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rfc:rfc4346

Network Working Group T. Dierks Request for Comments: 4346 Independent Obsoletes: 2246 E. Rescorla Category: Standards Track RTFM, Inc.

                                                            April 2006
            The Transport Layer Security (TLS) Protocol
                            Version 1.1

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.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document specifies Version 1.1 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.

Dierks & Rescorla Standards Track [Page 1] RFC 4346 The TLS Protocol April 2006

Table of Contents

 1. Introduction ....................................................4
    1.1. Differences from TLS 1.0 ...................................5
    1.2. Requirements Terminology ...................................5
 2. Goals ...........................................................5
 3. Goals of This Document ..........................................6
 4. Presentation Language ...........................................6
    4.1. Basic Block Size ...........................................7
    4.2. Miscellaneous ..............................................7
    4.3. Vectors ....................................................7
    4.4. Numbers ....................................................8
    4.5. Enumerateds ................................................8
    4.6. Constructed Types ..........................................9
         4.6.1. Variants ...........................................10
    4.7. Cryptographic Attributes ..................................11
    4.8. Constants .................................................12
 5. HMAC and the Pseudorandom Function .............................12
 6. The TLS Record Protocol ........................................14
    6.1. Connection States .........................................15
    6.2. Record layer ..............................................17
         6.2.1. Fragmentation ......................................17
         6.2.2. Record Compression and Decompression ...............19
         6.2.3. Record Payload Protection ..........................19
                6.2.3.1. Null or Standard Stream Cipher ............20
                6.2.3.2. CBC Block Cipher ..........................21
    6.3. Key Calculation ...........................................24
 7. The TLS Handshaking Protocols ..................................24
    7.1. Change Cipher Spec Protocol ...............................25
    7.2. Alert Protocol ............................................26
         7.2.1. Closure Alerts .....................................27
         7.2.2. Error Alerts .......................................28
    7.3. Handshake Protocol Overview ...............................31
    7.4. Handshake Protocol ........................................34
         7.4.1. Hello Messages .....................................35
                7.4.1.1. Hello request .............................35
                7.4.1.2. Client Hello ..............................36
                7.4.1.3. Server Hello ..............................39
         7.4.2. Server Certificate .................................40
         7.4.3. Server Key Exchange Message ........................42
         7.4.4. Certificate request ................................44
         7.4.5. Server Hello Done ..................................46
         7.4.6. Client certificate .................................46
         7.4.7. Client Key Exchange Message ........................47
                7.4.7.1. RSA Encrypted Premaster Secret Message ....47
                7.4.7.2. Client Diffie-Hellman Public Value ........50
         7.4.8. Certificate verify .................................50
         7.4.9. Finished ...........................................51

Dierks & Rescorla Standards Track [Page 2] RFC 4346 The TLS Protocol April 2006

 8. Cryptographic Computations .....................................52
    8.1. Computing the Master Secret ...............................52
         8.1.1. RSA ................................................53
         8.1.2. Diffie-Hellman .....................................53
 9. Mandatory Cipher Suites ........................................53
 10. Application Data Protocol .....................................53
 11. Security Considerations .......................................53
 12. IANA Considerations ...........................................54
 A. Appendix - Protocol constant values ............................55
         A.1. Record layer .........................................55
         A.2. Change cipher specs message ..........................56
         A.3. Alert messages .......................................56
         A.4. Handshake protocol ...................................57
         A.4.1. Hello messages .....................................57
         A.4.2. Server authentication and key exchange messages ....58
         A.4.3. Client authentication and key exchange messages ....59
         A.4.4.Handshake finalization message ......................60
         A.5. The CipherSuite ......................................60
         A.6. The Security Parameters ..............................63
 B. Appendix - Glossary ............................................64
 C. Appendix - CipherSuite definitions .............................68
 D. Appendix - Implementation Notes ................................69
         D.1 Random Number Generation and Seeding ..................70
         D.2 Certificates and authentication .......................70
         D.3 CipherSuites ..........................................70
 E. Appendix - Backward Compatibility With SSL .....................71
         E.1. Version 2 client hello ...............................72
         E.2. Avoiding man-in-the-middle version rollback ..........74
 F. Appendix - Security analysis ...................................74
         F.1. Handshake protocol ...................................74
         F.1.1. Authentication and key exchange ....................74
         F.1.1.1. Anonymous key exchange ...........................75
         F.1.1.2. RSA key exchange and authentication ..............75
         F.1.1.3. Diffie-Hellman key exchange with authentication ..76
         F.1.2. Version rollback attacks ...........................77
         F.1.3. Detecting attacks against the handshake protocol ...77
         F.1.4. Resuming sessions ..................................78
         F.1.5. MD5 and SHA ........................................78
         F.2. Protecting application data ..........................78
         F.3. Explicit IVs .........................................79
         F.4  Security of Composite Cipher Modes ...................79
         F.5  Denial of Service ....................................80
         F.6. Final notes ..........................................80
 Normative References ..............................................81
 Informative References ............................................82

Dierks & Rescorla Standards Track [Page 3] RFC 4346 The TLS Protocol April 2006

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., DES [DES], 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, MD5, 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], DSS [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.
 One advantage of TLS is that it is application protocol independent.
 Higher level protocols can layer on top of the TLS Protocol

Dierks & Rescorla Standards Track [Page 4] RFC 4346 The TLS Protocol April 2006

 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. Differences from TLS 1.0

 This document is a revision of the TLS 1.0 [TLS1.0] protocol, and
 contains some small security improvements, clarifications, and
 editorial improvements.  The major changes are:
  1. The implicit Initialization Vector (IV) is replaced with an

explicit IV to protect against CBC attacks [CBCATT].

  1. Handling of padding errors is changed to use the bad_record_mac

alert rather than the decryption_failed alert to protect against

    CBC attacks.
  1. IANA registries are defined for protocol parameters.
  1. Premature closes no longer cause a session to be nonresumable.
  1. Additional informational notes were added for various new attacks

on TLS.

 In addition, a number of minor clarifications and editorial
 improvements were made.

1.2. Requirements Terminology

 In this document, the keywords "MUST", "MUST NOT", "REQUIRED",
 "SHOULD", "SHOULD NOT" and "MAY" are to be interpreted as described
 in RFC 2119 [REQ].

2. Goals

 The goals of TLS Protocol, in order of their 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.

Dierks & Rescorla Standards Track [Page 5] RFC 4346 The TLS Protocol April 2006

 3. Extensibility: TLS seeks to provide a framework into which new
    public key and bulk encryption methods can be incorporated as
    necessary.  This will also accomplish two sub-goals: preventing
    the need to create a new protocol (and risking the introduction of
    possible new weaknesses) and avoiding the need to implement an
    entire new security library.
 4. Relative efficiency: Cryptographic operations tend to be highly
    CPU intensive, particularly public key operations.  For this
    reason, the TLS protocol has incorporated an optional session
    caching scheme to reduce the number of connections that need to be
    established from scratch.  Additionally, care has been taken to
    reduce network activity.

3. Goals of This Document

 This document and the TLS protocol itself are based on the SSL 3.0
 Protocol Specification as published by Netscape.  The differences
 between this protocol and SSL 3.0 are not dramatic, but they are
 significant enough that TLS 1.1, TLS 1.0, and SSL 3.0 do not
 interoperate (although each protocol incorporates a mechanism by
 which an implementation can back down 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.

Dierks & Rescorla Standards Track [Page 6] RFC 4346 The TLS Protocol April 2006

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 bytestream, a multi-byte item (a numeric in the
 example) is formed (using C notation) by:
     value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
             ... | byte[n-1];
 This byte ordering for multi-byte values is the commonplace network
 byte order or big endian format.

4.2. Miscellaneous

 Comments begin with "/*" and end with "*/".
 Optional components are denoted by enclosing them in "[[ ]]" double
 brackets.
 Single-byte entities containing uninterpreted data are of type
 opaque.

4.3. Vectors

 A vector (single dimensioned array) is a stream of homogeneous data
 elements.  The size of the vector may be specified at documentation
 time or left unspecified until runtime.  In either case, the length
 declares the number of bytes, not the number of elements, in the
 vector.  The syntax for specifying a new type, T', that is a fixed-
 length vector of type T is
     T T'[n];
 Here, T' occupies n bytes in the data stream, where n is a multiple
 of the size of T.  The length of the vector is not included in the
 encoded stream.
 In the following example, Datum is defined to be three consecutive
 bytes that the protocol does not interpret, while Data is three
 consecutive Datum, consuming a total of nine bytes.
     opaque Datum[3];      /* three uninterpreted bytes */
     Datum Data[9];        /* 3 consecutive 3 byte vectors */
 Variable-length vectors are defined by specifying a subrange of legal
 lengths, inclusively, using the notation <floor..ceiling>.  When

Dierks & Rescorla Standards Track [Page 7] RFC 4346 The TLS Protocol April 2006

 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, sufficient to
 represent the value 400 (see Section 4.4).  On the other hand, longer
 can represent up to 800 bytes of data, or 400 uint16 elements, and it
 may be empty.  Its encoding will include a two-byte actual length
 field prepended to the vector.  The length of an encoded vector must
 be an even multiple of the length of a single element (for example, a
 17-byte vector of uint16 would be illegal).
     opaque mandatory<300..400>;
           /* length field is 2 bytes, cannot be empty */
     uint16 longer<0..800>;
           /* zero to 400 16-bit unsigned integers */

4.4. Numbers

 The basic numeric data type is an unsigned byte (uint8).  All larger
 numeric data types are formed from fixed-length series of bytes
 concatenated as described in Section 4.1 and are also unsigned.  The
 following numeric types are predefined.
     uint8 uint16[2];
     uint8 uint24[3];
     uint8 uint32[4];
     uint8 uint64[8];
 All values, here and elsewhere in the specification, are stored in
 "network" or "big-endian" order; the uint32 represented by the hex
 bytes 01 02 03 04 is equivalent to the decimal value 16909060.

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.

Dierks & Rescorla Standards Track [Page 8] RFC 4346 The TLS Protocol April 2006

     enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
 Enumerateds occupy as much space in the byte stream as would its
 maximal defined ordinal value.  The following definition would cause
 one byte to be used to carry fields of type Color.
     enum { red(3), blue(5), white(7) } Color;
 One may optionally specify a value without its associated tag to
 force the width definition without defining a superfluous element.
 In the following example, Taste will consume two bytes in the data
 stream but can only assume the values 1, 2, or 4.
     enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
 The names of the elements of an enumeration are scoped within the
 defined type.  In the first example, a fully qualified reference to
 the second element of the enumeration would be Color.blue.  Such
 qualification is not required if the target of the assignment is well
 specified.
     Color color = Color.blue;     /* overspecified, legal */
     Color color = blue;           /* correct, type implicit */
 For enumerateds that are never converted to external representation,
 the numerical information may be omitted.
     enum { low, medium, high } Amount;

4.6. Constructed Types

 Structure types may be constructed from primitive types for
 convenience.  Each specification declares a new, unique type.  The
 syntax for definition is much like that of C.
     struct {
       T1 f1;
       T2 f2;
       ...
       Tn fn;
     } [[T]];
 The fields within a structure may be qualified using the type's name,
 with a syntax much like that available for enumerateds.  For example,
 T.f2 refers to the second field of the previous declaration.
 Structure definitions may be embedded.

