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RFC 6101

 
 
 

The Secure Sockets Layer (SSL) Protocol Version 3.0

Part 3 of 3, p. 36 to 67
Prev RFC Part

 


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6.  Cryptographic Computations

   The key exchange, authentication, encryption, and MAC algorithms are
   determined by the cipher_suite selected by the server and revealed in
   the server hello message.

6.1.  Asymmetric Cryptographic Computations

   The asymmetric algorithms are used in the handshake protocol to
   authenticate parties and to generate shared keys and secrets.

   For Diffie-Hellman, RSA, and FORTEZZA, the same algorithm is used to
   convert the pre_master_secret into the master_secret.  The
   pre_master_secret should be deleted from memory once the
   master_secret has been computed.

        master_secret =
          MD5(pre_master_secret + SHA('A' + pre_master_secret +
              ClientHello.random + ServerHello.random)) +
          MD5(pre_master_secret + SHA('BB' + pre_master_secret +
              ClientHello.random + ServerHello.random)) +
          MD5(pre_master_secret + SHA('CCC' + pre_master_secret +
              ClientHello.random + ServerHello.random));

6.1.1.  RSA

   When RSA is used for server authentication and key exchange, a 48-
   byte pre_master_secret is generated by the client, encrypted under
   the server's public key, and sent to the server.  The server uses its
   private key to decrypt the pre_master_secret.  Both parties then
   convert the pre_master_secret into the master_secret, as specified
   above.

   RSA digital signatures are performed using PKCS #1 [PKCS1] block
   type 1.  RSA public key encryption is performed using PKCS #1 block
   type 2.

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6.1.2.  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.

6.1.3.  FORTEZZA

   A random 48-byte pre_master_secret is sent encrypted under the TEK
   and its IV.  The server decrypts the pre_master_secret and converts
   it into a master_secret, as specified above.  Bulk cipher keys and
   IVs for encryption are generated by the client's token and exchanged
   in the key exchange message; the master_secret is only used for MAC
   computations.

6.2.  Symmetric Cryptographic Calculations and the CipherSpec

   The technique used to encrypt and verify the integrity of SSL records
   is specified by the currently active CipherSpec.  A typical example
   would be to encrypt data using DES and generate authentication codes
   using MD5.  The encryption and MAC algorithms are set to
   SSL_NULL_WITH_NULL_NULL at the beginning of the SSL handshake
   protocol, indicating that no message authentication or encryption is
   performed.  The handshake protocol is used to negotiate a more secure
   CipherSpec and to generate cryptographic keys.

6.2.1.  The Master Secret

   Before secure encryption or integrity verification can be performed
   on records, the client and server need to generate shared secret
   information known only to themselves.  This value is a 48-byte
   quantity called the master secret.  The master secret is used to
   generate keys and secrets for encryption and MAC computations.  Some
   algorithms, such as FORTEZZA, may have their own procedure for
   generating encryption keys (the master secret is used only for MAC
   computations in FORTEZZA).

6.2.2.  Converting the Master Secret into Keys and MAC Secrets

   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 CipherSpec (see Appendix A.7).  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

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   values, such as FORTEZZA keys communicated in the KeyExchange
   message, are empty.  The following inputs are available to the key
   definition process:

          opaque MasterSecret[48]
          ClientHello.random
          ServerHello.random

   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 =
          MD5(master_secret + SHA(`A' + master_secret +
                                  ServerHello.random +
                                  ClientHello.random)) +
          MD5(master_secret + SHA(`BB' + master_secret +
                                  ServerHello.random +
                                  ClientHello.random)) +
          MD5(master_secret + SHA(`CCC' + master_secret +
                                  ServerHello.random +
                                  ClientHello.random)) + [...];

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

        client_write_MAC_secret[CipherSpec.hash_size]
        server_write_MAC_secret[CipherSpec.hash_size]
        client_write_key[CipherSpec.key_material]
        server_write_key[CipherSpec.key_material]
        client_write_IV[CipherSpec.IV_size] /* non-export ciphers */
        server_write_IV[CipherSpec.IV_size] /* non-export ciphers */

   Any extra key_block material is discarded.

