GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc2246

Network Working Group T. Dierks Request for Comments: 2246 Certicom Category: Standards Track C. Allen

                                                             Certicom
                                                         January 1999
                          The TLS Protocol
                            Version 1.0

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 (1999).  All Rights Reserved.

Abstract

 This document specifies Version 1.0 of the Transport Layer Security
 (TLS) protocol. The TLS protocol provides communications privacy over
 the Internet. The protocol allows client/server applications to
 communicate in a way that is designed to prevent eavesdropping,
 tampering, or message forgery.

Table of Contents

 1.       Introduction                                              3
 2.       Goals                                                     4
 3.       Goals of this document                                    5
 4.       Presentation language                                     5
 4.1.     Basic block size                                          6
 4.2.     Miscellaneous                                             6
 4.3.     Vectors                                                   6
 4.4.     Numbers                                                   7
 4.5.     Enumerateds                                               7
 4.6.     Constructed types                                         8
 4.6.1.   Variants                                                  9
 4.7.     Cryptographic attributes                                 10
 4.8.     Constants                                                11
 5.       HMAC and the pseudorandom function                       11
 6.       The TLS Record Protocol                                  13
 6.1.     Connection states                                        14

Dierks & Allen Standards Track [Page 1] RFC 2246 The TLS Protocol Version 1.0 January 1999

 6.2.     Record layer                                             16
 6.2.1.   Fragmentation                                            16
 6.2.2.   Record compression and decompression                     17
 6.2.3.   Record payload protection                                18
 6.2.3.1. Null or standard stream cipher                           19
 6.2.3.2. CBC block cipher                                         19
 6.3.     Key calculation                                          21
 6.3.1.   Export key generation example                            22
 7.       The TLS Handshake Protocol                               23
 7.1.     Change cipher spec protocol                              24
 7.2.     Alert protocol                                           24
 7.2.1.   Closure alerts                                           25
 7.2.2.   Error alerts                                             26
 7.3.     Handshake Protocol overview                              29
 7.4.     Handshake protocol                                       32
 7.4.1.   Hello messages                                           33
 7.4.1.1. Hello request                                            33
 7.4.1.2. Client hello                                             34
 7.4.1.3. Server hello                                             36
 7.4.2.   Server certificate                                       37
 7.4.3.   Server key exchange message                              39
 7.4.4.   Certificate request                                      41
 7.4.5.   Server hello done                                        42
 7.4.6.   Client certificate                                       43
 7.4.7.   Client key exchange message                              43
 7.4.7.1. RSA encrypted premaster secret message                   44
 7.4.7.2. Client Diffie-Hellman public value                       45
 7.4.8.   Certificate verify                                       45
 7.4.9.   Finished                                                 46
 8.       Cryptographic computations                               47
 8.1.     Computing the master secret                              47
 8.1.1.   RSA                                                      48
 8.1.2.   Diffie-Hellman                                           48
 9.       Mandatory Cipher Suites                                  48
 10.      Application data protocol                                48
 A.       Protocol constant values                                 49
 A.1.     Record layer                                             49
 A.2.     Change cipher specs message                              50
 A.3.     Alert messages                                           50
 A.4.     Handshake protocol                                       51
 A.4.1.   Hello messages                                           51
 A.4.2.   Server authentication and key exchange messages          52
 A.4.3.   Client authentication and key exchange messages          53
 A.4.4.   Handshake finalization message                           54
 A.5.     The CipherSuite                                          54
 A.6.     The Security Parameters                                  56
 B.       Glossary                                                 57
 C.       CipherSuite definitions                                  61

Dierks & Allen Standards Track [Page 2] RFC 2246 The TLS Protocol Version 1.0 January 1999

 D.       Implementation Notes                                     64
 D.1.     Temporary RSA keys                                       64
 D.2.     Random Number Generation and Seeding                     64
 D.3.     Certificates and authentication                          65
 D.4.     CipherSuites                                             65
 E.       Backward Compatibility With SSL                          66
 E.1.     Version 2 client hello                                   67
 E.2.     Avoiding man-in-the-middle version rollback              68
 F.       Security analysis                                        69
 F.1.     Handshake protocol                                       69
 F.1.1.   Authentication and key exchange                          69
 F.1.1.1. Anonymous key exchange                                   69
 F.1.1.2. RSA key exchange and authentication                      70
 F.1.1.3. Diffie-Hellman key exchange with authentication          71
 F.1.2.   Version rollback attacks                                 71
 F.1.3.   Detecting attacks against the handshake protocol         72
 F.1.4.   Resuming sessions                                        72
 F.1.5.   MD5 and SHA                                              72
 F.2.     Protecting application data                              72
 F.3.     Final notes                                              73
 G.       Patent Statement                                         74
          Security Considerations                                  75
          References                                               75
          Credits                                                  77
          Comments                                                 78
          Full Copyright Statement                                 80

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 [RC4], 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

Dierks & Allen Standards Track [Page 3] RFC 2246 The TLS Protocol Version 1.0 January 1999

     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
 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 up to the judgment of the designers and
 implementors of protocols which run on top of TLS.

2. Goals

 The goals of TLS Protocol, in order of their priority, are:
  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 will then be able to
     successfully exchange cryptographic parameters without knowledge
     of one another's code.
  3. Extensibility: TLS seeks to provide a framework into which new
     public key and bulk encryption methods can be incorporated as
     necessary. This will also accomplish two sub-goals: to prevent

Dierks & Allen Standards Track [Page 4] RFC 2246 The TLS Protocol Version 1.0 January 1999

     the need to create a new protocol (and risking the introduction
     of possible new weaknesses) and to avoid 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 1.0 and SSL 3.0 do not interoperate
 (although TLS 1.0 does incorporate a mechanism by which a TLS
 implementation can back down to SSL 3.0). This document is intended
 primarily for readers who will be implementing the protocol and 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 nor 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, not to
 have general application beyond that particular goal.

