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RFC 6101

The Secure Sockets Layer (SSL) Protocol Version 3.0

Pages: 67
Historic
Part 3 of 3 – Pages 36 to 67
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Top   ToC   RFC6101 - Page 36   prevText

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.
Top   ToC   RFC6101 - Page 37

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
Top   ToC   RFC6101 - Page 38
   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);
Top   ToC   RFC6101 - Page 39
   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.
Top   ToC   RFC6101 - Page 40

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.
Top   ToC   RFC6101 - Page 41
   [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".
Top   ToC   RFC6101 - Page 42

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];
Top   ToC   RFC6101 - Page 43
            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;
Top   ToC   RFC6101 - Page 44

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>;
Top   ToC   RFC6101 - Page 45
        } 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;
Top   ToC   RFC6101 - Page 46
        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;
Top   ToC   RFC6101 - Page 47
        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.
Top   ToC   RFC6101 - Page 48
     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 };
Top   ToC   RFC6101 - Page 49
   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;
Top   ToC   RFC6101 - Page 50

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].
Top   ToC   RFC6101 - Page 51
   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.
Top   ToC   RFC6101 - Page 52
   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