Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 5077, 5246, 5746 (if                           March 21,                             May 22, 2016
           approved)
Updates: 4492 4492, 6066, 6961 (if approved)
Intended status: Standards Track
Expires: September 22, November 23, 2016

        The Transport Layer Security (TLS) Protocol Version 1.3
                        draft-ietf-tls-tls13-12
                        draft-ietf-tls-tls13-13

Abstract

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 22, November 23, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   5
     1.2.  Major Differences from TLS 1.2  . . . . . . . . . . . . .   6
   2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
   3.  Goals of This Document  . . . . . . . . . . . . . . . . . . .  10
   4.  Presentation Language . . . . . . . . . . . . . . . . . . . .  10  11
     4.1.  Basic Block Size  . . . . . . . . . . . . . . . . . . . .  10  11
     4.2.  Miscellaneous . . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Vectors . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Enumerateds . . . . . . . . . . . . . . . . . . . . . . .  12  13
     4.6.  Constructed Types . . . . . . . . . . . . . . . . . . . .  13  14
       4.6.1.  Variants  . . . . . . . . . . . . . . . . . . . . . .  14
     4.7.  Constants . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.8.  Cryptographic Attributes  . . . . . . . . . . . . . . . .  15
       4.8.1.  Digital Signing . . . . . . . . . . . . . . . . . . .  16
       4.8.2.  Authenticated Encryption with Additional Data (AEAD)   17
   5.  The TLS Record Protocol . . . . . . . . . . . . . . . . . . .  17
     5.1.  Connection States . . . . . . . . . . . . . . . . . . . .  18
     5.2.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  20
       5.2.1.  Fragmentation . . . . . . . . . . . . . . . . . . . .  20
       5.2.2.  Record Payload Protection . . . . . . . . . . . . . .  21  22
       5.2.3.  Record Padding  . . . . . . . . . . . . . . . . . . .  24
   6.  The TLS Handshaking Protocols . . . . . . . . . . . . . . . .  25
     6.1.  Alert Protocol  . . . . . . . . . . . . . . . . . . . . .  25  26
       6.1.1.  Closure Alerts  . . . . . . . . . . . . . . . . . . .  26  27
       6.1.2.  Error Alerts  . . . . . . . . . . . . . . . . . . . .  28  29
     6.2.  Handshake Protocol Overview . . . . . . . . . . . . . . .  31  32
       6.2.1.  Incorrect DHE Share . . . . . . . . . . . . . . . . .  35
       6.2.2.  Zero-RTT Exchange  Resumption and Pre-Shared Key (PSK) . . . . . . . . .  36
       6.2.3.  Zero-RTT Data . . . . . . . . .  36
       6.2.3.  Resumption and Pre-Shared Key (PSK) . . . . . . . . .  37 . .  38

     6.3.  Handshake Protocol  . . . . . . . . . . . . . . . . . . .  39
       6.3.1.  Key Exchange Messages . . . . . . . . . . . . . . . .  40
       6.3.2.  Hello Extensions  . . . . . . . . . . . . . . . . . .  46
       6.3.3.  Server Parameters . . . . . . . . . . . . . . . . . .  59  60
       6.3.4.  Authentication Messages . . . . . . . . . . . . . . .  63
       6.3.5.  Post-Handshake Messages . . . . . . . . . . . . . . .  71
   7.  Cryptographic Computations  . . . . . . . . . . . . . . . . .  73  74
     7.1.  Key Schedule  . . . . . . . . . . . . . . . . . . . . . .  73  74
     7.2.  Updating Traffic Keys and IVs . . . . . . . . . . . . . .  76
     7.3.  Traffic Key Calculation . . . . . . . . . . . . . . . . .  76
       7.3.1.  The Handshake Hash  . . . . . . . . . . . . . . . . .  77
       7.3.2.  Diffie-Hellman  . . . . . . . . . . . . . . . . . . .  78
       7.3.3.  77
       7.3.2.  Elliptic Curve Diffie-Hellman . . . . . . . . . . . .  78
       7.3.4.
       7.3.3.  Exporters . . . . . . . . . . . . . . . . . . . . . .  78
   8.  Mandatory Algorithms  . . . . . . . . . . . . . . . . . . . .  79
     8.1.  MTI Cipher Suites . . . . . . . . . . . . . . . . . . . .  79
     8.2.  MTI Extensions  . . . . . . . . . . . . . . . . . . . . .  79
   9.  Application Data Protocol . . . . . . . . . . . . . . . . . .  80
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  80
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  81  80
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  83
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  84  83
     12.2.  Informative References . . . . . . . . . . . . . . . . .  86
   Appendix A.  Protocol Data Structures and Constant Values . . . .  92
     A.1.  Record Layer  . . . . . . . . . . . . . . . . . . . . . .  92
     A.2.  Alert Messages  . . . . . . . . . . . . . . . . . . . . .  92
     A.3.  Handshake Protocol  . . . . . . . . . . . . . . . . . . .  94
       A.3.1.  Key Exchange Messages . . . . . . . . . . . . . . . .  94
       A.3.2.  Server Parameters Messages  . . . . . . . . . . . . .  98
       A.3.3.  Authentication Messages . . . . . . . . . . . . . . .  99
       A.3.4.  Ticket Establishment  . . . . . . . . . . . . . . . . 100  99
     A.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 100
       A.4.1.  Unauthenticated Operation . . . . . . . . . . . . . . 105
     A.5.  The Security Parameters . . . . . . . . . . . . . . . . . 105
     A.6.  Changes to RFC 4492 . . . . . . . . . . . . . . . . . . . 106
   Appendix B.  Implementation Notes . . . . . . . . . . . . . . . . 107
     B.1.  Random Number Generation and Seeding  . . . . . . . . . . 107
     B.2.  Certificates and Authentication . . . . . . . . . . . . . 107
     B.3.  Cipher Suite Support  . . . . . . . . . . . . . . . . . . 107
     B.4.  Implementation Pitfalls . . . . . . . . . . . . . . . . . 107
     B.5.  Client Tracking Prevention  . . . . . . . . . . . . . . . 109
   Appendix C.  Backward Compatibility . . . . . . . . . . . . . . . 109
     C.1.  Negotiating with an older server  . . . . . . . . . . . . 109 110
     C.2.  Negotiating with an older client  . . . . . . . . . . . . 110 111
     C.3.  Zero-RTT backwards compatibility  . . . . . . . . . . . . 111
     C.4.  Backwards Compatibility Security Restrictions . . . . . . 110 111
   Appendix D.  Security Analysis  . . . . . . . . . . . . . . . . . 111 112
     D.1.  Handshake Protocol  . . . . . . . . . . . . . . . . . . . 112 113
       D.1.1.  Authentication and Key Exchange . . . . . . . . . . . 112 113
       D.1.2.  Version Rollback Attacks  . . . . . . . . . . . . . . 113 114
       D.1.3.  Detecting Attacks Against the Handshake Protocol  . . 113 114
     D.2.  Protecting Application Data . . . . . . . . . . . . . . . 113 114
     D.3.  Denial of Service . . . . . . . . . . . . . . . . . . . . 114 115
     D.4.  Final Notes . . . . . . . . . . . . . . . . . . . . . . . 114 115
   Appendix E.  Working Group Information  . . . . . . . . . . . . . 114 115
   Appendix F.  Contributors . . . . . . . . . . . . . . . . . . . . 114 115
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 118 119

1.  Introduction

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

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

   The primary goal of the TLS protocol is to provide privacy and data
   integrity between two communicating peers.  The TLS protocol is
   composed of two layers: the TLS Record Protocol and the TLS Handshake
   Protocol.  At the lowest level, layered on top of some reliable
   transport protocol (e.g., TCP [RFC0793]), is the TLS Record Protocol.
   The TLS Record Protocol provides connection security that has two
   basic properties:

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

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

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

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

   -  The peer's identity can be authenticated using asymmetric, or
      public key, asymmetric (public
      key) cryptography (e.g., RSA [RSA], ECDSA [ECDSA]).  This [ECDSA]) or a pre-shared
      symmetric key.  The TLS server is always authenticated; client
      authentication can be made optional, but is generally required for
      at least one of the peers. optional.

   -  The negotiation of a shared secret is secure: the negotiated
      secret is unavailable to eavesdroppers, and for any authenticated
      connection the secret cannot be obtained, even by an attacker who
      can place himself in the middle of the connection.

   -  The negotiation is reliable: no attacker can modify the
      negotiation communication without being detected by the parties to
      the communication.

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

1.1.  Conventions and Terminology

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

   The following terms are used:

   client: The endpoint initiating the TLS connection.

   connection: A transport-layer connection between two endpoints.

   endpoint: Either the client or server of the connection.

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

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

   receiver: An endpoint that is receiving records.

   sender: An endpoint that is transmitting records.

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

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

1.2.  Major Differences from TLS 1.2

   draft-13

   -  Allow server to send SupportedGroups.

   -  Remove 0-RTT client authentication

   -  Remove (EC)DHE 0-RTT.

   -  Flesh out 0-RTT PSK mode and shrink EarlyDataIndiation

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

   -  Move CertificateStatus to an extension

   -  Extra fields in NewSessionTicket.

   -  Restructure key schedule and add a resumption_context value.

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

   -  Remove the redundant length fields in KeyShareEntry.

   -  Define a cookie field for HRR.

   draft-12

   -  Provide a list of the PSK cipher sutes. suites.

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

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

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

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

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

   -  Define exporters.

   -  Editorial cleanup

   draft-11

   -  Port the CFRG curves & signatures work from RFC4492bis.

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

   -  Reorder values in HkdfLabel.

   -  Add support for version anti-downgrade mechanism.

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

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

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

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

   draft-10

   -  Remove ClientCertificateTypes field from CertificateRequest and
      add extensions.

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

   draft-09

   -  Change to RSA-PSS signatures for handshake messages.

   -  Remove support for DSA.

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

   -  Add support for per-record padding.

   -  Switch to encrypted record ContentType.

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

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

   -  Deprecate SHA-1 with signatures.

   -  Add MTI algorithms.

   draft-08

   -  Remove support for weak and lesser used named curves.

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

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

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

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

   -  Merge EarlyDataIndication and KnownConfiguration.

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

   -  Relax certificate_list ordering requirement to match current
      practice.

   draft-07

   -  Integration of semi-ephemeral DH proposal.

   -  Add initial 0-RTT support.

   -  Remove resumption and replace with PSK + tickets.

   -  Move ClientKeyShare into an extension.

   -  Move to HKDF.

   draft-06

   -  Prohibit RC4 negotiation for backwards compatibility.

   -  Freeze & deprecate record layer version field.

   -  Update format of signatures with context.

   -  Remove explicit IV.

   draft-05

   -  Prohibit SSL negotiation for backwards compatibility.

   -  Fix which MS is used for exporters.

   draft-04

   -  Modify key computations to include session hash.

   -  Remove ChangeCipherSpec.

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

   -  Remove renegotiation.

   -  Remove point format negotiation.

   draft-03

   -  Remove GMT time.

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

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

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

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

   draft-02

   -  Increment version number.

   -  Rework handshake to provide 1-RTT mode.

   -  Remove custom DHE groups.

   -  Remove support for compression.

   -  Remove support for static RSA and DH key exchange.

   -  Remove support for non-AEAD ciphers.

2.  Goals

   The goals of the TLS protocol, in order of priority, are as follows:

   1.  Cryptographic security: TLS should be used to establish a secure
       connection between two parties.

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

   3.  Extensibility: TLS seeks to provide a framework into which new
       public key and record protection methods can be incorporated as
       necessary.  This will also accomplish two sub-goals: preventing
       the need to create a new protocol (and risking the introduction
       of possible new weaknesses) and avoiding the need to implement an
       entire new security library.

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

3.  Goals of This Document

   This document and the TLS protocol itself have evolved from the SSL
   3.0 Protocol Specification as published by Netscape.  The differences
   between this version and previous versions are significant enough
   that the various versions of TLS and SSL 3.0 do not interoperate
   (although each protocol incorporates a mechanism by which an
   implementation can back down to prior versions).  This document is
   intended primarily for readers who will be implementing the protocol
   and for those doing cryptographic analysis of it.  The specification
   has been written with this in mind, and it is intended to reflect the
   needs of those two groups.  For that reason, many of the algorithm-
   dependent data structures and rules are included in the body of the
   text (as opposed to in an appendix), providing easier access to them.

   This document is not intended to supply any details of service
   definition or of interface definition, although it does cover select
   areas of policy as they are required for the maintenance of solid
   security.

4.  Presentation Language

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

4.1.  Basic Block Size

   The representation of all data items is explicitly specified.  The
   basic data block size is one byte (i.e., 8 bits).  Multiple byte data
   items are concatenations of bytes, from left to right, from top to
   bottom.  From the byte stream, a multi-byte item (a numeric in the
   example) is formed (using C notation) by:

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

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

4.2.  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.

4.3.  Vectors

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

      T T'[n];

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

   In the following example, Datum is defined to be three consecutive
   bytes that the protocol does not interpret, while Data is three
   consecutive Datum, consuming a total of nine bytes.

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

   Variable-length vectors are defined by specifying a subrange of legal
   lengths, inclusively, using the notation <floor..ceiling>.  When
   these are encoded, the actual length precedes the vector's contents
   in the byte stream.  The length will be in the form of a number
   consuming as many bytes as required to hold the vector's specified
   maximum (ceiling) length.  A variable-length vector with an actual
   length field of zero is referred to as an empty vector.

      T T'<floor..ceiling>;

   In the following example, mandatory is a vector that must contain
   between 300 and 400 bytes of type opaque.  It can never be empty.
   The actual length field consumes two bytes, a uint16, which is
   sufficient to represent the value 400 (see Section 4.4).  On the
   other hand, longer can represent up to 800 bytes of data, or 400
   uint16 elements, and it may be empty.  Its encoding will include a
   two-byte actual length field prepended to the vector.  The length of
   an encoded vector must be an even multiple of the length of a single
   element (for example, a 17-byte vector of uint16 would be illegal).

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

4.4.  Numbers

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

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

   All values, here and elsewhere in the specification, are stored in
   network byte (big-endian) order; the uint32 represented by the hex
   bytes 01 02 03 04 is equivalent to the decimal value 16909060.

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

4.5.  Enumerateds

   An additional sparse data type is available called enum.  A field of
   type enum can only assume the values declared in the definition.
   Each definition is a different type.  Only enumerateds of the same
   type may be assigned or compared.  Every element of an enumerated
   must be assigned a value, as demonstrated in the following example.
   Since the elements of the enumerated are not ordered, they can be
   assigned any unique value, in any order.

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

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

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

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

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

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

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

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

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

      enum { low, medium, high } Amount;

4.6.  Constructed Types

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

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

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

4.6.1.  Variants

   Defined structures may have variants based on some knowledge that is
   available within the environment.  The selector must be an enumerated
   type that defines the possible variants the structure defines.  There
   must be a case arm for every element of the enumeration declared in
   the select.  Case arms have limited fall-through: if two case arms
   follow in immediate succession with no fields in between, then they
   both contain the same fields.  Thus, in the example below, "orange"
   and "banana" both contain V2.  Note that this is a new piece of
   syntax in TLS 1.2.

   The body of the variant structure may be given a label for reference.
   The mechanism by which the variant is selected at runtime is not
   prescribed by the presentation language.

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

   For example:

      enum { apple, orange, banana } VariantTag;

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

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

      struct {
          select (VariantTag) { /* value of selector is implicit */
              case apple:
                V1;   /* VariantBody, tag = apple */
              case orange:
              case banana:
                V2;   /* VariantBody, tag = orange or banana */
          } variant_body;       /* optional label on variant */
      } VariantRecord;

4.7.  Constants

   Typed constants can be defined for purposes of specification by
   declaring a symbol of the desired type and assigning values to it.

   Under-specified types (opaque, variable-length vectors, and
   structures that contain opaque) cannot be assigned values.  No fields
   of a multi-element structure or vector may be elided.

   For example:

      struct {
          uint8 f1;
          uint8 f2;
      } Example1;

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

4.8.  Cryptographic Attributes

   The two cryptographic operations -- digital signing, and
   authenticated encryption with additional data (AEAD) -- are
   designated digitally-signed, and aead-ciphered, respectively.  A
   field's cryptographic processing is specified by prepending an
   appropriate key word designation before the field's type
   specification.  Cryptographic keys are implied by the current session
   state (see Section 5.1).

4.8.1.  Digital Signing

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

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

   The algorithm field specifies the algorithm used (see Section 6.3.2.1 6.3.2.2
   for the definition of this field).  The signature is a digital
   signature using those algorithms over the contents of the element.
   The contents themselves do not appear on the wire but are simply
   calculated.  The length of the signature is specified by the signing
   algorithm and key.

   In previous versions of TLS, the ServerKeyExchange format meant that
   attackers can obtain a signature of a message with a chosen, 32-byte
   prefix.  Because TLS 1.3 servers are likely to also implement prior
   versions, the contents of the element always start with 64 bytes of
   octet 32 in order to clear that chosen-prefix.

   Following that padding is a context string used to disambiguate
   signatures for different purposes.  The context string will be
   specified whenever a digitally-signed element is used.  A single 0
   byte is appended to the context to act as a separator.

   Finally, the specified contents of the digitally-signed structure
   follow the 0 byte after the context string.  (See the example at the
   end of this section.)

   The combined input is then fed into the corresponding signature
   algorithm to produce the signature value on the wire.  See
   Section 6.3.2.1 6.3.2.2 for algorithms defined in this specification.

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

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

      2020202020202020202020202020202020202020202020202020202020202020
      2020202020202020202020202020202020202020202020202020202020202020
      4578616d706c6500

   followed by the encoding of the inner struct (field3 and field4).

   The length of the structure, in bytes, would be equal to two bytes
   for field1 and field2, plus two bytes for the signature algorithm,
   plus two bytes for the length of the signature, plus the length of
   the output of the signing algorithm.  The length of the signature is
   known because the algorithm and key used for the signing are known
   prior to encoding or decoding this structure.

4.8.2.  Authenticated Encryption with Additional Data (AEAD)

   In AEAD encryption, the plaintext is simultaneously encrypted and
   integrity protected.  The input may be of any length, and aead-
   ciphered output is generally larger than the input in order to
   accommodate the integrity check value.

5.  The TLS Record Protocol

   The TLS Record Protocol is a layered protocol.  At each layer,
   messages may include fields for length, description, and content.
   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 decrypted and verified, reassembled,
   and then delivered to higher-level clients.

   Three protocols that use the TLS Record Protocol are described in
   this document: the TLS Handshake Protocol, the Alert Protocol, and
   the application data protocol.  In order to allow extension of the
   TLS protocol, additional record content types can be supported by the
   TLS Record Protocol.  New record content type values are assigned by
   IANA in the TLS Content Type Registry as described in Section 11.

   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 send an
   "unexpected_message" alert.

   Any protocol designed for use over TLS must be carefully designed to
   deal with all possible attacks against it.  As a practical matter,
   this means that the protocol designer must be aware of what security
   properties TLS does and does not provide and cannot safely rely on
   the latter.

   Note in particular that the length of a record or absence of traffic
   itself is not protected by encryption unless the sender uses the
   supplied padding mechanism - see Section 5.2.3 for more details.

5.1.  Connection States

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

   A TLS connection state is the operating environment of the TLS Record
   Protocol.  It specifies a record protection algorithm and its
   parameters as well as the record protection keys and IVs for the
   connection in both the read and the write directions.  The security
   parameters are set by the TLS Handshake Protocol, which also
   determines when new cryptographic keys are installed and used for
   record protection.  The initial current state always specifies that
   records are not protected.

   The security parameters for a TLS Connection read and write state are
   set by providing the following values:

   connection end
      Whether this entity is considered the "client" or the "server" in
      this connection.

   Hash algorithm
      An algorithm used to generate keys from the appropriate secret
      (see Section 7.1 and Section 7.3).

   record protection algorithm
      The algorithm to be used for record protection.  This algorithm
      must be of the AEAD type and thus provides integrity and
      confidentiality as a single primitive.  This specification
      includes the key size of this algorithm and of the nonce for the
      AEAD algorithm.

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

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

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

   These parameters are defined in the presentation language as:

      enum { server, client } ConnectionEnd;

      enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

      enum { aes_gcm } RecordProtAlgorithm;

      /* The algorithms specified in KDFAlgorithm and
         RecordProtAlgorithm may be added to. */

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

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

   The connection state will use the security parameters to generate the
   following four items:

      client write key
      server write key
      client write iv
      server write iv

   The client write parameters are used by the server when receiving and
   processing records and vice versa.  The algorithm used for generating
   these items from the security parameters is described in Section 7.3.