Dierks & Rescorla Standards Track [Page 9] RFC 4346 The TLS Protocol April 2006

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.  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 en: Ten;
         } [[fv]];
     } [[Tv]];
 For example:
     enum { apple, orange } 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: V2;  /* VariantBody, tag = orange */
         } variant_body;       /* optional label on variant */
     } VariantRecord;
 Variant structures may be qualified (narrowed) by specifying a value
 for the selector prior to the type.  For example, an
     orange VariantRecord
 is a narrowed type of a VariantRecord containing a variant_body of
 type V2.

Dierks & Rescorla Standards Track [Page 10] RFC 4346 The TLS Protocol April 2006

4.7. Cryptographic Attributes

 The four cryptographic operations digital signing, stream cipher
 encryption, block cipher encryption, and public key encryption are
 designated digitally-signed, stream-ciphered, block-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).
 In digital signing, one-way hash functions are used as input for a
 signing algorithm.  A digitally-signed element is encoded as an
 opaque vector <0..2^16-1>, where the length is specified by the
 signing algorithm and key.
 In RSA signing, a 36-byte structure of two hashes (one SHA and one
 MD5) is signed (encrypted with the private key).  It is encoded with
 PKCS #1 block type 1, as described in [PKCS1A].
 Note: The standard reference for PKCS#1 is now RFC 3447 [PKCS1B].
       However, to minimize differences with TLS 1.0 text, we are
       using the terminology of RFC 2313 [PKCS1A].
 In DSS, the 20 bytes of the SHA hash are run directly through the
 Digital Signing Algorithm with no additional hashing.  This produces
 two values, r and s.  The DSS signature is an opaque vector, as
 above, the contents of which are the DER encoding of:
     Dss-Sig-Value  ::=  SEQUENCE  {
          r       INTEGER,
          s       INTEGER
     }
 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 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 signing
 algorithm and key.

Dierks & Rescorla Standards Track [Page 11] RFC 4346 The TLS Protocol April 2006

 An RSA-encrypted value is encoded with PKCS #1 block type 2, as
 described in [PKCS1A].
 In the following example,
     stream-ciphered struct {
         uint8 field1;
         uint8 field2;
         digitally-signed opaque hash[20];
     } UserType;
 the contents of hash are used as input for the signing 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 length of the signature,
 plus the length of the output of the signing algorithm.  This 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

 A number of operations in the TLS record and handshake layer require
 a keyed MAC; this is a secure digest of some data protected by a
 secret.  Forging the MAC is infeasible without knowledge of the MAC
 secret.  The construction we use for this operation is known as HMAC,
 and is described in [HMAC].
 HMAC can be used with a variety of different hash algorithms.  TLS
 uses it in the handshake with two different algorithms, MD5 and SHA-
 1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
 data).  Additional hash algorithms can be defined by cipher suites

Dierks & Rescorla Standards Track [Page 12] RFC 4346 The TLS Protocol April 2006

 and used to protect record data, but MD5 and SHA-1 are hard coded
 into the description of the handshaking for this version of the
 protocol.
 In addition, a construction is required to do expansion of secrets
 into blocks of data for the purposes of key generation or validation.
 This pseudo-random function (PRF) takes as input a secret, a seed,
 and an identifying label and produces an output of arbitrary length.
 In order to make the PRF as secure as possible, it uses two hash
 algorithms in a way that should guarantee its security if either
 algorithm remains secure.
 First, we define a data expansion function, P_hash(secret, data) that
 uses a single hash function to expand a secret and seed into an
 arbitrary quantity of output:
     P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
                            HMAC_hash(secret, A(2) + seed) +
                            HMAC_hash(secret, A(3) + seed) + ...
 Where + indicates concatenation.
 A() is defined as:
     A(0) = seed
     A(i) = HMAC_hash(secret, A(i-1))
 P_hash can be iterated as many times as is necessary to produce the
 required quantity of data.  For example, if P_SHA-1 is being used to
 create 64 bytes of data, it will have to be iterated 4 times (through
 A(4)), creating 80 bytes of output data; the last 16 bytes of the
 final iteration will then be discarded, leaving 64 bytes of output
 data.
 TLS's PRF is created by splitting the secret into two halves and
 using one half to generate data with P_MD5 and the other half to
 generate data with P_SHA-1, then exclusive-ORing the outputs of these
 two expansion functions together.
 S1 and S2 are the two halves of the secret, and each is the same
 length.  S1 is taken from the first half of the secret, S2 from the
 second half.  Their length is created by rounding up the length of
 the overall secret, divided by two; thus, if the original secret is
 an odd number of bytes long, the last byte of S1 will be the same as
 the first byte of S2.

Dierks & Rescorla Standards Track [Page 13] RFC 4346 The TLS Protocol April 2006

     L_S = length in bytes of secret;
     L_S1 = L_S2 = ceil(L_S / 2);
 The secret is partitioned into two halves (with the possibility of
 one shared byte) as described above, S1 taking the first L_S1 bytes,
 and S2 the last L_S2 bytes.
 The PRF is then defined as the result of mixing the two pseudorandom
 streams by exclusive-ORing them together.
     PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
                                P_SHA-1(S2, 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
 Note that because MD5 produces 16-byte outputs and SHA-1 produces
 20-byte outputs, the boundaries of their internal iterations will not
 be aligned.  Generating an 80-byte output will require that P_MD5
 iterate through A(5), while P_SHA-1 will only iterate through A(4).

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 record protocol clients 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 types can be
 supported by the record protocol.  Any new record types SHOULD
 allocate type values immediately beyond the ContentType values for
 the four record types described here (see Appendix A.1).  All such
 values must be defined by RFC 2434 Standards Action.  See Section 11
 for IANA Considerations for ContentType values.
 If a TLS implementation receives a record type it does not
 understand, it SHOULD just ignore it.  Any protocol designed for use

Dierks & Rescorla Standards Track [Page 14] RFC 4346 The TLS Protocol April 2006

 over TLS MUST be carefully designed to deal with all possible attacks
 against it.  Note that because the type and length of a record are
 not protected by encryption, care SHOULD be taken to minimize the
 value of traffic analysis of these values.

6.1. Connection States

 A TLS connection state is the operating environment of the TLS Record
 Protocol.  It specifies a compression algorithm, and encryption
 algorithm, and a MAC algorithm.  In addition, the parameters for
 these algorithms are known: the MAC secret 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 Change Cipher Spec 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.
 bulk encryption algorithm
    An algorithm to be used for bulk encryption.  This specification
    includes the key size of this algorithm, how much of that key is
    secret, whether it is a block or stream cipher, and the block size
    of the cipher (if appropriate).
 MAC algorithm
    An algorithm to be used for message authentication.  This
    specification includes the size of the hash returned by the MAC
    algorithm.
 compression algorithm
    An algorithm to be used for data compression.  This specification
    must include all information the algorithm requires compression.
 master secret
    A 48-byte secret shared between the two peers in the connection.

Dierks & Rescorla Standards Track [Page 15] RFC 4346 The TLS Protocol April 2006

 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 { null, rc4, rc2, des, 3des, des40, idea, aes }
     BulkCipherAlgorithm;
     enum { stream, block } CipherType;
     enum { null, md5, sha } MACAlgorithm;
     enum { null(0), (255) } CompressionMethod;
     /* The algorithms specified in CompressionMethod,
        BulkCipherAlgorithm, and MACAlgorithm may be added to. */
     struct {
         ConnectionEnd          entity;
         BulkCipherAlgorithm    bulk_cipher_algorithm;
         CipherType             cipher_type;
         uint8                  key_size;
         uint8                  key_material_length;
         MACAlgorithm           mac_algorithm;
         uint8                  hash_size;
         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 four items:
     client write MAC secret
     server write MAC secret
     client write key
     server write key
 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.

Dierks & Rescorla Standards Track [Page 16] RFC 4346 The TLS Protocol April 2006

 Once the security parameters have been set and the keys have been
 generated, the connection states can be instantiated by making them
 the current states.  These current states MUST be updated for each
 record processed.  Each connection state includes the following
 elements:
 compression state
    The current state of the compression algorithm.
 cipher state
    The current state of the encryption algorithm.  This will consist
    of the scheduled key for that connection.  For stream ciphers,
    this will also contain whatever state information is necessary to
    allow the stream to continue to encrypt or decrypt data.
 MAC secret
    The MAC secret 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).

Dierks & Rescorla Standards Track [Page 17] RFC 4346 The TLS Protocol April 2006

     struct {
         uint8 major, minor;
     } ProtocolVersion;
     enum {
         change_cipher_spec(20), alert(21), handshake(22),
         application_data(23), (255)
     } ContentType;
     struct {
         ContentType type;
         ProtocolVersion version;
         uint16 length;
         opaque fragment[TLSPlaintext.length];
     } TLSPlaintext;
 type
    The higher-level protocol used to process the enclosed fragment.
 version
    The version of the protocol being employed.  This document
    describes TLS Version 1.1, which uses the version { 3, 2 }.  The
    version value 3.2 is historical: TLS version 1.1 is a minor
    modification to the TLS 1.0 protocol, which was itself a minor
    modification to the SSL 3.0 protocol, which bears the version
    value 3.0.  (See Appendix A.1.)
 length
    The length (in bytes) of the following TLSPlaintext.fragment.  The
    length should not exceed 2^14.
 fragment
    The application data.  This data is transparent and is treated as
    an independent block to be dealt with by the higher-level protocol
    specified by the type field.
 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.

Dierks & Rescorla Standards Track [Page 18] RFC 4346 The TLS Protocol April 2006

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.
 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 should report a fatal decompression failure error.
     struct {
         ContentType type;       /* same as TLSPlaintext.type */
         ProtocolVersion version;/* same as TLSPlaintext.version */
         uint16 length;
         opaque fragment[TLSCompressed.length];
     } TLSCompressed;
 length
    The length (in bytes) of the following TLSCompressed.fragment.
    The length should 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.

Dierks & Rescorla Standards Track [Page 19] RFC 4346 The TLS Protocol April 2006

     struct {
         ContentType type;
         ProtocolVersion version;
         uint16 length;
         select (CipherSpec.cipher_type) {
             case stream: GenericStreamCipher;
             case block: GenericBlockCipher;
         } 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 may not exceed 2^14 + 2048.
 fragment
    The encrypted form of TLSCompressed.fragment, with the MAC.

