Network Working Group                                          T. Dierks
Request for Comments: 5246                                   Independent
Obsoletes: 3268, 4346, 4366                                  E. Rescorla
Updates: 4492                                                 RTFM, Inc.
Category: Standards Track                                    August 2008

The Transport Layer Security (TLS) Protocol
Version 1.2

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document specifies Version 1.2 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.

   1. Introduction ....................................................4
      1.1. Requirements Terminology ...................................5
      1.2. Major Differences from TLS 1.1 .............................5
   2. Goals ...........................................................6
   3. Goals of This Document ..........................................7
   4. Presentation Language ...........................................7
      4.1. Basic Block Size ...........................................7
      4.2. Miscellaneous ..............................................8
      4.3. Vectors ....................................................8
      4.4. Numbers ....................................................9
      4.5. Enumerateds ................................................9
      4.6. Constructed Types .........................................10
           4.6.1. Variants ...........................................10
      4.7. Cryptographic Attributes ..................................12
      4.8. Constants .................................................14
   5. HMAC and the Pseudorandom Function .............................14
   6. The TLS Record Protocol ........................................15
      6.1. Connection States .........................................16
      6.2. Record Layer ..............................................19
           6.2.1. Fragmentation ......................................19

           6.2.2. Record Compression and Decompression ...............20
           6.2.3. Record Payload Protection ..........................21
                  6.2.3.1. Null or Standard Stream Cipher ............22
                  6.2.3.2. CBC Block Cipher ..........................22
                  6.2.3.3. AEAD Ciphers ..............................24
      6.3. Key Calculation ...........................................25
   7. The TLS Handshaking Protocols ..................................26
      7.1. Change Cipher Spec Protocol ...............................27
      7.2. Alert Protocol ............................................28
           7.2.1. Closure Alerts .....................................29
           7.2.2. Error Alerts .......................................30
      7.3. Handshake Protocol Overview ...............................33
      7.4. Handshake Protocol ........................................37
           7.4.1. Hello Messages .....................................38
                  7.4.1.1. Hello Request .............................38
                  7.4.1.2. Client Hello ..............................39
                  7.4.1.3. Server Hello ..............................42
                  7.4.1.4. Hello Extensions ..........................44
                           7.4.1.4.1. Signature Algorithms ...........45
           7.4.2. Server Certificate .................................47
           7.4.3. Server Key Exchange Message ........................50
           7.4.4. Certificate Request ................................53
           7.4.5. Server Hello Done ..................................55
           7.4.6. Client Certificate .................................55
           7.4.7. Client Key Exchange Message ........................57
                  7.4.7.1. RSA-Encrypted Premaster Secret Message ....58
                  7.4.7.2. Client Diffie-Hellman Public Value ........61
           7.4.8. Certificate Verify .................................62
           7.4.9. Finished ...........................................63
   8. Cryptographic Computations .....................................64
      8.1. Computing the Master Secret ...............................64
           8.1.1. RSA ................................................65
           8.1.2. Diffie-Hellman .....................................65
   9. Mandatory Cipher Suites ........................................65
   10. Application Data Protocol .....................................65
   11. Security Considerations .......................................65
   12. IANA Considerations ...........................................65
   Appendix A. Protocol Data Structures and Constant Values ..........68
      A.1. Record Layer ..............................................68
      A.2. Change Cipher Specs Message ...............................69
      A.3. Alert Messages ............................................69
      A.4. Handshake Protocol ........................................70
           A.4.1. Hello Messages .....................................71
           A.4.2. Server Authentication and Key Exchange Messages ....72
           A.4.3. Client Authentication and Key Exchange Messages ....74
           A.4.4. Handshake Finalization Message .....................74
      A.5. The Cipher Suite ..........................................75
      A.6. The Security Parameters ...................................77

      A.7. Changes to RFC 4492 .......................................78
   Appendix B. Glossary ..............................................78
   Appendix C. Cipher Suite Definitions ..............................83
   Appendix D. Implementation Notes ..................................85
      D.1. Random Number Generation and Seeding ......................85
      D.2. Certificates and Authentication ...........................85
      D.3. Cipher Suites .............................................85
      D.4. Implementation Pitfalls ...................................85
   Appendix E. Backward Compatibility ................................87
      E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0 ................87
      E.2. Compatibility with SSL 2.0 ................................88
      E.3. Avoiding Man-in-the-Middle Version Rollback ...............90
   Appendix F. Security Analysis .....................................91
      F.1. Handshake Protocol ........................................91
           F.1.1. Authentication and Key Exchange ....................91
                  F.1.1.1. Anonymous Key Exchange ....................91
                  F.1.1.2. RSA Key Exchange and Authentication .......92
                  F.1.1.3. Diffie-Hellman Key Exchange with
                           Authentication ............................92
           F.1.2. Version Rollback Attacks ...........................93
           F.1.3. Detecting Attacks Against the Handshake Protocol ...94
           F.1.4. Resuming Sessions ..................................94
      F.2. Protecting Application Data ...............................94
      F.3. Explicit IVs ..............................................95
      F.4. Security of Composite Cipher Modes ........................95
      F.5. Denial of Service .........................................96
      F.6. Final Notes ...............................................96
   Normative References ..............................................97
   Informative References ............................................98
   Working Group Information ........................................101
   Contributors .....................................................101

1. Introduction

   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 [TCP]), 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.

   -  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.  The Record Protocol
      can operate without a MAC, 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 [REQ].

