RFC 2246
This document is obsolete. Please refer to RFC 4346.

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


   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

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   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 Null or standard stream cipher                           19 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 Hello request                                            33 Client hello                                             34 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 RSA encrypted premaster secret message                   44 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

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   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

     - 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.

     - 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

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       this mode while another protocol is using the Record Protocol as
       a transport for negotiating security parameters.

   The TLS Record Protocol is used for encapsulation of various higher
   level protocols. One such encapsulated protocol, the TLS Handshake
   Protocol, allows the server and client to authenticate each other and
   to negotiate an encryption algorithm and cryptographic keys before
   the application protocol transmits or receives its first byte of
   data. The TLS Handshake Protocol provides connection security that
   has three basic properties:

     - The peer's identity can be authenticated using asymmetric, or
       public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
       authentication can be made optional, but is generally required
       for at least one of the peers.

     - The negotiation of a shared secret is secure: the negotiated
       secret is unavailable to eavesdroppers, and for any authenticated
       connection the secret cannot be obtained, even by an attacker who
       can place himself in the middle of the connection.

     - The negotiation is reliable: no attacker can modify the
       negotiation communication without being detected by the parties
       to the communication.

   One advantage of TLS is that it is application protocol independent.
   Higher level protocols can layer on top of the TLS Protocol
   transparently. The TLS standard, however, does not specify how
   protocols add security with TLS; the decisions on how to initiate TLS
   handshaking and how to interpret the authentication certificates
   exchanged are left 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

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       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

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.

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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

   Single byte entities containing uninterpreted data are of type

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 */

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   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

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   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

       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]];

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   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

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       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.

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   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,

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   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);

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   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.

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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

   master secret
       A 48 byte secret shared between the two peers in the connection.

   client random
       A 32 byte value provided by the client.

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   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.

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   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

   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;

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       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;

       The higher level protocol used to process the enclosed fragment.

       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).

       The length (in bytes) of the following TLSPlaintext.fragment.
       The length should not exceed 2^14.

       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.

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   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;

       The length (in bytes) of the following TLSCompressed.fragment.
       The length should not exceed 2^14 + 1024.

       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;

       The type field is identical to TLSCompressed.type.

       The version field is identical to TLSCompressed.version.

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

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

   where "+" denotes concatenation.

       The sequence number for this record.

       The hashing algorithm specified by

   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. 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.

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       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

       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.

       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

 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.

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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

   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 +

   until enough output has been generated. Then the key_block is
   partitioned as follows:


   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.

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   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 =
                                  "client write key",
                                  SecurityParameters.client_random +
       final_server_write_key =
                                  "server write key",
                                  SecurityParameters.client_random +

   Exportable encryption algorithms derive their IVs solely from the
   random values from the hello messages:

       iv_block = PRF("", "IV block", SecurityParameters.client_random +

   The iv_block is partitioned into two initialization vectors as the
   key_block was above:


   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_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]

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       final_client_write_key  = PRF(client_write_key,
                                     "client write key",
                                     client_random +
       final_server_write_key  = PRF(server_write_key,
                                     "server write key",
                                     client_random +

       iv_block                = PRF("", "IV block", client_random +
       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

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   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

7.2. 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 {

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       } 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.

       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.

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   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

       An inappropriate message was received. This alert is always fatal
       and should never be observed in communication between proper

       This alert is returned if a record is received with an incorrect
       MAC. This message is always fatal.

       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.

       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.

       The decompression function received improper input (e.g. data
       that would expand to excessive length). This message is always

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       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.

       A certificate was corrupt, contained signatures that did not
       verify correctly, etc.

       A certificate was of an unsupported type.

       A certificate was revoked by its signer.

       A certificate has expired or is not currently valid.

       Some other (unspecified) issue arose in processing the
       certificate, rendering it unacceptable.

       A field in the handshake was out of range or inconsistent with
       other fields. This is always fatal.

       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.

       A valid certificate was received, but when access control was
       applied, the sender decided not to proceed with negotiation.
       This message is always fatal.

       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.

       A handshake cryptographic operation failed, including being
       unable to correctly verify a signature, decrypt a key exchange,
       or validate a finished message.

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       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.

       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.

       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

       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.

       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.

       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

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   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:

     - Exchange hello messages to agree on algorithms, exchange random
       values, and check for session resumption.

     - Exchange the necessary cryptographic parameters to allow the
       client and server to agree on a premaster secret.

     - Exchange certificates and cryptographic information to allow the
       client and server to authenticate themselves.

     - Generate a master secret from the premaster secret and exchanged
       random values.

     - Provide security parameters to the record layer.

     - Allow the client and server to verify that their peer has
       calculated the same security parameters and that the handshake
       occurred without tampering by an attacker.

   Note that higher layers should not be overly reliant on 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.

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   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

   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

   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

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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                  -------->
                                   <--------      ServerHelloDone
      Finished                     -------->
                                   <--------             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.

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RFC 2246              The TLS Protocol Version 1.0          January 1999

      Client                                                Server

      ClientHello                   -------->
                                    <--------             Finished
      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;

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   The handshake protocol messages are presented below in the order they
   must be sent; sending handshake messages in an unexpected order
   results in a fatal error. Unneeded handshake messages can be omitted,
   however. Note one exception to the ordering: the Certificate message
   is used twice in the handshake (from server to client, then from
   client to server), but described only in its first position. The one
   message 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. 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.