   Once the security parameters have been set and the keys have been
   generated, the connection states can be instantiated by making them
   the current states.  These current states MUST be updated for each
   record processed.  Each connection state includes the following
   elements:

   cipher state
      The current state of the encryption algorithm.  This will consist
      of the scheduled key for that connection.

   sequence number
      Each connection state contains a sequence number, which is
      maintained separately for read and write states.  The sequence
      number is set to zero at the beginning of a connection, and
      whenever the key is changed.  The sequence number is incremented
      after each record: specifically, the first record transmitted
      under a particular connection state and record key MUST use
      sequence number 0.  Sequence numbers are of type uint64 and MUST
      NOT exceed 2^64-1.  Sequence numbers do not wrap.  If a TLS
      implementation would need to wrap a sequence number, it MUST
      either rekey (Section 6.3.5.3) or terminate the connection.

5.2.  Record Layer

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

5.2.1.  Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Client
   message  Message
   boundaries are not preserved in the record layer (i.e., multiple client
   messages of the same ContentType MAY be coalesced into a single
   TLSPlaintext record, or a single message MAY be fragmented across
   several records).  Alert messages Section 6.1 (Section 6.1) MUST NOT be
   fragmented across records.

      struct {
          uint8 major;
          uint8 minor;
      } ProtocolVersion;

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

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

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

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

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

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

   This document describes TLS Version 1.3, which uses the version { 3,
   4 }.  The version value 3.4 is historical, deriving from the use of {
   3, 1 } for TLS 1.0 and { 3, 0 } for SSL 3.0.  In order to maximize
   backwards compatibility, the record layer version identifies as
   simply TLS 1.0.  Endpoints supporting other versions negotiate the
   version to use by following the procedure and requirements in
   Appendix C.

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

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

5.2.2.  Record Payload Protection

   The record protection functions translate a TLSPlaintext structure
   into a TLSCiphertext.  The deprotection functions reverse the
   process.  In TLS 1.3 as opposed to previous versions of TLS, all
   ciphers are modeled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide a unified encryption
   and authentication operation which turns plaintext into authenticated
   ciphertext and back again.

   AEAD ciphers take as input a single key, a nonce, a plaintext, and
   "additional data" to be included 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 and in TLS 1.3 the
   additional data input is empty (zero length).

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
          opaque content[TLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
       } fragment;
   } TLSCiphertext;

   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 used accustomed to parsing previous versions of TLS.
      The actual content type of the record is found in fragment.type
      after decryption.

   record_version
      The record_version field is identical to
      TLSPlaintext.record_version and is always { 3, 1 }.  Note that 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.fragment.
      The length MUST NOT exceed 2^14 + 256.  An endpoint that receives
      a record that exceeds this length MUST generate a fatal
      "record_overflow" alert.

   fragment.content
      The cleartext of TLSPlaintext.fragment.

   fragment.type
      The actual content type of the record.

   fragment.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 5.2.3 for more
      details.

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

   The length of the per-record nonce (iv_length) is set to max(8 bytes,
   N_MIN) for 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 padded to the left with
       zeroes to iv_length.

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

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

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

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

   The AEAD output consists of the ciphertext output by the AEAD
   encryption operation.  The length of the plaintext is greater than
   TLSPlaintext.length due to the inclusion of TLSPlaintext.type and
   however much padding is supplied by the sender.  The length of
   aead_output will generally be larger than the plaintext, but by an
   amount that varies with the AEAD cipher.  Since the ciphers might
   incorporate padding, the amount of overhead could vary with different
   lengths of plaintext.  Symbolically,

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

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

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

   If the decryption fails, a fatal "bad_record_mac" alert MUST be
   generated.

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

5.2.3.  Record Padding

   All encrypted 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 fragment.content
   if the sender desires.  This permits generation of plausibly-sized
   cover traffic in contexts where the presence or absence of activity
   may be sensitive.  Implementations MUST NOT send Handshake or Alert
   records that have a zero-length fragment.content.

   The padding sent is automatically verified by the record protection
   mechanism: Upon successful decryption of a TLSCiphertext.fragment,
   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 should treat the record as
   having an unexpected ContentType, sending an "unexpected_message"
   alert.

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

   Versions of TLS prior to 1.3 had limited support for padding.  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.

   Selecting a padding policy that suggests when and how much to pad is
   a complex topic, and is beyond the scope of this specification.  If
   the application layer protocol atop TLS permits padding, it may be
   preferable to pad application_data TLS records within the application
   layer.  Padding for encrypted handshake and alert TLS records must
   still be handled at the TLS layer, though.  Later documents may
   define padding selection algorithms, or define a padding policy
   request mechanism through TLS extensions or some other means.

6.  The TLS Handshaking Protocols

   TLS has three two subprotocols that are used to allow peers to agree upon
   security parameters for the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to report error
   conditions to each other.

   The TLS Handshake Protocol is responsible for negotiating a session,
   which consists of the following items:

   peer certificate
      X509v3 [RFC5280] certificate of the peer.  This element of the
      state may be null.

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

   resumption master secret
      a secret shared between the client and server that can be used as
      a pre-shared symmetric key (PSK) in future connections.

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

6.1.  Alert Protocol

   One of the content types supported by the TLS record layer is the
   alert type.  Alert messages convey the severity of the message
   (warning or fatal) and a description of the alert.  Alert messages
   with a level of fatal result in the immediate termination of the
   connection.  In this case, other connections corresponding to the
   session may continue, but the session identifier MUST be invalidated,
   preventing the failed session from being used to establish new
   connections.  Like other messages, alert messages are encrypted as
   specified by the current connection state.

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

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

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

6.1.1.  Closure Alerts

   The client and the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.  Failure to properly
   close a connection does not prohibit a session from being resumed.

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

   end_of_early_data
      This alert is sent by the client to indicate that all 0-RTT
      application_data messages have been transmitted (or none will be
      sent at all) and that this is the end of the flight.  This alert
      MUST be at the warning level.  Servers MUST NOT send this alert
      and clients receiving it MUST terminate the connection with an
      "unexpected_message" alert.

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

   Either party MAY initiate a close by sending a "close_notify" alert.
   Any data received after a closure alert is ignored.  If a transport-
   level close is received prior to a "close_notify", the receiver
   cannot know that all the data that was sent has been received.

   Each party MUST send a "close_notify" alert before closing the write
   side of the connection, unless some other fatal alert has been
   transmitted.  The other party MUST respond with a "close_notify"
   alert of its own and close down the connection immediately,
   discarding any pending writes.  The initiator of the close need not
   wait for the responding "close_notify" alert before closing the read
   side of the connection.

   If the application protocol using TLS provides that any data may be
   carried over the underlying transport after the TLS connection is
   closed, the TLS implementation must receive the responding
   "close_notify" alert before indicating to the application layer that
   the TLS connection has ended.  If the application protocol will not
   transfer any additional data, but will only close the underlying
   transport connection, then the implementation MAY choose to close the
   transport without waiting for the responding "close_notify".  No part
   of this standard should be taken to dictate the manner in which a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

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

6.1.2.  Error Alerts

   Error handling in the TLS Handshake Protocol is very simple.  When an
   error is detected, the detecting party sends a message to its peer.
   Upon transmission or receipt of a fatal alert message, both parties
   immediately close the connection.  Servers and clients MUST forget
   any session-identifiers, keys, and secrets associated with a failed
   connection.  Thus, any connection terminated with a fatal alert MUST
   NOT be resumed.

   Whenever an implementation encounters a condition which is defined as
   a fatal alert, it MUST send the appropriate alert prior to closing
   the connection.  For all errors where an alert level is not
   explicitly specified, the sending party MAY determine at its
   discretion whether to treat this as a fatal error or not.  If the
   implementation chooses to send an alert but intends to close the
   connection immediately afterwards, it MUST send that alert at the
   fatal alert level.

   If an alert with a level of warning is sent and received, generally
   the connection can continue normally.  If the receiving party decides
   not to proceed with the connection (e.g., after having received a
   "user_canceled" alert that it is not willing to accept), it SHOULD
   send a fatal alert to terminate the connection.  Given this, the
   sending peer cannot, in general, know how the receiving party will
   behave.  Therefore, warning alerts are not very useful when the
   sending party wants to continue the connection, and thus are
   sometimes omitted.  For example, if a party decides to accept an
   expired certificate (perhaps after confirming this with the user) and
   wants to continue the connection, it would not generally send a
   "certificate_expired" alert.

   The following error alerts are defined:

   unexpected_message
      An inappropriate message was received.  This alert is always fatal
      and should never be observed 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, this alert is used for all deprotection failures.
      This alert is always fatal and should never be observed in
      communication between proper implementations (except when messages
      were corrupted in the network).

   record_overflow
      A TLSCiphertext record was received that had a length more than
      2^14 + 256 bytes, or a record decrypted to a TLSPlaintext record
      with more than 2^14 bytes.  This alert is always fatal and should
      never be observed in communication between proper implementations
      (except when messages were corrupted in the network).

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

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

   unsupported_certificate
      A certificate was of an unsupported type.

   certificate_revoked
      A certificate was revoked by its signer.

   certificate_expired
      A certificate has expired or is not currently valid.

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

   illegal_parameter
      A field in the handshake was out of range or inconsistent with
      other fields.  This alert is always fatal.

   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 couldn't be matched with a known, trusted CA.  This
      alert is always fatal.

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

   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 always fatal and should never be observed in
      communication between proper implementations (except when messages
      were corrupted in the network).

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

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

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

   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.  This alert is always fatal.

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

   missing_extension
      Sent by endpoints that receive a hello message not containing an
      extension that is mandatory to send for the offered TLS version.
      This message is always fatal.  [[TODO: IANA Considerations.]]

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

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

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

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

   bad_certificate_hash_value
      Sent by servers when a retrieved object does not have the correct
      hash provided by the client via the "client_certificate_url"
      extension [RFC6066].  This alert is always fatal.

   unknown_psk_identity
      Sent by servers when a PSK cipher suite is selected 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.
      [[TODO: This doesn't really make sense with the current PSK
      negotiation scheme where the client provides multiple PSKs in
      flight 1. https://github.com/tlswg/tls13-spec/issues/230]]

   New Alert values are assigned by IANA as described in Section 11.

6.2.  Handshake Protocol Overview

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

   TLS supports three basic key exchange modes:

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

   -  A pre-shared symmetric key (PSK)

   -  A combination of a symmetric key and Diffie-Hellman

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

   Ephemeral Secret (ES): A secret which is derived from fresh (EC)DHE
   shares for this connection.  Keying material derived from ES is
   intended to be forward secret (with the exception of pre-shared key
   only modes).

   Static Secret (SS): A secret which may be derived from static or
   semi-static keying material, such as a pre-shared key or the server's
   semi-static (EC)DH share.

   Master Secret (MS): A secret derived from both the static and the
   ephemeral secret.

   In some cases, as with the DH handshake shown in

   Figure 1, the
   ephemeral and shared secrets are 1 below shows the same, but having both allows for
   a uniform key derivation scheme for all cipher modes.

   The basic full TLS Handshake for DH is shown in Figure 1: handshake.

       Client                                               Server

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

              +  Indicates extensions sent in the
                 previously noted message.

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

              {} Indicates messages protected using keys
                 derived from the ephemeral secret. handshake_traffic_secret.

              [] Indicates messages protected using keys
                 derived from the master secret. traffic_secret_N

               Figure 1: Message flow for full TLS Handshake

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

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

   Server Parameters: establish other handshake parameters (whether the
   client is authenticated, support for 0-RTT, etc.) application layer protocol support, etc.)

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

   In the Key Exchange phase, the client sends the ClientHello
   (Section 6.3.1.1) message, which contains a random nonce
   (ClientHello.random), its offered protocol version, cipher suite, and
   extensions, and in general either one or more Diffie-Hellman key
   shares in (in the "key_share" extension Section 6.3.2.3. 6.3.2.4), one or more
   pre-shared key labels (in the "pre_shared_key" extension
   Section 6.3.2.5), or both.

   The server processes the ClientHello and determines the appropriate
   cryptographic parameters for the connection.  It then responds with
   its own ServerHello which indicates the negotiated connection
   parameters.  [Section 6.3.1.2] 6.3.1.2].  The combination of the ClientHello
   and the ServerHello determines the values of ES and SS, as described
   above.  If DH either a pure (EC)DHE or (EC)DHE-PSK cipher suite is in
   use, this then the ServerHello will contain a "key_share" extension with
   the server's ephemeral Diffie-Hellman share which MUST be in the same group as one of the shares offered by
   group.  If a pure PSK or an (EC)DHE-PSK cipher suite is negotiated,
   then the client.  The server's KeyShare and ServerHello will contain a "pre_shared_key" extension
   indicating which if the client's KeyShare
   corresponding to the negotiated key exchange are used together to
   derive the Static Secret and Ephemeral Secret (in this mode they are
   the same).  [Section 6.3.2.3] offered PSKs was selected.

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

   EncryptedExtensions  responses to any extensions which are not
      required in order to determine the cryptographic parameters.
      [Section 6.3.3.1]

   CertificateRequest  if certificate-based client authentication is
      desired, the desired parameters for that certificate.  This
      message will be omitted if client authentication is not desired.  [[OPEN ISSUE: See
      https://github.com/tlswg/tls13-spec/issues/184]].
      [Section 6.3.3.2]

   ServerConfiguration
      supplies a configuration for 0-RTT handshakes (see Section 6.2.2).
      [Section 6.3.3.3]

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

   Certificate
      the certificate of the endpoint.  This message is omitted if
      certificate authentication the
      server is not being used. authenticating with a certificate (i.e., with PSK or
      (EC)DHE-PSK cipher suites).  Note that if raw public keys
      [RFC7250] or the cached information extension
      [I-D.ietf-tls-cached-info] are in use, then this message will not
      contain a certificate but rather some other value corresponding to
      the server's long-term key.  [Section 6.3.4.1]

   CertificateVerify
      a signature over the entire handshake using the public key in the
      Certificate message.  This message will be is omitted if the server is not
      authenticating via a certificate. certificate (i.e., with PSK or (EC)DHE-PSK
      cipher suites).  [Section 6.3.4.2]

   Finished
      a MAC over the entire handshake.  This message provides key
      confirmation, binds the endpoint's identity to the exchanged keys,
      and in some modes (0-RTT and PSK) PSK mode also authenticates the handshake
      using the the Static Secret. handshake.
      [Section 6.3.4.3]

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

   At this point, the handshake is complete, and the client and server
   may exchange application layer data.  Application data MUST NOT be
   sent prior to sending the Finished message.  Note that while the
   server may send application data prior to receiving the client's
   Authentication messages, any data sent at that point is is, of course course,
   being sent to an unauthenticated peer.

   [[TODO: Move this elsewhere?  Note that higher layers should not be
   overly reliant on whether TLS always negotiates the strongest
   possible connection between two endpoints.  There are a number of
   ways in which a man-in-the-middle attacker can attempt to make two
   entities drop down to the least secure method they support (i.e.,
   perform a downgrade attack).  The TLS protocol has been designed to
   minimize this risk, but there are still attacks available: for
   example, an attacker could block access to the port a secure service
   runs on, or attempt to get the peers to negotiate an unauthenticated
   connection.  The fundamental rule is that higher levels must be
   cognizant of what their security requirements are and never transmit
   information over a channel less secure than what they require.  The
   TLS protocol is secure in that any cipher suite offers its promised
   level of security: if you negotiate AES-GCM [GCM] with a 255-bit
   ECDHE key exchange with a host whose certificate chain you have
   verified, you can expect that to be reasonably "secure" against
   algorithmic attacks, at least in the year 2015.]]

6.2.1.  Incorrect DHE Share

   If the client has not provided an appropriate "key_share" extension
   (e.g. it includes only DHE or ECDHE groups unacceptable or
   unsupported by the server), the server corrects the mismatch with a
   HelloRetryRequest and the client will need to restart the handshake
   with an appropriate "key_share" extension, as shown in Figure 2: 2.  If
   no common cryptographic parameters can be negotiated, the server will
   send a "handshake_failure" or "insufficient_security" fatal alert
   (see Section 6.1).

            Client                                               Server

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

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

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

   [[OPEN ISSUE: Should we restart

   Note: the handshake hash?
   https://github.com/tlswg/tls13-spec/issues/104.]] [[OPEN ISSUE: We
   need to make sure that this flow doesn't introduce downgrade issues.
   Potential options include continuing transcript includes the handshake hashes (as long as
   clients don't change their opinion of the server's capabilities with
   aborted handshakes) and requiring the client to send the same
   ClientHello (as initial ClientHello/
   HelloRetryRequest exchange.  It is currently done) and then checking you get the same
   negotiated parameters.]]

   If no common cryptographic parameters can be negotiated, not reset with the server
   will send a "handshake_failure" or "insufficient_security" fatal
   alert (see Section 6.1). new
   ClientHello.

   TLS also allows several optimized variants of the basic handshake, as
   described below.

6.2.2.  Zero-RTT Exchange  Resumption and Pre-Shared Key (PSK)

   Although TLS 1.3 supports a "0-RTT" mode in which the client PSKs can both
   authenticate be established out of band, PSKs can also be
   established in a previous session and send application on its first flight, thus reducing then reused ("session
   resumption").  Once a handshake latency.  In order to enable this functionality, has completed, the server
   provides can send
   the client a ServerConfiguration message containing PSK identity which corresponds to a long-term (EC)DH
   share.  On key derived from the
   initial handshake (See Section 6.3.5.1).  The client can then use
   that PSK identity in future connections handshakes to negotiate use of the same server, PSK;
   if the client server accepts it, then the security context of the original
   connection is tied to the new connection.  In TLS 1.2 and below, this
   functionality was provided by "session resumption" and "session
   tickets" [RFC5077].  Both mechanisms are obsoleted in TLS 1.3.

   PSK cipher suites can either use
   that share PSK in combination with an (EC)DHE
   exchange in order to protect provide forward secrecy in combination with
   shared keys, or can use PSKs alone, at the first-flight data. cost of losing forward
   secrecy.

   Figure 3 shows a pair of handshakes in which the first establishes a
   PSK and the second uses it:

          Client                                               Server

   Initial Handshake:
          ClientHello
           + key_share
              + early_data
         ^  (Certificate*)
   0-RTT |  (CertificateVerify*)
   Data  |  (Finished)
         v  (Application Data*)
            (end_of_early_data)              -------->
                                                          ServerHello
                                                          + early_data
                                                            + key_share
                                                {EncryptedExtensions}
                                                {CertificateRequest*}
                                                 {ServerConfiguration*}
                                                       {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.  When a client offers resumption
   via PSK it SHOULD also supply a "key_share" extension to the server
   as well; this allows server to decline resumption and fall back to a
   full handshake.  A "key_share" extension MUST also be sent if the
   client is attempting to negotiate an (EC)DHE-PSK cipher suite.

6.2.3.  Zero-RTT Data

   When resuming via a PSK with an appropriate ticket (i.e., one with
   the "allow_early_data" flag), clients can also send data on their
   first flight ("early data").  This data is encrypted solely under
   keys derived using the PSK as the static secret.  As shown in
   Figure 4, the Zero-RTT data is just added to the 1-RTT handshake in
   the first flight, the rest of the handshake uses the same messages.

            Client                                               Server

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

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

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

                  () Indicates messages protected using keys
                     derived from the static secret. early_traffic_secret.

                  {} Indicates messages protected using keys
                     derived from the ephemeral secret. handshake_traffic_secret.

                  [] Indicates messages protected using keys
                     derived from the master secret. traffic_secret_N

          Figure 3: 4: Message flow for a zero round trip handshake

   As shown in Figure 3, the Zero-RTT data is just added

   [[OPEN ISSUE: Should it be possible to combine 0-RTT with the 1-RTT
   handshake in the first flight.  Specifically, the client sends its
   Authentication messages after the ClientHello, followed by any
   application data.  The rest of the handshake messages are the same as
   with Figure 1.  This implies that the server can request client
   authentication even if the client offers a certificate on its first
   flight.  This is consistent with the server being able to ask for
   client authentication after the handshake is complete (see
   Section 6.3.5.2).  When offering PSK support, the "pre_shared_key"
   extension will be used instead of (or in addition to) the "key_share"
   extension as specified above.