1.2. Major Differences from TLS 1.1

   This document is a revision of the TLS 1.1 [TLS1.1] 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 [TLSAES].

   -  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 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 [XDR] 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 cryptographic operations -- digital signing, stream cipher
   encryption, block cipher encryption, 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, 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 for the definition of this field).  Note that the
   introduction of the algorithm field is a change from previous
   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 [PKCS1].  As
   discussed in [PKCS1], 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 encryption, the plaintext is exclusive-ORed with an
   identical amount 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 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 [PKCS1].

   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.  The length of this 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 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 [HMAC], which
   is based on a hash function.  Other cipher suites MAY define their
   own MAC constructions, if needed.

   In addition, 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.  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 the data, applies
   a MAC, encrypts, and transmits the result.  Received data is
   decrypted, 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, and a MAC algorithm.  In addition, the parameters for
   these algorithms are known: the MAC key and the bulk encryption keys
   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.

   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
      Sections 5 and 6.3).

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

   MAC algorithm
      An algorithm to be used for message authentication.  This
      specification includes the size of the value returned by the MAC
      algorithm.

   compression algorithm
      An algorithm to be used for data compression.  This specification
      must include all information the algorithm requires to do
      compression.

   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) } CompressionMethod;

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

      struct {
          ConnectionEnd          entity;
          PRFAlgorithm           prf_algorithm;
          BulkCipherAlgorithm    bulk_cipher_algorithm;
          CipherType             cipher_type;
          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 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, which uses the version { 3, 3 }.  The
      version value 3.3 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.  The compression algorithm translates a
   TLSPlaintext structure into a TLSCompressed structure.  Compression
   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 the decompression function encounters a
   TLSCompressed.fragment that would decompress to a length in excess of
   2^14 bytes, it MUST report 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 encryption and MAC functions translate a TLSCompressed
      structure into a TLSCiphertext.  The decryption functions reverse
      the process.  The MAC of the record also includes a sequence
      number so that missing, extra, or repeated messages are
      detectable.

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

   type
      The type field is identical to TLSCompressed.type.

   version
      The version field is identical to TLSCompressed.version.

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

fragment
The encrypted form of TLSCompressed.fragment, with the MAC.

6.2.3.1. Null or Standard Stream Cipher

   Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6)
   convert TLSCompressed.fragment structures 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 is computed before encryption.  The stream cipher
   encrypts the entire block, including the MAC.  For stream ciphers
   that do not use a synchronization vector (such as RC4), the stream
   cipher state from the end of one record is simply used on the
   subsequent packet.  If the cipher suite is TLS_NULL_WITH_NULL_NULL,
   encryption consists of the identity operation (i.e., the data is not
   encrypted, and the MAC size is zero, implying that no MAC is used).
   For both null and stream ciphers, TLSCiphertext.length 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;

   The MAC is generated as described in Section 6.2.3.1.

   IV
      The Initialization Vector (IV) SHOULD be chosen at random, and
      MUST be unpredictable.  Note that in versions of TLS prior to 1.1,
      there was no IV field, and the last ciphertext block of the
      previous record (the "CBC residue") was used as the IV.  This was
      changed to prevent the attacks described in [CBCATT].  For block
      ciphers, the IV length is of length
      SecurityParameters.record_iv_length, which is equal to the
      SecurityParameters.block_size.

   padding
      Padding 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 the
      block length.  Lengths longer than necessary might be desirable to
      frustrate attacks on a protocol that are based on analysis of the
      lengths of exchanged messages.  Each uint8 in the padding data
      vector MUST be filled with the padding length value.  The receiver
      MUST check this padding and MUST use the bad_record_mac alert to
      indicate padding errors.

   padding_length
      The padding length MUST be such that the total size of the
      GenericBlockCipher structure is a multiple of the cipher's block
      length.  Legal values range from zero to 255, inclusive.  This
      length specifies the length of the padding field exclusive of the
      padding_length field itself.

   The encrypted data length (TLSCiphertext.length) is one more than the
   sum 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 bytes, and the MAC length is 20 bytes,
   then the length before padding is 82 bytes (this does not include the

   IV.  Thus, the padding length modulo 8 must be equal to 6 in order to
   make the total length an even multiple of 8 bytes (the block length).
   The padding length can be 6, 14, 22, 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 the
   GenericBlockCipher before block encryption would be xx 06 06 06 06 06
   06 06, where xx is the last octet of the MAC.

   Note: With block ciphers in CBC mode (Cipher Block Chaining), it is
   critical that the entire plaintext of the record be known before any
   ciphertext is transmitted.  Otherwise, it is possible for the
   attacker to mount the attack described in [CBCATT].

   Implementation note: Canvel et al. [CBCTIME] have demonstrated a
   timing attack on CBC padding based on the time required to compute
   the MAC.  In order 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, the best way to
   do this is to compute the MAC even if the padding is incorrect, and
   only then reject the packet.  For instance, if the pad appears to be
   incorrect, the implementation might assume a zero-length pad and then
   compute the MAC.  This leaves a small timing channel, since MAC
   performance depends to some extent on the size of the data fragment,
   but it is not believed to be large enough to be exploitable, due to
   the large block size of existing MACs and the small size of the
   timing signal.

6.2.3.3. AEAD Ciphers

   For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function
   converts TLSCompressed.fragment structures to 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 [AEAD].  The key is either the
   client_write_key or the server_write_key.  No MAC key is used.

