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Versions: (draft-ietf-tls-rfc5246-bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if                           March 10, 2017
           approved)
Updates: 4492, 5705, 6066, 6961 (if
         approved)
Intended status: Standards Track
Expires: September 11, 2017


        The Transport Layer Security (TLS) Protocol Version 1.3
                        draft-ietf-tls-tls13-19

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
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on September 11, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   5
     1.2.  Major Differences from TLS 1.2  . . . . . . . . . . . . .   6
     1.3.  Updates Affecting TLS 1.2 . . . . . . . . . . . . . . . .  13
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  14
     2.1.  Incorrect DHE Share . . . . . . . . . . . . . . . . . . .  17
     2.2.  Resumption and Pre-Shared Key (PSK) . . . . . . . . . . .  18
     2.3.  Zero-RTT Data . . . . . . . . . . . . . . . . . . . . . .  20
   3.  Presentation Language . . . . . . . . . . . . . . . . . . . .  22
     3.1.  Basic Block Size  . . . . . . . . . . . . . . . . . . . .  22
     3.2.  Miscellaneous . . . . . . . . . . . . . . . . . . . . . .  22
     3.3.  Vectors . . . . . . . . . . . . . . . . . . . . . . . . .  23
     3.4.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  24
     3.5.  Enumerateds . . . . . . . . . . . . . . . . . . . . . . .  24
     3.6.  Constructed Types . . . . . . . . . . . . . . . . . . . .  25
     3.7.  Constants . . . . . . . . . . . . . . . . . . . . . . . .  25
     3.8.  Variants  . . . . . . . . . . . . . . . . . . . . . . . .  25
     3.9.  Decoding Errors . . . . . . . . . . . . . . . . . . . . .  26
   4.  Handshake Protocol  . . . . . . . . . . . . . . . . . . . . .  27
     4.1.  Key Exchange Messages . . . . . . . . . . . . . . . . . .  28
       4.1.1.  Cryptographic Negotiation . . . . . . . . . . . . . .  28
       4.1.2.  Client Hello  . . . . . . . . . . . . . . . . . . . .  29
       4.1.3.  Server Hello  . . . . . . . . . . . . . . . . . . . .  32
       4.1.4.  Hello Retry Request . . . . . . . . . . . . . . . . .  34
     4.2.  Extensions  . . . . . . . . . . . . . . . . . . . . . . .  35
       4.2.1.  Supported Versions  . . . . . . . . . . . . . . . . .  38
       4.2.2.  Cookie  . . . . . . . . . . . . . . . . . . . . . . .  39
       4.2.3.  Signature Algorithms  . . . . . . . . . . . . . . . .  39
       4.2.4.  Negotiated Groups . . . . . . . . . . . . . . . . . .  42
       4.2.5.  Key Share . . . . . . . . . . . . . . . . . . . . . .  44



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       4.2.6.  Pre-Shared Key Exchange Modes . . . . . . . . . . . .  47
       4.2.7.  Early Data Indication . . . . . . . . . . . . . . . .  47
       4.2.8.  Pre-Shared Key Extension  . . . . . . . . . . . . . .  50
     4.3.  Server Parameters . . . . . . . . . . . . . . . . . . . .  54
       4.3.1.  Encrypted Extensions  . . . . . . . . . . . . . . . .  54
       4.3.2.  Certificate Request . . . . . . . . . . . . . . . . .  54
     4.4.  Authentication Messages . . . . . . . . . . . . . . . . .  56
       4.4.1.  The Transcript Hash . . . . . . . . . . . . . . . . .  57
       4.4.2.  Certificate . . . . . . . . . . . . . . . . . . . . .  58
       4.4.3.  Certificate Verify  . . . . . . . . . . . . . . . . .  62
       4.4.4.  Finished  . . . . . . . . . . . . . . . . . . . . . .  64
     4.5.  End of Early Data . . . . . . . . . . . . . . . . . . . .  65
     4.6.  Post-Handshake Messages . . . . . . . . . . . . . . . . .  66
       4.6.1.  New Session Ticket Message  . . . . . . . . . . . . .  66
       4.6.2.  Post-Handshake Authentication . . . . . . . . . . . .  67
       4.6.3.  Key and IV Update . . . . . . . . . . . . . . . . . .  68
   5.  Record Protocol . . . . . . . . . . . . . . . . . . . . . . .  69
     5.1.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  69
     5.2.  Record Payload Protection . . . . . . . . . . . . . . . .  71
     5.3.  Per-Record Nonce  . . . . . . . . . . . . . . . . . . . .  73
     5.4.  Record Padding  . . . . . . . . . . . . . . . . . . . . .  74
     5.5.  Limits on Key Usage . . . . . . . . . . . . . . . . . . .  75
   6.  Alert Protocol  . . . . . . . . . . . . . . . . . . . . . . .  75
     6.1.  Closure Alerts  . . . . . . . . . . . . . . . . . . . . .  76
     6.2.  Error Alerts  . . . . . . . . . . . . . . . . . . . . . .  77
   7.  Cryptographic Computations  . . . . . . . . . . . . . . . . .  80
     7.1.  Key Schedule  . . . . . . . . . . . . . . . . . . . . . .  80
     7.2.  Updating Traffic Keys and IVs . . . . . . . . . . . . . .  83
     7.3.  Traffic Key Calculation . . . . . . . . . . . . . . . . .  84
     7.4.  (EC)DHE Shared Secret Calculation . . . . . . . . . . . .  84
       7.4.1.  Finite Field Diffie-Hellman . . . . . . . . . . . . .  84
       7.4.2.  Elliptic Curve Diffie-Hellman . . . . . . . . . . . .  85
     7.5.  Exporters . . . . . . . . . . . . . . . . . . . . . . . .  85
   8.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  86
     8.1.  Mandatory-to-Implement Cipher Suites  . . . . . . . . . .  86
     8.2.  Mandatory-to-Implement Extensions . . . . . . . . . . . .  86
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  88
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  88
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  89
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  89
     11.2.  Informative References . . . . . . . . . . . . . . . . .  91
   Appendix A.  State Machine  . . . . . . . . . . . . . . . . . . .  98
     A.1.  Client  . . . . . . . . . . . . . . . . . . . . . . . . .  98
     A.2.  Server  . . . . . . . . . . . . . . . . . . . . . . . . .  98
   Appendix B.  Protocol Data Structures and Constant Values . . . .  99
     B.1.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  99
     B.2.  Alert Messages  . . . . . . . . . . . . . . . . . . . . . 100
     B.3.  Handshake Protocol  . . . . . . . . . . . . . . . . . . . 102



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       B.3.1.  Key Exchange Messages . . . . . . . . . . . . . . . . 102
       B.3.2.  Server Parameters Messages  . . . . . . . . . . . . . 107
       B.3.3.  Authentication Messages . . . . . . . . . . . . . . . 108
       B.3.4.  Ticket Establishment  . . . . . . . . . . . . . . . . 109
       B.3.5.  Updating Keys . . . . . . . . . . . . . . . . . . . . 109
     B.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 109
   Appendix C.  Implementation Notes . . . . . . . . . . . . . . . . 110
     C.1.  API considerations for 0-RTT  . . . . . . . . . . . . . . 110
     C.2.  Random Number Generation and Seeding  . . . . . . . . . . 111
     C.3.  Certificates and Authentication . . . . . . . . . . . . . 111
     C.4.  Implementation Pitfalls . . . . . . . . . . . . . . . . . 111
     C.5.  Client Tracking Prevention  . . . . . . . . . . . . . . . 113
     C.6.  Unauthenticated Operation . . . . . . . . . . . . . . . . 113
   Appendix D.  Backward Compatibility . . . . . . . . . . . . . . . 113
     D.1.  Negotiating with an older server  . . . . . . . . . . . . 114
     D.2.  Negotiating with an older client  . . . . . . . . . . . . 115
     D.3.  Zero-RTT backwards compatibility  . . . . . . . . . . . . 115
     D.4.  Backwards Compatibility Security Restrictions . . . . . . 116
   Appendix E.  Overview of Security Properties  . . . . . . . . . . 116
     E.1.  Handshake . . . . . . . . . . . . . . . . . . . . . . . . 117
     E.2.  Record Layer  . . . . . . . . . . . . . . . . . . . . . . 120
   Appendix F.  Working Group Information  . . . . . . . . . . . . . 121
   Appendix G.  Contributors . . . . . . . . . . . . . . . . . . . . 122
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 127

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 key (PSK).






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   -  Confidentiality: Data sent over the channel is only visible to the
      endpoints.  TLS does not hide the length of the data it transmits,
      though endpoints are able to pad in order to obscure lengths.

   -  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 E for a more complete statement of the relevant security
   properties.

   TLS consists of two primary components:

   -  A handshake protocol (Section 4) that 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) that 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; 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:



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   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 subsequent interactions.

   peer: An endpoint.  When discussing a particular endpoint, "peer"
   refers to the endpoint that is not the primary subject of discussion.

   receiver: An endpoint that is receiving records.

   sender: An endpoint that is transmitting records.

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

1.2.  Major Differences from TLS 1.2

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

   draft-19

   -  Hash context_value input to Exporters (*)

   -  Add an additional Derive-Secret stage to Exporters (*).

   -  Hash ClientHello1 in the transcript when HRR is used.  This
      reduces the state that needs to be carried in cookies. (*)

   -  Restructure CertificateRequest to have the selectors in
      extensions.  This also allowed defining a
      "certificate_authorities" extension which can be used by the
      client instead of trusted_ca_keys (*).

   -  Tighten record framing requirements and require checking of them
      (*).

   -  Consolidate "ticket_early_data_info" and "early_data" into a
      single extension (*).

   -  Change end_of_early_data to be a handshake message (*).

   -  Add pre-extract Derive-Secret stages to key schedule (*).

   -  Remove spurious requirement to implement "pre_shared_key".



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   -  Clarify location of "early_data" from server (it goes in EE, as
      indicated by the table in S 10).

   -  Require peer public key validation

   -  Add state machine diagram.

   draft-18

   -  Remove unnecessary resumption_psk which is the only thing expanded
      from the resumption master secret. (*).

   -  Fix signature_algorithms entry in extensions table.

   -  Restate rule from RFC 6066 that you can't resume unless SNI is the
      same.

   draft-17

   -  Remove 0-RTT Finished and resumption_context, and replace with a
      psk_binder field in the PSK itself (*)

   -  Restructure PSK key exchange negotiation modes (*)

   -  Add max_early_data_size field to TicketEarlyDataInfo (*)

   -  Add a 0-RTT exporter and change the transcript for the regular
      exporter (*)

   -  Merge TicketExtensions and Extensions registry.  Changes
      ticket_early_data_info code point (*)

   -  Replace Client.key_shares in response to HRR (*)

   -  Remove redundant labels for traffic key derivation (*)

   -  Harmonize requirements about cipher suite matching: for resumption
      you need to match KDF but for 0-RTT you need whole cipher suite.
      This allows PSKs to actually negotiate cipher suites. (*)

   -  Move SCT and OCSP into Certificate.extensions (*)

   -  Explicitly allow non-offered extensions in NewSessionTicket

   -  Explicitly allow predicting ClientFinished for NST

   -  Clarify conditions for allowing 0-RTT with PSK




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   draft-16

   -  Revise version negotiation (*)

   -  Change RSASSA-PSS and EdDSA SignatureScheme codepoints for better
      backwards compatibility (*)

   -  Move HelloRetryRequest.selected_group to an extension (*)

   -  Clarify the behavior of no exporter context and make it the same
      as an empty context.(*)

   -  New KeyUpdate format that allows for requesting/not-requesting an
      answer.  This also means changes to the key schedule to support
      independent updates (*)

   -  New certificate_required alert (*)

   -  Forbid CertificateRequest with 0-RTT and PSK.

   -  Relax requirement to check SNI for 0-RTT.

   draft-15

   -  New negotiation syntax as discussed in Berlin (*)

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

   -  Forbid empty tickets (*)

   -  Forbid application data messages in between post-handshake
      messages from the same flight (*)

   -  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



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   -  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
      accommodate 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.

   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.




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   -  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.





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   -  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.





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   -  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.




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   -  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 4.2.3.

   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.






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2.  Protocol Overview

   The cryptographic parameters of the connection state are produced by
   the TLS handshake protocol, which a TLS client and server use when
   first communicating to 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.

   A failure of the handshake or other protocol error triggers the
   termination of the connection, optionally preceded by an alert
   message (Section 6).

   TLS supports three basic key exchange modes:

   -  (EC)DHE (Diffie-Hellman both the finite field and elliptic curve
      varieties),

   -  PSK-only, and

   -  PSK with (EC)DHE

   Figure 1 below shows the basic full TLS handshake:



























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       Client                                               Server

Key  ^ ClientHello
Exch | + key_share*
     | + psk_key_exchange_modes*
     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 noteworthy extensions sent in the
                 previously noted message.

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

              {} Indicates messages protected using keys
                 derived from a [sender]_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.




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   In the Key Exchange phase, the client sends the ClientHello
   (Section 4.1.2) message, which contains a random nonce
   (ClientHello.random); its offered protocol versions; a list of
   symmetric cipher/HKDF hash pairs; some set of Diffie-Hellman key
   shares (in the "key_share" extension Section 4.2.5), a set of pre-
   shared key labels (in the "pre_shared_key" extension Section 4.2.8)
   or 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 (Section 4.1.3), which indicates the negotiated
   connection parameters.  The combination of the ClientHello and the
   ServerHello determines the shared keys.  If (EC)DHE key establishment
   is in use, then the ServerHello contains a "key_share" extension with
   the server's ephemeral Diffie-Hellman share which MUST be in the same
   group as one of the client's shares.  If PSK key establishment is in
   use, then the ServerHello contains 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 that are not
      required to determine the cryptographic parameters, other than
      those that are specific to individual certificates.
      [Section 4.3.1]

   CertificateRequest:  if certificate-based client authentication is
      desired, the desired parameters for that certificate.  This
      message is omitted if client authentication is not desired.
      [Section 4.3.2]

   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 and any per-certificate
      extensions.  This message is omitted by the server if not
      authenticating with a certificate and by the client if the server
      did not send CertificateRequest (thus indicating that the client
      should not authenticate with a certificate).  Note that if raw
      public keys [RFC7250] or the cached information extension
      [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 4.4.2]




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   CertificateVerify:  a signature over the entire handshake using the
      private key corresponding to the public key in the Certificate
      message.  This message is omitted if the endpoint is not
      authenticating via a certificate.  [Section 4.4.3]

   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 4.4.4]

   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 to or
   unsupported by the server), the server corrects the mismatch with a
   HelloRetryRequest and the client needs 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 MUST
   abort the handshake with an appropriate alert.





















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            Client                                               Server

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

            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 connection and then reused ("session
   resumption").  Once a handshake has completed, the server can send
   the client a PSK identity that corresponds to a key derived from the
   initial handshake (see Section 4.6.1).  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 and the key derived
   from the initial handshake is used to bootstrap the cryptographic
   state instead of a full handshake.  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.






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   PSKs can be used with (EC)DHE key exchange in order to provide
   forward secrecy in combination with shared keys, or can 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
          + key_share*
          + psk_key_exchange_modes
          + pre_shared_key          -------->
                                                          ServerHello
                                                     + pre_shared_key
                                                         + key_share*
                                                {EncryptedExtensions}
                                                           {Finished}
                                    <--------     [Application Data*]
          {Finished}                -------->
          [Application Data]        <------->      [Application Data]

               Figure 3: Message flow for resumption and PSK

   As the server is authenticating via a PSK, it does not send a
   Certificate or a CertificateVerify message.  When a client offers
   resumption via PSK, it SHOULD also supply a "key_share" extension to
   the server to allow the server to decline resumption and fall back to
   a full handshake, if needed.  The server responds with a



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   "pre_shared_key" extension to negotiate 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.

   When PSKs are provisioned out of band, the PSK identity and the KDF
   to be used with the PSK MUST also be provisioned.

2.3.  Zero-RTT Data

   When clients and servers share a PSK (either obtained externally or
   via a previous handshake), TLS 1.3 allows clients to send data on the
   first flight ("early data").  The client uses the PSK to authenticate
   the server and to encrypt the early data.

   When clients use a PSK obtained externally then the following
   additional information MUST be provisioned to both parties:

   -  The cipher suite for use with this PSK

   -  The Application-Layer Protocol Negotiation (ALPN) protocol, if any
      is to be used

   -  The Server Name Indication (SNI), if any is to be used

   As shown in Figure 4, the 0-RTT data is just added to the 1-RTT
   handshake in the first flight.  The rest of the handshake uses the
   same messages as with a 1-RTT handshake with PSK resumption.
























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            Client                                               Server

            ClientHello
            + early_data
            + key_share*
            + psk_key_exchange_modes
            + pre_shared_key
            (Application Data*)     -------->
                                                            ServerHello
                                                       + pre_shared_key
                                                           + key_share*
                                                  {EncryptedExtensions}
                                                             {Finished}
                                    <--------       [Application Data*]
            (EndOfEarlyData)
            {Finished}              -------->

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

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

                  () Indicates messages protected using keys
                     derived from client_early_traffic_secret.

                  {} Indicates messages protected using keys
                     derived from a [sender]_handshake_traffic_secret.

                  [] Indicates messages protected using keys
                     derived from traffic_secret_N

          Figure 4: Message flow for a zero round trip handshake

   IMPORTANT NOTE: The security properties for 0-RTT data are weaker
   than those for other kinds of TLS data.  Specifically:

   1.  This data is not forward secret, as it is encrypted solely under
       keys derived using the offered 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 4.2.8.3 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



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       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.)

   Protocols MUST NOT use 0-RTT data without a profile that defines its
   use.  That profile needs to identify which messages or interactions
   are safe to use with 0-RTT.  In addition, to avoid accidental misuse,
   implementations SHOULD NOT enable 0-RTT unless specifically
   requested.  Implementations SHOULD provide special functions for
   0-RTT data to ensure that an application is always aware that it is
   sending or receiving data that might be replayed.