6.2.3.1. Null or Standard Stream Cipher

 Stream ciphers (including BulkCipherAlgorithm.null, see Appendix A.6)
 convert TLSCompressed.fragment structures to and from stream
 TLSCiphertext.fragment structures.
     stream-ciphered struct {
         opaque content[TLSCompressed.length];
         opaque MAC[CipherSpec.hash_size];
     } GenericStreamCipher;
 The MAC is generated as:
     HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type +
                   TLSCompressed.version + TLSCompressed.length +
                   TLSCompressed.fragment));
 where "+" denotes concatenation.
 seq_num
    The sequence number for this record.
 hash
    The hashing algorithm specified by
    SecurityParameters.mac_algorithm.

Dierks & Rescorla Standards Track [Page 20] RFC 4346 The TLS Protocol April 2006

 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 CipherSuite 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).
 TLSCiphertext.length is TLSCompressed.length plus
 CipherSpec.hash_size.

6.2.3.2. CBC Block Cipher

 For block ciphers (such as RC2, DES, or AES), the encryption and MAC
 functions convert TLSCompressed.fragment structures to and from block
 TLSCiphertext.fragment structures.
     block-ciphered struct {
         opaque IV[CipherSpec.block_length];
         opaque content[TLSCompressed.length];
         opaque MAC[CipherSpec.hash_size];
         uint8 padding[GenericBlockCipher.padding_length];
         uint8 padding_length;
     } GenericBlockCipher;
 The MAC is generated as described in Section 6.2.3.1.
 IV
    Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
    IV in order to prevent the attacks described by [CBCATT].  We
    recommend the following equivalently strong procedures.  For
    clarity we use the following notation.
    IV
       The transmitted value of the IV field in the GenericBlockCipher
       structure.
    CBC residue
       The last ciphertext block of the previous record.
    mask
       The actual value that the cipher XORs with the plaintext prior
       to encryption of the first cipher block of the record.
    In prior versions of TLS, there was no IV field and the CBC
    residue and mask were one and the same.  See Sections 6.1,
    6.2.3.2, and 6.3, of [TLS1.0] for details of TLS 1.0 IV handling.

Dierks & Rescorla Standards Track [Page 21] RFC 4346 The TLS Protocol April 2006

    One of the following two algorithms SHOULD be used to generate the
    per-record IV:
    (1) Generate a cryptographically strong random string R of length
        CipherSpec.block_length.  Place R in the IV field.  Set the
        mask to R.  Thus, the first cipher block will be encrypted as
        E(R XOR Data).
    (2) Generate a cryptographically strong random number R of length
        CipherSpec.block_length and prepend it to the plaintext prior
        to encryption.  In this case either:
        (a) The cipher may use a fixed mask such as zero.
        (b) The CBC residue from the previous record may be used as
            the mask.  This preserves maximum code compatibility with
            TLS 1.0 and SSL 3.  It also has the advantage that it does
            not require the ability to quickly reset the IV, which is
            known to be a problem on some systems.
        In either (2)(a) or (2)(b) the data (R || data) is fed into
        the encryption process.  The first cipher block (containing
        E(mask XOR R) is placed in the IV field.  The first block of
        content contains E(IV XOR data).
    The following alternative procedure MAY be used; however, it has
    not been demonstrated to be as cryptographically strong as the
    above procedures.  The sender prepends a fixed block F to the
    plaintext (or, alternatively, a block generated with a weak PRNG).
    He then encrypts as in (2), above, using the CBC residue from the
    previous block as the mask for the prepended block.  Note that in
    this case the mask for the first record transmitted by the
    application (the Finished) MUST be generated using a
    cryptographically strong PRNG.
    The decryption operation for all three alternatives is the same.
    The receiver decrypts the entire GenericBlockCipher structure and
    then discards the first cipher block, corresponding to the IV
    component.
 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

Dierks & Rescorla Standards Track [Page 22] RFC 4346 The TLS Protocol April 2006

    MUST check this padding and SHOULD 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 CipherSpec.block_length, TLSCompressed.length,
 CipherSpec.hash_size, and padding_length.
 Example: If the block length is 8 bytes, the content length
          (TLSCompressed.length) is 61 bytes, and the MAC length is 20
          bytes, then the length before padding is 82 bytes (this does
          not include the IV, which may or may not be encrypted, as
          discussed above).  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,

Dierks & Rescorla Standards Track [Page 23] RFC 4346 The TLS Protocol April 2006

                      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.

6.3. Key Calculation

 The Record Protocol requires an algorithm to generate keys, and MAC
 secrets from the security parameters provided by the handshake
 protocol.
 The master secret is hashed into a sequence of secure bytes, which
 are assigned to the MAC secrets and keys required by the current
 connection state (see Appendix A.6).  CipherSpecs require a client
 write MAC secret, a server write MAC secret, a client write key, and
 a server write key, each of which is generated from the master secret
 in that order.  Unused values are empty.
 When keys and MAC secrets 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_secret[SecurityParameters.hash_size]
     server_write_MAC_secret[SecurityParameters.hash_size]
     client_write_key[SecurityParameters.key_material_length]
     server_write_key[SecurityParameters.key_material_length]
 Implementation note: The currently defined cipher suite that requires
 the most material is AES_256_CBC_SHA, defined in [TLSAES].  It
 requires 2 x 32 byte keys, 2 x 20 byte MAC secrets, and 2 x 16 byte
 Initialization Vectors, for a total of 136 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.

Dierks & Rescorla Standards Track [Page 24] RFC 4346 The TLS Protocol April 2006

 The Handshake Protocol is responsible for negotiating a session,
 which consists of the following items:
 session identifier
    An arbitrary byte sequence chosen by the server to identify an
    active or resumable session state.
 peer certificate
    X509v3 [X509] 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 bulk data encryption algorithm (such as null, DES,
    etc.) and a MAC algorithm (such as MD5 or SHA).  It also defines
    cryptographic attributes such as the hash_size.  (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 change cipher spec 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.

Dierks & Rescorla Standards Track [Page 25] RFC 4346 The TLS Protocol April 2006

 Immediately after sending this message, the sender MUST instruct the
 record layer to make the write pending state the write active state.
 (See Section 6.1.)  The change cipher spec message is sent during the
 handshake after the security parameters have been agreed upon, but
 before the verifying finished message is sent (see Section 7.4.9).
 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 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(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),

Dierks & Rescorla Standards Track [Page 26] RFC 4346 The TLS Protocol April 2006

               access_denied(49),
               decode_error(50),
               decrypt_error(51),
               export_restriction_RESERVED(60),
               protocol_version(70),
               insufficient_security(71),
               internal_error(80),
               user_canceled(90),
               no_renegotiation(100),
               (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

Dierks & Rescorla Standards Track [Page 27] RFC 4346 The TLS Protocol April 2006

 transport without waiting for the responding close_notify.  No part
 of this standard should be taken to dictate the manner in which a
 usage profile for TLS manages its data transport, including when
 connections are opened or closed.
 Note: It is assumed that closing a connection reliably delivers
       pending data before destroying the transport.

7.2.2. Error Alerts

 Error handling in the TLS Handshake protocol is very simple.  When an
 error is detected, the detecting party sends a message to the other
 party.  Upon transmission or receipt of a fatal alert message, both
 parties immediately close the connection.  Servers and clients MUST
 forget any session-identifiers, keys, and secrets associated with a
 failed connection.  Thus, any connection terminated with a fatal
 alert MUST NOT be resumed.  The following error alerts are defined:
 unexpected_message
    An inappropriate message was received.  This alert is always fatal
    and should never be observed in communication between proper
    implementations.
 bad_record_mac
    This alert is returned if a record is received with an incorrect
    MAC.  This alert also MUST be returned if an alert is sent because
    a TLSCiphertext decrypted in an invalid way: either it wasn't an
    even multiple of the block length, or its padding values, when
    checked, weren't correct.  This message is always fatal.
 decryption_failed
    This alert MAY be returned if 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.
 Note: Differentiating between bad_record_mac and decryption_failed
       alerts may permit certain attacks against CBC mode as used in
       TLS [CBCATT].  It is preferable to uniformly use the
       bad_record_mac alert to hide the specific type of the error.
 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.

Dierks & Rescorla Standards Track [Page 28] RFC 4346 The TLS Protocol April 2006

 decompression_failure
       The decompression function received improper input (e.g., data
       that would expand to excessive length).  This message is always
       fatal.
 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 in TLS.  It should not be
       sent by compliant implementations.
 bad_certificate
       A certificate was corrupt, contained signatures that did not
       verify correctly, etc.
 unsupported_certificate
       A certificate was of an unsupported type.
 certificate_revoked
       A certificate was revoked by its signer.
 certificate_expired
       A certificate has expired or is not currently valid.
 certificate_unknown
       Some other (unspecified) issue arose in processing the
       certificate, rendering it unacceptable.
 illegal_parameter
       A field in the handshake was out of range or inconsistent with
       other fields.  This 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.

Dierks & Rescorla Standards Track [Page 29] RFC 4346 The TLS Protocol April 2006

 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.
 decrypt_error
       A handshake cryptographic operation failed, including being
       unable to correctly verify a signature, decrypt a key exchange,
       or validate a finished message.
 export_restriction_RESERVED
       This alert was used in TLS 1.0 but not TLS 1.1.
 protocol_version
       The protocol version the client has attempted to negotiate is
       recognized but not supported.  (For example, old protocol
       versions might be avoided for security reasons).  This message
       is always fatal.
 insufficient_security
       Returned instead of handshake_failure when a negotiation has
       failed specifically because the server requires ciphers more
       secure than those supported by the client.  This message is
       always fatal.
 internal_error
       An internal error unrelated to the peer or the correctness of
       the protocol (such as a memory allocation failure) makes it
       impossible to continue.  This message is always fatal.
 user_canceled
       This handshake is being canceled for some reason unrelated to a
       protocol failure.  If the user cancels an operation after the
       handshake is complete, just closing the connection by sending a
       close_notify is more appropriate.  This alert should be
       followed by a close_notify.  This message is generally a
       warning.
 no_renegotiation
       Sent by the client in response to a hello request or by the
       server in response to a client hello after initial handshaking.
       Either of these would normally lead to renegotiation; when that
       is not appropriate, the recipient should respond with this
       alert.  At that point, the original requester can decide
       whether to proceed with the connection.  One case where this
       would be appropriate is where a server has spawned a process to
       satisfy a request; the process might receive security
       parameters (key length, authentication, etc.) at startup and it

Dierks & Rescorla Standards Track [Page 30] RFC 4346 The TLS Protocol April 2006

       might be difficult to communicate changes to these parameters
       after that point.  This message is always a warning.
 For all errors where an alert level is not explicitly specified, the
 sending party MAY determine at its discretion whether this is a fatal
 error or not; if an alert with a level of warning is received, the
 receiving party MAY decide at its discretion whether to treat this as
 a fatal error or not.  However, all messages that are transmitted
 with a level of fatal MUST be treated as fatal messages.
 New alert values MUST be defined by RFC 2434 Standards Action.  See
 Section 11 for IANA Considerations for alert values.