Dierks & Allen Standards Track [Page 5] RFC 2246 The TLS Protocol Version 1.0 January 1999

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 */

Dierks & Allen Standards Track [Page 6] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Variable length vectors are defined by specifying a subrange of legal
 lengths, inclusively, using the notation <floor..ceiling>.  When
 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

Dierks & Allen Standards Track [Page 7] RFC 2246 The TLS Protocol Version 1.0 January 1999

 be assigned a value, as demonstrated in the following example.  Since
 the elements of the enumerated are not ordered, they can be assigned
 any unique value, in any order.
     enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
 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]];

Dierks & Allen Standards Track [Page 8] RFC 2246 The TLS Protocol Version 1.0 January 1999

 The fields within a structure may be qualified using the type's name
 using a syntax much like that available for enumerateds. For example,
 T.f2 refers to the second field of the previous declaration.
 Structure definitions may be embedded.

4.6.1. Variants

 Defined structures may have variants based on some knowledge that is
 available within the environment. The selector must be an enumerated
 type that defines the possible variants the structure defines. There
 must be a case arm for every element of the enumeration declared in
 the select. 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, a

Dierks & Allen Standards Track [Page 9] RFC 2246 The TLS Protocol Version 1.0 January 1999

     orange VariantRecord
 is a narrowed type of a VariantRecord containing a variant_body of
 type V2.

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 0 or type 1 as described in [PKCS1].
 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 which 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 & Allen Standards Track [Page 10] RFC 2246 The TLS Protocol Version 1.0 January 1999

 An RSA encrypted value is encoded with PKCS #1 block type 2 as
 described in [PKCS1].
 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,
 then the entire structure is encrypted with a stream cipher. The
 length of this structure, in bytes would be equal to 2 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
 due to the fact that 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 required
 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,
 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,

Dierks & Allen Standards Track [Page 11] RFC 2246 The TLS Protocol Version 1.0 January 1999

 data). Additional hash algorithms can be defined by cipher suites 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 which should guarantee its security if either
 algorithm remains secure.
 First, we define a data expansion function, P_hash(secret, data)
 which 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 was being used to
 create 64 bytes of data, it would have to be iterated 4 times
 (through A(4)), creating 80 bytes of output data; the last 16 bytes
 of the final iteration would 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-or'ing 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.
     L_S = length in bytes of secret;
     L_S1 = L_S2 = ceil(L_S / 2);

Dierks & Allen Standards Track [Page 12] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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-or'ing 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; to generate a 80 byte output will involve P_MD5 being
 iterated 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, and reassembled, 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.2). If a TLS
 implementation receives a record type it does not understand, it
 should just ignore it. Any protocol designed for use 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 take to minimize the value of traffic
 analysis of these values.

Dierks & Allen Standards Track [Page 13] RFC 2246 The TLS Protocol Version 1.0 January 1999

6.1. Connection states

 A TLS connection state is the operating environment of the TLS Record
 Protocol. It specifies a compression algorithm, encryption algorithm,
 and MAC algorithm. In addition, the parameters for these algorithms
 are known: the MAC secret and the bulk encryption keys and IVs for
 the connection in both the read and the write directions. Logically,
 there are always four connection states outstanding: the current read
 and write states, and the pending read and write states. All records
 are processed under the current read and write states. The security
 parameters for the pending states can be set by the TLS Handshake
 Protocol, and the Handshake Protocol 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 which 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, the block size of
     the cipher (if appropriate), and whether it is considered an
     "export" cipher.
 MAC algorithm
     An algorithm to be used for message authentication. This
     specification includes the size of the hash which is returned by
     the MAC algorithm.
 compression algorithm
     An algorithm to be used for data compression. This specification
     must include all information the algorithm requires to do
     compression.
 master secret
     A 48 byte secret shared between the two peers in the connection.
 client random
     A 32 byte value provided by the client.

Dierks & Allen Standards Track [Page 14] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 } BulkCipherAlgorithm;
     enum { stream, block } CipherType;
     enum { true, false } IsExportable;
     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;
         IsExportable           is_exportable;
         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 six items:
     client write MAC secret
     server write MAC secret
     client write key
     server write key
     client write IV (for block ciphers only)
     server write IV (for block ciphers only)
 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 & Allen Standards Track [Page 15] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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. In addition, for block
     ciphers running in CBC mode (the only mode specified for TLS),
     this will initially contain the IV for that connection state and
     be updated to contain the ciphertext of the last block encrypted
     or decrypted as records are processed. For stream ciphers, this
     will contain whatever the necessary state information is 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. A sequence number is incremented after each
     record: specifically, the first record which is transmitted under
     a particular connection state should use sequence number 0.

6.2. Record layer

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

6.2.1. Fragmentation

 The record layer fragments information blocks into TLSPlaintext
 records carrying data in chunks of 2^14 bytes or less. Client message
 boundaries are not preserved in the record layer (i.e., multiple
 client messages of the same ContentType may be coalesced into a
 single TLSPlaintext record, or a single message may be fragmented
 across several records).
     struct {
         uint8 major, minor;
     } ProtocolVersion;

Dierks & Allen Standards Track [Page 16] RFC 2246 The TLS Protocol Version 1.0 January 1999

     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 1.0, which uses the version { 3, 1 }. The
     version value 3.1 is historical: TLS version 1.0 is 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 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.

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.

Dierks & Allen Standards Track [Page 17] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.
     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.

Dierks & Allen Standards Track [Page 18] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.
 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 or DES), the encryption and MAC
 functions convert TLSCompressed.fragment structures to and from block
 TLSCiphertext.fragment structures.

