is used raw, with no
      processing.

   For X25519 HKDF-Extract and X448, see [RFC7748].

7.3.3.  Exporters

   [RFC5705] defines keying material exporters
   HKDF-Expand functions as defined for TLS in terms of the
   TLS PRF.  This document replaces HKDF [RFC5869], as well as the PRF with HKDF, thus requiring a
   new construction.
   functions defined below:

       HKDF-Expand-Label(Secret, Label, HashValue, Length) =
            HKDF-Expand(Secret, HkdfLabel, Length)

       Where HkdfLabel is 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 exporter interface remains Hash function and the same, however HKDF hash are the value cipher suite hash
   function.  Hash.Length is its output length.

   Given a set of n InputSecrets, the final "master secret" is computed as:

   HKDF-Expand-Label(exporter_secret,
                     label, context_value, key_length)

8.  Compliance Requirements
8.1.  MTI Cipher Suites

   In
   by iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc.  The initial secret is simply a string of zeroes
   as long as the absence size of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement 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 key exchange
   with X25519 [RFC7748].

   A TLS-compliant application SHOULD implement Hash that is 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 basis for the absence HKDF.
   Concretely, for the present version of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement TLS 1.3, secrets are added in
   the following
   TLS extensions:

   -  Signature Algorithms ("signature_algorithms"; Section 4.2.2) order:

   -  Negotiated Groups ("supported_groups"; Section 4.2.3)  PSK

   -  Key Share ("key_share"; Section 4.2.4)  (EC)DHE shared secret

   This produces a full key derivation schedule shown in the diagram
   below.  In this diagram, the following formatting conventions apply:

   -  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 use these extensions when offering
   applicable cipher suites:

   -  "signature_algorithms"  HKDF-Extract is REQUIRED for certificate authenticated
      cipher suites.

   -  "supported_groups" drawn as taking the Salt argument from the top and "key_share" are REQUIRED for DHE or ECDHE
      cipher suites.
      the IKM argument from the left.

   -  "pre_shared_key"  Derive-Secret's Secret argument is REQUIRED for PSK cipher suites.

   -  "cookie" indicated by the arrow coming
      in from the left.  For instance, the Early Secret is REQUIRED the Secret
      for all cipher suites.

   When negotiating use of applicable cipher suites, endpoints MUST
   abort generating the connection with a "missing_extension" alert if early_traffic_secret.

   Note that the required
   extension was 0-RTT Finished message is not provided.  Any endpoint included in the Derive-
   Secret operation.

                 0
                 |
                 v
   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 is that receives any invalid
   combination of cipher suites and extensions MAY abort the connection
   with a "missing_extension" alert, regardless of negotiated
   parameters.

   Additionally, all implementations MUST support use of secrets shown down the
   "server_name" extension with applications capable left side
   of using it.
   Servers MAY require clients the diagram are just raw entropy without context, whereas the
   secrets down the right side include handshake context and therefore
   can be used to send a valid "server_name" extension.
   Servers requiring this extension SHOULD respond derive working keys without additional context.  Note
   that the different calls to a ClientHello
   lacking a "server_name" extension Derive-Secret may take different Messages
   arguments, even with a fatal "missing_extension"
   alert.

   Servers MUST NOT send the "signature_algorithms" extension; if same secret.  In a
   client receives this extension it MUST respond 0-RTT exchange, Derive-
   Secret is called with four distinct transcripts; in a fatal
   "unsupported_extension" alert and close 1-RTT only
   exchange with three distinct transcripts.

   If a given secret is not available, then the connection.

9.  Security Considerations

   Security issues are discussed throughout this memo, especially in
   Appendices B, C, and D.

10.  IANA Considerations

   This document uses several registries 0-value consisting of a
   string of Hash.length zeroes is used.  Note that were originally created in
   [RFC4346].  IANA has updated these to reference this document.  The
   registries and their allocation policies are below:

   -  TLS Cipher Suite Registry: Values with the first byte 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 IVs

   Once the range
      0-254 (decimal) are assigned via Specification Required [RFC2434].
      Values with the first byte 255 (decimal) are reserved handshake is complete, it is possible for Private
      Use [RFC2434].  IANA [SHALL add/has added] a "Recommended" column either side to
   update its sending traffic keys using the cipher suite registry.  All cipher suites listed KeyUpdate handshake message
   defined in
      Appendix A.4 are marked as "Yes".  All other cipher suites are
      marked Section 4.4.3.  The next generation of traffic keys is
   computed by generating traffic_secret_N+1 from traffic_secret_N as "No".  IANA [SHALL add/has added] add a note to
   described in this
      column reading:

         Cipher suites marked as "Yes" are those allocated via Standards
         Track RFCs.  Cipher suites marked section then re-deriving the traffic keys 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
   described in [RFC4366].
   IANA has updated it to reference this document. Section 7.3.