   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 = MD5(client_write_key +
                                     ClientHello.random +
                                     ServerHello.random);
        final_server_write_key = MD5(server_write_key +
                                     ServerHello.random +
                                     ClientHello.random);

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   Exportable encryption algorithms derive their IVs from the random
   messages:

        client_write_IV = MD5(ClientHello.random + ServerHello.random);
        server_write_IV = MD5(ServerHello.random + ClientHello.random);

   MD5 outputs are trimmed to the appropriate size by discarding the
   least-significant bytes.

6.2.2.1.  Export Key Generation Example

   SSL_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.  MD5 produces 16 bytes
   of output per call, so three calls to MD5 are required.  The MD5
   outputs are concatenated into a 48-byte key_block with the first MD5
   call providing bytes zero through 15, the second providing bytes 16
   through 31, etc.  The key_block is partitioned, and the write keys
   are salted because this is an exportable encryption algorithm.

        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]
        final_client_write_key = MD5(client_write_key +
                                     ClientHello.random +
                                     ServerHello.random)[0..15];
        final_server_write_key = MD5(server_write_key +
                                     ServerHello.random +
                                     ClientHello.random)[0..15];
        client_write_IV = MD5(ClientHello.random +
                              ServerHello.random)[0..7];
        server_write_IV = MD5(ServerHello.random +
                              ClientHello.random)[0..7];

7.  Security Considerations

   See Appendix F.

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8.  Informative References

   [DH1]      Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory V.
              IT-22, n. 6, pp. 74-84, June 1977.

   [SSL-2]    Hickman, K., "The SSL Protocol", February 1995.

   [3DES]     Tuchman, W., "Hellman Presents No Shortcut Solutions To
              DES", IEEE Spectrum, v. 16, n. 7, pp 40-41, July 1979.

   [DES]      ANSI X3.106, "American National Standard for Information
              Systems-Data Link Encryption", American National
              Standards Institute, 1983.

   [DSS]      NIST FIPS PUB 186, "Digital Signature Standard", National
              Institute of Standards and Technology U.S. Department of
              Commerce, May 1994.

   [FOR]      NSA X22, "FORTEZZA: Application Implementers Guide",
              Document # PD4002103-1.01, April 1995.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

   [RFC1832]  Srinivasan, R., "XDR: External Data Representation
              Standard", RFC 1832, August 1995.

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   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [IDEA]     Lai, X., "On the Design and Security of Block Ciphers",
              ETH Series in Information Processing, v. 1, Konstanz:
              Hartung-Gorre Verlag, 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.

   [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the ACM v. 21, n. 2 pp.
              120-126., February 1978.

   [SCH]      Schneier, B., "Applied Cryptography: Protocols,
              Algorithms, and Source Code in C", John Wiley & Sons,
              1994.

   [SHA]      NIST FIPS PUB 180-1, "Secure Hash Standard", May 1994.

              National Institute of Standards and Technology, U.S.
              Department of Commerce, DRAFT

   [X509]     CCITT, "The Directory - Authentication Framework",
              Recommendation X.509 , 1988.

   [RSADSI]   RSA Data Security, Inc., "Unpublished works".

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Appendix A.  Protocol Constant Values

   This section describes protocol types and constants.

A.1.  Record Layer

        struct {
            uint8 major, minor;
        } ProtocolVersion;

        ProtocolVersion version = { 3,0 };

        enum {
            change_cipher_spec(20), alert(21), handshake(22),
            application_data(23), (255)
        } ContentType;

        struct {
            ContentType type;
            ProtocolVersion version;
            uint16 length;
            opaque fragment[SSLPlaintext.length];
        } SSLPlaintext;

        struct {
            ContentType type;
            ProtocolVersion version;
            uint16 length;
            opaque fragment[SSLCompressed.length];
        } SSLCompressed;

        struct {
            ContentType type;
            ProtocolVersion version;
            uint16 length;
            select (CipherSpec.cipher_type) {
                case stream: GenericStreamCipher;
                case block:  GenericBlockCipher;
            } fragment;
        } SSLCiphertext;

        stream-ciphered struct {
            opaque content[SSLCompressed.length];
            opaque MAC[CipherSpec.hash_size];
        } GenericStreamCipher;