   The same warnings apply to any use of the early_exporter_secret.

   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.

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.





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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 exact multiple of the length of a single
   element (e.g., 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 */





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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.

3.5.  Enumerateds

   An additional sparse data type is available called enum.  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;

   Future extensions or additions to the protocol may define new values.
   Implementations need to be able to parse and ignore unknown values
   unless the definition of the field states otherwise.

   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 in the current
   version of the protocol.

      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



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   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 */

   The names assigned to enumerateds do not need to be unique.  The
   numerical value can describe a range over which the same name
   applies.  The value includes the minimum and maximum inclusive values
   in that range, separated by two period characters.  This is
   principally useful for reserving regions of the space.

      enum { sad(0), meh(1..254), happy(255) } Mood;

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.  Anonymous structs may also be
   defined inside other structures.

3.7.  Constants

   Fields and variables may be assigned a fixed value using "=", as in:

      struct {
          T1 f1 = 8;  /* T.f1 must always be 8 */
          T2 f2;
      } T;

3.8.  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.  The



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   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 en: Ten;
          } [[fv]];
      } [[Tv]];

   For example:

      enum { apple(0), orange(1) } VariantTag;

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

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

      struct {
          VariantTag type;
          select (VariantRecord.type) {
              case apple:  V1;
              case orange: V2;
          };
      } VariantRecord;

3.9.  Decoding Errors

   TLS defines two generic alerts (see Section 6) to use upon failure to
   parse a message.  Peers which receive a message which cannot be
   parsed according to the syntax (e.g., have a length extending beyond
   the message boundary or contain an out-of-range length) MUST
   terminate the connection with a "decode_error" alert.  Peers which
   receive a message which is syntactically correct but semantically




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   invalid (e.g., a DHE share of p - 1, or an invalid enum) MUST
   terminate the connection with an "illegal_parameter" alert.

4.  Handshake Protocol

   The handshake protocol is used to negotiate the secure attributes of
   a connection.  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 connection state.

      enum {
          client_hello(1),
          server_hello(2),
          new_session_ticket(4),
          end_of_early_data(5),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          certificate_request(13),
          certificate_verify(15),
          finished(20),
          key_update(24),
          message_hash(254),
          (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (Handshake.msg_type) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case end_of_early_data:     EndOfEarlyData;
              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).  A peer which receives a handshake
   message in an unexpected order MUST abort the handshake with an



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   "unexpected_message" alert.  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.  Cryptographic Negotiation

   TLS cryptographic negotiation proceeds by the client offering the
   following four sets of options in its ClientHello:

   -  A list of cipher suites which indicates the AEAD algorithm/HKDF
      hash pairs which the client supports.

   -  A "supported_groups" (Section 4.2.4) extension which indicates the
      (EC)DHE groups which the client supports and a "key_share"
      (Section 4.2.5) extension which contains (EC)DHE shares for some
      or all of these groups.

   -  A "signature_algorithms" (Section 4.2.3) extension which indicates
      the signature algorithms which the client can accept.

   -  A "pre_shared_key" (Section 4.2.8) extension which contains a list
      of symmetric key identities known to the client and a
      "psk_key_exchange_modes" (Section 4.2.6) extension which indicates
      the key exchange modes that may be used with PSKs.

   If the server does not select a PSK, then the first three of these
   options are entirely orthogonal: the server independently selects a
   cipher suite, an (EC)DHE group and key share for key establishment,
   and a signature algorithm/certificate pair to authenticate itself to
   the client.  If there is no overlap in "supported_groups" then the
   server MUST abort the handshake.

   If the server selects a PSK, then it MUST also select a key
   establishment mode from the set indicated by client's
   "psk_key_exchange_modes" extension (PSK alone or with (EC)DHE).  Note
   that if the PSK can be used without (EC)DHE then non-overlap in the
   "supported_groups" parameters need not be fatal, as it is in the non-
   PSK case discussed in the previous paragraph.





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   If the server selects an (EC)DHE group and the client did not offer a
   compatible "key_share" extension in the initial ClientHello, the
   server MUST respond with a HelloRetryRequest (Section 4.1.4) message.

   If the server successfully selects parameters and does not require a
   HelloRetryRequest, it indicates the selected parameters in the
   ServerHello as follows:

   -  If PSK is being used, then the server will send a "pre_shared_key"
      extension indicating the 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 server will also provide a "key_share"
      extension.

   -  When authenticating via a certificate, the server will send the
      Certificate (Section 4.4.2) and CertificateVerify (Section 4.4.3)
      messages.  In TLS 1.3 as defined by this document, either a PSK or
      a certificate is always used, but not both.  Future documents may
      define how to use them together.

   If the server is unable to negotiate a supported set of parameters
   (i.e., there is no overlap between the client and server parameters),
   it MUST abort the handshake with either a "handshake_failure" or
   "insufficient_security" fatal alert (see Section 6).

4.1.2.  Client Hello

   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 has responded to its ClientHello with a
   HelloRetryRequest.  In that case, the client MUST send the same
   ClientHello (without modification) except:

   -  If a "key_share" extension was supplied in the HelloRetryRequest,
      replacing the list of shares with a list containing a single
      KeyShareEntry from the indicated group.

   -  Removing the "early_data" extension (Section 4.2.7) if one was
      present.  Early data is not permitted after HelloRetryRequest.

   -  Including a "cookie" extension if one was provided in the
      HelloRetryRequest.

   -  Updating the "pre_shared_key" extension if present by recomputing
      the "obfuscated_ticket_age" and binder values and (optionally)



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      removing any PSKs which are incompatible with the server's
      indicated cipher suite.

   Because TLS 1.3 forbids renegotiation, if a server receives a
   ClientHello at any other time, it MUST terminate the connection.

   If a server established a TLS connection with a previous version of
   TLS and receives a TLS 1.3 ClientHello in a renegotiation, it MUST
   retain the previous protocol version.  In particular, it MUST NOT
   negotiate TLS 1.3.

   Structure of this message:

      uint16 ProtocolVersion;
      opaque Random[32];

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

      struct {
          ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
          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<8..2^16-1>;
      } ClientHello;

   All versions of TLS allow extensions to optionally follow the
   compression_methods field as an extensions field.  TLS 1.3
   ClientHellos will contain at least two extensions,
   "supported_versions" and either "key_share" or "pre_shared_key".  The
   presence of extensions can be detected by determining whether there
   are bytes following the compression_methods at the end of the
   ClientHello.  Note that this method of detecting optional data
   differs from the normal TLS method of having a variable-length field,
   but it is used for compatibility with TLS before extensions were
   defined.  TLS 1.3 servers will need to perform this check first and
   only attempt to negotiate TLS 1.3 if a "supported_version" extension
   is present.

   legacy_version  In previous versions of TLS, this field was used for
      version negotiation and represented the highest version number
      supported by the client.  Experience has shown that many servers
      do not properly implement version negotiation, leading to "version
      intolerance" in which the server rejects an otherwise acceptable
      ClientHello with a version number higher than it supports.  In TLS
      1.3, the client indicates its version preferences in the
      "supported_versions" extension (Section 4.2.1) and the



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      legacy_version field MUST be set to 0x0303, which was the version
      number for TLS 1.2.  (See Appendix D for details about backward
      compatibility.)

   random  32 bytes generated by a secure random number generator.  See
      Appendix C 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 be 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 symmetric cipher options
      supported by the client, specifically the record protection
      algorithm (including secret key length) and a hash to be used with
      HKDF, in descending order of client preference.  If the list
      contains cipher suites the server does not recognize, support, or
      wish to use, the server MUST ignore those cipher suites, and
      process the remaining ones as usual.  Values are defined in
      Appendix B.4.  If the client is attempting a PSK key
      establishment, it SHOULD advertise at least one cipher suite
      containing a Hash associated with the PSK.

   legacy_compression_methods  Versions of TLS before 1.3 supported
      compression with the list of supported compression methods being
      sent 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.  If a TLS
      1.3 ClientHello is received with any other value in this field,
      the server MUST abort the handshake with an "illegal_parameter"
      alert.  Note that TLS 1.3 servers might receive TLS 1.2 or prior
      ClientHellos which contain other compression methods and MUST
      follow the procedures for the appropriate prior version of 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 TLS 1.3, use of certain
      extensions is mandatory, as functionality is moved into extensions
      to preserve ClientHello compatibility with previous versions of
      TLS.  Servers MUST ignore unrecognized extensions.

   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
   messages always contain extensions (minimally they must contain
   "supported_versions" or they will be interpreted as TLS 1.2



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   ClientHello messages).  TLS 1.3 servers might receive ClientHello
   messages from versions of TLS prior to 1.3 that do not contain
   extensions.  If negotiating a version of TLS prior to 1.3, a server
   MUST check that the message either contains no data after
   legacy_compression_methods or that it contains a valid extensions
   block with no data following.  If not, then it MUST abort the
   handshake with a "decode_error" alert.

   After sending the ClientHello message, the client waits for a
   ServerHello or HelloRetryRequest message.

4.1.3.  Server Hello

   The server will send this message in response to a ClientHello
   message if it is able to find an acceptable set of parameters and the
   ClientHello contains sufficient information to proceed with the
   handshake.

   Structure of this message:

      struct {
          ProtocolVersion version;
          Random random;
          CipherSuite cipher_suite;
          Extension extensions<6..2^16-1>;
      } ServerHello;

   version  This field contains the version of TLS negotiated for this
      connection.  Servers MUST select a version from the list in
      ClientHello.supported_versions extension.  A client which receives
      a version that was not offered MUST abort the handshake.  For this
      version of the specification, the version is 0x0304.  (See
      Appendix D for details about backward compatibility.)

   random  32 bytes generated by a secure random number generator.  See
      Appendix C for additional information.  The last eight bytes MUST
      be overwritten as described below if negotiating TLS 1.2 or TLS
      1.1.  This structure is generated by the server and MUST be
      generated independently of the ClientHello.random.

   cipher_suite  The single cipher suite selected by the server from the
      list in ClientHello.cipher_suites.  A client which receives a
      cipher suite that was not offered MUST abort the handshake.

   extensions  A list of extensions.  The ServerHello MUST only include
      extensions which are required to establish the cryptographic
      context.  Currently the only such extensions are "key_share" and
      "pre_shared_key".  All current TLS 1.3 ServerHello messages will



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      contain one of these two extensions, or both when using a PSK with
      (EC)DHE key establishment.

   TLS 1.3 has a downgrade protection mechanism embedded in the server's
   random value.  TLS 1.3 servers which negotiate TLS 1.2 or below in
   response to a ClientHello MUST set the last eight bytes of their
   Random value specially.

   If negotiating TLS 1.2, TLS 1.3 servers MUST set the last eight bytes
   of their Random value to the bytes:

     44 4F 57 4E 47 52 44 01

   If negotiating TLS 1.1, TLS 1.3 servers MUST and TLS 1.2 servers
   SHOULD set the last eight bytes of their Random value to 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 the last eight bytes are not equal to either of these values.
   TLS 1.2 clients SHOULD also check that the last eight bytes are not
   equal to the second value if the ServerHello indicates TLS 1.1 or
   below.  If a match is found, the client MUST abort the handshake with
   an "illegal_parameter" alert.  This mechanism provides limited
   protection against downgrade attacks over and above that provided by
   the Finished exchange: because the ServerKeyExchange, a message
   present in TLS 1.2 and below, includes a signature over both random
   values, it is not possible for an active attacker to modify the
   random values without detection as long as ephemeral ciphers are
   used.  It does not provide downgrade protection when static RSA is
   used.

   Note: This is a change from [RFC5246], so in practice many TLS 1.2
   clients and servers will not behave as specified above.

   A client that receives a TLS 1.3 ServerHello during renegotiation
   MUST abort the handshake with a "protocol_version" alert.

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH Implementations of
   draft versions (see Section 4.2.1.1) of this specification SHOULD NOT
   implement this mechanism on either client and server.  A pre-RFC
   client connecting to RFC servers, or vice versa, will appear to
   downgrade to TLS 1.2.  With the mechanism enabled, this will cause an
   interoperability failure.







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4.1.4.  Hello Retry Request

   The server will send this message in response to a ClientHello
   message if it is able to find an acceptable set of parameters but the
   ClientHello does not contain sufficient information to proceed with
   the handshake.

   Structure of this message:

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

   The version, cipher_suite, and extensions fields have the same
   meanings as their corresponding values in the ServerHello.  The
   server SHOULD send only the extensions necessary for the client to
   generate a correct ClientHello pair.  As with ServerHello, a
   HelloRetryRequest MUST NOT contain any extensions that were not first
   offered by the client in its ClientHello, with the exception of
   optionally the "cookie" (see Section 4.2.2) extension.

   Upon receipt of a HelloRetryRequest, the client MUST verify that the
   extensions block is not empty and otherwise MUST abort the handshake
   with a "decode_error" alert.  Clients MUST abort the handshake with
   an "illegal_parameter" alert if the HelloRetryRequest would not
   result in any change in the ClientHello.  If a client receives a
   second HelloRetryRequest in the same connection (i.e., where the
   ClientHello was itself in response to a HelloRetryRequest), it MUST
   abort the handshake with an "unexpected_message" alert.

   Otherwise, the client MUST process all extensions in the
   HelloRetryRequest and send a second updated ClientHello.  The
   HelloRetryRequest extensions defined in this specification are:

   -  cookie (see Section 4.2.2)

   -  key_share (see Section 4.2.5)

   In addition, in its updated ClientHello, the client SHOULD NOT offer
   any pre-shared keys associated with a hash other than that of the
   selected cipher suite.  This allows the client to avoid having to
   compute partial hash transcripts for multiple hashes in the second
   ClientHello.  A client which receives a cipher suite that was not
   offered MUST abort the handshake.  Servers MUST ensure that they
   negotiate the same cipher suite when receiving a conformant updated
   ClientHello (if the server selects the cipher suite as the first step



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   in the negotiation, then this will happen automatically).  Upon
   receiving the ServerHello, clients MUST check that the cipher suite
   supplied in the ServerHello is the same as that in the
   HelloRetryRequest and otherwise abort the handshake with an
   "illegal_parameter" alert.

4.2.  Extensions

   A number of TLS messages contain tag-length-value encoded extensions
   structures.

      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),
          supported_versions(43),
          cookie(44),
          psk_key_exchange_modes(45),
          certificate_authorities(47),
          oid_filters(48),
          (65535)
      } ExtensionType;

   Here:

   -  "extension_type" identifies the particular extension type.

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

   The list of extension types is maintained by IANA as described in
   Section 10.

   Extensions are generally structured in a request/response fashion,
   though some extensions are just indications with no corresponding
   response.  The client sends its extension requests in the ClientHello
   message and the server sends its extension responses in the
   ServerHello, EncryptedExtensions and HelloRetryRequest messages.  The
   server sends extension requests in the CertificateRequest message
   which a client MAY respond to with a Certificate message.  The server




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   MAY also send unsolicited extensions in the NewSessionTicket, though
   the client does not respond directly to these.

   Implementations MUST NOT send extension responses if the remote
   endpoint did not send the corresponding extension requests, with the
   exception of the "cookie" extension in HelloRetryRequest.  Upon
   receiving such an extension, an endpoint MUST abort the handshake
   with an "unsupported_extension" alert.

   The table below indicates the messages where a given extension may
   appear, using the following notation: CH (ClientHello), SH
   (ServerHello), EE (EncryptedExtensions), CT (Certificate), CR
   (CertificateRequest), NST (NewSessionTicket) and HRR
   (HelloRetryRequest).  If an implementation receives an extension
   which it recognizes and which is not specified for the message in
   which it appears it MUST abort the handshake with an
   "illegal_parameter" alert.

    +--------------------------------------------------+-------------+
    | Extension                                        |     TLS 1.3 |
    +--------------------------------------------------+-------------+
    | server_name [RFC6066]                            |      CH, EE |
    |                                                  |             |
    | max_fragment_length [RFC6066]                    |      CH, EE |
    |                                                  |             |
    | client_certificate_url [RFC6066]                 |      CH, EE |
    |                                                  |             |
    | status_request [RFC6066]                         |  CH, CR, CT |
    |                                                  |             |
    | user_mapping [RFC4681]                           |      CH, EE |
    |                                                  |             |
    | cert_type [RFC6091]                              |      CH, EE |
    |                                                  |             |
    | supported_groups [RFC7919]                       |      CH, EE |
    |                                                  |             |
    | signature_algorithms [RFC5246]                   |      CH, CR |
    |                                                  |             |
    | use_srtp [RFC5764]                               |      CH, EE |
    |                                                  |             |
    | heartbeat [RFC6520]                              |      CH, EE |
    |                                                  |             |
    | application_layer_protocol_negotiation [RFC7301] |      CH, EE |
    |                                                  |             |
    | signed_certificate_timestamp [RFC6962]           |  CH, CR, CT |
    |                                                  |             |
    | client_certificate_type [RFC7250]                |      CH, EE |
    |                                                  |             |
    | server_certificate_type [RFC7250]                |      CH, CT |



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    |                                                  |             |
    | padding [RFC7685]                                |          CH |
    |                                                  |             |
    | key_share [[this document]]                      | CH, SH, HRR |
    |                                                  |             |
    | pre_shared_key [[this document]]                 |      CH, SH |
    |                                                  |             |
    | psk_key_exchange_modes [[this document]]         |          CH |
    |                                                  |             |
    | early_data [[this document]]                     | CH, EE, NST |
    |                                                  |             |
    | cookie [[this document]]                         |     CH, HRR |
    |                                                  |             |
    | supported_versions [[this document]]             |          CH |
    |                                                  |             |
    | certificate_authorities [[this document]]        |      CH, CR |
    |                                                  |             |
    | oid_filters [[this document]]                    |          CR |
    +--------------------------------------------------+-------------+

   When multiple extensions of different types are present, the
   extensions MAY appear in any order, with the exception of
   "pre_shared_key" Section 4.2.8 which MUST be the last extension in
   the ClientHello.  There MUST NOT be more than one extension of the
   same type.