   The registry next-generation traffic_secret is computed as:

    traffic_secret_N+1 = HKDF-Expand-Label(
                             traffic_secret_N,
                             "application traffic secret", "", Hash.Length)

   Once traffic_secret_N+1 and its
   allocation policy associated traffic keys have been
   computed, implementations SHOULD delete traffic_secret_N.  Once the
   directional keys are no longer needed, they SHOULD be deleted as
   well.

7.3.  Traffic Key Calculation

   The traffic keying material is listed below: generated from the following input
   values:

   -  TLS ExtensionType Registry: Values with  A secret value

   -  A phase value indicating the first byte in phase of the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with protocol the first byte 255 (decimal) keys are reserved
      being generated 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",

   -  A purpose value indicating that
      they shall be in the ServerHello.  "Encrypted", indicating that
      they shall be in specific value being generated

   -  The length of the EncryptedExtensions block, and "No"
      indicating that they are not used in TLS 1.3.  This column [shall
      be/has been] initially populated with key

   The keying material is computed using:

      key = HKDF-Expand-Label(Secret,
                              phase + ", " + purpose,
                              "",
                              key_length)

   The following table describes the values in this document.
      IANA [shall update/has updated] this registry inputs 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".

   +-------------------------------+-----------+-----------------------+ key calculation for
   each class of traffic keys:

   +-------------+--------------------------+--------------------------+
   | Extension Record Type | Recommend Secret                   |               TLS 1.3 Phase                    |
   +-------------+--------------------------+--------------------------+
   | 0-RTT       |        ed early_traffic_secret     | "early handshake key     |
   +-------------------------------+-----------+-----------------------+
   | server_name [RFC6066] Handshake   |       Yes                          |             Encrypted expansion"               |
   |             |                          |                          |
   | max_fragment_length [RFC6066] 0-RTT       |       Yes early_traffic_secret     |             Encrypted "early application data  |
   | Application |                          | key expansion"           |
   | client_certificate_url             |       Yes                          |             Encrypted                          |
   | [RFC6066] Handshake   | handshake_traffic_secret | "handshake key           |
   |             |                          | expansion"               |
   | trusted_ca_keys [RFC6066]             |       Yes                          |             Encrypted                          |
   | Application | traffic_secret_N         | "application data key    |
   | truncated_hmac [RFC6066] Data        |       Yes                          |                    No expansion"               |
   +-------------+--------------------------+--------------------------+

   The following table indicates the purpose values for each type of
   key:

                 +------------------+--------------------+
                 | Key Type         | Purpose            |
                 +------------------+--------------------+
                 | client_write_key | status_request [RFC6066] "client write key" |       Yes
                 |                    No                  |                    |
                 | server_write_key | "server write key" |
                 | user_mapping [RFC4681]                  |       Yes                    |             Encrypted
                 | client_write_iv  | "client write iv"  |
                 |                  |                    | 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
                 | server_write_iv  | "server write iv"  |           |                       |
   | key_share [[this document]]   |       Yes |                 Clear |
   |                               |           |                       |
   | pre_shared_key [[this         |       Yes |                 Clear |
   | document]]                    |           |                       |
   |                               |           |                       |
   | early_data [[this document]]  |       Yes |             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
                 +------------------+--------------------+

   All the first byte in traffic keying material is recomputed whenever the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with underlying
   Secret changes (e.g., when changing from the first byte 255 (decimal) are reserved
      for Private Use [RFC2434].  This registry SHALL have handshake to application
   data keys or upon a
      "Recommended" column. key update).

7.3.1.  Diffie-Hellman

   A conventional Diffie-Hellman computation is performed.  The registry [shall be/ has been] initially
      populated
   negotiated key (Z) is converted to byte string by encoding in big-
   endian, padded with zeros up to the values described size of the prime.  This byte
   string is used as the shared secret, and is used in Section 4.2.2.  The
      following values SHALL be marked the key schedule
   as "Recommended":
      ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, rsa_pss_sha256,
      rsa_pss_sha384, rsa_pss_sha512, ed25519.

11.  References

11.1.  Normative References

   [AES]      National Institute specified above.

   Note that this construction differs from previous versions of Standards TLS
   which remove leading zeros.

7.3.2.  Elliptic Curve Diffie-Hellman

   For secp256r1, secp384r1 and Technology,
              "Specification for the Advanced Encryption Standard
              (AES)", NIST FIPS 197, November 2001.

   [DH]       Diffie, W. secp521r1, ECDH calculations (including
   parameter and M. Hellman, "New Directions key generation as well as the shared secret
   calculation) are performed according 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 the ECDH shared secret
   elliptic curve point represented as an octet string.  Note that this
   octet string (Z in
              Cryptography", IEEE Transactions on Information Theory,
              V.IT-22 n.6 , June 1977.