        block-ciphered struct {
            opaque content[SSLCompressed.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 {
            close_notify(0),
            unexpected_message(10),
            bad_record_mac(20),
            decompression_failure(30),
            handshake_failure(40),
            no_certificate(41),
            bad_certificate(42),
            unsupported_certificate(43),
            certificate_revoked(44),
            certificate_expired(45),
            certificate_unknown(46),
            illegal_parameter (47),
            (255)
        } 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_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_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<0..2^16-1>;
            CompressionMethod compression_methods<0..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, fortezza_kea } 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 {
            opaque r_s [128]
        } ServerFortezzaParams

        struct {
            select (KeyExchangeAlgorithm) {
                case diffie_hellman:
                    ServerDHParams params;
                    Signature signed_params;
                case rsa:
                    ServerRSAParams params;
                    Signature signed_params;
                case fortezza_kea:
                    ServerFortezzaParams params;
            };
        } ServerKeyExchange;

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        enum { anonymous, rsa, dsa } SignatureAlgorithm;

        digitally-signed struct {
            select(SignatureAlgorithm) {
                case anonymous: struct { };
                case rsa:
                    opaque md5_hash[16];
                    opaque sha_hash[20];
                case dsa:
                    opaque sha_hash[20];
            };
        } Signature;


        enum {
            RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3),
            DSS_fixed_DH(4), RSA_ephemeral_DH(5), DSS_ephemeral_DH(6),
            FORTEZZA_MISSI(20), (255)
        } CertificateType;

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

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

        struct { } ServerHelloDone;

A.5.  Client Authentication and Key Exchange Messages

        struct {
            select (KeyExchangeAlgorithm) {
                case rsa: EncryptedPreMasterSecret;
                case diffie_hellman: DiffieHellmanClientPublicValue;
                case fortezza_kea: FortezzaKeys;
            } exchange_keys;
        } ClientKeyExchange;

        struct {
            ProtocolVersion client_version;
            opaque random[46];
        } PreMasterSecret;

        struct {
            public-key-encrypted PreMasterSecret pre_master_secret;
        } EncryptedPreMasterSecret;

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        struct {
            opaque y_c<0..128>;
            opaque r_c[128];
            opaque y_signature[40];
            opaque wrapped_client_write_key[12];
            opaque wrapped_server_write_key[12];
            opaque client_write_iv[24];
            opaque server_write_iv[24];
            opaque master_secret_iv[24];
            opaque encrypted_preMasterSecret[48];
        } FortezzaKeys;

        enum { implicit, explicit } PublicValueEncoding;

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

        struct {
            Signature signature;
        } CertificateVerify;

A.5.1.  Handshake Finalization Message

        struct {
            opaque md5_hash[16];
            opaque sha_hash[20];
        } Finished;

A.6.  The CipherSuite

   The following values define the CipherSuite codes used in the client
   hello and server hello messages.

   A CipherSuite defines a cipher specifications supported in SSL
   version 3.0.

     CipherSuite SSL_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.

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     CipherSuite SSL_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
     CipherSuite SSL_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
     CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
     CipherSuite SSL_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
     CipherSuite SSL_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
     CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
     CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
     CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
     CipherSuite SSL_RSA_WITH_DES_CBC_SHA               = { 0x00,0x09 };
     CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA          = { 0x00,0x0A };

   The following CipherSuite definitions are used for server-
   authenticated (and optionally client-authenticated) Diffie-Hellman.
   DH denotes cipher suites in which the server's certificate contains
   the Diffie-Hellman parameters signed by the certificate authority
   (CA).  DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
   parameters are signed by a DSS or RSA certificate, which has been
   signed by the CA.  The signing algorithm used is specified after the
   DH or DHE parameter.  In all cases, the client must have the same
   type of certificate, and must use the Diffie-Hellman parameters
   chosen by the server.

     CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
     CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA            = { 0x00,0x0C };
     CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x0D };
     CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
     CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA            = { 0x00,0x0F };
     CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x10 };
     CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
     CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA           = { 0x00,0x12 };
     CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x13 };
     CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
     CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA           = { 0x00,0x15 };
     CipherSuite SSL_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 strongly discouraged.

     CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
     CipherSuite SSL_DH_anon_WITH_RC4_128_MD5           = { 0x00,0x18 };
     CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };
     CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
     CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };

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   The final cipher suites are for the FORTEZZA token.