7.3. Handshake Protocol Overview

 The cryptographic parameters of the session state are produced by the
 TLS Handshake Protocol, which operates on top of the TLS Record
 Layer.  When a TLS client and server first start communicating, they
 agree on a protocol version, select cryptographic algorithms,
 optionally authenticate each other, and use public-key encryption
 techniques to generate shared secrets.
 The TLS Handshake Protocol involves the following steps:
  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

Dierks & Rescorla Standards Track [Page 31] RFC 4346 The TLS Protocol April 2006

 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.
 However, one SHOULD never send data over a link encrypted with 40-bit
 security unless one feels that data is worth no more than the effort
 required to break that encryption.
 These goals are achieved by the handshake protocol, which can be
 summarized as follows: The client sends a client hello message to
 which the server must respond with a server hello message, or else a
 fatal error will occur and the connection will fail.  The client
 hello and server hello are used to establish security enhancement
 capabilities between client and server.  The client hello and server
 hello establish the following attributes: Protocol Version, Session
 ID, Cipher Suite, and Compression Method.  Additionally, two random
 values are generated and exchanged: ClientHello.random and
 ServerHello.random.
 The actual key exchange uses up to four messages: the server
 certificate, the server key exchange, the client certificate, and the
 client key exchange.  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 48 to 128 bytes in
 length.
 Following the hello messages, the server will send its certificate,
 if it is to be authenticated.  Additionally, a server key exchange
 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 server hello done 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
 certificate request message, the client must send the certificate
 message.  The client key exchange message is now sent, and the
 content of that message will depend on the public key algorithm
 selected between the client hello and the server hello.  If the
 client has sent a certificate with signing ability, a digitally-

Dierks & Rescorla Standards Track [Page 32] RFC 4346 The TLS Protocol April 2006

 signed certificate verify message is sent to explicitly verify the
 certificate.
 At this point, a change cipher spec 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 change cipher spec 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
 TLS_NULL_WITH_NULL_NULL is established).
    Client                                               Server
    ClientHello                  -------->
                                                    ServerHello
                                                   Certificate*
                                             ServerKeyExchange*
                                            CertificateRequest*
                                 <--------      ServerHelloDone
    Certificate*
    ClientKeyExchange
    CertificateVerify*
    [ChangeCipherSpec]
    Finished                     -------->
                                             [ChangeCipherSpec]
                                 <--------             Finished
    Application Data             <------->     Application Data
           Fig. 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.

Dierks & Rescorla Standards Track [Page 33] RFC 4346 The TLS Protocol April 2006

 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 change cipher spec messages and proceed
 directly to finished messages.  Once the re-establishment is
 complete, the client and server MAY begin to exchange application
 layer data.  (See flow chart below.)  If a Session ID match is not
 found, the server generates a new session ID and the TLS client and
 server perform a full handshake.
    Client                                                Server
    ClientHello                   -------->
                                                     ServerHello
                                              [ChangeCipherSpec]
                                  <--------             Finished
    [ChangeCipherSpec]
    Finished                      -------->
    Application Data              <------->     Application Data
        Fig. 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;

Dierks & Rescorla Standards Track [Page 34] RFC 4346 The TLS Protocol April 2006

            case server_hello:        ServerHello;
            case certificate:         Certificate;
            case server_key_exchange: ServerKeyExchange;
            case certificate_request: CertificateRequest;
            case server_hello_done:   ServerHelloDone;
            case certificate_verify:  CertificateVerify;
            case client_key_exchange: ClientKeyExchange;
            case finished:            Finished;
        } body;
    } Handshake;
 The handshake protocol messages are presented below in the order they
 MUST be sent; sending handshake messages in an unexpected order
 results in a fatal error.  Unneeded handshake messages can be
 omitted, however.  Note one exception to the ordering: the
 Certificate message is used twice in the handshake (from server to
 client, then from client to server), but is described only in its
 first position.  The one message that is not bound by these ordering
 rules is the Hello Request 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 type values MUST be defined via RFC 2434
 Standards Action.  See Section 11 for IANA Considerations for these
 values.

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 hello request message MAY be sent by the server at any time.
 Meaning of this message:
    Hello request is a simple notification that the client should
    begin the negotiation process anew by sending a client hello
    message when convenient.  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

Dierks & Rescorla Standards Track [Page 35] RFC 4346 The TLS Protocol April 2006

    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
    hello request but does not receive a client hello in response, it
    may close the connection with a fatal alert.
    After sending a hello request, servers SHOULD not repeat the
    request until the subsequent handshake negotiation is complete.
       Structure of this message:
           struct { } HelloRequest;
 Note: 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 client hello as its first message.  The client can also send a
    client hello in response to a hello request or on its own
    initiative in order to renegotiate the security parameters in an
    existing connection.
 Structure of this message:
    The client hello 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, GMT,
    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.
       random_bytes
           28 bytes generated by a secure random number generator.

Dierks & Rescorla Standards Track [Page 36] RFC 4346 The TLS Protocol April 2006

 The client hello 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,
 from 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 CipherSuite list, passed from the client to the server in the
 client hello message, contains the combinations of cryptographic
 algorithms supported by the client in order of the client's
 preference (favorite choice first).  Each CipherSuite defines a key
 exchange algorithm, a bulk encryption algorithm (including secret key
 length), and a MAC algorithm.  The server will select a cipher suite
 or, if no acceptable choices are presented, return a handshake
 failure alert and close the connection.
    uint8 CipherSuite[2];    /* Cryptographic suite selector */
 The client hello includes a list of compression algorithms supported
 by the client, ordered according to the client's preference.

Dierks & Rescorla Standards Track [Page 37] RFC 4346 The TLS Protocol April 2006

    enum { null(0), (255) } CompressionMethod;
    struct {
        ProtocolVersion client_version;
        Random random;
        SessionID session_id;
        CipherSuite cipher_suites<2..2^16-1>;
        CompressionMethod compression_methods<1..2^8-1>;
    } ClientHello;
 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 3.2.  (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 should be empty if no session_id is available or if the
    client wishes to generate new security parameters.
 cipher_suites
    This is a list of the cryptographic options supported by the
    client, with the client's first preference first.  If the
    session_id field is not empty (implying a session resumption
    request) this vector MUST include at least the cipher_suite from
    that session.  Values are defined in Appendix A.5.
 compression_methods
    This is a list of the compression methods supported by the client,
    sorted by client preference.  If the session_id field is not empty
    (implying a session resumption request) it MUST include the
    compression_method from that session.  This vector MUST contain,
    and all implementations MUST support, CompressionMethod.null.
    Thus, a client and server will always be able to agree on a
    compression method.
 After sending the client hello message, the client waits for a server
 hello message.  Any other handshake message returned by the server
 except for a hello request is treated as a fatal error.
 Forward compatibility note:  In the interests of forward
 compatibility, it is permitted that a client hello message include
 extra data after the compression methods.  This data MUST be included

Dierks & Rescorla Standards Track [Page 38] RFC 4346 The TLS Protocol April 2006

 in the handshake hashes, but must otherwise be ignored.  This is the
 only handshake message for which this is legal; for all other
 messages, the amount of data in the message MUST match the
 description of the message precisely.
    Note: For the intended use of trailing data in the ClientHello,
       see RFC 3546 [TLSEXT].

7.4.1.3. Server Hello

 The server will send this message in response to a client hello
 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;
     } 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 3.2.  (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.

Dierks & Rescorla Standards Track [Page 39] RFC 4346 The TLS Protocol April 2006

 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.
 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.

7.4.2. Server Certificate

 When this message will be sent:
    The server MUST send a certificate whenever the agreed-upon key
    exchange method is not an anonymous one.  This message will always
    immediately follow the server hello message.
 Meaning of this message:
    The certificate type MUST be appropriate for the selected cipher
    suite's key exchange algorithm, and is generally an X.509v3
    certificate.  It MUST contain a key that matches the key exchange
    method, as follows.  Unless otherwise specified, the signing
    algorithm for the certificate MUST be the same as the algorithm
    for the certificate key.  Unless otherwise specified, the public
    key MAY be of any length.

Dierks & Rescorla Standards Track [Page 40] RFC 4346 The TLS Protocol April 2006

    Key Exchange Algorithm  Certificate Key Type
    RSA                     RSA public key; the certificate MUST
                            allow the key to be used for encryption.
    DHE_DSS                 DSS public key.
    DHE_RSA                 RSA public key that can be used for
                            signing.
    DH_DSS                  Diffie-Hellman key. The algorithm used
                            to sign the certificate MUST be DSS.
    DH_RSA                  Diffie-Hellman key. The algorithm used
                            to sign the certificate MUST be RSA.
 All certificate profiles and key and cryptographic formats are
 defined by the IETF PKIX working group [PKIX].  When a key usage
 extension is present, the digitalSignature bit MUST be set for the
 key to be eligible for signing, as described above, and the
 keyEncipherment bit MUST be present to allow encryption, as described
 above.  The keyAgreement bit must be set on Diffie-Hellman
 certificates.
 As CipherSuites that specify new key exchange methods are specified
 for the TLS Protocol, they will imply certificate format and the
 required encoded keying information.
 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 X.509v3 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 optionally 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

Dierks & Rescorla Standards Track [Page 41] RFC 4346 The TLS Protocol April 2006

 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.

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 server hello message, if this is an anonymous
    negotiation).
    The server key exchange 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 server key exchange message for the
    following key exchange methods:
         RSA
         DH_DSS
         DH_RSA
 Meaning of this message:
    This message conveys cryptographic information to allow the client
    to communicate the premaster secret: either an RSA public key with
    which to encrypt the premaster secret, or a Diffie-Hellman public
    key with which the client can complete a key exchange (with the
    result being the premaster secret).
 As additional CipherSuites 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.

Dierks & Rescorla Standards Track [Page 42] RFC 4346 The TLS Protocol April 2006

 Structure of this message:
    enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
    struct {
        opaque rsa_modulus<1..2^16-1>;
        opaque rsa_exponent<1..2^16-1>;
    } ServerRSAParams;
    rsa_modulus
        The modulus of the server's temporary RSA key.
    rsa_exponent
        The public exponent of the server's temporary RSA key.
    struct {
        opaque dh_p<1..2^16-1>;
        opaque dh_g<1..2^16-1>;
        opaque dh_Ys<1..2^16-1>;
    } ServerDHParams;     /* Ephemeral DH parameters */
    dh_p
        The prime modulus used for the Diffie-Hellman operation.
    dh_g
        The generator used for the Diffie-Hellman operation.
    dh_Ys
      The server's Diffie-Hellman public value (g^X mod p).
    struct {
        select (KeyExchangeAlgorithm) {
            case diffie_hellman:
                ServerDHParams params;
                Signature signed_params;
            case rsa:
                ServerRSAParams params;
                Signature signed_params;
        };
    } ServerKeyExchange;

Dierks & Rescorla Standards Track [Page 43] RFC 4346 The TLS Protocol April 2006

    struct {
        select (KeyExchangeAlgorithm) {
            case diffie_hellman:
                ServerDHParams params;
            case rsa:
                ServerRSAParams params;
        };
     } ServerParams;
    params
        The server's key exchange parameters.
    signed_params
        For non-anonymous key exchanges, a hash of the corresponding
        params value, with the signature appropriate to that hash
        applied.
    md5_hash
        MD5(ClientHello.random + ServerHello.random + ServerParams);
    sha_hash
        SHA(ClientHello.random + ServerHello.random + ServerParams);
    enum { anonymous, rsa, dsa } SignatureAlgorithm;
    struct {
        select (SignatureAlgorithm) {
            case anonymous: struct { };
            case rsa:
                digitally-signed struct {
                    opaque md5_hash[16];
                    opaque sha_hash[20];
                };
            case dsa:
                digitally-signed struct {
                    opaque sha_hash[20];
                };
            };
        };
    } Signature;

7.4.4. Certificate request

 When this message will be sent:
    A non-anonymous server can optionally request a certificate from
    the client, if it is appropriate for the selected cipher suite.

