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rfc:rfc8446

Internet Engineering Task Force (IETF) E. Rescorla Request for Comments: 8446 Mozilla Obsoletes: 5077, 5246, 6961 August 2018 Updates: 5705, 6066 Category: Standards Track ISSN: 2070-1721

      The Transport Layer Security (TLS) Protocol Version 1.3

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.
 This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077,
 5246, and 6961.  This document also specifies new requirements for
 TLS 1.2 implementations.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8446.

Rescorla Standards Track [Page 1] RFC 8446 TLS August 2018

Copyright Notice

 Copyright (c) 2018 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
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Rescorla Standards Track [Page 2] RFC 8446 TLS August 2018

Table of Contents

 1. Introduction ....................................................6
    1.1. Conventions and Terminology ................................7
    1.2. Major Differences from TLS 1.2 .............................8
    1.3. Updates Affecting TLS 1.2 ..................................9
 2. Protocol Overview ..............................................10
    2.1. Incorrect DHE Share .......................................14
    2.2. Resumption and Pre-Shared Key (PSK) .......................15
    2.3. 0-RTT Data ................................................17
 3. Presentation Language ..........................................19
    3.1. Basic Block Size ..........................................19
    3.2. Miscellaneous .............................................20
    3.3. Numbers ...................................................20
    3.4. Vectors ...................................................20
    3.5. Enumerateds ...............................................21
    3.6. Constructed Types .........................................22
    3.7. Constants .................................................23
    3.8. Variants ..................................................23
 4. Handshake Protocol .............................................24
    4.1. Key Exchange Messages .....................................25
         4.1.1. Cryptographic Negotiation ..........................26
         4.1.2. Client Hello .......................................27
         4.1.3. Server Hello .......................................31
         4.1.4. Hello Retry Request ................................33
    4.2. Extensions ................................................35
         4.2.1. Supported Versions .................................39
         4.2.2. Cookie .............................................40
         4.2.3. Signature Algorithms ...............................41
         4.2.4. Certificate Authorities ............................45
         4.2.5. OID Filters ........................................45
         4.2.6. Post-Handshake Client Authentication ...............47
         4.2.7. Supported Groups ...................................47
         4.2.8. Key Share ..........................................48
         4.2.9. Pre-Shared Key Exchange Modes ......................51
         4.2.10. Early Data Indication .............................52
         4.2.11. Pre-Shared Key Extension ..........................55
    4.3. Server Parameters .........................................59
         4.3.1. Encrypted Extensions ...............................60
         4.3.2. Certificate Request ................................60
    4.4. Authentication Messages ...................................61
         4.4.1. The Transcript Hash ................................63
         4.4.2. Certificate ........................................64
         4.4.3. Certificate Verify .................................69
         4.4.4. Finished ...........................................71
    4.5. End of Early Data .........................................72

Rescorla Standards Track [Page 3] RFC 8446 TLS August 2018

    4.6. Post-Handshake Messages ...................................73
         4.6.1. New Session Ticket Message .........................73
         4.6.2. Post-Handshake Authentication ......................75
         4.6.3. Key and Initialization Vector Update ...............76
 5. Record Protocol ................................................77
    5.1. Record Layer ..............................................78
    5.2. Record Payload Protection .................................80
    5.3. Per-Record Nonce ..........................................82
    5.4. Record Padding ............................................83
    5.5. Limits on Key Usage .......................................84
 6. Alert Protocol .................................................85
    6.1. Closure Alerts ............................................87
    6.2. Error Alerts ..............................................88
 7. Cryptographic Computations .....................................90
    7.1. Key Schedule ..............................................91
    7.2. Updating Traffic Secrets ..................................94
    7.3. Traffic Key Calculation ...................................95
    7.4. (EC)DHE Shared Secret Calculation .........................95
         7.4.1. Finite Field Diffie-Hellman ........................95
         7.4.2. Elliptic Curve Diffie-Hellman ......................96
    7.5. Exporters .................................................97
 8. 0-RTT and Anti-Replay ..........................................98
    8.1. Single-Use Tickets ........................................99
    8.2. Client Hello Recording ....................................99
    8.3. Freshness Checks .........................................101
 9. Compliance Requirements .......................................102
    9.1. Mandatory-to-Implement Cipher Suites .....................102
    9.2. Mandatory-to-Implement Extensions ........................103
    9.3. Protocol Invariants ......................................104
 10. Security Considerations ......................................106
 11. IANA Considerations ..........................................106
 12. References ...................................................109
    12.1. Normative References ....................................109
    12.2. Informative References ..................................112
 Appendix A. State Machine ........................................120
   A.1. Client ....................................................120
   A.2. Server ....................................................121
 Appendix B. Protocol Data Structures and Constant Values .........122
   B.1. Record Layer ..............................................122
   B.2. Alert Messages ............................................123
   B.3. Handshake Protocol ........................................124
     B.3.1. Key Exchange Messages .................................125
     B.3.2. Server Parameters Messages ............................131
     B.3.3. Authentication Messages ...............................132
     B.3.4. Ticket Establishment ..................................132
     B.3.5. Updating Keys .........................................133
   B.4. Cipher Suites .............................................133

Rescorla Standards Track [Page 4] RFC 8446 TLS August 2018

 Appendix C. Implementation Notes .................................134
   C.1. Random Number Generation and Seeding ......................134
   C.2. Certificates and Authentication ...........................135
   C.3. Implementation Pitfalls ...................................135
   C.4. Client Tracking Prevention ................................137
   C.5. Unauthenticated Operation .................................137
 Appendix D. Backward Compatibility ...............................138
   D.1. Negotiating with an Older Server ..........................139
   D.2. Negotiating with an Older Client ..........................139
   D.3. 0-RTT Backward Compatibility ..............................140
   D.4. Middlebox Compatibility Mode ..............................140
   D.5. Security Restrictions Related to Backward Compatibility ...141
 Appendix E. Overview of Security Properties ......................142
   E.1. Handshake .................................................142
     E.1.1. Key Derivation and HKDF ...............................145
     E.1.2. Client Authentication .................................146
     E.1.3. 0-RTT .................................................146
     E.1.4. Exporter Independence .................................146
     E.1.5. Post-Compromise Security ..............................146
     E.1.6. External References ...................................147
   E.2. Record Layer ..............................................147
     E.2.1. External References ...................................148
   E.3. Traffic Analysis ..........................................148
   E.4. Side-Channel Attacks ......................................149
   E.5. Replay Attacks on 0-RTT ...................................150
     E.5.1. Replay and Exporters ..................................151
   E.6. PSK Identity Exposure .....................................152
   E.7. Sharing PSKs ..............................................152
   E.8. Attacks on Static RSA .....................................152
 Contributors .....................................................153
 Author's Address .................................................160

Rescorla Standards Track [Page 5] RFC 8446 TLS August 2018

1. Introduction

 The primary goal of TLS is to provide a secure channel between two
 communicating peers; the only requirement from the underlying
 transport is a reliable, in-order data stream.  Specifically, the
 secure channel should provide the following properties:
  1. 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], the Elliptic Curve Digital Signature Algorithm (ECDSA)
    [ECDSA], or the Edwards-Curve Digital Signature Algorithm (EdDSA)
    [RFC8032]) or a symmetric pre-shared key (PSK).
  1. Confidentiality: Data sent over the channel after establishment is

only visible to the endpoints. TLS does not hide the length of

    the data it transmits, though endpoints are able to pad TLS
    records in order to obscure lengths and improve protection against
    traffic analysis techniques.
  1. Integrity: Data sent over the channel after establishment cannot

be modified by attackers without detection.

 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:
  1. 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.
  1. 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.

Rescorla Standards Track [Page 6] RFC 8446 TLS August 2018

 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 by both
 peers.
 This document supersedes and obsoletes previous versions of TLS,
 including version 1.2 [RFC5246].  It also obsoletes the TLS ticket
 mechanism defined in [RFC5077] and replaces it with the mechanism
 defined in Section 2.2.  Because TLS 1.3 changes the way keys are
 derived, it updates [RFC5705] as described in Section 7.5.  It also
 changes how Online Certificate Status Protocol (OCSP) messages are
 carried and therefore updates [RFC6066] and obsoletes [RFC6961] as
 described in Section 4.4.2.1.

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
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 The following terms are used:
 client:  The endpoint initiating the TLS connection.
 connection:  A transport-layer connection between two endpoints.
 endpoint:  Either the client or server of the connection.
 handshake:  An initial negotiation between client and server that
    establishes the parameters of their subsequent interactions
    within TLS.
 peer:  An endpoint.  When discussing a particular endpoint, "peer"
    refers to the endpoint that is not the primary subject of
    discussion.

Rescorla Standards Track [Page 7] RFC 8446 TLS August 2018

 receiver:  An endpoint that is receiving records.
 sender:  An endpoint that is transmitting records.
 server:  The endpoint that did not initiate the TLS connection.

1.2. Major Differences from TLS 1.2

 The following is a list of the major functional differences between
 TLS 1.2 and TLS 1.3.  It is not intended to be exhaustive, and there
 are many minor differences.
  1. The list of supported symmetric encryption algorithms has been

pruned of all algorithms that are considered legacy. Those that

    remain are all Authenticated Encryption with Associated Data
    (AEAD) algorithms.  The cipher suite concept has been changed to
    separate the authentication and key exchange mechanisms from the
    record protection algorithm (including secret key length) and a
    hash to be used with both the key derivation function and
    handshake message authentication code (MAC).
  1. A zero round-trip time (0-RTT) mode was added, saving a round trip

at connection setup for some application data, at the cost of

    certain security properties.
  1. Static RSA and Diffie-Hellman cipher suites have been removed; all

public-key based key exchange mechanisms now provide forward

    secrecy.
  1. All handshake messages after the ServerHello are now encrypted.

The newly introduced EncryptedExtensions message allows various

    extensions previously sent in the clear in the ServerHello to also
    enjoy confidentiality protection.
  1. The key derivation functions have been redesigned. The new design

allows easier analysis by cryptographers due to their improved key

    separation properties.  The HMAC-based Extract-and-Expand Key
    Derivation Function (HKDF) is used as an underlying primitive.
  1. The handshake state machine has been significantly restructured to

be more consistent and to remove superfluous messages such as

    ChangeCipherSpec (except when needed for middlebox compatibility).
  1. Elliptic curve algorithms are now in the base spec, and new

signature algorithms, such as EdDSA, are included. TLS 1.3

    removed point format negotiation in favor of a single point format
    for each curve.

Rescorla Standards Track [Page 8] RFC 8446 TLS August 2018

  1. Other cryptographic improvements were made, including changing the

RSA padding to use the RSA Probabilistic Signature Scheme

    (RSASSA-PSS), and the removal of compression, the Digital
    Signature Algorithm (DSA), and custom Ephemeral Diffie-Hellman
    (DHE) groups.
  1. The TLS 1.2 version negotiation mechanism has been deprecated in

favor of a version list in an extension. This increases

    compatibility with existing servers that incorrectly implemented
    version negotiation.
  1. Session resumption with and without server-side state as well as

the PSK-based cipher suites of earlier TLS versions have been

    replaced by a single new PSK exchange.
  1. References have been updated to point to the updated versions of

RFCs, as appropriate (e.g., RFC 5280 rather than RFC 3280).

1.3. Updates Affecting TLS 1.2

 This document defines several changes that optionally affect
 implementations of TLS 1.2, including those which do not also support
 TLS 1.3:
  1. A version downgrade protection mechanism is described in

Section 4.1.3.

  1. RSASSA-PSS signature schemes are defined in Section 4.2.3.
  1. The "supported_versions" ClientHello extension can be used to

negotiate the version of TLS to use, in preference to the

    legacy_version field of the ClientHello.
  1. The "signature_algorithms_cert" extension allows a client to

indicate which signature algorithms it can validate in X.509

    certificates.
 Additionally, this document clarifies some compliance requirements
 for earlier versions of TLS; see Section 9.3.

Rescorla Standards Track [Page 9] RFC 8446 TLS August 2018

2. Protocol Overview

 The cryptographic parameters used by the secure channel are produced
 by the TLS handshake protocol.  This sub-protocol of TLS is used by
 the client and server when first communicating with each other.  The
 handshake protocol allows peers to negotiate 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 the
 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:
  1. (EC)DHE (Diffie-Hellman over either finite fields or elliptic

curves)

  1. PSK-only
  1. PSK with (EC)DHE

Rescorla Standards Track [Page 10] RFC 8446 TLS August 2018

 Figure 1 below shows the basic full TLS handshake:
     Client                                           Server

Key ^ ClientHello Exch | + key_share*

   | + signature_algorithms*
   | + 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 [sender]_application_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):
  1. Key Exchange: Establish shared keying material and select the

cryptographic parameters. Everything after this phase is

    encrypted.
  1. Server Parameters: Establish other handshake parameters

(whether the client is authenticated, application-layer protocol

    support, etc.).

Rescorla Standards Track [Page 11] RFC 8446 TLS August 2018

  1. Authentication: Authenticate the server (and, optionally, the

client) and provide key confirmation and handshake integrity.

 In the Key Exchange phase, the client sends the ClientHello
 (Section 4.1.2) message, which contains a random nonce
 (ClientHello.random); its offered protocol versions; a list of
 symmetric cipher/HKDF hash pairs; either a set of Diffie-Hellman key
 shares (in the "key_share" (Section 4.2.8) extension), a set of
 pre-shared key labels (in the "pre_shared_key" (Section 4.2.11)
 extension), or both; and potentially additional extensions.
 Additional fields and/or messages may also be present for middlebox
 compatibility.
 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; the server's share 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 ClientHello 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]

Rescorla Standards Track [Page 12] RFC 8446 TLS August 2018

 Finally, the client and server exchange Authentication messages.  TLS
 uses the same set of messages every time that certificate-based
 authentication is needed.  (PSK-based authentication happens as a
 side effect of key exchange.)  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]
 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
 derive the keying material required by the record layer to exchange
 application-layer data protected through authenticated encryption.
 Application Data MUST NOT be sent prior to sending the Finished
 message, except as specified in Section 2.3.  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.

Rescorla Standards Track [Page 13] RFC 8446 TLS August 2018

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.
      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 incorporates 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.

Rescorla Standards Track [Page 14] RFC 8446 TLS August 2018

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 used to establish a new
 connection ("session resumption" or "resuming" with a PSK).  Once a
 handshake has completed, the server can send the client a PSK
 identity that corresponds to a unique key derived from the initial
 handshake (see Section 4.6.1).  The client can then use that PSK
 identity in future handshakes to negotiate the use of the associated
 PSK.  If the server accepts the PSK, then the security context of the
 new connection is cryptographically 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.
 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 for the application
 data.

Rescorla Standards Track [Page 15] RFC 8446 TLS August 2018

 Figure 3 shows a pair of handshakes in which the first handshake
 establishes a PSK and the second handshake uses it:
        Client                                               Server
 Initial Handshake:
        ClientHello
        + key_share               -------->
                                                        ServerHello
                                                        + key_share
                                              {EncryptedExtensions}
                                              {CertificateRequest*}
                                                     {Certificate*}
                                               {CertificateVerify*}
                                                         {Finished}
                                  <--------     [Application Data*]
        {Certificate*}
        {CertificateVerify*}
        {Finished}                -------->
                                  <--------      [NewSessionTicket]
        [Application Data]        <------->      [Application Data]
 Subsequent Handshake:
        ClientHello
        + key_share*
        + pre_shared_key          -------->
                                                        ServerHello
                                                   + pre_shared_key
                                                       + key_share*
                                              {EncryptedExtensions}
                                                         {Finished}
                                  <--------     [Application Data*]
        {Finished}                -------->
        [Application Data]        <------->      [Application Data]
             Figure 3: Message Flow for Resumption and PSK
 As the server is authenticating via a PSK, it does not send a
 Certificate or a CertificateVerify message.  When a client offers
 resumption via a 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
 "pre_shared_key" extension to negotiate the 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.

Rescorla Standards Track [Page 16] RFC 8446 TLS August 2018

 When PSKs are provisioned out of band, the PSK identity and the KDF
 hash algorithm to be used with the PSK MUST also be provisioned.
 Note:  When using an out-of-band provisioned pre-shared secret, a
    critical consideration is using sufficient entropy during the key
    generation, as discussed in [RFC4086].  Deriving a shared secret
    from a password or other low-entropy sources is not secure.  A
    low-entropy secret, or password, is subject to dictionary attacks
    based on the PSK binder.  The specified PSK authentication is not
    a strong password-based authenticated key exchange even when used
    with Diffie-Hellman key establishment.  Specifically, it does not
    prevent an attacker that can observe the handshake from performing
    a brute-force attack on the password/pre-shared key.

2.3. 0-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.
 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 for a 1-RTT handshake with PSK resumption.

Rescorla Standards Track [Page 17] RFC 8446 TLS August 2018

       Client                                               Server
       ClientHello
       + early_data
       + key_share*
       + psk_key_exchange_modes
       + pre_shared_key
       (Application Data*)     -------->
                                                       ServerHello
                                                  + pre_shared_key
                                                      + key_share*
                                             {EncryptedExtensions}
                                                     + early_data*
                                                        {Finished}
                               <--------       [Application Data*]
       (EndOfEarlyData)
       {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 client_early_traffic_secret.
             {} Indicates messages protected using keys
                derived from a [sender]_handshake_traffic_secret.
             [] Indicates messages protected using keys
                derived from [sender]_application_traffic_secret_N.
             Figure 4: Message Flow for a 0-RTT Handshake

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 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.
     Protection against replay for ordinary TLS 1.3 1-RTT data is
     provided via the server's Random value, but 0-RTT data does not
     depend on the ServerHello and therefore has weaker guarantees.
     This is especially relevant if the data is authenticated either
     with TLS client authentication or inside the application
     protocol.  The same warnings apply to any use of the
     early_exporter_master_secret.
 0-RTT data cannot be duplicated within a connection (i.e., the server
 will not process the same data twice for the same connection), and an
 attacker will not be able to make 0-RTT data appear to be 1-RTT data
 (because it is protected with different keys).  Appendix E.5 contains
 a description of potential attacks, and Section 8 describes
 mechanisms which the server can use to limit the impact of replay.

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
 following 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.

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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.
 A type alias T' for an existing type T is defined by:
    T T';

3.3. Numbers

 The basic numeric data type is an unsigned byte (uint8).  All larger
 numeric data types are constructed from a 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 transmitted
 in network byte (big-endian) order; the uint32 represented by the hex
 bytes 01 02 03 04 is equivalent to the decimal value 16909060.

3.4. 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.

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 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];        /* three 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 3.3).  Similarly,
 "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 two bytes, cannot be empty */
    uint16 longer<0..800>;
          /* zero to 400 16-bit unsigned integers */

3.5. Enumerateds

 An additional sparse data type, called "enum" or "enumerated", is
 available.  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.

