Network Working Group                                          T. Dierks
Internet-Draft                                               Independent
Obsoletes: 3268, 4346, 4366, 5246                            E. Rescorla
(if approved)                                                 RTFM, Inc.
Updates: 4492 (if approved)                               April 17,                                 July 7, 2014
Intended status: Standards Track
Expires: October 19, 2014 January 8, 2015

        The Transport Layer Security (TLS) Protocol Version 1.3
                        draft-ietf-tls-tls13-01
                        draft-ietf-tls-tls13-02

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 19, 2014. January 8, 2015.

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   document authors.  All rights reserved.

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   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Requirements Terminology . . . . . . . . . . . . . . . . .  6
     1.2.  Major Differences from TLS 1.2 . . . . . . . . . . . . . .  6
     1.3.  Major Differences from TLS 1.1 . . . . . . . . . . . . . .  6
   2.  Goals  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7  8
   3.  Goals of This Document . . . . . . . . . . . . . . . . . . . .  8
   4.  Presentation Language  . . . . . . . . . . . . . . . . . . . .   8  9
     4.1.  Basic Block Size . . . . . . . . . . . . . . . . . . . .   8 .  9
     4.2.  Miscellaneous  . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Vectors  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.4.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  Enumerateds  . . . . . . . . . . . . . . . . . . . . . . .  10 11
     4.6.  Constructed Types  . . . . . . . . . . . . . . . . . . . .  11 12
       4.6.1.  Variants . . . . . . . . . . . . . . . . . . . . . . . 12
     4.7.  Cryptographic Attributes . . . . . . . . . . . . . . . . . 13
     4.8.  Constants  . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.  HMAC and the  The Pseudorandom Function  . . . . . . . . . . . . . . . . . . 15
   6.  The TLS Record Protocol  . . . . . . . . . . . . . . . . . . .  17 16
     6.1.  Connection States  . . . . . . . . . . . . . . . . . . . . 17
     6.2.  Record Layer . . . . . . . . . . . . . . . . . . . . . .  20 . 19
       6.2.1.  Fragmentation  . . . . . . . . . . . . . . . . . . . .  20 19
       6.2.2.  Record Compression and Decompression  . . . . . . . .  22
       6.2.3.  Record Payload Protection  . . . . . . . . . . . . . .  22 20
     6.3.  Key Calculation  . . . . . . . . . . . . . . . . . . . . .  27 22
   7.  The TLS Handshaking Protocols  . . . . . . . . . . . . . . . .  28 23
     7.1.  Change Cipher Spec Protocol  . . . . . . . . . . . . . . .  29 24
     7.2.  Alert Protocol . . . . . . . . . . . . . . . . . . . . .  29 . 24
       7.2.1.  Closure Alerts . . . . . . . . . . . . . . . . . . .  30 . 25
       7.2.2.  Error Alerts . . . . . . . . . . . . . . . . . . . .  31 . 26
     7.3.  Handshake Protocol Overview  . . . . . . . . . . . . . . .  35 30
     7.4.  Handshake Protocol . . . . . . . . . . . . . . . . . . .  38 . 34
       7.4.1.  Hello Messages . . . . . . . . . . . . . . . . . . .  39
       7.4.2.  Server Certificate  . . . . 35
       7.4.2.  Client Key Exchange Message  . . . . . . . . . . . . .  49 39
       7.4.3.  Server Key Exchange Message  . . . . . . . . . . . . .  51 47
       7.4.4.  Certificate Request  Encrypted Extensions . . . . . . . . . . . . . . . . .  54 48
       7.4.5.  Server Hello Done Certificate . . . . . . . . . . . . . . . . . .  56 49
       7.4.6.  Client  Certificate Request  . . . . . . . . . . . . . . . . .  57 52
       7.4.7.  Client Key Exchange Message . . . . . . . . . . . . .  58
       7.4.8.  Server Certificate Verify  . . . . . . . . . . . . . . 53
       7.4.8.  Server Finished  . . . .  63
       7.4.9.  Finished . . . . . . . . . . . . . . . 55
       7.4.9.  Client Certificate . . . . . . . .  64
   8.  Cryptographic Computations . . . . . . . . . . 56
       7.4.10. Client Certificate Verify  . . . . . . .  66
     8.1.  Computing the Master Secret . . . . . . . 58
   8.  Cryptographic Computations . . . . . . . .  66
       8.1.1.  RSA . . . . . . . . . . 58
     8.1.  Computing the Master Secret  . . . . . . . . . . . . . . .  66
       8.1.2. 59
       8.1.1.  Diffie-Hellman . . . . . . . . . . . . . . . . . . .  67 . 59
   9.  Mandatory Cipher Suites  . . . . . . . . . . . . . . . . . . .  67 59
   10. Application Data Protocol  . . . . . . . . . . . . . . . . . .  67 59
   11. Security Considerations  . . . . . . . . . . . . . . . . . . .  67 59
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  67 59
   13. References . . . . . . . . . . . . . . . . . . . . . . . . .  69 . 61
     13.1. Normative References . . . . . . . . . . . . . . . . . .  69 . 61
     13.2. Informative References . . . . . . . . . . . . . . . . .  70 . 63
   Appendix A.  Protocol Data Structures and Constant Values  . . . .  73 67
     A.1.  Record Layer . . . . . . . . . . . . . . . . . . . . . .  73 . 67
     A.2.  Change Cipher Specs Message  . . . . . . . . . . . . . . .  74 67
     A.3.  Alert Messages . . . . . . . . . . . . . . . . . . . . .  75 . 68
     A.4.  Handshake Protocol . . . . . . . . . . . . . . . . . . .  76 . 69
       A.4.1.  Hello Messages . . . . . . . . . . . . . . . . . . .  76 . 69
       A.4.2.  Server Authentication and Key Exchange Messages  . . .  78 71
       A.4.3.  Client Authentication and Key Exchange Messages  . . .  79 72
       A.4.4.  Handshake Finalization Message . . . . . . . . . . .  80 . 72
     A.5.  The Cipher Suite . . . . . . . . . . . . . . . . . . . .  80 . 72
     A.6.  The Security Parameters  . . . . . . . . . . . . . . . . .  82 74
     A.7.  Changes to RFC 4492  . . . . . . . . . . . . . . . . . . .  83 74
   Appendix B.  Glossary  . . . . . . . . . . . . . . . . . . . . . .  83 75
   Appendix C.  Cipher Suite Definitions  . . . . . . . . . . . . . .  87 78
   Appendix D.  Implementation Notes  . . . . . . . . . . . . . . . .  89 79
     D.1.  Random Number Generation and Seeding . . . . . . . . . .  89 . 79
     D.2.  Certificates and Authentication  . . . . . . . . . . . . .  89 79
     D.3.  Cipher Suites  . . . . . . . . . . . . . . . . . . . . . .  90 79
     D.4.  Implementation Pitfalls  . . . . . . . . . . . . . . . . .  90 79
   Appendix E.  Backward Compatibility  . . . . . . . . . . . . . . .  91 81
     E.1.  Compatibility with TLS 1.0/1.1 and SSL 3.0 . . . . . . .  91 . 81
     E.2.  Compatibility with SSL 2.0 . . . . . . . . . . . . . . .  93 . 82
     E.3.  Avoiding Man-in-the-Middle Version Rollback  . . . . . . .  94 84
   Appendix F.  Security Analysis . . . . . . . . . . . . . . . . .  95 . 84
     F.1.  Handshake Protocol . . . . . . . . . . . . . . . . . . .  95 . 84
       F.1.1.  Authentication and Key Exchange  . . . . . . . . . . .  95 85
       F.1.2.  Version Rollback Attacks . . . . . . . . . . . . . .  98 . 86
       F.1.3.  Detecting Attacks Against the Handshake Protocol . .  98 . 87
       F.1.4.  Resuming Sessions  . . . . . . . . . . . . . . . . . .  98 87
     F.2.  Protecting Application Data  . . . . . . . . . . . . . . .  99 88
     F.3.  Explicit IVs  Denial of Service  . . . . . . . . . . . . . . . . . . . . 88
     F.4.  Final Notes  . .  99
     F.4.  Security of Composite Cipher Modes . . . . . . . . . . .  99
     F.5.  Denial of Service . . . . . . . . . . 88
   Appendix G.  Working Group Information . . . . . . . . . . 100
     F.6.  Final Notes . . . . 89
   Appendix H.  Contributors  . . . . . . . . . . . . . . . . . . . 101
   Appendix G.  Working Group Information  . . . . . . . . . . . . . 101
   Appendix H.  Contributors . . . . . . . . . . . . . . . . . . . . 101 89

1.  Introduction

   DISCLAIMER: This document is simply a copy WIP draft of RFC 5246 translated
   into markdown format with no intentional technical or editorial
   changes beyond updating the references TLS 1.3 and minor reformatting
   introduced by the translation.  It is being submitted as-is to create
   a clearer revision history has not yet seen
   significant security analysis.

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for future versions.  Any errata this
   draft is maintained in TLS
   1.2 remain 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 this version.  Thanks to Mark Nottingham for doing GitHub, but any substantive change should be discussed on
   the
   markdown translation. TLS mailing list.

   The primary goal of the TLS protocol is to provide privacy and data
   integrity between two communicating applications.  The 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], RC4 [SCH], etc.).  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).  The Record Protocol can also be used
      without encryption. encryption, i.e., in integrity-only modes.

   -  The connection is reliable.  Message transport includes a message
      integrity check using a keyed MAC.  Secure hash functions (e.g.,
      SHA-1, etc.) are used for MAC computations.  Messages include an authentication
      tag which protects them against modification.

   -  The Record Protocol can operate without a MAC, in an insecure mode but is
      generally only used in this mode while another protocol is using
      the 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, cryptography (e.g., RSA [RSA], DSA [DSS], etc.).  This
      authentication can be made optional, but is generally required for
      at least one of the peers.

   -  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.  Requirements Terminology

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

1.2.  Major Differences from TLS 1.2

   draft-02

   -  Increment version number.

   -  Reworked handshake to provide 1-RTT mode.

   -  Remove custom DHE groups.

   -  Removed support for compression.

   -  Removed support for static RSA and DH key exchange.

   -  Removed support for non-AEAD ciphers

1.3.  Major Differences from TLS 1.1

   This document is a revision of the TLS 1.1 [RFC4346] protocol which
   contains improved flexibility, particularly for negotiation of
   cryptographic algorithms.  The major changes are:

   -  The MD5/SHA-1 combination in the pseudorandom function (PRF) has
      been replaced with cipher-suite-specified PRFs.  All cipher suites
      in this document use P_SHA256.

   -  The MD5/SHA-1 combination in the digitally-signed element has been
      replaced with a single hash.  Signed elements now include a field
      that explicitly specifies the hash algorithm used.

   -  Substantial cleanup to the client's and server's ability to
      specify which hash and signature algorithms they will accept.
      Note that this also relaxes some of the constraints on signature
      and hash algorithms from previous versions of TLS.

   -  Addition of support for authenticated encryption with additional
      data modes.

   -  TLS Extensions definition and AES Cipher Suites were merged in
      from external [TLSEXT] and [RFC3268].

   -  Tighter checking of EncryptedPreMasterSecret version numbers.

   -  Tightened up a number of requirements.

   -  Verify_data length now depends on the cipher suite (default is
      still 12).

   -  Cleaned up description of Bleichenbacher/Klima attack defenses.

   -  Alerts MUST now be sent in many cases.

   -  After a certificate_request, if no certificates are available,
      clients now MUST send an empty certificate list.

   -  TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement
      cipher suite.

   -  Added HMAC-SHA256 cipher suites.

   -  Removed IDEA and DES cipher suites.  They are now deprecated and
      will be documented in a separate document.

   -  Support for the SSLv2 backward-compatible hello is now a MAY, not
      a SHOULD, with sending it a SHOULD NOT.  Support will probably
      become a SHOULD NOT in the future.

   -  Added limited "fall-through" to the presentation language to allow
      multiple case arms to have the same encoding.

   -  Added an Implementation Pitfalls sections

   -  The usual clarifications and editorial work.

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 bulk encryption 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 are based on the SSL 3.0
   Protocol Specification as published by Netscape.  The differences
   between this protocol and SSL 3.0 are not dramatic, but they 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., leading zero octets are not
   required 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.  Cryptographic Attributes

   The five two cryptographic operations -- digital signing, stream cipher
   encryption, block cipher encryption, and
   authenticated encryption with additional data (AEAD) encryption, and public key encryption -- are
   designated digitally-signed, stream-ciphered, block-ciphered, aead-
   ciphered, and public-key-encrypted, 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 6.1).

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

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

   The algorithm field specifies the algorithm used (see
   Section 7.4.1.4.1 7.4.2.3.1 for the definition of this field).  Note that the
   introduction of the
   algorithm field was introduced in TLS 1.2, and is a change from previous not in earlier
   versions.  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 RSA signing, the opaque vector contains the signature generated
   using the RSASSA-PKCS1-v1_5 signature scheme defined in [RFC3447].
   As discussed in [RFC3447], the DigestInfo MUST be DER-encoded [X680]
   [X690].  For hash algorithms without parameters (which includes
   SHA-1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be
   NULL, but implementations MUST accept both without parameters and
   with NULL parameters.  Note that earlier versions of TLS used a
   different RSA signature scheme that did not include a DigestInfo
   encoding.

   In DSA, the 20 bytes of the SHA-1 hash are run directly through the
   Digital Signing Algorithm with no additional hashing.  This produces
   two values, r and s.  The DSA signature is an opaque vector, as
   above, the contents of which are the DER encoding of:

      Dss-Sig-Value ::= SEQUENCE {
          r INTEGER,
          s INTEGER
      }

   Note: In current terminology, DSA refers to the Digital Signature
   Algorithm and DSS refers to the NIST standard.  In the original SSL
   and TLS specs, "DSS" was used universally.  This document uses "DSA"
   to refer to the algorithm, "DSS" to refer to the standard, and it
   uses "DSS" in the code point definitions for historical continuity.

   In stream cipher AEAD encryption, the plaintext is exclusive-ORed with an
   identical amount simultaneously encrypted and
   integrity protected.  The input may be of output generated from a cryptographically secure
   keyed pseudorandom number generator.

   In block cipher encryption, every block of plaintext encrypts to a
   block of ciphertext.  All block cipher encryption is done in CBC
   (Cipher Block Chaining) mode, and all items that are block-ciphered
   will be an exact multiple of the cipher block length.

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

   In public key encryption, a public key algorithm is used to encrypt
   data in such a way that it can be decrypted only with the matching
   private key.  A public-key-encrypted element is encoded as an opaque
   vector <0..2^16-1>, where the length is specified by the encryption
   algorithm and key.

   RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme
   defined in [RFC3447].

   In the following example

      stream-ciphered

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

   The contents of the inner struct (field3 and field4) are used as
   input for the signature/hash algorithm, and then the entire structure
   is encrypted with a stream cipher. algorithm.  The length of this the structure,
   in bytes, would be equal to two bytes for field1 and field2, plus two
   bytes for the signature and hash 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.  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 */

5.  HMAC and the  The Pseudorandom Function

   The TLS record layer uses a keyed Message Authentication Code (MAC)
   to protect message integrity.  The cipher suites defined in this
   document use a construction known as HMAC, described in [RFC2104],
   which is based on a hash function.  Other cipher suites MAY define
   their own MAC constructions, if needed.

   In addition, a

   A construction is required to do expansion of secrets into blocks of
   data for the purposes of key generation or validation.  This
   pseudorandom function (PRF) takes as input a secret, a seed, and an
   identifying label and produces an output of arbitrary length.

   In this section, we define one PRF, based on HMAC. HMAC [RFC2104].  This
   PRF with the SHA-256 hash function is used for all cipher suites
   defined in this document and in TLS documents published prior to this
   document when TLS 1.2 is negotiated.  New cipher suites MUST
   explicitly specify a PRF and, in general, SHOULD use the TLS PRF with
   SHA-256 or a stronger standard hash function.

   First, we define a data expansion function, P_hash(secret, data),
   that uses a single hash function to expand a secret and seed into an
   arbitrary quantity of output:

      P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
                             HMAC_hash(secret, A(2) + seed) +
                             HMAC_hash(secret, A(3) + seed) + ...

   where + indicates concatenation.

   A() is defined as:

      A(0) = seed
      A(i) = HMAC_hash(secret, A(i-1))

   P_hash can be iterated as many times as necessary to produce the
   required quantity of data.  For example, if P_SHA256 is being used to
   create 80 bytes of data, it will have to be iterated three times
   (through A(3)), creating 96 bytes of output data; the last 16 bytes
   of the final iteration will then be discarded, leaving 80 bytes of
   output data.

   TLS's PRF is created by applying P_hash to the secret as:

      PRF(secret, label, seed) = P_<hash>(secret, label + seed)

   The label is an ASCII string.  It should be included in the exact
   form it is given without a length byte or trailing null character.
   For example, the label "slithy toves" would be processed by hashing
   the following bytes:

      73 6C 69 74 68 79 20 74 6F 76 65 73

6.  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 Record Protocol takes messages to be transmitted, fragments the
   data into manageable blocks, optionally compresses protects the data, applies
   a MAC, encrypts, records, and transmits the
   result.  Received data is
   decrypted, decrypted and verified, decompressed, reassembled, and
   then delivered to higher-level clients.

   Four protocols that use the record protocol are described in this
   document: the handshake protocol, the alert protocol, the change
   cipher spec protocol, and the application data protocol.  In order to
   allow extension of the TLS protocol, additional record content types
   can be supported by the record protocol.  New record content type
   values are assigned by IANA in the TLS Content Type Registry as
   described in Section 12.

   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 type and length of a record are not protected
   by encryption.  If this information is itself sensitive, application
   designers may wish to take steps (padding, cover traffic) to minimize
   information leakage.

6.1.  Connection States

   A TLS connection state is the operating environment of the TLS Record
   Protocol.  It specifies a compression algorithm, an encryption
   algorithm, record protection algorithm and a MAC algorithm.  In addition, the its
   parameters for
   these algorithms are known: the MAC key and as well as the bulk encryption record protection keys and IVs for the
   connection in both the read and the write directions.  Logically,
   there are always four connection states outstanding: the current read
   and write states, and the pending read and write states.  All records
   are processed under the current read and write states.  The security
   parameters for the pending states can be set by the TLS Handshake
   Protocol, and the ChangeCipherSpec can selectively make either of the
   pending states current, in which case the appropriate current state
   is disposed of and replaced with the pending state; the pending state
   is then reinitialized to an empty state.  It is illegal to make a
   state that has not been initialized with security parameters a
   current state.  The initial current state always specifies that no encryption, compression, or MAC will be used.
   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.

   PRF algorithm
      An algorithm used to generate keys from the master secret (see
      Section 5 and Section 6.3).

   bulk encryption

   record protection algorithm
      An
      The algorithm to be used for bulk encryption. record protection.  This specification
      includes the key size of this algorithm, whether it is a block,
      stream, or AEAD cipher, the block size algorithm
      must be of the cipher (if
      appropriate), AEAD type and the lengths of explicit thus provides integrity and implicit
      initialization vectors (or nonces).

   MAC algorithm
      An algorithm
      confidentiality as a single primitive.  It is possible to be used have
      AEAD algorithms which do not provide any confidentiality and
      Section 6.2.2 defines a special NULL_NULL AEAD algorithm for message authentication. use
      in the initial handshake).  This specification includes the key
      size of the value returned by the MAC
      algorithm.

   compression algorithm
      An this algorithm to be used for data compression.  This specification
      must include all information and the algorithm requires to do
      compression. lengths of explicit and implicit
      initialization vectors (or nonces).

   master secret
      A 48-byte secret shared between the two peers in the connection.

   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_prf_sha256 } PRFAlgorithm;

      enum { null, rc4, 3des, aes }
        BulkCipherAlgorithm;

      enum { stream, block, aead } CipherType;

      enum { null, hmac_md5, hmac_sha1, hmac_sha256,
           hmac_sha384, hmac_sha512} MACAlgorithm;

      enum { null(0), (255) aes_gcm } CompressionMethod; RecordProtAlgorithm;

      /* The algorithms specified in CompressionMethod, PRFAlgorithm,
         BulkCipherAlgorithm, PRFAlgorithm and MACAlgorithm
         RecordProtAlgorithm may be added to. */

      struct {
          ConnectionEnd          entity;
          PRFAlgorithm           prf_algorithm;
          BulkCipherAlgorithm    bulk_cipher_algorithm;
          CipherType             cipher_type;
          RecordProtAlgorithm    record_prot_algorithm;
          uint8                  enc_key_length;
          uint8                  block_length;
          uint8                  fixed_iv_length;
          uint8                  record_iv_length;
          MACAlgorithm           mac_algorithm;
          uint8                  mac_length;
          uint8                  mac_key_length;
          CompressionMethod      compression_algorithm;
          opaque                 master_secret[48];
          opaque                 client_random[32];
          opaque                 server_random[32];
      } SecurityParameters;

   The record layer will use the security parameters to generate the
   following six four items (some of which are not required by all ciphers,
   and are thus empty):

      client write MAC key
      server write MAC key
      client write encryption key
      server write encryption 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 6.3

   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:

   compression state
      The current state of the compression algorithm.

   cipher state
      The current state of the encryption algorithm.  This will consist
      of the scheduled key for that connection.  For stream ciphers,
      this will also contain whatever state information is necessary to
      allow the stream to continue to encrypt or decrypt data.

   MAC key
      The MAC key for this connection, as generated above.

   sequence number
      Each connection state contains a sequence number, which is
      maintained separately for read and write states.  The sequence
      number MUST be set to zero whenever a connection state is made the
      active state.  Sequence numbers are of type uint64 and may not
      exceed 2^64-1.  Sequence numbers do not wrap.  If a TLS
      implementation would need to wrap a sequence number, it must
      renegotiate instead.  A sequence number is incremented after each
      record: specifically, the first record transmitted under a
      particular connection state MUST use sequence number 0.

6.2.  Record Layer

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

6.2.1.  Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks of 2^14 bytes or less.  Client
   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).

      struct {
          uint8 major;
          uint8 minor;
      } ProtocolVersion;

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

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

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

   version
      The version of the protocol being employed.  This document
      describes TLS Version 1.2, 1.3, which uses the version { 3, 3 4 }.  The
      version value 3.3 3.4 is historical, deriving from the use of {3, 1}
      for TLS 1.0.  (See Appendix A.1.)  Note that a client that
      supports multiple versions of TLS may not know what version will
      be employed before it receives the ServerHello.  See Appendix E
      for discussion about what record layer version number should be
      employed for ClientHello.

   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.

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

   Note: Data of different TLS record layer content types MAY be
   interleaved.  Application data is generally of lower precedence for
   transmission than other content types.  However, records MUST be
   delivered to the network in the same order as they are protected by
   the record layer.  Recipients MUST receive and process interleaved
   application layer traffic during handshakes subsequent to the first
   one on a connection.

6.2.2.  Record Compression and Decompression

   All records are compressed using the compression algorithm defined in
   the current session state.  There is always an active compression
   algorithm; however, initially it is defined as
   CompressionMethod.null. Payload Protection

   The compression algorithm translates record protection functions translate a TLSPlaintext structure
   into a TLSCompressed structure.  Compression TLSCiphertext.  The deprotection functions are initialized with default state information whenever a
   connection state is made active.  [RFC3749] describes compression
   algorithms for TLS.

