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RFC 2246


The TLS Protocol Version 1.0

Part 3 of 3, p. 49 to 80
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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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        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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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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    } 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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                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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        } 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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   (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

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 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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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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   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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   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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       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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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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      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.

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   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

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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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 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

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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 };

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       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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       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.

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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

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   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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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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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.

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   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.

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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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       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.

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   [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.

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   [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



   Christopher Allen                  Tim Dierks
   Certicom                           Certicom

   EMail:         EMail:

   Authors' Addresses

   Tim Dierks                         Philip L. Karlton
   Certicom                           Netscape Communications


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   Alan O. Freier                     Paul C. Kocher
   Netscape Communications            Independent Consultant

   EMail:         EMail:

   Other contributors

   Martin Abadi                       Robert Relyea
   Digital Equipment Corporation      Netscape Communications

   EMail:               EMail:

   Ran Canetti                        Jim Roskind
   IBM Watson Research Center         Netscape Communications

   EMail:      EMail:

   Taher Elgamal                      Micheal J. Sabin, Ph. D.
   Securify                           Consulting Engineer

   EMail:        EMail:

   Anil R. Gangolli                   Dan Simon
   Structured Arts Computing Corp.    Microsoft

   EMail: EMail:

   Kipp E.B. Hickman                  Tom Weinstein
   Netscape Communications            Netscape Communications

   EMail:           EMail:

   Hugo Krawczyk
   IBM Watson Research Center



   The discussion list for the IETF TLS working group is located at the
   e-mail address <>. Information on the
   group and information on how to subscribe to the list is at

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   Archives of the list can be found at:

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Full Copyright Statement

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