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rfc:rfc5246

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

Dierks & Rescorla Standards Track [Page 1] RFC 5246 TLS August 2008

         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

Dierks & Rescorla Standards Track [Page 2] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 3] 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:
  1. 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.
  1. 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:
  1. 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.
  1. 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.
  1. The negotiation is reliable: no attacker can modify the

negotiation communication without being detected by the parties to

    the communication.

Dierks & Rescorla Standards Track [Page 4] RFC 5246 TLS August 2008

 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:
  1. 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.
  1. 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.
  1. 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.
  1. Addition of support for authenticated encryption with additional

data modes.

  1. TLS Extensions definition and AES Cipher Suites were merged in

from external [TLSEXT] and [TLSAES].

  1. Tighter checking of EncryptedPreMasterSecret version numbers.
  1. Tightened up a number of requirements.
  1. Verify_data length now depends on the cipher suite (default is

still 12).

  1. Cleaned up description of Bleichenbacher/Klima attack defenses.

Dierks & Rescorla Standards Track [Page 5] RFC 5246 TLS August 2008

  1. Alerts MUST now be sent in many cases.
  1. After a certificate_request, if no certificates are available,

clients now MUST send an empty certificate list.

  1. TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement

cipher suite.

  1. Added HMAC-SHA256 cipher suites.
  1. Removed IDEA and DES cipher suites. They are now deprecated and

will be documented in a separate document.

  1. 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.
  1. Added limited "fall-through" to the presentation language to allow

multiple case arms to have the same encoding.

  1. Added an Implementation Pitfalls sections
  1. 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.

Dierks & Rescorla Standards Track [Page 6] 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.

Dierks & Rescorla Standards Track [Page 7] RFC 5246 TLS August 2008

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

Dierks & Rescorla Standards Track [Page 8] 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;

Dierks & Rescorla Standards Track [Page 9] 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

Dierks & Rescorla Standards Track [Page 10] 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;

Dierks & Rescorla Standards Track [Page 11] RFC 5246 TLS August 2008

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
    }

Dierks & Rescorla Standards Track [Page 12] RFC 5246 TLS August 2008

 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

Dierks & Rescorla Standards Track [Page 13] 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:

Dierks & Rescorla Standards Track [Page 14] 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.

Dierks & Rescorla Standards Track [Page 15] RFC 5246 TLS August 2008

 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).

Dierks & Rescorla Standards Track [Page 16] RFC 5246 TLS August 2008

 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. */

Dierks & Rescorla Standards Track [Page 17] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 18] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 19] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 20] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 21] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 22] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 23] RFC 5246 TLS August 2008

 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

Dierks & Rescorla Standards Track [Page 24] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 25] RFC 5246 TLS August 2008

 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:

Dierks & Rescorla Standards Track [Page 26] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 27] RFC 5246 TLS August 2008

 (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),

Dierks & Rescorla Standards Track [Page 28] RFC 5246 TLS August 2008

        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

Dierks & Rescorla Standards Track [Page 29] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 30] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 31] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 32] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 33] RFC 5246 TLS August 2008

 The TLS Handshake Protocol involves the following steps:
  1. Exchange hello messages to agree on algorithms, exchange random

values, and check for session resumption.

  1. Exchange the necessary cryptographic parameters to allow the

client and server to agree on a premaster secret.

  1. Exchange certificates and cryptographic information to allow the

client and server to authenticate themselves.

  1. Generate a master secret from the premaster secret and exchanged

random values.

  1. Provide security parameters to the record layer.
  1. 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.

Dierks & Rescorla Standards Track [Page 34] RFC 5246 TLS August 2008

 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).

Dierks & Rescorla Standards Track [Page 35] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 36] RFC 5246 TLS August 2008

    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;

Dierks & Rescorla Standards Track [Page 37] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 38] RFC 5246 TLS August 2008

    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,

Dierks & Rescorla Standards Track [Page 39] RFC 5246 TLS August 2008

 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;

Dierks & Rescorla Standards Track [Page 40] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 41] RFC 5246 TLS August 2008

    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;

Dierks & Rescorla Standards Track [Page 42] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 43] RFC 5246 TLS August 2008

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:
  1. "extension_type" identifies the particular extension type.
  1. "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.

Dierks & Rescorla Standards Track [Page 44] RFC 5246 TLS August 2008

 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:
  1. 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.
  1. Extensions should, as far as possible, be designed to prevent any

attack that forces use (or non-use) of a particular feature by

    manipulation of handshake messages.  This principle should be
    followed regardless of whether the feature is believed to cause a
    security problem.
    Often the fact that the extension fields are included in the
    inputs to the Finished message hashes will be sufficient, but
    extreme care is needed when the extension changes the meaning of
    messages sent in the handshake phase.  Designers and implementors
    should be aware of the fact that until the handshake has been
    authenticated, active attackers can modify messages and insert,
    remove, or replace extensions.
  1. 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.