Dierks & Allen Standards Track [Page 19] RFC 2246 The TLS Protocol Version 1.0 January 1999

     block-ciphered struct {
         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.
 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 long, 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 based on analysis of
     the lengths of exchanged messages. Each uint8 in the padding data
     vector must be filled with the padding length value.
 padding_length
     The padding length should 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 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, the length before padding is 82 bytes. 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) the

     initialization vector (IV) for the first record is generated with
     the other keys and secrets when the security parameters are set.
     The IV for subsequent records is the last ciphertext block from
     the previous record.

Dierks & Allen Standards Track [Page 20] RFC 2246 The TLS Protocol Version 1.0 January 1999

6.3. Key calculation

 The Record Protocol requires an algorithm to generate keys, IVs, 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, keys, and non-export IVs 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, a server write key, a client write IV, and a server write IV,
 which are generated from the master secret in that order. Unused
 values are empty.
 When generating keys and MAC secrets, the master secret is used as an
 entropy source, and the random values provide unencrypted salt
 material and IVs for exportable ciphers.
 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]
     client_write_IV[SecurityParameters.IV_size]
     server_write_IV[SecurityParameters.IV_size]
 The client_write_IV and server_write_IV are only generated for non-
 export block ciphers. For exportable block ciphers, the
 initialization vectors are generated later, as described below. Any
 extra key_block material is discarded.
 Implementation note:
     The cipher spec which is defined in this document which requires
     the most material is 3DES_EDE_CBC_SHA: it requires 2 x 24 byte
     keys, 2 x 20 byte MAC secrets, and 2 x 8 byte IVs, for a total of
     104 bytes of key material.

Dierks & Allen Standards Track [Page 21] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Exportable encryption algorithms (for which CipherSpec.is_exportable
 is true) require additional processing as follows to derive their
 final write keys:
     final_client_write_key =
     PRF(SecurityParameters.client_write_key,
                                "client write key",
                                SecurityParameters.client_random +
                                SecurityParameters.server_random);
     final_server_write_key =
     PRF(SecurityParameters.server_write_key,
                                "server write key",
                                SecurityParameters.client_random +
                                SecurityParameters.server_random);
 Exportable encryption algorithms derive their IVs solely from the
 random values from the hello messages:
     iv_block = PRF("", "IV block", SecurityParameters.client_random +
                    SecurityParameters.server_random);
 The iv_block is partitioned into two initialization vectors as the
 key_block was above:
     client_write_IV[SecurityParameters.IV_size]
     server_write_IV[SecurityParameters.IV_size]
 Note that the PRF is used without a secret in this case: this just
 means that the secret has a length of zero bytes and contributes
 nothing to the hashing in the PRF.

6.3.1. Export key generation example

 TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for
 each of the two encryption keys and 16 bytes for each of the MAC
 keys, for a total of 42 bytes of key material. The PRF output is
 stored in the key_block. The key_block is partitioned, and the write
 keys are salted because this is an exportable encryption algorithm.
     key_block               = PRF(master_secret,
                                   "key expansion",
                                   server_random +
                                   client_random)[0..41]
     client_write_MAC_secret = key_block[0..15]
     server_write_MAC_secret = key_block[16..31]
     client_write_key        = key_block[32..36]
     server_write_key        = key_block[37..41]

Dierks & Allen Standards Track [Page 22] RFC 2246 The TLS Protocol Version 1.0 January 1999

     final_client_write_key  = PRF(client_write_key,
                                   "client write key",
                                   client_random +
                                   server_random)[0..15]
     final_server_write_key  = PRF(server_write_key,
                                   "server write key",
                                   client_random +
                                   server_random)[0..15]
     iv_block                = PRF("", "IV block", client_random +
                                   server_random)[0..15]
     client_write_IV = iv_block[0..7]
     server_write_IV = iv_block[8..15]

7. The TLS Handshake Protocol

 The TLS Handshake Protocol consists of a suite of three sub-protocols
 which are used to allow peers to agree upon security parameters for
 the record layer, authenticate themselves, instantiate negotiated
 security parameters, and report error conditions to each other.
 The Handshake Protocol is responsible for negotiating a session,
 which consists of the following items:
 session identifier
     An arbitrary byte sequence chosen by the server to identify an
     active or resumable session state.
 peer certificate
     X509v3 [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.

Dierks & Allen Standards Track [Page 23] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 server
 to notify the receiving party that subsequent records will be
 protected under the newly negotiated CipherSpec and keys. Reception
 of this message causes the receiver to instruct the Record Layer to
 immediately copy the read pending state into the read current state.
 Immediately after sending this message, the sender should 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).

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),

Dierks & Allen Standards Track [Page 24] RFC 2246 The TLS Protocol Version 1.0 January 1999

         decompression_failure(30),
         handshake_failure(40),
         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(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. The session becomes
     unresumable if any connection is terminated without proper
     close_notify messages with level equal to warning.
 Either party may initiate a close by sending a close_notify alert.
 Any data received after a closure alert is ignored.
 Each party is required to send a close_notify alert before closing
 the write side of the connection. It is required that the other party
 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.

Dierks & Allen Standards Track [Page 25] RFC 2246 The TLS Protocol Version 1.0 January 1999

 If the application protocol using TLS provides that any data may be
 carried over the underlying transport after the TLS connection is
 closed, the TLS implementation must receive the responding
 close_notify alert before indicating to the application layer that
 the TLS connection has ended. If the application protocol will not
 transfer any additional data, but will only close the underlying
 transport connection, then the implementation may choose to close the
 transport without waiting for the responding close_notify. No part of
 this standard should be taken to dictate the manner in which a usage
 profile for TLS manages its data transport, including when
 connections are opened or closed.
 NB: 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 an fatal alert message, both
 parties immediately close the connection. Servers and clients are
 required to forget any session-identifiers, keys, and secrets
 associated with a failed connection. 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 message is always fatal.
 decryption_failed
     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.
 record_overflow
     A TLSCiphertext record was received which 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.
 decompression_failure
     The decompression function received improper input (e.g. data
     that would expand to excessive length). This message is always
     fatal.

Dierks & Allen Standards Track [Page 26] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.
 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.
 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.