   IMPORTANT NOTE: The security properties for 0-RTT data (regardless of server
   authenticating via a signature https://github.com/tlswg/tls13-spec/
   issues/443]]
   IMPORTANT NOTE: The security properties for 0-RTT data (regardless of
   the cipher suite) are weaker than those for other kinds of TLS data.
   Specifically:

   1.  This data is not forward secret, because it is encrypted solely
       with the server's semi-static (EC)DH share. PSK.

   2.  There are no guarantees of non-replay between connections.
       Unless the server takes special measures outside those provided
       by TLS (See Section 6.3.2.5.2), 6.3.2.7.2), the server has no guarantee that
       the same 0-RTT data was not transmitted on multiple 0-RTT
       connections.  This is especially relevant if the data is
       authenticated either with TLS client authentication or inside the
       application layer protocol.  However, 0-RTT data cannot be
       duplicated within a connection (i.e., the server will not process
       the same data twice for the same connection) and also cannot be
       sent as if it were ordinary TLS data.

   3.  If the server key is compromised, then the an attacker can tamper
       with the will
       not be able to make 0-RTT data without detection.  If the client's ephemeral
       share appear to be 1-RTT data (because
       it is compromised protected with different keys.)

   The contents and client authentication significance of each message will be presented in
   detail in the following sections.

6.3.  Handshake Protocol

   The TLS Handshake Protocol is used, then one of the
       attacker can impersonate defined higher-level clients
   of the client on subsequent connections.

6.2.3.  Resumption and Pre-Shared Key (PSK)

   Finally, TLS provides a pre-shared key (PSK) mode which allows a
   client and server who share an existing secret (e.g., a key
   established out of band) Record Protocol.  This protocol is used to establish a connection authenticated by
   that key.  PSKs can also be established in a previous session and
   then reused ("session resumption").  Once a handshake has completed,
   the server can send negotiate the client
   secure attributes of a PSK identity which corresponds session.  Handshake messages are supplied to a
   key derived from
   the initial handshake (See Section 6.3.5.1).  The
   client can then use that PSK identity in future handshakes to
   negotiate use of the PSK; if the server accepts it, then the security
   context of the original connection is tied to the new connection.  In TLS 1.2 record layer, where they are encapsulated within one or more
   TLSPlaintext or TLSCiphertext structures, which are processed and below, this functionality was provided
   transmitted as specified by "session
   resumption" and "session tickets" [RFC5077].  Both mechanisms are
   obsoleted the current active session state.

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

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

   The TLS 1.3.

   PSK cipher suites can either use PSK Handshake Protocol messages are presented below in combination with an (EC)DHE
   exchange the order
   they MUST be sent; sending handshake messages in an unexpected order to provide forward secrecy
   results in combination with
   shared keys, or an "unexpected_message" fatal error.  Unneeded handshake
   messages can use PSKs alone, at the cost of losing forward
   secrecy.

   Figure 4 shows a pair of handshakes be omitted, however.

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

6.3.1.  Key Exchange Messages

   The key exchange messages are used to exchange security capabilities
   between the first establishes a
   PSK client and server and to establish the second uses it:

          Client                                               Server

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

               Figure 4: Message flow for resumption traffic keys used
   to protect the handshake and PSK

   As the server is authenticating via data.

6.3.1.1.  Client Hello

   When this message will be sent:

      When a PSK, client first connects to a server, it does not is required to send
      the ClientHello as its first message.  The client will also send a
   Certificate or a CertificateVerify.  PSK-based resumption cannot be
   used
      ClientHello when the server has responded to provide its ClientHello with
      a new ServerConfiguration.  Note ServerHello that selects cryptographic parameters that don't
      match the client's "key_share" extension.  In that case, the
      client
   supplies MUST send the same ClientHello (without modification)
      except including a KeyShare new KeyShareEntry as the lowest priority share
      (i.e., appended to the server as well, which allows list of shares in the "key_share"
      extension).  If a server to
   decline resumption and fall back to receives a full handshake.

   The contents ClientHello at any other time,
      it MUST send a fatal "unexpected_message" alert and significance close the
      connection.

   Structure of each this message:

      The ClientHello message will be presented includes a random structure, which is used
      later in
   detail the protocol.

   The cipher suite list, passed from the client to the server in the following sections.

6.3.  Handshake Protocol

   The TLS Handshake Protocol is one
   ClientHello message, contains the combinations of cryptographic
   algorithms supported by the defined higher-level clients client in order of the TLS Record Protocol.  This protocol is used client's
   preference (favorite choice first).  Each cipher suite defines a key
   exchange algorithm, a record protection algorithm (including secret
   key length) and a hash to negotiate the
   secure attributes of be used with HKDF.  The server will select
   a session.  Handshake messages cipher suite or, if no acceptable choices are supplied to presented, return a
   "handshake_failure" alert and close the TLS record layer, where they are encapsulated within one or more
   TLSPlaintext connection.  If the list
   contains cipher suites the server does not recognize, support, or TLSCiphertext structures, which are processed
   wish to use, the server MUST ignore those cipher suites, and
   transmitted as specified by process
   the current active session state.

      enum {
          client_hello(1),
          server_hello(2),
          session_ticket(4),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          certificate_request(13),
          certificate_verify(15),
          server_configuration(17),
          finished(20),
          key_update(24),
          (255)
      } HandshakeType; remaining ones as usual.

      struct {
          HandshakeType msg_type;
          opaque random_bytes[32];
      } Random;

      uint8 CipherSuite[2];    /* handshake type Cryptographic suite selector */
          uint24 length;

      struct {
          ProtocolVersion client_version = { 3, 4 };    /* bytes in message TLS v1.3 */
          select (HandshakeType) {
              case client_hello:          ClientHello;
              case server_hello:          ServerHello;
              case hello_retry_request:   HelloRetryRequest;
              case encrypted_extensions:  EncryptedExtensions;
              case certificate_request:   CertificateRequest;
              case server_configuration:  ServerConfiguration;
              case certificate:           Certificate;
              case certificate_verify:    CertificateVerify;
              case finished:              Finished;
              case session_ticket:        NewSessionTicket;
              case key_update:            KeyUpdate;
          } body;
          Random random;
          opaque legacy_session_id<0..32>;
          CipherSuite cipher_suites<2..2^16-2>;
          opaque legacy_compression_methods<1..2^8-1>;
          Extension extensions<0..2^16-1>;
      } Handshake;

   The ClientHello;

   TLS Handshake Protocol messages are presented below in allows extensions to follow the order
   they MUST be sent; sending handshake messages in an unexpected order
   results compression_methods field in an "unexpected_message" fatal error.  Unneeded handshake
   messages
   extensions block.  The presence of extensions can be omitted, however.

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

6.3.1.  Key Exchange Messages

   The key exchange messages
   determining whether there are used to exchange security capabilities
   between the client and server and to establish bytes following the traffic keys used
   to protect compression_methods
   at the handshake and end of the data.

6.3.1.1.  Client Hello

   When ClientHello.  Note that this message will be sent:

      When a client first connects to method of detecting
   optional data differs from the normal TLS method of having a server,
   variable-length field, but it is required to send
      the ClientHello as its first message.  The client used for compatibility with TLS
   before extensions were defined.  As of TLS 1.3, all clients and
   servers will also send a
      ClientHello when the server has responded to its ClientHello with
      a ServerHello that selects cryptographic parameters that don't
      match the client's at least one extension (at least "key_share" extension.  In that case, the
      client MUST send or
   "pre_shared_key").

   client_version
      The version of the same ClientHello (without modification)
      except including a new KeyShareEntry as TLS protocol by which the lowest priority share
      (i.e., appended client wishes to
      communicate during this session.  This SHOULD be the list of shares in the KeyShare message).
      [[OPEN ISSUE: New random values?  See: https://github.com/tlswg/
      tls13-spec/issues/185]] If a server receives a ClientHello at any
      other time, it MUST send a fatal "unexpected_message" alert and
      close latest
      (highest valued) version supported by the connection.

   Structure client.  For this
      version of this message:

      The ClientHello message includes a random structure, which is used
      later in the protocol.

      struct specification, the version will be {
          opaque random_bytes[32];
      } Random;

   random_bytes 3, 4 }. (See
      Appendix C for details about backward compatibility.)

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

   legacy_session_id
      Versions of TLS before TLS 1.3 server implementations which respond to supported a ClientHello session resumption
      feature which has been merged with Pre-Shared Keys in this version
      (see Section 6.2.2).  This field MUST be ignored by a
   client_version indicating server
      negotiating TLS 1.2 or below MUST 1.3 and SHOULD be set the first eight
   bytes of their Random value to the bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to as a ClientHello with zero length vector
      (i.e., a
   client_version indicating TLS 1.1 or below SHOULD single zero byte length field) by clients which do not
      have a cached session ID set by a pre-TLS 1.3 server.

   cipher_suites
      This is a list of the cryptographic options supported by the
      client, with the client's first eight
   bytes preference first.  Values are
      defined in Appendix A.4.

   legacy_compression_methods
      Versions of their Random value to the bytes:

     44 4F 57 4E 47 52 44 00 TLS before 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
   that supported compression and the top eight octets are not equal to either list of these values.
      compression methods was supplied in this field.  For any TLS 1.2 clients SHOULD also perform 1.3
      ClientHello, this check if vector MUST contain exactly one byte set to
      zero, which corresponds to the ServerHello
   indicates TLS 1.1 or below. "null" compression method in prior
      versions of TLS.  If a match TLS 1.3 ClientHello is found received with any
      other value in this field, the client server MUST
   abort the handshake with generate a fatal
      "illegal_parameter" alert.  This
   mechanism provides limited protection against downgrade attacks over
   and above  Note that provided TLS 1.3 servers might
      receive TLS 1.2 or prior ClientHellos which contain other
      compression methods and MUST follow the procedures for the
      appropriate prior version of TLS.

   extensions
      Clients request extended functionality from servers by sending
      data in the Finished exchange: because extensions field.  The actual "Extension" format is
      defined in Section 6.3.2.

   In the
   ServerKeyExchange includes event that a signature over both random values, it client requests additional functionality using
   extensions, and this functionality is not possible for an active attacker to modify supplied by the randoms without
   detection as long as ephemeral ciphers are used.  It does not provide
   downgrade protection when static RSA is used. server, the
   client MAY abort the handshake.  Note: This is an update to TLS 1.2 so in practice many TLS 1.2
   clients 1.3 ClientHello messages
   MUST always contain extensions, and servers will not behave as specified above.

   Note: Versions of a TLS prior 1.3 server MUST respond to
   any TLS 1.3 used the top 32 bits of the
   Random value to encode the time since the UNIX epoch.  The sentinel
   value above was selected to avoid conflicting ClientHello without extensions with any valid a fatal
   "decode_error" alert.  TLS 1.3 servers may receive TLS 1.2
   Random value and to have
   ClientHello messages without extensions.  If negotiating TLS 1.2, a low (2^{-64}) probability of colliding
   with randomly selected Random values.

   The cipher suite list, passed from the client to the
   server MUST check that the amount of data in the message precisely
   matches one of these formats; if not, then it MUST send a fatal
   "decode_error" alert.

   After sending the ClientHello message, contains the combinations of cryptographic
   algorithms supported by the client in order of the client's
   preference (favorite choice first).  Each cipher suite defines a key
   exchange algorithm, a record protection algorithm (including secret
   key length) and waits for a hash to
   ServerHello or HelloRetryRequest message.

6.3.1.2.  Server Hello

   When this message will be used with HKDF. sent:

      The server will select send this message in response to a cipher suite or, if no ClientHello
      message when it was able to find an acceptable choices are presented, return a
   "handshake_failure" alert set of algorithms
      and close the connection. client's "key_share" extension was acceptable.  If the list
   contains cipher suites the server does
      client proposed groups are not recognize, support, or
   wish to use, the server MUST ignore those cipher suites, and process acceptable by the remaining ones as usual.

      uint8 CipherSuite[2];    /* Cryptographic suite selector */ server, it will
      respond with a "handshake_failure" fatal alert.

   Structure of this message:

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

   TLS allows extensions to follow the compression_methods field in an
   extensions block.  The presence ServerHello;

   In prior versions of TLS, the extensions can field could be detected by
   determining whether there are bytes following the compression_methods
   at the end of the omitted
   entirely if not needed, similar to 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.  As of TLS 1.3, all
   clients and servers will send at least one extension (at least
   "key_share" or "pre_shared_key").

   client_version
      The version of

   server_version
      This field will contain the TLS protocol lower of that suggested by which the client wishes to
      communicate during this session.  This SHOULD be
      in the latest
      (highest valued) version ClientHello and the highest supported by the client. server.  For
      this version of the specification, the version will be is { 3, 4 }.  (See
      Appendix C for details about backward compatibility.)

   random
      A client-generated random structure.

   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 6.2.3).
      This field MUST be ignored structure is generated by a the server
      negotiating TLS 1.3 and SHOULD MUST be set as a zero length vector
      (i.e., a single zero byte length field) by clients which do not
      have a cached session ID set by a pre-TLS 1.3 server.

   cipher_suites
      This is a list generated
      independently of the cryptographic options supported ClientHello.random.

   cipher_suite
      The single cipher suite selected by the
      client, with the client's first preference first.  Values are
      defined in Appendix A.4.

   legacy_compression_methods
      Versions of TLS before 1.3 supported compression and server from the list of
      compression methods was supplied in this field.
      ClientHello.cipher_suites.  For any TLS 1.3
      ClientHello, resumed sessions, 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 field is received with any
      other value in this field, the server MUST generate a fatal
      "illegal_parameter" alert.  Note that TLS 1.3 servers might
      receive TLS 1.2 or prior ClientHellos which contain other
      compression methods and MUST follow
      the procedures for value from the
      appropriate prior version state of TLS. the session being resumed.  [[TODO:
      interaction with PSK.]]

   extensions
      Clients request extended functionality from servers
      A list of extensions.  Note that only extensions offered by sending
      data in the extensions field.  The actual "Extension" format is
      defined
      client can appear in Section 6.3.2.

   In the event that a client requests additional functionality using
   extensions, and this functionality is not supplied by server's list.  In TLS 1.3 as opposed to
      previous versions of TLS, the server, server's extensions are split
      between the
   client MAY abort ServerHello and the handshake.  A server EncryptedExtensions
      Section 6.3.3.1 message.  The ServerHello MUST accept ClientHello
   messages both with and without only include
      extensions which are required to establish the cryptographic
      context.  Currently the only such extensions field, are "key_share",
      "pre_shared_key", and (as for all
   other messages) it "early_data".  Clients MUST check that the amount of data in
      ServerHello for the message
   precisely matches one presence of these formats; any forbidden extensions and if not, then it
      any are found MUST send a
   fatal "decode_error" alert.

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

6.3.1.2.  Server Hello

   When this message will be sent:

      The
      "illegal_parameter" alert.

   TLS 1.3 server will send this message in response implementations which respond to a ClientHello
      message when it was able to find an acceptable with a
   client_version indicating TLS 1.2 or below MUST set of algorithms
      and the client's "key_share" extension was acceptable.  If the
      client proposed groups are not acceptable by first eight
   bytes of their Random value to the server, it will bytes:

     44 4F 57 4E 47 52 44 01

   TLS 1.2 server implementations which respond to a ClientHello with a "handshake_failure" fatal alert.

   Structure of this message:

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

   In prior versions
   client_version indicating TLS 1.1 or below SHOULD set the first eight
   bytes of TLS, their Random value to the extensions field could be omitted
   entirely if bytes:

     44 4F 57 4E 47 52 44 00

   TLS 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
   that the top eight octets are not needed, similar equal to ClientHello.  As either of these values.
   TLS 1.3, all 1.2 clients and servers will send at least one extension (at least
   "key_share" or "pre_shared_key").

   server_version
      This field will contain SHOULD also perform this check if the lower of that suggested by ServerHello
   indicates TLS 1.1 or below.  If a match is found, the client
      in MUST
   abort the ClientHello handshake with a fatal "illegal_parameter" alert.  This
   mechanism provides limited protection against downgrade attacks over
   and the highest supported above that provided by the server.  For
      this version of the specification, Finished exchange: because the version is { 3, 4 }.  (See
      Appendix C for details about backward compatibility.)
   ServerKeyExchange includes a signature over both random
      This structure values, it is generated by the server and MUST be generated
      independently of the ClientHello.random.

   cipher_suite
      The single cipher suite selected by the server from
   not possible for an active attacker to modify the list in
      ClientHello.cipher_suites.  For resumed sessions, this field randoms without
   detection as long as ephemeral ciphers are used.  It does not provide
   downgrade protection when static RSA is
      the value from the state of the session being resumed.  [[TODO:
      interaction with PSK.]]

   extensions
      A list of extensions.  Note that only extensions offered by the
      client can appear used.

   Note: This is an update to TLS 1.2 so in the server's list.  In practice many TLS 1.3 1.2
   clients and servers will not behave as opposed specified above.

   Note: Versions of TLS prior to
      previous versions TLS 1.3 used the top 32 bits of TLS, the server's extensions are split
      between
   Random value to encode the ServerHello and time since the EncryptedExtensions
      Section 6.3.3.1 message. UNIX epoch.  The ServerHello MUST only include
      extensions which are required sentinel
   value above was selected to establish the cryptographic
      context.  Currently the only such extensions are "key_share",
      "pre_shared_key", and "early_data".  Clients MUST check the
      ServerHello for the presence of avoid conflicting with any forbidden extensions valid TLS 1.2
   Random value and if
      any are found MUST terminate the handshake with to have a
      "illegal_parameter" alert. low (2^{-64}) probability of colliding
   with randomly selected Random values.

6.3.1.3.  Hello Retry Request

   When this message will be sent:

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

   Structure of this message:

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

   [[OPEN ISSUE: Merge in DTLS Cookies?]]

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

   The server_version, cipher_suite, and extensions fields have the same
   meanings as their corresponding values in the ServerHello.  The
   server SHOULD send only the extensions necessary for the client to
   generate a correct ClientHello pair.  As with ServerHello, a
   HelloRetryRequest MUST NOT contain any extensions that were not first
   offered by the client in its ClientHello.

   Upon receipt of a HelloRetryRequest, the client MUST first verify
   that the selected_group field corresponds to a group which was
   provided in the "supported_groups" extension in the original
   ClientHello.  It MUST then verify that 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 a fatal
   "handshake_failure" alert.  Clients SHOULD also abort with
   "handshake_failure" in response to any second HelloRetryRequest which
   was sent in the same connection (i.e., where the ClientHello was
   itself in response to a HelloRetryRequest).

   Otherwise, the client MUST send a ClientHello with an updated
   KeyShare extension to the server.  The client MUST append a new
   KeyShareEntry for the group indicated in the selected_group field to
   the groups in its original KeyShare.

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

   [[OPEN ISSUE: https://github.com/tlswg/tls13-spec/issues/104]]

6.3.2.  Hello Extensions

   The extension format is:

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

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

   Here:

   -  "extension_type" identifies the particular extension type.

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

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

   An extension type MUST NOT appear in the ServerHello or
   HelloRetryRequest unless the same extension type appeared in the
   corresponding ClientHello.  If a client receives an extension type in
   ServerHello or HelloRetryRequest that it did not request in the
   associated ClientHello, it MUST abort the handshake with an
   "unsupported_extension" fatal alert.

   Nonetheless, "server-oriented" extensions may be provided in the
   future within this framework.  Such an extension (say, of type x)
   would require the client to first send an extension of type x in a
   ClientHello with empty extension_data to indicate that it supports
   the extension type.  In this case, the client is offering the
   capability to understand the extension type, and the server is taking
   the client up on its offer.

   When multiple extensions of different types are present in the
   ClientHello or ServerHello messages, the extensions MAY appear in any
   order.  There MUST NOT be more than one extension of the same type.

   Finally, note that extensions can be sent both when starting a new
   session and when requesting session resumption or 0-RTT mode.
   Indeed, a client that requests session resumption does not in general
   know whether the server will accept this request, and therefore it
   SHOULD send the same extensions as it would send if it were not
   attempting resumption.

   In general, the specification of each extension type needs to
   describe the effect of the extension both during full handshake and
   session resumption.  Most current TLS extensions are relevant only
   when a session is initiated: when an older session is resumed, the
   server does not process these extensions in ClientHello, and does not
   include them in ServerHello.  However, some extensions may specify
   different behavior during session resumption.  [[TODO: update this
   and the previous paragraph to cover PSK-based resumption.]]