Dierks & Rescorla Standards Track [Page 44] RFC 4346 The TLS Protocol April 2006

    This message, if sent, will immediately follow the Server Key
    Exchange message (if it is sent; otherwise, the Server 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>;
        DistinguishedName certificate_authorities<0..2^16-1>;
    } CertificateRequest;
    certificate_types
       This field is a list of the types of certificates requested,
       sorted in order of the server's preference.
    certificate_authorities
       A list of the distinguished names of acceptable certificate
       authorities.  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 both known roots and 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.
 ClientCertificateType values are divided into three groups:
    1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
       reserved for IETF Standards Track protocols.
    2. Values from 64 decimal (0x40) through 223 decimal (0xDF)
       inclusive are reserved for assignment for non-Standards Track
       methods.
    3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
       inclusive are reserved for private use.

Dierks & Rescorla Standards Track [Page 45] RFC 4346 The TLS Protocol April 2006

 Additional information describing the role of IANA in the allocation
 of ClientCertificateType code points is described in Section 11.
 Note: Values listed as RESERVED may not be used.  They were used in
       SSLv3.
 Note: DistinguishedName is derived from [X501].  DistinguishedNames
       are represented in DER-encoded format.
 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 server hello done message is sent by the server to indicate
    the end of the server hello 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 server hello done 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
    server hello done message.  This message is only sent if the
    server requests a certificate.  If no suitable certificate is
    available, the client SHOULD send a certificate message containing
    no certificates.  That is, the certificate_list structure has a
    length of zero.  If client authentication is required by the
    server for the handshake to continue, it may respond with a fatal
    handshake failure alert.  Client certificates are sent using the
    Certificate structure defined in Section 7.4.2.

Dierks & Rescorla Standards Track [Page 46] RFC 4346 The TLS Protocol April 2006

 Note: When using a static Diffie-Hellman based key exchange method
    (DH_DSS or DH_RSA), if client authentication is requested, the
    Diffie-Hellman group and generator encoded in the client's
    certificate MUST match the server specified Diffie-Hellman
    parameters if the client's parameters are to be used for the key
    exchange.

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 server hello done message.
 Meaning of this message:
    With this message, the premaster secret is set, either though
    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 key
    exchange method is DH_RSA or DH_DSS, client certification has been
    requested, and the client was able to respond with a certificate
    that contained a Diffie-Hellman public key whose parameters (group
    and generator) matched those specified by the server in its
    certificate, this message MUST not contain any data.
 Structure of this message:
    The choice of messages depends on which key exchange method has
    been selected.  See Section 7.4.3 for the KeyExchangeAlgorithm
    definition.
    struct {
        select (KeyExchangeAlgorithm) {
            case rsa: EncryptedPreMasterSecret;
            case diffie_hellman: 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 or the temporary RSA key

Dierks & Rescorla Standards Track [Page 47] RFC 4346 The TLS Protocol April 2006

    provided in a server key exchange message, and sends the result in
    an encrypted premaster secret message.  This structure is a
    variant of the client key exchange 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 roll-back attacks.
       Upon receiving the premaster secret, the server SHOULD check
       that this value matches the value transmitted by the client in
       the client hello message.
    random
        46 securely-generated random bytes.
    struct {
        public-key-encrypted PreMasterSecret pre_master_secret;
    } EncryptedPreMasterSecret;
    pre_master_secret
        This random value is generated by the client and is used to
        generate the master secret, as specified in Section 8.1.
 Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be
       used to attack a TLS server that is using PKCS#1 v 1.5 encoded
       RSA.  The attack takes advantage of the fact that, by failing
       in different ways, a TLS server can be coerced into revealing
       whether a particular message, when decrypted, is properly
       PKCS#1 v1.5 formatted or not.
       The best way to avoid vulnerability to this attack is to treat
       incorrectly formatted messages in a manner indistinguishable
       from correctly formatted RSA blocks.  Thus, when a server
       receives an incorrectly formatted RSA block, it should generate
       a random 48-byte value and proceed using it as the premaster
       secret.  Thus, the server will act identically whether the
       received RSA block is correctly encoded or not.
       [PKCS1B] defines a newer version of PKCS#1 encoding that is
       more secure against the Bleichenbacher attack.  However, for
       maximal compatibility with TLS 1.0, TLS 1.1 retains the
       original encoding.  No variants of the Bleichenbacher attack

Dierks & Rescorla Standards Track [Page 48] RFC 4346 The TLS Protocol April 2006

       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, encoding the
                      RSA encrypted data directly in the
                      ClientKeyExchange message.
                      This specification requires correct encoding of
                      the EncryptedPreMasterSecret complete with
                      length bytes.  The resulting PDU is incompatible
                      with many SSLv3 implementations.  Implementors
                      upgrading from SSLv3 must modify their
                      implementations to generate and accept the
                      correct encoding.  Implementors who wish to be
                      compatible with both SSLv3 and TLS should make
                      their implementation's behavior dependent on the
                      protocol version.
 Implementation Note: It is now known that remote timing-based attacks
                      on SSL are possible, at least when the client
                      and server are on the same LAN.  Accordingly,
                      implementations that use static RSA keys SHOULD
                      use RSA blinding or some other anti-timing
                      technique, as described in [TIMING].
 Note: The version number in the PreMasterSecret MUST be the version
       offered by the client in the ClientHello, not the version
       negotiated for the connection.  This feature is designed to
       prevent rollback attacks.  Unfortunately, many 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
       and Server implementations MAY, check the version number.  In
       practice, since the TLS handshake MACs prevent downgrade and no
       good attacks are known on those MACs, ambiguity is not
       considered a serious security risk.  Note that if servers
       choose to check the version number, they should randomize the

Dierks & Rescorla Standards Track [Page 49] RFC 4346 The TLS Protocol April 2006

       PreMasterSecret in case of error, rather than generate an
       alert, in order to avoid variants on the Bleichenbacher attack.
       [KPR03]

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 certificate already contains a suitable Diffie-
        Hellman key, then Yc is implicit and does not need to be sent
        again.  In this case, the client key exchange message will be
        sent, but it MUST be empty.
    explicit
        Yc needs to be sent.
    struct {
        select (PublicValueEncoding) {
            case implicit: struct { };
            case explicit: opaque dh_Yc<1..2^16-1>;
        } dh_public;
    } ClientDiffieHellmanPublic;
    dh_Yc
        The client's Diffie-Hellman public value (Yc).

7.4.8. Certificate verify

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

Dierks & Rescorla Standards Track [Page 50] RFC 4346 The TLS Protocol April 2006

 Structure of this message:
    struct {
         Signature signature;
    } CertificateVerify;
    The Signature type is defined in 7.4.3.
    CertificateVerify.signature.md5_hash
        MD5(handshake_messages);
    CertificateVerify.signature.sha_hash
        SHA(handshake_messages);
 Here handshake_messages refers to all handshake messages sent or
 received starting at client hello 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 7.4, exchanged thus far.

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 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.
    struct {
        opaque verify_data[12];
    } Finished;
    verify_data
        PRF(master_secret, finished_label, MD5(handshake_messages) +
        SHA-1(handshake_messages)) [0..11];

Dierks & Rescorla Standards Track [Page 51] RFC 4346 The TLS Protocol April 2006

    finished_label
        For Finished messages sent by the client, the string "client
        finished".  For Finished messages sent by the server, the
        string "server finished".
    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
        7.4, exchanged thus far.
 It is a fatal error if a finished message is not preceded by a change
 cipher spec message at the appropriate point in the handshake.
 The value handshake_messages includes all handshake messages starting
 at client hello 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 certificate verify 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: Change cipher spec messages, alerts, and any other record types
       are not handshake messages and are not included in the hash
       computations.  Also, Hello Request 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 server hello 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.

Dierks & Rescorla Standards Track [Page 52] RFC 4346 The TLS Protocol April 2006

     master_secret = PRF(pre_master_secret, "master secret",
                         ClientHello.random + ServerHello.random)
     [0..47];
 The master secret is always exactly 48 bytes in length.  The length
 of the premaster secret will vary depending on key exchange method.

8.1.1. RSA

 When RSA is used for server authentication and key exchange, a 48-
 byte pre_master_secret is generated by the client, encrypted under
 the server's public key, and sent to the server.  The server uses its
 private key to decrypt the pre_master_secret.  Both parties then
 convert the pre_master_secret into the master_secret, as specified
 above.
 RSA digital signatures are performed using PKCS #1 [PKCS1] block type
 1. RSA public key encryption is performed using PKCS #1 block type 2.

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_3DES_EDE_CBC_SHA.

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.

Dierks & Rescorla Standards Track [Page 53] RFC 4346 The TLS Protocol April 2006

12. IANA Considerations

 This document describes a number of new registries that have been
 created by IANA.  We recommended that they be placed as individual
 registries items under a common TLS category.
 Section 7.4.3 describes a TLS ClientCertificateType Registry to be
 maintained by the IANA, defining a number of such code point
 identifiers.  ClientCertificateType identifiers with values in the
 range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards
 Action.  Values from the range 64-223 (decimal) inclusive are
 assigned via [RFC2434] Specification Required.  Identifier values
 from 224-255 (decimal) inclusive are reserved for RFC 2434 Private
 Use.  The registry will initially be populated with the values in
 this document, Section 7.4.4.
 Section A.5 describes a TLS Cipher Suite Registry to be maintained by
 the IANA, and it defines a number of such cipher suite identifiers.
 Cipher suite values with the first byte in the range 0-191 (decimal)
 inclusive are assigned via RFC 2434 Standards Action.  Values with
 the first byte in the range 192-254 (decimal) are assigned via RFC
 2434 Specification Required.  Values with the first byte 255
 (decimal) are reserved for RFC 2434 Private Use.  The registry will
 initially be populated with the values from Section A.5 of this
 document, [TLSAES], and from Section 3 of [TLSKRB].
 Section 6 requires that all ContentType values be defined by RFC 2434
 Standards Action.  IANA has created a TLS ContentType registry,
 initially populated with values from Section 6.2.1 of this document.
 Future values MUST be allocated via Standards Action as described in
 [RFC2434].
 Section 7.2.2 requires that all Alert values be defined by RFC 2434
 Standards Action.  IANA has created a TLS Alert registry, initially
 populated with values from Section 7.2 of this document and from
 Section 4 of [TLSEXT].  Future values MUST be allocated via Standards
 Action as described in [RFC2434].
 Section 7.4 requires that all HandshakeType values be defined by RFC
 2434 Standards Action.  IANA has created a TLS HandshakeType
 registry, initially populated with values from Section 7.4 of this
 document and from Section 2.4 of [TLSEXT].  Future values MUST be
 allocated via Standards Action as described in [RFC2434].