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 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
 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 used for definitions is much like that of C.
    struct {
        T1 f1;
        T2 f2;
        ...
        Tn fn;
    } T;
 Fixed- and variable-length vector fields are allowed using the
 standard vector syntax.  Structures V1 and V2 in the variants example
 (Section 3.8) demonstrate this.

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 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.

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.  Each
 arm of the select (below) specifies the type of that variant's field
 and an optional field label.  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 [[fe1]];
            case e2: Te2 [[fe2]];
            ....
            case en: Ten [[fen]];
        };
    } Tv;

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 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;

4. Handshake Protocol

 The handshake protocol is used to negotiate the security parameters
 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.

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    enum {
        client_hello(1),
        server_hello(2),
        new_session_ticket(4),
        end_of_early_data(5),
        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;             /* remaining bytes in message */
        select (Handshake.msg_type) {
            case client_hello:          ClientHello;
            case server_hello:          ServerHello;
            case end_of_early_data:     EndOfEarlyData;
            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;
        };
    } Handshake;
 Protocol messages MUST be sent in the order defined in Section 4.4.1
 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 "unexpected_message" alert.
 New handshake message types are assigned by IANA as described in
 Section 11.

4.1. Key Exchange Messages

 The key exchange messages are used to determine the security
 capabilities of the client and the server and to establish shared
 secrets, including the traffic keys used to protect the rest of the
 handshake and the data.

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4.1.1. Cryptographic Negotiation

 In TLS, the cryptographic negotiation proceeds by the client offering
 the following four sets of options in its ClientHello:
  1. A list of cipher suites which indicates the AEAD algorithm/HKDF

hash pairs which the client supports.

  1. A "supported_groups" (Section 4.2.7) extension which indicates the

(EC)DHE groups which the client supports and a "key_share"

    (Section 4.2.8) extension which contains (EC)DHE shares for some
    or all of these groups.
  1. A "signature_algorithms" (Section 4.2.3) extension which indicates

the signature algorithms which the client can accept. A

    "signature_algorithms_cert" extension (Section 4.2.3) may also be
    added to indicate certificate-specific signature algorithms.
  1. A "pre_shared_key" (Section 4.2.11) extension which contains a

list of symmetric key identities known to the client and a

    "psk_key_exchange_modes" (Section 4.2.9) 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 between the received
 "supported_groups" and the groups supported by the server, then the
 server MUST abort the handshake with a "handshake_failure" or an
 "insufficient_security" alert.
 If the server selects a PSK, then it MUST also select a key
 establishment mode from the set indicated by the client's
 "psk_key_exchange_modes" extension (at present, 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.
 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.

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 If the server successfully selects parameters and does not require a
 HelloRetryRequest, it indicates the selected parameters in the
 ServerHello as follows:
  1. If PSK is being used, then the server will send a "pre_shared_key"

extension indicating the selected key.

  1. When (EC)DHE is in use, the server will also provide a "key_share"

extension. If PSK is not being used, then (EC)DHE and

    certificate-based authentication are always used.
  1. 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 TLS 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 as follows:
  1. 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.
  1. Removing the "early_data" extension (Section 4.2.10) if one was

present. Early data is not permitted after a HelloRetryRequest.

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

HelloRetryRequest.

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  1. Updating the "pre_shared_key" extension if present by recomputing

the "obfuscated_ticket_age" and binder values and (optionally)

    removing any PSKs which are incompatible with the server's
    indicated cipher suite.
  1. Optionally adding, removing, or changing the length of the

"padding" extension [RFC7685].

  1. Other modifications that may be allowed by an extension defined in

the future and present in the HelloRetryRequest.

 Because TLS 1.3 forbids renegotiation, if a server has negotiated
 TLS 1.3 and receives a ClientHello at any other time, it MUST
 terminate the connection with an "unexpected_message" alert.
 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;

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 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
    legacy_version field MUST be set to 0x0303, which is the version
    number for TLS 1.2.  TLS 1.3 ClientHellos are identified as having
    a legacy_version of 0x0303 and a supported_versions extension
    present with 0x0304 as the highest version indicated therein.
    (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).  A client which has a
    cached session ID set by a pre-TLS 1.3 server SHOULD set this
    field to that value.  In compatibility mode (see Appendix D.4),
    this field MUST be non-empty, so a client not offering a
    pre-TLS 1.3 session MUST generate a new 32-byte value.  This value
    need not be random but SHOULD be unpredictable to avoid
    implementations fixating on a specific value (also known as
    ossification).  Otherwise, it MUST be set as a zero-length vector
    (i.e., a zero-valued single byte length field).
 cipher_suites:  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.  Values are defined in
    Appendix B.4.  If the list contains cipher suites that the server
    does not recognize, support, or wish to use, the server MUST
    ignore those cipher suites and process the remaining ones as
    usual.  If the client is attempting a PSK key establishment, it
    SHOULD advertise at least one cipher suite indicating a Hash
    associated with the PSK.

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 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 (if negotiating such a prior version) 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, the use of certain
    extensions is mandatory, as functionality has moved into
    extensions to preserve ClientHello compatibility with previous
    versions of TLS.  Servers MUST ignore unrecognized extensions.
 All versions of TLS allow an extensions field to optionally follow
 the compression_methods field.  TLS 1.3 ClientHello messages always
 contain extensions (minimally "supported_versions", otherwise, they
 will be interpreted as TLS 1.2 ClientHello messages).  However,
 TLS 1.3 servers might receive ClientHello messages without an
 extensions field from prior versions of TLS.  The presence of
 extensions can be detected by determining whether there are bytes
 following the compression_methods field 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 the "supported_versions"
 extension is present.  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.
 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.
 After sending the ClientHello message, the client waits for a
 ServerHello or HelloRetryRequest message.  If early data is in use,
 the client may transmit early Application Data (Section 2.3) while
 waiting for the next handshake message.

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4.1.3. Server Hello

 The server will send this message in response to a ClientHello
 message to proceed with the handshake if it is able to negotiate an
 acceptable set of handshake parameters based on the ClientHello.
 Structure of this message:
    struct {
        ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
        Random random;
        opaque legacy_session_id_echo<0..32>;
        CipherSuite cipher_suite;
        uint8 legacy_compression_method = 0;
        Extension extensions<6..2^16-1>;
    } ServerHello;
 legacy_version:  In previous versions of TLS, this field was used for
    version negotiation and represented the selected version number
    for the connection.  Unfortunately, some middleboxes fail when
    presented with new values.  In TLS 1.3, the TLS server indicates
    its version using the "supported_versions" extension
    (Section 4.2.1), and the legacy_version field MUST be set to
    0x0303, which is 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.  The last 8 bytes MUST be
    overwritten as described below if negotiating TLS 1.2 or TLS 1.1,
    but the remaining bytes MUST be random.  This structure is
    generated by the server and MUST be generated independently of the
    ClientHello.random.
 legacy_session_id_echo:  The contents of the client's
    legacy_session_id field.  Note that this field is echoed even if
    the client's value corresponded to a cached pre-TLS 1.3 session
    which the server has chosen not to resume.  A client which
    receives a legacy_session_id_echo field that does not match what
    it sent in the ClientHello MUST abort the handshake with an
    "illegal_parameter" alert.
 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 with an
    "illegal_parameter" alert.
 legacy_compression_method:  A single byte which MUST have the
    value 0.

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 extensions:  A list of extensions.  The ServerHello MUST only include
    extensions which are required to establish the cryptographic
    context and negotiate the protocol version.  All TLS 1.3
    ServerHello messages MUST contain the "supported_versions"
    extension.  Current ServerHello messages additionally contain
    either the "pre_shared_key" extension or the "key_share"
    extension, or both (when using a PSK with (EC)DHE key
    establishment).  Other extensions (see Section 4.2) are sent
    separately in the EncryptedExtensions message.
 For reasons of backward compatibility with middleboxes (see
 Appendix D.4), the HelloRetryRequest message uses the same structure
 as the ServerHello, but with Random set to the special value of the
 SHA-256 of "HelloRetryRequest":
   CF 21 AD 74 E5 9A 61 11 BE 1D 8C 02 1E 65 B8 91
   C2 A2 11 16 7A BB 8C 5E 07 9E 09 E2 C8 A8 33 9C
 Upon receiving a message with type server_hello, implementations MUST
 first examine the Random value and, if it matches this value, process
 it as described in Section 4.1.4).
 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 8 bytes of their Random
 value specially in their ServerHello.
 If negotiating TLS 1.2, TLS 1.3 servers MUST set the last 8 bytes of
 their Random value to the bytes:
   44 4F 57 4E 47 52 44 01
 If negotiating TLS 1.1 or below, TLS 1.3 servers MUST, and TLS 1.2
 servers SHOULD, set the last 8 bytes of their ServerHello.Random
 value to the bytes:
   44 4F 57 4E 47 52 44 00
 TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below
 MUST check that the last 8 bytes are not equal to either of these
 values.  TLS 1.2 clients SHOULD also check that the last 8 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 what is 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

Rescorla Standards Track [Page 32] RFC 8446 TLS August 2018

 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 legacy TLS client performing renegotiation with TLS 1.2 or prior
 and which receives a TLS 1.3 ServerHello during renegotiation MUST
 abort the handshake with a "protocol_version" alert.  Note that
 renegotiation is not possible when TLS 1.3 has been negotiated.

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.  As discussed in Section 4.1.3, the HelloRetryRequest
 has the same format as a ServerHello message, and the legacy_version,
 legacy_session_id_echo, cipher_suite, and legacy_compression_method
 fields have the same meaning.  However, for convenience we discuss
 "HelloRetryRequest" throughout this document as if it were a distinct
 message.
 The server's extensions MUST contain "supported_versions".
 Additionally, it SHOULD contain the minimal set of extensions
 necessary for the client to generate a correct ClientHello pair.  As
 with the 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 check the
 legacy_version, legacy_session_id_echo, cipher_suite, and
 legacy_compression_method as specified in Section 4.1.3 and then
 process the extensions, starting with determining the version using
 "supported_versions".  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.

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 Otherwise, the client MUST process all extensions in the
 HelloRetryRequest and send a second updated ClientHello.  The
 HelloRetryRequest extensions defined in this specification are:
  1. supported_versions (see Section 4.2.1)
  1. cookie (see Section 4.2.2)
  1. key_share (see Section 4.2.8)
 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 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.
 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.
 The value of selected_version in the HelloRetryRequest
 "supported_versions" extension MUST be retained in the ServerHello,
 and a client MUST abort the handshake with an "illegal_parameter"
 alert if the value changes.

Rescorla Standards Track [Page 34] RFC 8446 TLS August 2018

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 {
      server_name(0),                             /* RFC 6066 */
      max_fragment_length(1),                     /* RFC 6066 */
      status_request(5),                          /* RFC 6066 */
      supported_groups(10),                       /* RFC 8422, 7919 */
      signature_algorithms(13),                   /* RFC 8446 */
      use_srtp(14),                               /* RFC 5764 */
      heartbeat(15),                              /* RFC 6520 */
      application_layer_protocol_negotiation(16), /* RFC 7301 */
      signed_certificate_timestamp(18),           /* RFC 6962 */
      client_certificate_type(19),                /* RFC 7250 */
      server_certificate_type(20),                /* RFC 7250 */
      padding(21),                                /* RFC 7685 */
      pre_shared_key(41),                         /* RFC 8446 */
      early_data(42),                             /* RFC 8446 */
      supported_versions(43),                     /* RFC 8446 */
      cookie(44),                                 /* RFC 8446 */
      psk_key_exchange_modes(45),                 /* RFC 8446 */
      certificate_authorities(47),                /* RFC 8446 */
      oid_filters(48),                            /* RFC 8446 */
      post_handshake_auth(49),                    /* RFC 8446 */
      signature_algorithms_cert(50),              /* RFC 8446 */
      key_share(51),                              /* RFC 8446 */
      (65535)
  } ExtensionType;

Rescorla Standards Track [Page 35] RFC 8446 TLS August 2018

 Here:
  1. "extension_type" identifies the particular extension type.
  1. "extension_data" contains information specific to the particular

extension type.

 The list of extension types is maintained by IANA as described in
 Section 11.
 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, HelloRetryRequest, and Certificate
 messages.  The server sends extension requests in the
 CertificateRequest message which a client MAY respond to with a
 Certificate message.  The server 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 the 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.

Rescorla Standards Track [Page 36] RFC 8446 TLS August 2018

 +--------------------------------------------------+-------------+
 | Extension                                        |     TLS 1.3 |
 +--------------------------------------------------+-------------+
 | server_name [RFC6066]                            |      CH, EE |
 |                                                  |             |
 | max_fragment_length [RFC6066]                    |      CH, EE |
 |                                                  |             |
 | status_request [RFC6066]                         |  CH, CR, CT |
 |                                                  |             |
 | supported_groups [RFC7919]                       |      CH, EE |
 |                                                  |             |
 | signature_algorithms (RFC 8446)                  |      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, EE |
 |                                                  |             |
 | padding [RFC7685]                                |          CH |
 |                                                  |             |
 | key_share (RFC 8446)                             | CH, SH, HRR |
 |                                                  |             |
 | pre_shared_key (RFC 8446)                        |      CH, SH |
 |                                                  |             |
 | psk_key_exchange_modes (RFC 8446)                |          CH |
 |                                                  |             |
 | early_data (RFC 8446)                            | CH, EE, NST |
 |                                                  |             |
 | cookie (RFC 8446)                                |     CH, HRR |
 |                                                  |             |
 | supported_versions (RFC 8446)                    | CH, SH, HRR |
 |                                                  |             |
 | certificate_authorities (RFC 8446)               |      CH, CR |
 |                                                  |             |
 | oid_filters (RFC 8446)                           |          CR |
 |                                                  |             |
 | post_handshake_auth (RFC 8446)                   |          CH |
 |                                                  |             |
 | signature_algorithms_cert (RFC 8446)             |      CH, CR |
 +--------------------------------------------------+-------------+

Rescorla Standards Track [Page 37] RFC 8446 TLS August 2018

 When multiple extensions of different types are present, the
 extensions MAY appear in any order, with the exception of
 "pre_shared_key" (Section 4.2.11) which MUST be the last extension in
 the ClientHello (but can appear anywhere in the ServerHello
 extensions block).  There MUST NOT be more than one extension of the
 same type in a given extension block.
 In TLS 1.3, unlike TLS 1.2, extensions are negotiated for 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 4.2.10).
 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:
  1. Some cases where a server does not agree to an extension are error

conditions (e.g., the handshake cannot continue), 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.
  1. 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
    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.

Rescorla Standards Track [Page 38] RFC 8446 TLS August 2018

4.2.1. Supported Versions

    struct {
        select (Handshake.msg_type) {
            case client_hello:
                 ProtocolVersion versions<2..254>;
            case server_hello: /* and HelloRetryRequest */
                 ProtocolVersion selected_version;
        };
    } SupportedVersions;
 The "supported_versions" extension is used by the client to indicate
 which versions of TLS it supports and by the server to indicate which
 version it is using.  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 in the
 ClientHello 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 allowed to be negotiated, they MUST
 be present as well).
 If this extension is not present, servers which are compliant with
 this specification and which also support TLS 1.2 MUST negotiate
 TLS 1.2 or prior as specified in [RFC5246], even if
 ClientHello.legacy_version is 0x0304 or later.  Servers MAY abort the
 handshake upon receiving a ClientHello with legacy_version 0x0304 or
 later.
 If this extension is present in the ClientHello, servers MUST NOT use
 the ClientHello.legacy_version value for version negotiation 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 that are present
 in that extension.  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.  Servers MUST be prepared to
 receive ClientHellos that include this extension but do not include
 0x0304 in the list of versions.
 A server which negotiates a version of TLS prior to TLS 1.3 MUST set
 ServerHello.version and MUST NOT send the "supported_versions"
 extension.  A server which negotiates TLS 1.3 MUST respond by sending
 a "supported_versions" extension containing the selected version
 value (0x0304).  It MUST set the ServerHello.legacy_version field to
 0x0303 (TLS 1.2).  Clients MUST check for this extension prior to
 processing the rest of the ServerHello (although they will have to

Rescorla Standards Track [Page 39] RFC 8446 TLS August 2018

 parse the ServerHello in order to read the extension).  If this
 extension is present, clients MUST ignore the
 ServerHello.legacy_version value and MUST use only the
 "supported_versions" extension to determine the selected version.  If
 the "supported_versions" extension in the ServerHello contains a
 version not offered by the client or contains a version prior to
 TLS 1.3, the client MUST abort the handshake with an
 "illegal_parameter" alert.

4.2.2. Cookie

    struct {
        opaque cookie<1..2^16-1>;
    } Cookie;
 Cookies serve two primary purposes:
  1. 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).
  1. Allowing the server to offload state to the client, thus allowing

it to send a HelloRetryRequest without storing any state. The

    server can do this by storing the hash of the ClientHello in the
    HelloRetryRequest cookie (protected with some suitable integrity
    protection 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 copy
 the contents of the extension received in the HelloRetryRequest into
 a "cookie" extension in the new ClientHello.  Clients MUST NOT use
 cookies in their initial ClientHello in subsequent connections.
 When a server is operating statelessly, it may receive an unprotected
 record of type change_cipher_spec between the first and second
 ClientHello (see Section 5).  Since the server is not storing any
 state, this will appear as if it were the first message to be
 received.  Servers operating statelessly MUST ignore these records.

Rescorla Standards Track [Page 40] RFC 8446 TLS August 2018

4.2.3. Signature Algorithms

 TLS 1.3 provides two extensions for indicating which signature
 algorithms may be used in digital signatures.  The
 "signature_algorithms_cert" extension applies to signatures in
 certificates, and the "signature_algorithms" extension, which
 originally appeared in TLS 1.2, applies to signatures in
 CertificateVerify messages.  The keys found in certificates MUST also
 be of appropriate type for the signature algorithms they are used
 with.  This is a particular issue for RSA keys and PSS signatures, as
 described below.  If no "signature_algorithms_cert" extension is
 present, then the "signature_algorithms" extension also applies to
 signatures appearing in certificates.  Clients which desire the
 server to authenticate itself via a certificate MUST send the
 "signature_algorithms" 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 9.2).
 The "signature_algorithms_cert" extension was added to allow
 implementations which supported different sets of algorithms for
 certificates and in TLS itself to clearly signal their capabilities.
 TLS 1.2 implementations SHOULD also process this extension.
 Implementations which have the same policy in both cases MAY omit the
 "signature_algorithms_cert" extension.

Rescorla Standards Track [Page 41] RFC 8446 TLS August 2018

 The "extension_data" field of these extensions contains a
 SignatureSchemeList value:
    enum {
        /* RSASSA-PKCS1-v1_5 algorithms */
        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 with public key OID rsaEncryption */
        rsa_pss_rsae_sha256(0x0804),
        rsa_pss_rsae_sha384(0x0805),
        rsa_pss_rsae_sha512(0x0806),
        /* EdDSA algorithms */
        ed25519(0x0807),
        ed448(0x0808),
        /* RSASSA-PSS algorithms with public key OID RSASSA-PSS */
        rsa_pss_pss_sha256(0x0809),
        rsa_pss_pss_sha384(0x080a),
        rsa_pss_pss_sha512(0x080b),
        /* Legacy algorithms */
        rsa_pkcs1_sha1(0x0201),
        ecdsa_sha1(0x0203),
        /* 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.