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

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

   -  Extensions should, as far as possible, be designed to prevent any
      attack that forces use (or non-use) of a particular feature by
      manipulation of handshake messages.  This principle should be
      followed regardless of whether the feature is believed to cause a
      security problem.  Often the fact that the extension fields are
      included in the inputs to the Finished message hashes will be
      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.

   -  It would be technically possible to use extensions to change major
      aspects of the design of TLS; for example example, the design of cipher
      suite negotiation.  This is not recommended; it would be more
      appropriate to define a new version of TLS -- particularly since
      the TLS handshake algorithms have specific protection against
      version rollback attacks based on the version number, and the
      possibility of version rollback should be a significant
      consideration in any major design change.

6.3.2.1.  Cookie

      struct {
          opaque cookie<0..255>;
      } Cookie;

   Cookies serve two primary purposes:

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

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

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

6.3.2.2.  Signature Algorithms

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

   Clients which offer one or more cipher suites which use certificate
   authentication (i.e., any non-PSK cipher suite) MUST send the
   "signature_algorithms" extension.  If this extension is not provided
   and no alternative cipher suite is available, the server MUST close
   the connection with a fatal "missing_extension" alert.  (see
   Section 8.2)

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

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

          // DSA algorithms (deprecated).
          dsa_sha1 (0x0202),
          dsa_sha256 (0x0402),
          dsa_sha384 (0x0502),
          dsa_sha512 (0x0602),

          //
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

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

          //

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

          //

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

          //

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

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

   Note: This production is named "SignatureScheme" because there is
   already a SignatureAlgorithm type in TLS 1.2.  We use the term
   "signature algorithm" throughout the text.

   Each SignatureScheme value lists a single signature algorithm that
   the client is willing to verify.  The values are indicated in
   descending order of preference.  Note that a signature algorithm
   takes as input an arbitrary-length message, rather than a digest.
   Algorithms which traditionally act on a digest should be defined in
   TLS to first hash the input with a specified hash function and then
   proceed as usual.

   rsa_pkcs1_sha1, etc.  The code point groups listed above have the
   following meanings:

   RSASSA-PKCS-v1_5 algorithms
      Indicates a signature algorithm using RSASSA-PKCS1-v1_5 [RFC3447]
      with the corresponding hash algorithm as defined in [SHS].  These
      values refer solely to signatures which appear in certificates
      (see Section 6.3.4.1.1) and are not defined for use in signed TLS
      handshake messages (see Section 4.8.1).

   ecdsa_secp256r1_sha256, etc.

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

   rsa_pss_sha256, etc.

   RSASSA-PSS algorithms
      Indicates a signature algorithm using RSASSA-PSS [RFC3447] with
      MGF1.  The digest used in the mask generation function and the
      digest being signed are both the corresponding hash algorithm as
      defined in [SHS].  When used in signed TLS handshake messages (see
      Section 4.8.1), the length of the salt MUST be equal to the length
      of the digest output.

   ed25519, ed448

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

   The semantics of this extension are somewhat complicated because the
   cipher suite adds additional constraints on signature algorithms.
   Section 6.3.4.1.1 describes the appropriate rules.

   rsa_pkcs1_sha1 and dsa_sha1 SHOULD NOT be offered.  Clients offering
   these values for backwards compatibility MUST list them as the lowest
   priority (listed after all other algorithms in the
   supported_signature_algorithms vector).  TLS 1.3 servers MUST NOT
   offer a SHA-1 signed certificate unless no valid certificate chain
   can be produced without it (see Section 6.3.4.1.1).

   The signatures on certificates that are self-signed or certificates
   that are trust anchors are not validated since they begin a
   certification path (see [RFC5280], Section 3.2).  A certificate that
   begins a certification path MAY use a signature algorithm that is not
   advertised as being supported in the "signature_algorithms"
   extension.

   Note that TLS 1.2 defines this extension differently.  TLS 1.3
   implementations willing to negotiate TLS 1.2 MUST behave in
   accordance with the requirements of [RFC5246] when negotiating that
   version.  In particular:

   -  TLS 1.2 ClientHellos may omit this extension.

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

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

6.3.2.2.

6.3.2.3.  Negotiated Groups

   When sent by the client, the "supported_groups" extension indicates
   the named groups which the client supports, 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
   [RFC4492] and [I-D.ietf-tls-negotiated-ff-dhe].  This extension was
   also used to negotiate ECDSA curves.  Signature algorithms are now
   negotiated independently (see Section 6.3.2.1). 6.3.2.2).

   Clients which offer one or more (EC)DHE cipher suites MUST send at
   least one supported NamedGroup value and servers MUST NOT negotiate
   any of these cipher suites unless a supported value was provided.  If
   this extension is not provided and no alternative cipher suite is
   available, the server MUST close the connection with a fatal
   "missing_extension" alert.  (see Section 8.2) If the extension is
   provided, but no compatible group is offered, the server MUST NOT
   negotiate a cipher suite of the relevant type.  For instance, if a
   client supplies only ECDHE groups, the server MUST NOT negotiate
   finite field Diffie-Hellman.  If no acceptable group can be selected
   across all cipher suites, then the server MUST generate a fatal
   "handshake_failure" alert.

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

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

          //

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

          //

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

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

   secp256r1, etc.

   Elliptic Curve Groups (ECDHE)
      Indicates support of the corresponding named curve.  Note that
      some curves are also recommended in ANSI X9.62 [X962] and FIPS
      186-4 [DSS].  Others are recommended in [I-D.irtf-cfrg-curves]. [RFC7748].  Values 0xFE00
      through 0xFEFF are reserved for private use.

   ffdhe2048, etc.

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

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

   As an example, of TLS 1.3, servers are permitted to send the "supported_groups"
   extension to the client.  If the server has a client that only supports secp256r1 (aka NIST P-256;
   value 23 = 0x0017) and secp384r1 (aka NIST P-384; value 24 = 0x0018)
   and group it prefers to use secp256r1 would include a TLS the
   ones in the "key_share" extension consisting
   of but is still willing to accept the following octets.  Note that
   ClientHello, it SHOULD send "supported_groups" to update the first two octets indicate client's
   view of its preferences.  Clients MUST NOT act upon any information
   found in "supported_groups" prior to successful completion of the
   extension type ("supported_groups" extension):

      00 0A 00 06 00 04 00 17 00 18
   handshake, but MAY use the information learned from a successfully
   completed handshake to change what groups they offer to a server in
   subsequent connections.

   [[TODO: IANA Considerations.]]

6.3.2.3.

6.3.2.4.  Key Share

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

   Clients which offer one or more (EC)DHE cipher suites MUST send this
   extension and SHOULD send at least one supported KeyShareEntry value.
   Servers MUST NOT negotiate any of these cipher suites unless a
   supported value was provided.  If this extension is not provided in a
   ServerHello or ClientHello, and the peer is offering (EC)DHE cipher
   suites, then the endpoint MUST close the connection with a fatal
   "missing_extension" alert.  (see Section 8.2) Clients MAY send an
   empty client_shares vector in order to request group selection from
   the server at the cost of an additional round trip.  (see
   Section 6.3.1.3)

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

   group
      The named group for the key being exchanged.  Finite Field Diffie-
      Hellman [DH] parameters are described in Section 6.3.2.3.1; 6.3.2.4.1;
      Elliptic Curve Diffie-Hellman parameters are described in
      Section 6.3.2.3.2. 6.3.2.4.2.

   key_exchange
      Key exchange information.  The contents of this field are
      determined by the specified group and its corresponding
      definition.  Endpoints MUST NOT send empty or otherwise invalid
      key_exchange values for any reason.

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

      struct {
          select (role) {
              case client:
                  KeyShareEntry client_shares<4..2^16-1>; client_shares<0..2^16-1>;

              case server:
                  KeyShareEntry server_share;
          }
      } KeyShare;

   client_shares
      A list of offered KeyShareEntry values in descending order of
      client preference.  This vector MAY be empty if the client is
      requesting a HelloRetryRequest.  The ordering of values here
      SHOULD match that of the ordering of offered support in the
      "supported_groups" extension.

   server_share
      A single KeyShareEntry value for the negotiated cipher suite.

   Servers offer exactly one KeyShareEntry value, which corresponds to
   the key exchange used for the negotiated cipher suite.

   Clients offer an arbitrary number of KeyShareEntry values, each
   representing a single set of key exchange parameters.  For instance,
   a client might offer shares for several elliptic curves or multiple
   integer DH
   FFDHE groups.  The key_exchange values for each KeyShareEntry MUST by
   generated independently.  Clients MUST NOT offer multiple
   KeyShareEntry values for the same parameters. group and servers receiving
   multiple KeyShareEntry values for the same group MUST abort the
   connection with a fatal "illegal_parameter" alert.  Clients and
   servers MUST NOT offer or accept any KeyShareEntry values for groups
   not listed in the client's "supported_groups" extension.  Servers
   MUST NOT offer a KeyShareEntry value for a group not offered by the
   client in its corresponding KeyShare.  Implementations receiving any KeyShare
   containing any of these prohibited values MUST abort the connection
   with a fatal "illegal_parameter" alert.

   If the server selects an (EC)DHE cipher suite and no mutually
   supported group is available between the two endpoints' KeyShare
   offers, yet there is a mutually supported group that can be found via
   the "supported_groups" extension, then the server MUST reply with a
   HelloRetryRequest.  If there is no mutually supported group at all,
   the server MUST NOT negotiate an (EC)DHE cipher suite.

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

6.3.2.3.1.

6.3.2.4.1.  Diffie-Hellman Parameters

   Diffie-Hellman [DH] parameters for both clients and servers are
   encoded in the opaque key_exchange field of a KeyShareEntry in a
   KeyShare structure.  The opaque value contains the Diffie-Hellman
   public value (dh_Y = g^X mod p), encoded as a big-endian integer.

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

6.3.2.3.2. value (Y = g^X mod p), encoded as a big-endian integer, padded
   with zeros to the size of p.

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

6.3.2.4.2.  ECDHE Parameters

   ECDHE parameters for both clients and servers are encoded in the the
   opaque key_exchange field of a KeyShareEntry in a KeyShare structure.
   The opaque value conveys the Elliptic Curve Diffie-Hellman public
   value (ecdh_Y) represented as a byte string ECPoint.point.

      opaque point <1..2^8-1>;

   point

   For secp256r1, secp384r1 and secp521r1, this is the contents are the byte
   string representation of an elliptic curve point public value following the
   conversion routine in Section 4.3.6 of ANSI X9.62 [X962].  For x25519 and
      x448, this is raw opaque octet-string representation of point (in
      the format those functions use), 32 octets for x25519 and 56
      octets for x448.

   Although X9.62 supports multiple point formats, any given curve MUST
   specify only a single point format.  All curves currently specified
   in this document MUST only be used with the uncompressed point format
   (the format for all ECDH functions is considered uncompressed).

   For x25519 and x448, the contents 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 negotiation; TLS
   1.3 removes this feature in favor of a single point format for each
   curve.

6.3.2.4.

6.3.2.5.  Pre-Shared Key Extension

   The "pre_shared_key" extension is used to indicate the identity of
   the pre-shared key to be used with a given handshake in association
   with a PSK or (EC)DHE-PSK cipher suite (see [RFC4279] for
   background).

   Clients which offer one or more PSK cipher suites MUST send at least
   one supported psk_identity value and servers MUST NOT negotiate any
   of these cipher suites unless a supported value was provided.  If
   this extension is not provided and no alternative cipher suite is
   available, the server MUST close the connection with a fatal
   "missing_extension" alert.  (see Section 8.2)

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

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

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

              case server:
                  psk_identity identity;
                  uint16 selected_identity;
          }
      } PreSharedKeyExtension;

   identity
      An opaque label

   identities
      A list of the identities (labels for keys) that the pre-shared key. client is
      willing to negotiate with the server.

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

   If no suitable identity is provided, the server MUST NOT negotiate a
   PSK cipher suite and MAY respond with an "unknown_psk_identity" alert
   message.  Sending this alert is OPTIONAL; servers MAY instead choose
   to send a "decrypt_error" alert to merely indicate an invalid PSK
   identity or instead negotiate use of a non-PSK cipher suite, if
   available.

   If the server selects a PSK cipher suite, it MUST send a
   "pre_shared_key" extension with the identity that it selected.  The
   client MUST verify that the server has selected one of server's selected_identity is within the identities
   that
   range supplied by the client supplied. client.  If any other identity value is returned, the
   client MUST generate a fatal "unknown_psk_identity" alert and close
   the connection.

6.3.2.6.  OCSP Status Extensions

   [RFC6066] and [RFC6961] provide extensions to negotiate the server
   sending OCSP responses to the client.  In TLS 1.2 and below, the
   server sends 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 EncryptedExtensions.  Specifically: The body of the
   "status_request" or "status_request_v2" extension from the server
   MUST be a CertificateStatus structure as defined in [RFC6066] and
   [RFC6961] respectively.

   Note: this means that the certificate status appears prior to the
   certificates it applies to.  This is returned, slightly anomalous but matches
   the
   client MUST generate a fatal "unknown_psk_identity" alert existing behavior for SignedCertificateTimestamps [RFC6962], and close
   is more easily extensible in the connection.

6.3.2.5. handshake state machine.

6.3.2.7.  Early Data Indication

   In cases where TLS clients have previously interacted with the server
   and the server has supplied a ServerConfiguration (Section 6.3.3.3),

   When PSK resumption is used, the client can send application data and in
   its Certificate/
   CertificateVerify messages (if client authentication is required). first flight of messages.  If the client opts to do so, it MUST
   supply an "early_data" extension as well as the "pre_shared_key"
   extension.

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

      struct {
          select (Role) {
              case client:
                  opaque configuration_id<1..2^16-1>;
                  CipherSuite cipher_suite;
                  Extension extensions<0..2^16-1>;
                  opaque context<0..255>;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

   configuration_id
      The label for the configuration in question.

   cipher_suite
      The cipher suite which the client is using to encrypt the early
      data.

   extensions
      The extensions required to define the cryptographic configuration
      for the clients early data (see below for details).

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

   The client specifies

   All of the cryptographic configuration parameters for the 0-RTT data using the "configuration_id", "cipher_suite", and "extensions"
   values.  For configurations received in-band (in a previous TLS
   connection) the client MUST:

   -  Send (symmetric cipher suite,
   ALPN, etc.)  MUST be those which were negotiated in the same cryptographic determining parameters
      (Section Section 6.3.2.5.1) with connection
   which established the previous connection.  If a
      0-RTT handshake is being used with a PSK.  The PSK that was negotiated via a
      non-PSK handshake, then used to encrypt the client early data
   MUST use be the same symmetric
      cipher parameters as were negotiated on that handshake but with a first PSK cipher suite.

   -  Indicate the same parameters as the server indicated listed in that
      connection. the client's "pre_shared_key"
   extension.

   0-RTT messages sent in the first flight have the same content types
   as their corresponding messages sent in other flights (handshake,
   application_data, and alert respectively) but are protected under
   different keys.  After all the 0-RTT application data messages (if
   any) have been sent, a "end_of_early_data" alert of type "warning" is
   sent to indicate the end of the flight.  Clients which do 0-RTT MUST
   always send "end_of_early_data" even if the ServerConfiguration
   indicates that no application data is allowed (EarlyDataType of
   "client_authentication"), though in that case it MUST NOT send any
   non-empty data records (i.e., those which consist a "end_of_early_data" alert of anything other
   than padding). type "warning" is
   sent to indicate the end of the flight.  0-RTT MUST always be
   followed by an "end_of_early_data" alert.

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

   -  Ignore the extension and return no response.  This indicates that
      the server has ignored any early data and an ordinary 1-RTT
      handshake is required.

   -  Return an empty extension, 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.

   [[OPEN ISSUE: are the rules below correct? https://github.com/tlswg/
   tls13-spec/issues/451]] Prior to accepting the "early_data"
   extension, the server MUST
   perform the following checks:

   -  The configuration_id matches a known server configuration.

   -  The client's cryptographic determining parameters match validate that the session ticket
   parameters are consistent with its current configuration.  It MUST
   also validate that the server has extensions negotiated based on in the rest of previous
   connection are identical to those being negotiated in the
      ClientHello.  If (EC)DHE is selected, this includes verifying that
      (1)
   ServerHello, with the ClientHello contains a key from exception of the same group that is
      indicated by following extensions:

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

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

   -  The values of "key_share", "pre_shared_key", and (2) "early_data",
      which MUST be as defined in this document.

   In addition, it MUST validate that the server has
      negotiated that group and will therefore include ticket_age is within a share from that
      group in its own "key_share" extension. small
   tolerance of the time since the ticket was issued (see
   Section 6.3.2.7.2).

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

   If the server chooses to accept the "early_data" extension, then it
   MUST comply with the same error handling requirements specified for
   all records when processing early data records.  Specifically,
   decryption failure of any 0-RTT record following an accepted
   "early_data" extension MUST produce a fatal "bad_record_mac" alert as
   per Section 5.2.2.

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

   [[OPEN ISSUE: This just specifies  Implementations SHOULD determine the signaling security
   parameters for 0-RTT but not the
   the 0-RTT cryptographic transforms, including:

   -  What is in the handshake hash (including potentially some
      speculative data from the server).

   -  What is signed in the client's CertificateVerify.

   -  Whether we really want the Finished to not include the server's
      data at all.

   What's here now needs a lot 1-RTT phase of cleanup the connection entirely before it is clear
   processing the EncryptedExtensions and
   correct.]]

6.3.2.5.1.  Cryptographic Determining Parameters

   In order Finished, using those values
   solely to allow the server determine whether to decrypt accept or reject 0-RTT data, data.

   [[TODO: How does the client needs
   to provide enough information to allow the server to decrypt behave if the
   traffic without negotiation.  This indication is accomplished by having the
   client indicate the "cryptographic determining parameters" in its
   ClientHello, which rejected.]]

6.3.2.7.1.  Processing Order

   Clients are necessary permitted to decrypt the client's packets
   (i.e., those present in the ServerHello).  This includes the
   following values:

   -  The cipher suite identifier.

   -  If (EC)DHE is being used, "stream" 0-RTT data until they receive the
   server's version of the "key_share"
      extension.

   -  If PSK is being used, Finished, only then sending the server's version of "end_of_early_data" alert.
   In order to avoid deadlock, when accepting "early_data", servers MUST
   process the "pre_shared_key"
      extension (indicating client's Finished and then immediately send the PSK
   ServerHello, rather than waiting for the client is using).

6.3.2.5.2. client's "end_of_early_data"
   alert.

6.3.2.7.2.  Replay Properties

   As noted in Section 6.2.2, 6.2.3, TLS does not provide any inter-connection provides only a limited inter-
   connection mechanism for replay protection for data sent by the
   client in the first flight.  As a special case, implementations where the server
   configuration, is delivered out of band (as has been proposed for
   DTLS-SRTP [RFC5763]), MAY use a unique server configuration
   identifier for each connection, thus preventing replay.
   Implementations are responsible for ensuring uniqueness of the
   identifier in this case.

6.3.3.  Server Parameters

6.3.3.1.  Encrypted Extensions

   When this message will be sent:

   The EncryptedExtensions message MUST be "ticket_age" extension sent immediately after by the
      ServerHello message.  This is client SHOULD be used by
   servers to limit the first message that is encrypted
      under keys derived from ES.

   Meaning of this message:

      The EncryptedExtensions message simply contains any extensions time over which should the first flight might be protected, i.e., any
   replayed.  A server can store the time at which are not needed it sends a server
   configuration to
      establish a client, or encode the cryptographic context.  The same extension types
      MUST NOT appear time in both a ticket.  Then,
   each time it receives an early_data extension, it can check to see if
   the ServerHello and EncryptedExtensions.
      If value used by the same extension appears in both locations, client matches its expectations.

   The "ticket_age" value provided by the client MUST
      rely only will be shorter than
   the actual time elapsed on the value in server by a single round trip time.
   This difference is comprised of the EncryptedExtensions block.  All
      server-sent extensions other than those explicitly listed in
      Section 6.3.1.2 or designated delay in sending the IANA registry MUST only
      appear in EncryptedExtensions.  Extensions which are designated
   NewSessionTicket message to
      appear in ServerHello MUST NOT appear in EncryptedExtensions.
      Clients MUST check EncryptedExtensions for the presence of any
      forbidden extensions and if any are found MUST terminate client, plus the
      handshake with a "illegal_parameter" alert.

   Structure of this message:

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

   extensions
      A list of extensions.

6.3.3.2.  Certificate Request

   When time taken to send
   the ClientHello to the server.  For this message will be sent:

      A non-anonymous server can optionally request reason, a certificate from server SHOULD
   measure the client, if appropriate round trip time prior to sending the NewSessionTicket
   message and account for that in the selected cipher suite.  This
      message, if sent, will follow EncryptedExtensions.