     CipherSuite SSL_FORTEZZA_KEA_WITH_NULL_SHA         = { 0X00,0X1C };
     CipherSuite SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA = { 0x00,0x1D };
     CipherSuite SSL_FORTEZZA_KEA_WITH_RC4_128_SHA      = { 0x00,0x1E };

   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.

A.7.  The CipherSpec

   A cipher suite identifies a CipherSpec.  These structures are part of
   the SSL session state.  The CipherSpec includes:

        enum { stream, block } CipherType;

        enum { true, false } IsExportable;

        enum { null, rc4, rc2, des, 3des, des40, fortezza }
            BulkCipherAlgorithm;

        enum { null, md5, sha } MACAlgorithm;

        struct {
            BulkCipherAlgorithm bulk_cipher_algorithm;
            MACAlgorithm mac_algorithm;
            CipherType cipher_type;
            IsExportable is_exportable
            uint8 hash_size;
            uint8 key_material;
            uint8 IV_size;
        } CipherSpec;

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Appendix B.  Glossary

   application protocol:  An application protocol is a protocol that
      normally layers directly on top of the transport layer (e.g.,
      TCP/IP [RFC0793]/[RFC0791]).  Examples include HTTP [RFC1945],
      TELNET [RFC0959], FTP [RFC0854], and SMTP.

   asymmetric cipher:  See public key cryptography.

   authentication:  Authentication is the ability of one entity to
      determine the identity of another entity.

   block cipher:  A block cipher is an algorithm that operates on
      plaintext in groups of bits, called blocks. 64 bits is a typical
      block size.

   bulk cipher:  A symmetric encryption algorithm used to encrypt large
      quantities of data.

   cipher block chaining (CBC) mode:  CBC is a mode in which every
      plaintext block encrypted with the block cipher is first
      exclusive-ORed with the previous ciphertext block (or, in the case
      of the first block, with the initialization vector).

   certificate:  As part of the X.509 protocol (a.k.a.  ISO
      Authentication framework), certificates are assigned by a trusted
      certificate authority and provide verification of a party's
      identity and may also supply its public key.

   client:  The application entity that initiates a connection to a
      server.

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

   client write MAC secret:  The secret data used to authenticate data
      written by the client.

   connection:  A connection is a transport (in the OSI layering model
      definition) that provides a suitable type of service.  For SSL,
      such connections are peer-to-peer relationships.  The connections
      are transient.  Every connection is associated with one session.

   Data Encryption Standard (DES):  DES is a very widely used symmetric
      encryption algorithm.  DES is a block cipher [DES] [3DES].

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   Digital Signature Standard:  (DSS) A standard for digital signing,
      including the Digital Signature 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.

   FORTEZZA:  A PCMCIA card that provides both encryption and digital
      signing.

   handshake:  An initial negotiation between client and server that
      establishes the parameters of their transactions.

   Initialization Vector (IV):  When a block cipher is used in CBC mode,
      the initialization vector is exclusive-ORed with the first
      plaintext block prior to encryption.

   IDEA:  A 64-bit block cipher designed by Xuejia Lai and James Massey
      [IDEA].

   Message Authentication Code (MAC):  A Message Authentication Code is
      a one-way hash computed from a message and some secret data.  Its
      purpose is to detect if the message has been altered.

   master secret:  Secure secret data used for generating encryption
      keys, MAC secrets, and IVs.

   MD5:  MD5 [RFC1321] is a secure hashing function that converts an
      arbitrarily long data stream into a digest of fixed size.

   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.

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   RC2, RC4:  Proprietary bulk ciphers from RSA Data Security, Inc.
      (There is no good reference to these as they are unpublished
      works; however, see [RSADSI]).  RC2 is a block cipher and RC4 is a
      stream cipher.

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

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

   server:  The server is the application entity that responds to
      requests for connections from clients.  The server is passive,
      waiting for requests from clients.

   session:  An SSL session is an association between a client and a
      server.  Sessions are created by the handshake protocol.  Sessions
      define a set of cryptographic security parameters, which can be
      shared among multiple connections.  Sessions are used to avoid the
      expensive negotiation of new security parameters for each
      connection.

   session identifier:  A session identifier is a value generated by a
      server that identifies a particular session.

   server write key:  The key used to encrypt data written by the
      server.

   server write MAC secret:  The secret data used to authenticate data
      written by the server.