Rescorla Standards Track [Page 42] RFC 8446 TLS August 2018

 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 [RFC8017] 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, although they
    MAY appear in "signature_algorithms" and
    "signature_algorithms_cert" for backward compatibility with
    TLS 1.2.
 ECDSA algorithms:  Indicates a signature algorithm using ECDSA
    [ECDSA], the corresponding curve as defined in ANSI X9.62 [ECDSA]
    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 RSAE algorithms:  Indicates a signature algorithm using
    RSASSA-PSS [RFC8017] 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].
    The length of the Salt MUST be equal to the length of the output
    of the digest algorithm.  If the public key is carried in an X.509
    certificate, it MUST use the rsaEncryption OID [RFC5280].
 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.
 RSASSA-PSS PSS algorithms:  Indicates a signature algorithm using
    RSASSA-PSS [RFC8017] 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].
    The length of the Salt MUST be equal to the length of the digest
    algorithm.  If the public key is carried in an X.509 certificate,
    it MUST use the RSASSA-PSS OID [RFC5756].  When used in
    certificate signatures, the algorithm parameters MUST be DER
    encoded.  If the corresponding public key's parameters are
    present, then the parameters in the signature MUST be identical to
    those in the public key.

Rescorla Standards Track [Page 43] RFC 8446 TLS August 2018

 Legacy algorithms:  Indicates algorithms which are being deprecated
    because they use algorithms with known weaknesses, specifically
    SHA-1 which is used in this context with either (1) RSA using
    RSASSA-PKCS1-v1_5 or (2) ECDSA.  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, although
    they MAY appear in "signature_algorithms" and
    "signature_algorithms_cert" for backward compatibility with
    TLS 1.2.  Endpoints SHOULD NOT negotiate these algorithms but are
    permitted to do so solely for backward compatibility.  Clients
    offering these values 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:
  1. TLS 1.2 ClientHellos MAY omit this extension.
  1. 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], SHA-224, and DSA
    MUST NOT be used.
  1. 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.
  1. 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.

Rescorla Standards Track [Page 44] RFC 8446 TLS August 2018

4.2.4. Certificate Authorities

 The "certificate_authorities" extension is used to indicate the
 certificate authorities (CAs) 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
    a 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 [RFC6066], which serves a similar
 purpose 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).

4.2.5. 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, if provided by the server, 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;

Rescorla Standards Track [Page 45] RFC 8446 TLS August 2018

 filters:  A list of certificate extension OIDs [RFC5280] with their
    allowed value(s) and 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
    filters 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 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.  Any given OID MUST NOT
    appear more than once in the filters list.
 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]:
  1. 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.
  1. 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.

Rescorla Standards Track [Page 46] RFC 8446 TLS August 2018

4.2.6. Post-Handshake Client Authentication

 The "post_handshake_auth" extension is used to indicate that a client
 is willing to perform post-handshake authentication (Section 4.6.2).
 Servers MUST NOT send a post-handshake CertificateRequest to clients
 which do not offer this extension.  Servers MUST NOT send this
 extension.
    struct {} PostHandshakeAuth;
 The "extension_data" field of the "post_handshake_auth" extension is
 zero length.

4.2.7. Supported 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.
 Note: In versions of TLS prior to TLS 1.3, this extension was named
 "elliptic_curves" and only contained elliptic curve groups.  See
 [RFC8422] 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;

Rescorla Standards Track [Page 47] RFC 8446 TLS August 2018

 Elliptic Curve Groups (ECDHE):  Indicates support for the
    corresponding named curve, defined in either FIPS 186-4 [DSS] or
    [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for
    Private Use [RFC8126].
 Finite Field Groups (DHE):  Indicates support for 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 sender's
 preferences (most preferred choice first).
 As of TLS 1.3, servers are permitted to send the "supported_groups"
 extension to the client.  Clients MUST NOT act upon any information
 found in "supported_groups" prior to successful completion of the
 handshake but MAY use the information learned from a successfully
 completed handshake to change what groups they use in their
 "key_share" extension in subsequent connections.  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.

4.2.8. 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.
 key_exchange:  Key exchange information.  The contents of this field
    are determined by the specified group and its corresponding
    definition.  Finite Field Diffie-Hellman [DH76] parameters are
    described in Section 4.2.8.1; Elliptic Curve Diffie-Hellman
    parameters are described in Section 4.2.8.2.

Rescorla Standards Track [Page 48] RFC 8446 TLS August 2018

 In the ClientHello message, the "extension_data" field of this
 extension contains a "KeyShareClientHello" value:
    struct {
        KeyShareEntry client_shares<0..2^16-1>;
    } KeyShareClientHello;
 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 MAY be a non-contiguous subset
 of the "supported_groups" extension and MAY omit the most preferred
 groups.  Such a situation could arise if the most preferred groups
 are new and unlikely to be supported in enough places to make
 pregenerating key shares for them efficient.
 Clients can offer as many KeyShareEntry values as the number of
 supported groups it is offering, 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.
 In a HelloRetryRequest message, the "extension_data" field of this
 extension contains a KeyShareHelloRetryRequest value:
    struct {
        NamedGroup selected_group;
    } KeyShareHelloRetryRequest;
 selected_group:  The mutually supported group the server intends to
    negotiate and is requesting a retried ClientHello/KeyShare for.
 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

Rescorla Standards Track [Page 49] RFC 8446 TLS August 2018

 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.
 In a ServerHello message, the "extension_data" field of this
 extension contains a KeyShareServerHello value:
    struct {
        KeyShareEntry server_share;
    } KeyShareServerHello;
 server_share:  A single KeyShareEntry value that is in the same group
    as one of the client's shares.
 If using (EC)DHE key establishment, servers offer exactly one
 KeyShareEntry in the ServerHello.  This value MUST be in the same
 group as 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
 client's "supported_groups" extension and MUST NOT send a
 KeyShareEntry when using the "psk_ke" PskKeyExchangeMode.  If using
 (EC)DHE key establishment and a HelloRetryRequest containing a
 "key_share" extension 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.

4.2.8.1. Diffie-Hellman Parameters

 Diffie-Hellman [DH76] 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 to
 the left 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.

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4.2.8.2. ECDHE Parameters

 ECDHE parameters for both clients and servers are encoded in the
 opaque key_exchange field of a KeyShareEntry in a KeyShare structure.
 For secp256r1, secp384r1, and secp521r1, the contents are the
 serialized value of the following struct:
    struct {
        uint8 legacy_form = 4;
        opaque X[coordinate_length];
        opaque Y[coordinate_length];
    } UncompressedPointRepresentation;
 X and Y, respectively, are the binary representations of the x and y
 values in network byte order.  There are no internal length markers,
 so each number representation occupies as many octets as implied by
 the curve parameters.  For P-256, this means that each of X and Y use
 32 octets, padded on the left by zeros if necessary.  For P-384, they
 take 48 octets each.  For P-521, they take 66 octets each.
 For the curves secp256r1, secp384r1, and secp521r1, peers MUST
 validate each other's public value Q by ensuring that the point is a
 valid point on the elliptic curve.  The appropriate validation
 procedures are defined in Section 4.3.7 of [ECDSA] and alternatively
 in Section 5.6.2.3 of [KEYAGREEMENT].  This process consists of three
 steps: (1) verify that Q is not the point at infinity (O), (2) verify
 that for Q = (x, y) both integers x and y are in the correct
 interval, and (3) ensure that (x, y) is a correct solution to the
 elliptic curve equation.  For these curves, implementors 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.

4.2.9. 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.

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 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 server MUST supply "key_share" values as described in
    Section 4.2.8.
 Any future values that are allocated must ensure that the transmitted
 protocol messages unambiguously identify which mode was selected by
 the server; at present, this is indicated by the presence of the
 "key_share" in the ServerHello.

4.2.10. Early Data Indication

 When a PSK is used and early data is allowed for that PSK, the client
 can send Application Data in its first flight of messages.  If the
 client opts to do so, it MUST supply both the "pre_shared_key" and
 "early_data" extensions.
 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 Section 4.6.1 for details regarding the use of the
 max_early_data_size field.
 The parameters for the 0-RTT data (version, symmetric cipher suite,
 Application-Layer Protocol Negotiation (ALPN) [RFC7301] protocol,
 etc.) are those associated with the PSK in use.  For externally
 provisioned PSKs, the associated values are those provisioned along
 with the key.  For PSKs established via a NewSessionTicket message,
 the associated values are 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.
 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 8).  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.
 0-RTT messages sent in the first flight have the same (encrypted)
 content types as messages of the same type sent in other flights
 (handshake and application_data) 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.

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 A server which receives an "early_data" extension MUST behave in one
 of three ways:
  1. Ignore the extension and return a regular 1-RTT response. The

server then skips past early data by attempting to deprotect

    received records using the handshake traffic key, discarding
    records which fail deprotection (up to the configured
    max_early_data_size).  Once a record is deprotected successfully,
    it is treated as the start of the client's second flight and the
    server proceeds as with an ordinary 1-RTT handshake.
  1. 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 an external
    content type of "application_data" (indicating that they are
    encrypted), up to the configured max_early_data_size.
  1. Return its own "early_data" 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.  Even though the server sends a message accepting early
    data, the actual early data itself may already be in flight by the
    time the server generates this message.
 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 the same as those associated with the
 selected PSK:
  1. The TLS version number
  1. The selected cipher suite
  1. The selected ALPN [RFC7301] protocol, if any
 These requirements are a superset of those needed to perform a 1-RTT
 handshake using the PSK in question.  For externally established
 PSKs, the associated values are those provisioned along with the key.
 For PSKs established via a NewSessionTicket message, the associated
 values are those negotiated in the connection during which the ticket
 was established.
 Future extensions MUST define their interaction with 0-RTT.

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 If any of these checks fail, the server MUST NOT respond with the
 extension and must discard all the first-flight data using one of the
 first two mechanisms listed above (thus falling back to 1-RTT or
 2-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 use trial decryption (either with
 the 1-RTT handshake keys or by looking for a cleartext ClientHello in
 the case of a HelloRetryRequest) to find the first non-0-RTT message.
 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 a 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 the Application Data previously
 sent in early data once the handshake has been completed.  Note that
 automatic retransmission of early data could result in incorrect
 assumptions regarding the status of the connection.  For instance,
 when the negotiated connection selects a different ALPN protocol from
 what was used for the early data, an application might need to
 construct different messages.  Similarly, if early data assumes
 anything about the connection state, it might be sent in error after
 the handshake completes.
 A TLS implementation SHOULD NOT automatically resend early data;
 applications are in a better position to decide when retransmission
 is appropriate.  A TLS implementation MUST NOT automatically resend
 early data unless the negotiated connection selects the same ALPN
 protocol.

4.2.11. Pre-Shared Key Extension

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

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 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 {
        PskIdentity identities<7..2^16-1>;
        PskBinderEntry binders<33..2^16-1>;
    } OfferedPsks;
    struct {
        select (Handshake.msg_type) {
            case client_hello: OfferedPsks;
            case server_hello: uint16 selected_identity;
        };
    } PreSharedKeyExtension;
 identity:  A label for a key.  For instance, a ticket (as defined in
    Appendix B.3.4) or a label for a pre-shared key established
    externally.
 obfuscated_ticket_age:  An obfuscated version of the age of the key.
    Section 4.2.11.1 describes how to form this value for identities
    established via the NewSessionTicket message.  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 4.2.10), the first identity is the one used
    for 0-RTT data.
 binders:  A series of HMAC values, one for each value in the
    identities list 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 KDF
 Hash algorithm on the connection where the ticket was established.
 For externally established PSKs, the Hash algorithm MUST be set when

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 the PSK is established or default to SHA-256 if no such algorithm is
 defined.  The server MUST ensure that it selects a compatible PSK
 (if any) and cipher suite.
 In TLS versions prior to TLS 1.3, the Server Name Identification
 (SNI) value was intended to be associated with the session (Section 3
 of [RFC6066]), with the server being required to enforce that the SNI
 value associated with the session matches the one specified in the
 resumption handshake.  However, in reality the implementations were
 not consistent on which of two supplied SNI values they would use,
 leading to the consistency requirement being de facto enforced by the
 clients.  In TLS 1.3, the SNI value is always explicitly specified in
 the resumption handshake, and there is no need for the server to
 associate an SNI value with the ticket.  Clients, however, SHOULD
 store the SNI with the PSK to fulfill the requirements of
 Section 4.6.1.
 Implementor's note: When session resumption is the primary use case
 of PSKs, 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., ones
 not in the PSK database or 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.  If backward
 compatibility is important, client-provided, externally established
 PSKs SHOULD influence cipher suite selection.
 Prior to accepting PSK key establishment, the server MUST validate
 the corresponding binder value (see Section 4.2.11.2 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.  See Section 8.2 and
 Appendix E.6 for the security rationale for this requirement.  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
 indicating a Hash associated with the PSK, and that a server
 "key_share" extension is present if required by the ClientHello
 "psk_key_exchange_modes" extension.  If these values are not
 consistent, the client MUST abort the handshake with an
 "illegal_parameter" alert.

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 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.
 The "pre_shared_key" 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.

4.2.11.1. Ticket Age

 The client's view of the age of a ticket is the time since the
 receipt of the NewSessionTicket message.  Clients MUST NOT attempt to
 use tickets which have ages greater than the "ticket_lifetime" value
 which was provided with the ticket.  The "obfuscated_ticket_age"
 field of each PskIdentity contains an obfuscated version of the
 ticket age formed by taking the age in milliseconds and adding the
 "ticket_age_add" value that was included with the ticket (see
 Section 4.6.1), modulo 2^32.  This addition prevents passive
 observers from correlating connections unless tickets are reused.
 Note that the "ticket_lifetime" field in the NewSessionTicket message
 is in seconds but the "obfuscated_ticket_age" is in milliseconds.
 Because ticket lifetimes are restricted to a week, 32 bits is enough
 to represent any plausible age, even in milliseconds.

4.2.11.2. PSK Binder

 The PSK binder value forms a binding between a PSK and the current
 handshake, as well as a binding between the handshake in which the
 PSK was generated (if via a NewSessionTicket message) and the current
 handshake.  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 PskBinderEntry 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).

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 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(Truncate(ClientHello1))
 Where Truncate() removes the binders list from the ClientHello.
 If the server responds with a HelloRetryRequest and the client then
 sends ClientHello2, its binder will be computed over:
    Transcript-Hash(ClientHello1,
                    HelloRetryRequest,
                    Truncate(ClientHello2))
 The full ClientHello1/ClientHello2 is included in all other handshake
 hash computations.  Note that in the first flight,
 Truncate(ClientHello1) is hashed directly, but in the second flight,
 ClientHello1 is hashed and then reinjected as a "message_hash"
 message, as described in Section 4.4.1.

4.2.11.3. Processing Order

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

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.

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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 that can 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;

Rescorla Standards Track [Page 60] RFC 8446 TLS August 2018

 certificate_request_context:  An opaque string which identifies the
    certificate request and which will be echoed in the client's
    Certificate message.  The certificate_request_context MUST be
    unique within the scope of this connection (thus 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.  When requesting post-handshake
    authentication, the server SHOULD make the context unpredictable
    to the client (e.g., by randomly generating it) in order to
    prevent an attacker who has temporary access to the client's
    private key from pre-computing valid CertificateVerify messages.
 extensions:  A set of extensions describing the parameters of the
    certificate being requested.  The "signature_algorithms" extension
    MUST be specified, and other extensions may optionally be included
    if defined for this message.  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" and optionally "signature_algorithms_cert"
 extensions.  The latter is expressed by sending the
 "certificate_authorities" extension (see Section 4.2.4).
 Servers which are authenticating with a PSK MUST NOT send the
 CertificateRequest message in the main handshake, though they MAY
 send it in post-handshake authentication (see Section 4.6.2) provided
 that the client has sent the "post_handshake_auth" extension (see
 Section 4.2.6).

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 PSK
 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 the
 [sender]_handshake_traffic_secret.

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 The computations for the Authentication messages all uniformly take
 the following inputs:
  1. The certificate and signing key to be used.
  1. A Handshake Context consisting of the set of messages to be

included in the transcript hash.

  1. A Base Key to be used to compute a MAC key.
 Based on these inputs, the messages then contain:
 Certificate:  The certificate to be used for authentication, and any
    supporting certificates in the chain.  Note that certificate-based
    client authentication is not available in PSK handshake flows
    (including 0-RTT).
 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   | server_handshake_traffic_   |
 |           | of EncryptedExtensions/ | secret                      |
 |           | CertificateRequest      |                             |
 |           |                         |                             |
 | Client    | ClientHello ... later   | client_handshake_traffic_   |
 |           | of server               | secret                      |
 |           | Finished/EndOfEarlyData |                             |
 |           |                         |                             |
 | Post-     | ClientHello ... client  | client_application_traffic_ |
 | Handshake | Finished +              | secret_N                    |
 |           | CertificateRequest      |                             |
 +-----------+-------------------------+-----------------------------+

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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.,
Transcript-Hash(ClientHello1, HelloRetryRequest, ... Mn) =
    Hash(message_hash ||        /* Handshake type */
         00 00 Hash.length  ||  /* Handshake message length (bytes) */
         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).
 For concreteness, the transcript hash 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.
 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.

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4.4.2. Certificate

 This message conveys the endpoint's certificate chain to the peer.
 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).
 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).  A Finished message MUST be sent regardless
 of whether the Certificate message is empty.
 Structure of this message:
    enum {
        X509(0),
        RawPublicKey(2),
        (255)
    } CertificateType;
    struct {
        select (certificate_type) {
            case RawPublicKey:
              /* From RFC 7250 ASN.1_subjectPublicKeyInfo */
              opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
            case X509:
              opaque cert_data<1..2^24-1>;
        };
        Extension extensions<0..2^16-1>;
    } CertificateEntry;
    struct {
        opaque certificate_request_context<0..2^8-1>;
        CertificateEntry certificate_list<0..2^24-1>;
    } Certificate;

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 certificate_request_context:  If this message is in response to a
    CertificateRequest, the value of certificate_request_context in
    that message.  Otherwise (in the case of server authentication),
    this field SHALL be zero length.
 certificate_list:  A sequence (chain) of CertificateEntry structures,
    each containing a single certificate and set of extensions.
 extensions:  A set of extension values for the CertificateEntry.  The
    "Extension" format is defined in Section 4.2.  Valid extensions
    for server certificates at present include the OCSP Status
    extension [RFC6066] and the SignedCertificateTimestamp extension
    [RFC6962]; future extensions may be defined for this message as
    well.  Extensions in the Certificate message from the server MUST
    correspond to ones from the ClientHello message.  Extensions in
    the Certificate message from the client MUST correspond to
    extensions in the CertificateRequest message from the server.  If
    an extension applies to the entire chain, it SHOULD be included in
    the first CertificateEntry.
 If the corresponding certificate type extension
 ("server_certificate_type" or "client_certificate_type") was not
 negotiated in EncryptedExtensions, or the X.509 certificate type was
 negotiated, then each CertificateEntry contains a DER-encoded X.509
 certificate.  The sender's certificate MUST come in the first
 CertificateEntry in the list.  Each following certificate SHOULD
 directly certify the one immediately 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.
 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.
 If the RawPublicKey certificate type was negotiated, then the
 certificate_list MUST contain no more than one CertificateEntry,
 which contains an ASN1_subjectPublicKeyInfo value as defined in
 [RFC7250], Section 3.