   Structure value it saves.

   There are several potential sources of this message:

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

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

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

   certificate_request_context
      An opaque string which identifies the certificate request error that make an exact
   measurement of time difficult.  Variations in client and
      which will server
   clocks are likely to be echoed minimal, outside of gross time corrections.
   Network propagation delays are most likely causes of a mismatch in
   legitimate values for elapsed time.  Both the client's Certificate message.  The
      certificate_request_context MUST NewSessionTicket and
   ClientHello messages might be unique within retransmitted and therefore delayed,
   which might be hidden by TCP.

   A small allowance for errors in clocks and variations in measurements
   is advisable.  However, any allowance also increases the scope of opportunity
   for replay.  In this connection (thus preventing case, it is better to reject early data than to
   risk greater exposure to replay of attacks.

6.3.2.8.  Ticket Age

      struct {
          uint32 ticket_age;
      } TicketAge;

   When the client
      CertificateVerify messages).

   supported_signature_algorithms
      A list of sends the signature algorithms that "early_data" extension, it MUST also send a
   "ticket_age" extension in its EncryptedExtensions block.  This value
   contains the time elapsed since the client learned about the server
   configuration that it is able to
      verify, listed using, in descending order of preference.  Any
      certificates provided milliseconds.  This value can be
   used by the client MUST server to limit the time over which early data can be signed using
   replayed.  Note: because ticket lifetimes are restricted to a
      signature algorithm found in supported_signature_algorithms.

   certificate_authorities
      A list of the distinguished names [X501] of acceptable
      certificate_authorities, represented week,
   32 bits is enough to represent any plausible age, even in DER-encoded [X690] format.
      These distinguished names may specify a desired distinguished name
      for a root CA or for a subordinate CA; thus,
   milliseconds.

6.3.3.  Server Parameters

6.3.3.1.  Encrypted Extensions

   When this message can will be
      used to describe known roots as well as a desired authorization
      space.  If sent:

      In all handshakes, the certificate_authorities list server MUST send the EncryptedExtensions
      message immediately after the ServerHello message.  This is empty, then the
      client MAY send any certificate
      first message that meets the rest of is encrypted under keys derived from
      handshake_traffic_secret.  If the
      selection criteria client indicates "early_data" in
      its ClientHello, it MUST also send EncryptedExtensions immediately
      following the CertificateRequest, unless there is some
      external arrangement ClientHello and immediately prior to the contrary.

   certificate_extensions
      A list Finished.

   Meaning of certificate this message:

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

   The same extension OIDs [RFC5280] with their allowed
      values, represented types MUST NOT appear in DER-encoded [X690] format.  Some
      certificate extension OIDs allow multiple values (e.g.  Extended
      Key Usage).  If the server has included a non-empty
      certificate_extensions list, both the client certificate MUST contain
      all of ServerHello and
   EncryptedExtensions.  If the specified same extension OIDs that appears in both
   locations, the client recognizes.
      For each extension OID recognized by the client, all of the
      specified values MUST be present in rely only on the client certificate (but value in the certificate MAY have
   EncryptedExtensions block.  All server-sent extensions other values as well).  However, than
   those explicitly listed in Section 6.3.1.2 or designated in the
      client IANA
   registry MUST ignore and skip any unrecognized certificate extension
      OIDs.  If only appear in EncryptedExtensions.  Extensions which
   are designated to appear in ServerHello MUST NOT appear in
   EncryptedExtensions.  Clients MUST check EncryptedExtensions for the client has ignored some
   presence of the required certificate
      extension OIDs, any forbidden extensions and supplied a certificate that does not satisfy
      the request, the server MAY at its discretion either continue the
      session without client authentication, or if any are found MUST
   terminate the session handshake with a fatal unsupported_certificate an "illegal_parameter" alert.  PKIX RFCs define a
      variety of certificate extension OIDs and their corresponding
      value types.  Depending on

   The client's EncryptedExtensions apply only to the type, matching certificate
      extension values are not necessarily bitwise-equal.  It is
      expected that TLS implementations will rely on their PKI libraries early data with
   which they appear.  Servers MUST NOT use them to perform certificate selection using certificate extension OIDs.
      This document defines matching rules for two standard certificate negotiate the rest
   of the handshake.  Only those extensions defined explicitly designated as
   being included in [RFC5280]:

      o  The Key Usage extension 0-RTT Encrypted Extensions in a certificate matches the request
         when all key usage bits asserted IANA registry can
   be sent in the client's EncryptedExtensions.

   Structure of this message:

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

   extensions
      A list of extensions.

6.3.3.2.  Certificate Request

   When this message will be sent:

      A non-anonymous server can optionally request are also
         asserted in the Key Usage certificate extension.

      o  The Extended Key Usage extension in a certificate matches from
      the
         request when all key purpose OIDs present in client, if appropriate for the selected cipher suite.  This
      message, if sent, will follow EncryptedExtensions.

   Structure of this message:

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

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

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

   certificate_request_context
      An opaque string which identifies the certificate request are
         also found and
      which will be echoed in the Extended Key Usage certificate extension. client's Certificate message.  The special anyExtendedKeyUsage OID
      certificate_request_context MUST be unique within the scope of
      this connection (thus preventing replay of client
      CertificateVerify messages).

   supported_signature_algorithms
      A list of the signature algorithms that the server is able to
      verify, listed in descending order of preference.  Any
      certificates provided by the client MUST NOT be used signed using a
      signature algorithm found in supported_signature_algorithms.

   certificate_authorities
      A list of the
         request.

      Separate specifications distinguished names [X501] of acceptable
      certificate_authorities, represented in DER-encoded [X690] format.

      These distinguished names may define matching rules specify a desired distinguished name
      for other
      certificate extensions.

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

6.3.3.3.  Server Configuration

   When a subordinate CA; thus, this message will can be sent:

      This message is
      used to provide describe known roots as well as a server configuration which the
      client can use in desired authorization
      space.  If the future to skip handshake negotiation and
      (optionally) to allow 0-RTT handshakes.  The ServerConfiguration
      message certificate_authorities list is sent as empty, then the last message before
      client MAY send any certificate that meets the CertificateVerify.

   Structure rest of this Message:

        enum { (65535) } ConfigurationExtensionType;

        enum { client_authentication(1), early_data(2),
               client_authentication_and_data(3), (255) } EarlyDataType;

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

        struct {
            opaque configuration_id<1..2^16-1>;
            uint32 expiration_date;
            KeyShareEntry static_key_share;
            EarlyDataType early_data_type;
            ConfigurationExtension extensions<0..2^16-1>;
        } ServerConfiguration;

   configuration_id
      The configuration identifier to be used the
      selection criteria in 0-RTT mode.

   expiration_date
      The last time when this configuration the CertificateRequest, unless there is expected some
      external arrangement to be valid (in
      seconds since the Unix epoch).  Servers MUST NOT use any value
      more than 604800 seconds (7 days) in the future.  Clients MUST NOT
      cache configurations for longer than 7 days, regardless contrary.

   certificate_extensions
      A list of certificate extension OIDs [RFC5280] with their allowed
      values, represented in DER-encoded [X690] format.  Some
      certificate extension OIDs allow multiple values (e.g.  Extended
      Key Usage).  If the
      expiration_date.  [[OPEN ISSUE: Is this server has included a non-empty
      certificate_extensions list, the right value?  The idea
      is just to minimize exposure.]]

   static_key_share
      The long-term DH key that is being established for this
      configuration.

   early_data_type
      The type client certificate MUST contain
      all of 0-RTT handshake the specified extension OIDs that this configuration is to be used
      for (see Section 6.3.2.5).  If "client_authentication" or
      "client_authentication_and_data", then the client recognizes.
      For each extension OID recognized by the client, all of the
      specified values MUST select be present in the client certificate for future handshakes based on (but
      the CertificateRequest
      parameters certificate MAY have other values as well).  However, the
      client MUST ignore and skip any unrecognized certificate extension
      OIDs.  If the client has ignored some of the required certificate
      extension OIDs, and supplied in this handshake.  The a certificate that does not satisfy
      the request, the server MUST NOT send MAY at its discretion either of these two options unless it also requested continue the
      session without client authentication, or terminate the session
      with a fatal unsupported_certificate alert.  PKIX RFCs define a
      variety of certificate extension OIDs and their corresponding
      value types.  Depending on this handshake.  [[OPEN ISSUE: Should we relax this?]]

   extensions
      This field is a placeholder for future extensions to the
      ServerConfiguration format.

   The semantics of this message type, matching certificate
      extension values are not necessarily bitwise-equal.  It is
      expected that TLS implementations will rely on their PKI libraries
      to establish a shared state between
   the client and server perform certificate selection using certificate extension OIDs.
      This document defines matching rules for use with the "early_data" two standard certificate
      extensions defined in [RFC5280]:

      o  The Key Usage extension with in a certificate matches the request
         when all key specified usage bits asserted in "static_key_share" and with the handshake
   parameters negotiated by this handshake.

   When the ServerConfiguration message is sent, the server MUST also
   send a Certificate message and a CertificateVerify message, even if the "early_data" request are also
         asserted in the Key Usage certificate extension.

      o  The Extended Key Usage extension was used for this handshake, thus
   requiring in a signature over certificate matches the configuration before it can be used by
         request when all key purpose OIDs present in the client.  Clients request are
         also found in the Extended Key Usage certificate extension.
         The special anyExtendedKeyUsage OID MUST NOT rely on the ServerConfiguration message
   until successfully receiving and processing be used in the server's Certificate,
   CertificateVerify, and Finished.  If there
         request.

      Separate specifications may define matching rules for other
      certificate extensions.

   Note: It is a failure in processing
   those messages, the fatal "handshake_failure" alert for an anonymous server
   to request client MUST discard the ServerConfiguration. authentication.

6.3.4.  Authentication Messages

   As discussed in Section 6.2, TLS uses a common set of messages for
   authentication, key confirmation, and handshake integrity:
   Certificate, CertificateVerify, and Finished.  These messages are
   always sent as the 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.

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

   -  The certificate and signing key to be used.

   -  A Handshake Context based on the handshake hash (see
      Section 7.3.1). of the handshake messages

   -  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 the 0-RTT case.

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

   Finished
      A MAC over the hash of Handshake value Hash(Handshake Context + Certificate +
      CertificateVerify
      CertificateVerify) + Hash(resumption_context) using a MAC key
      derived from the base key.

   Because the CertificateVerify signs the Handshake Context +
   Certificate and the Finished MACs the Handshake Context + Certificate
   + CertificateVerify, this is mostly equivalent to keeping a running
   hash of the handshake messages (exactly so in the pure 1-RTT cases).
   Note, however, that subsequent post-handshake authentications do not
   include each other, just the messages through the end of the main
   handshake.

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

   +----------------+-----------------------------------------+--------+

   +------------+--------------------------------+---------------------+
   | Mode       | Handshake Context              | Base   |
   |                |                                         | Key            |
   +----------------+-----------------------------------------+--------+
   +------------+--------------------------------+---------------------+
   | 0-RTT      | ClientHello + ServerConfiguration +     | xSS    |
   |                | Server Certificate + CertificateRequest                    | early_traffic_secre |
   |            | (where ServerConfiguration, etc. are                                | t                   |
   |            | from the previous handshake)                                |                     |
   | 1-RTT      | ClientHello ... later of Encry | handshake_traffic_s |
   | 1-RTT (Server)   | ClientHello ... ServerConfiguration ptedExtensions/CertificateRequ | master ecret               |
   |            | est                            | secret                     |
   |            |                                |                     |
   | 1-RTT (Client)      | ClientHello ... ServerFinished | master handshake_traffic_s |
   | (Client)   |                                | secret ecret               |
   |            |                                |                     |
   | Post-Handshake Post-      | ClientHello ... ClientFinished + | master traffic_secret_0    |
   | Handshake  | + CertificateRequest           | secret                     |
   +----------------+-----------------------------------------+--------+

   Note 1:  The ServerConfiguration, CertificateRequest, and Server
      Certificate in the 0-RTT case are the messages from the handshake
      where the ServerConfiguration was established.

   Note 2:
   +------------+--------------------------------+---------------------+

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

6.3.4.1.  Certificate

   When this message will be sent:

      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 whenever the if and only if server
      has requested client authentication via a CertificateRequest
      message (Section 6.3.3.2) or when the EarlyDataType provided with the
      server configuration indicates a need for client authentication.
      This message is only sent if 6.3.3.2).  If the server requests a certificate via
      one of these mechanisms.  If 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).

   Meaning of this message:

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

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

   Structure of this message:

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

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

   certificate_request_context:
      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 or client authentication in 0-RTT,
      this field SHALL be zero length.

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

   Note: Prior to TLS 1.3, "certificate_list" ordering required each
   certificate to certify the one immediately preceding it, however some
   implementations allowed some flexibility.  Servers sometimes send
   both a current and deprecated intermediate for transitional purposes,
   and others are simply configured incorrectly, but these cases can
   nonetheless be validated properly.  For maximum compatibility, all
   implementations SHOULD be prepared to handle potentially extraneous
   certificates and arbitrary orderings from any TLS version, with the
   exception of the end-entity certificate which MUST be first.

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

6.3.4.1.1.  Server Certificate Selection

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

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

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

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

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

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

   All certificates provided by the server MUST be signed by a signature
   algorithm that appears in the "signature_algorithms" extension
   provided by the client, if they are able to provide such a chain (see
   Section 6.3.2.1). 6.3.2.2).  Certificates that are self-signed or certificates
   that are expected to be trust anchors are not validated as part of
   the chain and therefore MAY be signed with any algorithm.

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

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

   As cipher suites that specify new key exchange methods are specified
   for the TLS protocol, they will imply the certificate format and the
   required encoded keying information.

6.3.4.1.2.  Client Certificate Selection

   The following rules apply to certificates sent by the client:

   In particular:

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

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

   -  The certificates MUST be signed using an acceptable hash/
      signature algorithm pair, as described in Section 6.3.3.2.  Note
      that this relaxes the constraints on certificate-signing
      algorithms found in prior versions of TLS.

   -  If the certificate_extensions list in the certificate request
      message was non-empty, the end-entity certificate MUST match the
      extension OIDs recognized by the client, as described in
      Section 6.3.3.2.

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

6.3.4.1.3.  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
   terminate the handshake with a fatal "decode_error" alert.

   If the client does not send any certificates, the server MAY at its
   discretion either continue the handshake without client
   authentication, or respond with a fatal "handshake_failure" 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 send a fatal alert.

   Any endpoint receiving any certificate signed using any signature
   algorithm using an MD5 hash MUST send a "bad_certificate" alert
   message and close the connection.

   SHA-1 is deprecated and therefore NOT RECOMMENDED.  Endpoints that
   reject certification paths due to use of a deprecated hash MUST send
   a fatal "bad_certificate" alert message before closing the
   connection.  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 a an ECDSA key).

6.3.4.2.  Certificate Verify

   When this message will be sent:

      This message is used to provide explicit proof that an endpoint
      possesses the private key corresponding to its certificate and
      also provides integrity for the handshake up to this point.
      Servers MUST send this message when using a cipher suite which is
      authenticated via a certificate.  Clients MUST send this message
      whenever authenticating via a Certificate (i.e., when the
      Certificate message is non-empty).  When sent, this message MUST
      appear immediately after the Certificate Message and immediately
      prior to the Finished message.

   Structure of this message:

      struct {
           digitally-signed struct {
              opaque hashed_data[hash_length];
           };
      } CertificateVerify;

      Where hashed_data is the hash output described in Section 6.3.4,
      namely Hash(Handshake Context + Certificate). Certificate) +
      Hash(resumption_context).  For concreteness, this means that the
      value that is signed is:

          padding + context_string + 00 + hashed_data

      The context string for a server signature is "TLS 1.3, server
      CertificateVerify" and for a client signature is "TLS 1.3, client
      CertificateVerify".  A hash of the handshake messages is signed
      rather than the messages themselves because the digitally-signed
      format requires padding and context bytes at the beginning of the
      input.  Thus, by signing a digest of the messages, an
      implementation need only needs to maintain one a single running hash per
      hash type for CertificateVerify, Finished and other messages.

      If 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 6.3.2.1). 6.3.2.2).  Note that there is a possibility for
      inconsistencies here.  For instance, the client might offer
      ECDHE_ECDSA key exchange but omit any ECDSA and EdDSA values from
      its "signature_algorithms" extension.  In order to negotiate
      correctly, the server MUST check any candidate cipher suites
      against the "signature_algorithms" extension before selecting
      them.  This is somewhat inelegant but is a compromise designed to
      minimize changes to the original cipher suite design.

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

      In addition, the signature algorithm MUST be compatible with the
      key in the sender's end-entity certificate.  RSA signatures MUST
      use an RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS-
      v1_5 algorithms appear in "signature_algorithms".  SHA-1 MUST NOT
      be used in any signatures in CertificateVerify.  (Note that
      rsa_pkcs1_sha1 and dsa_sha1, the only defined SHA-1 signature
      algorithms, are undefined for CertificateVerify signatures.)

   Note: When used with non-certificate-based handshakes (e.g., PSK),
   the client's signature does not cover the server's certificate
   directly, although it does cover the server's Finished message, which
   transitively includes the server's certificate when the PSK derives
   from a certificate-authenticated handshake.  [PSK-FINISHED] describes
   a concrete attack on this mode if the Finished is omitted from the
   signature.  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.  In order to ensure this,
   implementations MUST NOT mix certificate-based client authentication
   with pure PSK modes (i.e., those where the PSK was not derived from a
   previous non-PSK handshake).

6.3.4.3.  Finished

   When this message will be sent:

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

   Meaning of this message:

      Recipients of Finished messages MUST verify that the contents are
      correct.  Once a side has sent its Finished message and received
      and validated the Finished message from its peer, it may begin to
      send and receive application data over the connection.  This data
      will be protected under keys derived from the ephemeral secret
      (see Section 7).

   The key used to compute the finished message is computed from the
   Base key defined in Section 6.3.4 using HKDF (see Section 7.1).
   Specifically:

   client_finished_key =
       HKDF-Expand-Label(BaseKey, "client finished", "", L)

   server_finished_key =
       HKDF-Expand-Label(BaseKey, "server finished", "", L)

   Structure of this message:

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

   The verify_data value is computed as follows:

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

      * Only included if present.

   Where HMAC [RFC2104] uses the Hash algorithm for 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 the current version of TLS, it is the size of the HMAC
   output for the Hash used for the handshake.

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

6.3.5.  Post-Handshake Messages

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

6.3.5.1.  New Session Ticket Message

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

     resumption_psk = HKDF-Expand-Label(resumption_secret,
                                        "resumption psk", "", L)

     resumption_context = HKDF-Expand-Label(resumption_secret,
                                            "resumption context", "", L)

   The client MAY use this PSK for future handshakes by including it the
   ticket value in the "pre_shared_key" extension in its ClientHello
   (Section 6.3.2.4) 6.3.2.5) and supplying a suitable PSK cipher suite.  Servers
   may send multiple tickets on a single connection, for instance after
   post-handshake authentication.  For handshakes that do not use a
   resumption_psk, the resumption_context is a string of L zeroes.

    enum { (65535) } TicketExtensionType;

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

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

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

   flags
      A 32-bit value indicating the ways in which this ticket may be
      used (as an OR of the flags values).

   ticket_lifetime
      Indicates the lifetime in seconds as a 32-bit unsigned integer in
      network byte order from the time of ticket issuance.  Servers MUST
      NOT use any value more than 604800 seconds (7 days).  The value of
      zero indicates that the ticket should be discarded immediately.
      Clients MUST NOT cache session tickets for longer than 7 days,
      regardless of the ticket_lifetime.  It MAY delete the ticket
      earlier based on local policy.  A server MAY treat a ticket as
      valid for a shorter period of time than what is stated in the
      ticket_lifetime.

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

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

   [[TODO: Should we require that tickets

   The meanings of the flags are as follows:

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

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

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

   In all cases, the existing PSK or (EC)DHE-PSK cipher suites that the client
   offers/uses MUST have the same symmetric parameters (cipher/hash) as
   the cipher suite.  See suite negotiated for this connection.  If no flags are set
   that the TODO above about early_data and
   PSK.??] client recognizes, it MUST ignore the ticket.