   [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 1363 terminology) as output by FE2OSP, the
   Field Element to Octet String Conversion Primitive, has constant
   length for Transport Layer Security (TLS)",
              draft-mattsson-tls-ecdhe-psk-aead-05 (work any given field; leading zeros found in progress),
              April 2016.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing this octet string
   MUST NOT be truncated.

   (Note that this use of the identity KDF is a technicality.  The
   complete picture is that ECDH is employed with a non-trivial KDF
   because TLS does not directly use this secret for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words anything other than
   for use in RFCs computing other secrets.)

   ECDH functions are used as follows:

   -  The public key to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2434]  Narten, T. put into the KeyShareEntry.key_exchange
      structure is the result of applying the ECDH function to the
      secret key of appropriate length (into scalar input) 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 the
      standard public basepoint (into u-coordinate point input).

   -  The ECDH shared secret is the result of applying ECDH function to
      the secret key (into scalar input) 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., the peer's public key (into
      u-coordinate point input).  The output is used raw, with no
      processing.

   For X25519 and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites X448, see [RFC7748].

7.3.3.  Exporters

   [RFC5705] defines keying material exporters 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 TLS in terms of the
   TLS PRF.  This document replaces the PRF 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 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, key_length)

8.  Compliance Requirements
8.1.  MTI Cipher Suites for TLS

   In the absence of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement the
   TLS_AES_128_GCM_SHA256 cipher suite and SHOULD implement the
   TLS_AES_256_GCM_SHA384 and TLS_CHACHA20_POLY1305_SHA256 cipher
   suites.

   A TLS-compliant application MUST support digital signatures with SHA-
              256/384
   rsa_pkcs1_sha256 (for certificates), rsa_pss_sha256 (for
   CertificateVerify 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. certificates), 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., ecdsa_secp256r1_sha256.  A
   TLS-compliant application MUST support key exchange with secp256r1
   (NIST P-256) and D. Kwon, "Addition of SHOULD support key exchange with X25519 [RFC7748].

8.2.  MTI Extensions

   In 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 absence of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement 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. following
   TLS extensions:

   -  Signature Algorithms ("signature_algorithms"; Section 4.2.2)

   -  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 D. Bailey, "AES-CCM Cipher Suites use these extensions when offering
   applicable cipher suites:

   -  "signature_algorithms" is REQUIRED 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>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) certificate authenticated
      cipher suites.

   -  "supported_groups" and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <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 "key_share" are REQUIRED 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 DHE or ECDHE
      cipher suites.

   -  "pre_shared_key" is REQUIRED 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 PSK cipher suites.

   -  "cookie" is REQUIRED for Transport Layer Security (TLS)",
              RFC 7905, DOI 10.17487/RFC7905, June 2016,
              <http://www.rfc-editor.org/info/rfc7905>.

   [SHS]      National Institute all cipher suites.

   When negotiating use of Standards applicable cipher suites, endpoints MUST
   abort the connection with a "missing_extension" alert if the required
   extension was not provided.  Any endpoint that receives any invalid
   combination of cipher suites and Technology, U.S.
              Department extensions MAY abort the connection
   with a "missing_extension" alert, regardless of Commerce, "Secure Hash Standard", NIST FIPS
              PUB 180-4, March 2012.

   [X690]     ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification negotiated
   parameters.

   Additionally, all implementations MUST support use 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.

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 the
   "server_name" extension with applications capable of TLS 1.3: 0-RTT,
              Resumption 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 Delayed Authentication", Proceedings of
              IEEE Symposium on close the connection.

9.  Security Considerations

   Security issues are discussed throughout this memo, especially in
   Appendices B, C, 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 D.

10.  IANA Considerations

   This document uses several registries that were originally created in
   [RFC4346].  IANA has updated these to reference this document.  The
   registries 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 their allocation policies are below:

   -  TLS Cipher Suite Registry: Values with the Financial Services Industry: 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 add/has added] a "Recommended" column
      to the cipher suite registry.  All cipher suites listed in
      Appendix A.4 are marked as "Yes".  All other cipher suites are
      marked as "No".  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
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI ANS X9.62-2005, November 2005.

   [FGSW16]   Fischlin, M., Guenther, F., Schmidt, B., registry and B. Warinschi,
              "Key Confirmation its
   allocation policy is listed below:

   -  TLS ExtensionType Registry: Values with the first byte in Key Exchange: A Formal Treatment and
              Implications the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal) are reserved
      for TLS 1.3", Proceedings of IEEE Symposium
              on Security Private Use [RFC2434].  IANA [SHALL update/has updated] this
      registry to include the "key_share", "pre_shared_key", 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.
      "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 H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-23 (work "No"
      indicating that they are not used 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 TLS 1.3.  This column [shall
      be/has been] initially populated with the values in progress), June 2015.