   In TLS 1.3, unlike TLS 1.2, extensions are renegotiated with each
   handshake even when in resumption-PSK mode.  However, 0-RTT
   parameters are those negotiated in the previous handshake; mismatches
   may require rejecting 0-RTT (see Section 4.2.7).

   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 in overall security.  The following
   considerations should be taken into account when designing new
   extensions:

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

   -  Extensions should, as far as possible, be designed to prevent any
      attack 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 to cause a
      security problem.  Often the fact that the extension fields are
      included in the inputs to the Finished message hashes will be



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      sufficient, but extreme care is needed when the extension changes
      the meaning of messages sent in the handshake phase.  Designers
      and implementors should be aware of the fact that until the
      handshake has been authenticated, active attackers can modify
      messages and insert, remove, or replace extensions.

4.2.1.  Supported Versions

      struct {
          ProtocolVersion versions<2..254>;
      } SupportedVersions;

   The "supported_versions" extension is used by the client to indicate
   which versions of TLS it supports.  The extension contains a list of
   supported versions in preference order, with the most preferred
   version first.  Implementations of this specification MUST send this
   extension containing all versions of TLS which they are prepared to
   negotiate (for this specification, that means minimally 0x0304, but
   if previous versions of TLS are supported, they MUST be present as
   well).

   If this extension is not present, servers which are compliant with
   this specification MUST negotiate TLS 1.2 or prior as specified in
   [RFC5246], even if ClientHello.legacy_version is 0x0304 or later.

   If this extension is present, servers MUST ignore the
   ClientHello.legacy_version value and MUST use only the
   "supported_versions" extension to determine client preferences.
   Servers MUST only select a version of TLS present in that extension
   and MUST ignore any unknown versions.  Note that this mechanism makes
   it possible to negotiate a version prior to TLS 1.2 if one side
   supports a sparse range.  Implementations of TLS 1.3 which choose to
   support prior versions of TLS SHOULD support TLS 1.2.

   The server MUST NOT send the "supported_versions" extension.  The
   server's selected version is contained in the ServerHello.version
   field as in previous versions of TLS.

4.2.1.1.  Draft Version Indicator

   RFC EDITOR: PLEASE REMOVE THIS SECTION

   While the eventual version indicator for the RFC version of TLS 1.3
   will be 0x0304, implementations of draft versions of this
   specification SHOULD instead advertise 0x7f00 | draft_version in
   ServerHello.version, and HelloRetryRequest.server_version.  For
   instance, draft-17 would be encoded as the 0x7f11.  This allows pre-




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   RFC implementations to safely negotiate with each other, even if they
   would otherwise be incompatible.

4.2.2.  Cookie

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

   Cookies serve two primary purposes:

   -  Allowing the server to force 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 the server to offload state to the client, thus allowing
      it to send a HelloRetryRequest without storing any state.  The
      server does this by storing the hash of the ClientHello in the
      HelloRetryRequest cookie (protected with some suitable integrity
      algorithm).

   When sending a HelloRetryRequest, the server MAY provide a "cookie"
   extension to the client (this is an exception to the usual rule that
   the only extensions that may be sent are those that appear in the
   ClientHello).  When sending the new ClientHello, the client MUST echo
   the value of the extension.  Clients MUST NOT use cookies in
   subsequent connections.

4.2.3.  Signature Algorithms

   The client uses the "signature_algorithms" extension to indicate to
   the server which signature algorithms may be used in digital
   signatures.  Clients which desire the server to authenticate itself
   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 abort the
   handshake with a "missing_extension" alert (see Section 8.2).

   The "extension_data" field of this extension in a ClientHello
   contains a SignatureSchemeList value:









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      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(0x0804),
          rsa_pss_sha384(0x0805),
          rsa_pss_sha512(0x0806),

          /* EdDSA algorithms */
          ed25519(0x0807),
          ed448(0x0808),

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

      struct {
          SignatureScheme supported_signature_algorithms<2..2^16-2>;
      } SignatureSchemeList;

   Note: This enum is named "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
   the client is willing to verify.  The values are indicated in
   descending order of preference.  Note that a signature algorithm
   takes as input an arbitrary-length message, rather than a digest.
   Algorithms which traditionally act on a digest should be defined in
   TLS to first hash the input with a specified hash algorithm 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] with the corresponding hash algorithm
      as defined in [SHS].  These values refer solely to signatures
      which appear in certificates (see Section 4.4.2.2) and are not
      defined for use in signed TLS handshake messages.



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   ECDSA algorithms  Indicates a signature algorithm using ECDSA
      [ECDSA], the corresponding curve as defined in ANSI X9.62 [X962]
      and FIPS 186-4 [DSS], and the corresponding hash algorithm as
      defined in [SHS].  The signature is represented as a DER-encoded
      [X690] ECDSA-Sig-Value structure.

   RSASSA-PSS algorithms  Indicates a signature algorithm using RSASSA-
      PSS [RFC3447] with mask generation function 1.  The digest used in
      the 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 length of the salt MUST be
      equal to the length of the digest output.  This codepoint is also
      defined for use with TLS 1.2.

   EdDSA algorithms  Indicates a signature algorithm using EdDSA as
      defined in [RFC8032] or its successors.  Note that these
      correspond to the "PureEdDSA" algorithms and not the "prehash"
      variants.

   rsa_pkcs1_sha1, dsa_sha1, and ecdsa_sha1 SHOULD NOT be offered.
   Clients offering these values for backwards compatibility MUST list
   them as the lowest priority (listed after all other algorithms in
   SignatureSchemeList).  TLS 1.3 servers MUST NOT offer a SHA-1 signed
   certificate unless no valid certificate chain can be produced without
   it (see Section 4.4.2.2).

   The signatures on certificates that are self-signed or certificates
   that are trust anchors are not validated since they begin a
   certification path (see [RFC5280], Section 3.2).  A certificate that
   begins a certification path MAY use a signature algorithm that is not
   advertised as being supported 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 in
   accordance with the requirements of [RFC5246] when negotiating that
   version.  In particular:

   -  TLS 1.2 ClientHellos MAY omit this extension.

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




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   -  ECDSA signature schemes align with TLS 1.2's ECDSA hash/signature
      pairs.  However, the old semantics did not constrain the signing
      curve.  If TLS 1.2 is negotiated, implementations MUST be prepared
      to accept a signature that uses any curve that they advertised in
      the "supported_groups" extension.

   -  Implementations that advertise support for RSASSA-PSS (which is
      mandatory in TLS 1.3), MUST be prepared to accept a signature
      using that scheme even when TLS 1.2 is negotiated.  In TLS 1.2,
      RSASSA-PSS is used with RSA cipher suites.

4.2.3.1.  Certificate Authorities

   The "certificate_authorities" extension is used to indicate the
   certificate authorities which an endpoint supports and which SHOULD
   be used by the receiving endpoint to guide certificate selection.

   The body of the "certificate_authorities" extension consists of a
   CertificateAuthoritiesExtension structure.

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

      struct {
          DistinguishedName authorities<3..2^16-1>;
      } CertificateAuthoritiesExtension;

   authorities  A list of the distinguished names [X501] of acceptable
      certificate authorities, represented in DER-encoded [X690] format.
      These distinguished names specify a desired distinguished name for
      trust anchor or subordinate CA; thus, this message can be used to
      describe known trust anchors as well as a desired authorization
      space.

   The client MAY send the "certificate_authorities" extension in the
   ClientHello message.  The server MAY send it in the
   CertificateRequest message.

   The "trusted_ca_keys" extension, which serves a similar purpose
   [RFC6066], but is more complicated, is not used in TLS 1.3 (although
   it may appear in ClientHello messages from clients which are offering
   prior versions of TLS).

4.2.4.  Negotiated Groups

   When sent by the client, the "supported_groups" extension indicates
   the named groups which the client supports for key exchange, ordered
   from most preferred to least preferred.




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   Note: In versions of TLS prior to TLS 1.3, this extension was named
   "elliptic_curves" and only contained elliptic curve groups.  See
   [RFC4492] and [RFC7919].  This extension was also used to negotiate
   ECDSA curves.  Signature algorithms are now negotiated independently
   (see Section 4.2.3).

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

      enum {
          /* Elliptic Curve Groups (ECDHE) */
          secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
          x25519(0x001D), x448(0x001E),

          /* Finite Field Groups (DHE) */
          ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096 (0x0102),
          ffdhe6144(0x0103), ffdhe8192(0x0104),

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

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

   Elliptic Curve Groups (ECDHE)  Indicates support for the
      corresponding named curve, defined either in FIPS 186-4 [DSS] or
      in [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for
      private use.

   Finite Field Groups (DHE)  Indicates support of the corresponding
      finite field group, defined in [RFC7919].  Values 0x01FC through
      0x01FF are reserved for private use.

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

   As of TLS 1.3, servers are permitted to send the "supported_groups"
   extension to the client.  If the server has a group it prefers to the
   ones in the "key_share" extension but is still willing to accept the
   ClientHello, it SHOULD send "supported_groups" to update the client's
   view of its preferences; this extension SHOULD contain all groups the
   server supports, regardless of whether they are currently supported
   by the client.  Clients MUST NOT act upon any information found in
   "supported_groups" prior to successful completion of the handshake,



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   but MAY use the information learned from a successfully completed
   handshake to change what groups they use in their "key_share"
   extension in subsequent connections.

4.2.5.  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 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 4.2.5.1;
      Elliptic Curve Diffie-Hellman parameters are described in
      Section 4.2.5.2.

   key_exchange  Key exchange information.  The contents of this field
      are determined by the specified group and its corresponding
      definition.

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

      struct {
          select (Handshake.msg_type) {
              case client_hello:
                  KeyShareEntry client_shares<0..2^16-1>;

              case hello_retry_request:
                  NamedGroup selected_group;

              case server_hello:
                  KeyShareEntry server_share;
          };
      } KeyShare;

   client_shares  A list of offered KeyShareEntry values in descending
      order of client preference.  This vector MAY be empty if the
      client is requesting a HelloRetryRequest.  Each KeyShareEntry
      value MUST correspond to a group offered in the "supported_groups"
      extension and MUST appear in the same order.  However, the values



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      MAY be a non-contiguous subset of the "supported_groups" extension
      and MAY omit the most preferred groups.

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

   server_share  A single KeyShareEntry value that is in the same group
      as one of the client's shares.

   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 be
   generated independently.  Clients MUST NOT offer multiple
   KeyShareEntry values for the same group.  Clients MUST NOT offer any
   KeyShareEntry values for groups not listed in the client's
   "supported_groups" extension.  Servers MAY check for violations of
   these rules and abort the handshake with an "illegal_parameter" alert
   if one is violated.

   Upon receipt of this extension in a HelloRetryRequest, the client
   MUST verify that (1) the selected_group field corresponds to a group
   which was provided in the "supported_groups" extension in the
   original ClientHello; and (2) the selected_group field does not
   correspond to a group which was provided in the "key_share" extension
   in the original ClientHello.  If either of these checks fails, then
   the client MUST abort the handshake with an "illegal_parameter"
   alert.  Otherwise, when sending the new ClientHello, the client MUST
   replace the original "key_share" extension with one containing only a
   new KeyShareEntry for the group indicated in the selected_group field
   of the triggering HelloRetryRequest.

   If using (EC)DHE key establishment, servers offer exactly one
   KeyShareEntry in the ServerHello.  This value MUST correspond to the
   KeyShareEntry value offered by the client that the server has
   selected for the negotiated key exchange.  Servers MUST NOT send a
   KeyShareEntry for any group not indicated in the "supported_groups"
   extension and MUST NOT send a KeyShareEntry when using the "psk_ke"
   PskKeyExchangeMode.  If a HelloRetryRequest was received by the
   client, the client MUST verify that the selected NamedGroup in the
   ServerHello is the same as that in the HelloRetryRequest.  If this
   check fails, the client MUST abort the handshake with an
   "illegal_parameter" alert.








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4.2.5.1.  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) for the specified group (see [RFC7919]
   for group definitions) encoded as a big-endian integer and padded
   with zeros to the size of p in bytes.

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

   Peers MUST validate each other's public key Y by ensuring that 1 < Y
   < p-1.  This check ensures that the remote peer is properly behaved
   and isn't forcing the local system into a small subgroup.

4.2.5.2.  ECDHE Parameters

   ECDHE parameters for both clients and servers are encoded in the the
   opaque key_exchange field of a KeyShareEntry in a KeyShare structure.

   For secp256r1, secp384r1 and secp521r1, the contents are the byte
   string representation of an elliptic curve public value following the
   conversion routine in Section 4.3.6 of ANSI X9.62 [X962].

   Although X9.62 supports multiple point formats, any given curve MUST
   specify only a single point format.  All curves currently specified
   in this document MUST only be used with the uncompressed point format
   (the format for all ECDH functions is considered uncompressed).
   Peers MUST validate each other's public value Y by ensuring that the
   point is a valid point on the elliptic curve.

   For the curves secp256r1, secp384r1 and secp521r1, the appropriate
   validation procedures are defined in Section 4.3.7 of [X962] and
   alternatively in Section 5.6.2.6 of [KEYAGREEMENT].  This process
   consists of three steps: (1) verify that Y is not the point at
   infinity (O), (2) verify that for Y = (x, y) both integers are in the
   correct interval, (3) ensure that (x, y) is a correct solution to the
   elliptic curve equation.  For these curves, implementers do not need
   to verify membership in the correct subgroup.

   For X25519 and X448, the contents of the public value are the byte
   string inputs and outputs of the corresponding functions defined in
   [RFC7748], 32 bytes for X25519 and 56 bytes for X448.

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



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4.2.6.  Pre-Shared Key Exchange Modes

   In order to use PSKs, clients MUST also send a
   "psk_key_exchange_modes" extension.  The semantics of this extension
   are that the client only supports the use of PSKs with these modes,
   which restricts both the use of PSKs offered in this ClientHello and
   those which the server might supply via NewSessionTicket.

   A client MUST provide a "psk_key_exchange_modes" extension if it
   offers a "pre_shared_key" extension.  If clients offer
   "pre_shared_key" without a "psk_key_exchange_modes" extension,
   servers MUST abort the handshake.  Servers MUST NOT select a key
   exchange mode that is not listed by the client.  This extension also
   restricts the modes for use with PSK resumption; servers SHOULD NOT
   send NewSessionTicket with tickets that are not compatible with the
   advertised modes; however, if a server does so, the impact will just
   be that the client's attempts at resumption fail.

   The server MUST NOT send a "psk_key_exchange_modes" extension.

      enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;

      struct {
          PskKeyExchangeMode ke_modes<1..255>;
      } PskKeyExchangeModes;

   psk_ke  PSK-only key establishment.  In this mode, the server MUST
      NOT supply a "key_share" value.

   psk_dhe_ke  PSK with (EC)DHE key establishment.  In this mode, the
      client and servers MUST supply "key_share" values as described in
      Section 4.2.5.

4.2.7.  Early Data Indication

   When a PSK is used, the client can send application data 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" field of this extension contains an
   "EarlyDataIndication" value.










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      struct {} Empty;

      struct {
          select (Handshake.msg_type) {
              case new_session_ticket:   uint32 max_early_data_size;
              case client_hello:         Empty;
              case encrypted_extensions: Empty;
          };
      } EarlyDataIndication;

   See Appendix B.3.4 for the use of the max_early_data_size field.

   For PSKs provisioned via NewSessionTicket, a server MUST validate
   that the ticket age for the selected PSK identity (computed by
   subtracting ticket_age_add from PskIdentity.obfuscated_ticket_age
   modulo 2^32) is within a small tolerance of the time since the ticket
   was issued (see Section 4.2.8.3).  If it is not, the server SHOULD
   proceed with the handshake but reject 0-RTT, and SHOULD NOT take any
   other action that assumes that this ClientHello is fresh.

   The parameters for the 0-RTT data (symmetric cipher suite, ALPN
   protocol, etc.) are the same as those which were negotiated in the
   connection which established the PSK.  The PSK used to encrypt the
   early data MUST be the first PSK listed in the client's
   "pre_shared_key" extension.

   0-RTT messages sent in the first flight have the same (encrypted)
   content types as their corresponding messages sent in other flights
   (handshake, application_data, and alert respectively) but are
   protected under different keys.  After receiving the server's
   Finished message, if the server has accepted early data, an
   EndOfEarlyData message will be sent to indicate the key change.  This
   message will be encrypted with the 0-RTT traffic keys.

   A server which receives an "early_data" extension MUST behave in one
   of three ways:

   -  Ignore the extension and return a regular 1-RTT response.  The
      server then ignores early data using trial decryption until it is
      able to receive the client's second flight and complete an
      ordinary 1-RTT handshake.

   -  Request that the client send another ClientHello by responding
      with a HelloRetryRequest.  A client MUST NOT include the
      "early_data" extension in its followup ClientHello.  The server
      then ignores early data by skipping all records with external
      content type of "application_data" (indicating that they are
      encrypted).



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   -  Return its own extension in EncryptedExtensions, indicating that
      it intends to process the early data.  It is not possible for the
      server to accept only a subset of the early data messages.

   In order to accept early data, the server MUST have accepted a PSK
   cipher suite and selected the first key offered in the client's
   "pre_shared_key" extension.  In addition, it MUST verify that the
   following values are consistent with those negotiated in the
   connection during which the ticket was established.

   -  The TLS version number and cipher suite.

   -  The selected ALPN [RFC7301] protocol, if any.

   Future extensions MUST define their interaction with 0-RTT.