   SHA:  The Secure Hash Algorithm is defined in FIPS PUB 180-1.  It
      produces a 20-byte output [SHA].

   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.

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Appendix C.  CipherSuite Definitions

CipherSuite                  Is         Key            Cipher       Hash
                             Exportable Exchange

SSL_NULL_WITH_NULL_NULL               * NULL           NULL         NULL
SSL_RSA_WITH_NULL_MD5                 * RSA            NULL         MD5
SSL_RSA_WITH_NULL_SHA                 * RSA            NULL         SHA
SSL_RSA_EXPORT_WITH_RC4_40_MD5        * RSA_EXPORT     RC4_40       MD5
SSL_RSA_WITH_RC4_128_MD5                RSA            RC4_128      MD5
SSL_RSA_WITH_RC4_128_SHA                RSA            RC4_128      SHA
SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5    * RSA_EXPORT     RC2_CBC_40   MD5
SSL_RSA_WITH_IDEA_CBC_SHA               RSA            IDEA_CBC     SHA
SSL_RSA_EXPORT_WITH_DES40_CBC_SHA     * RSA_EXPORT     DES40_CBC    SHA
SSL_RSA_WITH_DES_CBC_SHA                RSA            DES_CBC      SHA
SSL_RSA_WITH_3DES_EDE_CBC_SHA           RSA            3DES_EDE_CBC SHA
SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA  * DH_DSS_EXPORT  DES40_CBC    SHA
SSL_DH_DSS_WITH_DES_CBC_SHA             DH_DSS         DES_CBC      SHA
SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA        DH_DSS         3DES_EDE_CBC SHA
SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA  * DH_RSA_EXPORT  DES40_CBC    SHA
SSL_DH_RSA_WITH_DES_CBC_SHA             DH_RSA         DES_CBC      SHA
SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA        DH_RSA         3DES_EDE_CBC SHA
SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC    SHA
SSL_DHE_DSS_WITH_DES_CBC_SHA            DHE_DSS        DES_CBC      SHA
SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA       DHE_DSS        3DES_EDE_CBC SHA
SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC    SHA
SSL_DHE_RSA_WITH_DES_CBC_SHA            DHE_RSA        DES_CBC      SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA       DHE_RSA        3DES_EDE_CBC SHA
SSL_DH_anon_EXPORT_WITH_RC4_40_MD5    * DH_anon_EXPORT RC4_40       MD5
SSL_DH_anon_WITH_RC4_128_MD5            DH_anon        RC4_128      MD5
SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA   DH_anon        DES40_CBC    SHA
SSL_DH_anon_WITH_DES_CBC_SHA            DH_anon        DES_CBC      SHA
SSL_DH_anon_WITH_3DES_EDE_CBC_SHA       DH_anon        3DES_EDE_CBC SHA
SSL_FORTEZZA_KEA_WITH_NULL_SHA          FORTEZZA_KEA   NULL         SHA
SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA  FORTEZZA_KEA   FORTEZZA_CBC SHA
SSL_FORTEZZA_KEA_WITH_RC4_128_SHA       FORTEZZA_KEA   RC4_128      SHA

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   +----------------+------------------------------+-------------------+
   |  Key Exchange  |          Description         |   Key Size Limit  |
   |    Algorithm   |                              |                   |
   +----------------+------------------------------+-------------------+
   |     DHE_DSS    |     Ephemeral DH with DSS    |        None       |
   |                |          signatures          |                   |
   | DHE_DSS_EXPORT |     Ephemeral DH with DSS    |   DH = 512 bits   |
   |                |          signatures          |                   |
   |     DHE_RSA    |     Ephemeral DH with RSA    |        None       |
   |                |          signatures          |                   |
   | DHE_RSA_EXPORT |     Ephemeral DH with RSA    |   DH = 512 bits,  |
   |                |          signatures          |     RSA = none    |
   |     DH_anon    |  Anonymous DH, no signatures |        None       |
   | DH_anon_EXPORT |  Anonymous DH, no signatures |   DH = 512 bits   |
   |     DH_DSS     |       DH with DSS-based      |        None       |
   |                |         certificates         |                   |
   |  DH_DSS_EXPORT |       DH with DSS-based      |   DH = 512 bits   |
   |                |         certificates         |                   |
   |     DH_RSA     |       DH with RSA-based      |        None       |
   |                |         certificates         |                   |
   |  DH_RSA_EXPORT |       DH with RSA-based      |   DH = 512 bits,  |
   |                |         certificates         |     RSA = none    |
   |  FORTEZZA_KEA  |     FORTEZZA KEA. Details    |        N/A        |
   |                |          unpublished         |                   |
   |      NULL      |        No key exchange       |        N/A        |
   |       RSA      |       RSA key exchange       |        None       |
   |   RSA_EXPORT   |       RSA key exchange       |   RSA = 512 bits  |
   +----------------+------------------------------+-------------------+