Dierks & Allen Standards Track [Page 27] RFC 2246 The TLS Protocol Version 1.0 January 1999

 export_restriction
     A negotiation not in compliance with export restrictions was
     detected; for example, attempting to transfer a 1024 bit
     ephemeral RSA key for the RSA_EXPORT handshake method. This
     message is always fatal.
 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 makes it impossible to continue (such as a memory
     allocation failure). 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 would be where a server has spawned a process to
     satisfy a request; the process might receive security parameters
     (key length, authentication, etc.) at startup and it might be
     difficult to communicate changes to these parameters after that
     point. This message is always a warning.
 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

Dierks & Allen Standards Track [Page 28] RFC 2246 The TLS Protocol Version 1.0 January 1999

 receiving party may decide at its discretion whether to treat this as
 a fatal error or not. However, all messages which are transmitted
 with a level of fatal must be treated as fatal messages.

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 TLS always
 negotiating the strongest possible connection between two peers:
 there are a number of ways a man in the middle attacker can attempt
 to make two entities drop down to the least secure method they
 support. The protocol has been designed to minimize this risk, but
 there are still attacks available: for example, an attacker could
 block access to the port a secure service runs on, or attempt to get
 the peers to negotiate an unauthenticated connection. The fundamental
 rule is that higher levels must be cognizant of what their security
 requirements are and never transmit information over a channel less
 secure than what they require. The TLS protocol is secure, in that
 any cipher suite offers its promised level of security: if you
 negotiate 3DES with a 1024 bit RSA key exchange with a host whose
 certificate you have verified, you can expect to be that secure.

Dierks & Allen Standards Track [Page 29] RFC 2246 The TLS Protocol Version 1.0 January 1999

 However, you should never send data over a link encrypted with 40 bit
 security unless you feel 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 defining the use of the
 messages to allow the client and server to agree upon a shared
 secret. This secret should be quite long; currently defined key
 exchange methods exchange secrets which 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 their 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. Now 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-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

Dierks & Allen Standards Track [Page 30] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.)
    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.  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.

Dierks & Allen Standards Track [Page 31] RFC 2246 The TLS Protocol Version 1.0 January 1999

    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;
             case server_hello:        ServerHello;
             case certificate:         Certificate;
             case server_key_exchange: ServerKeyExchange;
             case certificate_request: CertificateRequest;
             case server_hello_done:   ServerHelloDone;
             case certificate_verify:  CertificateVerify;
             case client_key_exchange: ClientKeyExchange;
             case finished:            Finished;
         } body;
     } Handshake;

Dierks & Allen Standards Track [Page 32] RFC 2246 The TLS Protocol Version 1.0 January 1999

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

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
     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 should never be included in the message hashes which

     are maintained throughout the handshake and used in the finished
     messages and the certificate verify message.

Dierks & Allen Standards Track [Page 33] RFC 2246 The TLS Protocol Version 1.0 January 1999

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) 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.
 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, this
 connection, or 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, while 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 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>;

Dierks & Allen Standards Track [Page 34] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.
     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 3.1 (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 the
     client wishes to generate new security parameters.

Dierks & Allen Standards Track [Page 35] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 for a
     client hello message to include extra data after the compression
     methods. This data must be included 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.

7.4.1.3. Server hello

 When this message will be sent:
     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;

Dierks & Allen Standards Track [Page 36] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 3.1 (See
     Appendix E for details about backward compatibility).
 random
     This structure is generated by the server and must be different
     from (and independent of) ClientHello.random.
 session_id
     This is the identity of the session corresponding to this
     connection. If the ClientHello.session_id was non-empty, the
     server will look in its session cache for a match. If a match is
     found and the server is willing to establish the new connection
     using the specified session state, the server will respond with
     the same value as was supplied by the client. This indicates a
     resumed session and dictates that the parties must proceed
     directly to the finished messages. Otherwise this field will
     contain a different value identifying the new session. The server
     may return an empty session_id to indicate that the session will
     not be cached and therefore cannot be resumed. If a session is
     resumed, it must be resumed using the same cipher suite it was
     originally negotiated with.
 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 which matches the key exchange
     method, as follows. Unless otherwise specified, the signing

Dierks & Allen Standards Track [Page 37] RFC 2246 The TLS Protocol Version 1.0 January 1999

     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.
     Key Exchange Algorithm  Certificate Key Type
     RSA                     RSA public key; the certificate must
                             allow the key to be used for encryption.
     RSA_EXPORT              RSA public key of length greater than
                             512 bits which can be used for signing,
                             or a key of 512 bits or shorter which
                             can be used for either encryption or
                             signing.
     DHE_DSS                 DSS public key.
     DHE_DSS_EXPORT          DSS public key.
     DHE_RSA                 RSA public key which can be used for
                             signing.
     DHE_RSA_EXPORT          RSA public key which can be used for
                             signing.
     DH_DSS                  Diffie-Hellman key. The algorithm used
                             to sign the certificate should be DSS.
     DH_RSA                  Diffie-Hellman key. The algorithm used
                             to sign the certificate should be RSA.
 All certificate profiles, 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 which 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;

Dierks & Allen Standards Track [Page 38] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 which 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
 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:
         RSA_EXPORT (if the public key in the server certificate is
         longer than 512 bits)
         DHE_DSS
         DHE_DSS_EXPORT
         DHE_RSA
         DHE_RSA_EXPORT
         DH_anon
     It is not legal to send the server key exchange message for the
     following key exchange methods:
         RSA
         RSA_EXPORT (when the public key in the server certificate is
         less than or equal to 512 bits in length)
         DH_DSS
         DH_RSA

Dierks & Allen Standards Track [Page 39] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Meaning of this message:
     This message conveys cryptographic information to allow the
     client to communicate the premaster secret: either an RSA public
     key to encrypt the premaster secret with, 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 which 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.