Dierks & Rescorla Standards Track [Page 45] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 46] RFC 5246 TLS August 2008

 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:
  1. 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}.
  1. If the negotiated key exchange algorithm is one of (DHE_DSS,

DH_DSS), behave as if the client had sent the value {sha1,dsa}.

  1. 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.

Dierks & Rescorla Standards Track [Page 47] RFC 5246 TLS August 2008

 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:
  1. The certificate type MUST be X.509v3, unless explicitly negotiated

otherwise (e.g., [TLSPGP]).

  1. 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].

Dierks & Rescorla Standards Track [Page 48] RFC 5246 TLS August 2008

    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].
  1. 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.

Dierks & Rescorla Standards Track [Page 49] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 50] RFC 5246 TLS August 2008

 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).

Dierks & Rescorla Standards Track [Page 51] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 52] RFC 5246 TLS August 2008

 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

Dierks & Rescorla Standards Track [Page 53] RFC 5246 TLS August 2008

 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:
  1. Any certificates provided by the client MUST be signed using a

hash/signature algorithm pair found in

    supported_signature_algorithms.
  1. 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.
  1. 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.

Dierks & Rescorla Standards Track [Page 54] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 55] RFC 5246 TLS August 2008

 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:
  1. The certificate type MUST be X.509v3, unless explicitly negotiated

otherwise (e.g., [TLSPGP]).

  1. 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.
  1. 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.

Dierks & Rescorla Standards Track [Page 56] RFC 5246 TLS August 2008

  1. 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.

Dierks & Rescorla Standards Track [Page 57] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 58] RFC 5246 TLS August 2008

 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:

Dierks & Rescorla Standards Track [Page 59] RFC 5246 TLS August 2008

    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

Dierks & Rescorla Standards Track [Page 60] RFC 5246 TLS August 2008

 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).

Dierks & Rescorla Standards Track [Page 61] RFC 5246 TLS August 2008

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.

Dierks & Rescorla Standards Track [Page 62] RFC 5246 TLS August 2008

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.

Dierks & Rescorla Standards Track [Page 63] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 64] RFC 5246 TLS August 2008

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.

Dierks & Rescorla Standards Track [Page 65] RFC 5246 TLS August 2008

  1. 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].
  1. 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].
  1. 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.
  1. TLS ContentType Registry: Future values are allocated via

Standards Action [RFC2434].

  1. TLS Alert Registry: Future values are allocated via Standards

Action [RFC2434].

  1. 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:
  1. 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:
  1. 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].

Dierks & Rescorla Standards Track [Page 66] RFC 5246 TLS August 2008

  1. 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.

Dierks & Rescorla Standards Track [Page 67] RFC 5246 TLS August 2008

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),

Dierks & Rescorla Standards Track [Page 69] RFC 5246 TLS August 2008

     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;

Dierks & Rescorla Standards Track [Page 70] RFC 5246 TLS August 2008

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;

Dierks & Rescorla Standards Track [Page 71] RFC 5246 TLS August 2008

 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 */

Dierks & Rescorla Standards Track [Page 72] RFC 5246 TLS August 2008

 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;

Dierks & Rescorla Standards Track [Page 73] RFC 5246 TLS August 2008

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.

Dierks & Rescorla Standards Track [Page 75] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 76] RFC 5246 TLS August 2008

 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;

Dierks & Rescorla Standards Track [Page 77] RFC 5246 TLS August 2008

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.

Dierks & Rescorla Standards Track [Page 78] RFC 5246 TLS August 2008

 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

Dierks & Rescorla Standards Track [Page 79] RFC 5246 TLS August 2008

    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.

Dierks & Rescorla Standards Track [Page 80] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 81] RFC 5246 TLS August 2008

 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.

Dierks & Rescorla Standards Track [Page 84] RFC 5246 TLS August 2008

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:
  1. 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.
  1. Do you ignore the TLS record layer version number in all TLS

records before ServerHello (see Appendix E.1)?

  1. Do you handle TLS extensions in ClientHello correctly, including

omitting the extensions field completely?

  1. Do you support renegotiation, both client and server initiated?

While renegotiation is an optional feature, supporting it is

    highly recommended.
  1. When the server has requested a client certificate, but no

suitable certificate is available, do you correctly send an empty

    Certificate message, instead of omitting the whole message (see
    Section 7.4.6)?
 Cryptographic details:
  1. 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)?
  1. What countermeasures do you use to prevent timing attacks against

RSA decryption and signing operations (see Section 7.4.7.1)?

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

leading zero bytes from the negotiated key (see Section 8.1.2)?

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

by the server are acceptable (see Section F.1.1.3)?

Dierks & Rescorla Standards Track [Page 86] RFC 5246 TLS August 2008

  1. How do you generate unpredictable IVs for CBC mode ciphers (see

Section 6.2.3.2)?

  1. Do you accept long CBC mode padding (up to 255 bytes; see Section

6.2.3.2)?

  1. How do you address CBC mode timing attacks (Section 6.2.3.2)?
  1. 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].

Dierks & Rescorla Standards Track [Page 90] 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

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Dierks & Rescorla Standards Track [Page 104]

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