   [IEEE1363]
              IEEE, "Standard Specifications for Public Key
              Cryptography", IEEE 1363 , 2000.

   [LXZFH16]  Li, X., Xu, J., Feng, D., Zhang, Z., 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 H. Hu, "Multiple
              Handshakes Security of all others
      as "No".

   +-------------------------------+-----------+-----------------------+
   | Extension                     | Recommend |               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>. |
   |                               |        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 |             Encrypted |
   |                               |           |                       |
   | 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]  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>.    |       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]  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.,   |       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 |             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 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.

11.  References

11.1.  Normative References

   [AES]      National Institute of Standards and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol Technology,
              "Specification for TLS
              Authentication", RFC 5054, DOI 10.17487/RFC5054, the Advanced Encryption Standard
              (AES)", NIST FIPS 197, November
              2007, <http://www.rfc-editor.org/info/rfc5054>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., 2001.

   [DH]       Diffie, W. 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 M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory,
              V.IT-22 n.6 , June 1977.

   [I-D.irtf-cfrg-eddsa]
              Josefsson, S. and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5246]  Dierks, T. I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-06
              (work in progress), August 2016.

   [I-D.mattsson-tls-ecdhe-psk-aead]
              Mattsson, J. and E. Rescorla, "The D. Migault, "ECDHE_PSK with AES-GCM and
              AES-CCM Cipher Suites for 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, (TLS)",
              draft-mattsson-tls-ecdhe-psk-aead-05 (work in progress),
              April 2016.

   [RFC2104]  Krawczyk, H., Bellare, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 5746, 2104,
              DOI 10.17487/RFC5746, 10.17487/RFC2104, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 5763, 2119,
              DOI 10.17487/RFC5763, May
              2010, <http://www.rfc-editor.org/info/rfc5763>.

   [RFC5764]  McGrew, D. 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2434]  Narten, T. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys H. Alvestrand, "Guidelines for the Secure
              Real-time Transport Protocol (SRTP)", Writing an
              IANA Considerations Section in RFCs", RFC 5764, 2434,
              DOI 10.17487/RFC5764, May 2010,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC5878]  Brown, M. 10.17487/RFC2434, October 1998,
              <http://www.rfc-editor.org/info/rfc2434>.

   [RFC3447]  Jonsson, J. and R. 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, "Transport Layer Security (TLS)
              Authorization Extensions", R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5878, 5280, DOI 10.17487/RFC5878, 10.17487/RFC5280, May 2010, <http://www.rfc-editor.org/info/rfc5878>.

   [RFC5929]  Altman, 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5288]  Salowey, J., Williams, N., Choudhury, A., and L. Zhu, "Channel Bindings D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5929, 5288,
              DOI 10.17487/RFC5929, July 2010,
              <http://www.rfc-editor.org/info/rfc5929>.

   [RFC6091]  Mavrogiannopoulos, N. 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 D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication", AES Galois Counter Mode (GCM)", RFC 6091, 5289,
              DOI 10.17487/RFC6091, February 2011,
              <http://www.rfc-editor.org/info/rfc6091>.

   [RFC6176]  Turner, S. 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 T. Polk, "Prohibiting Secure Sockets Layer
              (SSL) Version 2.0", AES Galois Counter Mode", RFC 6176, 5487,
              DOI 10.17487/RFC6176, 10.17487/RFC5487, March
              2011, <http://www.rfc-editor.org/info/rfc6176>.

   [RFC6347] 2009,
              <http://www.rfc-editor.org/info/rfc5487>.

   [RFC5705]  Rescorla, E. and N. Modadugu, "Datagram E., "Keying Material Exporters for Transport
              Layer Security Version 1.2", (TLS)", RFC 6347, 5705, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6520]  Seggelmann, R., Tuexen, M., 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC5869]  Krawczyk, H. and M. Williams, 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) 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",
              Extensions: Extension Definitions", RFC 7230, 6066,
              DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6209]  Kim, W., Lee, J., Park, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in D. Kwon, "Addition of the
              ARIA Cipher Suites to Transport Layer Security (TLS) (TLS)",
              RFC 6209, DOI 10.17487/RFC6209, April 2011,
              <http://www.rfc-editor.org/info/rfc6209>.

   [RFC6367]  Kanno, S. and Datagram M. Kanda, "Addition of the Camellia Cipher
              Suites to Transport Layer Security (DTLS)", (TLS)", RFC 7250, 6367,
              DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., 10.17487/RFC6367, September 2011,
              <http://www.rfc-editor.org/info/rfc6367>.