   Each AEAD cipher suite MUST specify how the nonce supplied to the
   AEAD operation is constructed, and what is the length of the
   GenericAEADCipher.nonce_explicit part.  In many cases, it is

   appropriate to use the partially implicit nonce technique described
   in Section 3.2.1 of [AEAD]; 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.

   The plaintext is the TLSCompressed.fragment.

   The additional authenticated data, which we denote as
   additional_data, is defined as follows:

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

   where "+" denotes concatenation.

   The aead_output consists of the ciphertext output by the AEAD
   encryption operation.  The length will generally be larger than
   TLSCompressed.length, but by an amount that varies with the AEAD
   cipher.  Since the ciphers might incorporate padding, 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 to decrypt and verify, the cipher takes as input the key,
   nonce, the "additional_data", and the AEADEncrypted value.  The
   output is either the plaintext or an error indicating that the
   decryption failed.  There is no separate integrity check.  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

   The Record Protocol requires an algorithm to generate keys required
   by the current connection state (see Appendix A.6) from the security
   parameters provided by the handshake protocol.

   The master secret is expanded into a sequence of secure bytes, which
   is then split to a client write MAC key, a server write MAC key, a
   client write encryption key, and a server write encryption key.  Each
   of these is generated from the byte sequence in that order.  Unused
   values are empty.  Some AEAD ciphers 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, the master secret is used as an
   entropy source.

   To generate the 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 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, the client_write_IV and server_write_IV are only generated
   for implicit nonce techniques as described in Section 3.2.1 of
   [AEAD].

   Implementation note: The currently defined cipher suite which
   requires the most 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 has three subprotocols that are used to allow peers to agree upon
   security parameters for the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to report error
   conditions to each other.

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

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

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

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

   cipher spec
      Specifies the pseudorandom function (PRF) used 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 the session can be used to initiate 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 encrypted and compressed under the current (not the pending)
   connection state.  The message consists of a single byte of value 1.

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

   The ChangeCipherSpec message is sent by both the client and the
   server to notify the receiving party that subsequent records will be
   protected under the newly negotiated CipherSpec and keys.  Reception
   of this message causes the receiver to instruct the record layer to
   immediately copy the read pending state into the read current state.
   Immediately after sending this message, the sender MUST instruct the
   record layer to make the write pending state the write active state.

   (See Section 6.1.)  The ChangeCipherSpec message 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 to send data using the old
   CipherSpec.  However, once the ChangeCipherSpec has been sent, the
   new CipherSpec MUST be used.  The first side to send the
   ChangeCipherSpec does not know that the other 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, with modern machines this interval is likely to be fairly
   short.

7.2. Alert Protocol

   One of the content types supported by the TLS record layer is the
   alert type.  Alert messages convey the severity of the message
   (warning or fatal) and a description of the alert.  Alert messages
   with a level of fatal result in the immediate termination of the
   connection.  In this case, other connections corresponding to the
   session may continue, but the session identifier MUST be invalidated,
   preventing the failed session from being used to establish new
   connections.  Like other messages, alert messages are encrypted 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 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 be returned if an alert 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.  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 record
      with more than 2^14+1024 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 and should never be observed in communication between proper
      implementations.

   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 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 with a 1024-bit RSA 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 the server must respond 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 the following attributes: Protocol Version,
   Session ID, Cipher Suite, and Compression Method.  Additionally, two
   random values are generated and exchanged: ClientHello.random and
   ServerHello.random.

   The actual key exchange uses up to four messages: the server
   Certificate, the ServerKeyExchange, the client Certificate, and the
   ClientKeyExchange.  New key exchange methods can be created by
   specifying a format for these messages and by defining the use of the
   messages to allow the client and server to agree upon a shared
   secret.  This secret MUST be quite long; currently defined key
   exchange methods exchange secrets that range from 46 bytes upwards.

   Following the hello messages, the server will 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 server is authenticated, it may request a certificate
   from the client, if that is appropriate to the cipher suite selected.
   Next, the server will send the ServerHelloDone message, indicating
   that the hello-message phase of the handshake is complete.  The
   server will then wait for a client response.  If the server has sent
   a CertificateRequest message, the client MUST send the Certificate
   message.  The ClientKeyExchange message is now sent, and the content
   of that message will depend on the public key algorithm selected
   between the ClientHello and the ServerHello.  If the client has sent
   a certificate with signing ability, a digitally-signed
   CertificateVerify message is sent to explicitly verify possession of
   the private key in the certificate.

   At this point, a 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 the Finished message under
   the new algorithms, keys, and secrets.  In response, the server will
   send its own ChangeCipherSpec message, transfer the pending to the
   current Cipher Spec, and send its Finished message under the new
   Cipher Spec.  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 the first handshake (before a cipher suite other than
   TLS_NULL_WITH_NULL_NULL is established).

      Client                                               Server

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

          Figure 2.  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 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;

   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 initialized to 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.

   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 or
   immediate MAC 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 algorithm (including secret key
   length), a MAC algorithm, 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 (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 of the compression methods supported by the client,
      sorted by client preference.  If the session_id field is not empty
      (implying a session resumption request), it MUST include the

      compression_method from that session.  This vector MUST contain,
      and all implementations MUST support, CompressionMethod.null.
      Thus, a client and server will always be able to agree on a
      compression method.

   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.