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 The OpenPGP certificate type [RFC6091] MUST NOT be used with TLS 1.3.
 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
 "status_request" extension from the server MUST be a
 CertificateStatus structure as defined in [RFC6066], which is
 interpreted as defined in [RFC6960].
 Note: The status_request_v2 extension [RFC6961] is deprecated.
 TLS 1.3 servers MUST NOT act upon its presence or information in it
 when processing ClientHello messages; in particular, they MUST NOT
 send the status_request_v2 extension in the EncryptedExtensions,
 CertificateRequest, or Certificate messages.  TLS 1.3 servers MUST be
 able to process ClientHello messages that include it, as it MAY be
 sent by clients that wish to use it in earlier protocol versions.
 A server MAY request that a client present an OCSP response with its
 certificate by sending an empty "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 1.2 and below.  In TLS 1.3, the server's SCT information is
 carried in an extension in the CertificateEntry.

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4.4.2.2. Server Certificate Selection

 The following rules apply to the certificates sent by the server:
  1. The certificate type MUST be X.509v3 [RFC5280], unless explicitly

negotiated otherwise (e.g., [RFC7250]).

  1. The server's end-entity certificate's public key (and associated

restrictions) MUST be compatible with the selected authentication

    algorithm from the client's "signature_algorithms" extension
    (currently RSA, ECDSA, or EdDSA).
  1. 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"/"signature_algorithms_cert" extensions (see
    Section 4.2.3).
  1. The "server_name" [RFC6066] and "certificate_authorities"

extensions 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 advertised by the client if it is 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 SHOULD NOT use the deprecated SHA-1 hash
 algorithm in general, but MAY do so if the client's advertisement
 permits it, and MUST NOT do so otherwise.
 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 appropriate certificate-related alert (by
 default, "unsupported_certificate"; see Section 6.2 for more
 information).
 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).

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4.4.2.3. Client Certificate Selection

 The following rules apply to certificates sent by the client:
  1. The certificate type MUST be X.509v3 [RFC5280], unless explicitly

negotiated otherwise (e.g., [RFC7250]).

  1. If the "certificate_authorities" extension in the

CertificateRequest message was present, at least one of the

    certificates in the certificate chain SHOULD be issued by one of
    the listed CAs.
  1. 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.
  1. If the CertificateRequest message contained a non-empty

"oid_filters" extension, the end-entity certificate MUST match the

    extension OIDs that are recognized by the client, as described in
    Section 4.2.5.

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 (i.e., it sends an empty
 Certificate message), 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 server MAY at its discretion either
 continue the handshake (considering the client unauthenticated) or
 abort the handshake.
 Any endpoint receiving any certificate which it would need to
 validate 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
 which it would need to validate using any signature algorithm using a
 SHA-1 hash abort the handshake with a "bad_certificate" alert.  For
 clarity, this means that endpoints can accept these algorithms for
 certificates that are self-signed or are trust anchors.

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 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.  The
 CertificateVerify message 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 type).  The signature is a
 digital signature using that algorithm.  The content that is covered
 under the signature is the hash output as described in Section 4.4.1,
 namely:
    Transcript-Hash(Handshake Context, Certificate)
 The digital signature is then computed over the concatenation of:
  1. A string that consists of octet 32 (0x20) repeated 64 times
  1. The context string
  1. A single 0 byte which serves as the separator
  1. The content to be signed

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 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".  The context string 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:
  1. The content covered by the digital signature
  1. 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 "signature_algorithms" extension in 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".  The SHA-1 algorithm
 MUST NOT be used in any signatures of CertificateVerify messages.

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 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 message MUST verify the signature
 field.  The verification process takes as input:
  1. The content covered by the digital signature
  1. The public key contained in the end-entity certificate found in

the associated Certificate message

  1. 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.

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 has received and
 validated the Finished message from its peer, it may begin to send
 and receive Application Data over the connection.  There are two
 settings in which it is permitted to send data prior to receiving the
 peer's Finished:
 1.  Clients sending 0-RTT data as described in Section 4.2.10.
 2.  Servers MAY send data after sending their first flight, but
     because the handshake is not yet complete, they have no assurance
     of either the peer's identity or its liveness (i.e., the
     ClientHello might have been replayed).

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 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:
 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.
 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 non-handshake record types are not
 handshake messages and are not included in the hash computations.
 Any records following a 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;
 If the server sent an "early_data" extension in EncryptedExtensions,
 the client MUST send an EndOfEarlyData message after receiving the
 server Finished.  If the server does not send an "early_data"
 extension in EncryptedExtensions, then the client MUST NOT send an
 EndOfEarlyData message.  This message indicates that all 0-RTT
 application_data messages, if any, have been transmitted and that the

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 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.

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 unique association between the ticket value and a secret
 PSK derived from the resumption master secret (see Section 7).
 The client MAY use this PSK for future handshakes by including the
 ticket value in the "pre_shared_key" extension in its ClientHello
 (Section 4.2.11).  Servers MAY send multiple tickets on a single
 connection, either immediately after each other or after specific
 events (see Appendix C.4).  For instance, the server might send a new
 ticket after post-handshake authentication in order to encapsulate
 the additional client authentication state.  Multiple tickets are
 useful for clients for a variety of purposes, including:
  1. Opening multiple parallel HTTP connections.
  1. Performing connection racing across interfaces and address

families via (for example) Happy Eyeballs [RFC8305] or related

    techniques.
 Any ticket MUST only be resumed with a cipher suite that has the same
 KDF hash algorithm as that used to establish the original connection.
 Clients MUST only resume if the new SNI value is valid for the server
 certificate presented in the original session and SHOULD only resume
 if the SNI value matches the one used in the original session.  The
 latter is a performance optimization: normally, there is no reason to
 expect that different servers covered by a single certificate would
 be able to accept each other's tickets; hence, attempting resumption
 in that case would waste a single-use ticket.  If such an indication
 is provided (externally or by any other means), clients MAY resume
 with a different SNI value.

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 On resumption, if reporting an SNI value to the calling application,
 implementations MUST use the value sent in the resumption ClientHello
 rather than the value sent in the previous session.  Note that if a
 server implementation declines all PSK identities with different SNI
 values, these two values are always the same.
 Note: Although the resumption master secret depends on the client's
 second flight, a server which does 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_nonce<0..255>;
        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 greater than
    604800 seconds (7 days).  The value of zero indicates that the
    ticket should be discarded immediately.  Clients MUST NOT cache
    tickets for longer than 7 days, regardless of the ticket_lifetime,
    and MAY delete tickets 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.  The server MUST generate a fresh value
    for each ticket it sends.
 ticket_nonce:  A per-ticket value that is unique across all tickets
    issued on this connection.

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 ticket:  The value of the ticket to be used as the PSK identity.  The
    ticket itself is an opaque label.  It MAY be either a database
    lookup key or a self-encrypted and self-authenticated value.
 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 4.2.10).  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 servers
    that reject early data due to lack of cryptographic material will
    be unable to differentiate padding from content, so clients
    SHOULD NOT depend on being able to send large quantities of
    padding in early data records.
 The PSK associated with the ticket is computed as:
     HKDF-Expand-Label(resumption_master_secret,
                      "resumption", ticket_nonce, Hash.length)
 Because the ticket_nonce value is distinct for each NewSessionTicket
 message, a different PSK will be derived for each ticket.
 Note that in principle it is possible to continue issuing new tickets
 which 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

 When the client has sent the "post_handshake_auth" extension (see
 Section 4.2.6), a server MAY request client authentication at any
 time after the handshake has completed by sending a
 CertificateRequest message.  The client MUST respond with the
 appropriate Authentication messages (see Section 4.4).  If the client
 chooses to authenticate, it MUST send Certificate, CertificateVerify,

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 and Finished.  If it declines, it MUST send a Certificate message
 containing no certificates followed by Finished.  All of the client's
 messages for a given response MUST appear consecutively on the wire
 with no intervening messages of other types.
 A client that receives a CertificateRequest message without having
 sent the "post_handshake_auth" extension MUST send an
 "unexpected_message" fatal alert.
 Note: Because client authentication could involve 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 Initialization Vector Update

 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.
    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.
 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

Rescorla Standards Track [Page 76] RFC 8446 TLS August 2018

 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
 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, 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 four content types: handshake, application_data, alert, and
 change_cipher_spec.  The change_cipher_spec record is used only for
 compatibility purposes (see Appendix D.4).
 An implementation may receive an unencrypted record of type
 change_cipher_spec consisting of the single byte value 0x01 at any
 time after the first ClientHello message has been sent or received
 and before the peer's Finished message has been received and MUST
 simply drop it without further processing.  Note that this record may
 appear at a point at the handshake where the implementation is
 expecting protected records, and so it is necessary to detect this
 condition prior to attempting to deprotect the record.  An
 implementation which receives any other change_cipher_spec value or
 which receives a protected change_cipher_spec record MUST abort the
 handshake with an "unexpected_message" alert.  If an implementation
 detects a change_cipher_spec record received before the first
 ClientHello message or after the peer's Finished message, it MUST be
 treated as an unexpected record type (though stateless servers may
 not be able to distinguish these cases from allowed cases).

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 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 ContentType
 registry as described in Section 11.

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:
  1. 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.
  1. Handshake messages MUST NOT span key changes. Implementations

MUST verify that all messages immediately preceding a key change

    align with a record boundary; if not, then they MUST terminate the
    connection with an "unexpected_message" alert.  Because the
    ClientHello, EndOfEarlyData, ServerHello, Finished, and KeyUpdate
    messages can immediately precede a key change, implementations
    MUST send these messages in alignment with a record boundary.
 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.  Application Data
 fragments MAY be split across multiple records or coalesced into a
 single record.

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    enum {
        invalid(0),
        change_cipher_spec(20),
        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:  MUST be set to 0x0303 for all records
    generated by a TLS 1.3 implementation other than an initial
    ClientHello (i.e., one not generated after a HelloRetryRequest),
    where it MAY also be 0x0301 for compatibility purposes.  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.
 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 backward
 compatibility, a record containing an initial ClientHello SHOULD have
 version 0x0301 (reflecting TLS 1.0) and a record containing a second
 ClientHello or a ServerHello MUST have version 0x0303 (reflecting
 TLS 1.2).  When negotiating prior versions of TLS, endpoints follow
 the procedure and requirements provided in Appendix D.

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 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.  Note that Application Data
 records MUST NOT be written to the wire unprotected (see Section 2
 for details).

5.2. Record Payload Protection

 The record protection functions translate a TLSPlaintext structure
 into a TLSCiphertext structure.  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 Associated
 Data" (AEAD) [RFC5116].  AEAD functions provide a 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 = application_data; /* 23 */
        ProtocolVersion legacy_record_version = 0x0303; /* TLS v1.2 */
        uint16 length;
        opaque encrypted_record[TLSCiphertext.length];
    } TLSCiphertext;
 content:  The TLSPlaintext.fragment value, containing the byte
    encoding of a handshake or an alert message, or the raw bytes of
    the application's data to send.
 type:  The TLSPlaintext.type value containing the content type of the
    record.
 zeros:  An arbitrary-length run of zero-valued bytes may appear in
    the cleartext after the type field.  This provides an opportunity
    for senders to pad any TLS record by a chosen amount as long as
    the total stays within record size limits.  See Section 5.4 for
    more details.

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 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
    0x0303.  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.  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 and the client_write_iv or server_write_iv (see
 Section 5.3), and the additional data input is the record header.
 I.e.,
    additional_data = TLSCiphertext.opaque_type ||
                      TLSCiphertext.legacy_record_version ||
                      TLSCiphertext.length
 The plaintext input to the AEAD algorithm is the encoded
 TLSInnerPlaintext structure.  Derivation of traffic keys is defined
 in Section 7.3.
 The AEAD output consists of the ciphertext output from the AEAD
 encryption operation.  The length of the plaintext is greater than
 the corresponding TLSPlaintext.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.

Rescorla Standards Track [Page 81] RFC 8446 TLS August 2018

 Since the ciphers might incorporate padding, the amount of overhead
 could vary with different lengths of plaintext.  Symbolically,
    AEADEncrypted =
        AEAD-Encrypt(write_key, nonce, additional_data, plaintext)
 The encrypted_record field of TLSCiphertext is set to AEADEncrypted.
 In order to decrypt and verify, the cipher takes as input the key,
 nonce, additional data, and the AEADEncrypted value.  The output is
 either the plaintext or an error indicating that the decryption
 failed.  There is no separate integrity check.  Symbolically,
    plaintext of encrypted_record =
        AEAD-Decrypt(peer_write_key, nonce,
                     additional_data, 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 TLSInnerPlaintext 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.  The appropriate sequence number is incremented by
 one after reading or writing each record.  Each sequence number is
 set to zero at the beginning of a connection and whenever the key is
 changed; the first record transmitted under a particular traffic key
 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 rekey (Section 4.6.3) or terminate the connection.

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 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.
 2.  The padded sequence number is XORed with either 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 and 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.

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 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 encoded TLSInnerPlaintext MUST NOT exceed 2^14
 + 1 octets.  If the maximum fragment length is reduced -- as, for
 example, by the record_size_limit extension from [RFC8449] -- then
 the reduced limit applies to the full plaintext, including the
 content type and padding.
 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 on top of TLS has its own padding, it
 may be preferable to pad Application Data TLS records within the
 application layer.  Padding for encrypted Handshake or Alert 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.

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6. Alert Protocol

 TLS provides an Alert content type to indicate closure information
 and errors.  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 level of the message in previous versions
 of TLS.  Alerts are divided into two classes: closure alerts and
 error alerts.  In TLS 1.3, the severity is implicit in the type of
 alert being sent, and the "level" field can safely be ignored.  The
 "close_notify" alert is used to indicate orderly closure of one
 direction of the connection.  Upon receiving such an alert, the TLS
 implementation SHOULD indicate end-of-data to the application.
 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 the secret values and keys established in failed
 connections, with the exception of the PSKs associated with session
 tickets, which SHOULD be discarded if possible.
 All the alerts listed in Section 6.2 MUST be sent with
 AlertLevel=fatal and MUST be treated as error alerts when received
 regardless of the AlertLevel in the message.  Unknown Alert types
 MUST be treated as error alerts.
 Note: 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 invalid (e.g., a DHE share of p - 1, or an invalid enum)
 MUST terminate the connection with an "illegal_parameter" alert.

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    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),
        unrecognized_name(112),
        bad_certificate_status_response(113),
        unknown_psk_identity(115),
        certificate_required(116),
        no_application_protocol(120),
        (255)
    } AlertDescription;
    struct {
        AlertLevel level;
        AlertDescription description;
    } Alert;

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6.1. Closure Alerts

 The client and the server must share knowledge that the connection is
 ending in order to avoid a truncation attack.
 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 alert has been received 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 generally has AlertLevel=warning.
 Either party MAY initiate a close of its write side of the connection
 by sending a "close_notify" alert.  Any data received after a closure
 alert has been received 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 its write
 side of the connection, unless it has already sent some error alert.
 This does not have any effect on its read side of the connection.
 Note that this is a change from versions of TLS prior to TLS 1.3 in
 which implementations were required to react to a "close_notify" by
 discarding pending writes and sending an immediate "close_notify"
 alert of their own.  That previous requirement could cause truncation
 in the read side.  Both parties need not wait to receive a
 "close_notify" alert before closing their read side of the
 connection, though doing so would introduce the possibility of
 truncation.
 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 a "close_notify" alert
 before indicating end-of-data to the application layer.  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 the write side of a connection
 reliably delivers pending data before destroying the transport.

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6.2. Error Alerts

 Error handling in TLS 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.
 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 an X
 alert" and "abort the handshake with an X alert" mean that the
 implementation MUST send alert X if it sends any alert.  All alerts
 defined below in this section, as well as all unknown alerts, are
 universally considered fatal as of TLS 1.3 (see Section 6).  The
 implementation SHOULD provide a way to facilitate logging the sending
 and receiving of alerts.
 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 (or some other
    negotiated limit).  This alert should never be observed in
    communication between proper implementations, except when messages
    were corrupted in the network.
 handshake_failure:  Receipt 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.

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 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.
 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 (not record layer) 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]).

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 missing_extension:  Sent by endpoints that receive a handshake
    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 handshake
    message containing an extension known to be prohibited for
    inclusion in the given handshake message, or including any
    extensions in a ServerHello or Certificate not first offered in
    the corresponding ClientHello or CertificateRequest.
 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]).
 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.
 no_application_protocol:  Sent by servers when a client
    "application_layer_protocol_negotiation" extension advertises only
    protocols that the server does not support (see [RFC7301]).
 New Alert values are assigned by IANA as described in Section 11.

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 from 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.

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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:
     HKDF-Expand-Label(Secret, Label, Context, Length) =
          HKDF-Expand(Secret, HkdfLabel, Length)
     Where HkdfLabel is specified as:
     struct {
         uint16 length = Length;
         opaque label<7..255> = "tls13 " + Label;
         opaque context<0..255> = Context;
     } HkdfLabel;
     Derive-Secret(Secret, Label, Messages) =
          HKDF-Expand-Label(Secret, Label,
                            Transcript-Hash(Messages), Hash.length)
 The Hash function used by Transcript-Hash and HKDF is the cipher
 suite hash algorithm.  Hash.length is its output length in bytes.
 Messages is 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 Context (indicated by "") is passed to HKDF-Expand-Label.  The
 labels specified in this document are all ASCII strings and do not
 include a trailing NUL byte.
 Note: With common hash functions, any label longer than 12 characters
 requires an additional iteration of the hash function to compute.
 The labels in this specification have all been chosen to fit within
 this limit.

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 Keys are derived from two input secrets using the HKDF-Extract and
 Derive-Secret functions.  The general pattern for adding a new secret
 is to use HKDF-Extract with the Salt being the current secret state
 and the Input Keying Material (IKM) being the new secret to be added.
 In this version of TLS 1.3, the two input secrets are:
  1. PSK (a pre-shared key established externally or derived from the

resumption_master_secret value from a previous connection)

  1. (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:
  1. HKDF-Extract is drawn as taking the Salt argument from the top and

the IKM argument from the left, with its output to the bottom and

    the name of the output on the right.
  1. 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.
  1. "0" indicates a string of Hash.length bytes set to zero.