Dierks & Rescorla Standards Track [Page 54] RFC 4346 The TLS Protocol April 2006

Appendix A. Protocol Constant Values

 This section describes protocol types and constants.

A.1. Record Layer

 struct {
     uint8 major, minor;
 } ProtocolVersion;
 ProtocolVersion version = { 3, 2 };     /* TLS v1.1 */
 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 (CipherSpec.cipher_type) {
         case stream: GenericStreamCipher;
         case block:  GenericBlockCipher;
     } fragment;
 } TLSCiphertext;
 stream-ciphered struct {
     opaque content[TLSCompressed.length];
     opaque MAC[CipherSpec.hash_size];
 } GenericStreamCipher;
 block-ciphered struct {
     opaque IV[CipherSpec.block_length];

Dierks & Rescorla Standards Track [Page 55] RFC 4346 The TLS Protocol April 2006

     opaque content[TLSCompressed.length];
     opaque MAC[CipherSpec.hash_size];
     uint8 padding[GenericBlockCipher.padding_length];
     uint8 padding_length;
 } GenericBlockCipher;

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(21),
         record_overflow(22),
         decompression_failure(30),
         handshake_failure(40),
         no_certificate_RESERVED (41),
         bad_certificate(42),
         unsupported_certificate(43),
         certificate_revoked(44),
         certificate_expired(45),
         certificate_unknown(46),
         illegal_parameter(47),
         unknown_ca(48),
         access_denied(49),
         decode_error(50),
         decrypt_error(51),
         export_restriction_RESERVED(60),
         protocol_version(70),
         insufficient_security(71),
         internal_error(80),
         user_canceled(90),
         no_renegotiation(100),
         (255)
     } AlertDescription;
 struct {
     AlertLevel level;
     AlertDescription description;
 } Alert;

Dierks & Rescorla Standards Track [Page 56] RFC 4346 The TLS Protocol April 2006

A.4. Handshake Protocol

 enum {
     hello_request(0), client_hello(1), server_hello(2),
     certificate(11), server_key_exchange (12),
     certificate_request(13), server_hello_done(14),
     certificate_verify(15), client_key_exchange(16),
     finished(20), (255)
 } HandshakeType;
 struct {
     HandshakeType msg_type;
     uint24 length;
     select (HandshakeType) {
         case hello_request:       HelloRequest;
         case client_hello:        ClientHello;
         case server_hello:        ServerHello;
         case certificate:         Certificate;
         case server_key_exchange: ServerKeyExchange;
         case certificate_request: CertificateRequest;
         case server_hello_done:   ServerHelloDone;
         case certificate_verify:  CertificateVerify;
         case client_key_exchange: ClientKeyExchange;
         case finished:            Finished;
     } body;
 } Handshake;

A.4.1. Hello messages

 struct { } HelloRequest;
 struct {
     uint32 gmt_unix_time;
     opaque random_bytes[28];
 } Random;
 opaque SessionID<0..32>;
 uint8 CipherSuite[2];
 enum { null(0), (255) } CompressionMethod;
 struct {
     ProtocolVersion client_version;
     Random random;
     SessionID session_id;
     CipherSuite cipher_suites<2..2^16-1>;
     CompressionMethod compression_methods<1..2^8-1>;

Dierks & Rescorla Standards Track [Page 57] RFC 4346 The TLS Protocol April 2006

 } ClientHello;
 struct {
     ProtocolVersion server_version;
     Random random;
     SessionID session_id;
     CipherSuite cipher_suite;
     CompressionMethod compression_method;
 } ServerHello;

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 { rsa, diffie_hellman } KeyExchangeAlgorithm;
 struct {
     opaque rsa_modulus<1..2^16-1>;
     opaque rsa_exponent<1..2^16-1>;
 } ServerRSAParams;
 struct {
     opaque dh_p<1..2^16-1>;
     opaque dh_g<1..2^16-1>;
     opaque dh_Ys<1..2^16-1>;
 } ServerDHParams;
 struct {
     select (KeyExchangeAlgorithm) {
         case diffie_hellman:
             ServerDHParams params;
             Signature signed_params;
         case rsa:
             ServerRSAParams params;
             Signature signed_params;
     };
 } ServerKeyExchange;
 enum { anonymous, rsa, dsa } SignatureAlgorithm;
 struct {
     select (KeyExchangeAlgorithm) {
         case diffie_hellman:
             ServerDHParams params;

Dierks & Rescorla Standards Track [Page 58] RFC 4346 The TLS Protocol April 2006

         case rsa:
             ServerRSAParams params;
     };
 } ServerParams;
 struct {
     select (SignatureAlgorithm) {
         case anonymous: struct { };
         case rsa:
             digitally-signed struct {
                 opaque md5_hash[16];
                 opaque sha_hash[20];
             };
         case dsa:
             digitally-signed struct {
                 opaque sha_hash[20];
             };
         };
     };
 } Signature;
 enum {
     rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
  rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
  fortezza_dms_RESERVED(20),
  (255)
 } ClientCertificateType;
 opaque DistinguishedName<1..2^16-1>;
 struct {
     ClientCertificateType certificate_types<1..2^8-1>;
     DistinguishedName certificate_authorities<0..2^16-1>;
 } CertificateRequest;
 struct { } ServerHelloDone;

A.4.3. Client Authentication and Key Exchange Messages

 struct {
     select (KeyExchangeAlgorithm) {
         case rsa: EncryptedPreMasterSecret;
         case diffie_hellman: ClientDiffieHellmanPublic;
     } exchange_keys;
 } ClientKeyExchange;

Dierks & Rescorla Standards Track [Page 59] RFC 4346 The TLS Protocol April 2006

 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 {
     Signature signature;
 } CertificateVerify;

A.4.4. Handshake Finalization Message

 struct {
     opaque verify_data[12];
 } Finished;

A.5. The CipherSuite

 The following values define the CipherSuite codes used in the client
 hello and server hello messages.
 A CipherSuite defines a cipher specification supported in TLS Version
 1.1.
 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 either an RSA or a DSS signature-capable certificate in the
 certificate request message.

Dierks & Rescorla Standards Track [Page 60] RFC 4346 The TLS Protocol April 2006

  CipherSuite TLS_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
  CipherSuite TLS_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
  CipherSuite TLS_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
  CipherSuite TLS_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
  CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
  CipherSuite TLS_RSA_WITH_DES_CBC_SHA               = { 0x00,0x09 };
  CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA          = { 0x00,0x0A };
 The following CipherSuite 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 DSS or RSA certificate that has been
 signed by the CA.  The signing algorithm used is specified after the
 DH or DHE parameter.  The server can request an RSA or DSS
 signature-capable certificate from the client for client
 authentication or it may request a Diffie-Hellman certificate.  Any
 Diffie-Hellman certificate provided by the client must use the
 parameters (group and generator) described by the server.
  CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA            = { 0x00,0x0C };
  CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x0D };
  CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA            = { 0x00,0x0F };
  CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x10 };
  CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA           = { 0x00,0x12 };
  CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x13 };
  CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA           = { 0x00,0x15 };
  CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x16 };
 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 and is therefore deprecated.
  CipherSuite TLS_DH_anon_WITH_RC4_128_MD5           = { 0x00,0x18 };
  CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
  CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };
 When SSLv3 and TLS 1.0 were designed, the United States restricted
 the export of cryptographic software containing certain strong
 encryption algorithms.  A series of cipher suites were designed to
 operate at reduced key lengths in order to comply with those
 regulations.  Due to advances in computer performance, these
 algorithms are now unacceptably weak, and export restrictions have
 since been loosened.  TLS 1.1 implementations MUST NOT negotiate
 these cipher suites in TLS 1.1 mode.  However, for backward
 compatibility they may be offered in the ClientHello for use with TLS

Dierks & Rescorla Standards Track [Page 61] RFC 4346 The TLS Protocol April 2006

 1.0 or SSLv3-only servers.  TLS 1.1 clients MUST check that the
 server did not choose one of these cipher suites during the
 handshake.  These ciphersuites are listed below for informational
 purposes and to reserve the numbers.
  CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
  CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
  CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
  CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
  CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
  CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
  CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
  CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
  CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };
 The following cipher suites were defined in [TLSKRB] and are included
 here for completeness.  See [TLSKRB] for details:
  CipherSuite    TLS_KRB5_WITH_DES_CBC_SHA           = { 0x00,0x1E }:
  CipherSuite    TLS_KRB5_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1F };
  CipherSuite    TLS_KRB5_WITH_RC4_128_SHA           = { 0x00,0x20 };
  CipherSuite    TLS_KRB5_WITH_IDEA_CBC_SHA          = { 0x00,0x21 };
  CipherSuite    TLS_KRB5_WITH_DES_CBC_MD5           = { 0x00,0x22 };
  CipherSuite    TLS_KRB5_WITH_3DES_EDE_CBC_MD5      = { 0x00,0x23 };
  CipherSuite    TLS_KRB5_WITH_RC4_128_MD5           = { 0x00,0x24 };
  CipherSuite    TLS_KRB5_WITH_IDEA_CBC_MD5          = { 0x00,0x25 };
 The following exportable cipher suites were defined in [TLSKRB] and
 are included here for completeness.  TLS 1.1 implementations MUST NOT
 negotiate these cipher suites.
  CipherSuite  TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA    = { 0x00,0x26};
  CipherSuite  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA    = { 0x00,0x27};
  CipherSuite  TLS_KRB5_EXPORT_WITH_RC4_40_SHA        = { 0x00,0x28};
  CipherSuite  TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5    = { 0x00,0x29};
  CipherSuite  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5    = { 0x00,0x2A};
  CipherSuite  TLS_KRB5_EXPORT_WITH_RC4_40_MD5        = { 0x00,0x2B};
 The following cipher suites were defined in [TLSAES] and are included
 here for completeness.  See [TLSAES] for details:
       CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA      = { 0x00, 0x2F };
       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_anon_WITH_AES_128_CBC_SHA  = { 0x00, 0x34 };