   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

   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 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.  (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. Hello Extensions

   The extension format is:

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

      enum {
          signature_algorithms(13), (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. 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 [MD5], 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
      [PKCS1] 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.  Sections 7.4.2 and 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, 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), behave as if the client had sent the value {sha1,dsa}.

   -  If the negotiated key exchange algorithm is one of (ECDH_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 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
      ServerHello message.

   Meaning of this message:

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

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

   Structure of this message:

      opaque ASN.1Cert<1..2^24-1>;

      struct {
          ASN.1Cert certificate_list<0..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 the 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 structure will be used for the client's
   response to a certificate request message.  Note that a client MAY
   send no certificates if it does not have an appropriate certificate
   to send in response to the 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 SET rather than a SEQUENCE, making the task
   of parsing the list more difficult.

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

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

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

      Key Exchange Alg.  Certificate Key Type

      RSA                RSA public key; the certificate MUST allow the
      RSA_PSK            key to be used for encryption (the
                         keyEncipherment bit MUST be set if the key
                         usage extension is present).
                         Note: RSA_PSK is defined in [TLSPSK].

      DHE_RSA            RSA 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 the signature
                         scheme and hash algorithm that will be employed
                         in the server key exchange message.
                         Note: ECDHE_RSA is defined in [TLSECC].

      DHE_DSS            DSA public key; the certificate MUST allow the
                         key to be used for signing with the hash
                         algorithm that will be employed in the server
                         key exchange message.

      DH_DSS             Diffie-Hellman public key; the keyAgreement bit
      DH_RSA             MUST be set if the key usage extension is
                         present.

      ECDH_ECDSA         ECDH-capable public key; the public key MUST
      ECDH_RSA           use a curve and point format supported by the
                         client, as described in [TLSECC].

      ECDHE_ECDSA        ECDSA-capable public key; the certificate MUST
                         allow the key to be used for signing with the
                         hash algorithm that will be employed in the
                         server key exchange message.  The public key
                         MUST use a curve and point format supported by
                         the client, as described in  [TLSECC].

   -  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.  Note that this also implies that the DH_DSS, DH_RSA,
   ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the
   algorithm used to sign 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 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.

7.4.3. Server Key Exchange Message

   When this message will be sent:

      This message will be sent immediately after the server Certificate
      message (or the ServerHello message, if this is an anonymous
      negotiation).

      The ServerKeyExchange message is sent by the server only when the
      server Certificate 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 not legal to send the ServerKeyExchange message for the
      following key exchange methods:

         RSA
         DH_DSS
         DH_RSA

      Other key exchange algorithms, such as those defined in [TLSECC],
      MUST specify whether the ServerKeyExchange message is sent or not;
      and if the message is sent, its contents.

   Meaning of this message:

      This message conveys cryptographic information to allow the client
      to communicate the premaster secret: a Diffie-Hellman public key
      with which the client can complete a key exchange (with the result
      being the premaster secret) or a public key for some other
      algorithm.

   Structure of this message:

      enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa
            /* 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 */

      dh_p
         The prime modulus used for the Diffie-Hellman operation.

      dh_g
         The generator used 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 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 [TLSECC] */
          };
      } ServerKeyExchange;

      params
         The server's key exchange parameters.

      signed_params
         For non-anonymous key exchanges, a signature over the server's
         key exchange parameters.

   If the client has offered the "signature_algorithms" extension, the
   signature algorithm and hash algorithm MUST be a pair listed in that
   extension.  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 to negotiate correctly, the server MUST check any candidate
   cipher suites against the "signature_algorithms" extension before
   selecting them.  This is somewhat inelegant but is a compromise
   designed to minimize changes to the original cipher suite design.

   In addition, the hash and signature algorithms MUST be compatible
   with the key in the server'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 [PKIX] may specify mechanisms for certificates to
   indicate which digest algorithms are to be used with DSA.

   As additional cipher suites are defined for TLS that include new key
   exchange algorithms, the server key exchange message will be sent if
   and only if the certificate type associated with the key exchange
   algorithm does not provide enough information for the client to
   exchange a premaster secret.

7.4.4. 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>;
          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. Server Hello Done

   When this message will be sent:

      The ServerHelloDone message is sent by the server to indicate the
      end of the ServerHello and associated messages.  After sending
      this message, the server will wait for a client response.

   Meaning of this message:

      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 the server provided a valid certificate, if required,
      and check that the server hello parameters are acceptable.

   Structure of this message:

      struct { } ServerHelloDone;

7.4.6. Client Certificate

   When this message will be sent:

      This is the first message the client can send after receiving a
      ServerHelloDone message.  This message is only sent if the server
      requests a certificate.  If no suitable certificate is available,
      the client MUST send a certificate message containing no
      certificates.  That is, the certificate_list structure has a
      length of zero.  If the client does not send any certificates, the
      server MAY at its discretion either continue the handshake without
      client authentication, or respond with a fatal handshake_failure
      alert.  Also, if some aspect of the certificate chain was
      unacceptable (e.g., it was not signed by a known, trusted CA), the
      server MAY at its discretion either continue the handshake
      (considering the client unauthenticated) or send a fatal alert.

      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 to the server;
      the server will use it when verifying the CertificateVerify
      message (when the client authentication is based on signing) or
      calculating the premaster secret (for non-ephemeral Diffie-
      Hellman).  The certificate MUST be appropriate for the negotiated
      cipher suite's key exchange algorithm, and any negotiated
      extensions.