Rescorla Standards Track [Page 92] RFC 8446 TLS August 2018

           0
           |
           v
 PSK ->  HKDF-Extract = Early Secret
           |
           +-----> Derive-Secret(., "ext binder" | "res binder", "")
           |                     = binder_key
           |
           +-----> Derive-Secret(., "c e traffic", ClientHello)
           |                     = client_early_traffic_secret
           |
           +-----> Derive-Secret(., "e exp master", ClientHello)
           |                     = early_exporter_master_secret
           v
     Derive-Secret(., "derived", "")
           |
           v
 (EC)DHE -> HKDF-Extract = Handshake Secret
           |
           +-----> Derive-Secret(., "c hs traffic",
           |                     ClientHello...ServerHello)
           |                     = client_handshake_traffic_secret
           |
           +-----> Derive-Secret(., "s hs traffic",
           |                     ClientHello...ServerHello)
           |                     = server_handshake_traffic_secret
           v
     Derive-Secret(., "derived", "")
           |
           v
 0 -> HKDF-Extract = Master Secret
           |
           +-----> Derive-Secret(., "c ap traffic",
           |                     ClientHello...server Finished)
           |                     = client_application_traffic_secret_0
           |
           +-----> Derive-Secret(., "s ap traffic",
           |                     ClientHello...server Finished)
           |                     = server_application_traffic_secret_0
           |
           +-----> Derive-Secret(., "exp master",
           |                     ClientHello...server Finished)
           |                     = exporter_master_secret
           |
           +-----> Derive-Secret(., "res master",
                                 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, it is called with three distinct transcripts.
 If a given secret is not available, then the 0-value consisting of a
 string of Hash.length bytes set to zeros 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_key, the label is "ext binder" for external PSKs (those
 provisioned outside of TLS) and "res binder" 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.
 Once all the values which are to be derived from a given secret have
 been computed, that secret SHOULD be erased.

7.2. Updating Traffic Secrets

 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_application_traffic_secret_N+1
 from client_/server_application_traffic_secret_N as described in this
 section and then re-deriving the traffic keys as described in
 Section 7.3.
 The next-generation application_traffic_secret is computed as:
     application_traffic_secret_N+1 =
         HKDF-Expand-Label(application_traffic_secret_N,
                           "traffic upd", "", Hash.length)
 Once client_/server_application_traffic_secret_N+1 and its associated
 traffic keys have been computed, implementations SHOULD delete
 client_/server_application_traffic_secret_N and its associated
 traffic keys.

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7.3. Traffic Key Calculation

 The traffic keying material is generated from the following input
 values:
  1. A secret value
  1. A purpose value indicating the specific value being generated
  1. The length of the key being generated
 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 value of Secret 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]_application_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 [DH76]
 computation is performed.  The negotiated key (Z) is converted to a
 byte string by encoding in big-endian form and left-padded with zeros
 up to the size of the prime.  This byte string is used as the shared
 secret in the key schedule as specified above.
 Note that this construction differs from previous versions of TLS
 which removed leading zeros.

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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 the 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.)
 For X25519 and X448, the ECDH calculations are as follows:
  1. The public key to put into the KeyShareEntry.key_exchange

structure is the result of applying the ECDH scalar multiplication

    function to the secret key of appropriate length (into scalar
    input) and the standard public basepoint (into u-coordinate point
    input).
  1. The ECDH shared secret is the result of applying the ECDH scalar

multiplication 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 these curves, 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 implementors use an alternative
 implementation of these elliptic curves, they SHOULD perform the
 additional checks specified in Section 7 of [RFC7748].

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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.
 The exporter value is computed as:
 TLS-Exporter(label, context_value, key_length) =
     HKDF-Expand-Label(Derive-Secret(Secret, label, ""),
                       "exporter", Hash(context_value), key_length)
 Where Secret is either the early_exporter_master_secret or the
 exporter_master_secret.  Implementations MUST use the
 exporter_master_secret unless explicitly specified by the
 application.  The early_exporter_master_secret is defined for use in
 settings where an exporter is needed for 0-RTT data.  A separate
 interface for the early exporter is RECOMMENDED; this avoids the
 exporter user accidentally using an early exporter when a regular one
 is desired or vice versa.
 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 than 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].

Rescorla Standards Track [Page 97] RFC 8446 TLS August 2018

8. 0-RTT and Anti-Replay

 As noted in Section 2.3 and Appendix E.5, TLS does not provide
 inherent replay protections for 0-RTT data.  There are two potential
 threats to be concerned with:
  1. Network attackers who mount a replay attack by simply duplicating

a flight of 0-RTT data.

  1. Network attackers who take advantage of client retry behavior to

arrange for the server to receive multiple copies of an

    application message.  This threat already exists to some extent
    because clients that value robustness respond to network errors by
    attempting to retry requests.  However, 0-RTT adds an additional
    dimension for any server system which does not maintain globally
    consistent server state.  Specifically, if a server system has
    multiple zones where tickets from zone A will not be accepted in
    zone B, then an attacker can duplicate a ClientHello and early
    data intended for A to both A and B.  At A, the data will be
    accepted in 0-RTT, but at B the server will reject 0-RTT data and
    instead force a full handshake.  If the attacker blocks the
    ServerHello from A, then the client will complete the handshake
    with B and probably retry the request, leading to duplication on
    the server system as a whole.
 The first class of attack can be prevented by sharing state to
 guarantee that the 0-RTT data is accepted at most once.  Servers
 SHOULD provide that level of replay safety by implementing one of the
 methods described in this section or by equivalent means.  It is
 understood, however, that due to operational concerns not all
 deployments will maintain state at that level.  Therefore, in normal
 operation, clients will not know which, if any, of these mechanisms
 servers actually implement and hence MUST only send early data which
 they deem safe to be replayed.
 In addition to the direct effects of replays, there is a class of
 attacks where even operations normally considered idempotent could be
 exploited by a large number of replays (timing attacks, resource
 limit exhaustion and others, as described in Appendix E.5).  Those
 can be mitigated by ensuring that every 0-RTT payload can be replayed
 only a limited number of times.  The server MUST ensure that any
 instance of it (be it a machine, a thread, or any other entity within
 the relevant serving infrastructure) would accept 0-RTT for the same
 0-RTT handshake at most once; this limits the number of replays to
 the number of server instances in the deployment.  Such a guarantee
 can be accomplished by locally recording data from recently received
 ClientHellos and rejecting repeats, or by any other method that

Rescorla Standards Track [Page 98] RFC 8446 TLS August 2018

 provides the same or a stronger guarantee.  The "at most once per
 server instance" guarantee is a minimum requirement; servers SHOULD
 limit 0-RTT replays further when feasible.
 The second class of attack cannot be prevented at the TLS layer and
 MUST be dealt with by any application.  Note that any application
 whose clients implement any kind of retry behavior already needs to
 implement some sort of anti-replay defense.

8.1. Single-Use Tickets

 The simplest form of anti-replay defense is for the server to only
 allow each session ticket to be used once.  For instance, the server
 can maintain a database of all outstanding valid tickets, deleting
 each ticket from the database as it is used.  If an unknown ticket is
 provided, the server would then fall back to a full handshake.
 If the tickets are not self-contained but rather are database keys,
 and the corresponding PSKs are deleted upon use, then connections
 established using PSKs enjoy forward secrecy.  This improves security
 for all 0-RTT data and PSK usage when PSK is used without (EC)DHE.
 Because this mechanism requires sharing the session database between
 server nodes in environments with multiple distributed servers, it
 may be hard to achieve high rates of successful PSK 0-RTT connections
 when compared to self-encrypted tickets.  Unlike session databases,
 session tickets can successfully do PSK-based session establishment
 even without consistent storage, though when 0-RTT is allowed they
 still require consistent storage for anti-replay of 0-RTT data, as
 detailed in the following section.

8.2. Client Hello Recording

 An alternative form of anti-replay is to record a unique value
 derived from the ClientHello (generally either the random value or
 the PSK binder) and reject duplicates.  Recording all ClientHellos
 causes state to grow without bound, but a server can instead record
 ClientHellos within a given time window and use the
 "obfuscated_ticket_age" to ensure that tickets aren't reused outside
 that window.
 In order to implement this, when a ClientHello is received, the
 server first verifies the PSK binder as described in Section 4.2.11.
 It then computes the expected_arrival_time as described in the next
 section and rejects 0-RTT if it is outside the recording window,
 falling back to the 1-RTT handshake.

Rescorla Standards Track [Page 99] RFC 8446 TLS August 2018

 If the expected_arrival_time is in the window, then the server checks
 to see if it has recorded a matching ClientHello.  If one is found,
 it either aborts the handshake with an "illegal_parameter" alert or
 accepts the PSK but rejects 0-RTT.  If no matching ClientHello is
 found, then it accepts 0-RTT and then stores the ClientHello for as
 long as the expected_arrival_time is inside the window.  Servers MAY
 also implement data stores with false positives, such as Bloom
 filters, in which case they MUST respond to apparent replay by
 rejecting 0-RTT but MUST NOT abort the handshake.
 The server MUST derive the storage key only from validated sections
 of the ClientHello.  If the ClientHello contains multiple PSK
 identities, then an attacker can create multiple ClientHellos with
 different binder values for the less-preferred identity on the
 assumption that the server will not verify it (as recommended by
 Section 4.2.11).  I.e., if the client sends PSKs A and B but the
 server prefers A, then the attacker can change the binder for B
 without affecting the binder for A.  If the binder for B is part of
 the storage key, then this ClientHello will not appear as a
 duplicate, which will cause the ClientHello to be accepted, and may
 cause side effects such as replay cache pollution, although any 0-RTT
 data will not be decryptable because it will use different keys.  If
 the validated binder or the ClientHello.random is used as the storage
 key, then this attack is not possible.
 Because this mechanism does not require storing all outstanding
 tickets, it may be easier to implement in distributed systems with
 high rates of resumption and 0-RTT, at the cost of potentially weaker
 anti-replay defense because of the difficulty of reliably storing and
 retrieving the received ClientHello messages.  In many such systems,
 it is impractical to have globally consistent storage of all the
 received ClientHellos.  In this case, the best anti-replay protection
 is provided by having a single storage zone be authoritative for a
 given ticket and refusing 0-RTT for that ticket in any other zone.
 This approach prevents simple replay by the attacker because only one
 zone will accept 0-RTT data.  A weaker design is to implement
 separate storage for each zone but allow 0-RTT in any zone.  This
 approach limits the number of replays to once per zone.  Application
 message duplication of course remains possible with either design.
 When implementations are freshly started, they SHOULD reject 0-RTT as
 long as any portion of their recording window overlaps the startup
 time.  Otherwise, they run the risk of accepting replays which were
 originally sent during that period.

Rescorla Standards Track [Page 100] RFC 8446 TLS August 2018

 Note: If the client's clock is running much faster than the server's,
 then a ClientHello may be received that is outside the window in the
 future, in which case it might be accepted for 1-RTT, causing a
 client retry, and then acceptable later for 0-RTT.  This is another
 variant of the second form of attack described in Section 8.

8.3. Freshness Checks

 Because the ClientHello indicates the time at which the client sent
 it, it is possible to efficiently determine whether a ClientHello was
 likely sent reasonably recently and only accept 0-RTT for such a
 ClientHello, otherwise falling back to a 1-RTT handshake.  This is
 necessary for the ClientHello storage mechanism described in
 Section 8.2 because otherwise the server needs to store an unlimited
 number of ClientHellos, and is a useful optimization for self-
 contained single-use tickets because it allows efficient rejection of
 ClientHellos which cannot be used for 0-RTT.
 In order to implement this mechanism, a server needs to store the
 time that the server generated the session ticket, offset by an
 estimate of the round-trip time between client and server.  I.e.,
     adjusted_creation_time = creation_time + estimated_RTT
 This value can be encoded in the ticket, thus avoiding the need to
 keep state for each outstanding ticket.  The server can determine the
 client's view of the age of the ticket by subtracting the ticket's
 "ticket_age_add" value from the "obfuscated_ticket_age" parameter in
 the client's "pre_shared_key" extension.  The server can determine
 the expected_arrival_time of the ClientHello as:
   expected_arrival_time = adjusted_creation_time + clients_ticket_age
 When a new ClientHello is received, the expected_arrival_time is then
 compared against the current server wall clock time and if they
 differ by more than a certain amount, 0-RTT is rejected, though the
 1-RTT handshake can be allowed to complete.

Rescorla Standards Track [Page 101] RFC 8446 TLS August 2018

 There are several potential sources of error that might cause
 mismatches between the expected_arrival_time and the measured time.
 Variations in client and server clock rates are likely to be minimal,
 though potentially the absolute times may be off by large values.
 Network propagation delays are the 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.  For clients on the Internet, this
 implies windows on the order of ten seconds to account for errors in
 clocks and variations in measurements; other deployment scenarios may
 have different needs.  Clock skew distributions are not symmetric, so
 the optimal tradeoff may involve an asymmetric range of permissible
 mismatch values.
 Note that freshness checking alone is not sufficient to prevent
 replays because it does not detect them during the error window,
 which -- depending on bandwidth and system capacity -- could include
 billions of replays in real-world settings.  In addition, this
 freshness checking is only done at the time the ClientHello is
 received and not when subsequent early Application Data records are
 received.  After early data is accepted, records may continue to be
 streamed to the server over a longer time period.

9. Compliance Requirements

9.1. 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
 [GCM] cipher suite and SHOULD implement the TLS_AES_256_GCM_SHA384
 [GCM] and TLS_CHACHA20_POLY1305_SHA256 [RFC8439] cipher suites (see
 Appendix B.4).
 A TLS-compliant application MUST support digital signatures with
 rsa_pkcs1_sha256 (for certificates), rsa_pss_rsae_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].

Rescorla Standards Track [Page 102] RFC 8446 TLS August 2018

9.2. Mandatory-to-Implement Extensions

 In the absence of an application profile standard specifying
 otherwise, a TLS-compliant application MUST implement the following
 TLS extensions:
  1. Supported Versions ("supported_versions"; Section 4.2.1)
  1. Cookie ("cookie"; Section 4.2.2)
  1. Signature Algorithms ("signature_algorithms"; Section 4.2.3)
  1. Signature Algorithms Certificate ("signature_algorithms_cert";

Section 4.2.3)

  1. Negotiated Groups ("supported_groups"; Section 4.2.7)
  1. Key Share ("key_share"; Section 4.2.8)
  1. Server Name Indication ("server_name"; Section 3 of [RFC6066])
 All implementations MUST send and use these extensions when offering
 applicable features:
  1. "supported_versions" is REQUIRED for all ClientHello, ServerHello,

and HelloRetryRequest messages.

  1. "signature_algorithms" is REQUIRED for certificate authentication.
  1. "supported_groups" is REQUIRED for ClientHello messages using DHE

or ECDHE key exchange.

  1. "key_share" is REQUIRED for DHE or ECDHE key exchange.
  1. "pre_shared_key" is REQUIRED for PSK key agreement.
  1. "psk_key_exchange_modes" is REQUIRED for PSK key agreement.

Rescorla Standards Track [Page 103] RFC 8446 TLS August 2018

 A client is considered to be attempting to negotiate using this
 specification if the ClientHello contains a "supported_versions"
 extension with 0x0304 contained in its body.  Such a ClientHello
 message MUST meet the following requirements:
  1. If not containing a "pre_shared_key" extension, it MUST contain

both a "signature_algorithms" extension and a "supported_groups"

    extension.
  1. 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 the 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.

9.3. Protocol Invariants

 This section describes invariants that TLS endpoints and middleboxes
 MUST follow.  It also applies to earlier versions of TLS.
 TLS is designed to be securely and compatibly extensible.  Newer
 clients or servers, when communicating with newer peers, should
 negotiate the most preferred common parameters.  The TLS handshake
 provides downgrade protection: Middleboxes passing traffic between a
 newer client and newer server without terminating TLS should be
 unable to influence the handshake (see Appendix E.1).  At the same
 time, deployments update at different rates, so a newer client or
 server MAY continue to support older parameters, which would allow it
 to interoperate with older endpoints.

Rescorla Standards Track [Page 104] RFC 8446 TLS August 2018

 For this to work, implementations MUST correctly handle extensible
 fields:
  1. A client sending a ClientHello MUST support all parameters

advertised in it. Otherwise, the server may fail to interoperate

    by selecting one of those parameters.
  1. A server receiving a ClientHello MUST correctly ignore all

unrecognized cipher suites, extensions, and other parameters.

    Otherwise, it may fail to interoperate with newer clients.  In
    TLS 1.3, a client receiving a CertificateRequest or
    NewSessionTicket MUST also ignore all unrecognized extensions.
  1. A middlebox which terminates a TLS connection MUST behave as a

compliant TLS server (to the original client), including having a

    certificate which the client is willing to accept, and also as a
    compliant TLS client (to the original server), including verifying
    the original server's certificate.  In particular, it MUST
    generate its own ClientHello containing only parameters it
    understands, and it MUST generate a fresh ServerHello random
    value, rather than forwarding the endpoint's value.
    Note that TLS's protocol requirements and security analysis only
    apply to the two connections separately.  Safely deploying a TLS
    terminator requires additional security considerations which are
    beyond the scope of this document.
  1. A middlebox which forwards ClientHello parameters it does not

understand MUST NOT process any messages beyond that ClientHello.

    It MUST forward all subsequent traffic unmodified.  Otherwise, it
    may fail to interoperate with newer clients and servers.
    Forwarded ClientHellos may contain advertisements for features not
    supported by the middlebox, so the response may include future TLS
    additions the middlebox does not recognize.  These additions MAY
    change any message beyond the ClientHello arbitrarily.  In
    particular, the values sent in the ServerHello might change, the
    ServerHello format might change, and the TLSCiphertext format
    might change.
 The design of TLS 1.3 was constrained by widely deployed
 non-compliant TLS middleboxes (see Appendix D.4); however, it does
 not relax the invariants.  Those middleboxes continue to be
 non-compliant.

Rescorla Standards Track [Page 105] RFC 8446 TLS August 2018

10. Security Considerations

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

11. IANA Considerations

 This document uses several registries that were originally created in
 [RFC4346] and updated in [RFC8447].  IANA has updated these to
 reference this document.  The registries and their allocation
 policies are below:
  1. TLS Cipher Suites registry: values with the first byte in the

range 0-254 (decimal) are assigned via Specification Required

    [RFC8126].  Values with the first byte 255 (decimal) are reserved
    for Private Use [RFC8126].
    IANA 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 "Y" for each new cipher suite.
  1. TLS ContentType registry: Future values are allocated via

Standards Action [RFC8126].

  1. TLS Alerts registry: Future values are allocated via Standards

Action [RFC8126]. IANA has populated this registry with the

    values from Appendix B.2.  The "DTLS-OK" column is marked as "Y"
    for all such values.  Values marked as "_RESERVED" have comments
    describing their previous usage.
  1. TLS HandshakeType registry: Future values are allocated via

Standards Action [RFC8126]. IANA has updated this registry to

    rename item 4 from "NewSessionTicket" to "new_session_ticket" and
    populated this registry with the values from Appendix B.3.  The
    "DTLS-OK" column is marked as "Y" for all such values.  Values
    marked "_RESERVED" have comments describing their previous or
    temporary usage.