Note: According to current US export law, RSA moduli larger than 512

     bits may not be used for key exchange in software exported from
     the US. With this message, the larger RSA keys encoded in
     certificates may be used to sign temporary shorter RSA keys for
     the RSA_EXPORT key exchange method.
 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).

Dierks & Allen Standards Track [Page 40] RFC 2246 The TLS Protocol Version 1.0 January 1999

     struct {
         select (KeyExchangeAlgorithm) {
             case diffie_hellman:
                 ServerDHParams params;
                 Signature signed_params;
             case rsa:
                 ServerRSAParams params;
                 Signature signed_params;
         };
     } ServerKeyExchange;
     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;
     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 appropriate for the selected cipher suite. This
     message, if sent, will immediately follow the Server Key Exchange
     message (if it is sent; otherwise, the Server Certificate
     message).

Dierks & Allen Standards Track [Page 41] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Structure of this message:
     enum {
         rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
         (255)
     } ClientCertificateType;
     opaque DistinguishedName<1..2^16-1>;
     struct {
         ClientCertificateType certificate_types<1..2^8-1>;
         DistinguishedName certificate_authorities<3..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 both to describe known roots
         and a desired authorization space.

Note: DistinguishedName is derived from [X509].

Note: It is a fatal handshake_failure alert for an anonymous server to

     request client identification.

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;

Dierks & Allen Standards Track [Page 42] RFC 2246 The TLS Protocol Version 1.0 January 1999

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. 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.

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 will immediately
     follow the client certificate message, if it is sent. Otherwise
     it will 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 which 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 which contained a Diffie-Hellman public key whose
     parameters (group and generator) matched those specified by the
     server in its certificate, this message will 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;

Dierks & Allen Standards Track [Page 43] RFC 2246 The TLS Protocol Version 1.0 January 1999

         } 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 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, 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;

Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used

     to attack a TLS server which is using PKCS#1 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 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 it receives an
     incorrectly formatted RSA block, a server 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.

Dierks & Allen Standards Track [Page 44] RFC 2246 The TLS Protocol Version 1.0 January 1999

     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.

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, 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 will 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 will immediately follow the client key exchange message.
 Structure of this message:
     struct {
          Signature signature;
     } CertificateVerify;

Dierks & Allen Standards Track [Page 45] RFC 2246 The TLS Protocol Version 1.0 January 1999

     The Signature type is defined in 7.4.3.
     CertificateVerify.signature.md5_hash
         MD5(handshake_messages);
     Certificate.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];
     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 handshake messages up to but not
         including this message. This is only data visible at the
         handshake layer and does not include record layer headers.

Dierks & Allen Standards Track [Page 46] RFC 2246 The TLS Protocol Version 1.0 January 1999

         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 hash contained in finished messages sent by the server
 incorporate Sender.server; those sent by the client incorporate
 Sender.client. 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 which 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.
     master_secret = PRF(pre_master_secret, "master secret",
                         ClientHello.random + ServerHello.random)
     [0..47];
 The master secret is always exactly 48 bytes in length. The length of
 the premaster secret will vary depending on key exchange method.

Dierks & Allen Standards Track [Page 47] RFC 2246 The TLS Protocol Version 1.0 January 1999

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.

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_DHE_DSS_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.

Dierks & Allen Standards Track [Page 48] RFC 2246 The TLS Protocol Version 1.0 January 1999

A. Protocol constant values

 This section describes protocol types and constants.

A.1. Record layer

  struct {
      uint8 major, minor;
  } ProtocolVersion;
  ProtocolVersion version = { 3, 1 };     /* TLS v1.0 */
  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 content[TLSCompressed.length];

Dierks & Allen Standards Track [Page 49] RFC 2246 The TLS Protocol Version 1.0 January 1999

      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),
          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(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 & Allen Standards Track [Page 50] RFC 2246 The TLS Protocol Version 1.0 January 1999

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 & Allen Standards Track [Page 51] RFC 2246 The TLS Protocol Version 1.0 January 1999

  } 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<1..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;
  select (SignatureAlgorithm)
  {   case anonymous: struct { };
      case rsa:
          digitally-signed struct {

Dierks & Allen Standards Track [Page 52] RFC 2246 The TLS Protocol Version 1.0 January 1999

              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),
      (255)
  } ClientCertificateType;
  opaque DistinguishedName<1..2^16-1>;
  struct {
      ClientCertificateType certificate_types<1..2^8-1>;
      DistinguishedName certificate_authorities<3..2^16-1>;
  } CertificateRequest;
  struct { } ServerHelloDone;

A.4.3. Client authentication and key exchange messages

  struct {
      select (KeyExchangeAlgorithm) {
          case rsa: EncryptedPreMasterSecret;
          case diffie_hellman: DiffieHellmanClientPublicValue;
      } exchange_keys;
  } ClientKeyExchange;
  struct {
      ProtocolVersion client_version;
      opaque random[46];
  } PreMasterSecret;
  struct {
      public-key-encrypted PreMasterSecret pre_master_secret;
  } EncryptedPreMasterSecret;
  enum { implicit, explicit } PublicValueEncoding;
  struct {
      select (PublicValueEncoding) {
          case implicit: struct {};
          case explicit: opaque DH_Yc<1..2^16-1>;

Dierks & Allen Standards Track [Page 53] RFC 2246 The TLS Protocol Version 1.0 January 1999

      } 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
 1.0.
 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.
  CipherSuite TLS_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
  CipherSuite TLS_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
  CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
  CipherSuite TLS_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
  CipherSuite TLS_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
  CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
  CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
  CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
  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

Dierks & Allen Standards Track [Page 54] RFC 2246 The TLS Protocol Version 1.0 January 1999

 (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
 parameters are signed by a DSS or RSA certificate, which 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_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
  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_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
  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_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
  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_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
  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_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
  CipherSuite TLS_DH_anon_WITH_RC4_128_MD5           = { 0x00,0x18 };
  CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };
  CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
  CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };

Note: All cipher suites whose first byte is 0xFF are considered

     private and can be used for defining local/experimental
     algorithms. Interoperability of such types is a local matter.