   If any of these checks fail, the server MUST NOT respond with the
   extension and must discard all the remaining first flight data (thus
   falling back to 1-RTT).  If the client attempts a 0-RTT handshake but
   the server rejects it, the server will generally not have the 0-RTT
   record protection keys and must instead trial decrypt each record
   with the 1-RTT handshake keys until it finds one that decrypts
   properly, and then pick up the handshake from that point.

   If the server chooses to accept the "early_data" extension, then it
   MUST comply with the same error handling requirements specified for
   all records when processing early data records.  Specifically, if the
   server fails to decrypt any 0-RTT record following an accepted
   "early_data" extension it MUST terminate the connection with a
   "bad_record_mac" alert as per Section 5.2.

   If the server rejects the "early_data" extension, the client
   application MAY opt to retransmit early data once the handshake has
   been completed.  A TLS implementation SHOULD NOT automatically re-
   send early data; applications are in a better position to decide when
   re-transmission is appropriate.  Automatic re-transmission of early
   data could result in assumptions about the status of the connection
   being incorrect.  In particular, a TLS implementation MUST NOT
   automatically re-send early data unless the negotiated connection
   selects the same ALPN protocol.  An application might need to
   construct different messages if a different protocol is selected.
   Similarly, if early data assumes anything about the connection state,
   it might be sent in error after the handshake completes.








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4.2.8.  Pre-Shared Key Extension

   The "pre_shared_key" extension is used to indicate the identity of
   the pre-shared key to be used with a given handshake in association
   with PSK key establishment.

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

      struct {
          opaque identity<1..2^16-1>;
          uint32 obfuscated_ticket_age;
      } PskIdentity;

      opaque PskBinderEntry<32..255>;

      struct {
          select (Handshake.msg_type) {
              case client_hello:
                  PskIdentity identities<7..2^16-1>;
                  PskBinderEntry binders<33..2^16-1>;

              case server_hello:
                  uint16 selected_identity;
          };

      } PreSharedKeyExtension;

   identity  A label for a key.  For instance, a ticket defined in
      Appendix B.3.4, or a label for a pre-shared key established
      externally.

   obfuscated_ticket_age  For each ticket, the time since the client
      learned about the server configuration that it is using, in
      milliseconds.  This value is added modulo 2^32 to the
      "ticket_age_add" value that was included with the ticket, see
      Section 4.6.1.  This addition prevents passive observers from
      correlating connections unless tickets are reused.  Note: because
      ticket lifetimes are restricted to a week, 32 bits is enough to
      represent any plausible age, even in milliseconds.  For identities
      established externally an obfuscated_ticket_age of 0 SHOULD be
      used, and servers MUST ignore the value.

   identities  A list of the identities that the client is willing to
      negotiate with the server.  If sent alongside the "early_data"
      extension (see Section 4.2.7), the first identity is the one used
      for 0-RTT data.




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   binders  A series of HMAC values, one for each PSK offered in the
      "pre_shared_keys" extension and in the same order, computed as
      described below.

   selected_identity  The server's chosen identity expressed as a
      (0-based) index into the identities in the client's list.

   Each PSK is associated with a single Hash algorithm.  For PSKs
   established via the ticket mechanism (Section 4.6.1), this is the
   Hash used for the KDF.  For externally established PSKs, the Hash
   algorithm MUST be set when the PSK is established.  The server must
   ensure that it selects a compatible PSK (if any) and cipher suites.

   Implementor's note: the most straightforward way to implement the
   PSK/cipher suite matching requirements is to negotiate the cipher
   suite first and then exclude any incompatible PSKs.  Any unknown PSKs
   (e.g., they are not in the PSK database or are encrypted with an
   unknown key) SHOULD simply be ignored.  If no acceptable PSKs are
   found, the server SHOULD perform a non-PSK handshake if possible.

   Prior to accepting PSK key establishment, the server MUST validate
   the corresponding binder value (see Section 4.2.8.1 below).  If this
   value is not present or does not validate, the server MUST abort the
   handshake.  Servers SHOULD NOT attempt to validate multiple binders;
   rather they SHOULD select a single PSK and validate solely the binder
   that corresponds to that PSK.  In order to accept PSK key
   establishment, the server sends a "pre_shared_key" extension
   indicating the selected identity.

   Clients MUST verify that the server's selected_identity is within the
   range supplied by the client, that the server selected a cipher suite
   containing a Hash associated with the PSK and that a server
   "key_share" extension is present if required by the ClientHello
   "psk_key_exchange_modes".  If these values are not consistent the
   client MUST abort the handshake with an "illegal_parameter" alert.

   If the server supplies an "early_data" extension, the client MUST
   verify that the server's selected_identity is 0.  If any other value
   is returned, the client MUST abort the handshake with an
   "illegal_parameter" alert.

   This extension MUST be the last extension in the ClientHello (this
   facilitates implementation as described below).  Servers MUST check
   that it is the last extension and otherwise fail the handshake with
   an "illegal_parameter" alert.






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4.2.8.1.  PSK Binder

   The PSK binder value forms a binding between a PSK and the current
   handshake, as well as between the handshake in which the PSK was
   generated (if via a NewSessionTicket message) and the handshake where
   it was used.  Each entry in the binders list is computed as an HMAC
   over a transcript hash (see Section 4.4.1) containing a partial
   ClientHello up to and including the PreSharedKeyExtension.identities
   field.  That is, it includes all of the ClientHello but not the
   binders list itself.  The length fields for the message (including
   the overall length, the length of the extensions block, and the
   length of the "pre_shared_key" extension) are all set as if binders
   of the correct lengths were present.

   The binding_value is computed in the same way as the Finished message
   (Section 4.4.4) but with the BaseKey being the binder_key derived via
   the key schedule from the corresponding PSK which is being offered
   (see Section 7.1).

   If the handshake includes a HelloRetryRequest, the initial
   ClientHello and HelloRetryRequest are included in the transcript
   along with the new ClientHello.  For instance, if the client sends
   ClientHello1, its binder will be computed over:

      Transcript-Hash(ClientHello1[truncated])

   If the server responds with HelloRetryRequest, and the client then
   sends ClientHello2, its binder will be computed over:

      Transcript-Hash(ClientHello1,
                      HelloRetryRequest,
                      ClientHello2[truncated])

   The full ClientHello is included in all other handshake hash
   computations.  Note that in the first flight, ClientHello1[truncated]
   is hashed directly, but in the second flight, it is hashed and then
   reinjected as a "handshake_hash" message, as described in
   Section 4.4.1.

4.2.8.2.  Processing Order

   Clients are permitted to "stream" 0-RTT data until they receive the
   server's Finished, only then sending the EndOfEarlyData message.  In
   order to avoid deadlocks, when accepting "early_data", servers MUST
   process the client's ClientHello and then immediately send the
   ServerHello, rather than waiting for the client's EndOfEarlyData
   message.




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4.2.8.3.  Replay Properties

   As noted in Section 2.3, TLS provides a limited mechanism for replay
   protection for data sent by the client in the first flight.

   The "obfuscated_ticket_age" parameter in the client's
   "pre_shared_key" extension SHOULD be used by servers to limit the
   time over which the first flight might be replayed.  A server can
   store the time at which it sends a ticket to the client, or encode
   the time in the ticket.  Then, each time it receives an
   "pre_shared_key" 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 will be shorter than the actual time elapsed on the
   server by a single round trip time.  This difference is comprised of
   the delay in sending the NewSessionTicket message to the client, plus
   the time taken to send the ClientHello to the server.  For this
   reason, a server SHOULD measure the round trip time prior to sending
   the NewSessionTicket message and account for that in the value it
   saves.

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

   -  The time that the server generated the session ticket and the
      estimated round trip time can be added together to form a baseline
      time.

   -  The "ticket_age_add" parameter from the NewSessionTicket is needed
      to recover the 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 and server clock
   rates are likely to be minimal, though potentially with gross time
   corrections.  Network propagation delays are most likely causes of a
   mismatch in legitimate values for elapsed time.  Both the
   NewSessionTicket and 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 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




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   skew distributions are not symmetric, so the optimal tradeoff may
   involve an asymmetric replay window.

4.3.  Server Parameters

   The next two messages from the server, EncryptedExtensions and
   CertificateRequest, contain information from the server that
   determines the rest of the handshake.  These messages are encrypted
   with keys derived from the server_handshake_traffic_secret.

4.3.1.  Encrypted Extensions

   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 the
   server_handshake_traffic_secret.

   The EncryptedExtensions message contains extensions which should be
   protected, i.e., any which are not needed to establish the
   cryptographic context, but which are not associated with individual
   certificates.  The client MUST check EncryptedExtensions for the
   presence of any forbidden extensions and if any are found MUST abort
   the handshake with an "illegal_parameter" alert.

   Structure of this message:

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

   extensions  A list of extensions.  For more information, see the
      table in Section 4.2.

4.3.2.  Certificate Request

   A server which is authenticating with a certificate MAY optionally
   request a certificate from the client.  This message, if sent, MUST
   follow EncryptedExtensions.

   Structure of this message:

      struct {
          opaque certificate_request_context<0..2^8-1>;
          Extension extensions<2..2^16-1>;
      } CertificateRequest;

   certificate_request_context  An opaque string which identifies the
      certificate request and which will be echoed in the client's



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      Certificate message.  The certificate_request_context MUST be
      unique within the scope of this connection (thus preventing replay
      of client CertificateVerify messages).  This field SHALL be zero
      length unless used for the post-handshake authentication exchanges
      described in Section 4.6.2.

   extensions  An optional set of extensions describing the parameters
      of the certificate being requested.  The "signature_algorithms"
      extension MUST be specified.  Clients MUST ignore unrecognized
      extensions.

   In prior versions of TLS, the CertificateRequest message carried a
   list of signature algorithms and certificate authorities which the
   server would accept.  In TLS 1.3 the former is expressed by sending
   the "signature_algorithms" extension.  The latter is expressed by
   sending the "certificate_authorities" extension (see
   Section 4.2.3.1).

   Servers which are authenticating with a PSK MUST NOT send the
   CertificateRequest message.

4.3.2.1.  OID Filters

   The "oid_filters" extension allows servers to provide a set of OID/
   value pairs which it would like the client's certificate to match.
   This extension MUST only be sent in the CertificateRequest message.

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

      struct {
          OIDFilter filters<0..2^16-1>;
      } OIDFilterExtension;

   filters  A list of certificate extension OIDs [RFC5280] with their
      allowed values, represented in DER-encoded [X690] format.  Some
      certificate extension OIDs allow multiple values (e.g., Extended
      Key Usage).  If the server has included a non-empty
      certificate_extensions list, the client certificate included in
      the response MUST contain all of the specified extension OIDs that
      the client recognizes.  For each extension OID recognized by the
      client, all of the specified values MUST be present in the client
      certificate (but the certificate MAY have other values as well).
      However, the client MUST ignore and skip any unrecognized
      certificate extension OIDs.  If the client ignored some of the
      required certificate extension OIDs and supplied a certificate



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      that does not satisfy the request, the server MAY at its
      discretion either continue the connection without client
      authentication, or abort the handshake with an
      "unsupported_certificate" alert.  PKIX RFCs define a variety of
      certificate extension OIDs and their corresponding value types.
      Depending on the type, matching certificate extension values are
      not necessarily bitwise-equal.  It is expected that TLS
      implementations will rely on their PKI libraries to perform
      certificate selection using certificate extension OIDs.  This
      document defines matching rules for two standard certificate
      extensions defined in [RFC5280]:

      o  The Key Usage extension in a certificate matches the request
         when all key usage bits asserted in the request are also
         asserted in the Key Usage certificate extension.

      o  The Extended Key Usage extension in a certificate matches the
         request when all key purpose OIDs present in the request are
         also found in the Extended Key Usage certificate extension.
         The special anyExtendedKeyUsage OID MUST NOT be used in the
         request.

      Separate specifications may define matching rules for other
      certificate extensions.

4.4.  Authentication Messages

   As discussed in Section 2, TLS generally uses a common set of
   messages for authentication, key confirmation, and handshake
   integrity: Certificate, CertificateVerify, and Finished.  (The
   PreSharedKey binders also perform key confirmation, in a similar
   fashion.)  These three messages are always sent as the last messages
   in their handshake flight.  The Certificate and CertificateVerify
   messages are only sent under certain circumstances, as defined below.
   The Finished message is always sent as part of the Authentication
   block.  These messages are encrypted under keys derived from
   [sender]_handshake_traffic_secret.

   The computations for the Authentication messages all uniformly take
   the following inputs:

   -  The certificate and signing key to be used.

   -  A Handshake Context consisting of the set of messages to be
      included in the transcript hash.

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




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   Based on these inputs, the messages then contain:

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

   CertificateVerify  A signature over the value Transcript-
      Hash(Handshake Context, Certificate)

   Finished  A MAC over the value Transcript-Hash(Handshake Context,
      Certificate, CertificateVerify) using a MAC key derived from the
      base key.

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

   +------------+-----------------------------+------------------------+
   | Mode       | Handshake Context           | Base Key               |
   +------------+-----------------------------+------------------------+
   | Server     | ClientHello ... later of En | server_handshake_traff |
   |            | cryptedExtensions/Certifica | ic_secret              |
   |            | teRequest                   |                        |
   |            |                             |                        |
   | Client     | ClientHello ...             | client_handshake_traff |
   |            | ServerFinished              | ic_secret              |
   |            |                             |                        |
   | Post-      | ClientHello ...             | client_traffic_secret_ |
   | Handshake  | ClientFinished +            | N                      |
   |            | CertificateRequest          |                        |
   +------------+-----------------------------+------------------------+

4.4.1.  The Transcript Hash

   Many of the cryptographic computations in TLS make use of a
   transcript hash.  This value is computed by hashing the concatenation
   of each included handshake message, including the handshake message
   header carrying the handshake message type and length fields, but not
   including record layer headers.  I.e.,

    Transcript-Hash(M1, M2, ... MN) = Hash(M1 || M2 ... MN)

   As an exception to this general rule, when the server responds to a
   ClientHello with a HelloRetryRequest, the value of ClientHello1 is
   replaced with a special synthetic handshake message of handshake type
   "message_hash" containing Hash(ClientHello1).  I.e.,






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    Transcript-Hash(ClientHello1, HelloRetryRequest, ... MN) =
        Hash(message_hash ||                 // Handshake Type
             00 00 Hash.length ||   // Handshake message length
             Hash(ClientHello1) ||  // Hash of ClientHello1
             HelloRetryRequest ... MN)

   The reason for this construction is to allow the server to do a
   stateless HelloRetryRequest by storing just the hash of ClientHello1
   in the cookie, rather than requiring it to export the entire
   intermediate hash state (see Section 4.2.2).

   In general, implementations can implement the transcript by keeping a
   running transcript hash value based on the negotiated hash.  Note,
   however, that subsequent post-handshake authentications do not
   include each other, just the messages through the end of the main
   handshake.

4.4.2.  Certificate

   The server MUST send a Certificate message whenever the agreed-upon
   key exchange method uses certificates for authentication (this
   includes all key exchange methods defined in this document except
   PSK).  This message conveys the endpoint's certificate chain to the
   peer.

   The client MUST send a Certificate message if and only if the server
   has requested client authentication via a CertificateRequest message
   (Section 4.3.2).  If the server requests client authentication but no
   suitable certificate is available, the client MUST send a Certificate
   message containing no certificates (i.e., with the "certificate_list"
   field having length 0).

   Structure of this message:

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

      struct {
          ASN1Cert cert_data;
          Extension extensions<0..2^16-1>;
      } CertificateEntry;

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

   certificate_request_context  If this message is in response to a
      CertificateRequest, the value of certificate_request_context in



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

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

   extensions:  A set of extension values for the CertificateEntry.  The
      "Extension" format is defined in Section 4.2.  Valid extensions
      include OCSP Status extensions ([RFC6066] and [RFC6961]) and
      SignedCertificateTimestamps ([RFC6962]).  An extension MUST only
      be present in a Certificate message if the corresponding
      ClientHello extension was presented in the initial handshake.  If
      an extension applies to the entire chain, it SHOULD be included in
      the first CertificateEntry.

   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 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, 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
   MUST be first.

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

4.4.2.1.  OCSP Status and SCT Extensions

   [RFC6066] and [RFC6961] provide extensions to negotiate the server
   sending OCSP responses to the client.  In TLS 1.2 and below, the
   server replies with an empty extension to indicate negotiation of
   this extension and the OCSP information is carried in a
   CertificateStatus message.  In TLS 1.3, the server's OCSP information
   is carried in an extension in the CertificateEntry containing the
   associated certificate.  Specifically: The body of the




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   "status_request" extension from the server MUST be a
   CertificateStatus structure as defined in [RFC6066].

   A server MAY request that a client present an OCSP response with its
   certificate by sending a "status_request" extension in its
   CertificateRequest message.  If the client opts to send an OCSP
   response, the body of its "status_request" extension MUST be a
   CertificateStatus structure as defined in [RFC6066].

   Similarly, [RFC6962] provides a mechanism for a server to send a
   Signed Certificate Timestamp (SCT) as an extension in the
   ServerHello.  In TLS 1.3, the server's SCT information is carried in
   an extension in CertificateEntry.

4.4.2.2.  Server Certificate Selection

   The following rules apply to the certificates sent by the server:

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

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

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

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

   All certificates provided by the server MUST be signed by a signature
   algorithm that appears in the "signature_algorithms" extension
   provided by the client, if they are able to provide such a chain (see
   Section 4.2.3).  Certificates that are self-signed or certificates
   that are expected to be trust anchors are not validated as part of
   the chain and therefore MAY be signed with any algorithm.

