    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.

    Table of Contents

       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

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               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

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          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

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    RFC 5246                          TLS                        August 2008

    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.

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       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.

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       -  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.

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    RFC 5246                          TLS                        August 2008

    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.

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    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

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    RFC 5246                          TLS                        August 2008

       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;

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    RFC 5246                          TLS                        August 2008

       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

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    RFC 5246                          TLS                        August 2008

       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;

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    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
          }

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       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

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    RFC 5246                          TLS                        August 2008

       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:

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    RFC 5246                          TLS                        August 2008

          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.

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       Implementations MUST NOT send record types not defined in this
       document unless negotiated by some extension.  If a TLS
       implementation receives an unexpected record type, it MUST 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).

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       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. */

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          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.

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       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.

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       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.

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          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.

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       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.

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          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

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       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

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       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.

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       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:

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       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.

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       (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),

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              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

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       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.

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       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.

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       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.

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       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.

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       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.

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       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).

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          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.

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          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;

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       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.

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          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,

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       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;

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          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

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          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;

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       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.

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    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.

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       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.

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          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

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       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.

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       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].

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          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.

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       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.

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       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).

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          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

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       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

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       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.

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       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.

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       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.

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       -  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.

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          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.

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       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:

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          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

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       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).

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    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.

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    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.

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          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.

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    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.

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       -  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].

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       -  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.

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    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;

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       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),

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           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;

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    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;

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       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 */

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       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;

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    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;

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    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.

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          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.

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       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;

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    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.

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       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

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          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.

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       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.

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       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).

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    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

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                            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.

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    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

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       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)?

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       -  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

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       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.

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       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.

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       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].

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    RFC 5246                          TLS                        August 2008

    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

    Dierks & Rescorla           Standards Track                    [Page 91]

    RFC 5246                          TLS                        August 2008

       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.,

    Dierks & Rescorla           Standards Track                    [Page 92]

    RFC 5246                          TLS                        August 2008

       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

    Dierks & Rescorla           Standards Track                    [Page 93]

    RFC 5246                          TLS                        August 2008

       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

    Dierks & Rescorla           Standards Track                    [Page 94]

    RFC 5246                          TLS                        August 2008

       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.

    Dierks & Rescorla           Standards Track                    [Page 95]

    RFC 5246                          TLS                        August 2008

       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.

    Dierks & Rescorla           Standards Track                    [Page 96]

    RFC 5246                          TLS                        August 2008

       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.

    Dierks & Rescorla           Standards Track                    [Page 97]

    RFC 5246                          TLS                        August 2008

       [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.

    Dierks & Rescorla           Standards Track                    [Page 98]

    RFC 5246                          TLS                        August 2008

       [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.

    Dierks & Rescorla           Standards Track                    [Page 99]

    RFC 5246                          TLS                        August 2008

       [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.

    Dierks & Rescorla           Standards Track                   [Page 100]

    RFC 5246                          TLS                        August 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

    Dierks & Rescorla           Standards Track                   [Page 101]

    RFC 5246                          TLS                        August 2008

       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

    Dierks & Rescorla           Standards Track                   [Page 102]

    RFC 5246                          TLS                        August 2008

       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

    Dierks & Rescorla           Standards Track                   [Page 103]

    RFC 5246                          TLS                        August 2008

    Full Copyright Statement

       Copyright (C) The IETF Trust (2008).

       This document is subject to the rights, licenses and restrictions
       contained in BCP 78, and except as set forth therein, the authors
       retain all their rights.

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    Intellectual Property

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    Dierks & Rescorla           Standards Track                   [Page 104]