   In particular:

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

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

      Client Cert. Type   Certificate Key Type

      rsa_sign            RSA public key; the certificate MUST allow the
                          key to be used for signing with the signature
                          scheme and hash algorithm that will be
                          employed in the certificate verify message.

      dss_sign            DSA public key; the certificate MUST allow the
                          key to be used for signing with the hash
                          algorithm that will be employed in the
                          certificate verify message.

      ecdsa_sign          ECDSA-capable public key; the certificate MUST
                          allow the key to be used for signing with the
                          hash algorithm that will be employed in the
                          certificate verify message; the public key
                          MUST use a curve and 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
                          point format supported by the server.

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

   -  The certificates MUST be signed using an acceptable hash/
      signature algorithm pair, as described in Section 7.4.4.  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.

7.4.7. Client Key Exchange Message

   When this message will be sent:

      This message is always sent by the client.  It MUST immediately
      follow the client certificate message, if it is sent.  Otherwise,
      it MUST be the first message sent by the client after it receives
      the ServerHelloDone message.

   Meaning of this message:

      With this message, the premaster secret is set, either by direct
      transmission of the RSA-encrypted secret or by the transmission of
      Diffie-Hellman parameters that will allow each side to agree upon
      the same premaster secret.

      When the client is using an ephemeral Diffie-Hellman exponent,
      then this message contains 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 but 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 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 from the server's certificate, and sends the result in
      an encrypted premaster secret message.  This structure is a
      variant of the ClientKeyExchange message and is 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
         This random value is generated by the client and is used to
         generate the master secret, as specified in Section 8.1.

   Note: The version number in the PreMasterSecret is the version
   offered by the client in the ClientHello.client_version, not the
   version negotiated for the connection.  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 in
   PreMasterSecret.  If ClientHello.client_version is TLS 1.1 or higher,
   server implementations MUST check the version number as described in
   the note below.  If the version number is TLS 1.0 or earlier, server
   implementations SHOULD check the version number, but MAY have a
   configuration option to disable the check.  Note that if the check
   fails, the PreMasterSecret SHOULD be randomized as described below.

   Note: Attacks discovered by Bleichenbacher [BLEI] 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 the correct
   version number.

   As described by Klima [KPR03], these vulnerabilities can be avoided
   by treating incorrectly formatted 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 the message to recover the plaintext M

      3. If the PKCS#1 padding is not correct, or the length of message
         M is not exactly 48 bytes:
            pre_master_secret = ClientHello.client_version || R
         else If ClientHello.client_version <= TLS 1.0, and version
         number check 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 the
   client has sent the wrong version in the original pre_master_secret.

   An alternative approach is to treat a version number mismatch as a
   PKCS-1 formatting error and randomize the premaster secret
   completely:

      1. Generate a string R of 48 random bytes

      2. Decrypt the message to recover the plaintext M

      3. If the PKCS#1 padding is not correct, or 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
   the first construction described is RECOMMENDED.

   In any case, a TLS server MUST NOT generate an alert if processing an
   RSA-encrypted premaster secret message fails, or the version number
   is not as expected.  Instead, it MUST continue the handshake with a
   randomly generated premaster secret.  It may be useful to log the
   real cause 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 [PKCS1] 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 the Bleichenbacher
   attack are known 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, the RSA-encrypted
   PreMasterSecret in a ClientKeyExchange is preceded by two length
   bytes.  These bytes are redundant in the case of RSA because the
   EncryptedPreMasterSecret is the only data in the ClientKeyExchange
   and its length can therefore be unambiguously determined.  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 the RSA-encrypted data
   directly in 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 MUST modify their implementations to generate
   and accept the correct encoding.  Implementors who wish to be
   compatible with both SSLv3 and TLS should make their implementation's
   behavior dependent on the protocol version.

   Implementation note: It is now known that remote timing-based attacks
   on TLS are possible, at least when the client and server are on the
   same LAN.  Accordingly, implementations that use static RSA keys MUST
   use RSA blinding or some other anti-timing technique, as described in
   [TIMING].

7.4.7.2. Client Diffie-Hellman Public Value

   Meaning of this message:

      This structure conveys the client's Diffie-Hellman public value
      (Yc) if it was not already included in the client's certificate.
      The encoding used for Yc is determined by the enumerated
      PublicValueEncoding.  This structure is a variant of the client
      key exchange message, and not a message in itself.

   Structure of this message:

      enum { implicit, explicit } PublicValueEncoding;

      implicit
         If the client has sent a certificate which contains a suitable
         Diffie-Hellman key (for fixed_dh client authentication), then
         Yc is implicit and does not need to be sent again.  In this
         case, the client key exchange message will be sent, but it 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. 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 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 of all the
      Handshake structures (as defined 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.

      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 [PKIX] 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 to verify that the key exchange and
      authentication processes were successful.  It is essential that a
      change cipher spec message be received between the other handshake
      messages and the Finished message.

   Meaning of this message:

      The Finished message is the first one protected with the just
      negotiated algorithms, keys, and secrets.  Recipients of Finished
      messages MUST verify that the contents are correct.  Once a side
      has sent its Finished message and received and validated the
      Finished message from its peer, it may begin to send and receive
      application data over the connection.

   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 client, the string
         "client finished".  For Finished messages sent by the server,
         the string "server finished".

      Hash denotes a Hash of the handshake messages.  For the PRF
      defined in Section 5, the Hash MUST be the Hash used as the basis
      for the PRF.  Any cipher suite which defines a different PRF MUST
      also define the Hash to use in the Finished computation.