   If the server cannot produce a certificate chain that is signed only
   via the indicated supported algorithms, then it SHOULD continue the
   handshake by sending the client a certificate chain of its choice
   that may include algorithms that are not known to be supported by the
   client.  This fallback chain MAY use the deprecated SHA-1 hash
   algorithm only if the "signature_algorithms" extension provided by



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   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 abort the handshake with an
   "unsupported_certificate" alert.

   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).

4.4.2.3.  Client Certificate Selection

   The following rules apply to certificates sent by the client:

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

   -  If the certificate_authorities list in the CertificateRequest
      message was non-empty, at least one of the certificates in the
      certificate chain SHOULD be issued by one of the listed CAs.

   -  The certificates MUST be signed using an acceptable signature
      algorithm, as described in Section 4.3.2.  Note that this relaxes
      the constraints on certificate-signing algorithms found in prior
      versions of TLS.

   -  If the certificate_extensions list in the CertificateRequest
      message was non-empty, the end-entity certificate MUST match the
      extension OIDs recognized by the client, as described in
      Section 4.3.2.

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

4.4.2.4.  Receiving a Certificate Message

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

   If the server supplies an empty Certificate message, the client MUST
   abort the handshake with a "decode_error" alert.

   If the client does not send any certificates, the server MAY at its
   discretion either continue the handshake without client
   authentication, or abort the handshake with a "certificate_required"
   alert.  Also, if some aspect of the certificate chain was
   unacceptable (e.g., it was not signed by a known, trusted CA), the



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   server MAY at its discretion either continue the handshake
   (considering the client unauthenticated) or abort the handshake.

   Any endpoint receiving any certificate signed using any signature
   algorithm using an MD5 hash MUST abort the handshake with a
   "bad_certificate" alert.  SHA-1 is deprecated and it is RECOMMENDED
   that any endpoint receiving any certificate signed using any
   signature algorithm using a SHA-1 hash abort the handshake with a
   "bad_certificate" alert.  All endpoints are RECOMMENDED to transition
   to SHA-256 or better as soon as possible to maintain interoperability
   with 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 signature algorithm (for instance, an
   RSA key signed with an ECDSA key).

4.4.3.  Certificate Verify

   This message is used to provide explicit proof that an endpoint
   possesses the private key corresponding to its certificate and also
   provides integrity for the handshake up to this point.  Servers MUST
   send this message when authenticating 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 MUST appear immediately after the Certificate message and
   immediately prior to the Finished message.

   Structure of this message:

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

   The algorithm field specifies the signature algorithm used (see
   Section 4.2.3 for the definition of this field).  The signature is a
   digital signature using that algorithm.  The content to be signed is
   the hash output as described in Section 4.4 namely:

      Transcript-Hash(Handshake Context, Certificate)

   The digital signature is then computed over the concatenation of:

   -  A string that consists of octet 32 (0x20) repeated 64 times

   -  The context string




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   -  A single 0 byte which serves as the separator

   -  The content to be signed

   This structure is intended to prevent an attack on previous versions
   of TLS in which the ServerKeyExchange format meant that attackers
   could obtain a signature of a message with a chosen 32-byte prefix
   (ClientHello.random).  The initial 64-byte pad clears that prefix
   along with the server-controlled ServerHello.random.

   The context string for a server signature is "TLS 1.3, server
   CertificateVerify" and for a client signature is "TLS 1.3, client
   CertificateVerify".  It is used to provide separation between
   signatures made in different contexts, helping against potential
   cross-protocol attacks.

   For example, if the transcript hash was 32 bytes of 01 (this length
   would make sense for SHA-256), the content covered by the digital
   signature for a server CertificateVerify would be:

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      544c5320312e332c207365727665722043657274696669636174655665726966
      79
      00
      0101010101010101010101010101010101010101010101010101010101010101

   On the sender side the process for computing the signature field of
   the CertificateVerify message takes as input:

   -  The content covered by the digital signature

   -  The private signing key corresponding to the certificate sent in
      the previous message

   If the CertificateVerify message is sent by a server, the signature
   algorithm MUST be one offered in the client's "signature_algorithms"
   extension unless no valid certificate chain can be produced without
   unsupported algorithms (see Section 4.2.3).

   If sent by a client, the signature algorithm used in the signature
   MUST be one of those present in the supported_signature_algorithms
   field of the CertificateRequest message.

   In addition, the signature algorithm MUST be compatible with the key
   in the sender's end-entity certificate.  RSA signatures MUST use an
   RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5
   algorithms appear in "signature_algorithms".  SHA-1 MUST NOT be used



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   in any signatures in CertificateVerify.  All SHA-1 signature
   algorithms in this specification are defined solely for use in legacy
   certificates, and are not valid for CertificateVerify signatures.

   The receiver of a CertificateVerify MUST verify the signature field.
   The verification process takes as input:

   -  The content covered by the digital signature

   -  The public key contained in the end-entity certificate found in
      the associated Certificate message.

   -  The digital signature received in the signature field of the
      CertificateVerify message

   If the verification fails, the receiver MUST terminate the handshake
   with a "decrypt_error" alert.

   Note: When used with non-certificate-based handshakes (e.g., PSK),
   the client's signature does not cover the server's certificate
   directly.  When the PSK was established through a NewSessionTicket,
   the client's signature transitively covers the server's certificate
   through the PSK binder.  [PSK-FINISHED] describes a concrete attack
   on constructions that do not bind to the server's certificate.  It is
   unsafe to use certificate-based client authentication when the client
   might potentially share the same PSK/key-id pair with two different
   endpoints and implementations MUST NOT combine external PSKs with
   certificate-based authentication.

4.4.4.  Finished

   The Finished message is the final message in the authentication
   block.  It is essential for providing authentication of the handshake
   and of the computed keys.

   Recipients of Finished messages MUST verify that the contents are
   correct and if incorrect MUST terminate the connection with a
   "decrypt_error" alert.

   Once a side has sent its Finished message and received and validated
   the Finished message from its peer, it may begin to send and receive
   application data over the connection.  Early data may be sent prior
   to the receipt of the peer's Finished message, per Section 4.2.7.

   The key used to compute the finished message is computed from the
   Base key defined in Section 4.4 using HKDF (see Section 7.1).
   Specifically:




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   finished_key =
       HKDF-Expand-Label(BaseKey, "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,
               Transcript-Hash(Handshake Context,
                               Certificate*, CertificateVerify*))

      * 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 a running
   hash, i.e., just the handshake hash at this point.

   In previous versions of TLS, the verify_data was always 12 octets
   long.  In TLS 1.3, it is the size of the HMAC output for the Hash
   used for the handshake.

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

   Any records following a 1-RTT Finished message MUST be encrypted
   under the appropriate application traffic key as described in
   Section 7.2.  In particular, this includes any alerts sent by the
   server in response to client Certificate and CertificateVerify
   messages.

4.5.  End of Early Data

      struct {} EndOfEarlyData;

   The EndOfEarlyData message 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 the following records are protected under
   handshake traffic keys.  Servers MUST NOT send this message and
   clients receiving it MUST terminate the connection with an
   "unexpected_message" alert.  This message is encrypted under keys
   derived from the client_early_traffic_secret.





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4.6.  Post-Handshake Messages

   TLS also allows other messages to be sent after the main handshake.
   These messages use a handshake content type and are encrypted under
   the appropriate application traffic key.

4.6.1.  New Session Ticket Message

   At any time after the server has received the client Finished
   message, it MAY send a NewSessionTicket message.  This message
   creates a pre-shared key (PSK) binding between the ticket value and
   the resumption master secret.

   The client MAY use this PSK for future handshakes by including the
   ticket value in the "pre_shared_key" extension in its ClientHello
   (Section 4.2.8).  Servers MAY send multiple tickets on a single
   connection, either immediately after each other or after specific
   events.  For instance, the server might send a new ticket after post-
   handshake authentication in order to encapsulate the additional
   client authentication state.  Clients SHOULD attempt to use each
   ticket no more than once, with more recent tickets being used first.

   Any ticket MUST only be resumed with a cipher suite that has the same
   KDF hash as that used to establish the original connection, and only
   if the client provides the same SNI value as in the original
   connection, as described in Section 3 of [RFC6066].

   Note: Although the resumption master secret depends on the client's
   second flight, servers which do not request client authentication MAY
   compute the remainder of the transcript independently and then send a
   NewSessionTicket immediately upon sending its Finished rather than
   waiting for the client Finished.  This might be appropriate in cases
   where the client is expected to open multiple TLS connections in
   parallel and would benefit from the reduced overhead of a resumption
   handshake, for example.

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

   ticket_lifetime  Indicates the lifetime in seconds as a 32-bit
      unsigned integer in network byte order from the time of ticket
      issuance.  Servers MUST NOT use any value more than 604800 seconds
      (7 days).  The value of zero indicates that the ticket should be
      discarded immediately.  Clients MUST NOT cache tickets for longer



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      than 7 days, regardless of the ticket_lifetime, and MAY delete the
      ticket earlier based on local policy.  A server MAY treat a ticket
      as valid for a shorter period of time than what is stated in the
      ticket_lifetime.

   ticket_age_add  A securely generated, random 32-bit value that is
      used to obscure the age of the ticket that the client includes in
      the "pre_shared_key" extension.  The client-side ticket age is
      added to this value modulo 2^32 to obtain the value that is
      transmitted by the client.

   ticket  The value of the ticket to be used as the PSK identity.  The
      ticket itself is an opaque label.  It MAY either be a database
      lookup key or a self-encrypted and self-authenticated value.
      Section 4 of [RFC5077] describes a recommended ticket construction
      mechanism.

   extensions  A set of extension values for the ticket.  The
      "Extension" format is defined in Section 4.2.  Clients MUST ignore
      unrecognized extensions.

   The sole extension currently defined for NewSessionTicket is
   "early_data", indicating that the ticket may be used to send 0-RTT
   data (Section 4.2.7)).  It contains the following value:

   max_early_data_size  The maximum amount of 0-RTT data that the client
      is allowed to send when using this ticket, in bytes.  Only
      Application Data payload (i.e., plaintext but not padding or the
      inner content type byte) is counted.  A server receiving more than
      max_early_data_size bytes of 0-RTT data SHOULD terminate the
      connection with an "unexpected_message" alert.

   Note that in principle it is possible to continue issuing new tickets
   which continue to indefinitely extend the lifetime of the keying
   material originally derived from an initial non-PSK handshake (which
   was most likely tied to the peer's certificate).  It is RECOMMENDED
   that implementations place limits on the total lifetime of such
   keying material; these limits should take into account the lifetime
   of the peer's certificate, the likelihood of intervening revocation,
   and the time since the peer's online CertificateVerify signature.

4.6.2.  Post-Handshake Authentication

   The server is permitted to request client authentication at any time
   after the handshake has completed by sending a CertificateRequest
   message.  The client SHOULD respond with the appropriate
   Authentication messages.  If the client chooses to authenticate, it
   MUST send Certificate, CertificateVerify, and Finished.  If it



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   declines, it MUST send a Certificate message containing no
   certificates followed by Finished.

   Note: Because client authentication may require prompting the user,
   servers MUST be prepared for some delay, including receiving an
   arbitrary number of other messages between sending the
   CertificateRequest and receiving a response.  In addition, clients
   which receive multiple CertificateRequests in close succession MAY
   respond to them in a different order than they were received (the
   certificate_request_context value allows the server to disambiguate
   the responses).

4.6.3.  Key and IV Update

      enum {
          update_not_requested(0), update_requested(1), (255)
      } KeyUpdateRequest;

      struct {
          KeyUpdateRequest request_update;
      } KeyUpdate;

   request_update  Indicates whether the recipient of the KeyUpdate
      should respond with its own KeyUpdate.  If an implementation
      receives any other value, it MUST terminate the connection with an
      "illegal_parameter" alert.

   The KeyUpdate handshake message is used to indicate that the sender
   is updating its sending cryptographic keys.  This message can be sent
   by either peer after it has sent a Finished message.  Implementations
   that receive a KeyUpdate message prior to receiving a Finished
   message MUST terminate the connection with an "unexpected_message"
   alert.  After sending a KeyUpdate message, the sender SHALL send all
   its traffic using the next generation of keys, computed as described
   in Section 7.2.  Upon receiving a KeyUpdate, the receiver MUST update
   its receiving keys.

   If the request_update field is set to "update_requested" then the
   receiver MUST send a KeyUpdate of its own with request_update set to
   "update_not_requested" prior to sending its next application data
   record.  This mechanism allows either side to force an update to the
   entire connection, but causes an implementation which receives
   multiple KeyUpdates while it is silent to respond with a single
   update.  Note that implementations may receive an arbitrary number of
   messages between sending a KeyUpdate with request_update set to
   update_requested and receiving the peer's KeyUpdate, because those
   messages may already be in flight.  However, because send and receive
   keys are derived from independent traffic secrets, retaining the



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   receive traffic secret does not threaten the forward secrecy of data
   sent before the sender changed keys.

   If implementations independently send their own KeyUpdates with
   request_update set to "update_requested", and they cross in flight,
   then each side will also send a response, with the result that each
   side increments by two generations.

   Both sender and receiver MUST encrypt their KeyUpdate messages with
   the old keys.  Additionally, both sides MUST enforce that a KeyUpdate
   with the old key 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 to be transmitted, fragments
   the data into manageable blocks, protects the records, and transmits
   the result.  Received data is verified and decrypted, reassembled,
   and then delivered to higher-level clients.

   TLS records are typed, which allows multiple higher-level protocols
   to 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 in
   this document unless negotiated by some extension.  If a TLS
   implementation receives an unexpected record type, it MUST terminate
   the connection with an "unexpected_message" alert.  New record
   content type values are assigned by IANA in the TLS Content Type
   Registry as described in Section 10.

5.1.  Record Layer

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Message
   boundaries are handled differently depending on the underlying
   ContentType.  Any future content types MUST specify appropriate
   rules.  Note that these rules are stricter than what was enforced in
   TLS 1.2.

   Handshake messages MAY be coalesced into a single TLSPlaintext record
   or fragmented across several records, provided that:

   -  Handshake messages MUST NOT be interleaved with other record
      types.  That is, if a handshake message is split over two or more
      records, there MUST NOT be any other records between them.





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   -  Handshake messages MUST NOT span key changes.  Because the
      ClientHello, EndOfEarlyData, ServerHello, Finished, and KeyUpdate
      messages can arrive immediately prior to a key change, upon
      receiving these messages a receiver MUST verify that the end of
      these messages aligns with a record boundary; if not, then it MUST
      terminate the connection with an "unexpected_message" alert.

   Implementations MUST NOT send zero-length fragments of Handshake
   types, even if those fragments contain padding.

   Alert messages (Section 6) MUST NOT be fragmented across records and
   multiple Alert messages MUST NOT be coalesced into a single
   TLSPlaintext record.  In other words, a record with an Alert type
   MUST contain exactly one message.

   Application Data messages contain data that is opaque to TLS.
   Application Data messages are always protected.  Zero-length
   fragments of Application Data MAY be sent as they are potentially
   useful as a traffic analysis countermeasure.

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

      struct {
          ContentType type;
          ProtocolVersion legacy_record_version;
          uint16 length;
          opaque fragment[TLSPlaintext.length];
      } TLSPlaintext;

   type  The higher-level protocol used to process the enclosed
      fragment.

   legacy_record_version  This value MUST be set to 0x0301 for all
      records generated by a TLS 1.3 implementation.  This field is
      deprecated and MUST be ignored for all purposes.  Previous
      versions of TLS would use other values in this field under some
      circumstances.

   length  The length (in bytes) of the following TLSPlaintext.fragment.
      The length MUST NOT exceed 2^14 bytes.  An endpoint that receives
      a record that exceeds this length MUST terminate the connection
      with a "record_overflow" alert.




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

   This document describes TLS 1.3, which uses the version 0x0304.  This
   version value is historical, deriving from the use of 0x0301 for TLS
   1.0 and 0x0300 for SSL 3.0.  In order to maximize backwards
   compatibility, the record layer version identifies as simply TLS 1.0.
   Endpoints supporting multiple versions negotiate the version to use
   by following the procedure and requirements in Appendix D.

   When record protection has not yet been engaged, TLSPlaintext
   structures are written directly onto the wire.  Once record
   protection has started, TLSPlaintext records are protected and sent
   as described in the following section.

5.2.  Record Payload Protection

   The record protection functions translate a TLSPlaintext structure
   into a TLSCiphertext.  The deprotection functions reverse the
   process.  In TLS 1.3, as opposed to previous versions of TLS, all
   ciphers are modeled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide an unified encryption
   and 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 = 23; /* application_data */
          ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
          uint16 length;
          opaque encrypted_record[length];
      } TLSCiphertext;

   content  The byte encoding of a handshake or an alert message, or the
      raw bytes of the application's data to send.

   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



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      senders to pad any TLS record by a chosen amount as long as the
      total stays within record size limits.  See Section 5.4 for more
      details.

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

   legacy_record_version  The legacy_record_version field is always
      0x0301.  TLS 1.3 TLSCiphertexts are not generated until after TLS
      1.3 has been negotiated, so there are no historical compatibility
      concerns where other values might be received.  Implementations
      MAY verify that the legacy_record_version field is 0x0301 and
      abort the connection if it is not.  Note that the handshake
      protocol including the ClientHello and ServerHello messages
      authenticates the protocol version, so this value is redundant.

   length  The length (in bytes) of the following
      TLSCiphertext.encrypted_record, which is the sum of the lengths of
      the content and the padding, plus one for the inner content type,
      plus any expansion added by the AEAD algorithm.  The length MUST
      NOT exceed 2^14 + 256 bytes.  An endpoint that receives a record
      that exceeds this length MUST terminate the connection with a
      "record_overflow" alert.

   encrypted_record  The AEAD-encrypted form of the serialized
      TLSInnerPlaintext structure.