Rescorla Standards Track [Page 106] RFC 8446 TLS August 2018

 This document also uses the TLS ExtensionType Values registry
 originally created in [RFC4366].  IANA has updated it to reference
 this document.  Changes to the registry follow:
  1. IANA has updated the registration policy as follows:
    Values with the first byte in the range 0-254 (decimal) are
    assigned via Specification Required [RFC8126].  Values with the
    first byte 255 (decimal) are reserved for Private Use [RFC8126].
  1. IANA has updated this registry to include the "key_share",

"pre_shared_key", "psk_key_exchange_modes", "early_data",

    "cookie", "supported_versions", "certificate_authorities",
    "oid_filters", "post_handshake_auth", and
    "signature_algorithms_cert" extensions with the values defined in
    this document and the "Recommended" value of "Y".
  1. IANA has updated this registry to include a "TLS 1.3" column which

lists the messages in which the extension may appear. This column

    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.
 This document updates an entry in the TLS Certificate Types registry
 originally created in [RFC6091] and updated in [RFC8447].  IANA has
 updated the entry for value 1 to have the name "OpenPGP_RESERVED",
 "Recommended" value "N", and comment "Used in TLS versions prior
 to 1.3."
 This document updates an entry in the TLS Certificate Status Types
 registry originally created in [RFC6961].  IANA has updated the entry
 for value 2 to have the name "ocsp_multi_RESERVED" and comment "Used
 in TLS versions prior to 1.3".
 This document updates two entries in the TLS Supported Groups
 registry (created under a different name by [RFC4492]; now maintained
 by [RFC8422]) and updated by [RFC7919] and [RFC8447].  The entries
 for values 29 and 30 (x25519 and x448) have been updated to also
 refer to this document.

Rescorla Standards Track [Page 107] RFC 8446 TLS August 2018

 In addition, this document defines two new registries that are
 maintained by IANA:
  1. TLS SignatureScheme registry: Values with the first byte in the

range 0-253 (decimal) are assigned via Specification Required

    [RFC8126].  Values with the first byte 254 or 255 (decimal) are
    reserved for Private Use [RFC8126].  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 backward compatibility.
    This registry has a "Recommended" column.  The registry has been
    initially populated with the values described in Section 4.2.3.
    The following values are marked as "Recommended":
    ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
    rsa_pss_rsae_sha256, rsa_pss_rsae_sha384, rsa_pss_rsae_sha512,
    rsa_pss_pss_sha256, rsa_pss_pss_sha384, rsa_pss_pss_sha512, and
    ed25519.  The "Recommended" column is assigned a value of "N"
    unless explicitly requested, and adding a value with a
    "Recommended" value of "Y" requires Standards Action [RFC8126].
    IESG Approval is REQUIRED for a Y->N transition.
  1. TLS PskKeyExchangeMode registry: Values in the range 0-253

(decimal) are assigned via Specification Required [RFC8126].

    The values 254 and 255 (decimal) are reserved for Private Use
    [RFC8126].  This registry has a "Recommended" column.  The
    registry has been initially populated with psk_ke (0) and
    psk_dhe_ke (1).  Both are marked as "Recommended".  The
    "Recommended" column is assigned a value of "N" unless explicitly
    requested, and adding a value with a "Recommended" value of "Y"
    requires Standards Action [RFC8126].  IESG Approval is REQUIRED
    for a Y->N transition.

Rescorla Standards Track [Page 108] RFC 8446 TLS August 2018

12. References

12.1. Normative References

 [DH76]     Diffie, W. and M. Hellman, "New directions in
            cryptography", IEEE Transactions on Information
            Theory, Vol. 22 No. 6, pp. 644-654,
            DOI 10.1109/TIT.1976.1055638, November 1976.
 [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.
 [GCM]      Dworkin, M., "Recommendation for Block Cipher Modes of
            Operation: Galois/Counter Mode (GCM) and GMAC",
            NIST Special Publication 800-38D,
            DOI 10.6028/NIST.SP.800-38D, November 2007.
 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104,
            DOI 10.17487/RFC2104, February 1997,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
            Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
            <https://www.rfc-editor.org/info/rfc5116>.
 [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,
            <https://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, <https://www.rfc-editor.org/info/rfc5705>.
 [RFC5756]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
            "Updates for RSAES-OAEP and RSASSA-PSS Algorithm
            Parameters", RFC 5756, DOI 10.17487/RFC5756, January 2010,
            <https://www.rfc-editor.org/info/rfc5756>.

Rescorla Standards Track [Page 109] RFC 8446 TLS August 2018

 [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
            Key Derivation Function (HKDF)", RFC 5869,
            DOI 10.17487/RFC5869, May 2010,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc6655>.
 [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
            Galperin, S., and C. Adams, "X.509 Internet Public Key
            Infrastructure Online Certificate Status Protocol - OCSP",
            RFC 6960, DOI 10.17487/RFC6960, June 2013,
            <https://www.rfc-editor.org/info/rfc6960>.
 [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
            Multiple Certificate Status Request Extension", RFC 6961,
            DOI 10.17487/RFC6961, June 2013,
            <https://www.rfc-editor.org/info/rfc6961>.
 [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
            Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
            <https://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, <https://www.rfc-editor.org/info/rfc6979>.
 [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, <https://www.rfc-editor.org/info/rfc7301>.
 [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,
            <https://www.rfc-editor.org/info/rfc7507>.
 [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
            for Security", RFC 7748, DOI 10.17487/RFC7748,
            January 2016, <https://www.rfc-editor.org/info/rfc7748>.

Rescorla Standards Track [Page 110] RFC 8446 TLS August 2018

 [RFC7919]  Gillmor, D., "Negotiated Finite Field Diffie-Hellman
            Ephemeral Parameters for Transport Layer Security (TLS)",
            RFC 7919, DOI 10.17487/RFC7919, August 2016,
            <https://www.rfc-editor.org/info/rfc7919>.
 [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
            "PKCS #1: RSA Cryptography Specifications Version 2.2",
            RFC 8017, DOI 10.17487/RFC8017, November 2016,
            <https://www.rfc-editor.org/info/rfc8017>.
 [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
            Signature Algorithm (EdDSA)", RFC 8032,
            DOI 10.17487/RFC8032, January 2017,
            <https://www.rfc-editor.org/info/rfc8032>.
 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8439]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
            Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
            <https://www.rfc-editor.org/info/rfc8439>.
 [SHS]      Dang, Q., "Secure Hash Standard (SHS)", National Institute
            of Standards and Technology report,
            DOI 10.6028/NIST.FIPS.180-4, August 2015.
 [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 8825-1:2015, November 2015.

Rescorla Standards Track [Page 111] RFC 8446 TLS August 2018

12.2. Informative References

 [AEAD-LIMITS]
            Luykx, A. and K. Paterson, "Limits on Authenticated
            Encryption Use in TLS", August 2017,
            <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
 [BBFGKZ16]
            Bhargavan, K., Brzuska, C., Fournet, C., Green, M.,
            Kohlweiss, M., and S. Zanella-Beguelin, "Downgrade
            Resilience in Key-Exchange Protocols", Proceedings of IEEE
            Symposium on Security and Privacy (San Jose),
            DOI 10.1109/SP.2016.37, May 2016.
 [BBK17]    Bhargavan, K., Blanchet, B., and N. Kobeissi, "Verified
            Models and Reference Implementations for the TLS 1.3
            Standard Candidate", Proceedings of IEEE Symposium on
            Security and Privacy (San Jose), DOI 10.1109/SP.2017.26,
            May 2017.
 [BDFKPPRSZZ16]
            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",
            Proceedings of IEEE Symposium on Security and Privacy (San
            Jose), May 2017, <https://eprint.iacr.org/2016/1178>.
 [Ben17a]   Benjamin, D., "Presentation before the TLS WG at
            IETF 100", November 2017,
            <https://datatracker.ietf.org/meeting/100/materials/
            slides-100-tls-sessa-tls13/>.
 [Ben17b]   Benjamin, D., "Additional TLS 1.3 results from Chrome",
            message to the TLS mailing list, 18 December 2017,
            <https://www.ietf.org/mail-archive/web/tls/current/
            msg25168.html>.
 [Blei98]   Bleichenbacher, D., "Chosen Ciphertext Attacks against
            Protocols Based on RSA Encryption Standard PKCS #1",
            Proceedings of CRYPTO '98, 1998.
 [BMMRT15]  Badertscher, C., Matt, C., Maurer, U., Rogaway, P., and B.
            Tackmann, "Augmented Secure Channels and the Goal of the
            TLS 1.3 Record Layer", ProvSec 2015, September 2015,
            <https://eprint.iacr.org/2015/394>.

Rescorla Standards Track [Page 112] RFC 8446 TLS August 2018

 [BT16]     Bellare, M. and B. Tackmann, "The Multi-User Security of
            Authenticated Encryption: AES-GCM in TLS 1.3", Proceedings
            of CRYPTO 2016, July 2016,
            <https://eprint.iacr.org/2016/564>.
 [CCG16]    Cohn-Gordon, K., Cremers, C., and L. Garratt, "On
            Post-compromise Security", IEEE Computer Security
            Foundations Symposium, DOI 10.1109/CSF.2016.19, July 2015.
 [CHECKOWAY]
            Checkoway, S., Maskiewicz, J., Garman, C., Fried, J.,
            Cohney, S., Green, M., Heninger, N., Weinmann, R.,
            Rescorla, E., and H. Shacham, "A Systematic Analysis of
            the Juniper Dual EC Incident", Proceedings of the 2016 ACM
            SIGSAC Conference on Computer and Communications Security
            - CCS '16, DOI 10.1145/2976749.2978395, October 2016.
 [CHHSV17]  Cremers, C., Horvat, M., Hoyland, J., Scott, S., and T.
            van der Merwe, "Awkward Handshake: Possible mismatch of
            client/server view on client authentication in
            post-handshake mode in Revision 18", message to the TLS
            mailing list, 10 February 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 (San Jose),
            DOI 10.1109/SP.2016.35, May 2016,
            <https://ieeexplore.ieee.org/document/7546518/>.
 [CK01]     Canetti, R. and H. Krawczyk, "Analysis of Key-Exchange
            Protocols and Their Use for Building Secure Channels",
            Proceedings of Eurocrypt 2001,
            DOI 10.1007/3-540-44987-6_28, April 2001.
 [CLINIC]   Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
            Why You Went to the Clinic: Risks and Realization of HTTPS
            Traffic Analysis", Privacy Enhancing Technologies, pp.
            143-163, DOI 10.1007/978-3-319-08506-7_8, 2014.
 [DFGS15]   Dowling, B., Fischlin, M., Guenther, F., and D. Stebila,
            "A Cryptographic Analysis of the TLS 1.3 Handshake
            Protocol Candidates", Proceedings of ACM CCS 2015,
            October 2015, <https://eprint.iacr.org/2015/914>.

Rescorla Standards Track [Page 113] RFC 8446 TLS August 2018

 [DFGS16]   Dowling, B., Fischlin, M., Guenther, F., and D. Stebila,
            "A Cryptographic Analysis of the TLS 1.3 Full and
            Pre-shared Key Handshake Protocol", TRON 2016,
            February 2016, <https://eprint.iacr.org/2016/081>.
 [DOW92]    Diffie, W., van Oorschot, P., and M. Wiener,
            "Authentication and authenticated key exchanges", Designs,
            Codes and Cryptography, DOI 10.1007/BF00124891, June 1992.
 [DSS]      National Institute of Standards and Technology, U.S.
            Department of Commerce, "Digital Signature Standard
            (DSS)", NIST FIPS PUB 186-4, DOI 10.6028/NIST.FIPS.186-4,
            July 2013.
 [FG17]     Fischlin, M. and F. Guenther, "Replay Attacks on Zero
            Round-Trip Time: The Case of the TLS 1.3 Handshake
            Candidates", Proceedings of EuroS&P 2017, April 2017,
            <https://eprint.iacr.org/2017/082>.
 [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 (San Jose),
            DOI 10.1109/SP.2016.34, May 2016,
            <https://ieeexplore.ieee.org/document/7546517/>.
 [FW15]     Weimer, F., "Factoring RSA Keys With TLS Perfect Forward
            Secrecy", September 2015.
 [HCJC16]   Husak, M., Cermak, M., Jirsik, T., and P. Celeda, "HTTPS
            traffic analysis and client identification using passive
            SSL/TLS fingerprinting", EURASIP Journal on Information
            Security, Vol. 2016, DOI 10.1186/s13635-016-0030-7,
            February 2016.
 [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,
            August 2015.
 [IEEE1363]
            IEEE, "IEEE Standard Specifications for Public Key
            Cryptography", IEEE Std. 1363-2000,
            DOI 10.1109/IEEESTD.2000.92292.

Rescorla Standards Track [Page 114] RFC 8446 TLS August 2018

 [JSS15]    Jager, T., Schwenk, J., and J. Somorovsky, "On the
            Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
            v1.5 Encryption", Proceedings of ACM CCS 2015,
            DOI 10.1145/2810103.2813657, October 2015,
            <https://www.nds.rub.de/media/nds/
            veroeffentlichungen/2015/08/21/Tls13QuicAttacks.pdf>.
 [KEYAGREEMENT]
            Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
            Davis, "Recommendation for Pair-Wise Key Establishment
            Schemes Using Discrete Logarithm Cryptography", National
            Institute of Standards and Technology,
            DOI 10.6028/NIST.SP.800-56Ar3, April 2018.
 [Kraw10]   Krawczyk, H., "Cryptographic Extraction and Key
            Derivation: The HKDF Scheme", Proceedings of CRYPTO 2010,
            August 2010, <https://eprint.iacr.org/2010/264>.
 [Kraw16]   Krawczyk, H., "A Unilateral-to-Mutual Authentication
            Compiler for Key Exchange (with Applications to Client
            Authentication in TLS 1.3", Proceedings of ACM CCS 2016,
            October 2016, <https://eprint.iacr.org/2016/711>.
 [KW16]     Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3",
            Proceedings of EuroS&P 2016, March 2016,
            <https://eprint.iacr.org/2015/978>.
 [LXZFH16]  Li, X., Xu, J., Zhang, Z., Feng, D., and H. Hu, "Multiple
            Handshakes Security of TLS 1.3 Candidates", Proceedings of
            IEEE Symposium on Security and Privacy (San Jose),
            DOI 10.1109/SP.2016.36, May 2016,
            <https://ieeexplore.ieee.org/document/7546519/>.
 [Mac17]    MacCarthaigh, C., "Security Review of TLS1.3 0-RTT",
            March 2017, <https://github.com/tlswg/tls13-spec/
            issues/1001>.
 [PS18]     Patton, C. and T. Shrimpton, "Partially specified
            channels: The TLS 1.3 record layer without elision", 2018,
            <https://eprint.iacr.org/2018/634>.
 [PSK-FINISHED]
            Scott, S., Cremers, C., Horvat, M., and T. van der Merwe,
            "Revision 10: possible attack if client authentication is
            allowed during PSK", message to the TLS mailing list,
            31 October 2015, <https://www.ietf.org/
            mail-archive/web/tls/current/msg18215.html>.

Rescorla Standards Track [Page 115] RFC 8446 TLS August 2018

 [REKEY]    Abdalla, M. and M. Bellare, "Increasing the Lifetime of a
            Key: A Comparative Analysis of the Security of Re-keying
            Techniques", ASIACRYPT 2000, DOI 10.1007/3-540-44448-3_42,
            October 2000.
 [Res17a]   Rescorla, E., "Preliminary data on Firefox TLS 1.3
            Middlebox experiment", message to the TLS mailing list,
            5 December 2017, <https://www.ietf.org/
            mail-archive/web/tls/current/msg25091.html>.
 [Res17b]   Rescorla, E., "More compatibility measurement results",
            message to the TLS mailing list, 22 December 2017,
            <https://www.ietf.org/mail-archive/web/tls/current/
            msg25179.html>.
 [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
            Text on Security Considerations", BCP 72, RFC 3552,
            DOI 10.17487/RFC3552, July 2003,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4086>.
 [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346,
            DOI 10.17487/RFC4346, April 2006,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4492>.
 [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, <https://www.rfc-editor.org/info/rfc5077>.

Rescorla Standards Track [Page 116] RFC 8446 TLS August 2018

 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <https://www.rfc-editor.org/info/rfc5246>.
 [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,
            <https://www.rfc-editor.org/info/rfc5764>.
 [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
            for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
            <https://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,
            <https://www.rfc-editor.org/info/rfc6091>.
 [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
            Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
            DOI 10.17487/RFC6101, August 2011,
            <https://www.rfc-editor.org/info/rfc6101>.
 [RFC6176]  Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
            (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176,
            March 2011, <https://www.rfc-editor.org/info/rfc6176>.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
            January 2012, <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc7230>.

Rescorla Standards Track [Page 117] RFC 8446 TLS August 2018

 [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, <https://www.rfc-editor.org/info/rfc7250>.
 [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
            DOI 10.17487/RFC7465, February 2015,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc7627>.
 [RFC7685]  Langley, A., "A Transport Layer Security (TLS) ClientHello
            Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
            October 2015, <https://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,
            <https://www.rfc-editor.org/info/rfc7924>.
 [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
            Better Connectivity Using Concurrency", RFC 8305,
            DOI 10.17487/RFC8305, December 2017,
            <https://www.rfc-editor.org/info/rfc8305>.
 [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
            Curve Cryptography (ECC) Cipher Suites for Transport Layer
            Security (TLS) Versions 1.2 and Earlier", RFC 8422,
            DOI 10.17487/RFC8422, August 2018,
            <https://www.rfc-editor.org/info/rfc8422>.
 [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
            and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
            <https://www.rfc-editor.org/info/rfc8447>.
 [RFC8449]  Thomson, M., "Record Size Limit Extension for TLS",
            RFC 8449, DOI 10.17487/RFC8449, August 2018,
            <https://www.rfc-editor.org/info/rfc8449>.

Rescorla Standards Track [Page 118] RFC 8446 TLS August 2018

 [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
            Obtaining Digital Signatures and Public-Key
            Cryptosystems", Communications of the ACM, Vol. 21 No. 2,
            pp. 120-126, DOI 10.1145/359340.359342, 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,
            DOI 10.1007/978-3-540-45146-4_24, August 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), DOI 10.14722/ndss.2016.23418,
            February 2016.
 [SSL2]     Hickman, K., "The SSL Protocol", February 1995.
 [TIMING]   Boneh, D. and D. Brumley, "Remote Timing Attacks Are
            Practical", USENIX Security Symposium, August 2003.
 [TLS13-TRACES]
            Thomson, M., "Example Handshake Traces for TLS 1.3", Work
            in Progress, draft-ietf-tls-tls13-vectors-06, July 2018.
 [X501]     ITU-T, "Information Technology - Open Systems
            Interconnection - The Directory: Models", ITU-T X.501,
            October 2016, <https://www.itu.int/rec/T-REC-X.501/en>.