      In previous versions of TLS, the verify_data was always 12 octets
      long.  In the current version of TLS, it depends on 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
         and does not include record layer headers.  This is the
         concatenation of all the Handshake structures as defined in
         Section 7.4, exchanged thus far.

   It is a fatal error if a Finished message is not preceded by a
   ChangeCipherSpec message at the appropriate point in the handshake.

   The value handshake_messages includes all handshake messages starting
   at ClientHello up to, but not including, this Finished message.  This
   may be different from handshake_messages in Section 7.4.8 because it
   would include the CertificateVerify message (if sent).  Also, the
   handshake_messages for the Finished message sent by the client will
   be different from that for the Finished message sent 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 and are not included in the hash
   computations.  Also, HelloRequest messages are omitted from handshake
   hashes.

8. Cryptographic Computations

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

8.1. Computing the Master Secret

   For all key exchange methods, the same algorithm is used to convert
   the pre_master_secret into the master_secret.  The pre_master_secret
   should be deleted from memory once 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 in length.  The length
   of the premaster secret will vary depending on key exchange method.

8.1.1. RSA

   When RSA is used for server authentication and key exchange, a 48-
   byte pre_master_secret is generated by the client, encrypted under
   the server's public key, and sent to the server.  The server uses its
   private key 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.  The
   negotiated key (Z) is used as the pre_master_secret, and is converted
   into the master_secret, as specified above.  Leading bytes of Z that
   contain all zero bits are stripped before it is used as 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
   [TLS1.1].  IANA has updated these to reference this document.  The
   registries and their allocation policies (unchanged from [TLS1.1])
   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
      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 first byte in
      the range 0-191 (decimal) inclusive are assigned via Standards
      Action [RFC2434].  Values with the first byte in the range 192-254
      (decimal) are assigned via Specification Required [RFC2434].
      Values with the first byte 255 (decimal) 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
      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.

Appendix A. Protocol Data Structures and Constant Values

This section describes protocol types and constants.

A.1. Record Layer

   struct {
       uint8 major;
       uint8 minor;
   } ProtocolVersion;

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

   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];
      };
   } GenericAEADCipher;

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),
       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)
       (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>;

A.4.2. Server Authentication and Key Exchange Messages

   opaque ASN.1Cert<2^24-1>;

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

   enum { dhe_dss, dhe_rsa, dh_anon, rsa,dh_dss, dh_rsa
          /* 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 [TLSECC] */
   } 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 are used for 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      = { 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 };
      CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA256   = { 0x00,0x40 };
      CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA256   = { 0x00,0x67 };
      CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA256    = { 0x00,0x68 };
      CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA256    = { 0x00,0x69 };
      CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA256   = { 0x00,0x6A };
      CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA256   = { 0x00,0x6B };

   The following cipher suites are used for completely anonymous
   Diffie-Hellman communications in which neither party is
   authenticated.  Note that this mode is vulnerable to 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   = { 0x00,0x6C };
      CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA256   = { 0x00,0x6D };

   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 } BulkCipherAlgorithm;

   enum { stream, block, aead } CipherType;

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

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

   struct {
       ConnectionEnd          entity;
       PRFAlgorithm           prf_algorithm;
       BulkCipherAlgorithm    bulk_cipher_algorithm;
       CipherType             cipher_type;
       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 [TLSECC] 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 [PKIX].

   As described in Sections 7.4.2 and 7.4.6, 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 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 [3DES] 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 initialization vector
      is exclusive-ORed with the first plaintext block prior to
      encryption.

   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, and IVs.

   MD5
      MD5 [MD5] 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 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
                                        Exchange

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        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  SHA256
TLS_DHE_DSS_WITH_AES_128_CBC_SHA256     DHE_DSS      AES_128_CBC  SHA256
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256     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     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.

   [RANDOM] 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 Section 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.  The message is presented below in
   sufficient detail for TLS server implementors; the true definition is
   still assumed 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 the purposes of calculating
   Finished and CertificateVerify, the msg_length field is not
   considered to be a 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
      The highest bit MUST be 1; the remaining bits contain the length
      of the following data in bytes.

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

   version
      Equal to 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 that claims to
      support TLS 1.2.

   challenge_length
      The length in bytes of the client's challenge to the server to
      authenticate itself.  Historically, permissible values are between
      16 and 32 bytes inclusive.  When using the SSLv2 backward-
      compatible handshake the client SHOULD use a 32-byte challenge.

   cipher_specs
      This is a list of all CipherSpecs the client is willing and able
      to use.  In addition to the 2.0 cipher specs defined in [SSL2],
      this includes the TLS cipher suites normally sent in
      ClientHello.cipher_suites, with each cipher suite prefixed by a
      zero byte.  For example, the TLS cipher suite {0x00,0x0A} would be
      sent as {0x00,0x00,0x0A}.

   session_id
      This field MUST be empty.

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

   Note: Requests to resume a 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 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 the PKCS padding
   (not including the terminal null of the padding) for the RSA
   encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
   to 0x03 (the other padding bytes are random).

   When a 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 they are not, the server SHOULD generate a random
   value for SECRET-KEY-DATA, and continue the handshake (which will
   eventually fail since the keys will not match).  Note that reporting
   the error situation to the client could make the server vulnerable to
   attacks described in [BLEI].