Note: Additional cipher suites can be registered by publishing an RFC

     which specifies the cipher suites, including the necessary TLS
     protocol information, including message encoding, premaster
     secret derivation, symmetric encryption and MAC calculation and
     appropriate reference information for the algorithms involved.
     The RFC editor's office may, at its discretion, choose to publish
     specifications for cipher suites which are not completely
     described (e.g., for classified algorithms) if it finds the
     specification to be of technical interest and completely
     specified.

Dierks & Allen Standards Track [Page 55] RFC 2246 The TLS Protocol Version 1.0 January 1999

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, idea }
     BulkCipherAlgorithm;
     enum { stream, block } CipherType;
     enum { true, false } IsExportable;
     enum { null, md5, sha } MACAlgorithm;
 /* 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;
         IsExportable is_exportable;
         MACAlgorithm mac_algorithm;
         uint8 hash_size;
         CompressionMethod compression_algorithm;
         opaque master_secret[48];
         opaque client_random[32];
         opaque server_random[32];
     } SecurityParameters;

Dierks & Allen Standards Track [Page 56] RFC 2246 The TLS Protocol Version 1.0 January 1999

B. Glossary

 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).
 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.

Dierks & Allen Standards Track [Page 57] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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]
 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]

Dierks & Allen Standards Track [Page 58] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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].
 RC4
     A stream cipher licensed by RSA Data Security [RSADSI]. A
     compatible cipher is described in [RC4].
 RSA
     A very widely used public-key algorithm that can be used for
     either encryption or digital signing. [RSA]
 salt
     Non-secret random data used to make export encryption keys resist
     precomputation attacks.
 server
     The server is the application entity that responds to requests
     for connections from clients. See also under client.

Dierks & Allen Standards Track [Page 59] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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, which 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 180-1. 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.
 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.

Dierks & Allen Standards Track [Page 60] RFC 2246 The TLS Protocol Version 1.0 January 1999

C. CipherSuite definitions

CipherSuite Is Key Cipher Hash

                           Exportable Exchange

TLS_NULL_WITH_NULL_NULL * NULL NULL NULL TLS_RSA_WITH_NULL_MD5 * RSA NULL MD5 TLS_RSA_WITH_NULL_SHA * RSA NULL SHA TLS_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 MD5 TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5 TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_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_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5 TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA 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

  • Indicates IsExportable is True
    Key
    Exchange
    Algorithm       Description                        Key size limit
    DHE_DSS         Ephemeral DH with DSS signatures   None
    DHE_DSS_EXPORT  Ephemeral DH with DSS signatures   DH = 512 bits
    DHE_RSA         Ephemeral DH with RSA signatures   None
    DHE_RSA_EXPORT  Ephemeral DH with RSA signatures   DH = 512 bits,
                                                       RSA = none
    DH_anon         Anonymous DH, no signatures        None
    DH_anon_EXPORT  Anonymous DH, no signatures        DH = 512 bits

Dierks & Allen Standards Track [Page 61] RFC 2246 The TLS Protocol Version 1.0 January 1999

    DH_DSS          DH with DSS-based certificates     None
    DH_DSS_EXPORT   DH with DSS-based certificates     DH = 512 bits
    DH_RSA          DH with RSA-based certificates     None
    DH_RSA_EXPORT   DH with RSA-based certificates     DH = 512 bits,
                                                       RSA = none
    NULL            No key exchange                    N/A
    RSA             RSA key exchange                   None
    RSA_EXPORT      RSA key exchange                   RSA = 512 bits
 Key size limit
     The key size limit gives the size of the largest public key that
     can be legally used for encryption in cipher suites that are
     exportable.
                       Key      Expanded   Effective   IV    Block
  Cipher       Type  Material Key Material  Key Bits  Size   Size
  NULL       * Stream   0          0           0        0     N/A
  IDEA_CBC     Block   16         16         128        8      8
  RC2_CBC_40 * Block    5         16          40        8      8
  RC4_40     * Stream   5         16          40        0     N/A
  RC4_128      Stream  16         16         128        0     N/A
  DES40_CBC  * Block    5          8          40        8      8
  DES_CBC      Block    8          8          56        8      8
  3DES_EDE_CBC Block   24         24         168        8      8
  • Indicates IsExportable is true.
 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
 Effective Key Bits
     How much entropy material is in the key material being fed into
     the encryption routines.
 IV Size
     How much data needs to be generated for the initialization
     vector. Zero for stream ciphers; equal to the block size for
     block ciphers.

Dierks & Allen Standards Track [Page 62] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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

Dierks & Allen Standards Track [Page 63] RFC 2246 The TLS Protocol Version 1.0 January 1999

D. Implementation Notes

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

D.1. Temporary RSA keys

 US Export restrictions limit RSA keys used for encryption to 512
 bits, but do not place any limit on lengths of RSA keys used for
 signing operations. Certificates often need to be larger than 512
 bits, since 512-bit RSA keys are not secure enough for high-value
 transactions or for applications requiring long-term security. Some
 certificates are also designated signing-only, in which case they
 cannot be used for key exchange.
 When the public key in the certificate cannot be used for encryption,
 the server signs a temporary RSA key, which is then exchanged. In
 exportable applications, the temporary RSA key should be the maximum
 allowable length (i.e., 512 bits). Because 512-bit RSA keys are
 relatively insecure, they should be changed often. For typical
 electronic commerce applications, it is suggested that keys be
 changed daily or every 500 transactions, and more often if possible.
 Note that while it is acceptable to use the same temporary key for
 multiple transactions, it must be signed each time it is used.
 RSA key generation is a time-consuming process. In many cases, a
 low-priority process can be assigned the task of key generation.
 Whenever a new key is completed, the existing temporary key can be
 replaced with the new one.