   AEAD algorithms take as input a single key, a nonce, a plaintext, and
   "additional data" to be included in the authentication check, as
   described in Section 2.1 of [RFC5116].  The key is either the
   client_write_key or the server_write_key, the nonce is derived from
   the sequence number (see Section 5.3) and the client_write_iv or
   server_write_iv, and the additional data input is empty (zero
   length).  Derivation of traffic keys is defined in Section 7.3.

   The plaintext is the concatenation of TLSInnerPlaintext.fragment,
   TLSInnerPlaintext.type, and any padding bytes (zeros).

   The AEAD output consists of the ciphertext output from the AEAD
   encryption operation.  The length of the plaintext is greater than
   TLSInnerPlaintext.length due to the inclusion of
   TLSInnerPlaintext.type and any padding supplied by the sender.  The
   length of the AEAD output will generally be larger than the
   plaintext, but by an amount that varies with the AEAD algorithm.




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   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 verify, the cipher takes as input the key,
   nonce, and the AEADEncrypted value.  The output is either the
   plaintext or an error indicating that the decryption failed.  There
   is no separate integrity check.  That is:

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

   If the decryption fails, the receiver MUST terminate the connection
   with a "bad_record_mac" alert.

   An AEAD algorithm used in TLS 1.3 MUST NOT produce an expansion
   greater than 255 octets.  An endpoint that receives a record from its
   peer with TLSCipherText.length larger than 2^14 + 256 octets MUST
   terminate the connection with a "record_overflow" alert.  This limit
   is derived from the maximum TLSPlaintext length of 2^14 octets + 1
   octet for ContentType + the maximum AEAD expansion of 255 octets.

5.3.  Per-Record Nonce

   A 64-bit sequence number is maintained separately for reading and
   writing records.  Each sequence number is set to zero at the
   beginning of a connection and whenever the key is changed.

   The sequence number is incremented by one after reading or writing
   each record.  The first record transmitted under a particular set of
   traffic keys MUST use sequence number 0.

   Because the size of sequence numbers is 64-bit, they should not wrap.
   If a TLS implementation would need to wrap a sequence number, it MUST
   either re-key (Section 4.6.3) or terminate the connection.

   Each AEAD algorithm will specify a range of possible lengths for the
   per-record nonce, from N_MIN bytes to N_MAX bytes of input
   ([RFC5116]).  The length of the TLS per-record nonce (iv_length) is
   set to the larger of 8 bytes and N_MIN for the AEAD algorithm (see
   [RFC5116] Section 4).  An AEAD algorithm where N_MAX is less than 8
   bytes MUST NOT be used with TLS.  The per-record nonce for the AEAD
   construction is formed as follows:

   1.  The 64-bit record sequence number is encoded in network byte
       order and padded to the left with zeros to iv_length.



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

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

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

5.4.  Record Padding

   All encrypted TLS records can be padded to inflate the size of the
   TLSCipherText.  This allows the sender to hide the size of 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 is a string of zero-valued bytes appended to the
   ContentType field before encryption.  Implementations MUST set the
   padding octets to all zeros before encrypting.

   Application Data records may contain a zero-length
   TLSInnerPlaintext.content if the sender desires.  This permits
   generation of plausibly-sized cover traffic in contexts where the
   presence or absence of activity may be sensitive.  Implementations
   MUST NOT send Handshake or Alert records that have a zero-length
   TLSInnerPlaintext.content; if such a message is received, the
   receiving implementation MUST terminate the connection with an
   "unexpected_message" alert.

   The padding sent is automatically verified by the record protection
   mechanism; upon successful decryption of a
   TLSCiphertext.encrypted_record, the receiving implementation scans
   the field from the end toward the beginning until it finds a non-zero
   octet.  This non-zero octet is the content type of the message.  This
   padding scheme was selected because it allows padding of any
   encrypted TLS record by an arbitrary size (from zero up to TLS record
   size limits) without introducing new content types.  The design also
   enforces all-zero padding octets, which allows for quick detection of
   padding errors.

   Implementations MUST limit their scanning to the cleartext returned
   from the AEAD decryption.  If a receiving implementation does not
   find a non-zero octet in the cleartext, it MUST terminate the
   connection with an "unexpected_message" alert.

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



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   Selecting a padding policy that suggests when and how much to pad is
   a complex topic, and is beyond the scope of this specification.  If
   the application layer protocol atop TLS has its own padding, it may
   be preferable to pad application_data TLS records within the
   application layer.  Padding for encrypted handshake and alert TLS
   records must still be handled at 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 the amount of plaintext which can
   be safely encrypted under a given set of keys.  [AEAD-LIMITS]
   provides an analysis of these limits under the assumption that the
   underlying primitive (AES or ChaCha20) has no weaknesses.
   Implementations SHOULD do a key update as described in Section 4.6.3
   prior to reaching these limits.

   For AES-GCM, up to 2^24.5 full-size records (about 24 million) may be
   encrypted on a given connection while keeping a safety margin of
   approximately 2^-57 for Authenticated Encryption (AE) security.  For
   ChaCha20/Poly1305, the record sequence number would wrap before the
   safety limit is reached.

6.  Alert Protocol

   One of the content types supported by the TLS record layer is the
   alert type.  Like other messages, alert messages are encrypted as
   specified by the current connection state.

   Alert messages convey a description of the alert and a legacy field
   that conveyed the severity of the message in previous versions of
   TLS.  In TLS 1.3, the severity is implicit in the type of alert being
   sent, and can safely be ignored.  Some alerts are sent to indicate
   orderly closure of the connection or the end of early data (see
   Section 6.1).  Upon receiving such an alert, the TLS implementation
   SHOULD indicate end-of-data to the application, and if appropriate
   for the alert type, send a closure alert in response.

   Error alerts indicate abortive closure of the connection (see
   Section 6.2).  Upon receiving an error alert, the TLS implementation
   SHOULD indicate an error to the application and MUST NOT allow any
   further data to be sent or received on the connection.  Servers and
   clients MUST forget keys and secrets associated with a failed
   connection.  Stateful implementations of tickets (as in many clients)
   SHOULD discard tickets associated with failed connections.




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   All the alerts listed in Section 6.2 MUST be sent as fatal and MUST
   be treated as fatal regardless of the AlertLevel in the message.
   Unknown alert types MUST be treated as fatal.

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

      enum {
          close_notify(0),
          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),
          certificate_required(116),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

6.1.  Closure Alerts

   The client and the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.




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   close_notify  This alert notifies the recipient that the sender will
      not send any more messages on this connection.  Any data received
      after a closure MUST be ignored.

   user_canceled  This alert notifies the recipient that the sender is
      canceling the handshake for some reason unrelated to a protocol
      failure.  If a user cancels an operation after the handshake is
      complete, just closing the connection by sending a "close_notify"
      is more appropriate.  This alert SHOULD 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 MUST be ignored.  If a
   transport-level close is received prior to a "close_notify", the
   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 connection, unless some other fatal alert has been
   transmitted.  The other party MUST respond with a "close_notify"
   alert of its own and close down the connection immediately,
   discarding any pending writes.  The initiator of the close need not
   wait for the responding "close_notify" alert before closing the read
   side of the connection.

   If the application protocol using TLS provides that any data may be
   carried over the underlying transport after the TLS connection is
   closed, the TLS implementation MUST receive the responding
   "close_notify" alert before indicating to the application layer that
   the TLS connection has ended.  If the application protocol will not
   transfer any additional data, but will only close the underlying
   transport connection, then the implementation 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 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 transport.

6.2.  Error Alerts

   Error handling in the TLS Handshake Protocol is very simple.  When an
   error is detected, the detecting party sends a message to its peer.
   Upon transmission or receipt of a fatal alert message, both parties
   MUST immediately close the connection.





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   Whenever an implementation encounters a fatal error condition, it
   SHOULD send an appropriate fatal alert and MUST close the connection
   without sending or receiving any additional data.  In the rest of
   this specification, when the phrases "terminate the connection" and
   "abort the handshake" are used without a specific alert it means that
   the implementation SHOULD send the alert indicated by the
   descriptions below.  The phrases "terminate the connection with a X
   alert" and "abort the handshake with a X alert" mean that the
   implementation MUST send alert X if it sends any alert.  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 (e.g., the wrong
      handshake message, premature application data, etc.) 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 which
      cannot be deprotected.  Because AEAD algorithms combine decryption
      and verification, and also to avoid side channel attacks, 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
      length more than 2^14 + 256 bytes, or a record decrypted to a
      TLSPlaintext record with more than 2^14 bytes.  This alert should
      never be observed in communication between proper implementations,
      except when messages were corrupted in the network.

   handshake_failure  Reception of a "handshake_failure" alert message
      indicates that the sender was unable to negotiate an acceptable
      set of security parameters given the options available.

   bad_certificate  A certificate was corrupt, contained signatures that
      did 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 is not currently
      valid.

   certificate_unknown  Some other (unspecified) issue arose in
      processing the certificate, rendering it unacceptable.



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   illegal_parameter  A field in the handshake was incorrect or
      inconsistent with other fields.  This alert is used for errors
      which conform to the formal protocol syntax but are otherwise
      incorrect.

   unknown_ca  A valid certificate chain or partial chain was received,
      but the certificate was not accepted because the CA certificate
      could not be located or could not be matched with a known trust
      anchor.

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

   decode_error  A message could not be decoded because some field was
      out of the specified range or the length of the message was
      incorrect.  This alert is used for errors where the message does
      not conform to the formal protocol syntax.  This alert should
      never be observed in communication between proper implementations,
      except when messages were corrupted in the network.

   decrypt_error  A handshake cryptographic operation failed, including
      being unable to correctly verify a signature or validate a
      Finished message or a PSK binder.

   protocol_version  The protocol version the peer has attempted to
      negotiate is recognized but not supported. (see Appendix D)

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

   internal_error  An internal error unrelated to the peer or the
      correctness of the protocol (such as a memory allocation failure)
      makes it impossible to continue.

   inappropriate_fallback  Sent by a server in response to an invalid
      connection retry attempt from a client (see [RFC7507]).

   missing_extension  Sent by endpoints that receive a hello message not
      containing an extension that is mandatory to send for the offered
      TLS version or other negotiated parameters.

   unsupported_extension  Sent by endpoints receiving any hello message
      containing an extension known to be prohibited for inclusion in
      the given hello message, including any extensions in a ServerHello
      or Certificate not first offered in the corresponding ClientHello.




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

   unrecognized_name  Sent by servers when no server exists identified
      by the name provided by the client via the "server_name" extension
      (see [RFC6066]).

   bad_certificate_status_response  Sent by clients when an invalid or
      unacceptable OCSP response is provided by the server via the
      "status_request" extension (see [RFC6066]).

   bad_certificate_hash_value  Sent by servers when a retrieved object
      does not have the correct hash provided by the client via the
      "client_certificate_url" extension (see [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 alert is OPTIONAL; servers MAY instead choose to send
      a "decrypt_error" alert to merely indicate an invalid PSK
      identity.

   certificate_required  Sent by servers when a client certificate is
      desired but none was provided by the client.

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

7.  Cryptographic Computations

   The TLS handshake establishes one or more input secrets which are
   combined to create the actual working keying material, as detailed
   below.  The key derivation process incorporates both the input
   secrets and the handshake transcript.  Note that because the
   handshake transcript includes the random values in the Hello
   messages, any given handshake will have different traffic secrets,
   even if the same input secrets are used, as is the case when the same
   PSK is used for multiple connections

7.1.  Key Schedule

   The key derivation process makes use of the HKDF-Extract and HKDF-
   Expand functions as defined for HKDF [RFC5869], as well as the
   functions defined below:








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

       Where HkdfLabel is specified as:

       struct {
           uint16 length = Length;
           opaque label<10..255> = "TLS 1.3, " + Label;
           opaque hash_value<0..255> = HashValue;
       } HkdfLabel;

       Derive-Secret(Secret, Label, Messages) =
            HKDF-Expand-Label(Secret, Label,
                              Transcript-Hash(Messages), Hash.length)

   The Hash function and the HKDF hash are the cipher suite hash
   algorithm.  Hash.length is its output length in bytes.  Messages are
   the concatenation of the indicated handshake messages, including the
   handshake message type and length fields, but not including record
   layer headers.  Note that in some cases a zero-length HashValue
   (indicated by "") is passed to HKDF-Expand-Label.

   Given a set of n InputSecrets, the final "master secret" is computed
   by iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc.  The initial secret is simply a string of
   Hash.length zero bytes.  Concretely, for the present version of TLS
   1.3, secrets are added in the following order:

   -  PSK (a pre-shared key established externally or a
      resumption_master_secret value from a previous connection)

   -  (EC)DHE shared secret (Section 7.4)

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

   -  HKDF-Extract is drawn as taking the Salt argument from the top and
      the IKM argument from the left.

   -  Derive-Secret's Secret argument is indicated by the incoming
      arrow.  For instance, the Early Secret is the Secret for
      generating the client_early_traffic_secret.

                 0
                 |
                 v
   PSK ->  HKDF-Extract = Early Secret
                 |



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                 +-----> Derive-Secret(.,
                 |                     "external psk binder key" |
                 |                     "resumption psk binder key",
                 |                     "")
                 |                     = binder_key
                 |
                 +-----> Derive-Secret(., "client early traffic secret",
                 |                     ClientHello)
                 |                     = client_early_traffic_secret
                 |
                 +-----> Derive-Secret(., "early exporter master secret",
                 |                     ClientHello)
                 |                     = early_exporter_secret
                 v
           Derive-Secret(., "derived secret", "")
                 |
                 v
(EC)DHE -> HKDF-Extract = Handshake Secret
                 |
                 +-----> Derive-Secret(., "client handshake traffic secret",
                 |                     ClientHello...ServerHello)
                 |                     = client_handshake_traffic_secret
                 |
                 +-----> Derive-Secret(., "server handshake traffic secret",
                 |                     ClientHello...ServerHello)
                 |                     = server_handshake_traffic_secret
                 v
           Derive-Secret(., "derived secret", "")
                 |
                 v
      0 -> HKDF-Extract = Master Secret
                 |
                 +-----> Derive-Secret(., "client application traffic secret",
                 |                     ClientHello...Server Finished)
                 |                     = client_traffic_secret_0
                 |
                 +-----> Derive-Secret(., "server application traffic secret",
                 |                     ClientHello...Server Finished)
                 |                     = server_traffic_secret_0
                 |
                 +-----> Derive-Secret(., "exporter master secret",
                 |                     ClientHello...Server Finished)
                 |                     = exporter_secret
                 |
                 +-----> Derive-Secret(., "resumption master secret",
                                       ClientHello...Client Finished)
                                       = resumption_master_secret




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   The general pattern here is that the secrets shown down the left side
   of the diagram are just raw entropy without context, whereas the
   secrets down the right side include handshake context and therefore
   can be used to derive working keys without additional context.  Note
   that the different calls to Derive-Secret may take different Messages
   arguments, even with the same secret.  In a 0-RTT exchange, Derive-
   Secret is called with four distinct transcripts; in a 1-RTT-only
   exchange with three distinct transcripts.

   The complete transcript passed to Derive-Secret is always taken from
   the following sequence of handshake messages, starting at the first
   ClientHello and including only those messages that were sent:
   ClientHello, HelloRetryRequest, ClientHello, ServerHello,
   EncryptedExtensions, Server CertificateRequest, Server Certificate,
   Server CertificateVerify, Server Finished, EndOfEarlyData, Client
   Certificate, Client CertificateVerify, Client Finished.

   If a given secret is not available, then the 0-value consisting of a
   string of Hash.length zero bytes is used.  Note that this does not
   mean skipping rounds, so if PSK is not in use Early Secret will still
   be HKDF-Extract(0, 0).  For the computation of the binder_secret, the
   label is "external psk binder key" for external PSKs (those
   provisioned outside of TLS) and "resumption psk binder key" for
   resumption PSKs (those provisioned as the resumption master secret of
   a previous handshake).  The different labels prevent the substitution
   of one type of PSK for the other.

   There are multiple potential Early Secret values depending on which
   PSK the server ultimately selects.  The client will need to compute
   one for each potential PSK; if no PSK is selected, it will then need
   to compute the early secret corresponding to the zero PSK.

7.2.  Updating Traffic Keys and IVs

   Once the handshake is complete, it is possible for either side to
   update its sending traffic keys using the KeyUpdate handshake message
   defined in Section 4.6.3.  The next generation of traffic keys is
   computed by generating client_/server_traffic_secret_N+1 from
   client_/server_traffic_secret_N as described in this section then re-
   deriving 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)





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   Once client/server_traffic_secret_N+1 and its associated traffic keys
   have been computed, implementations SHOULD delete client_/
   server_traffic_secret_N and its associated traffic keys.

7.3.  Traffic Key Calculation

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

   -  A secret value

   -  A purpose value indicating the specific value being generated

   -  The length of the key

   The traffic keying material is generated from an input traffic secret
   value using:

    [sender]_write_key = HKDF-Expand-Label(Secret, "key", "", key_length)
    [sender]_write_iv = HKDF-Expand-Label(Secret, "iv", "", iv_length)

   [sender] denotes the sending side.  The Secret value for each record
   type is shown in the table below.

         +-------------------+-----------------------------------+
         | Record Type       | Secret                            |
         +-------------------+-----------------------------------+
         | 0-RTT Application | client_early_traffic_secret       |
         |                   |                                   |
         | Handshake         | [sender]_handshake_traffic_secret |
         |                   |                                   |
         | Application Data  | [sender]_traffic_secret_N         |
         +-------------------+-----------------------------------+

   All the traffic keying material is recomputed whenever the underlying
   Secret changes (e.g., when changing from the handshake to application
   data keys or upon a key update).

7.4.  (EC)DHE Shared Secret Calculation

7.4.1.  Finite Field Diffie-Hellman

   For finite field groups, a conventional Diffie-Hellman computation is
   performed.  The negotiated key (Z) is converted to a byte string by
   encoding in big-endian and padded with zeros up to the size of the
   prime.  This byte string is used as the shared secret, and is used in
   the key schedule as specified above.