Rescorla Standards Track [Page 119] RFC 8446 TLS August 2018

Appendix A. State Machine

 This appendix 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.  Actions which are taken only in certain circumstances
 are indicated in [].  The notation "K_{send,recv} = foo" means "set
 the send/recv key to the given key".

A.1. Client

                            START <----+
             Send ClientHello |        | Recv HelloRetryRequest
        [K_send = early data] |        |
                              v        |
         /                 WAIT_SH ----+
         |                    | Recv ServerHello
         |                    | K_recv = handshake
     Can |                    V
    send |                 WAIT_EE
   early |                    | Recv EncryptedExtensions
    data |           +--------+--------+
         |     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]
                            | K_send = handshake
                            | [Send Certificate [+ CertificateVerify]]
  Can send                  | Send Finished
  app data   -->            | K_send = K_recv = application
  after here                v
                        CONNECTED
 Note that with the transitions as shown above, clients may send
 alerts that derive from post-ServerHello messages in the clear or
 with the early data keys.  If clients need to send such alerts, they
 SHOULD first rekey to the handshake keys if possible.

Rescorla Standards Track [Page 120] RFC 8446 TLS August 2018

A.2. Server

                            START <-----+
             Recv ClientHello |         | Send HelloRetryRequest
                              v         |
                           RECVD_CH ----+
                              | Select parameters
                              v
                           NEGOTIATED
                              | Send ServerHello
                              | K_send = handshake
                              | Send EncryptedExtensions
                              | [Send CertificateRequest]

Can send | [Send Certificate + CertificateVerify] app data | Send Finished after –> | K_send = application here +——–+——–+

            No 0-RTT |                 | 0-RTT
                     |                 |
 K_recv = handshake  |                 | K_recv = early data

[Skip decrypt errors] | +——> WAIT_EOED -+

                     |    |       Recv |      | Recv EndOfEarlyData
                     |    | early data |      | K_recv = handshake
                     |    +------------+      |
                     |                        |
                     +> WAIT_FLIGHT2 <--------+
                              |
                     +--------+--------+
             No auth |                 | Client auth
                     |                 |
                     |                 v
                     |             WAIT_CERT
                     |        Recv |       | Recv Certificate
                     |       empty |       v
                     | Certificate |    WAIT_CV
                     |             |       | Recv
                     |             v       | CertificateVerify
                     +-> WAIT_FINISHED <---+
                              | Recv Finished
                              | K_recv = application
                              v
                          CONNECTED

Rescorla Standards Track [Page 121] RFC 8446 TLS August 2018

Appendix B. Protocol Data Structures and Constant Values

 This appendix provides the normative protocol types and the
 definitions for 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

    enum {
        invalid(0),
        change_cipher_spec(20),
        alert(21),
        handshake(22),
        application_data(23),
        heartbeat(24),  /* RFC 6520 */
        (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 = application_data; /* 23 */
        ProtocolVersion legacy_record_version = 0x0303; /* TLS v1.2 */
        uint16 length;
        opaque encrypted_record[TLSCiphertext.length];
    } TLSCiphertext;

Rescorla Standards Track [Page 122] RFC 8446 TLS August 2018

B.2. Alert Messages

    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_RESERVED(111),
        unrecognized_name(112),
        bad_certificate_status_response(113),
        bad_certificate_hash_value_RESERVED(114),
        unknown_psk_identity(115),
        certificate_required(116),
        no_application_protocol(120),
        (255)
    } AlertDescription;
    struct {
        AlertLevel level;
        AlertDescription description;
    } Alert;

Rescorla Standards Track [Page 123] RFC 8446 TLS August 2018

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_RESERVED(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),
        certificate_url_RESERVED(21),
        certificate_status_RESERVED(22),
        supplemental_data_RESERVED(23),
        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 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;
        };
    } Handshake;

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B.3.1. Key Exchange Messages

  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;
  struct {
      ProtocolVersion legacy_version = 0x0303;    /* TLS v1.2 */
      Random random;
      opaque legacy_session_id_echo<0..32>;
      CipherSuite cipher_suite;
      uint8 legacy_compression_method = 0;
      Extension extensions<6..2^16-1>;
  } ServerHello;

Rescorla Standards Track [Page 125] RFC 8446 TLS August 2018

  struct {
      ExtensionType extension_type;
      opaque extension_data<0..2^16-1>;
  } Extension;
  enum {
      server_name(0),                             /* RFC 6066 */
      max_fragment_length(1),                     /* RFC 6066 */
      status_request(5),                          /* RFC 6066 */
      supported_groups(10),                       /* RFC 8422, 7919 */
      signature_algorithms(13),                   /* RFC 8446 */
      use_srtp(14),                               /* RFC 5764 */
      heartbeat(15),                              /* RFC 6520 */
      application_layer_protocol_negotiation(16), /* RFC 7301 */
      signed_certificate_timestamp(18),           /* RFC 6962 */
      client_certificate_type(19),                /* RFC 7250 */
      server_certificate_type(20),                /* RFC 7250 */
      padding(21),                                /* RFC 7685 */
      RESERVED(40),                               /* Used but never
                                                     assigned */
      pre_shared_key(41),                         /* RFC 8446 */
      early_data(42),                             /* RFC 8446 */
      supported_versions(43),                     /* RFC 8446 */
      cookie(44),                                 /* RFC 8446 */
      psk_key_exchange_modes(45),                 /* RFC 8446 */
      RESERVED(46),                               /* Used but never
                                                     assigned */
      certificate_authorities(47),                /* RFC 8446 */
      oid_filters(48),                            /* RFC 8446 */
      post_handshake_auth(49),                    /* RFC 8446 */
      signature_algorithms_cert(50),              /* RFC 8446 */
      key_share(51),                              /* RFC 8446 */
      (65535)
  } ExtensionType;
  struct {
      NamedGroup group;
      opaque key_exchange<1..2^16-1>;
  } KeyShareEntry;
  struct {
      KeyShareEntry client_shares<0..2^16-1>;
  } KeyShareClientHello;
  struct {
      NamedGroup selected_group;
  } KeyShareHelloRetryRequest;

Rescorla Standards Track [Page 126] RFC 8446 TLS August 2018

  struct {
      KeyShareEntry server_share;
  } KeyShareServerHello;
  struct {
      uint8 legacy_form = 4;
      opaque X[coordinate_length];
      opaque Y[coordinate_length];
  } UncompressedPointRepresentation;
  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 {
      PskIdentity identities<7..2^16-1>;
      PskBinderEntry binders<33..2^16-1>;
  } OfferedPsks;
  struct {
      select (Handshake.msg_type) {
          case client_hello: OfferedPsks;
          case server_hello: uint16 selected_identity;
      };
  } PreSharedKeyExtension;

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B.3.1.1. Version Extension

    struct {
        select (Handshake.msg_type) {
            case client_hello:
                 ProtocolVersion versions<2..254>;
            case server_hello: /* and HelloRetryRequest */
                 ProtocolVersion selected_version;
        };
    } SupportedVersions;

B.3.1.2. Cookie Extension

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

Rescorla Standards Track [Page 128] RFC 8446 TLS August 2018

B.3.1.3. Signature Algorithm Extension

    enum {
        /* RSASSA-PKCS1-v1_5 algorithms */
        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 with public key OID rsaEncryption */
        rsa_pss_rsae_sha256(0x0804),
        rsa_pss_rsae_sha384(0x0805),
        rsa_pss_rsae_sha512(0x0806),
        /* EdDSA algorithms */
        ed25519(0x0807),
        ed448(0x0808),
        /* RSASSA-PSS algorithms with public key OID RSASSA-PSS */
        rsa_pss_pss_sha256(0x0809),
        rsa_pss_pss_sha384(0x080a),
        rsa_pss_pss_sha512(0x080b),
        /* Legacy algorithms */
        rsa_pkcs1_sha1(0x0201),
        ecdsa_sha1(0x0203),
        /* Reserved Code Points */
        obsolete_RESERVED(0x0000..0x0200),
        dsa_sha1_RESERVED(0x0202),
        obsolete_RESERVED(0x0204..0x0400),
        dsa_sha256_RESERVED(0x0402),
        obsolete_RESERVED(0x0404..0x0500),
        dsa_sha384_RESERVED(0x0502),
        obsolete_RESERVED(0x0504..0x0600),
        dsa_sha512_RESERVED(0x0602),
        obsolete_RESERVED(0x0604..0x06FF),
        private_use(0xFE00..0xFFFF),
        (0xFFFF)
    } SignatureScheme;
    struct {
        SignatureScheme supported_signature_algorithms<2..2^16-2>;
    } SignatureSchemeList;

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B.3.1.4. Supported Groups Extension

    enum {
        unallocated_RESERVED(0x0000),
        /* 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 are 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.

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B.3.2. Server Parameters Messages

    opaque DistinguishedName<1..2^16-1>;
    struct {
        DistinguishedName authorities<3..2^16-1>;
    } CertificateAuthoritiesExtension;
    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;
    struct {} PostHandshakeAuth;
    struct {
        Extension extensions<0..2^16-1>;
    } EncryptedExtensions;
    struct {
        opaque certificate_request_context<0..2^8-1>;
        Extension extensions<2..2^16-1>;
    } CertificateRequest;

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B.3.3. Authentication Messages

    enum {
        X509(0),
        OpenPGP_RESERVED(1),
        RawPublicKey(2),
        (255)
    } CertificateType;
    struct {
        select (certificate_type) {
            case RawPublicKey:
              /* From RFC 7250 ASN.1_subjectPublicKeyInfo */
              opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
            case X509:
              opaque cert_data<1..2^24-1>;
        };
        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;

B.3.4. Ticket Establishment

    struct {
        uint32 ticket_lifetime;
        uint32 ticket_age_add;
        opaque ticket_nonce<0..255>;
        opaque ticket<1..2^16-1>;
        Extension extensions<0..2^16-2>;
    } NewSessionTicket;

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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.
            +------------------------------+-------------+
            | 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} |
            +------------------------------+-------------+

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 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 [RFC8439].  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, cipher suites for TLS 1.2 and lower cannot be used with
 TLS 1.3.
 New cipher suite values are assigned by IANA as described in
 Section 11.

Appendix C. Implementation Notes

 The TLS protocol cannot prevent many common security mistakes.  This
 appendix provides several recommendations to assist implementors.
 [TLS13-TRACES] provides test vectors for TLS 1.3 handshakes.

C.1. Random Number Generation and Seeding

 TLS requires a cryptographically secure pseudorandom number generator
 (CSPRNG).  In most cases, the operating system provides an
 appropriate facility such as /dev/urandom, which should be used
 absent other (e.g., performance) concerns.  It is RECOMMENDED to use
 an existing CSPRNG implementation in preference to crafting a new
 one.  Many adequate cryptographic libraries are already available
 under favorable license terms.  Should those prove unsatisfactory,
 [RFC4086] provides guidance on the generation of random values.
 TLS uses random values (1) in public protocol fields such as the
 public Random values in the ClientHello and ServerHello and (2) to
 generate keying material.  With a properly functioning CSPRNG, this
 does not present a security problem, as it is not feasible to
 determine the CSPRNG state from its output.  However, with a broken
 CSPRNG, it may be possible for an attacker to use the public output
 to determine the CSPRNG internal state and thereby predict the keying
 material, as documented in [CHECKOWAY].  Implementations can provide
 extra security against this form of attack by using separate CSPRNGs
 to generate public and private values.

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C.2. Certificates and Authentication

 Implementations are responsible for verifying the integrity of
 certificates and should generally support certificate revocation
 messages.  Absent a specific indication from an application profile,
 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.3. 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:
  1. Do you correctly handle handshake messages that are fragmented to

multiple TLS records (see Section 5.1)? Do you correctly handle

    corner cases like a ClientHello that is split into 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.
  1. Do you ignore the TLS record layer version number in all

unencrypted TLS records (see Appendix D)?

  1. 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
    later, and that attempts to use these obsolete capabilities fail
    correctly (see Appendix D)?
  1. Do you handle TLS extensions in ClientHellos correctly, including

unknown extensions?

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  1. 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)?
  1. 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?
  1. Do you properly ignore unrecognized cipher suites (Section 4.1.2),

hello extensions (Section 4.2), named groups (Section 4.2.7), key

    shares (Section 4.2.8), supported versions (Section 4.2.1), and
    signature algorithms (Section 4.2.3) in the ClientHello?
  1. 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:
  1. What countermeasures do you use to prevent timing attacks

[TIMING]?

  1. When using Diffie-Hellman key exchange, do you correctly preserve

leading zero bytes in the negotiated key (see Section 7.4.1)?

  1. Does your TLS client check that the Diffie-Hellman parameters sent

by the server are acceptable (see Section 4.2.8.1)?

  1. Do you use a strong and, most importantly, properly seeded random

number generator (see Appendix C.1) 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].
  1. Do you zero-pad Diffie-Hellman public key values and shared

secrets to the group size (see Section 4.2.8.1 and Section 7.4.1)?

  1. Do you verify signatures after making them, to protect against

RSA-CRT key leaks [FW15]?

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C.4. 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.5. 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:
  1. Raw public keys [RFC7250].
  1. 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 a mechanism such as channel bindings (though
 the channel bindings described in [RFC5929] are not defined for
 TLS 1.3).  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.

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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 there is at least one
 protocol version supported by both the client and the server.
 Prior versions of TLS used the record layer version number
 (TLSPlaintext.legacy_record_version and
 TLSCiphertext.legacy_record_version) for various purposes.  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 is
 included in the additional data for deprotection but MAY otherwise be
 ignored or MAY be validated to match the fixed constant value.
 Version negotiation is performed using only the handshake versions
 (ClientHello.legacy_version and ServerHello.legacy_version, as well
 as the ClientHello, HelloRetryRequest, and ServerHello
 "supported_versions" extensions).  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 Finished, 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.

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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(s) 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 is not acceptable), the client MUST abort the handshake with a
 "protocol_version" alert.
 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 backward-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 "supported_versions" extension is absent and the
 server only supports versions greater than
 ClientHello.legacy_version, the server 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.

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D.3. 0-RTT Backward 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.  It can then retry
 the connection with 0-RTT disabled.  To avoid a downgrade attack, the
 client SHOULD NOT disable TLS 1.3, only 0-RTT.
 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.

D.4. Middlebox Compatibility Mode

 Field measurements [Ben17a] [Ben17b] [Res17a] [Res17b] have found
 that a significant number of middleboxes misbehave when a TLS
 client/server pair negotiates TLS 1.3.  Implementations can increase
 the chance of making connections through those middleboxes by making
 the TLS 1.3 handshake look more like a TLS 1.2 handshake:
  1. The client always provides a non-empty session ID in the

ClientHello, as described in the legacy_session_id section of

    Section 4.1.2.
  1. If not offering early data, the client sends a dummy

change_cipher_spec record (see the third paragraph of Section 5)

    immediately before its second flight.  This may either be before
    its second ClientHello or before its encrypted handshake flight.
    If offering early data, the record is placed immediately after the
    first ClientHello.
  1. The server sends a dummy change_cipher_spec record immediately

after its first handshake message. This may either be after a

    ServerHello or a HelloRetryRequest.
 When put together, these changes make the TLS 1.3 handshake resemble
 TLS 1.2 session resumption, which improves the chance of successfully
 connecting through middleboxes.  This "compatibility mode" is
 partially negotiated: the client can opt to provide a session ID or
 not, and the server has to echo it.  Either side can send

Rescorla Standards Track [Page 140] RFC 8446 TLS August 2018

 change_cipher_spec at any time during the handshake, as they must be
 ignored by the peer, but if the client sends a non-empty session ID,
 the server MUST send the change_cipher_spec as described in this
 appendix.

D.5. Security Restrictions Related to Backward Compatibility

 Implementations negotiating the use of older versions of TLS SHOULD
 prefer forward secret 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 [RFC6101] is considered insufficient for the
 reasons enumerated in [RFC7568], and it MUST NOT be negotiated for
 any reason.
 The security of SSL 2.0 [SSL2] is considered insufficient for the
 reasons enumerated in [RFC6176], and it 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.legacy_version set to 0x0300 or less.  Any endpoint
 receiving a Hello message with ClientHello.legacy_version or
 ServerHello.legacy_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.

Rescorla Standards Track [Page 141] RFC 8446 TLS August 2018

Appendix E. Overview of Security Properties

 A complete security analysis of TLS is outside the scope of this
 document.  In this appendix, we provide an informal description of
 the desired properties as well as references to more detailed work in
 the research literature which provides more formal definitions.
 We cover properties of the handshake separately from those of the
 record layer.

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:
  1. A set of "session keys" (the various secrets derived from the

master secret) from which can be derived a set of working keys.

  1. A set of cryptographic parameters (algorithms, etc.).
  1. The identities of the communicating parties.
 We assume the attacker to be an active network attacker, which means
 it has complete control over the network used to communicate 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], Definition 1, part 1).
 Secrecy of the session keys:  The shared session keys should be known
    only to the communicating parties and not to the attacker (see
    [CK01], Definition 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.

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 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 independent.
 Downgrade protection:  The cryptographic parameters should be the
    same on both sides and should be the same as if the peers had been
    communicating in the absence of an attack (see [BBFGKZ16],
    Definitions 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) is compromised after the handshake is
    complete, this does not compromise the security of the session key
    (see [DOW92]), as long as the session key itself has been erased.
    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, compromising the
    long-term secret of one actor should not break that actor's
    authentication of their peer in the given connection (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 design and 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

Rescorla Standards Track [Page 143] RFC 8446 TLS August 2018

 connection N, thus providing forward secrecy between the connections.
 In addition, if multiple tickets are established on the same
 connection, they are associated with different keys, so compromise of
 the PSK associated with one ticket does not lead to the compromise of
 connections established with PSKs associated with other tickets.
 This property is most interesting if tickets are stored in a database
 (and so can be deleted) rather than if they are self-encrypted.
 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 current session.  This binding transitively
 includes the original handshake transcript, because that transcript
 is digested into the values which produce the resumption 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.  See Appendix E.1.1 for more on this.  Note: The
 binder does not cover the binder values from other PSKs, though they
 are included in the Finished MAC.
 TLS does not currently permit the server to send a
 certificate_request message in non-certificate-based handshakes
 (e.g., PSK).  If this restriction were to be relaxed in future, the
 client's signature would not cover the server's certificate directly.
 However, if the PSK was established through a NewSessionTicket, the
 client's signature would transitively cover 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 (see
 also [Kraw16]).  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.  Implementations
 MUST NOT combine external PSKs with certificate-based authentication
 of either the client or the server unless negotiated by some
 extension.
 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.