   [RFC6655]  McGrew, D. and E. Stephan,
              "Transport D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", (TLS)", RFC 7301, 6655,
              DOI 10.17487/RFC7301, 10.17487/RFC6655, July 2014, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for 2012,
              <http://www.rfc-editor.org/info/rfc6655>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS)",
              Multiple Certificate Status Request Extension", RFC 7366, 6961,
              DOI 10.17487/RFC7366, September 2014,
              <http://www.rfc-editor.org/info/rfc7366>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 7465, 6979, 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", 10.17487/RFC6979, August
              2013, <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 7568, 7251, DOI 10.17487/RFC7568, 10.17487/RFC7251, June 2015,
              <http://www.rfc-editor.org/info/rfc7568>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., 2014,
              <http://www.rfc-editor.org/info/rfc7251>.

   [RFC7443]  Patil, P., Reddy, T., Salgueiro, G., and M. Ray, "Transport Layer Security (TLS) Petit-
              Huguenin, "Application-Layer Protocol Negotiation (ALPN)
              Labels for Session Hash and Extended Master Secret Extension", Traversal Utilities for NAT (STUN)
              Usages", RFC 7627, 7443, DOI 10.17487/RFC7627, September 10.17487/RFC7443, January 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

   [RFC7685]
              <http://www.rfc-editor.org/info/rfc7443>.

   [RFC7748]  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., Hamburg, M., and L. Adleman, "A Method S. Turner, "Elliptic Curves
              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.

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 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 are listed
   here S. Josefsson, "ChaCha20-Poly1305
              Cipher Suites for completeness.  TLS 1.3 implementations MUST NOT send them
   but might receive them from older TLS implementations.

A.1.  Record Transport Layer

   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;
       opaque 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),
          bad_record_mac(20),
          decryption_failed_RESERVED(21),
          record_overflow(22),
          decompression_failure_RESERVED(30),
          handshake_failure(40),
          no_certificate_RESERVED(41),
          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),
          export_restriction_RESERVED(60),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          inappropriate_fallback(86),
          user_canceled(90),
          no_renegotiation_RESERVED(100),
          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;

A.3.  Handshake Protocol

      enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          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 new_session_ticket:    NewSessionTicket;
              case key_update:            KeyUpdate;
          } body;
      } Handshake;

A.3.1. 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 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),
          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:
                  uint32 obfuscated_ticket_age;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

A.3.1.1.  Cookie Extension

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

A.3.1.2. Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 1998.

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 Algorithm Extension
      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 */
          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 (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 {
          /* 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 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 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-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 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 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
   unintentional server configuration issues.  They are no longer
   considered appropriate Establish Keys for general use 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 should be assumed to be
   potentially unsafe.  The set of curves specified here is sufficient
   for interoperability with all currently deployed 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 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], M. Williams, "Transport
              Layer Security (TLS) 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 {
           SignatureScheme algorithm;
           opaque signature<0..2^16-1>;
      } CertificateVerify;

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

A.3.4.  Ticket Establishment
    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;

A.4.  Cipher Suites

   A cipher suite defines a cipher specification supported in TLS 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
   negotiated via hello messages 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 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
              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 record protection       |
   |           |                                                       |
   | HASH      | The hash algorithm used with HKDF                     |
   |           |                                                       |
   | VALUE     | The two byte ID assigned 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>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <http://www.rfc-editor.org/info/rfc7924>.

   [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for this cipher suite        |
   +-----------+-------------------------------------------------------+
   The "CIPHER" component commonly has sub-components used to designate
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the cipher name, bits, 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 mode, if applicable.  For example,
   "AES_256_GCM" represents 256-bit AES its use in the GCM mode IKE
              protocols", Proceedings 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 CRYPTO 2003 , 2003.

   [SLOTH]    Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS is Ephemeral Diffie-
   Hellman [DH]. 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 finite field based version is denoted "DHE" Directory: Models", ITU-T X.501, 1993.

11.3.  URIs

   [1] mailto:tls@ietf.org

Appendix A.  Protocol Data Structures and Constant Values

   This section describes protocol types and
   the elliptic curve based version is denoted "ECDHE".  Prior constants.  Values listed
   as _RESERVED were used in previous versions of TLS supported non-ephemeral key exchanges, however these and are not
   supported by listed
   here for completeness.  TLS 1.3.

   See the definitions of each cipher 1.3 implementations MUST NOT send them
   but might receive them from older TLS implementations.