   Compression must be lossless and may not increase the content length
   by more than 1024 bytes.  If reverse the decompression function encounters a
   TLSCompressed.fragment that would decompress
   process.  In TLS 1.3 as opposed to a length in excess previous versions of
   2^14 bytes, it MUST report TLS, all
   ciphers are modelled as "Authenticated Encryption with Additional
   Data" (AEAD) [RFC5116].  AEAD functions provide a fatal decompression failure error.

      struct {
          ContentType type;       /* same as TLSPlaintext.type */
          ProtocolVersion version;/* same as TLSPlaintext.version */
          uint16 length;
          opaque fragment[TLSCompressed.length];
      } TLSCompressed;

   length
      The length (in bytes) of the following TLSCompressed.fragment.
      The length MUST NOT exceed 2^14 + 1024.

   fragment
      The compressed form of TLSPlaintext.fragment.

   Note: A CompressionMethod.null operation is an identity operation; no
   fields are altered.

   Implementation note: Decompression functions are responsible for
   ensuring that messages cannot cause internal buffer overflows.

6.2.3.  Record Payload Protection

   The unified encryption
   and MAC functions translate a TLSCompressed structure authentication operation which turns plaintext into authenticated
   ciphertext and back again.

   AEAD ciphers take as input a TLSCiphertext.  The decryption functions reverse single key, a nonce, a plaintext, and
   "additional data" to be included in the process.
   The MAC authentication check, as
   described in Section 2.1 of [RFC5116].  The key is either the record also includes a sequence number so that
   missing, extra,
   client_write_key or repeated messages are detectable. the server_write_key.

      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          select (SecurityParameters.cipher_type)
          opaque nonce_explicit[SecurityParameters.record_iv_length];
          aead-ciphered struct {
              case stream: GenericStreamCipher;
              case block:  GenericBlockCipher;
              case aead:   GenericAEADCipher;
             opaque content[TLSPlaintext.length];
          } fragment;
      } TLSCiphertext;

   type
      The type field is identical to TLSCompressed.type. TLSPlaintext.type.

   version
      The version field is identical to TLSCompressed.version. TLSPlaintext.version.

   length
      The length (in bytes) of the following TLSCiphertext.fragment.
      The length MUST NOT exceed 2^14 + 2048.

   fragment
      The AEAD encrypted form of TLSCompressed.fragment, with TLSPlaintext.fragment.

   Each AEAD cipher suite MUST specify how the MAC.

6.2.3.1.  Null or Standard Stream Cipher

   Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6)
   convert TLSCompressed.fragment structures nonce supplied to and from stream
   TLSCiphertext.fragment structures.

      stream-ciphered struct {
          opaque content[TLSCompressed.length];
          opaque MAC[SecurityParameters.mac_length];
      } GenericStreamCipher;

   The MAC is generated as:

      MAC(MAC_write_key, seq_num +
                            TLSCompressed.type +
                            TLSCompressed.version +
                            TLSCompressed.length +
                            TLSCompressed.fragment);

   where "+" denotes concatenation.

   seq_num
      The sequence number for this record.

   MAC
      The MAC algorithm specified by SecurityParameters.mac_algorithm.

   Note that the MAC
   AEAD operation is constructed, and what is computed before encryption.  The stream cipher
   encrypts the entire block, including length of the MAC.  For stream ciphers
   that do not
   TLSCiphertext.nonce_explicit part.  In many cases, it is appropriate
   to use a synchronization vector (such as RC4), the stream
   cipher state from the end partially implicit nonce technique described in Section
   3.2.1 of one record is simply used on [RFC5116]; with record_iv_length being the
   subsequent packet.  If length of the cipher suite is TLS_NULL_WITH_NULL_NULL,
   encryption consists of the identity operation (i.e.,
   explicit part.  In this case, the data is not
   encrypted, implicit part SHOULD be derived
   from key_block as client_write_iv and server_write_iv (as described
   in Section 6.3), and the MAC size is zero, implying that no MAC explicit part is used).
   For both null and stream ciphers, TLSCiphertext.length included in
   GenericAEAEDCipher.nonce_explicit.

   The plaintext is
   TLSCompressed.length plus SecurityParameters.mac_length.

6.2.3.2.  CBC Block Cipher

   For block ciphers (such as 3DES or AES), the encryption and MAC
   functions convert TLSCompressed.fragment structures to and from block
   TLSCiphertext.fragment structures.

      struct {
          opaque IV[SecurityParameters.record_iv_length];
          block-ciphered struct {
              opaque content[TLSCompressed.length];
              opaque MAC[SecurityParameters.mac_length];
              uint8 padding[GenericBlockCipher.padding_length];
              uint8 padding_length;
          };
      } GenericBlockCipher; TLSPlaintext.fragment.

   The MAC additional authenticated data, which we denote as
   additional_data, is generated defined as described in Section 6.2.3.1.

   IV follows:

      additional_data = seq_num + TLSPlaintext.type +
                        TLSPlaintext.version + TLSPlaintext.length;

   [[OPEN ISSUE: Fix length which gives us a problem here for algorithms
   which pad.  See: https://github.com/tlswg/tls13-spec/issues/47]]

   where "+" denotes concatenation.

   The Initialization Vector (IV) SHOULD be chosen at random, and
      MUST AEAD output consists of the ciphertext output by the AEAD
   encryption operation.  The length will generally be unpredictable.  Note larger than
   TLSPlaintext.length, but by an amount that in versions varies with the AEAD
   cipher.  Since the ciphers might incorporate padding, the amount of TLS prior
   overhead could vary with different TLSPlaintext.length values.  Each
   AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes.
   Symbolically,

      AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
                                   additional_data)

   [[OPEN ISSUE: Reduce these values?
   https://github.com/tlswg/tls13-spec/issues/55]]

   In order to 1.1,
      there was no IV field, decrypt and verify, the last ciphertext block of the
      previous record (the "CBC residue") was used cipher takes as input the IV.  This was
      changed to prevent key,
   nonce, the attacks described in [CBCATT].  For block
      ciphers, "additional_data", and the IV length is of length
      SecurityParameters.record_iv_length, which AEADEncrypted value.  The
   output is equal to either the
      SecurityParameters.block_size.

   padding
      Padding plaintext or an error indicating that is added to force the length of the plaintext to be
      an integral multiple of the block cipher's block length.  The
      padding MAY be any length up to 255 bytes, as long as it results
      in the TLSCiphertext.length being an integral multiple of
   decryption failed.  There is no separate integrity check.  That is:

      TLSPlaintext.fragment = AEAD-Decrypt(write_key, nonce,
                                           AEADEncrypted,
                                           additional_data)

   If the
      block length.  Lengths longer than necessary might decryption fails, a fatal bad_record_mac alert MUST be desirable to
      frustrate attacks on
   generated.

   As a protocol that are based on analysis of special case, we define the
      lengths of exchanged messages.  Each uint8 in NULL_NULL AEAD cipher which is
   simply the padding data
      vector identity operation and thus provides no security.  This
   cipher MUST ONLY be filled used with the padding length value. initial TLS_NULL_WITH_NULL_NULL
   cipher suite.

6.3.  Key Calculation

   [[OPEN ISSUE: This may be revised.  See
   https://github.com/tlswg/tls13-spec/issues/5]] The receiver
      MUST check this padding and MUST use the bad_record_mac alert Record Protocol
   requires an algorithm to
      indicate padding errors.

   padding_length
      The padding length MUST be such that the total size of the
      GenericBlockCipher structure is a multiple of generate keys required by the cipher's block
      length.  Legal values range current
   connection state (see Appendix A.6) from zero to 255, inclusive.  This
      length specifies the length of the padding field exclusive of security parameters
   provided by the
      padding_length field itself. handshake protocol.

   The encrypted data length (TLSCiphertext.length) master secret is one more than the
   sum expanded into a sequence of SecurityParameters.block_length, TLSCompressed.length,
   SecurityParameters.mac_length, and padding_length.

   Example: If the block length is 8 bytes, the content length
   (TLSCompressed.length) is 61 secure bytes, and the MAC length which
   is 20 bytes, then the length before padding split to a client write encryption key and a server write
   encryption key.  Each of these is 82 bytes (this does not include the
   IV.  Thus, generated from the padding length modulo 8 must be equal to 6 byte sequence in order to
   make the total length an even multiple of 8 bytes (the block length).
   The padding length can be 6, 14, 22,
   that order.  Unused values are empty.  Some ciphers may additionally
   require a client write IV and so on, through 254.  If the
   padding length were the minimum necessary, 6, the padding would be 6
   bytes, each containing the value 6.  Thus, the last 8 octets of a server write IV.

   When keys are generated, the
   GenericBlockCipher before block encryption would be xx 06 06 06 06 06
   06 06, where xx master secret is used as an entropy
   source.

   To generate the last octet of key material, compute

      key_block = PRF(SecurityParameters.master_secret,
                      "key expansion",
                      SecurityParameters.server_random +
                      SecurityParameters.client_random);

   until enough output has been generated.  Then, the MAC.

   Note: With block ciphers in CBC mode (Cipher Block Chaining), it key_block is
   critical that the entire plaintext of
   partitioned as follows:

      client_write_key[SecurityParameters.enc_key_length]
      server_write_key[SecurityParameters.enc_key_length]
      client_write_IV[SecurityParameters.fixed_iv_length]
      server_write_IV[SecurityParameters.fixed_iv_length]

   Currently, the record be known before any
   ciphertext is transmitted.  Otherwise, it is possible client_write_IV and server_write_IV are only generated
   for the
   attacker to mount the attack implicit nonce techniques as described in [CBCATT].

   Implementation note: Canvel et al.  [CBCTIME] have demonstrated a
   timing attack on CBC padding based on the time required Section 3.2.1 of
   [RFC5116].

7.  The TLS Handshaking Protocols

   TLS has three subprotocols that are used to compute
   the MAC.  In order allow peers to defend against this attack, implementations
   MUST ensure that record processing time is essentially the same
   whether or not the padding is correct.  In general, agree upon
   security parameters for the best way record layer, to
   do this is authenticate themselves,
   to compute the MAC even if the padding is incorrect, instantiate negotiated security parameters, and
   only then reject the packet.  For instance, if the pad appears to be
   incorrect, the implementation might assume report error
   conditions to each other.

   The Handshake Protocol is responsible for negotiating a zero-length pad and then
   compute session,
   which consists of the MAC.  This leaves a small timing channel, since MAC
   performance depends following items:

   session identifier
      An arbitrary byte sequence chosen by the server to some extent on identify an
      active or resumable session state.

   peer certificate
      X509v3 [RFC3280] certificate of the size peer.  This element of the data fragment,
   but it is not believed to be large enough to
      state may be exploitable, due to null.

   cipher spec
      Specifies the large block size of existing MACs authentication and key establishment algorithms, the small size of the
   timing signal.

6.2.3.3.  AEAD Ciphers

   For AEAD [RFC5116] ciphers (such as [CCM] or [GCM]), the AEAD
      pseudorandom function converts TLSCompressed.fragment structures (PRF) used to generate keying material, and from AEAD
   TLSCiphertext.fragment structures.

      struct {
         opaque nonce_explicit[SecurityParameters.record_iv_length];
         aead-ciphered struct {
             opaque content[TLSCompressed.length];
         };
      } GenericAEADCipher;

   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 record protection algorithm (See Appendix A.6 for formal
      definition.)

   master secret
      48-byte secret shared between the server_write_key.  No MAC key client and server.

   is used.

   Each AEAD cipher suite MUST specify how resumable
      A flag indicating whether the nonce supplied session can be used to initiate new
      connections.

   These items are then used to create security parameters for use by
   the
   AEAD operation is constructed, and what is record layer when protecting application data.  Many connections
   can be instantiated using the length of same session through the
   GenericAEADCipher.nonce_explicit part.  In many cases, it is
   appropriate to use resumption
   feature of the partially implicit nonce technique described
   in Section 3.2.1 of [RFC5116]; with record_iv_length being the length
   of the explicit part.  In this case, the implicit part SHOULD be
   derived from key_block as client_write_iv and server_write_iv (as
   described in Section 6.3), and the explicit part is included in
   GenericAEAEDCipher.nonce_explicit. TLS Handshake Protocol.

7.1.  Change Cipher Spec Protocol

   The plaintext is the TLSCompressed.fragment. change cipher spec protocol exists to signal transitions in
   ciphering strategies.  The additional authenticated data, protocol consists of a single message,
   which we denote as
   additional_data, is defined as follows:

      additional_data = seq_num + TLSCompressed.type +
                        TLSCompressed.version + TLSCompressed.length;

   where "+" denotes concatenation. encrypted under the current (not the pending) connection
   state.  The aead_output message consists of the ciphertext output a single byte of value 1.

      struct {
          enum { change_cipher_spec(1), (255) } type;
      } ChangeCipherSpec;

   The ChangeCipherSpec message is sent by both the AEAD
   encryption operation.  The length client and the
   server to notify the receiving party that subsequent records will generally be larger than
   TLSCompressed.length, but by an amount that varies with
   protected under the AEAD
   cipher.  Since newly negotiated CipherSpec and keys.  Reception
   of this message causes the ciphers might incorporate padding, receiver to instruct the amount of
   overhead could vary with different TLSCompressed.length values.  Each
   AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes.
   Symbolically,

      AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
                                   additional_data)

   In order record layer to decrypt and verify,
   immediately copy the cipher takes as input read pending state into the key,
   nonce, read current state.
   Immediately after sending this message, the "additional_data", and sender MUST instruct the AEADEncrypted value.  The
   output is either
   record layer to make the plaintext or an error indicating that write pending state the
   decryption failed.  There is no separate integrity check.  That is:

      TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce,
                                            AEADEncrypted,
                                            additional_data)

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

6.3.  Key Calculation write current state.
   (See Section 6.1.)  The Record Protocol requires an algorithm to generate keys required
   by ChangeCipherSpec message is sent during the current connection state (see Appendix A.6) from
   handshake after the security parameters provided by have been agreed upon, but
   before the handshake protocol.

   The master secret is expanded into first message protected with a sequence of secure bytes, which new CipherSpec is then split to a client write MAC key, a server write MAC key, a
   client write encryption key, and sent.

   Note: If a server write encryption key.  Each
   of these rehandshake occurs while data is generated from flowing on a connection,
   the byte sequence in that order.  Unused
   values are empty.  Some AEAD ciphers communicating parties may additionally require a
   client write IV and a server write IV (see Section 6.2.3.3).

   When keys and MAC keys are generated, continue to send data using the master secret is used as an
   entropy source.

   To generate old
   CipherSpec.  However, once the key material, compute

      key_block = PRF(SecurityParameters.master_secret,
                      "key expansion",
                      SecurityParameters.server_random +
                      SecurityParameters.client_random);

   until enough output ChangeCipherSpec has been generated.  Then, the key_block is
   partitioned as follows:

      client_write_MAC_key[SecurityParameters.mac_key_length]
      server_write_MAC_key[SecurityParameters.mac_key_length]
      client_write_key[SecurityParameters.enc_key_length]
      server_write_key[SecurityParameters.enc_key_length]
      client_write_IV[SecurityParameters.fixed_iv_length]
      server_write_IV[SecurityParameters.fixed_iv_length]

   Currently, sent, the client_write_IV and server_write_IV are only generated
   for implicit nonce techniques as described in Section 3.2.1 of
   [RFC5116].

   Implementation note:
   new CipherSpec MUST be used.  The currently defined cipher suite which
   requires first side to send the most
   ChangeCipherSpec does not know that the other side has finished
   computing the new keying material is AES_256_CBC_SHA256.  It requires 2 x 32
   byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key
   material.

7.  The TLS Handshaking Protocols

   TLS (e.g., if it has three subprotocols that are used to allow peers to agree upon
   security parameters for perform a time-
   consuming public key operation).  Thus, a small window of time,
   during which the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to report error
   conditions recipient must buffer the data, MAY exist.  In
   practice, with modern machines this interval is likely to each other.

   The Handshake be fairly
   short.  [[TODO: This text seems confusing.]]

7.2.  Alert Protocol is responsible for negotiating a session,
   which consists

   One of the following items:

   session identifier
      An arbitrary byte sequence chosen content types supported by the server to identify an
      active TLS record layer is the
   alert type.  Alert messages convey the severity of the message
   (warning or resumable session state.

   peer certificate
      X509v3 [RFC3280] certificate fatal) and a description of the peer.  This element alert.  Alert messages
   with a level of fatal result in the
      state may be null.

   compression method
      The algorithm used to compress data prior to encryption.

   cipher spec
      Specifies immediate termination of the pseudorandom function (PRF) used
   connection.  In this case, other connections corresponding to generate keying
      material, the bulk data encryption algorithm (such as null, AES,
      etc.) and the MAC algorithm (such as HMAC-SHA1).  It also defines
      cryptographic attributes such as the mac_length.  (See
      Appendix A.6 for formal definition.)

   master secret
      48-byte secret shared between the client and server.

   is resumable
      A flag indicating whether
   session may continue, but the session can identifier MUST be invalidated,
   preventing the failed session from being used to initiate establish new
   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 through the resumption
   feature of the TLS Handshake Protocol.

7.1.  Change Cipher Spec Protocol

   The change cipher spec protocol exists to signal transitions in
   ciphering strategies.  The protocol consists of a single message,
   which is  Like other messages, alert messages are encrypted and compressed under as
   specified by the current (not the pending) connection state.  The message consists of a single byte of value 1.

      struct

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

      enum { change_cipher_spec(1),
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decryption_failed_RESERVED(21),
          record_overflow(22),
          decompression_failure_RESERVED(30),
          handshake_failure(40),
          no_certificate_RESERVED(41),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),
          unknown_ca(48),
          access_denied(49),
          decode_error(50),
          decrypt_error(51),
          export_restriction_RESERVED(60),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          user_canceled(90),
          no_renegotiation(100),
          unsupported_extension(110),
          (255)
      } type; AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } ChangeCipherSpec; Alert;

7.2.1.  Closure Alerts

   The ChangeCipherSpec message is sent by both the client and the server to notify must share knowledge that the receiving connection is
   ending in order to avoid a truncation attack.  Either party that subsequent records will be
   protected under may
   initiate the newly negotiated CipherSpec and keys.  Reception exchange of this closing messages.

   close_notify
      This message causes the receiver to instruct the record layer to
   immediately copy the read pending state into notifies the read current state.
   Immediately after sending this message, recipient that the sender MUST instruct the
   record layer will not send
      any more messages on this connection.  Note that as of TLS 1.1,
      failure to make the write pending state the write active state.
   (See Section 6.1.)  The ChangeCipherSpec message properly close a connection no longer requires that a
      session not be resumed.  This is sent during the
   handshake after the security parameters have been agreed upon, but
   before the verifying Finished message is sent.

   Note: If a rehandshake occurs while data is flowing on a connection,
   the communicating parties may continue change from TLS 1.0 to send conform
      with widespread implementation practice.

   Either party may initiate a close by sending a close_notify alert.
   Any data using the old
   CipherSpec.  However, once the ChangeCipherSpec received after a closure alert is ignored.

   Unless some other fatal alert has been sent, the
   new CipherSpec MUST be used.  The first side transmitted, each party is
   required to send a close_notify alert before closing the
   ChangeCipherSpec does not know that the other write side has finished
   computing the new keying material (e.g., if it has to perform a time-
   consuming public key operation).  Thus, a small window
   of time,
   during which the recipient must buffer the data, MAY exist.  In
   practice, connection.  The other party MUST respond with modern machines this interval is likely to be fairly
   short.

7.2.  Alert Protocol

   One a close_notify
   alert of its own and close down the content types supported by the TLS record layer connection immediately,
   discarding any pending writes.  It is not required for the
   alert type.  Alert messages convey the severity initiator
   of the message
   (warning or fatal) and a description of close to wait for the alert.  Alert messages
   with a level of fatal result in responding close_notify alert before
   closing the immediate termination read side of the connection.  In this case, other connections corresponding to

   If the
   session application protocol using TLS provides that any data 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 and
   compressed, as specified by the current connection state.

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

      enum {
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decryption_failed_RESERVED(21),
          record_overflow(22),
          decompression_failure(30),
          handshake_failure(40),
          no_certificate_RESERVED(41),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),
          unknown_ca(48),
          access_denied(49),
          decode_error(50),
          decrypt_error(51),
          export_restriction_RESERVED(60),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          user_canceled(90),
          no_renegotiation(100),
          unsupported_extension(110),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

7.2.1.  Closure Alerts

   The client and the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.  Either party may
   initiate the exchange of closing messages.

   close_notify
      This message notifies the recipient that the sender will not send
      any more messages on this connection.  Note that as of TLS 1.1,
      failure to properly close a connection no longer requires that a
      session not be resumed.  This is a change from TLS 1.0 to conform
      with widespread implementation practice.

   Either party may initiate a close by sending a close_notify alert.
   Any data received after a closure alert is ignored.

   Unless some other fatal alert has been transmitted, each party is
   required to send a close_notify alert before closing the write side
   of the connection.  The other party MUST respond with a close_notify
   alert of its own and close down the connection immediately,
   discarding any pending writes.  It is not required for the initiator
   of the close to 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
   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.

7.2.2.  Error Alerts

   Error handling in the TLS Handshake protocol is very simple.  When an
   error is detected, the detecting party sends a message to the other
   party.  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
   no_renegotiation alert that it is not willing to accept), it SHOULD
   send a fatal alert to terminate the connection.  Given this, the
   sending party 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 peer 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 with an incorrect
      MAC.  This alert also MUST which cannot be returned if an alert
      deprotected.  Because AEAD algorithms combine decryption and
      verification, this message is sent because
      a TLSCiphertext decrypted in an invalid way: either it wasn't an
      even multiple of the block length, or its padding values, when
      checked, weren't correct. used for all deprotection failures.
      This message is always fatal and should never be observed in
      communication between proper implementations (except when messages
      were corrupted in the network).

   decryption_failed_RESERVED
      This alert was used in some earlier versions of TLS, and may have
      permitted certain attacks against the CBC mode [CBCATT].  It MUST
      NOT be sent by compliant implementations.

   record_overflow
      A TLSCiphertext record was received that had a length more than
      2^14+2048 bytes, or a record decrypted to a TLSCompressed TLSPlaintext record
      with more than 2^14+1024 2^14 bytes.  This message is always fatal and
      should never be observed in communication between proper
      implementations (except when messages were corrupted in the
      network).

   decompression_failure
      The decompression function received improper input (e.g., data
      that would expand to excessive length).
      This message is always
      fatal alert was used in previous versions of TLS.  TLS 1.3 does not
      include compression and should never be observed TLS 1.3 implementations MUST NOT send this
      alert when in communication between proper
      implementations. TLS 1.3 mode.

   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 is a fatal error.

   no_certificate_RESERVED
      This alert was used in SSLv3 but not any version of TLS.  It MUST
      NOT be sent by compliant implementations.

   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 message 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
      message is always fatal.

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

   export_restriction_RESERVED
      This alert was used in some earlier versions of TLS.  It MUST NOT
      be sent by compliant implementations.

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

   user_canceled
      This handshake is being canceled for some reason unrelated to a
      protocol failure.  If the 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 message is generally a warning.

   no_renegotiation
      Sent by the client in response to a hello request or by the server
      in response to a client hello after initial handshaking.  Either
      of these would normally lead to renegotiation; when that is not
      appropriate, the recipient should respond with this alert.  At
      that point, the original requester can decide whether to proceed
      with the connection.  One case where this would be appropriate is
      where a server has spawned a process to satisfy a request; the
      process might receive security parameters (key length,
      authentication, etc.) at startup, and it might be difficult to
      communicate changes to these parameters after that point.  This
      message is always a warning.

   unsupported_extension
      sent by clients that receive an extended server hello containing
      an extension that they did not put in the corresponding client
      hello.  This message is always fatal.

   New Alert values are assigned by IANA as described in Section 12.