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

7.4.2.  Elliptic Curve Diffie-Hellman

   For secp256r1, secp384r1 and secp521r1, ECDH calculations (including
   parameter and 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 IEEE 1363 terminology) as output 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 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 anything other than
   for computing other secrets.)

   ECDH functions are used as follows:

   -  The public key to 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 the
      standard public basepoint (into u-coordinate point input).

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

   For X25519 and X448, implementations SHOULD use the approach
   specified in [RFC7748] to calculate the Diffie-Hellman shared secret.
   Implementations MUST check whether the computed Diffie-Hellman shared
   secret is the all-zero value and abort if so, as described in
   Section 6 of [RFC7748].  If implementers use an alternative
   implementation of these elliptic curves, they SHOULD perform the
   additional checks specified in Section 7 of [RFC7748].

7.5.  Exporters

   [RFC5705] defines keying material exporters for TLS in terms of the
   TLS pseudorandom function (PRF).  This document replaces the PRF with
   HKDF, thus requiring a new construction.  The exporter interface
   remains the same.




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   The exporter value is computed as:

   HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
                     "exporter", Hash(context_value), key_length)

   Where Secret is either the early_exporter_secret or the
   exporter_secret.  Implementations MUST use the exporter_secret unless
   explicitly specified by the application.  A separate interface for
   the early exporter is RECOMMENDED, especially on a server where a
   single interface can make the early exporter inaccessible.

   If no context is provided, the context_value is zero-length.
   Consequently, providing no context computes the same value as
   providing an empty context.  This is a change from previous versions
   of TLS where an empty context produced a different output to an
   absent context.  As of this document's publication, no allocated
   exporter label is used both with and without a context.  Future
   specifications MUST NOT define a use of exporters that permit both an
   empty context and no context with the same label.  New uses of
   exporters SHOULD provide a context in all exporter computations,
   though the value could be empty.

   Requirements for the format of exporter labels are defined in section
   4 of [RFC5705].

8.  Compliance Requirements

8.1.  Mandatory-to-Implement Cipher Suites

   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. (see Appendix B.4)

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

8.2.  Mandatory-to-Implement Extensions

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

   -  Supported Versions ("supported_versions"; Section 4.2.1)



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   -  Cookie ("cookie"; Section 4.2.2)

   -  Signature Algorithms ("signature_algorithms"; Section 4.2.3)

   -  Negotiated Groups ("supported_groups"; Section 4.2.4)

   -  Key Share ("key_share"; Section 4.2.5)

   -  Server Name Indication ("server_name"; Section 3 of [RFC6066])

   All implementations MUST send and use these extensions when offering
   applicable features:

   -  "supported_versions" is REQUIRED for all ClientHello messages.

   -  "signature_algorithms" is REQUIRED for certificate authentication.

   -  "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE
      key exchange.

   -  "pre_shared_key" is REQUIRED for PSK key agreement.

   A client is considered to be attempting to negotiate using this
   specification if the ClientHello contains a "supported_versions"
   extension with a version indicating TLS 1.3.  Such a ClientHello
   message MUST meet the following requirements:

   -  If not containing a "pre_shared_key" extension, it MUST contain
      both a "signature_algorithms" extension and a "supported_groups"
      extension.

   -  If containing a "supported_groups" extension, it MUST also contain
      a "key_share" extension, and vice versa.  An empty
      KeyShare.client_shares vector is permitted.

   Servers receiving a ClientHello which does not conform to these
   requirements MUST abort the handshake with a "missing_extension"
   alert.

   Additionally, all implementations MUST support use of the
   "server_name" extension with applications capable of using it.
   Servers MAY require clients to send a valid "server_name" extension.
   Servers requiring this extension SHOULD respond to a ClientHello
   lacking a "server_name" extension by terminating the connection with
   a "missing_extension" alert.






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9.  Security Considerations

   Security issues are discussed throughout this memo, especially in
   Appendix C, Appendix D, and Appendix E.

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 their allocation policies are below:

   -  TLS Cipher Suite Registry: Values with the first byte in the range
      0-254 (decimal) are assigned via Specification Required [RFC5226].
      Values with the first byte 255 (decimal) are reserved for Private
      Use [RFC5226].

      IANA [SHALL add/has added] the cipher suites listed in
      Appendix B.4 to the registry.  The "Value" and "Description"
      columns are taken from the table.  The "DTLS-OK" and "Recommended"
      columns are both marked as "Yes" for each new cipher suite.
      [[This assumes [I-D.ietf-tls-iana-registry-updates] has been
      applied.]]

   -  TLS ContentType Registry: Future values are allocated via
      Standards Action [RFC5226].

   -  TLS Alert Registry: Future values are allocated via Standards
      Action [RFC5226].  IANA [SHALL update/has updated] this registry
      to include values for "missing_extension" and
      "certificate_required".

   -  TLS HandshakeType Registry: Future values are allocated via
      Standards Action [RFC5226].  IANA [SHALL update/has updated] this
      registry to rename item 4 from "NewSessionTicket" to
      "new_session_ticket" and to add the "hello_retry_request",
      "encrypted_extensions", "end_of_early_data", "key_update", and
      "handshake_hash" values.

   This document also uses a registry originally created in [RFC4366].
   IANA has updated it to reference this document.  The registry and its
   allocation policy is listed below:

   -  IANA [SHALL update/has updated] this registry to include the
      "key_share", "pre_shared_key", "psk_key_exchange_modes",
      "early_data", "cookie", "supported_versions",
      "certificate_authorities", and "oid_filters" extensions with the
      values defined in this document and the Recommended value of
      "Yes".



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   -  IANA [SHALL update/has updated] this registry to include a "TLS
      1.3" column which lists the messages in which the extension may
      appear.  This column [SHALL be/has been] initially populated from
      the table in Section 4.2 with any extension not listed there
      marked as "-" to indicate that it is not used by TLS 1.3.

   In addition, this document defines a new registry to be maintained by
   IANA:

   -  TLS SignatureScheme Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC5226].  Values with the first byte 255 (decimal) are reserved
      for Private Use [RFC5226].  Values with the first byte in the
      range 0-6 or with the second byte in the range 0-3 that are not
      currently allocated are reserved for backwards compatibility.
      This registry SHALL have a "Recommended" column.  The registry
      [shall be/ has been] initially populated with the values described
      in Section 4.2.3.  The following values SHALL be marked as
      "Recommended": ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
      rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519.

   Finally, this document obsoletes the TLS HashAlgorithm Registry and
   the TLS SignatureAlgorithm Registry, both originally created in
   [RFC5246].  IANA [SHALL update/has updated] the TLS HashAlgorithm
   Registry to list values 7-223 as "Reserved" and the TLS
   SignatureAlgorithm Registry to list values 4-223 as "Reserved".

11.  References

11.1.  Normative References

   [AES]      National Institute of Standards and Technology,
              "Specification for the Advanced Encryption Standard
              (AES)", NIST FIPS 197, November 2001.

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

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.



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   [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>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655,
              DOI 10.17487/RFC6655, July 2012,
              <http://www.rfc-editor.org/info/rfc6655>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <http://www.rfc-editor.org/info/rfc6979>.



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   [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
              <http://www.rfc-editor.org/info/rfc7507>.

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
              <http://www.rfc-editor.org/info/rfc7539>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

   [RFC7919]  Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for Transport Layer Security (TLS)",
              RFC 7919, DOI 10.17487/RFC7919, August 2016,
              <http://www.rfc-editor.org/info/rfc7919>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <http://www.rfc-editor.org/info/rfc8032>.

   [SHS]      National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard", NIST FIPS
              PUB 180-4, March 2012.

   [X690]     ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2002, 2002.

   [X962]     ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 1998.

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.



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   [CCG16]    Cohn-Gordon, K., Cremers, C., and L. Garratt, "On Post-
              Compromise Security", IEEE Computer Security Foundations
              Symposium , 2015.

   [CHHSV17]  Cremers, C., Horvat, M., Hoyland, J., van der Merwe, T.,
              and S. Scott, "Awkward Handshake: Possible mismatch of
              client/server view on client authentication in post-
              handshake mode in Revision 18", 2017,
              <https://www.ietf.org/mail-archive/web/tls/current/
              msg22382.html>.

   [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 , 1992.

   [DSS]      National Institute of Standards and Technology, U.S.
              Department of Commerce, "Digital Signature Standard,
              version 4", NIST FIPS PUB 186-4, 2013.

   [ECDSA]    American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry: The
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI ANS X9.62-2005, November 2005.

   [FGSW16]   Fischlin, M., Guenther, F., Schmidt, B., and B. Warinschi,
              "Key Confirmation in Key Exchange: A Formal Treatment and
              Implications for TLS 1.3", Proceedings of IEEE Symposium
              on Security and Privacy (Oakland) 2016 , 2016.

   [FW15]     Florian Weimer, ., "Factoring RSA Keys With TLS Perfect
              Forward Secrecy", September 2015.

   [GCM]      Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC",
              NIST Special Publication 800-38D, November 2007.






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   [HGFS15]   Hlauschek, C., Gruber, M., Fankhauser, F., and C. Schanes,
              "Prying Open Pandora's Box: KCI Attacks against TLS",
              Proceedings of USENIX Workshop on Offensive Technologies ,
              2015.

   [I-D.ietf-tls-iana-registry-updates]
              Salowey, J. and S. Turner, "D/TLS IANA Registry Updates",
              draft-ietf-tls-iana-registry-updates-00 (work in
              progress), January 2017.

   [I-D.ietf-tls-tls13-vectors]
              Thomson, M., "Example Handshake Traces for TLS 1.3",
              draft-ietf-tls-tls13-vectors-00 (work in progress),
              January 2017.

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

   [KEYAGREEMENT]
              Barker, E., Lily Chen, ., Roginsky, A., and M. Smid,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography", NIST Special
              Publication 800-38D, May 2013.

   [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>.

   [RECORD]   Bhargavan, K., Delignat-Lavaud, A., Fournet, C.,
              Kohlweiss, M., Pan, J., Protzenko, J., Rastogi, A., Swamy,
              N., Zanella-Beguelin, S., and J. Zinzindohoue,
              "Implementing and Proving the TLS 1.3 Record Layer",
              December 2016, <http://eprint.iacr.org/2016/1178>.




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   [REKEY]    Abdalla, M. and M. Bellare, "Increasing the Lifetime of a
              Key: A Comparative Analysis of the Security of Re-keying
              Techniques", ASIACRYPT2000 , October 2000.

   [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>.

   [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>.

   [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>.



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   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              DOI 10.17487/RFC5081, November 2007,
              <http://www.rfc-editor.org/info/rfc5081>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC5878]  Brown, M. and R. Housley, "Transport Layer Security (TLS)
              Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
              May 2010, <http://www.rfc-editor.org/info/rfc5878>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <http://www.rfc-editor.org/info/rfc5929>.

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

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





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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,
              <http://www.rfc-editor.org/info/rfc6520>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <http://www.rfc-editor.org/info/rfc7366>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [RFC7568]  Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
              DOI 10.17487/RFC7568, June 2015,
              <http://www.rfc-editor.org/info/rfc7568>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.






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   [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
              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 Diffie-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














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Appendix A.  State Machine

   This section provides a summary of the legal state transitions for
   the client and server handshakes.  State names (in all capitals,
   e.g., START) have no formal meaning but are provided for ease of
   comprehension.  Messages which are sent only sometimes are indicated
   in [].

A.1.  Client

                              START <----+
               Send ClientHello |        | Recv HelloRetryRequest
            /                   v        |
           |                  WAIT_SH ---+
       Can |                    | Recv ServerHello
      send |                    V
     early |                 WAIT_EE
      data |                    | Recv EncryptedExtensions
           |           +--------+--------+
           |     Using |                 | Using certificate
           |       PSK |                 v
           |           |            WAIT_CERT_CR
           |           |        Recv |       | Recv CertificateRequest
           |           | Certificate |       v
           |           |             |    WAIT_CERT
           |           |             |       | Recv Certificate
           |           |             v       v
           |           |              WAIT_CV
           |           |                 | Recv CertificateVerify
           |           +> WAIT_FINISHED <+
           |                  | Recv Finished
           \                  |
                              | [Send EndOfEarlyData]
                              | [Send Certificate [+ CertificateVerify]]
                              | Send Finished
    Can send                  v
    app data -->          CONNECTED
    after
    here

A.2.  Server










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                              START <-----+
               Recv ClientHello |         | Send HelloRetryRequest
                                v         |
                             RECVD_CH ----+
                                | Select parameters
                                v
                             NEGOTIATED
                                | Send ServerHello
                                | Send EncryptedExtensions
                                | [Send CertificateRequest]
 Can send                       | [Send Certificate + CertificateVerify]
 app data -->                   | Send Finished
 after                 +--------+--------+
 here         No 0-RTT |                 | 0-RTT
                       |                 v
                       |             WAIT_EOED <---+
                       |            Recv |   |     | Recv
                       |  EndOfEarlyData |   |     | early data
                       |                 |   +-----+
                       +> WAIT_FLIGHT2 <-+
                                |
                       +--------+--------+
               No auth |                 | Client auth
                       |                 |
                       |                 v
                       |             WAIT_CERT
                       |        Recv |       | Recv Certificate
                       |       empty |       v
                       | Certificate |    WAIT_CV
                       |             |       | Recv
                       |             v       | CertificateVerify
                       +-> WAIT_FINISHED <---+
                                | Recv Finished
                                v
                            CONNECTED

Appendix B.  Protocol Data Structures and Constant Values

   This section describes protocol types and constants.  Values listed
   as _RESERVED were used in previous versions of TLS and are listed
   here for completeness.  TLS 1.3 implementations MUST NOT send them
   but might receive them from older TLS implementations.

B.1.  Record Layer







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      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;
          uint16 length;
          opaque fragment[TLSPlaintext.length];
      } TLSPlaintext;

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

      struct {
          ContentType opaque_type = 23; /* application_data */
          ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
          uint16 length;
          opaque encrypted_record[length];
      } TLSCiphertext;

B.2.  Alert Messages





















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

      enum {
          close_notify(0),
          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),
          certificate_required(116),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;








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B.3.  Handshake Protocol

      enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          hello_verify_request_RESERVED(3),
          new_session_ticket(4),
          end_of_early_data(5),
          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),
          message_hash(254),
          (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (Handshake.msg_type) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case end_of_early_data:     EndOfEarlyData;
              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;

B.3.1.  Key Exchange Messages

      uint16 ProtocolVersion;
      opaque Random[32];

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




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      struct {
          ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
          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<8..2^16-1>;
      } ClientHello;

      struct {
          ProtocolVersion version;
          Random random;
          CipherSuite cipher_suite;
          Extension extensions<6..2^16-1>;
      } ServerHello;

      struct {
          ProtocolVersion server_version;
          CipherSuite cipher_suite;
          Extension extensions<2..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),
          supported_versions(43),
          cookie(44),
          psk_key_exchange_modes(45),
          certificate_authorities(47),
          oid_filters(48),
          (65535)
      } ExtensionType;

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

      struct {
          select (Handshake.msg_type) {



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              case client_hello:
                  KeyShareEntry client_shares<0..2^16-1>;

              case hello_retry_request:
                  NamedGroup selected_group;

              case server_hello:
                  KeyShareEntry server_share;
          };
      } KeyShare;

      enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;

      struct {
          PskKeyExchangeMode ke_modes<1..255>;
      } PskKeyExchangeModes;

      struct {} Empty;

      struct {
          select (Handshake.msg_type) {
              case new_session_ticket:   uint32 max_early_data_size;
              case client_hello:         Empty;
              case encrypted_extensions: Empty;
          };
      } EarlyDataIndication;

      struct {
          opaque identity<1..2^16-1>;
          uint32 obfuscated_ticket_age;
      } PskIdentity;

      opaque PskBinderEntry<32..255>;

      struct {
          select (Handshake.msg_type) {
              case client_hello:
                  PskIdentity identities<7..2^16-1>;
                  PskBinderEntry binders<33..2^16-1>;

              case server_hello:
                  uint16 selected_identity;
          };

      } PreSharedKeyExtension;






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B.3.1.1.  Version Extension

      struct {
          ProtocolVersion versions<2..254>;
      } SupportedVersions;

B.3.1.2.  Cookie Extension

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

B.3.1.3.  Signature Algorithm Extension






































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      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(0x0804),
          rsa_pss_sha384(0x0805),
          rsa_pss_sha512(0x0806),

          /* EdDSA algorithms */
          ed25519(0x0807),
          ed448(0x0808),

          /* 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;

      struct {
          SignatureScheme supported_signature_algorithms<2..2^16-2>;
      } SignatureSchemeList;

B.3.1.4.  Supported Groups Extension










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      enum {
          /* Elliptic Curve Groups (ECDHE) */
          obsolete_RESERVED(0x0001..0x0016),
          secp256r1(0x0017), secp384r1(0x0018), secp521r1(0x0019),
          obsolete_RESERVED(0x001A..0x001C),
          x25519(0x001D), x448(0x001E),

          /* Finite Field Groups (DHE) */
          ffdhe2048(0x0100), ffdhe3072(0x0101), ffdhe4096 (0x0102),
          ffdhe6144(0x0103), ffdhe8192(0x0104),

          /* Reserved Code Points */
          ffdhe_private_use(0x01FC..0x01FF),
          ecdhe_private_use(0xFE00..0xFEFF),
          obsolete_RESERVED(0xFF01..0xFF02),
          (0xFFFF)
      } NamedGroup;

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

   Values within "obsolete_RESERVED" ranges were used in previous
   versions of TLS and MUST NOT be offered or negotiated by TLS 1.3
   implementations.  The obsolete curves have various known/theoretical
   weaknesses or have had very little usage, in some cases only due to
   unintentional server configuration issues.  They are no longer
   considered appropriate for general use and should be assumed to be
   potentially unsafe.  The set of curves specified here is sufficient
   for interoperability with all currently deployed and properly
   configured TLS implementations.