Rescorla Standards Track [Page 144] RFC 8446 TLS August 2018

 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.  See
 [BBFGKZ16] for more details on TLS 1.3 and downgrade.
 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,
 even if the computed shared key is not authenticated.  Because the
 server authenticates before the client, the client can ensure that if
 it authenticates to the server, it only reveals its identity to an
 authenticated server.  Note that implementations must use the
 provided record-padding mechanism during the handshake to avoid
 leaking information about the identities due to length.  The client's
 proposed PSK identities are not encrypted, nor is the one that the
 server selects.

E.1.1. Key Derivation and HKDF

 Key derivation in TLS 1.3 uses HKDF as defined in [RFC5869] and its
 two components, HKDF-Extract and HKDF-Expand.  The full rationale for
 the HKDF construction can be found in [Kraw10] and the rationale for
 the way it is used in TLS 1.3 in [KW16].  Throughout this document,
 each application of HKDF-Extract is followed by one or more
 invocations of HKDF-Expand.  This ordering should always be followed
 (including in future revisions of this document); in particular, one
 SHOULD NOT use an output of HKDF-Extract as an input to another
 application of HKDF-Extract without an HKDF-Expand in between.
 Multiple applications of HKDF-Expand to some of the same inputs are
 allowed as long as these are differentiated via the key and/or the
 labels.
 Note that HKDF-Expand implements a pseudorandom function (PRF) with
 both inputs and outputs of variable length.  In some of the uses of
 HKDF in this document (e.g., for generating exporters and the
 resumption_master_secret), it is necessary that the application of
 HKDF-Expand be collision resistant; namely, it should be infeasible
 to find two different inputs to HKDF-Expand that output the same
 value.  This requires the underlying hash function to be collision
 resistant and the output length from HKDF-Expand to be of size at
 least 256 bits (or as much as needed for the hash function to prevent
 finding collisions).

Rescorla Standards Track [Page 145] RFC 8446 TLS August 2018

E.1.2. Client Authentication

 A client that has sent authentication data to a server, either during
 the handshake or in post-handshake authentication, cannot be sure
 whether 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.  In addition, the analysis of
 post-handshake authentication from [Kraw16] shows that the client
 identified by the certificate sent in the post-handshake phase
 possesses the traffic key.  This party is therefore the client that
 participated in the original handshake or one to whom the original
 client delegated the traffic key (assuming that the traffic key has
 not been compromised).

E.1.3. 0-RTT

 The 0-RTT mode of operation generally provides security properties
 similar to those of 1-RTT data, with the two exceptions that the
 0-RTT encryption keys do not provide full forward secrecy and that
 the server is not able to guarantee uniqueness of the handshake
 (non-replayability) without keeping potentially undue amounts of
 state.  See Section 8 for mechanisms to limit the exposure to replay.

E.1.4. Exporter Independence

 The exporter_master_secret and early_exporter_master_secret are
 derived to be independent of the traffic keys and therefore do not
 represent a threat to the security of traffic encrypted with those
 keys.  However, because these secrets can be used to compute any
 exporter value, they SHOULD be erased as soon as possible.  If the
 total set of exporter labels is known, then implementations SHOULD
 pre-compute the inner Derive-Secret stage of the exporter computation
 for all those labels, then erase the [early_]exporter_master_secret,
 followed by each inner value as soon as it is known that it will not
 be needed again.

E.1.5. Post-Compromise Security

 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 backward or future secrecy.
 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.

Rescorla Standards Track [Page 146] RFC 8446 TLS August 2018

E.1.6. External References

 The reader should refer to the following references for analysis of
 the TLS handshake: [DFGS15], [CHSV16], [DFGS16], [KW16], [Kraw16],
 [FGSW16], [LXZFH16], [FG17], and [BBK17].

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 secrecy 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
 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.  In order to prevent mass cryptanalysis when the same
 plaintext is repeatedly encrypted by different users under the same
 key (as is commonly the case for HTTP), the nonce is formed by mixing

Rescorla Standards Track [Page 147] RFC 8446 TLS August 2018

 the sequence number with a secret per-connection initialization
 vector derived along with the traffic keys.  See [BT16] for analysis
 of this construction.
 The rekeying technique in TLS 1.3 (see Section 7.2) follows the
 construction of the serial generator as discussed in [REKEY], which
 shows that rekeying can allow keys to be used for a larger number of
 encryptions than without rekeying.  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.

E.2.1. External References

 The reader should refer to the following references for analysis of
 the TLS record layer: [BMMRT15], [BT16], [BDFKPPRSZZ16], [BBK17], and
 [PS18].

E.3. Traffic Analysis

 TLS is susceptible to a variety of traffic analysis attacks based on
 observing the length and timing of encrypted packets [CLINIC]
 [HCJC16].  This is particularly easy when there is a small set of
 possible messages to be distinguished, such as for a video server
 hosting a fixed corpus of content, but still provides usable
 information even in more complicated scenarios.
 TLS does not provide any specific defenses against this form of
 attack but does include a padding mechanism for use by applications:
 The plaintext protected by the AEAD function consists of content plus
 variable-length padding, which allows the application to produce
 arbitrary-length encrypted records as well as padding-only cover
 traffic to conceal the difference between periods of transmission and
 periods of silence.  Because the padding is encrypted alongside the
 actual content, an 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 or trickling in records to see

Rescorla Standards Track [Page 148] RFC 8446 TLS August 2018

 which ones elicit a response from the server).  In general, it is not
 known how to remove all of these channels because even a
 constant-time padding removal function will likely feed the content
 into data-dependent functions.  At minimum, a fully constant-time
 server or client would require close cooperation with the
 application-layer protocol implementation, including making that
 higher-level protocol constant time.
 Note: Robust traffic analysis defenses will likely lead to inferior
 performance due to delays in transmitting packets and increased
 traffic volume.

E.4. Side-Channel Attacks

 In general, TLS does not have specific defenses against side-channel
 attacks (i.e., those which attack the communications via secondary
 channels such as timing), leaving those to the implementation of the
 relevant cryptographic primitives.  However, certain features of TLS
 are designed to make it easier to write side-channel resistant code:
  1. Unlike previous versions of TLS which used a composite MAC-then-

encrypt structure, TLS 1.3 only uses AEAD algorithms, allowing

    implementations to use self-contained constant-time
    implementations of those primitives.
  1. TLS uses a uniform "bad_record_mac" alert for all decryption

errors, which is intended to prevent an attacker from gaining

    piecewise insight into portions of the message.  Additional
    resistance is provided by terminating the connection on such
    errors; a new connection will have different cryptographic
    material, preventing attacks against the cryptographic primitives
    that require multiple trials.
 Information leakage through side channels can occur at layers above
 TLS, in application protocols and the applications that use them.
 Resistance to side-channel attacks depends on applications and
 application protocols separately ensuring that confidential
 information is not inadvertently leaked.

Rescorla Standards Track [Page 149] RFC 8446 TLS August 2018

E.5. Replay Attacks on 0-RTT

 Replayable 0-RTT data presents a number of security threats to TLS-
 using applications, unless those applications are specifically
 engineered to be safe under replay (minimally, this means idempotent,
 but in many cases may also require other stronger conditions, such as
 constant-time response).  Potential attacks include:
  1. Duplication of actions which cause side effects (e.g., purchasing

an item or transferring money) to be duplicated, thus harming the

    site or the user.
  1. Attackers can store and replay 0-RTT messages in order to reorder

them with respect to other messages (e.g., moving a delete to

    after a create).
  1. Exploiting cache timing behavior to discover the content of 0-RTT

messages by replaying a 0-RTT message to a different cache node

    and then using a separate connection to measure request latency,
    to see if the two requests address the same resource.
 If data can be replayed a large number of times, additional attacks
 become possible, such as making repeated measurements of the speed of
 cryptographic operations.  In addition, they may be able to overload
 rate-limiting systems.  For a further description of these attacks,
 see [Mac17].
 Ultimately, servers have the responsibility to protect themselves
 against attacks employing 0-RTT data replication.  The mechanisms
 described in Section 8 are intended to prevent replay at the TLS
 layer but do not provide complete protection against receiving
 multiple copies of client data.  TLS 1.3 falls back to the 1-RTT
 handshake when the server does not have any information about the
 client, e.g., because it is in a different cluster which does not
 share state or because the ticket has been deleted as described in
 Section 8.1.  If the application-layer protocol retransmits data in
 this setting, then it is possible for an attacker to induce message
 duplication by sending the ClientHello to both the original cluster
 (which processes the data immediately) and another cluster which will
 fall back to 1-RTT and process the data upon application-layer
 replay.  The scale of this attack is limited by the client's
 willingness to retry transactions and therefore only allows a limited
 amount of duplication, with each copy appearing as a new connection
 at the server.

Rescorla Standards Track [Page 150] RFC 8446 TLS August 2018

 If implemented correctly, the mechanisms described in Sections 8.1
 and 8.2 prevent a replayed ClientHello and its associated 0-RTT data
 from being accepted multiple times by any cluster with consistent
 state; for servers which limit the use of 0-RTT to one cluster for a
 single ticket, then a given ClientHello and its associated 0-RTT data
 will only be accepted once.  However, if state is not completely
 consistent, then an attacker might be able to have multiple copies of
 the data be accepted during the replication window.  Because clients
 do not know the exact details of server behavior, they MUST NOT send
 messages in early data which are not safe to have replayed and which
 they would not be willing to retry across multiple 1-RTT connections.
 Application 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 and how to handle the
 situation when the server rejects 0-RTT and falls back to 1-RTT.
 In addition, to avoid accidental misuse, TLS implementations MUST NOT
 enable 0-RTT (either sending or accepting) unless specifically
 requested by the application and MUST NOT automatically resend 0-RTT
 data if it is rejected by the server unless instructed by the
 application.  Server-side applications may wish to implement special
 processing for 0-RTT data for some kinds of application traffic
 (e.g., abort the connection, request that data be resent at the
 application layer, or delay processing until the handshake
 completes).  In order to allow applications to implement this kind of
 processing, TLS implementations MUST provide a way for the
 application to determine if the handshake has completed.

E.5.1. Replay and Exporters

 Replays of the ClientHello produce the same early exporter, thus
 requiring additional care by applications which use these exporters.
 In particular, if these exporters are used as an authentication
 channel binding (e.g., by signing the output of the exporter), an
 attacker who compromises the PSK can transplant authenticators
 between connections without compromising the authentication key.
 In addition, the early exporter SHOULD NOT be used to generate
 server-to-client encryption keys because that would entail the reuse
 of those keys.  This parallels the use of the early application
 traffic keys only in the client-to-server direction.

Rescorla Standards Track [Page 151] RFC 8446 TLS August 2018

E.6. PSK Identity Exposure

 Because implementations respond to an invalid PSK binder by aborting
 the handshake, it may be possible for an attacker to verify whether a
 given PSK identity is valid.  Specifically, if a server accepts both
 external-PSK handshakes and certificate-based handshakes, a valid PSK
 identity will result in a failed handshake, whereas an invalid
 identity will just be skipped and result in a successful certificate
 handshake.  Servers which solely support PSK handshakes may be able
 to resist this form of attack by treating the cases where there is no
 valid PSK identity and where there is an identity but it has an
 invalid binder identically.

E.7. Sharing PSKs

 TLS 1.3 takes a conservative approach to PSKs by binding them to a
 specific KDF.  By contrast, TLS 1.2 allows PSKs to be used with any
 hash function and the TLS 1.2 PRF.  Thus, any PSK which is used with
 both TLS 1.2 and TLS 1.3 must be used with only one hash in TLS 1.3,
 which is less than optimal if users want to provision a single PSK.
 The constructions in TLS 1.2 and TLS 1.3 are different, although they
 are both based on HMAC.  While there is no known way in which the
 same PSK might produce related output in both versions, only limited
 analysis has been done.  Implementations can ensure safety from
 cross-protocol related output by not reusing PSKs between TLS 1.3 and
 TLS 1.2.

E.8. Attacks on Static RSA

 Although TLS 1.3 does not use RSA key transport and so is not
 directly susceptible to Bleichenbacher-type attacks [Blei98], if TLS
 1.3 servers also support static RSA in the context of previous
 versions of TLS, then it may be possible to impersonate the server
 for TLS 1.3 connections [JSS15].  TLS 1.3 implementations can prevent
 this attack by disabling support for static RSA across all versions
 of TLS.  In principle, implementations might also be able to separate
 certificates with different keyUsage bits for static RSA decryption
 and RSA signature, but this technique relies on clients refusing to
 accept signatures using keys in certificates that do not have the
 digitalSignature bit set, and many clients do not enforce this
 restriction.

Rescorla Standards Track [Page 152] RFC 8446 TLS August 2018

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
 Richard Barnes
 Cisco
 rlb@ipv.sx
 Steven M. Bellovin
 Columbia University
 smb@cs.columbia.edu
 David Benjamin
 Google
 davidben@google.com
 Benjamin Beurdouche
 INRIA & Microsoft Research
 benjamin.beurdouche@ens.fr
 Karthikeyan Bhargavan
 (editor 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

Rescorla Standards Track [Page 153] RFC 8446 TLS August 2018

 Matt Caswell
 OpenSSL
 matt@openssl.org
 Stephen Checkoway
 University of Illinois at Chicago
 sfc@uic.edu
 Pete Chown
 Skygate Technology Ltd
 pc@skygate.co.uk
 Katriel Cohn-Gordon
 University of Oxford
 me@katriel.co.uk
 Cas Cremers
 University of Oxford
 cas.cremers@cs.ox.ac.uk
 Antoine Delignat-Lavaud
 (co-author of [RFC7627])
 INRIA
 antdl@microsoft.com
 Tim Dierks
 (co-author of TLS 1.0, co-editor of TLS 1.1 and 1.2)
 Independent
 tim@dierks.org
 Roelof DuToit
 Symantec Corporation
 roelof_dutoit@symantec.com
 Taher Elgamal
 Securify
 taher@securify.com
 Pasi Eronen
 Nokia
 pasi.eronen@nokia.com
 Cedric Fournet
 Microsoft
 fournet@microsoft.com

Rescorla Standards Track [Page 154] RFC 8446 TLS August 2018

 Anil Gangolli
 anil@busybuddha.org
 David M. Garrett
 dave@nulldereference.com
 Illya Gerasymchuk
 Independent
 illya@iluxonchik.me
 Alessandro Ghedini
 Cloudflare Inc.
 alessandro@cloudflare.com
 Daniel Kahn Gillmor
 ACLU
 dkg@fifthhorseman.net
 Matthew Green
 Johns Hopkins University
 mgreen@cs.jhu.edu
 Jens Guballa
 ETAS
 jens.guballa@etas.com
 Felix Guenther
 TU Darmstadt
 mail@felixguenther.info
 Vipul Gupta
 (co-author of [RFC4492])
 Sun Microsystems Laboratories
 vipul.gupta@sun.com
 Chris Hawk
 (co-author of [RFC4492])
 Corriente Networks LLC
 chris@corriente.net
 Kipp Hickman
 Alfred Hoenes
 David Hopwood
 Independent Consultant
 david.hopwood@blueyonder.co.uk

Rescorla Standards Track [Page 155] RFC 8446 TLS August 2018

 Marko Horvat
 MPI-SWS
 mhorvat@mpi-sws.org
 Jonathan Hoyland
 Royal Holloway, University of London
 jonathan.hoyland@gmail.com
 Subodh Iyengar
 Facebook
 subodh@fb.com
 Benjamin Kaduk
 Akamai Technologies
 kaduk@mit.edu
 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
 hugokraw@us.ibm.com
 Adam Langley
 (co-author of [RFC7627])
 Google
 agl@google.com
 Olivier Levillain
 ANSSI
 olivier.levillain@ssi.gouv.fr

Rescorla Standards Track [Page 156] RFC 8446 TLS August 2018

 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
 Colm MacCarthaigh
 Amazon Web Services
 colm@allcosts.net
 Carl Mehner
 USAA
 carl.mehner@usaa.com
 Jan Mikkelsen
 Transactionware
 janm@transactionware.com
 Bodo Moeller
 (co-author of [RFC4492])
 Google
 bodo@acm.org
 Kyle Nekritz
 Facebook
 knekritz@fb.com
 Erik Nygren
 Akamai Technologies
 erik+ietf@nygren.org
 Magnus Nystrom
 Microsoft
 mnystrom@microsoft.com
 Kazuho Oku
 DeNA Co., Ltd.
 kazuhooku@gmail.com

Rescorla Standards Track [Page 157] RFC 8446 TLS August 2018

 Kenny Paterson
 Royal Holloway, University of London
 kenny.paterson@rhul.ac.uk
 Christopher Patton
 University of Florida
 cjpatton@ufl.edu
 Alfredo Pironti
 (co-author of [RFC7627])
 INRIA
 alfredo.pironti@inria.fr
 Andrei Popov
 Microsoft
 andrei.popov@microsoft.com
 Marsh Ray
 (co-author of [RFC7627])
 Microsoft
 maray@microsoft.com
 Robert Relyea
 Netscape Communications
 relyea@netscape.com
 Kyle Rose
 Akamai Technologies
 krose@krose.org
 Jim Roskind
 Amazon
 jroskind@amazon.com
 Michael Sabin
 Joe Salowey
 Tableau Software
 joe@salowey.net
 Rich Salz
 Akamai
 rsalz@akamai.com
 David Schinazi
 Apple Inc.
 dschinazi@apple.com

Rescorla Standards Track [Page 158] RFC 8446 TLS August 2018

 Sam Scott
 Royal Holloway, University of London
 me@samjs.co.uk
 Thomas Shrimpton
 University of Florida
 teshrim@ufl.edu
 Dan Simon
 Microsoft, Inc.
 dansimon@microsoft.com
 Brian Smith
 Independent
 brian@briansmith.org
 Brian Sniffen
 Akamai Technologies
 ietf@bts.evenmere.org
 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
 Hannes Tschofenig
 Arm Limited
 Hannes.Tschofenig@arm.com
 Sean Turner
 sn3rd
 sean@sn3rd.com
 Steven Valdez
 Google
 svaldez@google.com

Rescorla Standards Track [Page 159] RFC 8446 TLS August 2018

 Filippo Valsorda
 Cloudflare Inc.
 filippo@cloudflare.com
 Thyla van der Merwe
 Royal Holloway, University of London
 tjvdmerwe@gmail.com
 Victor Vasiliev
 Google
 vasilvv@google.com
 Hoeteck Wee
 Ecole Normale Superieure, Paris
 hoeteck@alum.mit.edu
 Tom Weinstein
 David Wong
 NCC Group
 david.wong@nccgroup.trust
 Christopher A. Wood
 Apple Inc.
 cawood@apple.com
 Tim Wright
 Vodafone
 timothy.wright@vodafone.com
 Peter Wu
 Independent
 peter@lekensteyn.nl
 Kazu Yamamoto
 Internet Initiative Japan Inc.
 kazu@iij.ad.jp

Author's Address

 Eric Rescorla
 Mozilla
 Email: ekr@rtfm.com

Rescorla Standards Track [Page 160]

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