7.3.  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 use public-key encryption
   techniques to generate shared secrets.

   The TLS Handshake Protocol involves the following steps:

   -  Exchange hello messages to agree on a protocol version,
      algorithms, exchange random values, and check for session
      resumption.

   -  Exchange the necessary cryptographic parameters to allow the
      client and server to agree on a premaster secret.

   -  Exchange certificates and cryptographic information to allow the
      client and server to authenticate themselves.

   -  Generate a master secret from the premaster secret and exchanged
      random values.

   -  Provide security parameters to the record layer.

   -  Allow the client and server to verify that their peer has
      calculated the same security parameters and that the handshake
      occurred without tampering by an attacker.

   Note that higher layers should not be overly reliant on whether TLS
   always negotiates the strongest possible connection between two
   peers.  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.  The 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 3DES AES-GCM [GCM] with a 1024-bit RSA DHE
   key exchange with a host whose certificate you have verified, you can
   expect to be that secure.

   These goals are achieved by the handshake protocol, which can be
   summarized as follows: The client sends a ClientHello message to which
   contains a random nonce (ClientHello.random), its preferences for
   Protocol Version, Cipher Suite, and a variety of extensions.  In the
   same flight, it sends a ClientKeyExchange message which contains its
   share of the parameters for key agreement for some set of expected
   server must respond parameters (DHE/ECDHE groups, etc.).

   The server responds to the ClientHello with a ServerHello message, or
   else a fatal error will occur and the connection will fail.  The ClientHello
   and ServerHello are used to establish security enhancement
   capabilities between client and server.  The ClientHello and
   ServerHello establish contains the server's nonce (ServerHello.random), the
   server's choice of the following attributes: Protocol Version, Session ID, ID and Cipher Suite,
   and Compression Method.  Additionally, two
   random values are generated and exchanged: ClientHello.random and
   ServerHello.random.

   The actual key exchange uses up the server's response to four messages: the server
   Certificate, extensions the ServerKeyExchange, client offered.

   If the client Certificate, and has provided a ClientKeyExchange with an appropriate
   set of keying material, the
   ClientKeyExchange.  New key exchange methods server can be created by
   specifying a format for these messages then generate its own keying
   material share and by defining the use send a ServerKeyExchange message which contains
   its share of the
   messages to allow parameters for the client and key agreement.  The server to agree upon a can
   now compute the shared secret.  This secret MUST  At this point, a ChangeCipherSpec
   message is sent by the server, and the server copies the pending
   Cipher Spec into the current Cipher Spec.  The remainder of the
   server's handshake messages will be quite long; currently defined key
   exchange methods exchange secrets encrypted under that range from 46 bytes upwards. Cipher Spec.

   Following the hello these messages, the server will send an EncryptedExtensions
   message which contains a response to any client's extensions which
   are not necessary to establish the Cipher Suite.  The server will
   then send its certificate in a Certificate message if it is to be
   authenticated.  Additionally, a
   ServerKeyExchange message may be sent, if it is required (e.g., if
   the server has no certificate, or if its certificate is for signing
   only).  If the  The server is authenticated, it may optionally request a certificate from
   the client, client by sending a CertificateRequest message at this point.
   Finally, if that is appropriate to the cipher suite selected.
   Next, the server is authenticated, it will send a
   CertificateVerify message which provides a signature over the ServerHelloDone message, indicating
   that entire
   handshake up to this point.  This serves both to authenticate the hello-message phase
   server and to establish the integrity of the handshake is complete.  The
   server will then wait for a client response.  If negotiation.  Finally,
   the server has sent sends a CertificateRequest message, Finished message which includes an integrity check
   over the client MUST send handshake keyed by the Certificate
   message.  The ClientKeyExchange message is now sent, shared secret and the content
   of demonstrates that message will depend on the public key algorithm selected
   between
   the ClientHello server and client have agreed upon the ServerHello. same keys.  [[TODO: If the client has sent
   a certificate with signing ability, a digitally-signed
   CertificateVerify message
   server is sent to explicitly verify possession of not requesting client authentication, it MAY start sending
   application data following the private key in Finished, though the certificate. server has no way
   of knowing who will be receiving the data.  Add this.]]

   Once the client receives the ServerKeyExchange, it can also compute
   the shared key.  At this point, a point ChangeCipherSpec message is sent by
   the client, and the client copies the pending Cipher Spec into the
   current Cipher Spec.  The client then immediately sends remainder of the Finished message client's messages will be
   encrypted under
   the new algorithms, keys, and secrets.  In response, this Cipher Spec.  If the server will has sent a
   CertificateRequest message, the client MUST send its own ChangeCipherSpec the Certificate
   message, transfer though it may contain zero certificates.  If the pending client has
   sent a certificate, a digitally-signed CertificateVerify message is
   sent to explicitly verify possession of the
   current Cipher Spec, and send its Finished message under private key in the new
   Cipher Spec.
   certificate.  Finally, the client sends the Finished message.  At
   this point, the handshake is complete, and the client and server may begin to
   exchange application layer data.  (See flow chart below.)
   Application data MUST NOT be sent prior to the
   completion of Finished message.
   [[TODO: can we make this clearer and more clearly match the first handshake (before a cipher suite other than
   TLS_NULL_WITH_NULL_NULL is established). text
   above about server-side False Start.]]  Client Server

      ClientHello
      ClientKeyExchange            -------->
                                                      ServerHello
                                                ServerKeyExchange
                                               [ChangeCipherSpec]
                                             EncryptedExtensions*
                                                     Certificate*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                               CertificateVerify*
                                    <--------      ServerHelloDone            Finished
      [ChangeCipherSpec]
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                     -------->
                                               [ChangeCipherSpec]
                                   <--------             Finished
      Application Data             <------->     Application Data

               Figure 1.  Message flow for a full handshake

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

   Note: To help avoid pipeline stalls, ChangeCipherSpec is an
   independent TLS protocol content type, and is not actually a TLS
   handshake message.

   When

   If the client and server decide to resume a previous session or
   duplicate has not provided an existing session (instead of negotiating new security
   parameters), the message flow is as follows:

   The client sends a ClientHello using the Session ID of appropriate ClientKeyExchange (e.g.
   it includes only DHE or ECDHE groups unacceptable or unsupported by
   the session to
   be resumed.  The server then checks its session cache for a match.
   If a match is found, and server), the server is willing to re-establish corrects the
   connection under mismatch with the specified session state, it will send a ServerHello with
   (which the same Session ID value.  At this point, both client and server MUST send ChangeCipherSpec messages can detect by comparing the selected cipher suite
   and parameters with the ClientKeyExchange it offered) and the client
   will need to restart the handshake with an appropriate
   ClientKeyExchange, as shown in Figure 2:

      Client                                               Server

      ClientHello
      ClientKeyExchange            -------->
                                   <--------          ServerHello

      ClientHello
      ClientKeyExchange            -------->
                                                      ServerHello
                                                ServerKeyExchange
                                               [ChangeCipherSpec]
                                             EncryptedExtensions*
                                                     Certificate*
                                              CertificateRequest*
                                               CertificateVerify*
                                   <--------             Finished
      [ChangeCipherSpec]
      Certificate*
      CertificateVerify*
      Finished                     -------->
      Application Data             <------->     Application Data

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

   [[OPEN ISSUE: Do we restart the handshake hash?]]  [[OPEN ISSUE: We
   need to make sure that this flow doesn't introduce downgrade issues.
   Potential options include continuing 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 is currently done) and then checking you get the same
   negotiated parameters.]]

   When the client and server decide to resume a previous session or
   duplicate an existing session (instead of negotiating new security
   parameters), the message flow is as follows:

   The client sends a ClientHello using the Session ID of the session to
   be resumed.  The server then checks its session cache for a match.
   If a match is found, and the server is willing to re-establish the
   connection under the specified session state, it will send a
   ServerHello with the same Session ID value.  At this point, both
   client and server MUST send ChangeCipherSpec messages and proceed
   directly to Finished messages.  Once the re-establishment is
   complete, the client and server MAY begin to exchange application
   layer data.  (See flow chart below.)  If a Session ID match is not
   found, the server generates a new session ID, and the TLS client and
   server perform a full handshake.

      Client                                                Server

      ClientHello
      ClientKeyExhange              -------->
                                                       ServerHello
                                                [ChangeCipherSpec]
                                    <--------             Finished
      [ChangeCipherSpec]
      Finished                      -------->
      Application Data              <------->     Application Data

          Figure 2. 3.  Message flow for an abbreviated handshake

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

7.4.  Handshake Protocol

   The TLS Handshake Protocol is one of the defined higher-level clients
   of the TLS Record Protocol.  This protocol is used to negotiate the
   secure attributes of a session.  Handshake messages are supplied to
   the TLS record layer, where they are encapsulated within one or more
   TLSPlaintext structures, which are processed and transmitted as
   specified by the current active session state.

      enum {
          hello_request(0), client_hello(1), server_hello(2),
          certificate(11), server_key_exchange (12),
          certificate_request(13), server_hello_done(14), certificate_verify(15),
          client_key_exchange(16), finished(20), (255)
      } HandshakeType;

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case hello_request:       HelloRequest;
              case client_hello:        ClientHello;
              case client_key_exchange: ClientKeyExchange;
              case server_hello:        ServerHello;
              case certificate:         Certificate;
              case server_key_exchange: ServerKeyExchange;
              case certificate:         Certificate;
              case certificate_request: CertificateRequest;
              case server_hello_done:   ServerHelloDone;
              case certificate_verify:  CertificateVerify;
              case client_key_exchange: ClientKeyExchange;
              case finished:            Finished;
          } body;
      } Handshake;

   The handshake protocol messages are presented below in the order they
   MUST be sent; sending handshake messages in an unexpected order
   results in a fatal error.  Unneeded handshake messages can be
   omitted, however.  Note one exception to the ordering: the
   Certificate message is used twice in the handshake (from server to
   client, then from client to server), but described only in its first
   position.  The one message that is not bound by these
   ordering rules is the HelloRequest message, which can be sent at any
   time, but which SHOULD be ignored by the client if it arrives in the
   middle of a handshake.

   New handshake message types are assigned by IANA as described in
   Section 12.

7.4.1.  Hello Messages

   The hello phase messages are used to exchange security enhancement
   capabilities between the client and server.  When a new session
   begins, the record layer's connection state encryption, hash, and
   compression algorithms are AEAD algorithm is
   initialized to null. NULL_NULL.  The current connection state is used for
   renegotiation messages.

7.4.1.1.  Hello Request

   When this message will be sent:

      The HelloRequest message MAY be sent by the server at any time.

   Meaning of this message:

      HelloRequest is a simple notification that the client should begin
      the negotiation process anew.  In response, the client should send
      a ClientHello message when convenient.  This message is not
      intended to establish which side is the client or server but
      merely to initiate a new negotiation.  Servers SHOULD NOT send a
      HelloRequest immediately upon the client's initial connection.  It
      is the client's job to send a ClientHello at that time.

      This message will be ignored by the client if the client is
      currently negotiating a session.  This message MAY be ignored by
      the client if it does not wish to renegotiate a session, or the
      client may, if it wishes, respond with a no_renegotiation alert.
      Since handshake messages are intended to have transmission
      precedence over application data, it is expected that the
      negotiation will begin before no more than a few records are
      received from the client.  If the server sends a HelloRequest but
      does not receive a ClientHello in response, it may close the
      connection with a fatal alert.

      After sending a HelloRequest, servers SHOULD NOT repeat the
      request until the subsequent handshake negotiation is complete.

   Structure of this message:

      struct { } HelloRequest;

   This message MUST NOT be included in the message hashes that are
   maintained throughout the handshake and used in the Finished messages
   and the certificate verify message.

7.4.1.2.  Client Hello

   When this message will be sent:

      When a client first connects to a server, it is required to send
      the ClientHello as its first message.  The client can also send a
      ClientHello in response to a HelloRequest or on its own initiative
      in order to renegotiate the security parameters in an existing
      connection.  Finally, the client will 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
      ClientKeyExchange.  In that case, the client MUST send the same
      ClientHello (without modification) along with the new
      ClientKeyExchange.

   Structure of this message:

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

         struct {
             uint32 gmt_unix_time;
             opaque random_bytes[28];
         } Random;

   gmt_unix_time
      The current time and date in standard UNIX 32-bit format (seconds
      since the midnight starting Jan 1, 1970, UTC, ignoring leap
      seconds) according to the sender's internal clock.  Clocks are not
      required to be set correctly by the basic TLS protocol; higher-
      level or application protocols may define additional requirements.
      Note that, for historical reasons, the data element is named using
      GMT, the predecessor of the current worldwide time base, UTC.

   random_bytes
      28 bytes generated by a secure random number generator.

   The ClientHello message includes a variable-length session
   identifier.  If not empty, the value identifies a session between the
   same client and server whose security parameters the client wishes to
   reuse.  The session identifier MAY be from an earlier connection,
   this connection, or from another currently active connection.  The
   second option is useful if the client only wishes to update the
   random structures and derived values of a connection, and the third
   option makes it possible to establish several independent secure
   connections without repeating the full handshake protocol.  These
   independent connections may occur sequentially or simultaneously; a
   SessionID becomes valid when the handshake negotiating it completes
   with the exchange of Finished messages and persists until it is
   removed due to aging or because a fatal error was encountered on a
   connection associated with the session.  The actual contents of the
   SessionID are defined by the server.

      opaque SessionID<0..32>;

   Warning: Because the SessionID is transmitted without encryption confidentiality
   or
   immediate MAC integrity protection, servers MUST NOT place confidential
   information in session identifiers or let the contents of fake
   session identifiers cause any breach of security.  (Note that the
   content of the handshake as a whole, including the SessionID, is
   protected by the Finished messages exchanged at the end of the
   handshake.)
   The cipher suite list, passed from the client to the server in 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 bulk encryption record protection algorithm (including secret
   key
   length), a MAC algorithm, length) and a PRF.  The server will select a cipher suite or, if
   no acceptable choices are presented, return a handshake failure alert
   and close the connection.  If the list contains cipher suites the
   server does not recognize, support, or wish to use, the server MUST
   ignore those cipher suites, and process the remaining ones as usual.

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

   The ClientHello includes a list of compression algorithms supported
   by the client, ordered according to the client's preference.

      enum { null(0), (255) } CompressionMethod;

      struct {
          ProtocolVersion client_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suites<2..2^16-2>;
          CompressionMethod compression_methods<1..2^8-1>;
          select (extensions_present) {
              case false:
                  struct {};
              case true:
                  Extension extensions<0..2^16-1>;
          };
      } ClientHello;

   TLS allows extensions to follow the compression_methods field in an
   extensions block.  The presence of extensions can be detected by
   determining whether there are bytes following the compression_methods
   at the end of the ClientHello.  Note that this method of detecting
   optional data differs from the normal TLS method of having a
   variable-length field, but it is used for compatibility with TLS
   before extensions were defined.

   client_version
      The version of the TLS protocol by which the client wishes to
      communicate during this session.  This SHOULD be the latest
      (highest valued) version supported by the client.  For this
      version of the specification, the version will be 3.3 3.4 (see
      Appendix E for details about backward compatibility).

   random
      A client-generated random structure.

   session_id
      The ID of a session the client wishes to use for this connection.
      This field is empty if no session_id is available, or if the
      client wishes to generate new security parameters.

   cipher_suites
      This is a list of the cryptographic options supported by the
      client, with the client's first preference first.  If the
      session_id field is not empty (implying a session resumption
      request), this vector MUST include at least the cipher_suite from
      that session.  Values are defined in Appendix A.5.

   compression_methods
      This is a list
      Versions of TLS before 1.3 supported compression and the list of
      compression methods supported by the client,
      sorted by client preference.  If the session_id was supplied in this field.  For any TLS 1.3
      ClientHello, this field is not empty
      (implying a session resumption request), it MUST include contain only the
      compression_method from that session.  This vector MUST contain,
      and all implementations MUST support, CompressionMethod.null.
      Thus, "null" compression
      method with the code point of 0.  If a client and TLS 1.3 ClientHello is
      received with any other value in this field, the server will always be able to agree on MUST
      generate a fatal "illegal_parameter" alert.  Note that TLS 1.3
      servers may receive TLS 1.2 or prior ClientHellos which contain
      other compression method. methods and MUST follow the procedures for the
      appropriate prior version of TLS.

   extensions
      Clients MAY request extended functionality from servers by sending
      data in the extensions field.  The actual "Extension" format is
      defined in Section 7.4.1.4. 7.4.2.3.

   In the event that a client requests additional functionality using
   extensions, and this functionality is not supplied by the server, the
   client MAY abort the handshake.  A server MUST accept ClientHello
   messages both with and without the extensions field, and (as for all
   other messages) it 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, the client waits for a
   ServerHello message.  Any handshake message returned by the server,
   except for a HelloRequest, is treated as a fatal error.

7.4.1.3.  Server Hello

7.4.2.  Client Key Exchange Message

   When this message will be sent:

      The server will send this

      This message in response to a is always sent by the client.  It MUST immediately
      follow the ClientHello
      message when it was able to find an acceptable set of algorithms.
      If it cannot find such a match, message.  In backward compatibility mode
      (see Section XXX) it will respond with a handshake
      failure alert.

   Structure of this message:

      struct { be included in the EarlyData extension
      (Section 7.4.2.3.2) in the ClientHello.

   Meaning of this message:

      This message contains the client's cryptographic parameters for
      zero or more key establishment methods.

   Structure of this message:

      enum { dhe(1), (255) } KeyExchangeAlgorithm;

      struct {
          KeyExchangeAlgorithm algorithm;
          select (KeyExchangeAlgorithm) {
             dhe:
                 ClientDiffieHellmanParams;
          } exchange_keys;
      } ClientKeyExchangeOffer;

      struct {
          ClientKeyExchangeOffer offers<0..2^16-1>;
      } ClientKeyExchange;

   offers
      A list of ClientKeyExchangeOffer values.

   [[OPEN ISSUE: Should we rename CKE here?]]  Clients may offer an
   arbitrary number of ClientKeyExchangeOffer values, each representing
   a single set of key agreement parameters; for instance a client might
   offer shares for several elliptic curves or multiple integer DH
   groups.  The shares for each ClientKeyExchangeOffer MUST by generated
   independently.  Clients MUST NOT offer multiple
   ClientKeyExchangeOffers for the same parameters.  It is explicitly
   permitted to send an empty ClientKeyExchange message, as this is used
   to elicit the server's parameters if the client has no useful
   information.

   [TODO: Recommendation about what the client offers.  Presumably which
   integer DH groups and which curves.]  [TODO: Work out how this
   interacts with PSK and SRP.]

7.4.2.1.  Client Diffie-Hellman Parameters

   When one of the ClientKeyExchangeOffers is a Diffie-Hellman key, the
   client SHALL encode it using ClientDiffieHellmanParams.  This
   structure conveys the client's Diffie-Hellman public value (dh_Yc)
   and the group which it is being provided for.

   Structure of this message:

      struct {
          DiscreteLogDHEGroup group;  // from draft-gillmor
          opaque dh_Yc<1..2^16-1>;
      } ClientDiffieHellmanParams;

      group
         The DHE group to which these parameters correspond.

      dh_Yc
         The client's Diffie-Hellman public value (g^X mod p).

7.4.2.2.  Server Hello

   When this message will be sent:

      The server will send this message in response to a ClientHello
      message when it was able to find an acceptable set of algorithms.
      If it cannot find such a match, it will respond with a handshake
      failure alert.

   Structure of this message:

      struct {
          ProtocolVersion server_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suite;
          CompressionMethod compression_method;
          select (extensions_present) {
              case false:
                  struct {};
              case true:
                  Extension extensions<0..2^16-1>;
          };
      } ServerHello;

   The presence of extensions can be detected by determining whether
   there are bytes following the compression_method cipher_suite field at the end of the
   ServerHello.

   server_version
      This field will contain the lower of that suggested by the client
      in the client hello and the highest supported by the server.  For
      this version of the specification, the version is 3.3. 3.4.  (See
      Appendix E for details about backward compatibility.)

   random
      This structure is generated by the server and MUST be
      independently generated from the ClientHello.random.

   session_id
      This is the identity of the session corresponding to this
      connection.  If the ClientHello.session_id was non-empty, the
      server will look in its session cache for a match.  If a match is
      found and the server is willing to establish the new connection
      using the specified session state, the server will respond with
      the same value as was supplied by the client.  This indicates a
      resumed session and dictates that the parties must proceed
      directly to the Finished messages.  Otherwise, this field will
      contain a different value identifying the new session.  The server
      may return an empty session_id to indicate that the session will
      not be cached and therefore cannot be resumed.  If a session is
      resumed, it must be resumed using the same cipher suite it was
      originally negotiated with.  Note that there is no requirement
      that the server resume any session even if it had formerly
      provided a session_id.  Clients MUST be prepared to do a full
      negotiation -- including negotiating new cipher suites -- during
      any handshake.

   cipher_suite
      The single cipher suite selected by the server from the list in
      ClientHello.cipher_suites.  For resumed sessions, this field is
      the value from the state of the session being resumed.

   compression_method
      The single compression algorithm selected by the server from the
      list in ClientHello.compression_methods.  For resumed sessions,
      this field is the value from the resumed session state.

   extensions
      A list of extensions.  Note that only extensions offered by the
      client can appear in the server's list.

7.4.1.4.  In TLS 1.3 as opposed to
      previous versions of TLS, the server's extensions are split
      between the ServerHello and the EncryptedExtensions Section 7.4.4
      message.  The ServerHello MUST only include extensions which are
      required to establish the cryptographic context.

7.4.2.3.  Hello Extensions

   The extension format is:

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

      enum {
          signature_algorithms(13), early_data(TBD), (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 a companion document
   [TLSEXT].  The list of extension types is maintained by IANA as
   described in Section 12.

   An extension type MUST NOT appear in the ServerHello unless the same
   extension type appeared in the corresponding ClientHello.  If a
   client receives an extension type in ServerHello 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.  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 Client Hello, and does
   not include them in Server Hello.  However, some extensions may
   specify different behavior during session 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 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.

7.4.1.4.1.

7.4.2.3.1.  Signature Algorithms

   The client uses the "signature_algorithms" extension to indicate to
   the server which signature/hash algorithm pairs may be used in
   digital signatures.  The "extension_data" field of this extension
   contains a "supported_signature_algorithms" value.

      enum {
          none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
          sha512(6), (255)
      } HashAlgorithm;

      enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
        SignatureAlgorithm;

      struct {
            HashAlgorithm hash;
            SignatureAlgorithm signature;
      } SignatureAndHashAlgorithm;

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

   Each SignatureAndHashAlgorithm value lists a single hash/signature
   pair that the client is willing to verify.  The values are indicated
   in descending order of preference.

   Note: Because not all signature algorithms and hash algorithms may be
   accepted by an implementation (e.g., DSA with SHA-1, but not SHA-
   256), algorithms here are listed in pairs.

   hash
      This field indicates the hash algorithm which may be used.  The
      values indicate support for unhashed data, MD5 [RFC1321], SHA-1,
      SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively.  The
      "none" value is provided for future extensibility, in case of a
      signature algorithm which does not require hashing before signing.

   signature
      This field indicates the signature algorithm that may be used.
      The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
      [RFC3447] and DSA [DSS], and ECDSA [ECDSA], respectively.  The
      "anonymous" value is meaningless in this context but used in
      Section 7.4.3.  It MUST NOT appear in this extension.

   The semantics of this extension are somewhat complicated because the
   cipher suite indicates permissible signature algorithms but not hash
   algorithms.  Section 7.4.2 7.4.5 and Section 7.4.3 describe the appropriate
   rules.