A.1.  Record Layer

   enum {
       invalid_RESERVED(0),
       change_cipher_spec_RESERVED(20),
       alert(21),
       handshake(22),
       application_data(23)
       (255)
   } ContentType;

   struct {
       ContentType type;
       ProtocolVersion legacy_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 legacy_record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       opaque 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),
          bad_record_mac(20),
          decryption_failed_RESERVED(21),
          record_overflow(22),
          decompression_failure_RESERVED(30),
          handshake_failure(40),
          no_certificate_RESERVED(41),
          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),
          export_restriction_RESERVED(60),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          inappropriate_fallback(86),
          user_canceled(90),
          no_renegotiation_RESERVED(100),
          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;

A.3.  Handshake Protocol

      enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          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 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 in its specification
   document for the full details of each combination of algorithms that
   is specified.

   The following is a list of standards track server-authenticated (and
   optionally client-authenticated) cipher suites which are currently
   available in TLS 1.3:

   +----------------------------------------+-----------+--------------+
   | Cipher Suite Name                      | Value     | Specificatio |
   |                                        |           | n            |
   +----------------------------------------+-----------+--------------+
   | TLS_DHE_RSA_WITH_AES_128_GCM_SHA256    | {0x00,0x9 | [RFC5288]    |
   |                                        | E}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_AES_256_GCM_SHA384    | {0x00,0x9 | [RFC5288]    |
   |                                        | F}        |              |
   |                                        |           |              |
   | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA25 | {0xC0,0x2 | [RFC5289]    |
   | 6                                      | B}        |              |
   |                                        |           |              |
   | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA38 | {0xC0,0x2 | [RFC5289]    |
   | selector */

   struct {
       ProtocolVersion max_supported_version = { 3, 4                                      | C}        |              |
   |                                        |           |              |
   | TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256  | {0xC0,0x2 | [RFC5289]    |
   |                                        | F}        |              |
   |                                        |           |              |
   | TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384  | {0xC0,0x3 | [RFC5289]    |
   |                                        | 0}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_AES_128_CCM           | {0xC0,0x9 | [RFC6655]    |
   |                                        | E}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_AES_256_CCM           | {0xC0,0x9 | [RFC6655]    |
   |                                        | F}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_AES_128_CCM_8         | {0xC0,0xA | [RFC6655]    |
   |                                        | 2}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_AES_256_CCM_8         | {0xC0,0xA | [RFC6655]    |
   |                                        | 3}        |              |
   |                                        |           |              |
   | TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_S | {0xCC,0xA | [RFC7905]    |
   | HA256                                  | 8}        |              |
   |                                        |           |              |
   | TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 | {0xCC,0xA | [RFC7905]    |
   | _SHA256                                | 9}        |              |
   |                                        |           |              |
   | TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA | {0xCC,0xA | [RFC7905]    |
   | 256                                    | A}        |              |
   +----------------------------------------+-----------+--------------+

   Note: };    /* 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 version;
       Random random;
       CipherSuite cipher_suite;
       Extension extensions<0..2^16-1>;
   } ServerHello;

   struct {
       ProtocolVersion server_version;
       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),
       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;

   enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeModes;
   enum { psk_auth(0), psk_sign_auth(1), (255) } PskAuthenticationModes;

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

   struct {
       PskKeMode ke_modes<1..255>;
       PskAuthMode auth_modes<1..255>;
       opaque identity<0..2^16-1>;
   } PskIdentity;

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

           case server:
               uint16 selected_identity;
       }
   } PreSharedKeyExtension;

   struct {
       select (Role) {
           case client:
               uint32 obfuscated_ticket_age;

           case server:
              struct {};
       }
   } EarlyDataIndication;

A.3.1.1.  Cookie Extension

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

A.3.1.2.  Signature Algorithm Extension
      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 */
          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 (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.  Supported Groups 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 values listed for ChaCha/Poly are preliminary but are being obsolete curves have various known/theoretical
   weaknesses or will be used have had very little usage, in some cases only due to
   unintentional server configuration issues.  They are no longer
   considered appropriate for interop testing general use and therefore are likely to should be
   assigned.

   Note: ECDHE AES GCM was not yet standards track prior assumed to the
   publication be
   potentially unsafe.  The set of this specification.  This document promotes the above-
   listed ciphers to standards track. curves specified here is sufficient
   for interoperability with all currently deployed and properly
   configured TLS implementations.

A.3.1.4.  Deprecated Extensions

   The following is a list 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 {
           SignatureScheme algorithm;
           opaque signature<0..2^16-1>;
      } CertificateVerify;

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

A.3.4.  Ticket Establishment
    enum { (65535) } TicketExtensionType;

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

    struct {
        uint32 ticket_lifetime;
        PskKeMode ke_modes<1..255>;
        PskAuthMode auth_modes<1..255>;
        opaque ticket<1..2^16-1>;
        TicketExtension extensions<0..2^16-2>;
    } NewSessionTicket;

A.4.  Cipher Suites

   A symmetric cipher suite defines the pair of standards track ephemeral pre-shared key the AEAD cipher and hash
   function to be used with HKDF.  Cipher suites which are currently available in TLS 1.3:

   +------------------------------+----------+-------------------------+
   | follow the naming
   convention: 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 suite names follow the naming convention:

      CipherSuite TLS13_CIPHER_HASH = VALUE;

      +-----------+-------------------------------------------------+
      | Component | CM_SHA256 Contents                                        | 04}
      +-----------+-------------------------------------------------+
      | -psk-aead] TLS       | The string "TLS"                                |
      |           |                                                 |
      | TLS_ECDHE_PSK_WITH_AES_256_C CIPHER    | {0xD0,0x The symmetric cipher used for record protection | [I-D.mattsson-tls-ecdhe
      |           | CM_SHA384                                                 | 05}
      | -psk-aead] HASH      | The hash algorithm used with HKDF               |
      |           |                                                 |
      | TLS_ECDHE_PSK_WITH_CHACHA20_ VALUE     | {0xCC,0x The two byte ID assigned for this cipher suite  | [RFC7905]
      +-----------+-------------------------------------------------+

   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 Name            | POLY1305_SHA256 Value       | AC} Specification |
      +------------------------------+-------------+---------------+
      | TLS_AES_128_GCM_SHA256       | {0x13,0x01} | [This RFC]    |
      |                              | TLS_DHE_PSK_WITH_CHACHA20_PO             | {0xCC,0x               | [RFC7905]
      | TLS_AES_256_GCM_SHA384       | LY1305_SHA256 {0x13,0x02} | AD} [This RFC]    |
      |
   +------------------------------+----------+-------------------------+

   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                              |             |               |
      | TLS_CHACHA20_POLY1305_SHA256 | {0x13,0x03} | [This RFC]    |
      |                              |             |               |
      | TLS_AES_128_CCM_SHA256       | {0x13,0x04} | [This RFC]    |
      |                              |             |               |
      | TLS_AES_128_CCM_8_SHA256     | {0x13,0x05} | [This RFC]    |
      +------------------------------+-------------+---------------+

   Although TLS 1.3 uses the ordering of
   components within PSK AES CCM same cipher suite names.  The names above
   are space as defined.

   All previous
   versions of TLS, TLS 1.3 cipher suites in this section are specified defined differently, only
   specifying the symmetric ciphers, and cannot it be used for use with both TLS 1.2.
   Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS 1.3, as well as the corresponding versions of DTLS.  (see
   Appendix C)
   1.3.

   New cipher suite values are assigned by IANA as described in
   Section 10.

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 not yet been
   defined.]] If no such mechanism is used, then 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 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.  API considerations for 0-RTT

   0-RTT data has very different security properties from data
   transmitted after a completed handshake: it can be replayed.
   Implementations SHOULD provide different functions for reading and
   writing 0-RTT data and data transmitted after the handshake, and
   SHOULD NOT automatically resend 0-RTT data if it is rejected by the
   server.

B.2.  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,  In 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 cases, the counter value operating system provides an appropriate
   facility such as /dev/urandom, which should be used absent other
   (performance) concerns.  It is 16 bits or more.  Seeding a 128-bit generally preferrable to use an
   existing PRNG would
   thus require approximately 100 such timer values. implementation in preference to crafting a new one, and
   many adequate cryptographic libraries are already available under
   favorable license terms.  Should those prove unsatisfactory,
   [RFC4086] provides guidance on the generation of random values.

B.2.

B.3.  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.

B.4.  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.

B.5.  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 5.1)?  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 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 (Section 4.1.1), 4.1.2),
      hello extensions (Section 4.2), named groups (Section 4.2.3), and
      signature algorithms (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?  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, (see Section 4.2.4.1)?

   -  Do you use a strong and, most importantly, properly seeded random
      number generator (see Appendix B.1) B.2) when generating Diffie-Hellman
      private values, the ECDSA "k" parameter, and other 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 4.2.4.1)?

B.5.

B.6.  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  (TLSPlaintext.legacy_record_version &
   TLSCiphertext.record_version)
   TLSCiphertext.legacy_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
   (ClientHello.max_supported_version & ServerHello.server_version) ServerHello.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 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.
   ClientHello.max_supported_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. ClientHello.max_supported_version.  For example, if the server
   supports TLS 1.0, 1.1, and 1.2, and client_version max_supported_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, max_supported_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).
   (TLSPlaintext.legacy_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 attempts to use
   0-RTT, particularly against multi-server deployments.  For example, a
   deployment 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 could be downgraded to TLS 1.2.

   A client that attempts to send 0-RTT data MUST fail a connection if
   it receives a ServerHello with TLS 1.2 or older.  A client that
   attempts to repair this error SHOULD NOT send a TLS 1.2 ClientHello,
   but instead send a TLS 1.3 ClientHello without 0-RTT data.