B.3.2.  Server Parameters Messages


















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

      struct {
          DistinguishedName authorities<3..2^16-1>;
      } CertificateAuthoritiesExtension;

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

      struct {
          opaque certificate_request_context<0..2^8-1>;
          Extension extensions<2..2^16-1>;
      } CertificateRequest;

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

      struct {
          OIDFilter filters<0..2^16-1>;
      } OIDFilterExtension;

B.3.3.  Authentication Messages

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

      struct {
          ASN1Cert cert_data;
          Extension extensions<0..2^16-1>;
      } CertificateEntry;

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

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

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





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B.3.4.  Ticket Establishment

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

B.3.5.  Updating Keys

      struct {} EndOfEarlyData;

      enum {
          update_not_requested(0), update_requested(1), (255)
      } KeyUpdateRequest;

      struct {
          KeyUpdateRequest request_update;
      } KeyUpdate;

B.4.  Cipher Suites

   A symmetric cipher suite defines the pair of the AEAD algorithm and
   hash algorithm to be used with HKDF.  Cipher suite names follow the
   naming convention:

      CipherSuite TLS_AEAD_HASH = VALUE;

      +-----------+------------------------------------------------+
      | Component | Contents                                       |
      +-----------+------------------------------------------------+
      | TLS       | The string "TLS"                               |
      |           |                                                |
      | AEAD      | The AEAD algorithm used for record protection  |
      |           |                                                |
      | HASH      | The hash algorithm used with HKDF              |
      |           |                                                |
      | VALUE     | The two byte ID assigned for this cipher suite |
      +-----------+------------------------------------------------+

   This specification defines the following cipher suites for use with
   TLS 1.3.








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              +------------------------------+-------------+
              | Description                  | Value       |
              +------------------------------+-------------+
              | TLS_AES_128_GCM_SHA256       | {0x13,0x01} |
              |                              |             |
              | TLS_AES_256_GCM_SHA384       | {0x13,0x02} |
              |                              |             |
              | TLS_CHACHA20_POLY1305_SHA256 | {0x13,0x03} |
              |                              |             |
              | TLS_AES_128_CCM_SHA256       | {0x13,0x04} |
              |                              |             |
              | TLS_AES_128_CCM_8_SHA256     | {0x13,0x05} |
              +------------------------------+-------------+

   The corresponding AEAD algorithms AEAD_AES_128_GCM, AEAD_AES_256_GCM,
   and AEAD_AES_128_CCM are defined in [RFC5116].
   AEAD_CHACHA20_POLY1305 is defined in [RFC7539].  AEAD_AES_128_CCM_8
   is defined in [RFC6655].  The corresponding hash algorithms are
   defined in [SHS].

   Although TLS 1.3 uses the same cipher suite space as previous
   versions of TLS, TLS 1.3 cipher suites are defined differently, only
   specifying the symmetric ciphers, and cannot be used for TLS 1.2.
   Similarly, TLS 1.2 and lower cipher suites cannot be used with TLS
   1.3.

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

Appendix C.  Implementation Notes

   The TLS protocol cannot prevent many common security mistakes.  This
   section provides several recommendations to assist implementors.
   [I-D.ietf-tls-tls13-vectors] provides test vectors for TLS 1.3
   handshakes.

C.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.







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C.2.  Random Number Generation and Seeding

   TLS requires a cryptographically secure pseudorandom number generator
   (PRNG).  In most cases, the operating system provides an appropriate
   facility such as /dev/urandom, which should be used absent other
   (performance) concerns.  It is generally preferable to use an
   existing PRNG 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.

C.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 trust anchors should be done very carefully.  Users
   should be able to view information about the certificate and trust
   anchor.  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.

C.4.  Implementation Pitfalls

   Implementation experience has shown that certain parts of earlier TLS
   specifications are not easy to understand, and have been a source of
   interoperability and security problems.  Many of these areas have
   been clarified in this document, but this appendix contains a short
   list of the most important things that require special attention from
   implementors.

   TLS protocol issues:

   -  Do you correctly handle handshake messages that are fragmented to
      multiple TLS records (see Section 5.1)?  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 CertificateRequest
      handshake messages can be large enough to require fragmentation.

   -  Do you ignore the TLS record layer version number in all
      unencrypted TLS records? (see Appendix D)

   -  Have you ensured that all support for SSL, RC4, EXPORT ciphers,
      and MD5 (via the "signature_algorithms" extension) is completely
      removed from all possible configurations that support TLS 1.3 or



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      later, and that attempts to use these obsolete capabilities fail
      correctly? (see Appendix D)

   -  Do you handle TLS extensions in ClientHello correctly, including
      unknown extensions?

   -  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 4.4.2.3)?

   -  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?

   -  Do you properly ignore unrecognized cipher suites (Section 4.1.2),
      hello extensions (Section 4.2), named groups (Section 4.2.4), and
      signature algorithms (Section 4.2.3)?

   -  As a server, do you send a HelloRetryRequest to clients which
      support a compatible (EC)DHE group but do not predict it in the
      "key_share" extension?  As a client, do you correctly handle a
      HelloRetryRequest from the server?

   Cryptographic details:

   -  What countermeasures do you use to prevent timing attacks
      [TIMING]?

   -  When using Diffie-Hellman key exchange, do you correctly preserve
      leading zero bytes in the negotiated key (see Section 7.4.1)?

   -  Does your TLS client check that the Diffie-Hellman parameters sent
      by the server are acceptable, (see Section 4.2.5.1)?

   -  Do you use a strong and, most importantly, properly seeded random
      number generator (see Appendix C.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 4.2.5.1)?

   -  Do you verify signatures after making them to protect against RSA-
      CRT key leaks?  [FW15]




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C.5.  Client Tracking Prevention

   Clients SHOULD NOT reuse a ticket for multiple connections.  Reuse of
   a ticket allows passive observers to correlate different connections.
   Servers that issue tickets SHOULD offer at least as many 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 tickets with every connection.
   This ensures that clients are always able to use a new ticket when
   creating a new connection.

C.6.  Unauthenticated Operation

   Previous versions of TLS offered explicitly unauthenticated cipher
   suites based on anonymous Diffie-Hellman.  These modes have been
   deprecated in TLS 1.3.  However, it is still possible to negotiate
   parameters 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 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 D.  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.legacy_record_version and



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   TLSCiphertext.legacy_record_version) As of TLS 1.3, this field is
   deprecated.  The value of TLSPlaintext.legacy_record_version MUST be
   ignored by all implementations.  The value of
   TLSCiphertext.legacy_record_version MAY be ignored, or MAY be
   validated to match the fixed constant value.  Version negotiation is
   performed using only the handshake versions.
   (ClientHello.legacy_version, ClientHello "supported_versions"
   extension, and 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 4.4.2.2)

   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.

D.1.  Negotiating with an older server

   A TLS 1.3 client who wishes to negotiate with servers that do not
   support TLS 1.3 will send a normal TLS 1.3 ClientHello containing
   0x0303 (TLS 1.2) in ClientHello.legacy_version but with the correct
   version in the "supported_versions" extension.  If the server does
   not support TLS 1.3 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 using a ticket for resumption SHOULD initiate the connection
   using the version that was previously negotiated.

   Note that 0-RTT data is not compatible with older servers and SHOULD
   NOT be sent absent knowledge that the server supports TLS 1.3.  See
   Appendix D.3.

   If the version chosen by the server is not supported by the client
   (or not acceptable), the client MUST abort the handshake with a
   "protocol_version" alert.





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   Some legacy server implementations are known to not implement the TLS
   specification properly and might abort connections upon encountering
   TLS extensions or versions which they are 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.

D.2.  Negotiating with an older client

   A TLS server can also receive a ClientHello indicating a version
   number smaller than its highest supported version.  If the
   "supported_versions" extension is present, the server MUST negotiate
   using that extension as described in Section 4.2.1.  If the
   "supported_versions" extension is not present, the server MUST
   negotiate the minimum of ClientHello.legacy_version and TLS 1.2.  For
   example, if the server supports TLS 1.0, 1.1, and 1.2, and
   legacy_version is TLS 1.0, the server will proceed with a TLS 1.0
   ServerHello.  If the server only supports versions greater than
   ClientHello.legacy_version, it MUST abort the handshake with a
   "protocol_version" alert.

   Note that earlier versions of TLS did not clearly specify the record
   layer version number value in all cases
   (TLSPlaintext.legacy_record_version).  Servers will receive various
   TLS 1.x versions in this field, but its value MUST always be ignored.

D.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 will 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.




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D.4.  Backwards Compatibility Security Restrictions

   Implementations negotiating use of older versions of TLS SHOULD
   prefer forward secure and AEAD cipher suites, when 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 3.0 [SSL3] is considered insufficient for the
   reasons enumerated in [RFC7568], and MUST NOT be negotiated 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 a ClientHello.legacy_version or
   ServerHello.version set to 0x0300 or less.  Any endpoint receiving a
   Hello message with ClientHello.legacy_version or ServerHello.version
   set to 0x0300 MUST abort the handshake with a "protocol_version"
   alert.

   Implementations MUST NOT send any records with a version less than
   0x0300.  Implementations SHOULD NOT accept any records with a version
   less than 0x0300 (but may inadvertently do so if the record version
   number is ignored completely).

   Implementations MUST NOT use the Truncated HMAC extension, defined in
   Section 7 of [RFC6066], as it is not applicable to AEAD algorithms
   and has been shown to be insecure in some scenarios.

Appendix E.  Overview of Security Properties

   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.



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   We cover properties of the handshake separately from those of the
   record layer.

E.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 of the
   following values:

   -  A set of "session keys" (the various secrets derived from 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 keys.  The handshake needs to output
      the same set of session keys on both sides of the handshake,
      provided that it completes successfully on each endpoint (See
      [CK01]; defn 1, part 1).

   Secrecy of the session keys.  The shared session keys 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 keys:  Any two distinct handshakes should
      produce distinct, unrelated session keys.  Individual session keys
      produced by a handshake should also be distinct and unrelated.

   Downgrade protection.  The cryptographic parameters should be the
      same on both sides and should be the same as if the peers had been




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      communicating in the absence of an attack (See [BBFKZG16]; defns 8
      and 9}).

   Forward secret with respect to long-term keys  If the long-term
      keying material (in this case the signature keys in certificate-
      based authentication modes or the external/resumption PSK in PSK
      with (EC)DHE modes) are compromised after the handshake is
      complete, this does not compromise the security of the session key
      (See [DOW92]).  The forward secrecy property is not satisfied when
      PSK is used in the "psk_ke" PskKeyExchangeMode.

   Key Compromise Impersonation (KCI) resistance  In a mutually-
      authenticated connection with certificates, peer authentication
      should hold even if the local long-term secret was compromised
      before the connection was established (see [HGFS15]).  For
      example, if a client's signature key is compromised, it should not
      be possible to impersonate arbitrary servers to that client in
      subsequent handshakes.

   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 external PSK and resumption PSK bootstrap from a long-term shared
   secret into a unique per-connection set of short-term session keys.
   This secret may have been established in a previous handshake.  If
   PSK with (EC)DHE key establishment is used, these session keys will
   also be forward secret.  The resumption PSK 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.

   The PSK binder value forms a binding between a PSK and the current
   handshake, as well as between the session where the PSK was
   established and the session where it was used.  This binding
   transitively includes the original handshake transcript, because that
   transcript is digested into the values which produce the Resumption



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   Master Secret.  This requires that both the KDF used to produce the
   resumption master secret and the MAC used to compute the binder be
   collision resistant.  These are properties of HKDF and HMAC
   respectively when used with collision resistant hash functions and
   producing output of at least 256 bits.  Any future replacement of
   these functions MUST also provide collision resistance.  Note: The
   binder does not cover the binder values from other PSKs, though they
   are included in the Finished MAC.

   If an exporter is used, then it produces values which are unique and
   secret (because they are generated from a unique session key).
   Exporters computed with different labels and contexts are
   computationally independent, so it is not feasible to compute one
   from another or the session secret from the exported value.  Note:
   exporters can produce arbitrary-length values.  If exporters are to
   be used as channel bindings, the exported value MUST be large enough
   to provide collision resistance.  The exporters provided in TLS 1.3
   are derived from the same handshake contexts as the early traffic
   keys and the application traffic keys respectively, and thus have
   similar security properties.  Note that they do not include the
   client's certificate; future applications which wish to bind to the
   client's certificate may need to define a new exporter that includes
   the full handshake transcript.

   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 4.1.3
   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.

   A client that has sent authentication data to a server, either in the
   main handshake or in post-handshake authentication, cannot be sure if
   the server afterwards considers the client to be authenticated or
   not.  If the client needs to determine if the server considers the
   connection to be unilaterally or mutually authenticated, this has to
   be provisioned by the application layer.  See [CHHSV17] for details.

   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



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   server is not able to guarantee full uniqueness of the handshake
   (non-replayability) without keeping potentially undue amounts of
   state.  See Section 4.2.7 for one mechanism to limit the exposure to
   replay.

   TLS does not provide security for handshakes which take place after
   the peer's long-term secret (signature key or external PSK) is
   compromised.  It therefore does not provide post-compromise security
   [CCG16], sometimes also referred to as backwards or future security.
   This is in contrast to KCI resistance, which describes the security
   guarantees that a party has after its own long-term secret has been
   compromised.

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

E.2.  Record Layer

   The record layer depends on the handshake producing strong traffic
   secrets which can be used to derive bidirectional encryption 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.

   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 4.6.3 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



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   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.

   The re-keying technique in TLS 1.3 (see Section 7.2) follows the
   construction of the serial generator in [REKEY], which shows that re-
   keying can allow keys to be used for a larger number of encryptions
   than without re-keying.  This relies on the security of the HKDF-
   Expand-Label function as a pseudorandom function (PRF).  In addition,
   as long as this function is truly one way, it is not possible to
   compute traffic keys from prior to a key change (forward secrecy).

   TLS does not provide security for data which is communicated on a
   connection after a traffic secret of that connection is compromised.
   That is, TLS does not provide post-compromise security/future
   secrecy/backward secrecy with respect to the traffic secret.  Indeed,
   an attacker who learns a traffic secret can compute all future
   traffic secrets on that connection.  Systems which want such
   guarantees need to do a fresh handshake and establish a new
   connection with an (EC)DHE exchange.

   The reader should refer to [RECORD] for analysis of the TLS record
   layer.

Appendix F.  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://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






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Appendix G.  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
      INRIA & Microsoft Research - Joint Center
      benjamin.beurdouche@ens.fr

   -  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

   -  Katriel Cohn-Gordon
      University of Oxford
      me@katriel.co.uk

   -  Cas Cremers
      University of Oxford



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      cas.cremers@cs.ox.ac.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
      dave@nulldereference.com

   -  Alessandro Ghedini
      Cloudflare Inc.
      alessandro@cloudflare.com

   -  Jens Guballa
      ETAS
      jens.guballa@etas.com

   -  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




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   -  David Hopwood
      Independent Consultant
      david.hopwood@blueyonder.co.uk

   -  Marko Horvat
      MPI-SWS
      mhorvat@mpi-sws.org

   -  Jonathan Hoyland
      Royal Holloway, University of London

   -  Subodh Iyengar
      Facebook
      subodh@fb.com

   -  Benjamin Kaduk
      Akamai
      kaduk@mit.edu

   -  Daniel Kahn Gillmor
      ACLU
      dkg@fifthhorseman.net

   -  Hubert Kario
      Red Hat Inc.
      hkario@redhat.com

   -  Phil Karlton (co-author of SSL 3.0)

   -  Leon Klingele
      Independent
      mail@leonklingele.de

   -  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

   -  Olivier Levillain
      ANSSI
      olivier.levillain@ssi.gouv.fr



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   -  Xiaoyin Liu
      University of North Carolina at Chapel Hill
      xiaoyin.l@outlook.com

   -  Ilari Liusvaara
      Independent
      ilariliusvaara@welho.com

   -  Atul Luykx
      K.U.  Leuven
      atul.luykx@kuleuven.be

   -  Carl Mehner
      USAA
      carl.mehner@usaa.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
      Microsoft
      mnystrom@microsoft.com

   -  Kazuho Oku
      DeNA Co., Ltd.
      kazuhooku@gmail.com

   -  Kenny Paterson
      Royal Holloway, University of London
      kenny.paterson@rhul.ac.uk

   -  Alfredo Pironti (co-author of [RFC7627])
      INRIA
      alfredo.pironti@inria.fr

   -  Andrei Popov
      Microsoft
      andrei.popov@microsoft.com




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   -  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

   -  Rich Salz
      Akamai
      rsalz@akamai.com

   -  Sam Scott
      Royal Holloway, University of London
      me@samjs.co.uk

   -  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

   -  Tim Taubert
      Mozilla
      ttaubert@mozilla.com

   -  Martin Thomson
      Mozilla
      mt@mozilla.com

   -  Filippo Valsorda
      Cloudflare Inc.



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      filippo@cloudflare.com

   -  Thyla van der Merwe
      Royal Holloway, University of London
      tjvdmerwe@gmail.com

   -  Tom Weinstein

   -  Hoeteck Wee
      Ecole Normale Superieure, Paris
      hoeteck@alum.mit.edu

   -  David Wong
      NCC Group
      david.wong@nccgroup.trust

   -  Tim Wright
      Vodafone
      timothy.wright@vodafone.com

   -  Kazu Yamamoto
      Internet Initiative Japan Inc.
      kazu@iij.ad.jp

Author's Address

   Eric Rescorla
   RTFM, Inc.

   EMail: ekr@rtfm.com





















Rescorla               Expires September 11, 2017             [Page 127]


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