                                  Table 1

   Key size limit:  The key size limit gives the size of the largest
      public key that can be legally used for encryption in cipher
      suites that are exportable.

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   +--------------+--------+-----+-------+-------+-------+------+------+
   | Cipher       | Cipher | IsE |  Key  |  Exp. | Effec |  IV  | Bloc |
   |              |  Type  | xpo | Mater |  Key  |  tive | Size |   k  |
   |              |        | rta |  ial  | Mater |  Key  |      | Size |
   |              |        | ble |       |  ial  |  Bits |      |      |
   +--------------+--------+-----+-------+-------+-------+------+------+
   | NULL         | Stream |  *  |   0   |   0   |   0   |   0  |  N/A |
   | FORTEZZA_CBC |  Block |     |   NA  |   12  |   96  |  20  |   8  |
   |              |        |     |  (**) |  (**) |  (**) | (**) |      |
   | 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.
        ** FORTEZZA uses its own key and IV generation algorithms.

                                  Table 2

   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.

               +---------------+-----------+--------------+
               | Hash Function | Hash Size | Padding Size |
               +---------------+-----------+--------------+
               |      NULL     |     0     |       0      |
               |      MD5      |     16    |      48      |
               |      SHA      |     20    |      40      |
               +---------------+-----------+--------------+

                                  Table 3

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Appendix D.  Implementation Notes

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

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

   SSL 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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   Note: 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 that 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.

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

   SSL supports a range of key sizes and security levels, including some
   that provide no or minimal security.  A proper implementation will
   probably not support many cipher suites.  For example, 40-bit
   encryption is easily broken, so implementations requiring strong
   security should not allow 40-bit keys.  Similarly, anonymous Diffie-
   Hellman is strongly discouraged because it cannot prevent man-in-the-
   middle attacks.  Applications should also enforce minimum and maximum
   key sizes.  For example, certificate chains containing 512-bit RSA
   keys or signatures are not appropriate for high-security
   applications.

D.5.  FORTEZZA

   This section describes implementation details for cipher suites that
   make use of the FORTEZZA hardware encryption system.

D.5.1.  Notes on Use of FORTEZZA Hardware

   A complete explanation of all issues regarding the use of FORTEZZA
   hardware is outside the scope of this document.  However, there are a
   few special requirements of SSL that deserve mention.

   Because SSL is a full duplex protocol, two crypto states must be
   maintained, one for reading and one for writing.  There are also a
   number of circumstances that can result in the crypto state in the
   FORTEZZA card being lost.  For these reasons, it's recommended that
   the current crypto state be saved after processing a record, and
   loaded before processing the next.

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   After the client generates the TEK, it also generates two message
   encryption keys (MEKs), one for reading and one for writing.  After
   generating each of these keys, the client must generate a
   corresponding IV and then save the crypto state.  The client also
   uses the TEK to generate an IV and encrypt the premaster secret.  All
   three IVs are sent to the server, along with the wrapped keys and the
   encrypted premaster secret in the client key exchange message.  At
   this point, the TEK is no longer needed, and may be discarded.

   On the server side, the server uses the master IV and the TEK to
   decrypt the premaster secret.  It also loads the wrapped MEKs into
   the card.  The server loads both IVs to verify that the IVs match the
   keys.  However, since the card is unable to encrypt after loading an
   IV, the server must generate a new IV for the server write key.  This
   IV is discarded.

   When encrypting the first encrypted record (and only that record),
   the server adds 8 bytes of random data to the beginning of the
   fragment.  These 8 bytes are discarded by the client after
   decryption.  The purpose of this is to synchronize the state on the
   client and server resulting from the different IVs.