   To avoid this error condition, multi-server deployments 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 ClientHello.max_supported_version or
   ServerHello.server_version
   ServerHello.version set to { 3, 0 } or less.  Any endpoint receiving
   a Hello message with ClientHello.client_version ClientHello.max_supported_version or
   ServerHello.server_version
   ServerHello.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 Properties

   [[TODO: This section is still a WIP and needs a 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 intended to 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
      set of working keys.

   -  A set of cryptographic parameters (algorithms, etc.)

   -  The identities of the communicating parties.

   We assume that the 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 not necessarily independent, but reflect the
   protocol consumers' needs.

   Establishing the same session key.  The handshake needs to output the
      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 the communicating parties, not to the attacker (See
      [CK01]; defn 1, part 2).  Note that in a 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 client's view of the peer identity should
      reflect the server's identity.  If the client is authenticated,
      the 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 should be the
      same on both sides and should be the same as if the peers had been
      communicating in the absence of 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 modes or the
      PSK in PSK-(EC)DHE modes) are compromised after the handshake is
      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 passive attackers.  The
      client's identity should be protected against both passive and
      active attackers.

   Informally, the signature-based modes of TLS 1.3 provide for the
   establishment of a unique, secret, shared, key established by an
   (EC)DHE key exchange and authenticated by the server's signature over
   the handshake transcript, as well as tied to the server's identity by
   a MAC.  If the client is authenticated by a certificate, it also
   signs over the handshake transcript and provides a MAC tied to both
   identities.  [SIGMA] describes the analysis of this type of key
   exchange 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 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
   resumption master secret computed by connection N and needed to form
   connection N+1 is separate from the traffic keys used by connection
   N, thus providing forward secrecy between the connections.

   For all handshake modes, the Finished MAC (and where present, the
   signature), prevents downgrade attacks.  In addition, the use of
   certain bytes in the random nonces as described in Section 4.1.2 4.1.3
   allows the detection of downgrade to previous TLS versions.

   As soon as the client and the server have exchanged enough
   information to establish shared keys, the remainder of the handshake
   is encrypted, thus providing protection against passive attackers.
   Because the server authenticates before the client, the client can
   ensure that it only reveals its identity to an authenticated server.
   Note that implementations must use the provided record padding
   mechanism during the handshake to avoid leaking information about the
   identities due to length.

   The 0-RTT mode of operation generally provides the same security
   properties as 1-RTT data, with the two exceptions that the 0-RTT
   encryption keys do not provide full forward secrecy and that the the
   server is not 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 exposure to
   replay.

   The reader should refer to the following references for analysis of
   the TLS handshake [CHSV16] [FGSW16] [LXZFH16].

D.2.  Record Layer

   The record layer depends on the handshake producing a strong session
   key which can be used to derive bidirectional traffic keys and
   nonces.  Assuming that is true, and the keys are used for no more
   data than indicated in Section 5.5 then the record layer should
   provide the following guarantees:

   Confidentiality.  An attacker should not be able to determine the
      plaintext contents of a given record.

   Integrity.  An attacker should not be able to craft a new record
      which is different from an existing record which will be accepted
      by the receiver.

   Order protection/non-replayability  An attacker should not be able to
      cause the receiver to accept a record which it has already
      accepted or cause the receiver to accept record N+1 without having
      first processed record N.  [[TODO: If we merge in DTLS to this
      document, we will need to update this guarantee.]]

   Length concealment.  Given a record with a given external length, the
      attacker should not be able to determine the amount of the record
      that is content versus padding.

   Forward security after key change.  If the traffic key update
      mechanism described in Section 4.4.3 has been used and the
      previous generation key is deleted, an attacker who compromises
      the endpoint should not be able to decrypt traffic encrypted with
      the old key.

   Informally, TLS 1.3 provides these properties by AEAD-protecting the
   plaintext with a strong key.  AEAD encryption [RFC5116] provides
   confidentiality and integrity for the data.  Non-replayability is
   provided by using a separate nonce for each record, with the nonce
   being derived from the record sequence number (Section 5.3), with the
   sequence number being maintained independently at both sides thus
   records which are delivered out of order result in AEAD deprotection
   failures.

   The plaintext protected by the AEAD function consists of content plus
   variable-length padding.  Because the padding is also encrypted, the
   attacker cannot directly determine the length of the padding, but may
   be able to measure it indirectly by the use of timing channels
   exposed during record processing (i.e., seeing how long it takes to
   process a record).  In general, it is not known how to remove this
   type of channel because even a constant time padding removal function
   will then feed the 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 previous keys after a key
   change (forward secrecy).  However, TLS does not provide security for
   data which is sent after the traffic secret is compromised, even afer
   a key update (backward secrecy); systems which want backward secrecy
   must do a fresh handshake and establish a 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
   https://www.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