   If the client supports only the default hash and signature algorithms
   (listed in this section), it MAY omit the signature_algorithms
   extension.  If the client does not support the default algorithms, or
   supports other hash and signature algorithms (and it is willing to
   use them for verifying messages sent by the server, i.e., server
   certificates and server key exchange), it MUST send the
   signature_algorithms extension, listing the algorithms it is willing
   to accept.

   If the client does not send the signature_algorithms extension, the
   server MUST do the following:

   -  If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
      DH_RSA, RSA_PSK, ECDH_RSA, (DHE_RSA,
      ECDHE_RSA), behave as if client had sent the value {sha1,rsa}.

   -  If the negotiated key exchange algorithm is one of (DHE_DSS,
      DH_DSS), DHE_DSS, behave as if
      the client had sent the value {sha1,dsa}.

   -  If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
      ECDHE_ECDSA), ECDHE_ECDSA, behave as
      if the client had sent value {sha1,ecdsa}.

   Note: this is a change from TLS 1.1 where there are no explicit
   rules, but as a practical matter one can assume that the peer
   supports MD5 and SHA-1.

   Note: this extension is not meaningful for TLS versions prior to 1.2.
   Clients MUST NOT offer it if they are offering prior versions.
   However, even if clients do offer it, the rules specified in [TLSEXT]
   require servers to ignore extensions they do not understand.

   Servers MUST NOT send this extension.  TLS servers MUST support
   receiving this extension.

   When performing session resumption, this extension is not included in
   Server Hello, and the server ignores the extension in Client Hello
   (if present).

7.4.2.  Server Certificate

   When this message will be sent:

      The server MUST send

7.4.2.3.2.  Early Data Extension

   TLS versions before 1.3 have 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 DH_anon).  This strict message will always immediately ordering and do not
   permit additional messages to follow the
      ServerHello message.

   Meaning of this message:

      This message conveys the server's certificate chain ClientHello.  The EarlyData
   extension allows TLS messages which would otherwise be sent as
   separate records to be instead inserted in the client. ClientHello.  The certificate MUST be appropriate for
   extension simply contains the negotiated cipher
      suite's key exchange algorithm and any negotiated extensions.

   Structure of this message:

      opaque ASN.1Cert<1..2^24-1>; TLS records which would otherwise have
   been included in the client's first flight.

         struct {
          ASN.1Cert certificate_list<0..2^24-1>;
           TLSCipherText messages<5 .. 2^24-1>;
         } Certificate;

   certificate_list
      This is a sequence (chain) of certificates.  The sender's
      certificate MUST come first in the list.  Each following
      certificate MUST directly certify the one preceding it.  Because
      certificate validation requires that root keys be distributed
      independently, the self-signed certificate that specifies EarlyDataExtension;

   Extra messages for the root
      certificate authority client's first flight MAY either be omitted from the chain, under
   transmitted standalone or sent as EarlyData.  However, when a client
   does not know whether TLS 1.3 can be negotiated - e.g., because the
      assumption that the remote end must already possess it in order to
      validate it in any case.

   The same message type and structure will be used for the client's
   response to a certificate request message.  Note that
   server may support a client MAY
   send no certificates if prior version of TLS or because of network
   intermediaries - it does not have an appropriate certificate
   to SHOULD use the EarlyData extension.  If the
   EarlyData extension is used, then clients MUST NOT send in response to any messages
   other than the server's authentication request.

   Note: PKCS #7 [PKCS7] ClientHello in their initial flight.

   Any data included in EarlyData is not used as integrated into the format for handshake
   hashes directly.  E.g., if the certificate
   vector because PKCS #6 [PKCS6] extended certificates are not used.
   Also, PKCS #7 defines a SET rather than a SEQUENCE, making ClientKeyExchange is included in
   EarlyData, then the task handshake hashes consist of parsing ClientHello +
   ServerHello, etc.  However, because the list more difficult.

   The following rules apply to ClientKeyExchange is in a
   ClientHello extension, it is still hashed transitively.  This
   procedure guarantees that the certificates sent Finished message covers these messages
   even if they are ultimately ignored by the server:

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

   -  The end entity certificate's public key (and associated
      restrictions) because it
   is sent to a TLS 1.2 server).  TLS 1.3 servers MUST be compatible with the selected key exchange
      algorithm.

      Key Exchange Alg.  Certificate Key Type

      RSA                RSA public key; understand
   messages sent in EarlyData, and aside from hashing them differently,
   MUST treat them as if they had been sent immediately after the certificate
   ClientHello.

   Servers MUST allow NOT send the
      RSA_PSK            key to be used for encryption (the
                         keyEncipherment bit MUST be set if the key
                         usage extension EarlyData extension.  Negotiating TLS 1.3
   serves as acknowledgement that it was processed as described above.

   [[OPEN ISSUE: This is present).
                         Note: RSA_PSK a fairly general mechanism which is defined possibly
   overkill in [RFC4279].

      DHE_RSA            RSA public key; the certificate MUST allow the
      ECDHE_RSA          key to 1-RTT case, where it would potentially be used more
   attractive to just have a "ClientKeyExchange" extension.  However,
   for signing (the
                         digitalSignature bit MUST be set if the key
                         usage extension is present) with 0-RTT case we will want to send the signature
                         scheme Certificate,
   CertificateVerify, and hash algorithm that will be employed
                         in application data, so a more general extension
   seems appropriate at least until we have determined we don't need it
   for 0-RTT.]]

7.4.2.4.  Negotiated DL DHE Groups

   Previous versions of TLS before 1.3 allowed the server key exchange message.
                         Note: ECDHE_RSA is defined in [RFC4492].

      DHE_DSS            DSA public key; to specify a
   custom DHE group.  This version of TLS requires the certificate use of specific
   named groups.  [I-D.gillmor-tls-negotiated-dl-dhe] describes a
   mechanism for negotiating such groups.

   If the ClientHello contains a DHE cipher suite, it MUST allow also include
   a "negotiated_dl_dhe_groups" extension.  If the
                         key to be used for signing server selects a DHE
   cipher suite, it MUST respond with the hash
                         algorithm that will extension to indicate the
   selected group.  If no acceptable group can be employed in selected across all
   cipher suites, then the server
                         key exchange message.

      DH_DSS             Diffie-Hellman public key; the keyAgreement bit
      DH_RSA MUST generate a fatal
   "handshake_failure" alert.  [[TODO: Presumably we want to bring
   [I-D.gillmor-tls-negotiated-dl-dhe] into this specification.]]

7.4.3.  Server Key Exchange Message

   When this message will be set sent:

      This message will be sent immediately after the ServerHello
      message if the key usage extension client has provided a ClientKeyExchange message
      which is
                         present.

      ECDH_ECDSA         ECDH-capable public key; compatible with the public key MUST
      ECDH_RSA           use a curve selected cipher suite and point format supported by the
                         client, as described in [RFC4492].

      ECDHE_ECDSA        ECDSA-capable public key; the certificate MUST group
      parameters.

   Meaning of this message:

      This message conveys cryptographic information to allow the key client
      to be used for signing with the
                         hash algorithm that will be employed in compute the
                         server key exchange message.  The premaster secret: a Diffie-Hellman public key
                         MUST use a curve and point format supported by
                         the client, as described in  [RFC4492].

   -  The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are
      used to guide certificate selection.

   If with
      which the client provided can complete a "signature_algorithms" extension, then all
   certificates provided by key exchange (with the server MUST be signed by a hash/
   signature algorithm pair that appears in that extension.  Note that
   this implies that a certificate containing result
      being the premaster secret) or a public key for one signature
   algorithm MAY some other
      algorithm.

   Structure of this message:

      struct {
          opaque dh_Ys<1..2^16-1>;
      } ServerDiffieHellmanParams;     /* Ephemeral DH parameters */

      dh_Ys
         The server's Diffie-Hellman public value (g^X mod p).

      struct {
          select (KeyExchangeAlgorithm) {
              case dhe:
                  ServerDiffieHellmanParams;
              /* may be signed using a different signature algorithm (for
   instance, an RSA extended, e.g., for ECDH -- see [RFC4492] */
          } params;
      } ServerKeyExchange;

   params
      The server's key signed with a DSA key).  This is a departure
   from TLS 1.1, which required that exchange parameters.  These correspond to the algorithms be
      group indicated by the same.  Note
   that this also implies that ServerHello message using the DH_DSS, DH_RSA, ECDH_ECDSA, cipher suite
      and
   ECDH_RSA key exchange algorithms the "negotiated_dl_dhe_groups"
      [I-D.gillmor-tls-negotiated-dl-dhe] extension.  [[TODO:
      incorporate ECDHE if the WG decides to.]]  [[OPEN ISSUE: Note that
      we explicitly do not restrict indicate the algorithm used
   to sign group here since that's
      specified in the certificate.  Fixed DH certificates MAY be signed with
   any hash/signature algorithm pair appearing in the extension.  The
   names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are historical.

   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,
   etc.).  If the server has a single certificate, it SHOULD attempt to
   validate that ServerHello.  We could duplicate it meets these criteria.

   Note here, but
      that seems more confusing since there are certificates that use algorithms and/or algorithm
   combinations that cannot be currently used with TLS.  For example, a
   certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in
   SubjectPublicKeyInfo) cannot be used because TLS defines no
   corresponding signature algorithm.

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

7.4.3.  Server Key Exchange Message mismatch.]]

7.4.4.  Encrypted Extensions

   When this message will be sent:

      This

      If this message will is sent, it MUST be sent immediately after the server Certificate
      server's ChangeCipherSpec (and hence as the first handshake
      message (or after the ServerHello message, if ServerKeyExchange).

   Meaning of this is an anonymous
      negotiation). message:

      The ServerKeyExchange message is sent by the server only when the
      server Certificate EncryptedExtensions message (if sent) does not contain enough data
      to allow the client to exchange a premaster secret.  This is true
      for the following key exchange methods:

         DHE_DSS
         DHE_RSA
         DH_anon

      It is simply contains any extensions
      which should be protected, i.e., any which are not legal needed to send the ServerKeyExchange message for
      establish the
      following key exchange methods:

         RSA
         DH_DSS
         DH_RSA

      Other key exchange algorithms, such as those defined in [RFC4492], cryptographic context.  The same extension types
      MUST specify whether NOT appear in both the ServerKeyExchange message is sent or not; ServerHello and if EncryptedExtensions.
      If the message is sent, its contents.

   Meaning of this message:

      This message conveys cryptographic information to allow same extension appears in both locations, the client
      to communicate MUST
      rely only on the premaster secret: a Diffie-Hellman public key
      with which value in the client can complete EncryptedExtensions block.  [[OPEN
      ISSUE: Should we just produce a key exchange (with the result
      being canonical list of what goes where
      and have it be an error to have it in the premaster secret) or wrong place?  That seems
      simpler.  Perhaps have a public key for some other
      algorithm. whitelist of which extensions can be
      unencrypted and everything else MUST be encrypted.]]

   Structure of this message:

      enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa
            /* may be extended, e.g., for ECDH -- see [RFC4492] */
           } KeyExchangeAlgorithm;

      struct {
          opaque dh_p<1..2^16-1>;
          opaque dh_g<1..2^16-1>;
          opaque dh_Ys<1..2^16-1>;
          Extension extensions<0..2^16-1>;
      } ServerDHParams;     /* Ephemeral DH parameters */

      dh_p EncryptedExtensions;

   extensions
      A list of extensions.

7.4.5.  Server Certificate

   When this message will be sent:

      The prime modulus used 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 DH_anon).  This message will always immediately follow the Diffie-Hellman operation.

      dh_g
      ChangeCipherSpec which follows the server's ServerKeyExchange
      message.

   Meaning of this message:

      This message conveys the server's certificate chain to the client.

      The generator used certificate MUST be appropriate for the Diffie-Hellman operation.

      dh_Ys
         The server's Diffie-Hellman public value (g^X mod p).

      struct {
          select (KeyExchangeAlgorithm) {
              case dh_anon:
                  ServerDHParams params;
              case dhe_dss:
              case dhe_rsa:
                  ServerDHParams params;
                  digitally-signed negotiated cipher
      suite's key exchange algorithm and any negotiated extensions.

   Structure of this message:

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

      struct {
                      opaque client_random[32];
                      opaque server_random[32];
                      ServerDHParams params;
          ASN1Cert certificate_list<0..2^24-1>;
      } signed_params;
              case rsa:
              case dh_dss:
              case dh_rsa:
                  struct {} ;
                 /* message Certificate;

   certificate_list
      This is omitted for rsa, dh_dss, and dh_rsa */
              /* may be extended, e.g., for ECDH -- see [RFC4492] */
          };
      } ServerKeyExchange;

   params
      The server's key exchange parameters.

   signed_params
      For non-anonymous key exchanges, a signature over sequence (chain) of certificates.  The sender's
      certificate MUST come first in the server's key
      exchange parameters.

   If list.  Each following
      certificate MUST directly certify the client has offered one preceding it.  Because
      certificate validation requires that root keys be distributed
      independently, the "signature_algorithms" extension, self-signed certificate that specifies the
   signature algorithm root
      certificate authority MAY be omitted from the chain, under the
      assumption that the remote end must already possess it in order to
      validate it in any case.

   The same message type and hash algorithm MUST structure will be used for the client's
   response to a pair listed in that
   extension. certificate request message.  Note that there is a possibility for inconsistencies
   here.  For instance, the client might offer DHE_DSS key exchange but
   omit any DSA pairs from its "signature_algorithms" extension.  In
   order MAY
   send no certificates if it does not have an appropriate certificate
   to send in response to negotiate correctly, the server MUST check any candidate
   cipher suites against the "signature_algorithms" extension before
   selecting them.  This is somewhat inelegant but server's authentication request.

   Note: PKCS #7 [PKCS7] is not used as the format for the certificate
   vector because PKCS #6 [PKCS6] extended certificates are not used.
   Also, PKCS #7 defines a compromise
   designed to minimize changes SET rather than a SEQUENCE, making the task
   of parsing the list more difficult.

   The following rules apply to the original cipher suite design.

   In addition, certificates sent by the hash and signature algorithms server:

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

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

      Key Exchange Alg.  Certificate Key Type

      DHE_RSA            RSA keys MAY public key; the certificate MUST allow the
      ECDHE_RSA          key to be used for signing (the
                         digitalSignature bit MUST be set if the key
                         usage extension is present) with any permitted the signature
                         scheme and hash algorithm, subject to restrictions algorithm that will be employed
                         in the certificate, if any.

   Because DSA signatures do not contain any secure indication of hash
   algorithm, there server key exchange message.
                         Note: ECDHE_RSA is a risk of hash substitution if multiple hashes
   may be used with any key.  Currently, defined in [RFC4492].

      DHE_DSS            DSA [DSS] may only be used with
   SHA-1.  Future revisions of DSS [DSS-3] are expected to public key; the certificate MUST allow the use
   of other digest algorithms with DSA, as well as guidance as to which
   digest algorithms should be used with each
                         key size.  In addition,
   future revisions of [RFC3280] may specify mechanisms for certificates
   to indicate which digest algorithms are to be used with DSA.

   As additional cipher suites are defined for TLS signing with the hash
                         algorithm that include new key
   exchange algorithms, will be employed in the server
                         key exchange message will be sent if
   and only if message.

      ECDHE_ECDSA        ECDSA-capable public key; the certificate type associated with MUST
                         allow the key exchange
   algorithm does not provide enough information to be used for signing with the client to
   exchange a premaster secret.

7.4.4.  Certificate Request

   When this message
                         hash algorithm that will be sent:

      A non-anonymous employed in the
                         server can optionally request a certificate key exchange message.  The public key
                         MUST use a curve and point format supported by
                         the client, as described in  [RFC4492].

   -  The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are
      used to guide certificate selection.

   If the client provided a "signature_algorithms" extension, then all
   certificates provided by the server MUST be signed by a hash/
   signature algorithm pair that appears in that extension.  Note that
   this implies 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 DSA key).  This is a departure
   from TLS 1.1, which required that the algorithms be the same.

   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,
   etc.).  If the server has a single certificate, it SHOULD attempt to
   validate that it meets these criteria.

   Note that there are certificates that use algorithms and/or algorithm
   combinations that cannot be currently used with TLS.  For example, a
   certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in
   SubjectPublicKeyInfo) cannot be used because TLS defines no
   corresponding signature algorithm.

   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.

7.4.6.  Certificate Request

   When this message will be sent:

      A non-anonymous server can optionally request a certificate from
      the client, if appropriate for the selected cipher suite.  This
      message, if sent, will immediately follow the ServerKeyExchange
      message (if it is sent; otherwise, this message follows the server's Certificate
      message).

   Structure of this message:

      enum {
          rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
          rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
          fortezza_dms_RESERVED(20), (255)
      } ClientCertificateType;

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

      struct {
          ClientCertificateType certificate_types<1..2^8-1>;
          SignatureAndHashAlgorithm
            supported_signature_algorithms<2^16-1>;
            supported_signature_algorithms<2..2^16-2>;
          DistinguishedName certificate_authorities<0..2^16-1>;
      } CertificateRequest;

   certificate_types
      A list of the types of certificate types that the client may
      offer.

       rsa_sign        a certificate containing an RSA key
       dss_sign        a certificate containing a DSA key
       rsa_fixed_dh    a certificate containing a static DH key.
       dss_fixed_dh    a certificate containing a static DH key

   supported_signature_algorithms
      A list of the hash/signature algorithm pairs that the server is
      able to verify, listed in descending order of preference.

   certificate_authorities
      A list of the distinguished names [X501] of acceptable
      certificate_authorities, represented in DER-encoded format.  These
      distinguished names may specify a desired distinguished name for a
      root CA or for a subordinate CA; thus, this message can be used to
      describe known roots as well as a desired authorization space.  If
      the certificate_authorities list is empty, then the client MAY
      send any certificate of the appropriate ClientCertificateType,
      unless there is some external arrangement to the contrary.

   The interaction of the certificate_types and
   supported_signature_algorithms fields is somewhat complicated.
   certificate_types has been present in TLS since SSLv3, but was
   somewhat underspecified.  Much of its functionality is superseded by
   supported_signature_algorithms.  The following rules apply:

   -  Any certificates provided by the client MUST be signed using a
      hash/signature algorithm pair found in
      supported_signature_algorithms.

   -  The end-entity certificate provided by the client MUST contain a
      key that is compatible with certificate_types.  If the key is a
      signature key, it MUST be usable with some hash/signature
      algorithm pair in supported_signature_algorithms.

   -  For historical reasons, the names of some client certificate types
      include the algorithm used to sign the certificate.  For example,
      in earlier versions of TLS, rsa_fixed_dh meant a certificate
      signed with RSA and containing a static DH key.  In TLS 1.2, this
      functionality has been obsoleted by the
      supported_signature_algorithms, and the certificate type no longer
      restricts the algorithm used to sign the certificate.  For
      example, if the server sends dss_fixed_dh certificate type and
      {{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
      with a certificate containing a static DH key, signed with RSA-
      SHA1.

   New ClientCertificateType values are assigned by IANA as described in
   Section 12.

   Note: Values listed as RESERVED may not be used.  They were used in
   SSLv3.

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

7.4.5.

7.4.7.  Server Hello Done Certificate Verify

   When this message will be sent:

      The ServerHelloDone

      This message is sent by the server used to indicate provide explicit proof that the
      end of server
      possesses the ServerHello private key corresponding to its certificate.
      certificate and associated messages.  After sending
      this message, the server will wait also provides integrity for a client response.

   Meaning of the handshake up to
      this message: point.  This message means that the server is done sending messages to
      support the key exchange, and the client can proceed with its
      phase of the key exchange.

      Upon receipt of the ServerHelloDone message, the client SHOULD
      verify that only sent when the server provided is
      authenticated via a valid certificate, if required,
      and check that certificate.  When sent, it MUST be the last
      server hello parameters are acceptable. handshake message prior to the Finished.

   Structure of this message:

      struct {
           digitally-signed struct {
               opaque handshake_messages[handshake_messages_length];
           } ServerHelloDone;

7.4.6.  Client Certificate

   When
      } CertificateVerify;

      Here handshake_messages refers to all handshake messages sent or
      received, starting at client hello and up to, but not including,
      this message will be sent:

      This is message, including the first message type and length fields of the client can send after receiving a
      ServerHelloDone message.
      handshake messages.  This message is only sent if the server
      requests a certificate.  If no suitable certificate is available, concatenation of all the client MUST send a certificate message containing no
      certificates.  That is,
      Handshake structures (as defined in Section 7.4) exchanged thus
      far.  Note that this requires both sides to either buffer the certificate_list structure has
      messages or compute running hashes for all potential hash
      algorithms up to the time of the CertificateVerify computation.
      Servers can minimize this computation cost by offering a
      length
      restricted set of zero. digest algorithms in the CertificateRequest
      message.

      If the client does not send any certificates, has offered the
      server MAY at its discretion either continue "signature_algorithms" extension,
      the handshake without
      client authentication, or respond with signature algorithm and hash algorithm MUST be a fatal handshake_failure
      alert.  Also, if some aspect of the certificate chain was
      unacceptable (e.g., it was not signed by pair listed
      in that extension.  Note that there is a known, trusted CA), the
      server MAY at its discretion either continue the handshake
      (considering possibility for
      inconsistencies here.  For instance, the client unauthenticated) or send a fatal alert.

      Client certificates are sent using the Certificate structure
      defined in Section 7.4.2.

   Meaning of this message:

      This message conveys the client's certificate chain might offer
      DHE_DSS key exchange but omit any DSA pairs from its
      "signature_algorithms" extension.  In order to the server; negotiate
      correctly, the server will use it when verifying the CertificateVerify
      message (when MUST check any candidate cipher suites
      against the client authentication "signature_algorithms" extension before selecting
      them.  This is based on signing) or
      calculating the premaster secret (for non-ephemeral Diffie-
      Hellman).  The certificate MUST be appropriate for somewhat inelegant but is a compromise designed to
      minimize changes to the negotiated original cipher suite's key exchange algorithm, and any negotiated
      extensions. suite design.

      In particular:

   -  The certificate type addition, the hash and signature algorithms MUST be X.509v3, unless explicitly negotiated
      otherwise (e.g., [RFC5081]).

   -  The end-entity certificate's public key (and associated
      restrictions) has to be compatible
      with the certificate types
      listed key in CertificateRequest:

    Client Cert. Type   Certificate Key Type

    rsa_sign            RSA public key; the certificate MUST allow the
                        key to server's end-entity certificate.  RSA keys MAY
      be used for signing with the signature
                        scheme and any permitted hash algorithm that will be
                        employed algorithm, subject to restrictions
      in the certificate verify message.

    dss_sign certificate, if any.

      Because DSA public key; the certificate MUST allow the
                        key to signatures do not contain any secure indication of
      hash algorithm, there is a risk of hash substitution if multiple
      hashes may be used for signing with the hash
                        algorithm that will any key.  Currently, DSA [DSS] may only be employed in the
                        certificate verify message.

    ecdsa_sign          ECDSA-capable public key; the certificate MUST
      used with SHA-1.  Future revisions of DSS [DSS-3] are expected to
      allow the key use of other digest algorithms with DSA, as well as
      guidance as to which digest algorithms should be used with each
      key size.  In addition, future revisions of [RFC3280] may specify
      mechanisms for signing certificates to indicate which digest algorithms
      are to be used with the
                        hash algorithm that DSA.  [[TODO: Update this to deal with DSS-3
      and DSS-4. https://github.com/tlswg/tls13-spec/issues/59]]

7.4.8.  Server Finished

   When this message will be employed in sent:

      The Server's Finished message is the
                        certificate verify message; final message sent by the
      server and indicates that the public key exchange and authentication
      processes were successful.