6.3.5.2.  Post-Handshake Authentication

   The server is permitted to request client authentication at any time
   after the handshake has completed by sending a CertificateRequest
   message.  The client SHOULD respond with the appropriate
   Authentication messages.  If the client chooses to authenticate, it
   MUST send Certificate, CertificateVerify, and Finished.  If it
   declines, it MUST send a Certificate message containing no
   certificates followed by Finished.

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

6.3.5.3.  Key and IV Update

   struct {} KeyUpdate;

   The KeyUpdate handshake message is used to indicate that the sender
   is updating its sending cryptographic keys.  This message can be sent
   by the server after sending its first flight and the client after
   sending its second flight.  Implementations that receive a KeyUpdate
   message prior to receiving a Finished message as part of the 1-RTT
   handshake MUST generate a fatal "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
   their receiving keys and if they have not already updated their
   sending state up to or past the then current receiving generation
   MUST send their own KeyUpdate prior to sending any other messages.
   This mechanism allows either side to force an update to the entire
   connection.  Note that implementations may receive an arbitrary
   number of messages between sending a KeyUpdate and receiving the
   peer's KeyUpdate because those messages may already be in flight.

   Note that if implementations independently send their own KeyUpdates
   and they cross in flight, this only results in an update of one
   generation; when each side receives the other side's update it just
   updates its receive keys and notes that the generations match and
   thus no send update is needed.

   Note that the side which sends its KeyUpdate first needs to retain
   the traffic keys (though not the traffic secret) for the previous
   generation of keys until it receives the KeyUpdate from the other
   side.

   Both sender and receiver must 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.

7.  Cryptographic Computations

   In order to begin connection protection, the TLS Record Protocol
   requires specification of a suite of algorithms, a master secret, and
   the client and server random values.  The authentication, key
   exchange, and record protection algorithms are determined by the
   cipher_suite selected by the server and revealed in the ServerHello
   message.  The random values are exchanged in the hello messages.  All
   that remains is to calculate the key schedule.

7.1.  Key Schedule

   The TLS handshake establishes secret keying material which is then
   used to protect traffic.  This keying material is derived from the
   two input secret values: Static Secret (SS) and Ephemeral Secret
   (ES).

   The exact source of each of these secrets depends on values are exchanged in the operational
   mode (DHE, ECDHE, PSK, etc.) and hello messages.  All
   that remains is summarized in to calculate the table below:

   +-----------------+------------------------+------------------------+
   | Key Exchange    |     Static Secret (SS) |  Ephemeral Secret (ES) |
   +-----------------+------------------------+------------------------+
   | (EC)DHE (full   |    Client ephemeral w/ |    Client ephemeral w/ |
   | handshake)      |       server ephemeral |       server ephemeral |
   |                 |                        |                        |
   | (EC)DHE (w/     |    Client ephemeral w/ |    Client ephemeral w/ |
   | 0-RTT)          |          server static |       server ephemeral |
   |                 |                        |                        |
   | PSK             |         Pre-Shared Key |         Pre-shared key |
   |                 |                        |                        |
   | PSK + (EC)DHE   |         Pre-Shared schedule.

7.1.  Key |    Client ephemeral w/ |
   |                 |                        |       server ephemeral |
   +-----------------+------------------------+------------------------+

   These shared secret values Schedule

   The TLS handshake establishes one or more input secrets which are used
   combined to generate cryptographic keys create the actual working keying material, as
   shown detailed
   below.  The key derivation process is as follows, where L denotes the length makes use of the underlying hash function for following
   functions, based on HKDF [RFC5869].  SS and ES denote
   the sources from the table above. [RFC5869]:

   HKDF-Extract(Salt, IKM) as defined in {{RFC5869}}.

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

   Where HkdfLabel is specified as:

   struct HkdfLabel {
     uint16 length;
     opaque label<9..255>;
     opaque hash_value<0..255>;
   };

     Where:

   - HkdfLabel.length is Length
   - HkdfLabel.label is "TLS 1.3, " + Label
   - HkdfLabel.hash_value is HashValue.

     1. xSS = HKDF-Extract(0, SS). Note that HKDF-Extract always
        produces a value HashValue.

   Derive-Secret(Secret, Label, Messages) =
        HKDF-Expand-Label(Secret, Label,
                          Hash(Messages) + Hash(resumption_context), L))

   Given a set of n InputSecrets, the final "master secret" is computed
   by iteratively invoking HKDF-Extract with InputSecret_1,
   InputSecret_2, etc.  The initial secret is simply a string of 0s as
   long as the size of the Hash that is the basis for the HKDF.
   Concretely, for the present version of TLS 1.3, secrets are added in
   the following order:

   -  PSK

   -  (EC)DHE shared secret

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

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

   -  Derive-Secret's Secret argument is indicated by the arrow coming
      in from the left.  For instance, the Early Secret is the same length as Secret
      for generating the underlying hash
        function.

     2. xES = HKDF-Extract(0, ES)

     3. mSS early_traffic-secret.

                 0
                 |
                 v
   PSK ->  HKDF-Extract
                 |
                 v
           Early Secret  --> Derive-Secret(., "early traffic secret",
                 |                         ClientHello)
                 |                         = HKDF-Expand-Label(xSS, "expanded static early_traffic_secret
                 v
(EC)DHE -> HKDF-Extract
                 |
                 v
              Handshake
               Secret -----> Derive-Secret(., "handshake traffic secret",
                                handshake_hash, L)

     4. mES
                 |                         ClientHello + ServerHello)
                 |                         = HKDF-Expand-Label(xES, "expanded ephemeral handshake_traffic_secret
                 v
      0 -> HKDF-Extract
                 |
                 v
            Master Secret
                 |
                 +---------> Derive-Secret(., "application traffic secret",
                                handshake_hash, L)

     5. master_secret
                 |                         ClientHello...Server Finished)
                 |                         = HKDF-Extract(mSS, mES)

     6. traffic_secret_0 = HKDF-Expand-Label(master_secret,
                                             "traffic
                 |
                 +---------> Derive-Secret(., "exporter master secret",
                                             handshake_hash, L)

     Where handshake_hash includes all messages up through the
     server CertificateVerify message, but not including any
     0-RTT handshake messages (the server's Finished is not
     included because the master_secret is need to compute
     the finished key). [[OPEN ISSUE: Should we be including
     0-RTT handshake messages here and below?.
     https://github.com/tlswg/tls13-spec/issues/351
     ]] At this point,
     SS, ES, xSS, xES, mSS, and mES SHOULD be securely deleted,
     along with any ephemeral (EC)DH secrets.

     7. resumption_secret
                 |                         ClientHello...Client Finished)
                 |                         = HKDF-Expand-Label(master_secret,
                                              "resumption master secret"
                                              handshake_hash, L)

     8. exporter_secret = HKDF-Expand-Label(master_secret,
                                            "exporter
                 |
                 +---------> Derive-Secret(., "resumption master secret",
                                            handshake_hash, L)

     Where handshake_hash contains
                                           ClientHello...Client Finished)
                                           = resumption_secret

   The general pattern here is that the entire handshake up to
     and including secrets shown down the client's Finished, but not including
     any 0-RTT left side
   of the diagram are just raw entropy without context, whereas the
   secrets down the right side include handshake messages or post-handshake messages.
     AT this point master_secret SHOULD context and therefore
   can be securely deleted.

   The traffic used to derive working keys are computed from xSS, xES, and without additional context.  Note
   that the traffic_secret
   as described in Section 7.3 below.  The traffic_secret different calls to Derive-Secret may be updated
   throughout the connection as described in Section 7.2.

   Note: Although take different Messages
   arguments, even with the steps above are phrased as individual HKDF-Extract
   and HKDF-Expand operations, because each HKDF-Expand operation same secret.  In a 0-RTT exchange, Derive-
   Secret is
   paired called with an HKDF-Extract, it is possible to implement this key
   schedule four distinct transcripts; in a 1-RTT only
   exchange with three distinct transcripts.

   If a black-box HKDF API, albeit at some loss given secret is not available, then the 0-value consisting of efficiency
   as some HKDF-Extract operations will be repeated. a
   string of L zeroes is used.

7.2.  Updating Traffic Keys and IVs

   Once the handshake is complete, it is possible for either side to
   update its sending traffic keys using the KeyUpdate handshake message
   Section 6.3.5.3.  The next generation of traffic keys is computed by
   generating traffic_secret_N+1 from traffic_secret_N as described in
   this section then re-deriving the traffic keys as described in
   Section 7.3.

   The next-generation traffic_secret is computed as:

   traffic_secret_N+1 = HKDF-Expand-Label(traffic_secret_N, "traffic "application
   traffic secret", "", L)

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

7.3.  Traffic Key Calculation

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

   -  A secret value

   -  A phase value indicating the phase of the protocol the keys are
      being generated for.

   -  A purpose value indicating the specific value being generated

   -  The length of the key.

   -  The handshake context which is used to generate the keys.

   The keying material is computed using:

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

   The following table describes the inputs to the key calculation for
   each class of traffic keys:

   +-------------+---------+-----------------+-------------------------+

   +-------------+--------------------------+--------------------------+
   | Record Type | Secret                   | Phase                    |       Handshake Context |
   +-------------+---------+-----------------+-------------------------+
   +-------------+--------------------------+--------------------------+
   | 0-RTT       | xSS early_traffic_secret     | "early          |           ClientHello + |
   | Handshake   |         | handshake key     |   ServerConfiguration + |
   | Handshake   |                          | expansion"               |      Server Certificate |
   |
   |             |                          |                          |
   | 0-RTT       | xSS early_traffic_secret     | "early          |           ClientHello + |
   | Application |         | application     |   ServerConfiguration + |
   |             |         | data key        |      Server Certificate  |
   | Application |                          | key expansion"           |
   |             |                          |                          |
   |                         |
   | Handshake   | xES handshake_traffic_secret | "handshake key           |          ClientHello... |
   |             |                          | expansion"               |             ServerHello |
   |
   |             |                          |                          |
   | Application | traffic traffic_secret_N         | "application    |   ClientHello... Server |
   | Data        | secret  | data key    |                Finished |
   | Data        |                          | expansion"               |                         |
   +-------------+---------+-----------------+-------------------------+
   +-------------+--------------------------+--------------------------+

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

                 +------------------+--------------------+
                 | Key Type         | Purpose            |
                 +------------------+--------------------+
                 | Client Write Key | "client write key" |
                 |                  |                    |
                 | Server Write Key | "server write key" |
                 |                  |                    |
                 | Client Write IV  | "client write iv"  |
                 |                  |                    |
                 | Server Write IV  | "server write iv"  |
                 +------------------+--------------------+

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

7.3.1.  The Handshake Hash

   The handshake hash ("handshake_hash") is defined as the hash (using
   the Hash algorithm for the handshake) of all handshake messages sent
   or received, starting at ClientHello up to the present time, with the
   exception of the client's 0-RTT authentication messages (Certificate,
   CertificateVerify, and Finished) including the type and length fields
   of the handshake messages.  This is the concatenation of the
   exchanged Handshake structures in plaintext form (even if they were
   encrypted on the wire).  [[OPEN ISSUE: See https://github.com/tlswg/
   tls13-spec/issues/351 for the question of whether the 0-RTT handshake
   messages are hashed.]]

7.3.2.  Diffie-Hellman

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

   Note that contain all
   zero bits are stripped before it is used as the input to HKDF.

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

7.3.2.  Elliptic Curve Diffie-Hellman

   For secp256r1, secp384r1 and secp521r1, ECDH calculations (including
   parameter and key generation as well as the shared secret
   calculation) are performed according to [IEEE1363] using the ECKAS-
   DH1 scheme with the identity map as key derivation function (KDF), so
   that the shared secret is the x-coordinate of the ECDH shared secret
   elliptic curve point represented as an octet string.  Note that this
   octet string (Z in IEEE 1363 terminology) as output by FE2OSP, the
   Field Element to Octet String Conversion Primitive, has constant
   length for any given field; leading zeros found in this octet string
   MUST NOT be truncated.

   (Note that this use of the identity KDF is a technicality.  The
   complete picture is that ECDH is employed with a non-trivial KDF
   because TLS does not directly use this secret for anything other than
   for computing other secrets.)

   ECDH functions are used as follows:

   -  The public key to put into ECPoint.point the KeyShareEntry.key_exchange
      structure is the result of applying the ECDH function to the
      secret key of appropriate length (into scalar input) and the
      standard public basepoint (into u-coordinate point input).

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

   For X25519 and X448, see [RFC7748].

7.3.4.

7.3.3.  Exporters

   [RFC5705] defines keying material exporters for TLS in terms of the
   TLS PRF.  This document replaces the PRF with HKDF, thus requiring a
   new construction.  The exporter interface remains the same, however
   the value is computed as:

   HKDF-Expand-Label(HKDF-Extract(0, exporter_secret),

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

   Note: the inner HKDF-Extract is strictly unnecessary, but it
   maintains the invariant that HKDF Extract and Expand calls are
   paired. key_length)

8.  Mandatory Algorithms

8.1.  MTI Cipher Suites

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

       TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   These cipher suites MUST support both digital signatures and key
   exchange with secp256r1 (NIST P-256) and SHOULD support key exchange
   with X25519 [RFC7748].

   A TLS-compliant application SHOULD implement the following cipher
   suites:

       TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
       TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256

8.2.  MTI Extensions

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

   -  Signature Algorithms ("signature_algorithms"; Section 6.3.2.1) 6.3.2.2)

   -  Negotiated Groups ("supported_groups"; Section 6.3.2.2) 6.3.2.3)

   -  Key Share ("key_share"; Section 6.3.2.3) 6.3.2.4)

   -  Pre-Shared Key ("pre_shared_key"; Section 6.3.2.4) 6.3.2.5)

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

   -  Cookie ("cookie"; Section 6.3.2.1)

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

   -  "signature_algorithms" is REQUIRED for certificate authenticated
      cipher suites

   -  "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE
      cipher suites

   -  "pre_shared_key" is REQUIRED for PSK cipher suites

   -  "cookie" is REQUIRED for all cipher suites.

   When negotiating use of applicable cipher suites, endpoints MUST
   abort the connection with a "missing_extension" alert if the required
   extension was not provided.  Any endpoint that receives any invalid
   combination of cipher suites and extensions MAY abort the connection
   with a "missing_extension" alert, regardless of negotiated
   parameters.

   Additionally, all implementations MUST support use of 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 with a fatal "missing_extension"
   alert.

   Some of these extensions exist only for the client to provide
   additional data to the server in a backwards-compatible way and thus
   have no meaning when sent from a server.  The client-only extensions
   defined in this document are: "signature_algorithms" &
   "supported_groups".

   Servers MUST NOT send these extensions.  Clients
   receiving any of these extensions the "signature_algorithms" extension; if a
   client receives this extension it MUST respond with a fatal
   "unsupported_extension" alert and close the connection.

9.  Application Data Protocol

   Application data messages are carried by the record layer and are
   fragmented and encrypted based on the current connection state.  The
   messages are treated as transparent data to the record layer.

10.  Security Considerations

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

11.  IANA Considerations

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

   -  TLS Cipher Suite Registry: Values with the first byte in the range
      0-254 (decimal) are assigned via Specification Required [RFC2434].
      Values with the first byte 255 (decimal) are reserved for Private
      Use [RFC2434].  IANA [SHALL add/has added] a "Recommended" column
      to the cipher suite registry.  All cipher suites listed in
      Appendix A.4 are marked as "Yes".  All other cipher suites are
      marked as "No".  IANA [SHALL add/has added] add a note to this
      column reading:

         Cipher suites marked as "Yes" are those allocated via Standards
         Track RFCs.  Cipher suites marked as "No" are not; cipher
         suites marked "No" range from "good" to "bad" from a
         cryptographic standpoint.

   -  TLS ContentType Registry: Future values are allocated via
      Standards Action [RFC2434].

   -  TLS Alert Registry: Future values are allocated via Standards
      Action [RFC2434].

   -  TLS HandshakeType Registry: Future values are allocated via
      Standards Action [RFC2434].

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

   -  TLS ExtensionType Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal) are reserved
      for Private Use [RFC2434].  IANA [SHALL update/has updated] this
      registry to include the "key_share", "pre_shared_key", and
      "early_data" extensions as defined in this document.

      IANA [shall update/has updated] this registry to include a "TLS
      1.3" column with the following four values: "Client", indicating
      that the server shall not send them.  "Clear", indicating that
      they shall be in the ServerHello.  "Encrypted", indicating that
      they shall be in the EncryptedExtensions block, "Early",
      indicating that they shall be only in the client's 0-RTT
      EncryptedExtensions block, and "No" indicating that they are not
      used in TLS 1.3.  This column [shall be/has been] initially
      populated with the values in this document.  IANA [shall update/has update/
      has updated] this registry to add a "Recommended" column.  IANA
      [shall/has] initially populated this column with the values in the
      table below.  This table has been generated by marking Standards
      Track RFCs as "Yes" and all others as "No".

   +-----------------------------------------+-------------+-----------+

   +-------------------------------+-----------+-----------------------+
   | Extension                     | Recommended Recommend |               TLS 1.3 |
   +-----------------------------------------+-------------+-----------+
   |                               |        ed |                       |
   +-------------------------------+-----------+-----------------------+
   | server_name [RFC6066]         |       Yes |             Encrypted |
   |                               |           |                       |
   | max_fragment_length [RFC6066] |       Yes |             Encrypted |
   |                               |           |                       |
   | client_certificate_url [RFC6066]        |       Yes |             Encrypted |
   | [RFC6066]                     |           |                       |
   |                               |           |                       |
   | trusted_ca_keys [RFC6066]     |       Yes |             Encrypted |
   |                               |           |                       |
   | truncated_hmac [RFC6066]      |       Yes |                    No |
   |                               |           |                       |
   | status_request [RFC6066]      |       Yes |                    No |
   |                               |           |                       |
   | user_mapping [RFC4681]        |       Yes |             Encrypted |
   |                               |           |                       |
   | client_authz [RFC5878]        |        No |             Encrypted |
   |                               |           |                       |
   | server_authz [RFC5878]        |        No |             Encrypted |
   |                               |           |                       |
   | cert_type [RFC6091]           |       Yes |             Encrypted |
   |                               |           |                       |
   | supported_groups [RFC-ietf-tls- [RFC-ietf-   |       Yes |    Client             Encrypted |
   | negotiated-ff-dhe] tls-negotiated-ff-dhe]        |           |                       |
   |                               |           |                       |
   | ec_point_formats [RFC4492]    |       Yes |                    No |
   |                               |           |                       |
   | srp [RFC5054]                 |        No |                    No |
   |                               |           |                       |
   | signature_algorithms [RFC5246]          |       Yes |                Client |
   | [RFC5246]                     |           |                       |
   |                               |           |                       |
   | use_srtp [RFC5764]            |       Yes |             Encrypted |
   |                               |           |                       |
   | heartbeat [RFC6520]           |       Yes |             Encrypted |
   |                               |           |                       |
   | application_layer_protocol_negotiation application_layer_protocol_ne |       Yes |             Encrypted |
   | gotiation [RFC7301]           |           |                       |
   |                               |           |                       |
   | status_request_v2 [RFC6961]   |       Yes |             Encrypted |
   |                               |           |                       |
   | signed_certificate_timestamp [RFC6962]  |        No |             Encrypted |
   | [RFC6962]                     |           |                       |
   |                               |           |                       |
   | client_certificate_type [RFC7250]       |       Yes |             Encrypted |
   | [RFC7250]                     |           |                       |
   |                               |           |                       |
   | server_certificate_type [RFC7250]       |       Yes |             Encrypted |
   | [RFC7250]                     |           |                       |
   |                               |           |                       |
   | padding [RFC7685]             |       Yes |                Client |
   |                               |           |                       |
   | encrypt_then_mac [RFC7366]    |       Yes |                    No |
   |                               |           |                       |
   | extended_master_secret [RFC7627]        |       Yes |                    No |
   | [RFC7627]                     |           |                       |
   |                               |           |                       |
   | SessionTicket TLS [RFC4507]   |       Yes |                    No |
   |                               |           |                       |
   | renegotiation_info [RFC5746]  |       Yes |                    No |
   |                               |           |                       |
   | key_share [[this document]]   |       Yes |                 Clear |
   |                               |           |                       |
   | pre_shared_key [[this document]]         |       Yes |                 Clear |
   | document]]                    |           |                       |
   |                               |           |                       |
   | early_data [[this document]]  |       Yes |                 Clear |
   +-----------------------------------------+-------------+-----------+
   |                               |           |                       |
   | ticket_age [[this document]]  |       Yes |                 Early |
   |                               |           |                       |
   | cookie [[this document]]      |       Yes | Encrypted/HelloRetryR |
   |                               |           |                equest |
   +-------------------------------+-----------+-----------------------+

   In addition, this document defines two new registries to be
   maintained by IANA

   -  TLS SignatureScheme Registry: Values with the first byte in the
      range 0-254 (decimal) are assigned via Specification Required
      [RFC2434].  Values with the first byte 255 (decimal) are reserved
      for Private Use [RFC2434].  This registry SHALL have a
      "Recommended" column.  The registry [shall be/ has been] initially
      populated with the values described in Section 6.3.2.1. 6.3.2.2.  The
      following values SHALL be marked as "Recommended":
      ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, rsa_pss_sha256,
      rsa_pss_sha384, rsa_pss_sha512, ed25519.