Appendix F. Security Analysis

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

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 also optionally authenticate parties 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 unauthenticated client, and
   total anonymity.  Whenever the server is authenticated, the channel
   is secure against man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If the server is authenticated,
   its certificate message must provide a valid certificate chain
   leading to an acceptable certificate authority.  Similarly,
   authenticated clients must supply an acceptable certificate to the
   server.  Each party is responsible for verifying that the other's
   certificate is valid and has not expired or been revoked.

   The general goal of the key exchange process is to create a
   pre_master_secret known to the communicating parties and not to
   attackers.  The pre_master_secret will be used to generate the
   master_secret (see Section 8.1).  The master_secret is required to
   generate the Finished messages, encryption keys, and MAC keys (see
   Sections 7.4.9 and 6.3).  By sending a correct Finished message,
   parties thus prove that they 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 server key exchange message, and the client's are sent in the

   client key exchange message.  Eavesdroppers who do not know the
   private values should not be able to find the Diffie-Hellman result
   (i.e., 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 the Finished messages were not
   replaced by an attacker, server authentication 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 the server's certificate.  Note that
   compromise of the server's static RSA key results in a loss of
   confidentiality for all sessions protected under that static key.
   TLS users desiring Perfect Forward Secrecy should use DHE cipher
   suites.  The damage done by exposure of a private key can be limited
   by changing one's private key (and certificate) frequently.

   After verifying the server's certificate, the client encrypts a
   pre_master_secret with the server's public key.  By successfully
   decoding the pre_master_secret and producing a correct Finished
   message, the server demonstrates that it knows the private key
   corresponding to the server certificate.

   When RSA is used for key exchange, clients are authenticated using
   the certificate verify message (see Section 7.4.8).  The client signs
   a value derived from all preceding handshake messages.  These
   handshake messages include the server certificate, which binds the
   signature to the server, and ServerHello.random, which binds the
   signature to the current handshake process.

F.1.1.3. Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is used, the server can either
   supply a 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 hello.random values before
   signing to ensure that attackers do not replay old parameters.  In
   either case, the client can verify the certificate or signature to
   ensure that the parameters belong to the 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
   server will generate the 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 converted into the master_secret as soon as possible.
   Client Diffie-Hellman parameters must be compatible with those
   supplied by the server for the key exchange to work.

   If the client has a standard DSA or RSA certificate or is
   unauthenticated, it sends a set of temporary parameters to the server
   in the client key exchange message, then optionally uses a
   certificate verify message to authenticate itself.

   If the same DH keypair is to be used 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 the
   guidelines found in [SUBGROUP].

   Small subgroup attacks are most easily avoided by using one of the
   DHE cipher suites and generating a fresh DH private key (X) for each
   handshake.  If a suitable base (such as 2) is chosen, g^X mod p can
   be computed very quickly; therefore, the performance cost is
   minimized.  Additionally, using a fresh key for each handshake
   provides Perfect Forward Secrecy.  Implementations SHOULD generate a
   new X for each handshake when using DHE cipher suites.

   Because TLS allows the server to provide arbitrary DH groups, the
   client should verify that the DH group is of suitable size as defined
   by local policy.  The client SHOULD also verify that the DH public
   exponent appears to be of adequate size.  [KEYSIZ] provides a useful
   guide to the strength of various group sizes.  The server MAY choose
   to assist the client by providing a known group, such as those
   defined in [IKEALG] or [MODP].  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 if) two TLS-
   capable parties use an SSL 2.0 handshake.

   Although the solution using non-random PKCS #1 block type 2 message
   padding is inelegant, it provides a reasonably secure way for Version
   3.0 servers to detect the attack.  This solution is not secure
   against attackers who can brute-force the key and substitute a new
   ENCRYPTED-KEY-DATA message containing the same key (but with normal
   padding) before the application-specified wait threshold has expired.
   Altering the padding of the least-significant 8 bytes of the PKCS

   padding does not impact security for the size of the signed hashes
   and RSA key lengths used in the protocol, since this is essentially
   equivalent to increasing the input block size by 8 bytes.

F.1.3. Detecting Attacks Against the Handshake Protocol

   An attacker might try to influence the handshake exchange to make the
   parties select different encryption algorithms than they would
   normally choose.

   For this attack, an attacker must actively change one or more
   handshake messages.  If this occurs, the client and server will
   compute different values for the handshake message hashes.  As a
   result, the parties will not accept each others' Finished messages.
   Without the master_secret, the attacker cannot repair the Finished
   messages, so the attack will be discovered.

F.1.4. Resuming Sessions

   When a connection is established by resuming a session, new
   ClientHello.random and ServerHello.random values are hashed with the
   session's master_secret.  Provided that the master_secret has not
   been compromised and that the secure hash operations used to produce
   the encryption keys and MAC keys are secure, the connection should be
   secure and effectively independent from previous connections.
   Attackers cannot use known encryption keys or MAC secrets to
   compromise the master_secret without breaking the secure hash
   operations.

   Sessions cannot be resumed unless both the client and server agree.
   If either party suspects that the session may have been compromised,
   or that certificates may have expired or been revoked, it should
   force a full handshake.  An upper limit of 24 hours is suggested for
   session ID lifetimes, since an attacker who obtains a master_secret
   may be able to impersonate the compromised party until the
   corresponding session ID is retired.  Applications that may be 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 ClientHello.random and
   ServerHello.random to produce unique data encryption keys and MAC
   secrets for each connection.