Dierks & Rescorla Standards Track [Page 62] RFC 4346 The TLS Protocol April 2006

       CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA      = { 0x00, 0x35 };
       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_anon_WITH_AES_256_CBC_SHA  = { 0x00, 0x3A };
  The cipher suite space is divided into three regions:
    1. Cipher suite values with first byte 0x00 (zero) through decimal
       191 (0xBF) inclusive are reserved for the IETF Standards Track
       protocols.
    2. Cipher suite values with first byte decimal 192 (0xC0) through
       decimal 254 (0xFE) inclusive are reserved for assignment for
       non-Standards Track methods.
    3. Cipher suite values with first byte 0xFF are reserved for
       private use.
 Additional information describing the role of IANA in the allocation
 of cipher suite code points is described in Section 11.
 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 { null, rc4, rc2, des, 3des, des40, aes, idea }
          BulkCipherAlgorithm;
          enum { stream, block } CipherType;
          enum { null, md5, sha } MACAlgorithm;
       /* The algorithms specified in CompressionMethod,
       BulkCipherAlgorithm, and MACAlgorithm may be added to. */

Dierks & Rescorla Standards Track [Page 63] RFC 4346 The TLS Protocol April 2006

          struct {
              ConnectionEnd entity;
              BulkCipherAlgorithm bulk_cipher_algorithm;
              CipherType cipher_type;
              uint8 key_size;
              uint8 key_material_length;
              MACAlgorithm mac_algorithm;
              uint8 hash_size;
              CompressionMethod compression_algorithm;
              opaque master_secret[48];
              opaque client_random[32];
              opaque server_random[32];
          } SecurityParameters;

Appendix B. Glossary

 Advanced Encryption Standard (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. [AES] 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.
 authentication
    Authentication is the ability of one entity to determine the
    identity of another entity.
 block cipher
    A block cipher is an algorithm that operates on plaintext in
    groups of bits, called blocks. 64 bits 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).

Dierks & Rescorla Standards Track [Page 64] RFC 4346 The TLS Protocol April 2006

 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 secret
    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 is a very widely used symmetric encryption algorithm.  DES is
    a block cipher with a 56 bit key and an 8 byte block size.  Note
    that in TLS, for key generation purposes, DES is treated as having
    an 8 byte key length (64 bits), but it still only provides 56 bits
    of protection.  (The low bit of each key byte is presumed to be
    set to produce odd parity in that key byte.)  DES can also be
    operated in a mode 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.  [DES], [3DES]
 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, "Digital Signature
    Standard," published May 1994 by the U.S. Dept. of Commerce.
    [DSS]

Dierks & Rescorla Standards Track [Page 65] RFC 4346 The TLS Protocol April 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.
 IDEA
    A 64-bit block cipher designed by Xuejia Lai and James Massey.
    [IDEA]
 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 is a secure hashing function that converts an arbitrarily long
    data stream into a digest of fixed size (16 bytes).  [MD5]
 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.
 RC2
    A block cipher developed by Ron Rivest at RSA Data Security, Inc.
    [RSADSI] described in [RC2].

Dierks & Rescorla Standards Track [Page 66] RFC 4346 The TLS Protocol April 2006

 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 under 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 secret
    The secret data used to authenticate data written by the server.
 SHA
    The Secure Hash Algorithm is defined in FIPS PUB 180-2.  It
    produces a 20-byte output.  Note that all references to SHA
    actually use the modified SHA-1 algorithm.  [SHA]
 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.

Dierks & Rescorla Standards Track [Page 67] RFC 4346 The TLS Protocol April 2006

 Transport Layer Security (TLS)
    This protocol; also, the Transport Layer Security working group of
    the Internet Engineering Task Force (IETF).  See "Comments" at the
    end of this document.

Appendix C. CipherSuite Definitions

CipherSuite Key Exchange Cipher Hash

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_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_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_DES_CBC_SHA DH_anon DES_CBC SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA

    Key
    Exchange
    Algorithm     Description                        Key size limit
    DHE_DSS       Ephemeral DH with DSS signatures   None
    DHE_RSA       Ephemeral DH with RSA signatures   None
    DH_anon       Anonymous DH, no signatures        None
    DH_DSS        DH with DSS-based certificates     None
    DH_RSA        DH with RSA-based certificates     None
                                                     RSA = none
    NULL          No key exchange                    N/A
    RSA           RSA key exchange                   None

Dierks & Rescorla Standards Track [Page 68] RFC 4346 The TLS Protocol April 2006

                       Key      Expanded     IV    Block
  Cipher       Type  Material Key Material   Size   Size
  NULL         Stream   0          0         0     N/A
  IDEA_CBC     Block   16         16         8      8
  RC2_CBC_40   Block    5         16         8      8
  RC4_40       Stream   5         16         0     N/A
  RC4_128      Stream  16         16         0     N/A
  DES40_CBC    Block    5          8         8      8
  DES_CBC      Block    8          8         8      8
  3DES_EDE_CBC Block   24         24         8      8
 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.
 Expanded Key Material
    The number of bytes actually fed into the encryption algorithm.
 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.
 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.
       Hash      Hash      Padding
     function    Size       Size
       NULL       0          0
       MD5        16         48
       SHA        20         40

Appendix D. Implementation Notes

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

Dierks & Rescorla Standards Track [Page 69] RFC 4346 The TLS Protocol April 2006

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 MD5 and/or SHA, are
 acceptable, but cannot provide more security than the size of the
 random number generator state.  (For example, MD5-based PRNGs usually
 provide 128 bits of 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 CipherSuites

 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 example, 40-bit
 encryption is easily broken, so implementations requiring strong
 security should not allow 40-bit keys.  Similarly, 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.

Dierks & Rescorla Standards Track [Page 70] RFC 4346 The TLS Protocol April 2006

Appendix E. Backward Compatibility with SSL

 For historical reasons and in order to avoid a profligate consumption
 of reserved port numbers, application protocols that are secured by
 TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same
 connection port.  For example, the https protocol (HTTP secured by
 SSL or TLS) uses port 443 regardless of which security protocol it is
 using.  Thus, some mechanism must be determined to distinguish and
 negotiate among the various protocols.
 TLS versions 1.1 and 1.0, and SSL 3.0 are very similar; thus,
 supporting both is easy.  TLS clients who wish to negotiate with such
 older servers SHOULD send client hello messages using the SSL 3.0
 record format and client hello structure, sending {3, 2} for the
 version field to note that they support TLS 1.1. If the server
 supports only TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0
 server hello; if it supports TLS 1.1 it will respond with a TLS 1.1
 server hello.  The negotiation then proceeds as appropriate for the
 negotiated protocol.
 Similarly, a TLS 1.1  server that wishes to interoperate with TLS 1.0
 or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages and
 respond with a SSL 3.0 server hello if an SSL 3.0 client hello with a
 version field of {3, 0} is received, denoting that this client does
 not support TLS.  Similarly, if a SSL 3.0 or TLS 1.0 hello with a
 version field of {3, 1} is received, the server SHOULD respond with a
 TLS 1.0 hello with a version field of {3, 1}.
 Whenever a client already knows the highest protocol known to a
 server (for example, when resuming a session), it SHOULD initiate the
 connection in that native protocol.
 TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL
 Version 2.0 client hello messages [SSL2].  TLS servers SHOULD accept
 either client hello format if they wish to support SSL 2.0 clients on
 the same connection port.  The only deviations from the Version 2.0
 specification are the ability to specify a version with a value of
 three and the support for more ciphering types in the CipherSpec.
Warning: The ability to send Version 2.0 client hello messages will be
     phased out with all due haste.  Implementors SHOULD make every
     effort to move forward as quickly as possible.  Version 3.0
     provides better mechanisms for moving to newer versions.

Dierks & Rescorla Standards Track [Page 71] RFC 4346 The TLS Protocol April 2006

     The following cipher specifications are carryovers from SSL
     Version 2.0. These are assumed to use RSA for key exchange and
     authentication.
      V2CipherSpec TLS_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
      V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
      V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
      V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
                                                 = { 0x04,0x00,0x80 };
      V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
      V2CipherSpec TLS_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
      V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
     Cipher specifications native to TLS can be included in Version
     2.0 client hello messages using the syntax below.  Any
     V2CipherSpec element with its first byte equal to zero will be
     ignored by Version 2.0 servers.  Clients sending any of the above
     V2CipherSpecs SHOULD also include the TLS equivalent (see
     Appendix A.5):
      V2CipherSpec (see TLS name) = { 0x00, CipherSuite };
 Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
     handshakes for backward compatibility but MUST NOT negotiate them
     in TLS 1.1 mode.

E.1. Version 2 Client Hello

 The Version 2.0 client hello message is presented below using this
 document's presentation model.  The true definition is still assumed
 to be the SSL Version 2.0 specification.  Note that this message MUST
 be sent directly on the wire, not wrapped as an SSLv3 record
   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;

Dierks & Rescorla Standards Track [Page 72] RFC 4346 The TLS Protocol April 2006

 msg_length
    This field is the length of the following data in bytes.  The high
    bit MUST be 1 and is not part of the length.
 msg_type
    This field, in conjunction with the version field, identifies a
    version 2 client hello message.  The value SHOULD be one (1).
 version
    The highest version of the protocol supported by the client
    (equals ProtocolVersion.version; see Appendix A.1).
 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.
 challenge_length
    The length in bytes of the client's challenge to the server to
    authenticate itself.  When using the SSLv2 backward compatible
    handshake the client MUST use a 32-byte challenge.
 cipher_specs
    This is a list of all CipherSpecs the client is willing and able
    to use.  There MUST be at least one CipherSpec acceptable to the
    server.
 session_id
    This field MUST be empty.
 challenge The client challenge to the server for the server to
    identify itself is a (nearly) arbitrary-length random.  The TLS
    server will right-justify the challenge data to become the
    ClientHello.random data (padded with leading zeroes, if
    necessary), as specified in this protocol specification.  If the
    length of the challenge is greater than 32 bytes, only the last 32
    bytes are used.  It is legitimate (but not necessary) for a V3
    server to reject a V2 ClientHello that has fewer than 16 bytes of
    challenge data.
    Note: Requests to resume a TLS session MUST use a TLS client
          hello.

Dierks & Rescorla Standards Track [Page 73] RFC 4346 The TLS Protocol April 2006

E.2. Avoiding Man-in-the-Middle Version Rollback

 When TLS clients fall back to Version 2.0 compatibility mode, they
 SHOULD use special PKCS #1 block formatting.  This is done so that
 TLS servers will reject Version 2.0 sessions with TLS-capable
 clients.
 When TLS clients are in Version 2.0 compatibility mode, they 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).  After decrypting the
 ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an
 error if these eight padding bytes are 0x03.  Version 2.0 servers
 receiving blocks padded in this manner will proceed normally.