D.2. 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. To seed a 128-bit PRNG, one
 would thus require approximately 100 such timer values.

Dierks & Allen Standards Track [Page 64] RFC 2246 The TLS Protocol Version 1.0 January 1999

Warning: The seeding functions in RSAREF and versions of BSAFE prior to

        3.0 are order-independent. For example, if 1000 seed bits are
        supplied, one at a time, in 1000 separate calls to the seed
        function, the PRNG will end up in a state which depends only
        on the number of 0 or 1 seed bits in the seed data (i.e.,
        there are 1001 possible final states). Applications using
        BSAFE or RSAREF must take extra care to ensure proper seeding.
        This may be accomplished by accumulating seed bits into a
        buffer and processing them all at once or by processing an
        incrementing counter with every seed bit; either method will
        reintroduce order dependence into the seeding process.

D.3. 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.4. CipherSuites

 TLS supports a range of key sizes and security levels, including some
 which 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 & Allen Standards Track [Page 65] RFC 2246 The TLS Protocol Version 1.0 January 1999

E. Backward Compatibility With SSL

 For historical reasons and in order to avoid a profligate consumption
 of reserved port numbers, application protocols which are secured by
 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 version 1.0 and SSL 3.0 are very similar; thus, supporting both
 is easy. TLS clients who wish to negotiate with SSL 3.0 servers
 should send client hello messages using the SSL 3.0 record format and
 client hello structure, sending {3, 1} for the version field to note
 that they support TLS 1.0. If the server supports only SSL 3.0, it
 will respond with an SSL 3.0 server hello; if it supports TLS, with a
 TLS server hello. The negotiation then proceeds as appropriate for
 the negotiated protocol.
 Similarly, a TLS server which wishes to interoperate with SSL 3.0
 clients should accept SSL 3.0 client hello messages and respond with
 an SSL 3.0 server hello if an SSL 3.0 client hello is received which
 has a version field of {3, 0}, denoting that this client does not
 support TLS.
 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 1.0 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.
 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 };

Dierks & Allen Standards Track [Page 66] RFC 2246 The TLS Protocol Version 1.0 January 1999

     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 };

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.
     uint8 V2CipherSpec[3];
     struct {
         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];
         Random challenge;
     } V2ClientHello;
 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).

Dierks & Allen Standards Track [Page 67] RFC 2246 The TLS Protocol Version 1.0 January 1999

 session_id_length
     This field must have a value of either zero or 16. If zero, the
     client is creating a new session. If 16, the session_id field
     will contain the 16 bytes of session identification.
 challenge_length
     The length in bytes of the client's challenge to the server to
     authenticate itself. This value must be 32.
 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
     If this field's length is not zero, it will contain the
     identification for a session that the client wishes to resume.
 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 should use a TLS client hello.

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.

Dierks & Allen Standards Track [Page 68] RFC 2246 The TLS Protocol Version 1.0 January 1999

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
 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 certificate verify and 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

Dierks & Allen Standards Track [Page 69] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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 that 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 may be either 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 or
 DSS 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

     certificates but must comply with government-imposed size limits
     on keys used for key exchange.
 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.

Dierks & Allen Standards Track [Page 70] RFC 2246 The TLS Protocol Version 1.0 January 1999

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
 can 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.

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 be using
 40-bit encryption keys anyway. 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.

Dierks & Allen Standards Track [Page 71] RFC 2246 The TLS Protocol Version 1.0 January 1999

F.1.3. Detecting attacks against the handshake protocol

 An attacker might try to influence the handshake exchange to make the
 parties select different encryption algorithms than they would
 normally choose. Because many implementations will support 40-bit
 exportable encryption and some may even support null encryption or
 MAC algorithms, this attack is of particular concern.
 For this attack, an attacker must actively change one or more
 handshake messages. If this occurs, the client and server will
 compute different values for the handshake message hashes. As a
 result, the parties will not accept each others' finished messages.
 Without the master_secret, the attacker cannot repair the finished
 messages, so the attack will be discovered.

F.1.4. Resuming sessions

 When a connection is established by resuming a session, new
 ClientHello.random and ServerHello.random values are hashed with the
 session's master_secret. Provided that the master_secret has not been
 compromised and that the secure hash operations used to produce the
 encryption keys and MAC 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.

Dierks & Allen Standards Track [Page 72] RFC 2246 The TLS Protocol Version 1.0 January 1999

 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.
 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. 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 & Allen Standards Track [Page 73] RFC 2246 The TLS Protocol Version 1.0 January 1999

G. Patent Statement

 Some of the cryptographic algorithms proposed for use in this
 protocol have patent claims on them. In addition Netscape
 Communications Corporation has a patent claim on the Secure Sockets
 Layer (SSL) work that this standard is based on. The Internet
 Standards Process as defined in RFC 2026 requests that a statement be
 obtained from a Patent holder indicating that a license will be made
 available to applicants under reasonable terms and conditions.
 The Massachusetts Institute of Technology has granted RSA Data
 Security, Inc., exclusive sub-licensing rights to the following
 patent issued in the United States:
     Cryptographic Communications System and Method ("RSA"), No.
     4,405,829
 Netscape Communications Corporation has been issued the following
 patent in the United States:
     Secure Socket Layer Application Program Apparatus And Method
     ("SSL"), No. 5,657,390
 Netscape Communications has issued the following statement:
     Intellectual Property Rights
     Secure Sockets Layer
     The United States Patent and Trademark Office ("the PTO")
     recently issued U.S. Patent No. 5,657,390 ("the SSL Patent")  to
     Netscape for inventions described as Secure Sockets Layers
     ("SSL"). The IETF is currently considering adopting SSL as a
     transport protocol with security features.  Netscape encourages
     the royalty-free adoption and use of the SSL protocol upon the
     following terms and conditions:
  • If you already have a valid SSL Ref license today which

includes source code from Netscape, an additional patent

         license under the SSL patent is not required.
  • If you don't have an SSL Ref license, you may have a royalty

free license to build implementations covered by the SSL

         Patent Claims or the IETF TLS specification provided that you
         do not to assert any patent rights against Netscape or other
         companies for the implementation of SSL or the IETF TLS
         recommendation.