   Meaning of this message:

      Recipients of Finished messages MUST use verify that the contents are
      correct.  Once a curve side has sent its Finished message and point format supported by received
      and validated the server.

    rsa_fixed_dh        Diffie-Hellman public key; MUST use Finished message from its peer, it may begin to
      send and receive application data over the same
    dss_fixed_dh        parameters as server's key.

    rsa_fixed_ecdh      ECDH-capable public key; MUST use connection.

   Structure of this message:

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

      verify_data
         PRF(master_secret, finished_label, Hash(handshake_messages))
            [0..verify_data_length-1];

      finished_label
         For Finished messages sent by the
    ecdsa_fixed_ecdh    same curve as client, the server's key, and MUST use a
                        point format supported string
         "client finished".  For Finished messages sent by the server.

   -  If the certificate_authorities list in server,
         the certificate request
      message was non-empty, one string "server finished".

      Hash denotes a Hash of the certificates in the certificate
      chain SHOULD be issued by one of handshake messages.  For the listed CAs.

   -  The certificates MUST be signed using an acceptable hash/
      signature algorithm pair, as described PRF
      defined in Section 7.4.4.  Note
      that this relaxes the constraints on certificate-signing
      algorithms found in prior versions of TLS.

   Note that, as with 5, the server certificate, there are certificates
   that use algorithms/algorithm combinations that cannot Hash MUST be currently the Hash used with TLS.

7.4.7.  Client Key Exchange Message

   When this message will be sent:

      This message is always sent by as the client.  It MUST immediately
      follow basis
      for the client certificate message, if it is sent.  Otherwise,
      it PRF.  Any cipher suite which defines a different PRF MUST be the first message sent by the client after it receives
      also define the ServerHelloDone message.

   Meaning of this message:

      With this message, Hash to use in the premaster secret is set, either by direct
      transmission Finished computation.

      In previous versions of TLS, the RSA-encrypted secret or by verify_data was always 12 octets
      long.  In the transmission current version of
      Diffie-Hellman parameters that will allow each side to agree upon
      the same premaster secret.

      When TLS, it depends on the client is using an ephemeral Diffie-Hellman exponent,
      then cipher
      suite.  Any cipher suite which does not explicitly specify
      verify_data_length has a verify_data_length equal to 12.  This
      includes all existing cipher suites.  Note that this message contains
      representation has the client's Diffie-Hellman public
      value.  If the client is sending a certificate containing a static
      DH exponent (i.e., it is doing fixed_dh client authentication),
      then this message MUST be sent same encoding as with previous versions.
      Future cipher suites MAY specify other lengths but such length
      MUST be empty.

   Structure of this message:

      The choice of messages depends on which key exchange method has
      been selected.  See Section 7.4.3 for the KeyExchangeAlgorithm
      definition.

      struct {
          select (KeyExchangeAlgorithm) {
              case rsa:
                  EncryptedPreMasterSecret;
              case dhe_dss:
              case dhe_rsa:
              case dh_dss:
              case dh_rsa:
              case dh_anon:
                  ClientDiffieHellmanPublic;
          } exchange_keys;
      } ClientKeyExchange;

7.4.7.1.  RSA-Encrypted Premaster Secret Message

   Meaning at least 12 bytes.

   handshake_messages
      All of this message:

      If RSA is being used for key agreement and authentication, the
      client generates a 48-byte premaster secret, encrypts it using the
      public key data from the server's certificate, and sends the result all messages in
      an encrypted premaster secret this handshake (not including
      any HelloRequest messages) up to, but not including, this message.
      This structure is a
      variant of only data visible at the ClientKeyExchange message handshake layer and is does not a message in
      itself.

   Structure of this message:

      struct {
          ProtocolVersion client_version;
          opaque random[46];
      } PreMasterSecret;

      client_version
         The latest (newest) version supported by the client.  This is
         used to detect version rollback attacks.

      random
         46 securely-generated random bytes.

      struct {
          public-key-encrypted PreMasterSecret pre_master_secret;
      } EncryptedPreMasterSecret;

      pre_master_secret
      include record layer headers.  This random value is generated by the client and is used to
         generate concatenation of all
      the master secret, Handshake structures as specified in
         [Section 8.1].

   Note: The version number defined in the PreMasterSecret Section 7.4, exchanged thus
      far.

   It is the version
   offered a fatal error if a Finished message is not preceded by a
   ChangeCipherSpec message at the client appropriate point in the ClientHello.client_version, not the
   version negotiated for the connection. handshake.

   The value handshake_messages includes all handshake messages starting
   at ClientHello up to, but not including, this Finished message.  This feature is designed to
   prevent rollback attacks.  Unfortunately, some old implementations
   use the negotiated version instead, and therefore checking the
   version number
   may lead to failure to interoperate with such
   incorrect client implementations.

   Client implementations MUST always send the correct version number be different from handshake_messages in
   PreMasterSecret.  If ClientHello.client_version is TLS 1.1 Section 7.4.7 or higher,
   server implementations MUST check
   Section 7.4.10.  Also, the version number as described in handshake_messages for the note below.  If Finished
   message sent by the version number is TLS 1.0 or earlier, server
   implementations SHOULD check client will be different from that for the version number, but MAY have a
   configuration option to disable
   Finished message sent by the check.  Note that if server, because the check
   fails, one that is sent
   second will include the PreMasterSecret SHOULD be randomized as described below. prior one.

   Note: Attacks discovered by Bleichenbacher [BLEI] ChangeCipherSpec messages, alerts, and Klima et al.
   [KPR03] can be used to attack a TLS server that reveals whether a
   particular message, when decrypted, is properly PKCS#1 formatted,
   contains a valid PreMasterSecret structure, or has any other record types
   are not handshake messages and are not included in the correct
   version number.

   As described by Klima [KPR03], these vulnerabilities can hash
   computations.  Also, HelloRequest messages are omitted from handshake
   hashes.

7.4.9.  Client Certificate

   When this message will be avoided
   by treating incorrectly formatted sent:

      This message blocks and/or mismatched
   version numbers in a manner indistinguishable from correctly
   formatted RSA blocks.  In other words:

   1.  Generate a string R of 46 random bytes

   2.  Decrypt is the first handshake message to recover the plaintext M

   3.  If client can send
      after receiving the PKCS#1 padding server's Finished and having sent its own
      ChangeCipherSpecs.  This message is not correct, or only sent if the length of message M
       is not exactly 48 bytes:

     pre_master_secret = ClientHello.client_version || R

       else server
      requests a certificate.  If ClientHello.client_version <= TLS 1.0, and version number
       check no suitable certificate is explicitly disabled:

     pre_master_secret = M

       else:

     pre_master_secret = ClientHello.client_version || M[2..47]

   Note that explicitly constructing the pre_master_secret with the
   ClientHello.client_version produces an invalid master_secret if available,
      the client has sent the wrong version in the original pre_master_secret.

   An alternative approach is to treat a version number mismatch as MUST send a
   PKCS-1 formatting error and randomize certificate message containing no
      certificates.  That is, the premaster secret
   completely:

   1.  Generate certificate_list structure has a string R
      length of 48 random bytes

   2.  Decrypt the message to recover the plaintext M

   3. zero.  If the PKCS#1 padding is client does not correct, or send any certificates, the length of message M
       is not exactly 48 bytes:

     pre_master_secret = R

       else If ClientHello.client_version <= TLS 1.0, and version number
       check is explicitly disabled:

     premaster secret = M
       else If M[0..1] != ClientHello.client_version:

     premaster secret = R

       else:

     premaster secret = M

   Although no practical attacks against this construction are known,
   Klima et al.  [KPR03] describe some theoretical attacks, and
   therefore
      server MAY at its discretion either continue the first construction described is RECOMMENDED.

   In any case, handshake without
      client authentication, or respond with a TLS server MUST NOT generate an alert fatal handshake_failure
      alert.  Also, if processing an
   RSA-encrypted premaster secret message fails, or some aspect of the version number
   is not as expected.  Instead, certificate chain was
      unacceptable (e.g., it MUST was not signed by a known, trusted CA), the
      server MAY at its discretion either continue the handshake with
      (considering the client unauthenticated) or send a
   randomly generated premaster secret.  It may be useful to log fatal alert.

      Client certificates are sent using the
   real cause Certificate structure
      defined in Section 7.4.5.

   Meaning of failure for troubleshooting purposes; however, care
   must be taken to avoid leaking the information to an attacker
   (through, e.g., timing, log files, or other channels.)

   The RSAES-OAEP encryption scheme defined in [RFC3447] is more secure
   against the Bleichenbacher attack.  However, for maximal
   compatibility with earlier versions of TLS, this specification uses
   the RSAES-PKCS1-v1_5 scheme.  No variants of message:

      This message conveys the Bleichenbacher
   attack are known client's certificate chain to exist provided that the above recommendations are
   followed.

   Implementation note: Public-key-encrypted data is represented as an
   opaque vector <0..2^16-1> (see Section 4.7).  Thus, server;
      the RSA-encrypted
   PreMasterSecret in a ClientKeyExchange is preceded by two length
   bytes.  These bytes are redundant in server will use it when verifying the case of RSA because CertificateVerify
      message (when the
   EncryptedPreMasterSecret client authentication is based on signing) or
      calculating the only data in premaster secret (for non-ephemeral Diffie-
      Hellman).  The certificate MUST be appropriate for the ClientKeyExchange negotiated
      cipher suite's key exchange algorithm, and its length can therefore any negotiated
      extensions.

   In particular:

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

   -  The SSLv3
   specification was not clear about the encoding of public-key-
   encrypted data, and therefore many SSLv3 implementations do not
   include the length bytes -- they encode end-entity certificate's public key (and associated
      restrictions) has to be compatible with the RSA-encrypted data
   directly certificate types
      listed in CertificateRequest:

    Client Cert. Type   Certificate Key Type

    rsa_sign            RSA public key; the ClientKeyExchange message.

   This specification requires correct encoding of the
   EncryptedPreMasterSecret complete with length bytes.  The resulting
   PDU is incompatible with many SSLv3 implementations.  Implementors
   upgrading from SSLv3 certificate MUST modify their implementations to generate
   and accept allow the correct encoding.  Implementors who wish
                        key to be
   compatible used for signing with both SSLv3 the signature
                        scheme and TLS should make their implementation's
   behavior dependent on hash algorithm that will be
                        employed in the protocol version.

   Implementation note: It is now known that remote timing-based attacks
   on TLS are possible, at least when certificate verify message.

    dss_sign            DSA public key; the client and server are on certificate MUST allow the
   same LAN.  Accordingly, implementations
                        key to be used for signing with the hash
                        algorithm that use static RSA keys MUST
   use RSA blinding or some other anti-timing technique, as described will be employed in
   [TIMING].

7.4.7.2.  Client Diffie-Hellman Public Value

   Meaning of this message:

      This structure conveys the client's Diffie-Hellman
                        certificate verify message.

    ecdsa_sign          ECDSA-capable public value
      (Yc) if it was not already included in key; the client's certificate.
      The encoding certificate MUST
                        allow the key to be used for Yc is determined by signing with the enumerated
      PublicValueEncoding.  This structure is a variant of
                        hash algorithm that will be employed in the client
                        certificate verify message; the public key exchange message,
                        MUST use a curve and not point format supported by
                        the server.

    rsa_fixed_dh        Diffie-Hellman public key; MUST use the same
    dss_fixed_dh        parameters as server's key.

    rsa_fixed_ecdh      ECDH-capable public key; MUST use the
    ecdsa_fixed_ecdh    same curve as the server's key, and MUST use a message
                        point format supported by the server.

   -  If the certificate_authorities list in itself.

   Structure the certificate request
      message was non-empty, one of this message:

      enum { implicit, explicit } PublicValueEncoding;

      implicit
         If the client has sent a certificates in the certificate which contains a suitable
         Diffie-Hellman key (for fixed_dh client authentication), then
         Yc is implicit and does not need to
      chain SHOULD be sent again.  In this
         case, issued by one of the client key exchange message will be sent, but it listed CAs.

   -  The certificates MUST be empty.

      explicit
         Yc needs to be sent.

      struct {
          select (PublicValueEncoding) {
              case implicit: struct { };
              case explicit: opaque dh_Yc<1..2^16-1>;
          } dh_public;
      } ClientDiffieHellmanPublic;

   dh_Yc
      The client's Diffie-Hellman public value (Yc).

7.4.8. signed using an acceptable hash/
      signature algorithm pair, as described in Section 7.4.6.  Note
      that this relaxes the constraints on certificate-signing
      algorithms found in prior versions of TLS.

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

7.4.10.  Client Certificate Verify

   When this message will be sent:

      This message is used to provide explicit verification of a client
      certificate.  This message is only sent following a client
      certificate that has signing capability (i.e., all certificates
      except those containing fixed Diffie-Hellman parameters).  When
      sent, it MUST immediately follow the client key exchange client's Certificate message.

   Structure of this message:

      struct {
           digitally-signed struct {
               opaque handshake_messages[handshake_messages_length];
           }
      } CertificateVerify;

      Here handshake_messages refers to all handshake messages sent or
      received, starting at client hello and up to, but not including,
      this message, including the type and length fields of the
      handshake messages.  This is the concatenation
      The contents of all the
      Handshake structures (as defined message are computed as described in
      Section 7.4) exchanged thus
      far.  Note that this requires both sides to either buffer the
      messages or compute running hashes for all potential hash
      algorithms up to the time of the CertificateVerify computation.
      Servers can minimize this computation cost by offering a
      restricted set of digest algorithms in the CertificateRequest
      message. 7.4.7.

      The hash and signature algorithms used in the signature MUST be
      one of those present in the supported_signature_algorithms field
      of the CertificateRequest message.  In addition, the hash and
      signature algorithms MUST be compatible with the key in the
      client's end-entity certificate.  RSA keys MAY be used with any
      permitted hash algorithm, subject to restrictions in the
      certificate, if any.

      Because DSA signatures do not contain any secure indication of
      hash algorithm, there is a risk of hash substitution if multiple
      hashes may be used with any key.  Currently, DSA [DSS] may only be
      used with SHA-1.  Future revisions of DSS [DSS-3] are expected to
      allow the use of other digest algorithms with DSA, as well as
      guidance as to which digest algorithms should be used with each
      key size.  In addition, future revisions of [RFC3280] may specify
      mechanisms for certificates to indicate which digest algorithms
      are to be used with DSA.

7.4.9.  Finished

   When this message will be sent:

      A Finished message is always sent immediately after a change
      cipher spec message

8.  Cryptographic Computations

   In order to verify that begin connection protection, the key exchange and
      authentication processes were successful.  It is essential that TLS Record Protocol
   requires specification of a
      change cipher spec message be received between the other handshake
      messages suite of algorithms, a master secret, and
   the Finished message.

   Meaning of this message: client and server random values.  The Finished message is the first one protected with the just
      negotiated algorithms, keys, authentication, key
   agreement, and secrets.  Recipients of Finished
      messages MUST verify that the contents record protection algorithms 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.

   Structure of this message:

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

      verify_data
         PRF(master_secret, finished_label, Hash(handshake_messages))
            [0..verify_data_length-1];

      finished_label
         For Finished messages sent determined by the client, the string
         "client finished".  For Finished messages sent
   cipher_suite selected by the server, server and revealed in the string "server finished".

      Hash denotes a Hash of ServerHello
   message.  The random values are exchanged in the handshake hello messages.  For  All
   that remains is to calculate the PRF
      defined in Section 5, master secret.

8.1.  Computing the Hash MUST be Master Secret

   For all key exchange methods, the Hash same algorithm is used as to convert
   the basis
      for pre_master_secret into the PRF.  Any cipher suite which defines a different PRF MUST
      also define master_secret.  The pre_master_secret
   should be deleted from memory once the Hash to use master_secret has been
   computed.

      master_secret = PRF(pre_master_secret, "master secret",
                          ClientHello.random + ServerHello.random)
                          [0..47];

   The master secret is always exactly 48 bytes in the Finished computation.

      In previous versions length.  The length
   of TLS, the verify_data was always 12 octets
      long.  In the current version of TLS, it depends premaster secret will vary depending on key exchange method.

8.1.1.  Diffie-Hellman

   A conventional Diffie-Hellman computation is performed.  The
   negotiated key (Z) is used as the cipher
      suite.  Any cipher suite which does not explicitly specify
      verify_data_length has a verify_data_length equal to 12.  This
      includes all existing cipher suites.  Note that this
      representation has the same encoding as with previous versions.
      Future cipher suites MAY specify other lengths but such length
      MUST be at least 12 bytes.

   handshake_messages
      All of the data from all messages in this handshake (not including
      any HelloRequest messages) up to, but not including, this message.
      This is only data visible at the handshake layer pre_master_secret, and does not
      include record layer headers.  This is converted
   into the concatenation master_secret, as specified above.  Leading bytes of Z that
   contain all
      the Handshake structures as defined in Section 7.4, exchanged thus
      far.

   It is a fatal error if a Finished message zero bits are stripped before it is not preceded by a
   ChangeCipherSpec message at used as the appropriate point in
   pre_master_secret.

   Note: Diffie-Hellman parameters are specified by the handshake.

   The value handshake_messages includes all handshake messages starting
   at ClientHello up to, but not including, this Finished message.  This server and may
   be different from handshake_messages in Section 7.4.8 because it
   would include the CertificateVerify message (if sent).  Also, either ephemeral or contained within the
   handshake_messages for server's certificate.

9.  Mandatory Cipher Suites

   In the Finished message sent by absence of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement the client will cipher
   suite TODO:Needs to be different from that selected [1].  (See Appendix A.5 for the Finished message sent
   definition).

10.  Application Data Protocol

   Application data messages are carried by the server,
   because the one that is sent second will include the prior one.

   Note: ChangeCipherSpec messages, alerts, and any other record types
   are not handshake messages layer and are not included in
   fragmented and encrypted based on the hash
   computations.  Also, HelloRequest current connection state.  The
   messages are omitted from handshake
   hashes.

8.  Cryptographic Computations

   In order treated as transparent data 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, encryption,
   and MAC algorithms record layer.

11.  Security Considerations

   Security issues are determined by the cipher_suite selected by the
   server discussed throughout this memo, especially in
   Appendices D, E, and revealed F.

12.  IANA Considerations

   [[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]

   This document uses several registries that were originally created in the ServerHello message.

   [RFC4346].  IANA has updated these to reference this document.  The compression
   algorithm is negotiated in the hello messages,
   registries and the random values their allocation policies (unchanged from [RFC4346])
   are exchanged listed below.

   -  TLS ClientCertificateType Identifiers Registry: Future values in
      the hello messages.  All that remains is to
   calculate range 0-63 (decimal) inclusive are assigned via Standards
      Action [RFC2434].  Values in the master secret.

8.1.  Computing range 64-223 (decimal) inclusive
      are assigned via Specification Required [RFC2434].  Values from
      224-255 (decimal) inclusive are reserved for Private Use
      [RFC2434].

   -  TLS Cipher Suite Registry: Future values with the Master Secret

   For all key exchange methods, first byte in
      the same algorithm is used to convert range 0-191 (decimal) inclusive are assigned via Standards
      Action [RFC2434].  Values with the pre_master_secret into first byte in the master_secret.  The pre_master_secret
   should be deleted from memory once range 192-254
      (decimal) are assigned via Specification Required [RFC2434].
      Values with the master_secret has been
   computed.

      master_secret = PRF(pre_master_secret, "master secret",
                          ClientHello.random + ServerHello.random)
                          [0..47];

   The master secret is always exactly 48 bytes first byte 255 (decimal) are reserved for Private
      Use [RFC2434].

   -  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 length. [RFC4366].
   IANA has updated it to reference this document.  The length
   of the premaster secret will vary depending on key exchange method.

8.1.1.  RSA

   When RSA is used for server authentication registry and key exchange, a 48-
   byte pre_master_secret its
   allocation policy (unchanged from [RFC4366]) is generated by the client, encrypted under
   the server's public key, and sent listed below:

   -  TLS ExtensionType Registry: Future values are allocated via IETF
      Consensus [RFC2434].  IANA has updated this registry to include
      the server.  The server uses signature_algorithms extension and its
   private key corresponding value
      (see Section 7.4.2.3).

   In addition, this document defines two new registries to decrypt the pre_master_secret.  Both parties then
   convert the pre_master_secret into the master_secret, as specified
   above.

8.1.2.  Diffie-Hellman

   A conventional Diffie-Hellman computation is performed. be
   maintained by IANA:

   -  TLS SignatureAlgorithm Registry: The
   negotiated key (Z) is used as registry has been initially
      populated with the pre_master_secret, and is converted
   into values described in Section 7.4.2.3.1.  Future
      values in the master_secret, as specified above.  Leading bytes of Z that
   contain all zero bits range 0-63 (decimal) inclusive are stripped before it is used as assigned via
      Standards Action [RFC2434].  Values in the
   pre_master_secret.

   Note: Diffie-Hellman parameters are specified by the server and may
   be either ephemeral or contained within the server's certificate.

9.  Mandatory Cipher Suites

   In the absence of an application profile standard specifying
   otherwise, a TLS-compliant application MUST implement the cipher
   suite TLS_RSA_WITH_AES_128_CBC_SHA (see Appendix A.5 for the
   definition).

10.  Application Data Protocol

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

11.  Security Considerations

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

12.  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 (unchanged from [RFC4346])
   are listed below.

   -  TLS ClientCertificateType Identifiers Registry: Future values in
      the range 0-63 (decimal) inclusive are assigned via Standards
      Action [RFC2434].  Values in the range 64-223 (decimal) inclusive range 64-223 (decimal)
      inclusive are assigned via Specification Required [RFC2434].
      Values from 224-255 (decimal) inclusive are reserved for Private
      Use [RFC2434].

   -  TLS Cipher Suite HashAlgorithm Registry: Future values The registry has been initially
      populated with the first byte values described in Section 7.4.2.3.1.  Future
      values in the range 0-191 0-63 (decimal) inclusive are assigned via
      Standards Action [RFC2434].  Values with the first byte in the range 192-254 64-223 (decimal)
      inclusive are assigned via Specification Required [RFC2434].
      Values with the first byte 255 from 224-255 (decimal) inclusive are reserved for Private
      Use [RFC2434].

   -  This document defines several new HMAC-SHA256-based cipher suites,
      whose values (in Appendix A.5) have been allocated from the TLS
      Cipher Suite registry.

   -  TLS ContentType Registry: Future values are allocated via

13.  References

13.1.  Normative References

   [AES]                                National Institute of 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 (unchanged from [RFC4366]) is listed below:

   -  TLS ExtensionType Registry: Future values are allocated via IETF
      Consensus [RFC2434].  IANA has updated this registry to include
      the signature_algorithms extension and its corresponding value
      (see Section 7.4.1.4).

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

   -  TLS SignatureAlgorithm Registry: The registry has been initially
      populated with the values described in Section 7.4.1.4.1.  Future
      values in the range 0-63 (decimal) inclusive are assigned via
      Standards Action [RFC2434].  Values in the range 64-223 (decimal)
      inclusive are assigned via Specification Required [RFC2434].
      Values from 224-255 (decimal) inclusive are reserved for Private
      Use [RFC2434].

   -  TLS HashAlgorithm Registry: The registry has been initially
      populated with the values described in Section 7.4.1.4.1.  Future
      values in the range 0-63 (decimal) inclusive are assigned via
      Standards Action [RFC2434].  Values in the range 64-223 (decimal)
      inclusive are assigned via Specification Required [RFC2434].
      Values from 224-255 (decimal) inclusive are reserved for Private
      Use [RFC2434].