   -  TLS ConfigurationExtensionType Registry: Values with the first
      byte in the range 0-254 (decimal) are assigned via Specification
      Required [RFC2434].  Values with the first byte 255 (decimal) are
      reserved for Private Use [RFC2434].  This registry SHALL have a
      "Recommended" column.  The registry [shall be/ has been] initially
      populated with the values described in Section 6.3.3.3, with all
      values marked with "Recommended" value "Yes".

12.  References

12.1.  Normative References

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

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

   [I-D.ietf-tls-chacha20-poly1305]
              Langley, A., Chang, W., Mavrogiannopoulos, N.,
              Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
              Cipher Suites for Transport Layer Security (TLS)", draft-
              ietf-tls-chacha20-poly1305-04 (work in progress), December
              2015.

   [I-D.irtf-cfrg-curves]
              Langley, A. and M. Hamburg, "Elliptic Curves for
              Security", draft-irtf-cfrg-curves-11 Transport Layer Security (TLS)", draft-
              ietf-tls-chacha20-poly1305-04 (work in progress),
              October December
              2015.

   [I-D.irtf-cfrg-eddsa]
              Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-04 draft-irtf-cfrg-eddsa-05
              (work in progress), March 2016.

   [I-D.mattsson-tls-ecdhe-psk-aead]
              Mattsson, J. and D. Migault, "ECDHE_PSK with AES-GCM and
              AES-CCM Cipher Suites for Transport Layer Security (TLS)",
              draft-mattsson-tls-ecdhe-psk-aead-03
              draft-mattsson-tls-ecdhe-psk-aead-05 (work in progress),
              December 2015.
              April 2016.

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

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              DOI 10.17487/RFC2434, October 1998,
              <http://www.rfc-editor.org/info/rfc2434>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.

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

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              DOI 10.17487/RFC5288, August 2008,
              <http://www.rfc-editor.org/info/rfc5288>.

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,
              <http://www.rfc-editor.org/info/rfc5289>.

   [RFC5487]  Badra, M., "Pre-Shared Key Cipher Suites for TLS with SHA-
              256/384 and AES Galois Counter Mode", RFC 5487,
              DOI 10.17487/RFC5487, March 2009,
              <http://www.rfc-editor.org/info/rfc5487>.

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

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

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

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

   [RFC6367]  Kanno, S. and M. Kanda, "Addition of the Camellia Cipher
              Suites to Transport Layer Security (TLS)", RFC 6367,
              DOI 10.17487/RFC6367, September 2011,
              <http://www.rfc-editor.org/info/rfc6367>.

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

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

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

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
              <http://www.rfc-editor.org/info/rfc7251>.

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

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

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

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

12.2.  Informative References

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

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

   [FI06]     Finney, H., "Bleichenbacher's RSA signature forgery based
              on implementation error", August 2006, <https://www.ietf.org/
              mail-archive/web/openpgp/current/msg00999.html>.
              <https://www.ietf.org/mail-archive/web/openpgp/current/
              msg00999.html>.

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

   [I-D.ietf-tls-cached-info]
              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-23 (work in progress), May 2016.

   [I-D.ietf-tls-negotiated-ff-dhe]
              Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
              ff-dhe-10 (work in progress), June 2015.

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

   [PKCS6]    RSA Laboratories, "PKCS #6: RSA Extended Certificate
              Syntax Standard, version 1.5", November 1993.

   [PKCS7]    RSA Laboratories, "PKCS #7: RSA Cryptographic Message
              Syntax Standard, version 1.5", November 1993.

   [PSK-FINISHED]
              Cremers, C., Horvat, M., van der Merwe, T., and S. Scott,
              "Revision 10: possible attack if client authentication is
              allowed during PSK", 2015, <https://www.ietf.org/mail-
              archive/web/tls/current/msg18215.html>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, DOI 10.17487/RFC1948, May 1996,
              <http://www.rfc-editor.org/info/rfc1948>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <http://www.rfc-editor.org/info/rfc4279>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <http://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <http://www.rfc-editor.org/info/rfc4303>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <http://www.rfc-editor.org/info/rfc4346>.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
              <http://www.rfc-editor.org/info/rfc4366>.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <http://www.rfc-editor.org/info/rfc4506>.

   [RFC4507]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 4507, DOI 10.17487/RFC4507, May
              2006, <http://www.rfc-editor.org/info/rfc4507>.

   [RFC4681]  Santesson, S., Medvinsky, A., and J. Ball, "TLS User
              Mapping Extension", RFC 4681, DOI 10.17487/RFC4681,
              October 2006, <http://www.rfc-editor.org/info/rfc4681>.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, DOI 10.17487/RFC5054, November
              2007, <http://www.rfc-editor.org/info/rfc5054>.

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

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

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

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

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

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <http://www.rfc-editor.org/info/rfc5763>.

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

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

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

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

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

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

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

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

   [RFC6962]  Laurie, B., Langley, A.,

   [RFC7230]  Fielding, R., Ed. and E. Kasper, "Certificate
              Transparency", J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 6962, 7230, DOI 10.17487/RFC6962, 10.17487/RFC7230, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>. 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

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

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

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

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

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

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

   [RFC7685]  Langley, A., "A Transport Layer Security (TLS) ClientHello
              Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
              October 2015, <http://www.rfc-editor.org/info/rfc7685>.

   [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the ACM v. 21, n. 2, pp.
              120-126., February 1978.

   [SLOTH]    Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              Network and Distributed System Security Symposium (NDSS
              2016) , 2016.

   [SSL2]     Netscape Communications Corp.,     Hickman, K., "The SSL Protocol", February 1995.

   [SSL3]     Freier, A., Karlton, P., and P. Kocher, "The SSL 3.0
              Protocol", November 1996.

   [TIMING]   Boneh, D. and D. Brumley, "Remote timing attacks are
              practical", USENIX Security Symposium, 2003.

   [X501]     "Information Technology - Open Systems Interconnection -
              The Directory: Models", ITU-T X.501, 1993.

12.3.  URIs

   [1] mailto:tls@ietf.org

Appendix A.  Protocol Data Structures and Constant Values

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

A.1.  Record Layer

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

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

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

   struct {
       ContentType opaque_type = application_data(23); /* see fragment.type */
       ProtocolVersion record_version = { 3, 1 };    /* TLS v1.x */
       uint16 length;
       aead-ciphered struct {
          opaque content[TLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
       } fragment;
   } TLSCiphertext;

A.2.  Alert Messages
      enum { warning(1), fatal(2), (255) } AlertLevel;

      enum {
          close_notify(0),
          end_of_early_data(1),
          unexpected_message(10),               /* fatal */
          bad_record_mac(20),                   /* fatal */
          decryption_failed_RESERVED(21),       /* fatal */
          record_overflow(22),                  /* fatal */
          decompression_failure_RESERVED(30),   /* fatal */
          handshake_failure(40),                /* fatal */
          no_certificate_RESERVED(41),          /* fatal */
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),                /* fatal */
          unknown_ca(48),                       /* fatal */
          access_denied(49),                    /* fatal */
          decode_error(50),                     /* fatal */
          decrypt_error(51),                    /* fatal */
          export_restriction_RESERVED(60),      /* fatal */
          protocol_version(70),                 /* fatal */
          insufficient_security(71),            /* fatal */
          internal_error(80),                   /* fatal */
          inappropriate_fallback(86),           /* fatal */
          user_canceled(90),
          no_renegotiation_RESERVED(100),       /* fatal */
          missing_extension(109),               /* fatal */
          unsupported_extension(110),           /* fatal */
          certificate_unobtainable(111),
          unrecognized_name(112),
          bad_certificate_status_response(113), /* fatal */
          bad_certificate_hash_value(114),      /* fatal */
          unknown_psk_identity(115),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

A.3.  Handshake Protocol

      enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          session_ticket(4),
          hello_retry_request(6),
          encrypted_extensions(8),
          certificate(11),
          server_key_exchange_RESERVED(12),
          certificate_request(13),
          server_hello_done_RESERVED(14),
          certificate_verify(15),
          client_key_exchange_RESERVED(16),
          server_configuration(17),
          finished(20),
          key_update(24),
          (255)
      } HandshakeType;

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

A.3.1.  Key Exchange Messages

      struct {
          opaque random_bytes[32];
      } Random;

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

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

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

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

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

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

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

      struct {
          select (role) {
              case client:
                  KeyShareEntry client_shares<4..2^16-1>; client_shares<0..2^16-1>;

              case server:
                  KeyShareEntry server_share;

          }
      } KeyShare;

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

      opaque point <1..2^8-1>;

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

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

              case server:
                  psk_identity identity;
                  uint16 selected_identity;
          }
      } PreSharedKeyExtension;

      struct {
          select (Role) {
              case client:
                  opaque configuration_id<1..2^16-1>;
                  CipherSuite cipher_suite;
                  Extension extensions<0..2^16-1>;
                  opaque context<0..255>;

              case server:
                 struct {};
          }
      } EarlyDataIndication;

      struct {
          uint32 ticket_age;
      } TicketAge;

A.3.1.1.  Cookie Extension

      struct {
          opaque cookie<0..255>;
      } Cookie;

A.3.1.2.  Signature Algorithm Extension
      enum {
          //
          /* RSASSA-PKCS-v1_5 algorithms. algorithms */
          rsa_pkcs1_sha1 (0x0201),
          rsa_pkcs1_sha256 (0x0401),
          rsa_pkcs1_sha384 (0x0501),
          rsa_pkcs1_sha512 (0x0601),

          // DSA algorithms (deprecated).
          dsa_sha1 (0x0202),
          dsa_sha256 (0x0402),
          dsa_sha384 (0x0502),
          dsa_sha512 (0x0602),

          //

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

          //

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

          //

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

          //

          /* Reserved Code Points. Points */
          dsa_sha1_RESERVED (0x0202),
          dsa_sha256_RESERVED (0x0402),
          dsa_sha384_RESERVED (0x0502),
          dsa_sha512_RESERVED (0x0602),
          obsolete_RESERVED (0x0000..0x0200),
          obsolete_RESERVED (0x0203..0x0400),
          obsolete_RESERVED (0x0404..0x0500),
          obsolete_RESERVED (0x0504..0x0600),
          obsolete_RESERVED (0x0604..0x06FF),
          private_use (0xFE00..0xFFFF),
          (0xFFFF)
      } SignatureScheme;

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

A.3.1.2.

A.3.1.3.  Named Group Extension
      enum {
          //
          /* Elliptic Curve Groups (ECDHE). (ECDHE) */
          obsolete_RESERVED (1..22),
          secp256r1 (23), secp384r1 (24), secp521r1 (25),
          obsolete_RESERVED (26..28),
          x25519 (29), x448 (30),

          //

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

          //

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

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

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

A.3.1.3.

A.3.1.4.  Deprecated Extensions

   The following extensions are no longer applicable to TLS 1.3,
   although TLS 1.3 clients MAY send them if they are willing to
   negotiate them with prior versions of TLS.  TLS 1.3 servers MUST
   ignore these extensions if they are negotiating TLS 1.3:
   truncated_hmac [RFC6066], srp [RFC5054], encrypt_then_mac [RFC7366],
   extended_master_secret [RFC7627], SessionTicket [RFC5077], and
   renegotiation_info [RFC5746].

A.3.2.  Server Parameters Messages
      struct {
          Extension extensions<0..2^16-1>;
      } EncryptedExtensions;

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

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

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

        enum { (65535) } ConfigurationExtensionType;

        enum { client_authentication(1), early_data(2),
               client_authentication_and_data(3), (255) } EarlyDataType;

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

        struct {
            opaque configuration_id<1..2^16-1>;
            uint32 expiration_date;
            KeyShareEntry static_key_share;
            EarlyDataType early_data_type;
            ConfigurationExtension extensions<0..2^16-1>;
        } ServerConfiguration;

A.3.3.  Authentication Messages

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

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

      struct {
           digitally-signed struct {
              opaque hashed_data[hash_length];
           };
      } CertificateVerify;

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

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

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

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

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

A.4.  Cipher Suites

   A cipher suite defines a cipher specification supported in TLS and
   negotiated via hello messages in the TLS handshake.  Cipher suite
   names follow a general naming convention composed of a series of
   component algorithm names separated by underscores:

      CipherSuite TLS_KEA_AUTH_WITH_CIPHER_HASH = VALUE;

   +-----------+-------------------------------------------------------+
   | Component | Contents                                              |
   +-----------+-------------------------------------------------------+
   | TLS       | The string "TLS"                                      |
   |           |                                                       |
   | KEA       | The key exchange algorithm (e.g. ECDHE, DHE)          |
   |           |                                                       |
   | AUTH      | The authentication algorithm (e.g. certificates, PSK) |
   |           |                                                       |
   | WITH      | The string "WITH"                                     |
   |           |                                                       |
   | CIPHER    | The symmetric cipher used for record protection       |
   |           |                                                       |
   | HASH      | The hash algorithm used with HKDF                     |
   |           |                                                       |
   | VALUE     | The two byte ID assigned for this cipher suite        |
   +-----------+-------------------------------------------------------+
   The "CIPHER" component commonly has sub-components used to designate
   the cipher name, bits, and mode, if applicable.  For example,
   "AES_256_GCM" represents 256-bit AES in the GCM mode of operation.
   Cipher suite names that lack a "HASH" value that are defined for use
   with TLS 1.2 or later use the SHA-256 hash algorithm by default.

   The primary key exchange algorithm used in TLS is Ephemeral Diffie-
   Hellman [DH].  The finite field based version is denoted "DHE" and
   the elliptic curve based version is denoted "ECDHE".  Prior versions
   of TLS supported non-ephemeral key exchanges, however these are not
   supported by TLS 1.3.

   See the definitions of each cipher suite in its specification
   document for the full details of each combination of algorithms that
   is specified.

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

   +-------------------------------+----------+------------------------+
   | Cipher Suite Name             | Value    | Specification          |
   +-------------------------------+----------+------------------------+
   | TLS_DHE_RSA_WITH_AES_128_GCM_ | {0x00,0x | [RFC5288]              |
   | SHA256                        | 9E}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_GCM_ | {0x00,0x | [RFC5288]              |
   | SHA384                        | 9F}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_ECDSA_WITH_AES_128_ | {0xC0,0x | [RFC5289]              |
   | GCM_SHA256                    | 2B}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_ECDSA_WITH_AES_256_ | {0xC0,0x | [RFC5289]              |
   | GCM_SHA384                    | 2C}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_RSA_WITH_AES_128_GC | {0xC0,0x | [RFC5289]              |
   | M_SHA256                      | 2F}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_RSA_WITH_AES_256_GC | {0xC0,0x | [RFC5289]              |
   | M_SHA384                      | 30}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_128_CCM  | {0xC0,0x | [RFC6655]              |
   |                               | 9E}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_CCM  | {0xC0,0x | [RFC6655]              |
   |                               | 9F}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_128_CCM_ | {0xC0,0x | [RFC6655]              |
   | 8                             | A2}      |                        |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_AES_256_CCM_ | {0xC0,0x | [RFC6655]              |
   | 8                             | A3}      |                        |
   |                               |          |                        |
   | TLS_ECDHE_RSA_WITH_CHACHA20_P | {0xCC,0x | [I-D.ietf-tls-chacha20 |
   | OLY1305_SHA256                | A8}      | -poly1305]             |
   |                               |          |                        |
   | TLS_ECDHE_ECDSA_WITH_CHACHA20 | {0xCC,0x | [I-D.ietf-tls-chacha20 |
   | _POLY1305_SHA256              | A9}      | -poly1305]             |
   |                               |          |                        |
   | TLS_DHE_RSA_WITH_CHACHA20_POL | {0xCC,0x | [I-D.ietf-tls-chacha20 |
   | Y1305_SHA256                  | AA}      | -poly1305]             |
   +-------------------------------+----------+------------------------+

   Note: The values listed for ChaCha/Poly are preliminary but are being
   or will be used for interop testing and therefore are likely to be
   assigned.

   Note: ECDHE AES GCM was not yet standards track prior to the
   publication of this specification.  This document promotes the above-
   listed ciphers to standards track.

   The following is a list of standards track ephemeral pre-shared key
   cipher suites which are currently available in TLS 1.3:

   +------------------------------+----------+-------------------------+
   | Cipher Suite Name            | Value    | Specification           |
   +------------------------------+----------+-------------------------+
   | TLS_DHE_PSK_WITH_AES_128_GCM | {0x00,0x | [RFC5487]               |
   | _SHA256                      | AA}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_256_GCM | {0x00,0x | [RFC5487]               |
   | _SHA384                      | AB}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_128_CCM | {0xC0,0x | [RFC6655]               |
   |                              | A6}      |                         |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_AES_256_CCM | {0xC0,0x | [RFC6655]               |
   |                              | A7}      |                         |
   |                              |          |                         |
   | TLS_PSK_DHE_WITH_AES_128_CCM | {0xC0,0x | [RFC6655]               |
   | _8                           | AA}      |                         |
   |                              |          |                         |
   | TLS_PSK_DHE_WITH_AES_256_CCM | {0xC0,0x | [RFC6655]               |
   | _8                           | AB}      |                         |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA256                    | 01}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_256_G | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA384                    | 02}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_8_SHA256                  | 03}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_128_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA256                    | 04}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_AES_256_C | {0xD0,0x | [I-D.mattsson-tls-ecdhe |
   | CM_SHA384                    | 05}      | -psk-aead]              |
   |                              |          |                         |
   | TLS_ECDHE_PSK_WITH_CHACHA20_ | {0xCC,0x | [I-D.ietf-tls-chacha20- |
   | POLY1305_SHA256              | AC}      | poly1305]               |
   |                              |          |                         |
   | TLS_DHE_PSK_WITH_CHACHA20_PO | {0xCC,0x | [I-D.ietf-tls-chacha20- |
   | LY1305_SHA256                | AD}      | poly1305]               |
   +------------------------------+----------+-------------------------+

   Note: The values listed for ECDHE and ChaCha/Poly are preliminary but
   are being or will be used for interop testing and therefore are
   likely to be assigned.

   Note: [RFC6655] is inconsistent with respect to the ordering of
   components within PSK AES CCM cipher suite names.  The names above
   are as defined.

   All cipher suites in this section are specified for use with both TLS
   1.2 and TLS 1.3, as well as the corresponding versions of DTLS.  (see
   Appendix C)

   New cipher suite values are assigned by IANA as described in
   Section 11.

A.4.1.  Unauthenticated Operation

   Previous versions of TLS offered explicitly unauthenticated cipher
   suites based on anonymous Diffie-Hellman.  These cipher suites have
   been deprecated in TLS 1.3.  However, it is still possible to
   negotiate cipher suites that do not provide verifiable server
   authentication by several methods, including:

   -  Raw public keys [RFC7250].

   -  Using a public key contained in a certificate but without
      validation of the certificate chain or any of its contents.

   Either technique used alone is are vulnerable to man-in-the-middle
   attacks and therefore unsafe for general use.  However, it is also
   possible to bind such connections to an external authentication
   mechanism via out-of-band validation of the server's public key,
   trust on first use, or channel bindings [RFC5929].  [[NOTE: TLS 1.3
   needs a new channel binding definition that has not yet been
   defined.]] If no such mechanism is used, then the connection has no
   protection against active man-in-the-middle attack; applications MUST
   NOT use TLS in such a way absent explicit configuration or a specific
   application profile.