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 CipherSpec 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

Dierks & Rescorla Standards Track [Page 74] RFC 4346 The TLS Protocol April 2006

 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 secrets (see
 Sections 7.4.8, 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 RSA or Diffie-
 Hellman for key exchange.  With anonymous RSA, the client encrypts a
 pre_master_secret with the server's uncertified public key extracted
 from the server key exchange message.  The result is sent in a client
 key exchange message.  Since eavesdroppers do not know the server's
 private key, it will be infeasible for them to decode the
 pre_master_secret.
 Note: No anonymous RSA Cipher Suites are defined in this document.
 With Diffie-Hellman, the server's public parameters are contained in
 the server key exchange message and the client's are sent in the
 client key exchange message.  Eavesdroppers who do not know the
 private values should not be able to find the Diffie-Hellman result
 (i.e., the pre_master_secret).
 Warning: Completely anonymous connections only provide protection
          against passive eavesdropping.  Unless an independent
          tamper-proof channel is used to verify that the finished
          messages were not replaced by an attacker, server
          authentication is required in environments where active
          man-in-the-middle attacks are a concern.

F.1.1.2. RSA Key Exchange and Authentication

 With RSA, key exchange and server authentication are combined.  The
 public key either may be contained in the server's certificate or may
 be a temporary RSA key sent in a server key exchange message.  When
 temporary RSA keys are used, they are signed by the server's RSA
 certificate.  The signature includes the current ClientHello.random,
 so old signatures and temporary keys cannot be replayed.  Servers may
 use a single temporary RSA key for multiple negotiation sessions.
 Note: The temporary RSA key option is useful if servers need large

Dierks & Rescorla Standards Track [Page 75] RFC 4346 The TLS Protocol April 2006

       certificates but must comply with government-imposed size
       limits on keys used for key exchange.
 Note that if ephemeral RSA is not used, 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 the master_secret and 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 DSS or RSA certificate.
 Temporary parameters are hashed with the hello.random values before
 signing to ensure that attackers do not replay old parameters.  In
 either case, the client can verify the certificate or signature to
 ensure that the parameters belong to the server.
 If the client has a certificate containing fixed Diffie-Hellman
 parameters, its certificate contains the information required to
 complete the key exchange.  Note that in this case the client and
 server will generate the same Diffie-Hellman result (i.e.,
 pre_master_secret) every time they communicate.  To prevent the
 pre_master_secret from staying in memory any longer than necessary,
 it should be converted into the master_secret as soon as possible.
 Client Diffie-Hellman parameters must be compatible with those
 supplied by the server for the key exchange to work.
 If the client has a standard DSS 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.

Dierks & Rescorla Standards Track [Page 76] RFC 4346 The TLS Protocol April 2006

 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 ciphersuites 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 ciphersuites.

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.
 Parties concerned about attacks of this scale should not use 40-bit
 encryption keys.  Altering the padding of the least-significant 8
 bytes of the PKCS padding does not impact security for the size of
 the signed hashes and RSA key lengths used in the protocol, since
 this is essentially equivalent to increasing the input block size by
 8 bytes.

F.1.3. Detecting Attacks against the Handshake Protocol

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

Dierks & Rescorla Standards Track [Page 77] RFC 4346 The TLS Protocol April 2006

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 secrets 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 (which use both SHA and MD5).
 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.1.5. MD5 and SHA

 TLS uses hash functions very conservatively.  Where possible, both
 MD5 and SHA are used in tandem to ensure that non-catastrophic flaws
 in one algorithm will not break the overall protocol.

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

Dierks & Rescorla Standards Track [Page 78] RFC 4346 The TLS Protocol April 2006

 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 secrets 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
 ciphersuite.  The objective is to protect both the integrity and
 confidentiality of the transmitted data from malicious actions by
 active attackers in the network.  It turns out that the order in
 which encryption and authentication functions are applied to the data
 plays an important role for achieving this goal [ENCAUTH].
 The most robust method, called encrypt-then-authenticate, first
 applies encryption to the data and then applies a MAC to the
 ciphertext.  This method ensures that the integrity and
 confidentiality goals are obtained with ANY pair of encryption and
 MAC functions, provided that the former is secure against chosen
 plaintext attacks and that the MAC is secure against chosen-message
 attacks.  TLS uses another method, called authenticate-then-encrypt,
 in which first a MAC is computed on the plaintext and then the
 concatenation of plaintext and MAC is encrypted.  This method has
 been proven secure for CERTAIN combinations of encryption functions
 and MAC functions, but it is not guaranteed to be secure in general.
 In particular, it has been shown that there exist perfectly secure
 encryption functions (secure even in the information-theoretic sense)
 that combined with any secure MAC function, fail to provide the
 confidentiality goal against an active attack.  Therefore, new
 ciphersuites 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.

Dierks & Rescorla Standards Track [Page 79] RFC 4346 The TLS Protocol April 2006

 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
 pseudo-random generator and this pad is xor-ed 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 previous versions of SSL, 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 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
 denial of service 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-ESP] or ESP [AH-ESP].

F.6. Final Notes

 For TLS to be able to provide a secure connection, both the client
 and server systems, keys, and applications must be secure.  In
 addition, the implementation must be free of security errors.
 The system is only as strong as the weakest key exchange and
 authentication algorithm supported, and only trustworthy
 cryptographic functions should be used.  Short public keys, 40-bit
 bulk encryption 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.

Dierks & Rescorla Standards Track [Page 80] RFC 4346 The TLS Protocol April 2006

Normative References

 [AES]      National Institute of Standards and Technology,
            "Specification for the Advanced Encryption Standard (AES)"
            FIPS 197.  November 26, 2001.
 [3DES]     W. Tuchman, "Hellman Presents No Shortcut Solutions To
            DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp. 40-41.
 [DES]      ANSI X3.106, "American National Standard for Information
            Systems-Data Link Encryption," American National Standards
            Institute, 1983.
 [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.
 [IDEA]     X. Lai, "On the Design and Security of Block Ciphers," ETH
            Series in Information Processing, v. 1, Konstanz:
            Hartung-Gorre Verlag, 1992.
 [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321,
            April 1992.
 [PKCS1A]   B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1:
            RSA Cryptography Specifications Version 1.5", RFC 2313,
            March 1998.
 [PKCS1B]   J. Jonsson, 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.
 [RC2]      Rivest, R., "A Description of the RC2(r) Encryption
            Algorithm", RFC 2268, March 1998.
 [SCH]      B. Schneier. "Applied Cryptography: Protocols, Algorithms,
            and Source Code in C, 2ed", Published by John Wiley &
            Sons, Inc. 1996.

Dierks & Rescorla Standards Track [Page 81] RFC 4346 The TLS Protocol April 2006

 [SHA]      NIST FIPS PUB 180-2, "Secure Hash Standard," National
            Institute of Standards and Technology, U.S. Department of
            Commerce., August 2001.
 [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.
 [TLSAES]   Chown, P., "Advanced Encryption Standard (AES)
            Ciphersuites for Transport Layer Security (TLS)", RFC
            3268, June 2002.
 [TLSEXT]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
            and T. Wright, "Transport Layer Security (TLS)
            Extensions", RFC 3546, June 2003.
 [TLSKRB]   Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
            Suites to Transport Layer Security (TLS)", RFC 2712,
            October 1999.

Informative References

 [AH-ESP]   Kent, S., "IP Authentication Header", RFC 4302, December
            2005.
            Eastlake 3rd, D., "Cryptographic Algorithm Implementation
            Requirements for Encapsulating Security Payload (ESP) and
            Authentication Header (AH)", RFC 4305, 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., "Password Interception in a SSL/TLS Channel",
            http://lasecwww.epfl.ch/memo_ssl.shtml, 2003.
 [ENCAUTH]  Krawczyk, H., "The Order of Encryption and Authentication
            for Protecting Communications (Or: How Secure is SSL?)",
            Crypto 2001.

Dierks & Rescorla Standards Track [Page 82] RFC 4346 The TLS Protocol April 2006

 [KPR03]    Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
            Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
            March 2003.
 [PKCS6]    RSA Laboratories, "PKCS #6: RSA Extended Certificate
            Syntax Standard," version 1.5, November 1993.
 [PKCS7]    RSA Laboratories, "PKCS #7: RSA Cryptographic Message
            Syntax Standard," version 1.5, November 1993.
 [RANDOM]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005.
 [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. Frier, 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]      Hellstrom, G. and P. Jones, "RTP Payload for Text
            Conversation", RFC 4103, June 2005.
 [TIMING]   Boneh, D., Brumley, D., "Remote timing attacks are
            practical", USENIX Security Symposium 2003.
 [TLS1.0]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
            RFC 2246, January 1999.
 [X501]     ITU-T Recommendation X.501: Information Technology - Open
            Systems Interconnection - The Directory: Models, 1993.
 [X509]     ITU-T Recommendation X.509 (1997 E): Information
            Technology - Open Systems Interconnection - "The Directory
            - Authentication Framework". 1988.

Dierks & Rescorla Standards Track [Page 83] RFC 4346 The TLS Protocol April 2006

 [XDR]      Srinivasan, R., "XDR: External Data Representation
            Standard", RFC 1832, August 1995.

Authors' Addresses

 Working Group Chairs
 Win Treese
 EMail: treese@acm.org
 Eric Rescorla
 EMail: ekr@rtfm.com

Editors

 Tim Dierks
 Independent
 EMail: tim@dierks.org
 Eric Rescorla
 RTFM, Inc.
 EMail: ekr@rtfm.com

Other Contributors

 Christopher Allen (co-editor of TLS 1.0)
 Alacrity Ventures
 EMail: ChristopherA@AlacrityManagement.com
 Martin Abadi
 University of California, Santa Cruz
 EMail: abadi@cs.ucsc.edu
 Ran Canetti
 IBM
 EMail: canetti@watson.ibm.com

Dierks & Rescorla Standards Track [Page 84] RFC 4346 The TLS Protocol April 2006

 Taher Elgamal
 Securify
 EMail: taher@securify.com
 Anil Gangolli
 EMail: anil@busybuddha.org
 Kipp Hickman
 Phil Karlton (co-author of SSLv3)
 Paul Kocher (co-author of SSLv3)
 Cryptography Research
 EMail: paul@cryptography.com
 Hugo Krawczyk
 Technion Israel Institute of Technology
 EMail: hugo@ee.technion.ac.il
 Robert Relyea
 Netscape Communications
 EMail: relyea@netscape.com
 Jim Roskind
 Netscape Communications
 EMail: jar@netscape.com
 Michael Sabin
 Dan Simon
 Microsoft, Inc.
 EMail: dansimon@microsoft.com
 Tom Weinstein

Dierks & Rescorla Standards Track [Page 85] RFC 4346 The TLS Protocol April 2006

Comments

 The discussion list for the IETF TLS working group is located at the
 e-mail address <ietf-tls@lists.consensus.com>. Information on the
 group and information on how to subscribe to the list is at
 <http://lists.consensus.com/>.
 Archives of the list can be found at:
     <http://www.imc.org/ietf-tls/mail-archive/>

Dierks & Rescorla Standards Track [Page 86] RFC 4346 The TLS Protocol April 2006

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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Dierks & Rescorla Standards Track [Page 87]

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