   This document also uses the TLS Compression Method Identifiers
   Registry, defined in [RFC3749].  IANA has allocated value 0 for the
   "null" compression method.

13.  References

13.1.  Normative References

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

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

   [DSS]                                National Institute of Standards
                                        and Technology, U.S. Department
                                        of Commerce, "Digital Signature
                                        Standard", NIST FIPS PUB 186-2,
                                        2000.

   [RFC1321]                            Rivest, R., "The MD5 Message-Digest Message-
                                        Digest Algorithm", RFC 1321,
                                        April 1992.

   [RFC2104]                            Krawczyk, H., Bellare, M., and
                                        R. Canetti, "HMAC: Keyed-
                Hashing Keyed-Hashing
                                        for Message Authentication",
                                        RFC 2104, February 1997.

   [RFC2119]                            Bradner, S., "Key words for use
                                        in RFCs to Indicate Requirement
                                        Levels", BCP 14, RFC 2119,
                                        March 1997.

   [RFC2434]                            Narten, T. and H. Alvestrand,
                                        "Guidelines for Writing an IANA
                                        Considerations Section in RFCs",
                                        BCP 26, RFC 2434, October 1998.

   [RFC3280]                            Housley, R., Polk, W., Ford, W.,
                                        and D. Solo, "Internet X.509
                                        Public Key Infrastructure
                                        Certificate and Certificate
                                        Revocation List (CRL) Profile",
                                        RFC 3280, April 2002.

   [RFC3447]                            Jonsson, J. and B. Kaliski,
                                        "Public-Key Cryptography
                                        Standards (PKCS) #1: RSA
                                        Cryptography Specifications
                                        Version 2.1", RFC 3447,
                                        February 2003.

   [RFC5288]                            Salowey, J., Choudhury, A., and
                                        D. McGrew, "AES Galois Counter
                                        Mode (GCM) Cipher Suites for
                                        TLS", RFC 5288, August 2008.

   [SCH]                                Schneier, B., "Applied
                                        Cryptography: Protocols,
                                        Algorithms, and Source Code in
                                        C, 2nd ed.", 1996.

   [SHS]                                National Institute of Standards
                                        and Technology, U.S. Department
                                        of Commerce, "Secure Hash
                                        Standard", NIST FIPS PUB 180-2,
                                        August 2002.

   [TRIPLEDES]                          National Institute of Standards
                                        and Technology, "Recommendation
                                        for the Triple Data Encryption
                                        Algorithm (TDEA) Block Cipher",
                                        NIST Special Publication 800-67,
                                        May 2004.

   [X680]                               ITU-T, "Information technology -
                                        Abstract Syntax Notation One
                                        (ASN.1): Specification of basic
                                        notation", ISO/IEC 8824-1:2002,
                                        2002.

   [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 ISO/
                                        IEC 8825-1:2002, 2002.

13.2.  Informative References

   [BLEI]                               Bleichenbacher, D., "Chosen
                                        Ciphertext Attacks against
                                        Protocols Based on RSA
                                        Encryption Standard PKCS",
                                        CRYPTO98 LNCS vol. 1462, pages:
                                        1-12, 1998, Advances in
                                        Cryptology, 1998.

   [CBCATT]                             Moeller, B., "Security of CBC
                                        Ciphersuites in SSL/TLS:
                                        Problems and Countermeasures",
                                        May 2004,
                <http://www.openssl.org/~bodo/tls-cbc.txt>.

   [CBCTIME]    Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux,
                "Password Interception in a SSL/TLS Channel", CRYPTO
                2003 LNCS vol. 2729, 2003. <http://
                                        www.openssl.org/~bodo/
                                        tls-cbc.txt>.

   [CCM]                                "NIST Special Publication 800-38C: 800-
                                        38C: The CCM Mode for
                                        Authentication and
                                        Confidentiality", May 2004, <http://
                csrc.nist.gov/publications/nistpubs/800-38C/
                SP800-38C.pdf>. <htt
                                        p://csrc.nist.gov/publications/
                                        nistpubs/800-38C/SP800-38C.pdf>.

   [DES]                                "Data Encryption Standard
                                        (DES)", NIST FIPS PUB 46-3,
                                        October 1999.

   [DSS-3]                              National Institute of Standards
                                        and Technology, U.S., "Digital
                                        Signature Standard", NIST FIPS
                                        PUB 186-3 Draft, 2006.

   [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.

   [ENCAUTH]                            Krawczyk, H., "The Order of
                                        Encryption and Authentication
                                        for Protecting Communications
                                        (Or: How Secure is SSL?)", 2001.

   [FI06]                               "Bleichenbacher's RSA signature
                                        forgery based on implementation
                                        error", August 2006, <http://www.imc.org/
                ietf-openpgp/mail-archive/msg14307.html>. <http://
                                        www.imc.org/ietf-openpgp/
                                        mail-archive/msg14307.html>.

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

   [KPR03]      Klima, V., Pokorny, O., and T. Rosa, "Attacking RSA-
                based Sessions

   [I-D.gillmor-tls-negotiated-dl-dhe]  Gillmor, D., "Negotiated
                                        Discrete Log Diffie-Hellman
                                        Ephemeral Parameters for TLS", d
                                        raft-gillmor-tls-negotiated-dl-
                                        dhe-02 (work in SSL/TLS", March 2003,
                <http://eprint.iacr.org/2003/052/>. progress),
                                        April 2014.

   [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.

   [RFC0793]                            Postel, J., "Transmission
                                        Control Protocol", STD 7,
                                        RFC 793, September 1981.

   [RFC1948]                            Bellovin, S., "Defending Against
                                        Sequence Number Attacks",
                                        RFC 1948, May 1996.

   [RFC2246]                            Dierks, T. and C. Allen, "The
                                        TLS Protocol Version 1.0",
                                        RFC 2246, January 1999.

   [RFC2785]                            Zuccherato, R., "Methods for
                                        Avoiding the "Small-
                Subgroup" "Small-Subgroup"
                                        Attacks on the Diffie-Hellman
                                        Key Agreement Method for
                                        S/MIME", RFC 2785, March 2000.

   [RFC3268]                            Chown, P., "Advanced Encryption
                                        Standard (AES) Ciphersuites for
                                        Transport Layer Security (TLS)",
                                        RFC 3268, June 2002.

   [RFC3526]                            Kivinen, T. and M. Kojo, "More
                                        Modular Exponential (MODP)
                                        Diffie-Hellman groups for
                                        Internet Key Exchange (IKE)",
                                        RFC 3526, May 2003.

   [RFC3749]    Hollenbeck, S., "Transport Layer Security Protocol
                Compression Methods", RFC 3749, May 2004.

   [RFC3766]                            Orman, H. and P. Hoffman,
                                        "Determining Strengths For
                                        Public Keys Used For Exchanging
                                        Symmetric Keys", BCP 86,
                                        RFC 3766, April 2004.

   [RFC4086]                            Eastlake, D., Schiller, J., and
                                        S. Crocker, "Randomness
                                        Requirements for Security",
                                        BCP 106, RFC 4086, June 2005.

   [RFC4279]    Eronen, P. and H. Tschofenig, "Pre-Shared Key
                Ciphersuites for Transport Layer Security (TLS)",
                RFC 4279, December 2005.

   [RFC4302]                            Kent, S., "IP Authentication
                                        Header", RFC 4302,
                                        December 2005.

   [RFC4303]                            Kent, S., "IP Encapsulating
                                        Security Payload (ESP)",
                                        RFC 4303, December 2005.

   [RFC4307]                            Schiller, J., "Cryptographic
                                        Algorithms for Use in the
                                        Internet Key Exchange Version 2
                                        (IKEv2)", RFC 4307,
                                        December 2005.

   [RFC4346]                            Dierks, T. and E. Rescorla, "The
                                        Transport Layer Security (TLS)
                                        Protocol Version 1.1", RFC 4346,
                                        April 2006.

   [RFC4366]                            Blake-Wilson, S., Nystrom, M.,
                                        Hopwood, D., Mikkelsen, J., and
                                        T. Wright, "Transport Layer
                                        Security (TLS) Extensions",
                                        RFC 4366, April 2006.

   [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, May 2006.

   [RFC4506]                            Eisler, M., "XDR: External Data
                                        Representation Standard",
                                        STD 67, RFC 4506, May 2006.

   [RFC5081]                            Mavrogiannopoulos, N., "Using
                                        OpenPGP Keys for Transport Layer
                                        Security (TLS) Authentication",
                                        RFC 5081, November 2007.

   [RFC5116]                            McGrew, D., "An Interface and
                                        Algorithms for Authenticated
                                        Encryption", RFC 5116,
                                        January 2008.

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

   [SSL2]                               Netscape Communications Corp.,
                                        "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.

   [TLSEXT]                             Eastlake 3rd, D., "Transport
                                        Layer Security (TLS) Extensions:
                                        Extension Definitions",
                                        February 2008.

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

URIs

   [1]  <https://github.com/tlswg/tls13-spec/issues/32>

   [2]  <mailto:tls@ietf.org>

Appendix A.  Protocol Data Structures and Constant Values

   This section describes protocol types and constants.

   [[TODO: Clean this up to match the in-text description.]]

A.1.  Record Layer

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

   ProtocolVersion version = { 3, 3 4 };     /* TLS v1.2*/ v1.3*/

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

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

   struct {
       ContentType type;
       ProtocolVersion version;
       uint16 length;
       opaque fragment[TLSCompressed.length];
   } TLSCompressed;

   struct {
       ContentType type;
       ProtocolVersion version;
       uint16 length;
       select (SecurityParameters.cipher_type) {
           case stream: GenericStreamCipher;
           case block:  GenericBlockCipher;
           case aead:   GenericAEADCipher;
       } fragment;
   } TLSCiphertext;

   stream-ciphered struct {
       opaque content[TLSCompressed.length];
       opaque MAC[SecurityParameters.mac_length];
   } GenericStreamCipher;

   struct {
       opaque IV[SecurityParameters.record_iv_length];
       block-ciphered struct {
           opaque content[TLSCompressed.length];
           opaque MAC[SecurityParameters.mac_length];
           uint8 padding[GenericBlockCipher.padding_length];
           uint8 padding_length;
       };
   } GenericBlockCipher;

   struct {
      opaque nonce_explicit[SecurityParameters.record_iv_length];
       aead-ciphered struct {
           opaque content[TLSCompressed.length];
      }; content[TLSPlaintext.length];
       } fragment;
   } GenericAEADCipher; TLSCiphertext;

A.2.  Change Cipher Specs Message

   struct {
       enum { change_cipher_spec(1), (255) } type;
   } ChangeCipherSpec;

A.3.  Alert Messages

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

   enum {
       close_notify(0),
       unexpected_message(10),
       bad_record_mac(20),
       decryption_failed_RESERVED(21),
       record_overflow(22),
       decompression_failure(30),
       decompression_failure_RESERVED(30),
       handshake_failure(40),
       no_certificate_RESERVED(41),
       bad_certificate(42),
       unsupported_certificate(43),
       certificate_revoked(44),
       certificate_expired(45),
       certificate_unknown(46),
       illegal_parameter(47),
       unknown_ca(48),
       access_denied(49),
       decode_error(50),
       decrypt_error(51),
       export_restriction_RESERVED(60),
       protocol_version(70),
       insufficient_security(71),
       internal_error(80),
       user_canceled(90),
       no_renegotiation(100),
       unsupported_extension(110),           /* new */
       (255)
   } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;

A.4.  Handshake Protocol

   enum {
       hello_request(0), client_hello(1), server_hello(2),
       certificate(11), server_key_exchange (12),
       certificate_request(13), server_hello_done(14),
       certificate_verify(15), client_key_exchange(16),
       finished(20)
       finished(20),
       (255)
   } HandshakeType;

   struct {
       HandshakeType msg_type;
       uint24 length;
       select (HandshakeType) {
           case hello_request:       HelloRequest;
           case client_hello:        ClientHello;
           case server_hello:        ServerHello;
           case certificate:         Certificate;
           case server_key_exchange: ServerKeyExchange;
           case certificate_request: CertificateRequest;
           case server_hello_done:   ServerHelloDone;
           case certificate_verify:  CertificateVerify;
           case client_key_exchange: ClientKeyExchange;
           case finished:            Finished;
       } body;
   } Handshake;

A.4.1.  Hello Messages

   struct { } HelloRequest;

   struct {
       uint32 gmt_unix_time;
       opaque random_bytes[28];
   } Random;

   opaque SessionID<0..32>;

   uint8 CipherSuite[2];

   enum { null(0), (255) } CompressionMethod;

   struct {
       ProtocolVersion client_version;
       Random random;
       SessionID session_id;
       CipherSuite cipher_suites<2..2^16-2>;
       CompressionMethod compression_methods<1..2^8-1>;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ClientHello;

   struct {
       ProtocolVersion server_version;
       Random random;
       SessionID session_id;
       CipherSuite cipher_suite;
       CompressionMethod compression_method;
       select (extensions_present) {
           case false:
               struct {};
           case true:
               Extension extensions<0..2^16-1>;
       };
   } ServerHello;

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

   enum {
       signature_algorithms(13), (65535)
   } ExtensionType;

   enum{
       none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
       sha512(6), (255)
   } HashAlgorithm;
   enum {
      anonymous(0), rsa(1), dsa(2), ecdsa(3), (255)
   } SignatureAlgorithm;

   struct {
         HashAlgorithm hash;
         SignatureAlgorithm signature;
   } SignatureAndHashAlgorithm;

   SignatureAndHashAlgorithm
    supported_signature_algorithms<2..2^16-1>;
    supported_signature_algorithms<2..2^16-2>;

A.4.2.  Server Authentication and Key Exchange Messages
   opaque ASN.1Cert<2^24-1>; ASN1Cert<2^24-1>;

   struct {
       ASN.1Cert
       ASN1Cert certificate_list<0..2^24-1>;
   } Certificate;

   enum { dhe_dss, dhe_rsa, dh_anon, rsa,dh_dss, dh_rsa dh_anon
          /* may be extended, e.g., for ECDH -- see [TLSECC] */
        } KeyExchangeAlgorithm;

   struct {
       opaque dh_p<1..2^16-1>;
       opaque dh_g<1..2^16-1>;
       opaque dh_Ys<1..2^16-1>;
   } ServerDHParams;     /* Ephemeral DH parameters */

   struct {
       select (KeyExchangeAlgorithm) {
           case dh_anon:
               ServerDHParams params;
           case dhe_dss:
           case dhe_rsa:
               ServerDHParams params;
               digitally-signed struct {
                   opaque client_random[32];
                   opaque server_random[32];
                   ServerDHParams params;
               } signed_params;
           case rsa:
           case dh_dss:
           case dh_rsa:
               struct {} ;
              /* message is omitted for rsa, dh_dss, and dh_rsa */
           /* may be extended, e.g., for ECDH --- see [RFC4492] */
   } ServerKeyExchange;

   enum {
       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
       rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
       fortezza_dms_RESERVED(20),
       (255)
   } ClientCertificateType;

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

   struct {
       ClientCertificateType certificate_types<1..2^8-1>;
       DistinguishedName certificate_authorities<0..2^16-1>;
   } CertificateRequest;

   struct { } ServerHelloDone;

A.4.3.  Client Authentication and Key Exchange Messages

   struct {
       select (KeyExchangeAlgorithm) {
           case rsa:
               EncryptedPreMasterSecret;
           case dhe_dss:
           case dhe_rsa:
           case dh_dss:
           case dh_rsa:
           case dh_anon:
               ClientDiffieHellmanPublic;
       } exchange_keys;
   } ClientKeyExchange;

   struct {
       ProtocolVersion client_version;
       opaque random[46];
   } PreMasterSecret;

   struct {
       public-key-encrypted PreMasterSecret pre_master_secret;
   } EncryptedPreMasterSecret;

   enum { implicit, explicit } PublicValueEncoding;

   struct {
       select (PublicValueEncoding) {
           case implicit: struct {};
           case explicit: opaque DH_Yc<1..2^16-1>;
       } dh_public;
   } ClientDiffieHellmanPublic;

   struct {
        digitally-signed struct {
            opaque handshake_messages[handshake_messages_length];
        }
   } CertificateVerify;

A.4.4.  Handshake Finalization Message

   struct {
       opaque verify_data[verify_data_length];
   } Finished;

A.5.  The Cipher Suite

   The following values define the cipher suite codes used in the
   ClientHello and ServerHello messages.

   A cipher suite defines a cipher specification supported in TLS
   Version 1.2.

   TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
   TLS connection during the first handshake on that channel, but MUST
   NOT be negotiated, as it provides no more protection than an
   unsecured connection.

      CipherSuite TLS_NULL_WITH_NULL_NULL             = { 0x00,0x00 };

   The following CipherSuite definitions require that the server provide
   an RSA certificate that can be used for key exchange.  The server may
   request any signature-capable certificate in the certificate request
   message.

      CipherSuite TLS_RSA_WITH_NULL_MD5                 = { 0x00,0x01 };
      CipherSuite TLS_RSA_WITH_NULL_SHA                 = { 0x00,0x02 };
      CipherSuite TLS_RSA_WITH_NULL_SHA256              = { 0x00,0x3B };
      CipherSuite TLS_RSA_WITH_RC4_128_MD5              = { 0x00,0x04 };
      CipherSuite TLS_RSA_WITH_RC4_128_SHA              = { 0x00,0x05 };
      CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA         = { 0x00,0x0A };
      CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA          = { 0x00,0x2F };
      CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA          = { 0x00,0x35 };
      CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA256       = { 0x00,0x3C };
      CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA256       = { 0x00,0x3D };

   The following cipher suite definitions definitions, defined in {{RFC5288}, are
   used for server-
   authenticated server-authenticated (and optionally client-authenticated)
   Diffie-Hellman.  DH denotes cipher suites in which the server's
   certificate contains the Diffie-Hellman parameters signed by the
   certificate authority (CA).  DHE denotes ephemeral Diffie-Hellman,
   where the Diffie-Hellman parameters are signed by a signature-capable
   certificate, which has been signed by the CA.  The signing algorithm
   used by the server is specified after the DHE component of the
   CipherSuite name.  The server can request any signature-capable
   certificate from the client for client authentication, or it may
   request a Diffie-Hellman certificate.  Any Diffie-Hellman certificate
   provided by the client must use the parameters (group and generator)
   described by the server.

     CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA TLS_RSA_WITH_AES_128_GCM_SHA256 = { 0x00,0x0D };
      CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x10 };
      CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA     = { 0x00,0x13 };
      CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA     = { 0x00,0x16 };
      CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA       = { 0x00,0x30 };
      CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA       = { 0x00,0x31 };
      CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA      = { 0x00,0x32 };
      CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA      = { 0x00,0x33 };
      CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA       = { 0x00,0x36 };
      CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA       = { 0x00,0x37 };
      CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA      = { 0x00,0x38 };
      CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA      = { 0x00,0x39 };
      CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA256    = { 0x00,0x3E };
      CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA256    = { 0x00,0x3F }; {0x00,0x9C}
     CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 TLS_RSA_WITH_AES_256_GCM_SHA384 = { 0x00,0x40 }; {0x00,0x9D}
     CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = { 0x00,0x67 }; {0x00,0x9E}
     CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA256 TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = { 0x00,0x68 }; {0x00,0x9F}
     CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA256 TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = { 0x00,0x69 }; {0x00,0xA2}
     CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = { 0x00,0x6A };
      CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA256   = { 0x00,0x6B }; {0x00,0xA3}

   The following cipher suites suites, defined in {{RFC5288}, are used for
   completely anonymous Diffie-
   Hellman Diffie-Hellman communications in which neither
   party is authenticated.  Note that this mode is vulnerable to man-in-the- middle man-in-
   the-middle attacks.  Using this mode therefore is of limited use:
   These cipher suites MUST NOT be used by TLS 1.2 implementations
   unless the application layer has specifically requested to allow
   anonymous key exchange.  (Anonymous key exchange may sometimes be
   acceptable, for example, to support opportunistic encryption when no
   set-up for authentication is in place, or when TLS is used as part of
   more complex security protocols that have other means to ensure
   authentication.)

     CipherSuite TLS_DH_anon_WITH_RC4_128_MD5          = { 0x00,0x18 };
      CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA     = { 0x00,0x1B };
      CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA      = { 0x00,0x34 };
      CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA      = { 0x00,0x3A };
      CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA256 TLS_DH_anon_WITH_AES_128_GCM_SHA256 = { 0x00,0x6C }; {0x00,0xA6}
     CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA256 TLS_DH_anon_WITH_AES_256_GCM_SHA384 = { 0x00,0x6D }; {0x00,0xA7}

   [[TODO: Add all the defined AEAD ciphers.  This currently only lists
   GCM. https://github.com/tlswg/tls13-spec/issues/53]] Note that using
   non-anonymous key exchange without actually verifying the key
   exchange is essentially equivalent to anonymous key exchange, and the
   same precautions apply.  While non-anonymous key exchange will
   generally involve a higher computational and communicational cost
   than anonymous key exchange, it may be in the interest of
   interoperability not to disable non-anonymous key exchange when the
   application layer is allowing anonymous key exchange.

   New cipher suite values have been assigned by IANA as described in
   Section 12.

   Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
   reserved to avoid collision with Fortezza-based cipher suites in SSL
   3.

A.6.  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 { null(0), (255) } CompressionMethod;

   enum { server, client } ConnectionEnd;

   enum { tls_prf_sha256 } PRFAlgorithm;

   enum { null, rc4, 3des, aes aes_gcm } BulkCipherAlgorithm;

   enum { stream, block, aead } CipherType;

   enum { null, hmac_md5, hmac_sha1, hmac_sha256, hmac_sha384,
     hmac_sha512} MACAlgorithm; RecordProtAlgorithm;

   /* Other values may be added to the algorithms specified in
   CompressionMethod, PRFAlgorithm, BulkCipherAlgorithm,
   PRFAlgorithm and
   MACAlgorithm. RecordProtAlgorithm */

   struct {
       ConnectionEnd          entity;
       PRFAlgorithm           prf_algorithm;
       BulkCipherAlgorithm    bulk_cipher_algorithm;
       CipherType             cipher_type;
       RecordProtAlgorithm    record_prot_algorithm;
       uint8                  enc_key_length;
       uint8                  block_length;
       uint8                  fixed_iv_length;
       uint8                  record_iv_length;
       MACAlgorithm           mac_algorithm;
       uint8                  mac_length;
       uint8                  mac_key_length;
       CompressionMethod      compression_algorithm;
       opaque                 master_secret[48];
       opaque                 client_random[32];
       opaque                 server_random[32];
     } SecurityParameters;

A.7.  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 a "signature_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 [RFC3280].