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RFC 2246              The TLS Protocol Version 1.0          January 1999 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;

       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.

       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

       opaque SessionID<0..32>;

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       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

   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;

       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).

       A client-generated random structure.

       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.

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       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.

       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. 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;

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       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).

       This structure is generated by the server and must be different
       from (and independent of) ClientHello.random.

       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.

       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.

       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

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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

       DHE_DSS                 DSS public key.

       DHE_DSS_EXPORT          DSS public key.

       DHE_RSA                 RSA public key which can be used for

       DHE_RSA_EXPORT          RSA public key which can be used for

       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;

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       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)

       It is not legal to send the server key exchange message for the
       following key exchange methods:

           RSA_EXPORT (when the public key in the server certificate is
           less than or equal to 512 bits in length)

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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;

           The modulus of the server's temporary RSA key.

           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 */

           The prime modulus used for the Diffie-Hellman operation.

           The generator used for the Diffie-Hellman operation.

           The server's Diffie-Hellman public value (g^X mod p).

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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;

           The server's key exchange parameters.

           For non-anonymous key exchanges, a hash of the corresponding
           params value, with the signature appropriate to that hash

           MD5(ClientHello.random + ServerHello.random + ServerParams);

           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

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   Structure of this message:
       enum {
           rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
       } ClientCertificateType;

       opaque DistinguishedName<1..2^16-1>;

       struct {
           ClientCertificateType certificate_types<1..2^8-1>;
           DistinguishedName certificate_authorities<3..2^16-1>;
       } CertificateRequest;

              This field is a list of the types of certificates requested,
              sorted in order of the server's preference.

           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;

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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

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

   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

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

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RFC 2246              The TLS Protocol Version 1.0          January 1999

           } exchange_keys;
       } ClientKeyExchange; 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

   Structure of this message:
       struct {
           ProtocolVersion client_version;
           opaque random[46];
       } PreMasterSecret;

           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.

           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.

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           This random value is generated by the client and is used to
           generate the master secret, as specified in Section 8.1. 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;

           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.

           Yc needs to be sent.

       struct {
           select (PublicValueEncoding) {
               case implicit: struct { };
               case explicit: opaque dh_Yc<1..2^16-1>;
           } dh_public;
       } ClientDiffieHellmanPublic;

           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;

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RFC 2246              The TLS Protocol Version 1.0          January 1999

       The Signature type is defined in 7.4.3.



   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;

           PRF(master_secret, finished_label, MD5(handshake_messages) +
           SHA-1(handshake_messages)) [0..11];

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

           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.

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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

       master_secret = PRF(pre_master_secret, "master secret",
                           ClientHello.random + ServerHello.random)

   The master secret is always exactly 48 bytes in length. The length of
   the premaster secret will vary depending on key exchange method.

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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

   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

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

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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];

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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 {
        } AlertDescription;

    struct {
        AlertLevel level;
        AlertDescription description;
    } Alert;

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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>;

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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 {

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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),
    } 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>;

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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

   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

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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

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 }

       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;

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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 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).

       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.

       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

   client write key
       The key used to encrypt data written by the client.

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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.

       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.

   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.

       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

       A 64-bit block cipher designed by Xuejia Lai and James Massey.

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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 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.

       A block cipher developed by Ron Rivest at RSA Data Security, Inc.
       [RSADSI] described in [RC2].

       A stream cipher licensed by RSA Data Security [RSADSI]. A
       compatible cipher is described in [RC4].

       A very widely used public-key algorithm that can be used for
       either encryption or digital signing. [RSA]

       Non-secret random data used to make export encryption keys resist
       precomputation attacks.

       The server is the application entity that responds to requests
       for connections from clients. See also under client.

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RFC 2246              The TLS Protocol Version 1.0          January 1999

       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

   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.

       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]

       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.

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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_WITH_IDEA_CBC_SHA               RSA            IDEA_CBC     SHA
TLS_RSA_WITH_DES_CBC_SHA                RSA            DES_CBC      SHA
TLS_DH_DSS_WITH_DES_CBC_SHA             DH_DSS         DES_CBC      SHA
TLS_DH_RSA_WITH_DES_CBC_SHA             DH_RSA         DES_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

      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

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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

                         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.

       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.

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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

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

       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;

       This field, in conjunction with the version field, identifies a
       version 2 client hello message. The value should be one (1).

       The highest version of the protocol supported by the client
       (equals ProtocolVersion.version, see Appendix A.1).

       This field is the total length of the field cipher_specs. It
       cannot be zero and must be a multiple of the V2CipherSpec length

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RFC 2246              The TLS Protocol Version 1.0          January 1999

       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.

       The length in bytes of the client's challenge to the server to
       authenticate itself. This value must be 32.

       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

       If this field's length is not zero, it will contain the
       identification for a session that the client wishes to resume.

       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.

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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.

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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.

   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

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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

        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.


   [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

   [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-

   [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.

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.


   Win Treese
   Open Market

   EMail: treese@openmarket.com


   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


   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

Dierks & Allen              Standards Track                    [Page 78]

RFC 2246              The TLS Protocol Version 1.0          January 1999

   Archives of the list can be found at:

Dierks & Allen              Standards Track                    [Page 79]

RFC 2246              The TLS Protocol Version 1.0          January 1999

Full Copyright Statement

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   or assist in its implementation may be prepared, copied, published
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   This document and the information contained herein is provided on an

Dierks & Allen              Standards Track                    [Page 80]