D.5.2.  FORTEZZA Cipher Suites

   5) FORTEZZA_NULL_WITH_NULL_SHA: Uses the full FORTEZZA key exchange,
   including sending server and client write keys and IVs.

D.5.3.  FORTEZZA Session Resumption

   There are two possibilities for FORTEZZA session restart: 1) Never
   restart a FORTEZZA session. 2) Restart a session with the previously
   negotiated keys and IVs.

   Never restarting a FORTEZZA session:

   Clients who never restart FORTEZZA sessions should never send session
   IDs that were previously used in a FORTEZZA session as part of the
   ClientHello.  Servers who never restart FORTEZZA sessions should
   never send a previous session id on the ServerHello if the negotiated
   session is FORTEZZA.

   Restart a session:

   You cannot restart FORTEZZA on a session that has never done a
   complete FORTEZZA key exchange (that is, you cannot restart FORTEZZA
   if the session was an RSA/RC4 session renegotiated for FORTEZZA).  If
   you wish to restart a FORTEZZA session, you must save the MEKs and

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   IVs from the initial key exchange for this session and reuse them for
   any new connections on that session.  This is not recommended, but it
   is possible.

Appendix E.  Version 2.0 Backward Compatibility

   Version 3.0 clients that support version 2.0 servers must send
   version 2.0 client hello messages [SSL-2].  Version 3.0 servers
   should accept either client hello format.  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.  Implementers should make every
   effort to move forward as quickly as possible.  Version 3.0 provides
   better mechanisms for transitioning to newer versions.

   The following cipher specifications are carryovers from SSL version
   2.0.  These are assumed to use RSA for key exchange and
   authentication.

        V2CipherSpec SSL_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
        V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
        V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
        V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
                                                   = { 0x04,0x00,0x80 };
        V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
        V2CipherSpec SSL_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
        V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };

   Cipher specifications introduced in version 3.0 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 version 3.0 equivalent (see
   Appendix A.6):

        V2CipherSpec (see Version 3.0 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.

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        uint8 V2CipherSpec[3];

        struct {
            unit8 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;

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

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

   cipher_spec_length:  This field is the total length of the field
      cipher_specs.  It cannot be zero and must be a multiple of the
      V2CipherSpec length (3).

   session_id_length:  This field must have a value of either zero or
      16.  If zero, the client is creating a new session.  If 16, the
      session_id field will contain the 16 bytes of session
      identification.

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

   cipher_specs:  This is a list of all CipherSpecs the client is
      willing and able to use.  There must be at least one CipherSpec
      acceptable to the server.

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

   challenge:  The client's challenge to the server for the server to
      identify itself is a (nearly) arbitrary length random.  The
      version 3.0 server will right justify the challenge data to become
      the ClientHello.random data (padded with leading zeroes, if
      necessary), as specified in this version 3.0 protocol.  If the
      length of the challenge is greater than 32 bytes, then 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.

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   Note: Requests to resume an SSL 3.0 session should use an SSL 3.0
   client hello.

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

   When SSL version 3.0 clients fall back to version 2.0 compatibility
   mode, they use special PKCS #1 block formatting.  This is done so
   that version 3.0 servers will reject version 2.0 sessions with
   version 3.0-capable clients.

   When version 3.0 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 SSL 3.0 should issue
   an error if these eight padding bytes are 0x03.  Version 2.0 servers
   receiving blocks padded in this manner will proceed normally.

Appendix F.  Security Analysis

   The SSL 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 SSL 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 MasterSecret, 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

   SSL supports three authentication modes: authentication of both
   parties, server authentication with an unauthenticated client, and
   total anonymity.  Whenever the server is authenticated, the channel
   should be secure against man-in-the-middle attacks, but completely
   anonymous sessions are inherently vulnerable to such attacks.

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   Anonymous servers cannot authenticate clients, since the client
   signature in the certificate verify message may require a server
   certificate to bind the signature to a particular server.  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 6.1).  The master_secret is required to
   generate the finished messages, encryption keys, and MAC secrets (see
   Sections 5.6.9 and 6.2.2).  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, Diffie-
   Hellman, or FORTEZZA for key exchange.  With anonymous RSA, the
   client encrypts a pre_master_secret with the server's uncertified
   public key extracted from the server key exchange message.  The
   result is sent in a client key exchange message.  Since eavesdroppers
   do not know the server's private key, it will be infeasible for them
   to decode the pre_master_secret.