   As described in Section 7.4.2 7.4.5 and Section 7.4.6, 7.4.9, the restrictions on
   the signature algorithms used to sign certificates are no longer tied
   to the cipher suite (when used by the server) or the
   ClientCertificateType (when used by the client).  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.  Glossary

   Advanced Encryption Standard (AES)
      AES [AES] is a widely used symmetric encryption algorithm.  AES is
      a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte
      block size.  TLS currently only supports the 128- and 256-bit key
      sizes.

   application protocol
      An application protocol is a protocol that normally layers
      directly on top of the transport layer (e.g., TCP/IP).  Examples
      include HTTP, TELNET, FTP, and SMTP.

   asymmetric cipher
      See public key cryptography.

   authenticated encryption with additional data (AEAD)
      A symmetric encryption algorithm that simultaneously provides
      confidentiality and message integrity.

   authentication
      Authentication is the ability of one entity to determine the
      identity of another entity.

   block cipher
      A block cipher is an algorithm that operates on plaintext in
      groups of bits, called blocks. 64 bits was, and 128 bits is, a
      common block size.

   bulk cipher
      A symmetric encryption algorithm used to encrypt large quantities
      of data.

   cipher block chaining (CBC)
      CBC is a mode in which every plaintext block encrypted with a
      block cipher is first exclusive-ORed with the previous ciphertext
      block (or, in the case of the first block, with the initialization
      vector).  For decryption, every block is first decrypted, then
      exclusive-ORed with the previous ciphertext block (or IV).

   certificate
      As part of the X.509 protocol (a.k.a.  ISO Authentication
      framework), certificates are assigned by a trusted Certificate
      Authority and provide a strong binding between a party's identity
      or some other attributes and its public key.

   client
      The application entity that initiates a TLS connection to a
      server.  This may or may not imply that the client initiated the
      underlying transport connection.  The primary operational
      difference between the server and client is that the server is
      generally authenticated, while the client is only optionally
      authenticated.

   client write key
      The key used to encrypt data written by the client.

   client write MAC key
      The secret data used to authenticate protect data written by the client.

   connection
      A connection is a transport (in the OSI layering model definition)
      that provides a suitable type of service.  For TLS, such
      connections are peer-to-peer relationships.  The connections are
      transient.  Every connection is associated with one session.

   Data Encryption Standard
      DES [DES] still is a very widely used symmetric encryption
      algorithm although it is considered as rather weak now.  DES is a
      block cipher with a 56-bit key and an 8-byte block size.  Note
      that in TLS, for key generation purposes, DES is treated as having
      an 8-byte key length (64 bits), but it still only provides 56 bits
      of protection.  (The low bit of each key byte is presumed to be
      set to produce odd parity in that key byte.)  DES can also be
      operated in a mode [TRIPLEDES] where three independent keys and
      three encryptions are used for each block of data; this uses 168
      bits of key (24 bytes in the TLS key generation method) and
      provides the equivalent of 112 bits of security.

   Digital Signature Standard (DSS)
      A standard for digital signing, including the Digital Signing
      Algorithm, approved by the National Institute of Standards and
      Technology, defined in NIST FIPS PUB 186-2, "Digital Signature
      Standard", published January 2000 by the U.S. Department of
      Commerce [DSS].  A significant update [DSS-3] has been drafted and
      was published in March 2006.

   digital signatures
      Digital signatures utilize public key cryptography and one-way
      hash functions to produce a signature of the data that can be
      authenticated, and is difficult to forge or repudiate.

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

   Initialization Vector (IV)
      When a block cipher is used in CBC mode, the
      Some AEAD ciphers require an initialization vector
      is exclusive-ORed with to allow the first plaintext block prior
      cipher to
      encryption. safely protect multiple chunks of data with the same
      keying material.  The size of the IV depends on the cipher suite.

   Message Authentication Code (MAC)
      A Message Authentication Code is a one-way hash computed from a
      message and some secret data.  It is difficult to forge without
      knowing the secret data.  Its purpose is to detect if the message
      has been altered.

   master secret
      Secure secret data used for generating encryption keys, MAC
      secrets, keys and IVs.

   MD5
      MD5 [RFC1321] is a hashing function that converts an arbitrarily
      long data stream into a hash of fixed size (16 bytes).  Due to
      significant progress in cryptanalysis, at the time of publication
      of this document, MD5 no longer can be considered a 'secure'
      hashing function.

   public key cryptography
      A class of cryptographic techniques employing two-key ciphers.
      Messages encrypted with the public key can only be decrypted with
      the associated private key.  Conversely, messages signed with the
      private key can be verified with the public key.

   one-way hash function
      A one-way transformation that converts an arbitrary amount of data
      into a fixed-length hash.  It is computationally hard to reverse
      the transformation or to find collisions.  MD5 and SHA are
      examples of one-way hash functions.

   RC4
      A stream cipher invented by Ron Rivest.  A compatible cipher is
      described in [SCH].

   RSA
      A very widely used public key algorithm that can be used for
      either encryption or digital signing.  [RSA]

   server
      The server is the application entity that responds to requests for
      connections from clients.  See also "client".

   session
      A TLS session is an association between a client and a server.
      Sessions are created by the handshake protocol.  Sessions define a
      set of cryptographic security parameters that can be shared among
      multiple connections.  Sessions are used to avoid the expensive
      negotiation of new security parameters for each connection.

   session identifier
      A session identifier is a value generated by a server that
      identifies a particular session.

   server write key
      The key used to encrypt data written by the server.

   server write MAC key
      The secret data used to authenticate protect data written by the server.

   SHA
      The Secure Hash Algorithm [SHS] is defined in FIPS PUB 180-2.  It
      produces a 20-byte output.  Note that all references to SHA
      (without a numerical suffix) actually use the modified SHA-1
      algorithm.

   SHA-256
      The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2.
      It produces a 32-byte output.

   SSL
      Netscape's Secure Socket Layer protocol [SSL3].  TLS is based on
      SSL Version 3.0.

   stream cipher
      An encryption algorithm that converts a key into a
      cryptographically strong keystream, which is then exclusive-ORed
      with the plaintext.

   symmetric cipher
      See bulk cipher.

   Transport Layer Security (TLS)
      This protocol; also, the Transport Layer Security working group of
      the Internet Engineering Task Force (IETF).  See "Working Group
      Information" at the end of this document (see page 99).

Appendix C.  Cipher Suite Definitions

   Cipher Suite                          Key        Cipher         Mac        Record
                                         Exchange   Protection   PRF

   TLS_NULL_WITH_NULL_NULL               NULL         NULL         NULL
TLS_RSA_WITH_NULL_MD5                   RSA          NULL         MD5
TLS_RSA_WITH_NULL_SHA                   RSA          NULL         SHA
TLS_RSA_WITH_NULL_SHA256                RSA          NULL         SHA256
TLS_RSA_WITH_RC4_128_MD5                RSA          RC4_128      MD5
TLS_RSA_WITH_RC4_128_SHA                RSA          RC4_128      SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA           RSA          3DES_EDE_CBC SHA
TLS_RSA_WITH_AES_128_CBC_SHA            RSA          AES_128_CBC  SHA
TLS_RSA_WITH_AES_256_CBC_SHA            RSA          AES_256_CBC  SHA
TLS_RSA_WITH_AES_128_CBC_SHA256         RSA          AES_128_CBC  SHA256
TLS_RSA_WITH_AES_256_CBC_SHA256         RSA          AES_256_CBC  SHA256
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA        DH_DSS       3DES_EDE_CBC SHA
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA        DH_RSA       3DES_EDE_CBC SHA
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA       DHE_DSS      3DES_EDE_CBC SHA
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA       DHE_RSA      3DES_EDE_CBC SHA
TLS_DH_anon_WITH_RC4_128_MD5            DH_anon      RC4_128      MD5
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA       DH_anon      3DES_EDE_CBC SHA
TLS_DH_DSS_WITH_AES_128_CBC_SHA         DH_DSS       AES_128_CBC  SHA
TLS_DH_RSA_WITH_AES_128_CBC_SHA         DH_RSA       AES_128_CBC  SHA
TLS_DHE_DSS_WITH_AES_128_CBC_SHA        DHE_DSS      AES_128_CBC  SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA        DHE_RSA      AES_128_CBC  SHA
TLS_DH_anon_WITH_AES_128_CBC_SHA        DH_anon      AES_128_CBC  SHA
TLS_DH_DSS_WITH_AES_256_CBC_SHA         DH_DSS       AES_256_CBC  SHA
TLS_DH_RSA_WITH_AES_256_CBC_SHA         DH_RSA       AES_256_CBC  SHA
TLS_DHE_DSS_WITH_AES_256_CBC_SHA        DHE_DSS      AES_256_CBC  SHA
TLS_DHE_RSA_WITH_AES_256_CBC_SHA       NULL_NULL    N/A
   TLS_DHE_RSA_WITH_AES_128_GCM_SHA256   DHE_RSA      AES_256_CBC  SHA
TLS_DH_anon_WITH_AES_256_CBC_SHA        DH_anon      AES_256_CBC  SHA
TLS_DH_DSS_WITH_AES_128_CBC_SHA256      DH_DSS       AES_128_CBC  SHA256
TLS_DH_RSA_WITH_AES_128_CBC_SHA256      DH_RSA       AES_128_CBC    AES_128_GCM  SHA256
TLS_DHE_DSS_WITH_AES_128_CBC_SHA256     DHE_DSS      AES_128_CBC  SHA256
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
   TLS_DHE_RSA_WITH_AES_256_GCM_SHA384   DHE_RSA      AES_128_CBC  SHA256
TLS_DH_anon_WITH_AES_128_CBC_SHA256     DH_anon      AES_128_CBC  SHA256
TLS_DH_DSS_WITH_AES_256_CBC_SHA256      DH_DSS       AES_256_CBC  SHA256
TLS_DH_RSA_WITH_AES_256_CBC_SHA256      DH_RSA       AES_256_CBC  SHA256
TLS_DHE_DSS_WITH_AES_256_CBC_SHA256    AES_256_GCM  SHA384
   TLS_DHE_DSS_WITH_AES_128_GCM_SHA256   DHE_DSS      AES_256_CBC  SHA256
TLS_DHE_RSA_WITH_AES_256_CBC_SHA256     DHE_RSA      AES_256_CBC  SHA256
TLS_DH_anon_WITH_AES_256_CBC_SHA256     DH_anon      AES_256_CBC  SHA256

                       Key      IV   Block
Cipher        Type    Material  Size  Size
------------  ------  --------  ----  -----
NULL          Stream      0       0    N/A
RC4_128       Stream     16       0    N/A
3DES_EDE_CBC  Block      24       8      8
AES_128_CBC   Block      16      16     16
AES_256_CBC   Block      32      16     16

MAC       Algorithm    mac_length  mac_key_length
--------  -----------  ----------  --------------
NULL      N/A              0             0
MD5       HMAC-MD5        16            16
SHA       HMAC-SHA1       20            20
SHA256    HMAC-SHA256     32            32

   Type
      Indicates whether this is a stream cipher or a block cipher
      running in CBC mode.

   Key Material
      The number of bytes from the key_block that are used for
      generating the write keys.

   IV Size
      The amount of data needed to be generated for the initialization
      vector.  Zero for stream ciphers; equal to the block size for
      block ciphers (this is equal to
      SecurityParameters.record_iv_length).

   Block Size
      The amount of data a block cipher enciphers in one chunk; a block
      cipher running in CBC mode can only encrypt an even multiple of
      its block size.

Appendix D.  Implementation Notes

   The TLS protocol cannot prevent many common security mistakes.  This
   section provides several recommendations to assist implementors.

D.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-1, 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's 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.

D.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.

D.3.  Cipher Suites

   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.  For instance, anonymous
   Diffie-Hellman is strongly discouraged because it cannot prevent man-
   in-the-middle attacks.  Applications should also enforce minimum and
   maximum key sizes.  For example, certificate chains containing 512-
   bit RSA keys or signatures are not appropriate for high-security
   applications.

D.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 6.2.1)?  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 before ServerHello (see Appendix E.1)?

   -  Do you handle TLS extensions in ClientHello correctly, including
      omitting the extensions field completely?

   -  Do you support renegotiation, both client and server initiated?
      While renegotiation is an optional feature, supporting it is
      highly recommended.

   -  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 7.4.6)?

   Cryptographic details:

   -  In the RSA-encrypted Premaster Secret, do you correctly send and
      verify the version number?  When an error is encountered, do you
      continue the handshake to avoid the Bleichenbacher attack (see
      Section 7.4.7.1)?

   -  What countermeasures do you use to prevent timing attacks against
      RSA decryption and signing operations (see Section 7.4.7.1)?

   -  When verifying RSA signatures, do you accept both NULL and missing
      parameters (see Section 4.7)?  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
      leading zero bytes from the negotiated key (see Section 8.1.2)?

   -  Does your TLS client check that the Diffie-Hellman parameters sent
      by the server are acceptable (see Appendix F.1.1.3)?

   -  How do you generate unpredictable IVs for CBC mode ciphers (see
      Section 6.2.3.2)?

   -  Do you accept long CBC mode padding (up to 255 bytes; see
      Section 6.2.3.2?

   -  How do you address CBC mode timing attacks (Section 6.2.3.2)?

   -  Do you use a strong and, most importantly, properly seeded random
      number generator (see Appendix D.1) for generating the premaster
      secret (for RSA key exchange), Diffie-Hellman private values, the
      DSA "k" parameter, and other security-critical values?

Appendix E.  Backward Compatibility

E.1.  Compatibility with TLS 1.0/1.1 and SSL 3.0

   Since there are various versions of TLS (1.0, 1.1, 1.2, and any
   future versions) and SSL (2.0 and 3.0), means are needed to negotiate
   the specific protocol version to use.  The TLS protocol provides a
   built-in mechanism for version negotiation so as not to bother other
   protocol components with the complexities of version selection.

   TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
   compatible ClientHello messages; thus, supporting all of them is
   relatively easy.  Similarly, servers can easily 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.

   A TLS 1.2 client who wishes to negotiate with such older servers will
   send a normal TLS 1.2 ClientHello, containing { 3, 3 } (TLS 1.2) 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.

   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.

   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 server supports (or is
   willing to use) only versions greater than client_version, it MUST
   send a "protocol_version" alert message and close the connection.

   Whenever a client already knows the highest protocol version known to
   a server (for example, when resuming a session), it SHOULD initiate
   the connection in that native protocol.

   Note: some server implementations are known to implement version
   negotiation incorrectly.  For example, there are buggy TLS 1.0
   servers that simply close the connection when the client offers a
   version newer than TLS 1.0.  Also, it is known that some servers will
   refuse the connection if any TLS extensions are included in
   ClientHello.  Interoperability with such buggy servers is a complex
   topic beyond the scope of this document, and may require multiple
   connection attempts by the client.

   Earlier versions of the TLS specification were not fully clear on
   what the record layer version number (TLSPlaintext.version) should
   contain when sending ClientHello (i.e., before it is known which
   version of the protocol will be employed).  Thus, TLS servers
   compliant with this specification MUST accept any value {03,XX} as
   the record layer version number for ClientHello.

   TLS clients that wish to negotiate with older servers MAY send any
   value {03,XX} as the record layer version number.  Typical values
   would be {03,00}, the lowest version number supported by the client,
   and the value of ClientHello.client_version.  No single value will
   guarantee interoperability with all old servers, but this is a
   complex topic beyond the scope of this document.

E.2.  Compatibility with SSL 2.0

   TLS 1.2 clients that wish to support SSL 2.0 servers MUST send
   version 2.0 CLIENT-HELLO messages defined in [SSL2].  The message
   MUST contain the same version number as would be used for ordinary
   ClientHello, and MUST encode the supported TLS cipher suites in the
   CIPHER-SPECS-DATA field as described below.

   Warning: The ability to send version 2.0 CLIENT-HELLO messages will
   be phased out with all due haste, since the newer ClientHello format
   provides better mechanisms for moving to newer versions and
   negotiating extensions.  TLS 1.2 clients SHOULD NOT support SSL 2.0.

   However, even TLS servers that do not support SSL 2.0 MAY accept
   version 2.0 CLIENT-HELLO messages.    AES_128_GCM  SHA256
   TLS_DHE_DSS_WITH_AES_256_GCM_SHA384   DHE_DSS    AES_256_GCM  SHA384
   TLS_DH_anon_WITH_AES_128_GCM_SHA256   DH_anon    AES_128_GCM  SHA256
   TLS_DH_anon_WITH_AES_256_GCM_SHA384   DH_anon    AES_128_GCM  SHA384

                   Key      Implicit IV   Explicit IV
   Cipher         Material  Size          Size
   ------------   --------  ----------    -----------
   NULL               0          0             0
   AES_128_GCM       16          4             8
   AES_256_GCM       32          4             8

   Key Material
      The message is presented below in
   sufficient detail number of bytes from the key_block that are used for TLS server implementors;
      generating the true definition is
   still assumed write keys.

   Implicit IV Size
      The amount of data to be [SSL2].

   For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same
   way as a ClientHello with a "null" compression method and no
   extensions.  Note that this message MUST be sent directly on the
   wire, not wrapped as a TLS record.  For generated for the purposes per-connection part of calculating
   Finished and CertificateVerify,
      the msg_length field initialization vector.  This is not
   considered equal to
      SecurityParameters.fixed_iv_length).

   Explicit IV Size
      The amount of data needed to be a generated for the per-record part
      of the handshake message.

      uint8 V2CipherSpec[3];
      struct {
          uint16 msg_length;
          uint8 msg_type;
          Version version;
          uint16 cipher_spec_length;
          uint16 session_id_length;
          uint16 challenge_length;
          V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
          opaque session_id[V2ClientHello.session_id_length];
          opaque challenge[V2ClientHello.challenge_length;
      } V2ClientHello;

   msg_length initialization vector.  This is equal to
      SecurityParameters.record_iv_length).

Appendix D.  Implementation Notes

   The highest bit MUST TLS protocol cannot prevent many common security mistakes.  This
   section provides several recommendations to assist implementors.

D.1.  Random Number Generation and Seeding

   TLS requires a cryptographically secure pseudorandom number generator
   (PRNG).  Care must be 1; taken in designing and seeding PRNGs.  PRNGs
   based on secure hash operations, most notably SHA-1, are acceptable,
   but cannot provide more security than the remaining 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's 18.2 Hz
   timer provide 1 or 2 secure bits contain each, even though the length total size of
   the following data in bytes.

   msg_type
      This field, in conjunction with the version field, identifies a
      version 2 ClientHello message.  The counter value MUST be 1.

   version
      Equal to ClientHello.client_version.

   cipher_spec_length
      This field is 16 bits or more.  Seeding a 128-bit PRNG would
   thus require approximately 100 such timer values.

   [RFC4086] provides guidance on the total length generation of random values.

D.2.  Certificates and Authentication

   Implementations are responsible for verifying the field cipher_specs.  It
      cannot be zero integrity of
   certificates and MUST should generally support certificate revocation
   messages.  Certificates should always be verified to ensure proper
   signing by a multiple 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 V2CipherSpec length
      (3).

   session_id_length
      This field MUST have certificate and root CA.

D.3.  Cipher Suites

   TLS supports a value range of zero for a client key sizes and security levels, including some
   that claims to provide no or minimal security.  A proper implementation will
   probably not support TLS 1.2.

   challenge_length
      The length in bytes of the client's challenge to the server to
      authenticate itself.  Historically, permissible values many cipher suites.  For instance, anonymous
   Diffie-Hellman is strongly discouraged because it cannot prevent man-
   in-the-middle attacks.  Applications should also enforce minimum and
   maximum key sizes.  For example, certificate chains containing 512-
   bit RSA keys or signatures are not appropriate for high-security
   applications.

D.4.  Implementation Pitfalls

   Implementation experience has shown that certain parts of earlier TLS
   specifications are between
      16 not easy to understand, and 32 bytes inclusive.  When using the SSLv2 backward-
      compatible handshake the client SHOULD use a 32-byte challenge.

   cipher_specs
      This is have been a list source of all CipherSpecs the client is willing
   interoperability and able
      to use.  In addition to the 2.0 cipher specs defined security problems.  Many of these areas have
   been clarified in [SSL2], this includes the TLS cipher suites normally sent in
      ClientHello.cipher_suites, with each cipher suite prefixed by document, but this appendix contains a
      zero byte.  For example, short
   list of the most important things that require special attention from
   implementors.

   TLS cipher suite {0x00,0x0A} would be
      sent as {0x00,0x00,0x0A}.

   session_id
      This field MUST be empty.

   challenge
      Corresponds protocol issues:

   -  Do you correctly handle handshake messages that are fragmented to ClientHello.random.  If the challenge length
      multiple TLS records (see Section 6.2.1)?  Including corner cases
      like a ClientHello that is
      less than 32, split to several small fragments?  Do
      you fragment handshake messages that exceed the TLS server will pad maximum fragment
      size?  In particular, the data with leading (note:
      not trailing) zero bytes to make it 32 bytes long.

   Note: Requests certificate and certificate request
      handshake messages can be large enough to resume a require fragmentation.

   -  Do you ignore the TLS session MUST use a record layer version number in all TLS client hello.

E.3.  Avoiding Man-in-the-Middle Version Rollback

   When
      records before ServerHello (see Appendix E.1)?

   -  Do you handle TLS clients fall back to Version 2.0 compatibility mode, they
   MUST use special PKCS#1 block formatting.  This extensions in ClientHello correctly, including
      omitting the extensions field completely?

   -  Do you support renegotiation, both client and server initiated?
      While renegotiation is done so that TLS
   servers will reject Version 2.0 sessions with TLS-capable clients. an optional feature, supporting it is
      highly recommended.

   -  When the server has requested a client negotiates SSL 2.0 certificate, but also supports TLS, it MUST set
   the right-hand (least-significant) 8 random bytes no
      suitable certificate is available, do you correctly send an empty
      Certificate message, instead of omitting the PKCS padding
   (not including whole message (see
      Section 7.4.9)?

   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 4.7)?  Do you verify that the terminal null of RSA padding
      doesn't have additional data after the padding) for hash value?  [FI06]

   -  When using Diffie-Hellman key exchange, do you correctly strip
      leading zero bytes from the RSA
   encryption of negotiated key (see Section 8.1.1)?

   -  Does your TLS client check that the ENCRYPTED-KEY-DATA field of Diffie-Hellman parameters sent
      by the CLIENT-MASTER-KEY
   to 0x03 (the other padding bytes server are random).

   When acceptable (see Appendix F.1.1.2)?

   -  Do you use a TLS-capable server negotiates SSL 2.0 it SHOULD, after
   decrypting strong and, most importantly, properly seeded random
      number generator (see Appendix D.1) Diffie-Hellman private values,
      the ENCRYPTED-KEY-DATA field, check that these 8 padding
   bytes DSA "k" parameter, and other security-critical values?

Appendix E.  Backward Compatibility

E.1.  Compatibility with TLS 1.0/1.1 and SSL 3.0

   [[TODO: Revise backward compatibility section for TLS 1.3.
   https://github.com/tlswg/tls13-spec/issues/54]] Since there are 0x03.  If they
   various versions of TLS (1.0, 1.1, 1.2, and any future versions) and
   SSL (2.0 and 3.0), means are not, needed to negotiate the server SHOULD generate specific
   protocol version to use.  The TLS protocol provides a random
   value built-in
   mechanism for SECRET-KEY-DATA, and continue the handshake (which will
   eventually fail since the keys will version negotiation so as not match).  Note that reporting to bother other protocol
   components with the error situation complexities of version selection.

   TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
   compatible ClientHello messages; thus, supporting all of them is
   relatively easy.  Similarly, servers can easily handle clients trying
   to use future versions of TLS as long as the ClientHello format
   remains compatible, and the client could make supports the server vulnerable to
   attacks described highest protocol
   version available in [BLEI].

Appendix F.  Security Analysis

   The the server.

   A TLS protocol is designed to establish a secure connection between
   a 1.3 client and 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 communicating over does not support this
   version, it will respond with a ServerHello containing an insecure channel.  This
   document makes several traditional assumptions, including that
   attackers have substantial computational resources and cannot obtain
   secret information from sources outside older
   version number.  If the protocol.  Attackers are
   assumed client agrees to have use this version, the ability to capture, modify, delete, replay, and
   otherwise tamper with messages sent over
   negotiation will proceed as appropriate for the communication channel.
   This appendix outlines how TLS has been designed to resist a variety
   of attacks.

F.1.  Handshake Protocol

   The handshake protocol negotiated protocol.