A.5.  The Security Parameters

   These security parameters are determined by the TLS Handshake
   Protocol and provided as parameters to the TLS record layer in order
   to initialize a connection state.  SecurityParameters includes:

      enum { server, client } ConnectionEnd;

      enum { tls_kdf_sha256, tls_kdf_sha384 } KDFAlgorithm;

      enum { aes_gcm } RecordProtAlgorithm;

      /* The algorithms specified in KDFAlgorithm and
         RecordProtAlgorithm may be added to. */

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

A.6.  Changes to RFC 4492

   RFC 4492 [RFC4492] adds Elliptic Curve cipher suites to TLS.  This
   document changes some of the structures used in that document.  This
   section details the required changes for implementors of both RFC
   4492 and TLS 1.2.  Implementors of TLS 1.2 who are not implementing
   RFC 4492 do not need to read this section.

   This document adds an "algorithm" field to the digitally-signed
   element in order to identify the signature and digest algorithms used
   to create a signature.  This change applies to digital signatures
   formed using ECDSA as well, thus allowing ECDSA signatures to be used
   with digest algorithms other than SHA-1, provided such use is
   compatible with the certificate and any restrictions imposed by
   future revisions of [RFC5280].

   As described in Section 6.3.4.1.1, the restrictions on the signature
   algorithms used to sign certificates are no longer tied to the cipher
   suite.  Thus, the restrictions on the algorithm used to sign
   certificates specified in Sections 2 and 3 of RFC 4492 are also
   relaxed.  As in this document, the restrictions on the keys in the
   end-entity certificate remain.

Appendix B.  Implementation Notes

   The TLS protocol cannot prevent many common security mistakes.  This
   section provides several recommendations to assist implementors.

B.1.  Random Number Generation and Seeding

   TLS requires a cryptographically secure pseudorandom number generator
   (PRNG).  Care must be taken in designing and seeding PRNGs.  PRNGs
   based on secure hash operations, most notably SHA-256, are
   acceptable, but cannot provide more security than the size of the
   random number generator state.

   To estimate the amount of seed material being produced, add the
   number of bits of unpredictable information in each seed byte.  For
   example, keystroke timing values taken from a PC compatible 18.2 Hz
   timer provide 1 or 2 secure bits each, even though the total size of
   the counter value is 16 bits or more.  Seeding a 128-bit PRNG would
   thus require approximately 100 such timer values.

   [RFC4086] provides guidance on the generation of random values.

B.2.  Certificates and Authentication

   Implementations are responsible for verifying the integrity of
   certificates and should generally support certificate revocation
   messages.  Certificates should always be verified to ensure proper
   signing by a trusted Certificate Authority (CA).  The selection and
   addition of trusted CAs should be done very carefully.  Users should
   be able to view information about the certificate and root CA.

B.3.  Cipher Suite Support

   TLS supports a range of key sizes and security levels, including some
   that provide no or minimal security.  A proper implementation will
   probably not support many cipher suites.  Applications SHOULD also
   enforce minimum and maximum key sizes.  For example, certification
   paths containing keys or signatures weaker than 2048-bit RSA or
   224-bit ECDSA are not appropriate for secure applications.  See also
   Appendix C.3. C.4.

B.4.  Implementation Pitfalls

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

   TLS protocol issues:

   -  Do you correctly handle handshake messages that are fragmented to
      multiple TLS records (see Section 5.2.1)?  Including corner cases
      like a ClientHello that is split to several small fragments?  Do
      you fragment handshake messages that exceed the maximum fragment
      size?  In particular, the certificate and certificate request
      handshake messages can be large enough to require fragmentation.

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

   -  Have you ensured that all support for SSL, RC4, EXPORT ciphers,
      and MD5 (via the "signature_algorithm" extension) is completely
      removed from all possible configurations that support TLS 1.3 or
      later, and that attempts to use these obsolete capabilities fail
      correctly? (see Appendix C)

   -  Do you handle TLS extensions in ClientHello correctly, including
      unknown extensions or omitting the extensions field completely?

   -  When the server has requested a client certificate, but no
      suitable certificate is available, do you correctly send an empty
      Certificate message, instead of omitting the whole message (see
      Section 6.3.4.1.2)?

   -  When processing the plaintext fragment produced by AEAD-Decrypt
      and scanning from the end for the ContentType, do you avoid
      scanning past the start of the cleartext in the event that the
      peer has sent a malformed plaintext of all-zeros?

   -  When processing a ClientHello containing a version of { 3, 5 } or
      higher, do you respond with the highest common version of TLS
      rather than requiring an exact match?

   -  Do you ignore unrecognized cipher suites (see Section 6.3.1.1),
      named groups (see Section 6.3.2.3), and signature algorithms (see
      Section 6.3.2.2)?

   Cryptographic details:

   -  What countermeasures do you use to prevent timing attacks against
      RSA signing operations [TIMING]?

   -  When verifying RSA signatures, do you accept both NULL and missing
      parameters (see Section 4.8)?  Do you verify that the RSA padding
      doesn't have additional data after the hash value?  [FI06]

   -  When using Diffie-Hellman key exchange, do you correctly strip preserve
      leading zero bytes from in the negotiated key (see Section 7.3.2)? 7.3.1)?

   -  Does your TLS client check that the Diffie-Hellman parameters sent
      by the server are acceptable (see Appendix D.1.1.1)?

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

   -  Do you zero-pad Diffie-Hellman public key values to the group size
      (see Section 6.3.2.4.1)?

B.5.  Client Tracking Prevention

   Clients SHOULD NOT reuse a session ticket for multiple connections.
   Reuse of a session ticket allows passive observers to correlate
   different connections.  Servers that issue session tickets SHOULD
   offer at least as many session tickets as the number of connections
   that a client might use; for example, a web browser using HTTP/1.1
   [RFC7230] might open six connections to a server.  Servers SHOULD
   issue new session tickets with every connection.  This ensures that
   clients are always able to use a new session ticket when creating a
   new connection.

Appendix C.  Backward Compatibility

   The TLS protocol provides a built-in mechanism for version
   negotiation between endpoints potentially supporting different
   versions of TLS.

   TLS 1.x and SSL 3.0 use compatible ClientHello messages.  Servers can
   also handle clients trying to use future versions of TLS as long as
   the ClientHello format remains compatible and the client supports the
   highest protocol version available in the server.

   Prior versions of TLS used the record layer version number for
   various purposes.  (TLSPlaintext.record_version &
   TLSCiphertext.record_version) As of TLS 1.3, this field is deprecated
   and its value MUST be ignored by all implementations.  Version
   negotiation is performed using only the handshake versions.
   (ClientHello.client_version & ServerHello.server_version) In order to
   maximize interoperability with older endpoints, implementations that
   negotiate the use of TLS 1.0-1.2 SHOULD set the record layer version
   number to the negotiated version for the ServerHello and all records
   thereafter.

   For maximum compatibility with previously non-standard behavior and
   misconfigured deployments, all implementations SHOULD support
   validation of certification paths based on the expectations in this
   document, even when handling prior TLS versions' handshakes. (see
   Section 6.3.4.1.1)

   TLS 1.2 and prior supported an "Extended Master Secret" [RFC7627]
   extension which digested large parts of the handshake transcript into
   the master secret.  Because TLS 1.3 always hashes in the transcript
   up to the server CertificateVerify, implementations which support
   both TLS 1.3 and earlier versions SHOULD indicate the use of the
   Extended Master Secret extension in their APIs whenever TLS 1.3 is
   used.

C.1.  Negotiating with an older server

   A TLS 1.3 client who wishes to negotiate with such older servers will
   send a normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) in
   ClientHello.client_version.  If the server does not support this
   version it will respond with a ServerHello containing an older
   version number.  If the client agrees to use this version, the
   negotiation will proceed as appropriate for the negotiated protocol.
   A client resuming a session SHOULD initiate the connection using the
   version that was previously negotiated.

   Note that 0-RTT data is not compatible with older servers.  See
   Appendix C.3.

   If the version chosen by the server is not supported by the client
   (or not acceptable), the client MUST send a "protocol_version" alert
   message and close the connection.

   If a TLS server receives a ClientHello containing a version number
   greater than the highest version supported by the server, it MUST
   reply according to the highest version supported by the server.

   Some legacy server implementations are known to not implement the TLS
   specification properly and might abort connections upon encountering
   TLS extensions or versions which it is not aware of.
   Interoperability with buggy servers is a complex topic beyond the
   scope of this document.  Multiple connection attempts may be required
   in order to negotiate a backwards compatible connection, however this
   practice is vulnerable to downgrade attacks and is NOT RECOMMENDED.

C.2.  Negotiating with an older client

   A TLS server can also receive a ClientHello containing a version
   number smaller than the highest supported version.  If the server
   wishes to negotiate with old clients, it will proceed as appropriate
   for the highest version supported by the server that is not greater
   than ClientHello.client_version.  For example, if the server supports
   TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will
   proceed with a TLS 1.0 ServerHello.  If the server only supports
   versions greater than client_version, it MUST send a
   "protocol_version" alert message and close the connection.

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

C.3.  Zero-RTT backwards compatibility

   0-RTT data is not compatible with older servers.  An older server
   will respond to the ClientHello with an older ServerHello, but it
   will not correctly skip the 0-RTT data and fail to complete the
   handshake.  This can cause issues when a client offers 0-RTT,
   particularly against multi-server deployments.  For example, a
   deployment may deploy TLS 1.3 gradually with some servers
   implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3
   deployment may be downgraded to TLS 1.2.

   If a client accepts older versions of TLS and receives an older
   ServerHello after sending a ClientHello with 0-RTT data, it MAY retry
   the connection without 0-RTT.  It is NOT RECOMMENDED to retry the
   connection in response to a more generic error or advertise lower
   versions of TLS.

   Multi-server deployments are RECOMMENDED to ensure a stable
   deployment of TLS 1.3 without 0-RTT prior to enabling 0-RTT.

C.4.  Backwards Compatibility Security Restrictions

   If an implementation negotiates use of TLS 1.2, then negotiation of
   cipher suites also supported by TLS 1.3 SHOULD be preferred, if
   available.

   The security of RC4 cipher suites is considered insufficient for the
   reasons cited in [RFC7465].  Implementations MUST NOT offer or
   negotiate RC4 cipher suites for any version of TLS for any reason.

   Old versions of TLS permitted the use of very low strength ciphers.
   Ciphers with a strength less than 112 bits MUST NOT be offered or
   negotiated for any version of TLS for any reason.

   The security of SSL 2.0 [SSL2] is considered insufficient for the
   reasons enumerated in [RFC6176], and MUST NOT be negotiated for any
   reason.

   Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-
   HELLO.  Implementations MUST NOT negotiate TLS 1.3 or later using an
   SSL version 2.0 compatible CLIENT-HELLO.  Implementations are NOT
   RECOMMENDED to accept an SSL version 2.0 compatible CLIENT-HELLO in
   order to negotiate older versions of TLS.

   Implementations MUST NOT send or accept any records with a version
   less than { 3, 0 }.

   The security of SSL 3.0 [SSL3] is considered insufficient for the
   reasons enumerated in [RFC7568], and MUST NOT be negotiated for any
   reason.

   Implementations MUST NOT send a ClientHello.client_version or
   ServerHello.server_version set to { 3, 0 } or less.  Any endpoint
   receiving a Hello message with ClientHello.client_version or
   ServerHello.server_version set to { 3, 0 } MUST respond with a
   "protocol_version" alert message and close the connection.

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

Appendix D.  Security Analysis

   [[TODO: The entire security analysis needs a rewrite.]]

   The TLS protocol is designed to establish a secure connection between
   a client and a server communicating over an insecure channel.  This
   document makes several traditional assumptions, including that
   attackers have substantial computational resources and cannot obtain
   secret information from sources outside the protocol.  Attackers are
   assumed to have the ability to capture, modify, delete, replay, and
   otherwise tamper with messages sent over the communication channel.
   This appendix outlines how TLS has been designed to resist a variety
   of attacks.

D.1.  Handshake Protocol

   The TLS Handshake Protocol is responsible for selecting a cipher spec
   and generating a master secret, which together comprise the primary
   cryptographic parameters associated with a secure session.  The TLS
   Handshake Protocol can also optionally authenticate parties who have
   certificates signed by a trusted certificate authority.

D.1.1.  Authentication and Key Exchange

   TLS supports three authentication modes: authentication of both
   parties, server authentication with an unauthenticated client, and
   total anonymity.  Whenever the server is authenticated, the channel
   is secure against man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If the server is authenticated,
   its certificate message must provide a valid certificate chain
   leading to an acceptable certificate authority.  Similarly,
   authenticated clients must supply an acceptable certificate to the
   server.  Each party is responsible for verifying that the other's
   certificate is valid and has not expired or been revoked.

   [[TODO: Rewrite this because the master_secret is not used this way
   any more after Hugo's changes.]] The general goal of the key exchange
   process is to create a master_secret known to the communicating
   parties and not to attackers (see Section 7.1).  The master_secret is
   required to generate the Finished messages and record protection keys
   (see Section 6.3.4.3 and Section 7.3).  By sending a correct Finished
   message, parties thus prove that they know the correct master_secret.

D.1.1.1.  Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is used, the client and server use
   the "key_share" extension to send temporary Diffie-Hellman
   parameters.  The signature in the certificate verify message (if
   present) covers the entire handshake up to that point and thus
   attests the certificate holder's desire to use the the ephemeral DHE
   keys.

   Peers SHOULD validate each other's public key Y (dh_Ys offered by the
   server or DH_Yc offered by the client) by ensuring that 1 <
   Y < p-1.  This simple check ensures that the remote peer is properly
   behaved and isn't forcing the local system into a small subgroup.

   Additionally, using a fresh key for each handshake provides Perfect
   Forward Secrecy.  Implementations SHOULD generate a new X for each
   handshake when using DHE cipher suites.

D.1.2.  Version Rollback Attacks

   Because TLS includes substantial improvements over SSL Version 2.0,
   attackers may try to make TLS-capable clients and servers fall back
   to Version 2.0.  This attack can occur if (and only if) two TLS-
   capable parties use an SSL 2.0 handshake.  (See also Appendix C.3.) C.4.)

   Although the solution using non-random PKCS #1 block type 2 message
   padding is inelegant, it provides a reasonably secure way for Version
   3.0 servers to detect the attack.  This solution is not secure
   against attackers who can brute-force the key and substitute a new
   ENCRYPTED-KEY-DATA message containing the same key (but with normal
   padding) before the application-specified wait threshold has expired.
   Altering the padding of the least-significant 8 bytes of the PKCS
   padding does not impact security for the size of the signed hashes
   and RSA key lengths used in the protocol, since this is essentially
   equivalent to increasing the input block size by 8 bytes.

D.1.3.  Detecting Attacks Against the Handshake Protocol

   An attacker might try to influence the handshake exchange to make the
   parties select different encryption algorithms than they would
   normally choose.

   For this attack, an attacker must actively change one or more
   handshake messages.  If this occurs, the client and server will
   compute different values for the handshake message hashes.  As a
   result, the parties will not accept each others' Finished messages.
   Without the static secret, the attacker cannot repair the Finished
   messages, so the attack will be discovered.

D.2.  Protecting Application Data

   The shared secrets are hashed with the handshake transcript to
   produce unique record protection secrets for each connection.

   Outgoing data is protected using an AEAD algorithm before
   transmission.  The authentication data includes the sequence number,
   message type, message length, and the message contents.  The message
   type field is necessary to ensure that messages intended for one TLS
   record layer client are not redirected to another.  The sequence
   number ensures that attempts to delete or reorder messages will be
   detected.  Since sequence numbers are 64 bits long, they should never
   overflow.  Messages from one party cannot be inserted into the
   other's output, since they use independent keys.

D.3.  Denial of Service

   TLS is susceptible to a number of denial-of-service (DoS) attacks.
   In particular, an attacker who initiates a large number of TCP
   connections can cause a server to consume large amounts of CPU doing
   asymmetric crypto operations.  However, because TLS is generally used
   over TCP, it is difficult for the attacker to hide their point of
   origin if proper TCP SYN randomization is used [RFC1948] by the TCP
   stack.

   Because TLS runs over TCP, it is also susceptible to a number of DoS
   attacks on individual connections.  In particular, attackers can
   forge RSTs, thereby terminating connections, or forge partial TLS
   records, thereby causing the connection to stall.  These attacks
   cannot in general be defended against by a TCP-using protocol.
   Implementors or users who are concerned with this class of attack
   should use IPsec AH [RFC4302] or ESP [RFC4303].

D.4.  Final Notes

   For TLS to be able to provide a secure connection, both the client
   and server systems, keys, and applications must be secure.  In
   addition, the implementation must be free of security errors.

   The system is only as strong as the weakest key exchange and
   authentication algorithm supported, and only trustworthy
   cryptographic functions should be used.  Short public keys and
   anonymous servers should be used with great caution.  Implementations
   and users must be careful when deciding which certificates and
   certificate authorities are acceptable; a dishonest certificate
   authority can do tremendous damage.

Appendix E.  Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address tls@ietf.org [1].  Information on the group and
   information on how to subscribe to the list is at
   https://www1.ietf.org/mailman/listinfo/tls

   Archives of the list can be found at: https://www.ietf.org/mail-
   archive/web/tls/current/index.html

Appendix F.  Contributors

   -  Martin Abadi
      University of California, Santa Cruz
      abadi@cs.ucsc.edu

   -  Christopher Allen (co-editor of TLS 1.0)
      Alacrity Ventures
      ChristopherA@AlacrityManagement.com

   -  Steven M.  Bellovin
      Columbia University
      smb@cs.columbia.edu

   -  David Benjamin
      Google
      davidben@google.com

   -  Benjamin Beurdouche

   -  Karthikeyan Bhargavan (co-author of [RFC7627])
      INRIA
      karthikeyan.bhargavan@inria.fr

   -  Simon Blake-Wilson (co-author of [RFC4492])
      BCI
      sblakewilson@bcisse.com

   -  Nelson Bolyard (co-author of [RFC4492])
      Sun Microsystems, Inc.
      nelson@bolyard.com

   -  Ran Canetti
      IBM
      canetti@watson.ibm.com

   -  Pete Chown
      Skygate Technology Ltd
      pc@skygate.co.uk

   -  Antoine Delignat-Lavaud (co-author of [RFC7627])
      INRIA
      antoine.delignat-lavaud@inria.fr

   -  Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2)
      Independent
      tim@dierks.org

   -  Taher Elgamal
      Securify
      taher@securify.com

   -  Pasi Eronen
      Nokia
      pasi.eronen@nokia.com

   -  Cedric Fournet
      Microsoft
      fournet@microsoft.com

   -  Anil Gangolli
      anil@busybuddha.org

   -  David M.  Garrett

   -  Vipul Gupta (co-author of [RFC4492])
      Sun Microsystems Laboratories
      vipul.gupta@sun.com

   -  Chris Hawk (co-author of [RFC4492])
      Corriente Networks LLC
      chris@corriente.net

   -  Kipp Hickman

   -  Alfred Hoenes

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

   -  Subodh Iyengar
      Facebook
      subodh@fb.com

   -  Daniel Kahn Gillmor
      ACLU
      dkg@fifthhorseman.net

   -  Phil Karlton (co-author of SSL 3.0)

   -  Paul Kocher (co-author of SSL 3.0)
      Cryptography Research
      paul@cryptography.com

   -  Hugo Krawczyk
      IBM
      hugo@ee.technion.ac.il

   -  Adam Langley (co-author of [RFC7627])
      Google
      agl@google.com

   -  Ilari Liusvaara
      Independent
      ilariliusvaara@welho.com

   -  Jan Mikkelsen
      Transactionware
      janm@transactionware.com

   -  Bodo Moeller (co-author of [RFC4492])
      Google
      bodo@openssl.org

   -  Erik Nygren
      Akamai Technologies
      erik+ietf@nygren.org

   -  Magnus Nystrom
      RSA Security
      magnus@rsasecurity.com

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

   -  Andrei Popov
      Microsoft
      andrei.popov@microsoft.com

   -  Marsh Ray (co-author of [RFC7627])
      Microsoft
      maray@microsoft.com

   -  Robert Relyea
      Netscape Communications
      relyea@netscape.com

   -  Jim Roskind
      Netscape Communications
      jar@netscape.com

   -  Michael Sabin

   -  Dan Simon
      Microsoft, Inc.
      dansimon@microsoft.com

   -  Nick Sullivan
      CloudFlare Inc.

      nick@cloudflare.com

   -  Bjoern Tackmann
      University of California, San Diego
      btackmann@eng.ucsd.edu

   -  Martin Thomson
      Mozilla
      mt@mozilla.com

   -  Tom Weinstein

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

   -  Tim Wright
      Vodafone
      timothy.wright@vodafone.com

Author's Address

   Eric Rescorla
   RTFM, Inc.

   EMail: ekr@rtfm.com