   With Diffie-Hellman or FORTEZZA, 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) or the FORTEZZA token encryption
   key (TEK).

   Warning: Completely anonymous connections only provide protection
   against passive eavesdropping.  Unless an independent tamper-proof
   channel is used to verify that the finished messages were not
   replaced by an attacker, server authentication is required in
   environments where active man-in-the-middle attacks are a concern.

F.1.1.2.  RSA Key Exchange and Authentication

   With RSA, key exchange and server authentication are combined.  The
   public key either may be contained in the server's certificate or may
   be a temporary RSA key sent in a server key exchange message.  When
   temporary RSA keys are used, they are signed by the server's RSA or
   DSS certificate.  The signature includes the current

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   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 5.6.8).  The client signs
   a value derived from the master_secret and all preceding handshake
   messages.  These handshake messages include the server certificate,
   which binds the signature to the server, and ServerHello.random,
   which binds the signature to the current handshake process.

F.1.1.3.  Diffie-Hellman Key Exchange with Authentication

   When Diffie-Hellman key exchange is used, the server either can
   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.

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F.1.1.4.  FORTEZZA

   FORTEZZA's design is classified, but at the protocol level it is
   similar to Diffie-Hellman with fixed public values contained in
   certificates.  The result of the key exchange process is the token
   encryption key (TEK), which is used to wrap data encryption keys,
   client write key, server write key, and master secret encryption key.
   The data encryption keys are not derived from the pre_master_secret
   because unwrapped keys are not accessible outside the token.  The
   encrypted pre_master_secret is sent to the server in a client key
   exchange message.

F.1.2.  Version Rollback Attacks

   Because SSL version 3.0 includes substantial improvements over SSL
   version 2.0, attackers may try to make version 3.0-capable clients
   and servers fall back to version 2.0.  This attack is occurring if
   (and only if) two version 3.0-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,
   since this is essentially equivalent to increasing the input block
   size by 8 bytes.

F.1.3.  Detecting Attacks against the Handshake Protocol

   An attacker might try to influence the handshake exchange to make the
   parties select different encryption algorithms than they would
   normally choose.  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 other's finished messages.
   Without the master_secret, the attacker cannot repair the finished
   messages, so the attack will be discovered.

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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

   SSL 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.  FORTEZZA encryption keys are generated
   by the token, and are not derived from the master_secret.

   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 SSL
   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.

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   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 SSL 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.

Appendix G.  Acknowledgements

G.1.  Other Contributors

   Martin Abadi                  Robert Relyea
   Digital Equipment Corporation Netscape Communications
   ma@pa.dec.com                 relyea@netscape.com

   Taher Elgamal                 Jim Roskind
   Netscape Communications       Netscape Communications
   elgamal@netscape.com          jar@netscape.com

   Anil Gangolli                 Micheal J. Sabin, Ph.D.
   Netscape Communications       Consulting Engineer
   gangolli@netscape.com         msabin@netcom.com

   Kipp E.B. Hickman             Tom Weinstein
   Netscape Communications       Netscape Communications
   kipp@netscape.com             tomw@netscape.com

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G.2.  Early Reviewers

   Robert Baldwin                Clyde Monma
   RSA Data Security, Inc.       Bellcore
   baldwin@rsa.com               clyde@bellcore.com

   George Cox                    Eric Murray
   Intel Corporation             ericm@lne.com
   cox@ibeam.jf.intel.com

   Cheri Dowell                  Avi Rubin
   Sun Microsystems              Bellcore
   cheri@eng.sun.com             rubin@bellcore.com

   Stuart Haber                  Don Stephenson
   Bellcore                      Sun Microsystems
   stuart@bellcore.com           don.stephenson@eng.sun.com

   Burt Kaliski                  Joe Tardo
   RSA Data Security, Inc.       General Magic
   burt@rsa.com                  tardo@genmagic.com

Authors' Addresses

   Alan O. Freier
   Netscape Communications


   Philip Karlton
   Netscape Communications


   Paul C. Kocher
   Independent Consultant