   If the version chosen by the server is responsible for selecting not supported by the client
   (or not acceptable), the client MUST send a cipher spec "protocol_version" alert
   message and
   generating a master secret, which together comprise close the primary
   cryptographic parameters associated with connection.

   If a secure session.  The
   handshake protocol 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.

   A TLS server can also optionally authenticate parties who have
   certificates signed by receive a trusted certificate authority.

F.1.1.  Authentication and Key Exchange

   TLS supports three authentication modes: authentication of both
   parties, ClientHello containing a version
   number smaller than the highest supported version.  If the server authentication
   wishes to negotiate with an unauthenticated client, and
   total anonymity.  Whenever old clients, it will proceed as appropriate
   for the highest version supported by the server that is authenticated, not greater
   than ClientHello.client_version.  For example, if the channel server supports
   TLS 1.0, 1.1, and 1.2, and client_version is secure against man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If TLS 1.0, the server is authenticated,
   its certificate message must provide will
   proceed with 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 TLS 1.0 ServerHello.  If server supports (or is valid and has not expired or been revoked.

   The general goal of
   willing to use) only versions greater than client_version, it MUST
   send a "protocol_version" alert message and close the key exchange process is to create connection.

   Whenever a
   pre_master_secret client already knows the highest protocol version known to
   a server (for example, when resuming a session), it SHOULD initiate
   the communicating parties and not to
   attackers.  The pre_master_secret will be used connection in that native protocol.

   Note: some server implementations are known to generate implement version
   negotiation incorrectly.  For example, there are buggy TLS 1.0
   servers that simply close the
   master_secret (see Section 8.1).  The master_secret is required to
   generate connection when the Finished messages, encryption keys, and MAC keys (see
   Section 7.4.9 and Section 6.3).  By sending client offers a correct Finished
   message, parties thus prove
   version newer than TLS 1.0.  Also, it is known that they know some servers will
   refuse the correct
   pre_master_secret.

F.1.1.1.  Anonymous Key Exchange

   Completely anonymous sessions can be established using Diffie-Hellman
   for key exchange.  The server's public parameters connection if any TLS extensions are contained included in
   ClientHello.  Interoperability with such buggy servers is a complex
   topic beyond the server key exchange message, scope of this document, and may require multiple
   connection attempts by the client's are sent in client.

   Earlier versions of the
   client key exchange message.  Eavesdroppers who do TLS specification were not know fully clear on
   what the
   private record layer version number (TLSPlaintext.version) should
   contain when sending ClientHello (i.e., before it is known which
   version of the protocol will be employed).  Thus, TLS servers
   compliant with this specification MUST accept any value {03,XX} as
   the record layer version number for ClientHello.

   TLS clients that wish to negotiate with older servers MAY send any
   value {03,XX} as the record layer version number.  Typical values should not
   would be able to find {03,00}, the Diffie-Hellman result
   (i.e., lowest version number supported by the pre_master_secret).

   Warning: Completely anonymous connections only provide protection
   against passive eavesdropping.  Unless an independent tamper-proof
   channel is used to verify that client,
   and the Finished messages were not
   replaced by an attacker, server authentication value of ClientHello.client_version.  No single value will
   guarantee interoperability with all old servers, but this is required in
   environments where active man-in-the-middle attacks are a concern.

F.1.1.2.  RSA Key Exchange and Authentication

   With RSA, key exchange and server authentication are combined.  The
   public key is contained in
   complex topic beyond the server's certificate.  Note that
   compromise scope of the server's static RSA key results this document.

E.2.  Compatibility with SSL 2.0

   TLS 1.2 clients that wish to support SSL 2.0 servers MUST send
   version 2.0 CLIENT-HELLO messages defined in a loss of
   confidentiality [SSL2].  The message
   MUST contain the same version number as would be used for all sessions protected under that static key. ordinary
   ClientHello, and MUST encode the supported TLS users desiring Perfect Forward Secrecy should use DHE cipher
   suites. suites in the
   CIPHER-SPECS-DATA field as described below.

   Warning: The damage done by exposure of a private key can ability to send version 2.0 CLIENT-HELLO messages will
   be limited
   by changing one's private key (and certificate) frequently.

   After verifying the server's certificate, the client encrypts a
   pre_master_secret phased out with all due haste, since the server's public key.  By successfully
   decoding the pre_master_secret newer ClientHello format
   provides better mechanisms for moving to newer versions and producing a correct Finished
   message, the
   negotiating extensions.  TLS 1.2 clients SHOULD NOT support SSL 2.0.

   However, even TLS servers that do not support SSL 2.0 MAY accept
   version 2.0 CLIENT-HELLO messages.  The message is presented below in
   sufficient detail for TLS server demonstrates that it knows implementors; the private key
   corresponding true definition is
   still assumed to the server certificate.

   When RSA be [SSL2].

   For negotiation purposes, 2.0 CLIENT-HELLO is used for key exchange, clients are authenticated using interpreted the certificate verify message (see Section 7.4.8).  The client signs same
   way as a value derived from all preceding handshake messages.  These
   handshake messages include the server certificate, which binds ClientHello with a "null" compression method and no
   extensions.  Note that this message MUST be sent directly on the
   signature to
   wire, not wrapped as a TLS record.  For the server, purposes of calculating
   Finished and ServerHello.random, which binds CertificateVerify, the
   signature msg_length field is not
   considered to be a part of the current handshake process.

F.1.1.3.  Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is used, message.

      uint8 V2CipherSpec[3];
      struct {
          uint16 msg_length;
          uint8 msg_type;
          Version version;
          uint16 cipher_spec_length;
          uint16 session_id_length;
          uint16 challenge_length;
          V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
          opaque session_id[V2ClientHello.session_id_length];
          opaque challenge[V2ClientHello.challenge_length;
      } V2ClientHello;

   msg_length
      The highest bit MUST be 1; the server can either
   supply a certificate containing fixed Diffie-Hellman parameters or
   use remaining bits contain the server key exchange message to send a set length
      of temporary
   Diffie-Hellman parameters signed with a DSA or RSA certificate.
   Temporary parameters are hashed the following data in bytes.

   msg_type
      This field, in conjunction with the hello.random values before
   signing version field, identifies a
      version 2 ClientHello message.  The value MUST be 1.

   version
      Equal to ensure that attackers do not replay old parameters.  In
   either case, ClientHello.client_version.

   cipher_spec_length
      This field is the total length of the field cipher_specs.  It
      cannot be zero and MUST be a multiple of the V2CipherSpec length
      (3).

   session_id_length
      This field MUST have a value of zero for a client can verify that claims to
      support TLS 1.2.

   challenge_length
      The length in bytes of the certificate or signature client's challenge to
   ensure that the parameters belong server to
      authenticate itself.  Historically, permissible values are between
      16 and 32 bytes inclusive.  When using the server.

   If SSLv2 backward-
      compatible handshake the client has SHOULD use a certificate containing fixed Diffie-Hellman
   parameters, its certificate contains 32-byte challenge.

   cipher_specs
      This is a list of all CipherSpecs the information required client is willing and able
      to use.  In addition to
   complete the key exchange.  Note that 2.0 cipher specs defined in [SSL2],
      this case the client and
   server will generate the same Diffie-Hellman result (i.e.,
   pre_master_secret) every time they communicate.  To prevent includes the
   pre_master_secret from staying TLS cipher suites normally sent in memory any longer than necessary,
   it should be converted into
      ClientHello.cipher_suites, with each cipher suite prefixed by a
      zero byte.  For example, the master_secret as soon TLS cipher suite {0x00,0x0A} would be
      sent as possible.
   Client Diffie-Hellman parameters must {0x00,0x00,0x0A}.

   session_id
      This field MUST be compatible with those
   supplied by empty.

   challenge
      Corresponds to ClientHello.random.  If the challenge length is
      less than 32, the TLS server for will pad the key exchange data with leading (note:
      not trailing) zero bytes to work.

   If the client has make it 32 bytes long.

   Note: Requests to resume a standard DSA or RSA certificate or TLS session MUST use a TLS client hello.

E.3.  Avoiding Man-in-the-Middle Version Rollback

   When TLS clients fall back to Version 2.0 compatibility mode, they
   MUST use special PKCS#1 block formatting.  This is
   unauthenticated, it sends done so that TLS
   servers will reject Version 2.0 sessions with TLS-capable clients.

   When a client negotiates SSL 2.0 but also supports TLS, it MUST set
   the right-hand (least-significant) 8 random bytes of temporary parameters to the server
   in PKCS padding
   (not including the client key exchange message, then optionally uses a
   certificate verify message to authenticate itself.

   If terminal null of the same DH keypair is to be used padding) for multiple handshakes, either
   because the client or server has a certificate containing a fixed DH
   keypair or because the server is reusing DH keys, care must be taken
   to prevent small subgroup attacks.  Implementations SHOULD follow RSA
   encryption of the
   guidelines found in [RFC2785].

   Small subgroup attacks are most easily avoided by using one ENCRYPTED-KEY-DATA field of the
   DHE cipher suites and generating CLIENT-MASTER-KEY
   to 0x03 (the other padding bytes are random).

   When a fresh DH private key (X) for each
   handshake. TLS-capable server negotiates SSL 2.0 it SHOULD, after
   decrypting the ENCRYPTED-KEY-DATA field, check that these 8 padding
   bytes are 0x03.  If a suitable base (such as 2) is chosen, g^X mod p can
   be computed very quickly; therefore, they are not, the performance cost is
   minimized.  Additionally, using a fresh key for each handshake
   provides Perfect Forward Secrecy.  Implementations server SHOULD generate a
   new X random
   value for each handshake when using DHE cipher suites.

   Because TLS allows the server to provide arbitrary DH groups, the
   client should verify that SECRET-KEY-DATA, and continue the DH group is of suitable size as defined
   by local policy.  The client SHOULD also verify handshake (which will
   eventually fail since the keys will not match).  Note that reporting
   the DH public
   exponent appears to be of adequate size.  [RFC3766] provides a useful
   guide error situation to the strength of various group sizes.  The client could make the server MAY choose vulnerable to assist the client by providing a known group, such as those
   defined
   attacks described in [RFC4307] or [RFC3526].  These can be verified by simple
   comparison.

F.1.2.  Version Rollback Attacks

   Because [BLEI].

Appendix F.  Security Analysis

   The TLS includes substantial improvements protocol is designed to establish a secure connection between
   a client and a server communicating over SSL Version 2.0, an insecure channel.  This
   document makes several traditional assumptions, including that
   attackers may try to make TLS-capable clients have substantial computational resources and servers fall back cannot obtain
   secret information from sources outside the protocol.  Attackers are
   assumed to Version 2.0. have the ability to capture, modify, delete, replay, and
   otherwise tamper with messages sent over the communication channel.
   This attack appendix outlines how TLS has been designed to resist a variety
   of attacks.

F.1.  Handshake Protocol

   The 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
   handshake protocol can occur if (and only if) two TLS-
   capable also optionally authenticate parties use who have
   certificates signed by a trusted certificate authority.

F.1.1.  Authentication and Key Exchange

   TLS supports three authentication modes: authentication of both
   parties, server authentication with an SSL 2.0 handshake.

   Although the solution using non-random PKCS #1 block type 2 message
   padding unauthenticated client, and
   total anonymity.  Whenever the server is inelegant, it provides a reasonably secure way for Version
   3.0 servers to detect authenticated, the attack.  This solution channel
   is not secure against attackers who can brute-force man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If the key and substitute a new
   ENCRYPTED-KEY-DATA server is authenticated,
   its certificate message containing must provide a valid certificate chain
   leading to an acceptable certificate authority.  Similarly,
   authenticated clients must supply an acceptable certificate to the same key (but with normal
   padding) before
   server.  Each party is responsible for verifying that the application-specified wait threshold other's
   certificate is valid and has expired.
   Altering the padding of the least-significant 8 bytes of the PKCS
   padding does not impact security for the size expired or been revoked.

   The general goal of the signed hashes
   and RSA key lengths used in the protocol, since this exchange process is essentially
   equivalent to increasing the input block size by 8 bytes.

F.1.3.  Detecting Attacks Against create a
   pre_master_secret known to the Handshake Protocol

   An attacker might try communicating parties and not to influence
   attackers.  The pre_master_secret will be used to generate the handshake exchange
   master_secret (see Section 8.1).  The master_secret is required to make
   generate the Finished messages and record protection keys (see
   Section 7.4.8 and Section 6.3).  By sending a correct Finished
   message, parties select different encryption algorithms than thus prove that they would
   normally choose.

   For this attack, an attacker must actively change one or more
   handshake messages.  If this occurs, know the correct
   pre_master_secret.

F.1.1.1.  Anonymous Key Exchange

   Completely anonymous sessions can be established using Diffie-Hellman
   for key exchange.  The server's public parameters are contained in
   the client and server will
   compute different values for key exchange message, and the handshake message hashes.  As a
   result, client's are sent in the parties will
   client key exchange message.  Eavesdroppers who do not accept each others' Finished messages.
   Without know the master_secret,
   private values should not be able to find the attacker cannot repair Diffie-Hellman result
   (i.e., the Finished
   messages, so pre_master_secret).

   Warning: Completely anonymous connections only provide protection
   against passive eavesdropping.  Unless an independent tamper-proof
   channel is used to verify that the attack will be discovered.

F.1.4.  Resuming Sessions

   When Finished messages were not
   replaced by an attacker, server authentication is required in
   environments where active man-in-the-middle attacks are a connection concern.

F.1.1.2.  Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is established by resuming used, the server can either
   supply a session, new
   ClientHello.random and ServerHello.random values certificate containing fixed Diffie-Hellman parameters or
   use the server key exchange message to send a set of temporary
   Diffie-Hellman parameters signed with a DSA or RSA certificate.
   Temporary parameters are hashed with the
   session's master_secret.  Provided hello.random values before
   signing to ensure that the master_secret has attackers do not
   been compromised and replay old parameters.  In
   either case, the client can verify the certificate or signature to
   ensure that the secure hash operations used parameters belong to produce the encryption keys server.

   If the client has a certificate containing fixed Diffie-Hellman
   parameters, its certificate contains the information required to
   complete the key exchange.  Note that in this case the client and MAC keys are secure,
   server will generate the connection same Diffie-Hellman result (i.e.,
   pre_master_secret) every time they communicate.  To prevent the
   pre_master_secret from staying in memory any longer than necessary,
   it should be
   secure and effectively independent from previous connections.
   Attackers cannot use known encryption keys or MAC secrets to
   compromise converted into the master_secret without breaking the secure hash
   operations.

   Sessions cannot as soon as possible.
   Client Diffie-Hellman parameters must be resumed unless both compatible with those
   supplied by the client and server agree. for the key exchange to work.

   If either party suspects that the session may have been compromised, client has a standard DSA or that certificates may have expired RSA certificate or been revoked, is
   unauthenticated, it should
   force sends a full handshake.  An upper limit set of 24 hours is suggested for
   session ID lifetimes, since an attacker who obtains temporary parameters to the server
   in the client key exchange message, then optionally uses a master_secret
   may be able
   certificate verify message to impersonate authenticate itself.

   If the compromised party until same DH keypair is to be used for multiple handshakes, either
   because the
   corresponding session ID client or server has a certificate containing a fixed DH
   keypair or because the server is retired.  Applications that may reusing DH keys, care must be run in
   relatively insecure environments should not write session IDs taken
   to
   stable storage.

F.2.  Protecting Application Data

   The master_secret is hashed with prevent small subgroup attacks.  Implementations SHOULD follow the ClientHello.random and
   ServerHello.random to produce unique data encryption keys
   guidelines found in [RFC2785].

   Small subgroup attacks are most easily avoided by using one of the
   DHE cipher suites and MAC
   secrets generating a fresh DH private key (X) for each connection.

   Outgoing data is protected with a MAC before transmission.  To
   prevent message replay or modification attacks, the MAC
   handshake.  If a suitable base (such as 2) is chosen, g^X mod p can
   be computed
   from the MAC key, the sequence number, the message length, very quickly; therefore, the
   message contents, and two fixed character strings.  The message type
   field performance cost is necessary to ensure that messages intended
   minimized.  Additionally, using a fresh key for one each handshake
   provides Perfect Forward Secrecy.  Implementations SHOULD generate a
   new X for each handshake when using DHE cipher suites.

   Because TLS
   record layer client are not redirected allows the server to another. provide arbitrary DH groups, the
   client should verify that the DH group is of suitable size as defined
   by local policy.  The sequence
   number ensures client SHOULD also verify that attempts the DH public
   exponent appears 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 MAC keys.  Similarly, of adequate size.  [RFC3766] provides a useful
   guide to the strength of various group sizes.  The server write and MAY choose
   to assist the client write keys are independent, so stream cipher
   keys are used by providing a known group, such as those
   defined in [RFC4307] or [RFC3526].  These can be verified by simple
   comparison.

F.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 once.

   If an attacker does break if) two TLS-
   capable parties use an encryption key, all messages encrypted
   with SSL 2.0 handshake.

   Although the solution using non-random PKCS #1 block type 2 message
   padding is inelegant, it can be read.  Similarly, compromise of provides a MAC key can make
   message-modification attacks possible.  Because MACs are also
   encrypted, message-alteration attacks generally require breaking the
   encryption algorithm as well as reasonably secure way for Version
   3.0 servers to detect the MAC.

   Note: MAC keys may be larger than encryption keys, so messages attack.  This solution is not secure
   against attackers who can
   remain tamper resistant even if encryption keys are broken.

F.3.  Explicit IVs

   [CBCATT] describes a chosen plaintext attack on TLS that depends on
   knowing brute-force the IV for key and substitute a record.  Previous versions 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 TLS [RFC2246] used the CBC residue least-significant 8 bytes of the previous record as PKCS
   padding does not impact security for the IV and therefore
   enabled this attack.  This version uses an explicit IV in order to
   protect against this attack.

F.4.  Security size of Composite Cipher Modes

   TLS secures transmitted application data via the use of symmetric
   encryption signed hashes
   and authentication functions defined RSA key lengths used in the negotiated
   cipher suite.  The objective protocol, since this is essentially
   equivalent to protect both the integrity and
   confidentiality of increasing the transmitted data from malicious actions input block size by
   active attackers in 8 bytes.

F.1.3.  Detecting Attacks Against the network.  It turns out that Handshake Protocol

   An attacker might try to influence the order in
   which encryption and authentication functions are applied handshake exchange to make the data
   plays an important role for achieving this goal [ENCAUTH].

   The most robust method, called encrypt-then-authenticate, first
   applies
   parties select different encryption to algorithms than they would
   normally choose.

   For this attack, an attacker must actively change one or more
   handshake messages.  If this occurs, the data client and then applies server will
   compute different values for the handshake message hashes.  As a MAC to
   result, the
   ciphertext.  This method ensures that parties will not accept each others' Finished messages.
   Without the integrity and
   confidentiality goals are obtained with ANY pair of encryption and
   MAC functions, provided that master_secret, the former is secure against chosen
   plaintext attacks and that attacker cannot repair the MAC is secure against chosen-message
   attacks.  TLS uses another method, called authenticate-then-encrypt,
   in which first Finished
   messages, so the attack will be discovered.

F.1.4.  Resuming Sessions

   When a MAC connection is computed on the plaintext established by resuming a session, new
   ClientHello.random and then ServerHello.random values are hashed with the
   concatenation of plaintext and MAC is encrypted.  This method
   session's master_secret.  Provided that the master_secret has
   been proven secure for CERTAIN combinations of encryption functions
   and MAC functions, but it is not guaranteed to be secure in general.
   In particular, it has
   been shown compromised and that there exist perfectly secure
   encryption functions (secure even in the information-theoretic sense)
   that combined with any secure MAC function, fail hash operations used to provide produce
   the
   confidentiality goal against an active attack.  Therefore, new cipher
   suites record protection kayes are secure, the connection should be
   secure and operation modes adopted into TLS need effectively independent from previous connections.
   Attackers cannot use known keys to be analyzed under compromise the authenticate-then-encrypt method to verify that they achieve master_secret
   without breaking the
   stated integrity and confidentiality goals.

   Currently, secure hash operations.

   Sessions cannot be resumed unless both the security of client and server agree.
   If either party suspects that the authenticate-then-encrypt method has session may have been proven for some important cases.  One is the case of stream
   ciphers in which a computationally unpredictable pad of the length of
   the message, plus the length compromised,
   or that certificates may have expired or been revoked, it should
   force a full handshake.  An upper limit of the MAC tag, 24 hours is produced using suggested for
   session ID lifetimes, since an attacker who obtains a
   pseudorandom generator and this pad is exclusive-ORed with master_secret
   may be able to impersonate the
   concatenation of plaintext and MAC tag.  The other is compromised party until the case of CBC
   mode using a secure block cipher.  In this case, security can
   corresponding session ID is retired.  Applications that may be
   shown if one applies one CBC encryption pass run in
   relatively insecure environments should not write session IDs to
   stable storage.

F.2.  Protecting Application Data

   The master_secret is hashed with the concatenation of
   plaintext and MAC and uses a new, independent, ClientHello.random and unpredictable IV
   ServerHello.random to produce unique record protection secrets for
   each new pair of plaintext connection.

   Outgoing data is protected using an AEAD algorithm before
   transmission.  The authentication data includes the sequence number,
   message type, message length, and MAC.  In versions of TLS prior the message contents.  The message
   type field is necessary to
   1.1, CBC mode was used properly EXCEPT ensure that it used a predictable IV
   in the form of the last block of the previous ciphertext.  This made messages intended for one TLS open
   record layer client are not redirected to chosen plaintext attacks.  This version of the protocol
   is immune another.  The sequence
   number ensures that attempts to those attacks.  For exact details in 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 encryption
   modes proven secure, see [ENCAUTH].

F.5.
   other's output, since they use independent keys.

F.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 for doing RSA decryption.
   asymmetric crypto operations.  However, because TLS is generally used
   over TCP, it is difficult for the attacker to hide his 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].

F.6.

F.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 G.  Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address tls@ietf.org [1]. [2].  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:
   http://www.ietf.org/mail-archive/web/tls/current/index.html

Appendix H.  Contributors

   Christopher Allen (co-editor of TLS 1.0)
   Alacrity Ventures
   ChristopherA@AlacrityManagement.com

   Martin Abadi
   University of California, Santa Cruz
   abadi@cs.ucsc.edu

   Steven M. Bellovin
   Columbia University
   smb@cs.columbia.edu

   Simon Blake-Wilson
   BCI
   sblakewilson@bcisse.com

   Ran Canetti
   IBM
   canetti@watson.ibm.com

   Pete Chown
   Skygate Technology Ltd
   pc@skygate.co.uk

   Taher Elgamal
   taher@securify.com
   Securify

   Pasi Eronen
   pasi.eronen@nokia.com
   Nokia

   Anil Gangolli
   anil@busybuddha.org

   Kipp Hickman
   Alfred Hoenes

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

   Phil Karlton (co-author of SSLv3)

   Paul Kocher (co-author of SSLv3)
   Cryptography Research
   paul@cryptography.com

   Hugo Krawczyk
   IBM
   hugo@ee.technion.ac.il

   Jan Mikkelsen
   Transactionware
   janm@transactionware.com

   Magnus Nystrom
   RSA Security
   magnus@rsasecurity.com

   Robert Relyea
   Netscape Communications
   relyea@netscape.com

   Jim Roskind
   Netscape Communications
   jar@netscape.com

   Michael Sabin

   Dan Simon
   Microsoft, Inc.
   dansimon@microsoft.com

   Tom Weinstein

   Tim Wright
   Vodafone
   timothy.wright@vodafone.com

Authors' Addresses

   Tim Dierks
   Independent

   EMail: tim@dierks.org

   Eric Rescorla
   RTFM, Inc.

   EMail: ekr@rtfm.com