   Outgoing data is protected with a MAC before transmission.  To
   prevent message replay or modification attacks, the MAC is computed
   from the MAC key, the sequence number, the message length, the

   message contents, and two fixed character strings.  The message type
   field is necessary to ensure that messages intended for one TLS
   record layer client are not redirected to another.  The sequence
   number ensures that attempts to delete or reorder messages will be
   detected.  Since sequence numbers are 64 bits long, they should never
   overflow.  Messages from one party cannot be inserted into the
   other's output, since they use independent MAC keys.  Similarly, the
   server write and client write keys are independent, so stream cipher
   keys are used only once.

   If an attacker does break an encryption key, all messages encrypted
   with it can be read.  Similarly, compromise of 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 the MAC.

   Note: MAC keys may be larger than encryption keys, so messages 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 the IV for a record.  Previous versions of TLS [TLS1.0] used
   the CBC residue of the previous record as the IV and therefore
   enabled this attack.  This version uses an explicit IV in order to
   protect against this attack.

F.4. Security of Composite Cipher Modes

   TLS secures transmitted application data via the use of symmetric
   encryption and authentication functions defined in the negotiated
   cipher suite.  The objective is to protect both the integrity and
   confidentiality of the transmitted data from malicious actions by
   active attackers in the network.  It turns out that the order in
   which encryption and authentication functions are applied to the data
   plays an important role for achieving this goal [ENCAUTH].

   The most robust method, called encrypt-then-authenticate, first
   applies encryption to the data and then applies a MAC to the
   ciphertext.  This method ensures that the integrity and
   confidentiality goals are obtained with ANY pair of encryption and
   MAC functions, provided that the former is secure against chosen
   plaintext attacks and that the MAC is secure against chosen-message
   attacks.  TLS uses another method, called authenticate-then-encrypt,
   in which first a MAC is computed on the plaintext and then the
   concatenation of plaintext and MAC is encrypted.  This method 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 that there exist perfectly secure
   encryption functions (secure even in the information-theoretic sense)
   that combined with any secure MAC function, fail to provide the
   confidentiality goal against an active attack.  Therefore, new cipher
   suites and operation modes adopted into TLS need to be analyzed under
   the authenticate-then-encrypt method to verify that they achieve the
   stated integrity and confidentiality goals.

   Currently, the security of the authenticate-then-encrypt method has
   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 of the MAC tag, is produced using a
   pseudorandom generator and this pad is exclusive-ORed with the
   concatenation of plaintext and MAC tag.  The other is the case of CBC
   mode using a secure block cipher.  In this case, security can be
   shown if one applies one CBC encryption pass to the concatenation of
   plaintext and MAC and uses a new, independent, and unpredictable IV
   for each new pair of plaintext and MAC.  In versions of TLS prior to
   1.1, CBC mode was used properly EXCEPT that it used a predictable IV
   in the form of the last block of the previous ciphertext.  This made
   TLS open to chosen plaintext attacks.  This version of the protocol
   is immune to those attacks.  For exact details in the encryption
   modes proven secure, see [ENCAUTH].

F.5. 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.  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 [SEQNUM] 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 [AH] or ESP [ESP].

F.6. 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.

Normative References

   [AES]      National Institute of Standards and Technology,
              "Specification for the Advanced Encryption Standard (AES)"
              FIPS 197.  November 26, 2001.

   [3DES]     National Institute of Standards and Technology,
              "Recommendation for the Triple Data Encryption Algorithm
              (TDEA) Block Cipher", NIST Special Publication 800-67, May
              2004.

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

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

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

   [PKCS1]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [PKIX]     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.

   [SCH]      B. Schneier. "Applied Cryptography: Protocols, Algorithms,
              and Source Code in C, 2nd ed.", Published by John Wiley &
              Sons, Inc. 1996.

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

   [REQ]      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.

   [X680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
              Information technology - Abstract Syntax Notation One
              (ASN.1): Specification of basic notation.

   [X690]     ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER).

Informative References

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

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

   [BLEI]     Bleichenbacher D., "Chosen Ciphertext Attacks against
              Protocols Based on RSA Encryption Standard PKCS #1" in
              Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462,
              pages:  1-12, 1998.

   [CBCATT]   Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
              Problems and Countermeasures",
              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", Advances in
              Cryptology -- CRYPTO 2003, LNCS vol. 2729, 2003.

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

   [DES]      National Institute of Standards and Technology, "Data
              Encryption Standard (DES)", FIPS PUB 46-3, October 1999.

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

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

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

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

   [FI06]     Hal Finney, "Bleichenbacher's RSA signature forgery based
              on implementation error", ietf-openpgp@imc.org mailing
              list, 27 August 2006, http://www.imc.org/ietf-openpgp/
              mail-archive/msg14307.html.

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

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

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

   [KPR03]    Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
              Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
              March 2003.

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

   [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.

   [RANDOM]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

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

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

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

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

   [SSL2]     Hickman, Kipp, "The SSL Protocol", Netscape Communications
              Corp., Feb 9, 1995.

   [SSL3]     A. Freier, P. Karlton, and P. Kocher, "The SSL 3.0
              Protocol", Netscape Communications Corp., Nov 18, 1996.

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

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

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

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

   [TLSECC]   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.

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

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

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

   [TLS1.0]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [TLS1.1]   Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

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

Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address <tls@ietf.org>. 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>

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

Editors' Addresses

   Tim Dierks
   Independent
   EMail: tim@dierks.org

   Eric Rescorla
   RTFM, Inc.
   EMail: ekr@rtfm.com

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