Dierks & Allen Standards Track [Page 74] RFC 2246 The TLS Protocol Version 1.0 January 1999

     What are "Patent Claims":
     Patent claims are claims in an issued foreign or domestic patent
     that:
      1) must be infringed in order to implement methods or build
         products according to the IETF TLS specification;  or
      2) patent claims which require the elements of the SSL patent
         claims and/or their equivalents to be infringed.
 The Internet Society, Internet Architecture Board, Internet
 Engineering Steering Group and the Corporation for National Research
 Initiatives take no position on the validity or scope of the patents
 and patent applications, nor on the appropriateness of the terms of
 the assurance. The Internet Society and other groups mentioned above
 have not made any determination as to any other intellectual property
 rights which may apply to the practice of this standard.  Any further
 consideration of these matters is the user's own responsibility.

Security Considerations

 Security issues are discussed throughout this memo.

References

 [3DES]   W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
          IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
 [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.
 [DES]    ANSI X3.106, "American National Standard for Information
          Systems-Data Link Encryption," American National Standards
          Institute, 1983.
 [DH1]    W. Diffie and M. E. Hellman, "New Directions in
          Cryptography," IEEE Transactions on Information Theory, V.
          IT-22, n. 6, Jun 1977, pp. 74-84.
 [DSS]    NIST FIPS PUB 186, "Digital Signature Standard," National
          Institute of Standards and Technology, U.S. Department of
          Commerce, May 18, 1994.
 [FTP]    Postel J., and J. Reynolds, "File Transfer Protocol", STD 9,
          RFC 959, October 1985.

Dierks & Allen Standards Track [Page 75] RFC 2246 The TLS Protocol Version 1.0 January 1999

 [HTTP]   Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
          Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
 [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.
 [MD2]    Kaliski, B., "The MD2 Message Digest Algorithm", RFC 1319,
          April 1992.
 [MD5]    Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
          April 1992.
 [PKCS1]  RSA Laboratories, "PKCS #1: RSA Encryption Standard,"
          version 1.5, November 1993.
 [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.
 [PKIX]   Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
          Public Key Infrastructure: Part I: X.509 Certificate and CRL
          Profile", RFC 2459, January 1999.
 [RC2]    Rivest, R., "A Description of the RC2(r) Encryption
          Algorithm", RFC 2268, January 1998.
 [RC4]    Thayer, R. and K. Kaukonen, A Stream Cipher Encryption
          Algorithm, Work in Progress.
 [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.
 [RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782
 [SCH]    B. Schneier. Applied Cryptography: Protocols, Algorithms,
          and Source Code in C, Published by John Wiley & Sons, Inc.
          1994.

Dierks & Allen Standards Track [Page 76] RFC 2246 The TLS Protocol Version 1.0 January 1999

 [SHA]    NIST FIPS PUB 180-1, "Secure Hash Standard," National
          Institute of Standards and Technology, U.S. Department of
          Commerce, Work in Progress, May 31, 1994.
 [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.
 [TCP]    Postel, J., "Transmission Control Protocol," STD 7, RFC 793,
          September 1981.
 [TEL]    Postel J., and J. Reynolds, "Telnet Protocol
          Specifications", STD 8, RFC 854, May 1993.
 [TEL]    Postel J., and J. Reynolds, "Telnet Option Specifications",
          STD 8, RFC 855, May 1993.
 [X509]   CCITT. Recommendation X.509: "The Directory - Authentication
          Framework". 1988.
 [XDR]    R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External
          Data Representation Standard, August 1995.

Credits

 Win Treese
 Open Market
 EMail: treese@openmarket.com
 Editors
 Christopher Allen                  Tim Dierks
 Certicom                           Certicom
 EMail: callen@certicom.com         EMail: tdierks@certicom.com
 Authors' Addresses
 Tim Dierks                         Philip L. Karlton
 Certicom                           Netscape Communications
 EMail: tdierks@certicom.com

Dierks & Allen Standards Track [Page 77] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Alan O. Freier                     Paul C. Kocher
 Netscape Communications            Independent Consultant
 EMail: freier@netscape.com         EMail: pck@netcom.com
 Other contributors
 Martin Abadi                       Robert Relyea
 Digital Equipment Corporation      Netscape Communications
 EMail: ma@pa.dec.com               EMail: relyea@netscape.com
 Ran Canetti                        Jim Roskind
 IBM Watson Research Center         Netscape Communications
 EMail: canetti@watson.ibm.com      EMail: jar@netscape.com
 Taher Elgamal                      Micheal J. Sabin, Ph. D.
 Securify                           Consulting Engineer
 EMail: elgamal@securify.com        EMail: msabin@netcom.com
 Anil R. Gangolli                   Dan Simon
 Structured Arts Computing Corp.    Microsoft
 EMail: gangolli@structuredarts.com EMail:  dansimon@microsoft.com
 Kipp E.B. Hickman                  Tom Weinstein
 Netscape Communications            Netscape Communications
 EMail: kipp@netscape.com           EMail: tomw@netscape.com
 Hugo Krawczyk
 IBM Watson Research Center
 EMail: hugo@watson.ibm.com

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/>.

Dierks & Allen Standards Track [Page 78] RFC 2246 The TLS Protocol Version 1.0 January 1999

 Archives of the list can be found at:
     <http://www.imc.org/ietf-tls/mail-archive/>

Dierks & Allen Standards Track [Page 79] RFC 2246 The TLS Protocol Version 1.0 January 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Dierks & Allen Standards Track [Page 80]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2246.txt · Last modified: 1999/01/26 22:02 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki