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RFC 6101

 
 
 

The Secure Sockets Layer (SSL) Protocol Version 3.0

Part 2 of 3, p. 12 to 36
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5.  SSL Protocol

   SSL is a layered protocol.  At each layer, messages may include
   fields for length, description, and content.  SSL takes messages to
   be transmitted, fragments the data into manageable blocks, optionally
   compresses the data, applies a MAC, encrypts, and transmits the
   result.  Received data is decrypted, verified, decompressed, and
   reassembled, then delivered to higher level clients.

5.1.  Session and Connection States

   An SSL session is stateful.  It is the responsibility of the SSL
   handshake protocol to coordinate the states of the client and server,
   thereby allowing the protocol state machines of each to operate
   consistently, despite the fact that the state is not exactly
   parallel.  Logically, the state is represented twice, once as the
   current operating state and (during the handshake protocol) again as
   the pending state.  Additionally, separate read and write states are
   maintained.  When the client or server receives a change cipher spec
   message, it copies the pending read state into the current read
   state.  When the client or server sends a change cipher spec message,
   it copies the pending write state into the current write state.  When
   the handshake negotiation is complete, the client and server exchange
   change cipher spec messages (see Section 5.3), and they then
   communicate using the newly agreed-upon cipher spec.

   An SSL session may include multiple secure connections; in addition,
   parties may have multiple simultaneous sessions.

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   The session state includes the following elements:

   session identifier:  An arbitrary byte sequence chosen by the server
      to identify an active or resumable session state.

   peer certificate:  X509.v3 [X509] certificate of the peer.  This
      element of the state may be null.

   compression method:  The algorithm used to compress data prior to
      encryption.

   cipher spec:  Specifies the bulk data encryption algorithm (such as
      null, DES, etc.) and a MAC algorithm (such as MD5 or SHA).  It
      also defines cryptographic attributes such as the hash_size.  (See
      Appendix A.7 for formal definition.)

   master secret:  48-byte secret shared between the client and server.

   is resumable:  A flag indicating whether the session can be used to
      initiate new connections.

   The connection state includes the following elements:

   server and client random:  Byte sequences that are chosen by the
      server and client for each connection.

   server write MAC secret:  The secret used in MAC operations on data
      written by the server.

   client write MAC secret:  The secret used in MAC operations on data
      written by the client.

   server write key:  The bulk cipher key for data encrypted by the
      server and decrypted by the client.

   client write key:  The bulk cipher key for data encrypted by the
      client and decrypted by the server.

   initialization vectors:  When a block cipher in Cipher Block Chaining
      (CBC) mode is used, an initialization vector (IV) is maintained
      for each key.  This field is first initialized by the SSL
      handshake protocol.  Thereafter, the final ciphertext block from
      each record is preserved for use with the following record.

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   sequence numbers:  Each party maintains separate sequence numbers for
      transmitted and received messages for each connection.  When a
      party sends or receives a change cipher spec message, the
      appropriate sequence number is set to zero.  Sequence numbers are
      of type uint64 and may not exceed 2^64-1.

5.2.  Record Layer

   The SSL record layer receives uninterpreted data from higher layers
   in non-empty blocks of arbitrary size.

5.2.1.  Fragmentation

   The record layer fragments information blocks into SSLPlaintext
   records of 2^14 bytes or less.  Client message boundaries are not
   preserved in the record layer (i.e., multiple client messages of the
   same ContentType may be coalesced into a single SSLPlaintext record).

        struct {
            uint8 major, minor;
        } ProtocolVersion;

        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;

   type:  The higher level protocol used to process the enclosed
      fragment.

   version:  The version of protocol being employed.  This document
      describes SSL version 3.0 (see Appendix A.1).

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

   fragment:  The application data.  This data is transparent and
      treated as an independent block to be dealt with by the higher
      level protocol specified by the type field.

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   Note: Data of different SSL record layer content types may be
   interleaved.  Application data is generally of lower precedence for
   transmission than other content types.

5.2.2.  Record Compression and Decompression

   All records are compressed using the compression algorithm defined in
   the current session state.  There is always an active compression
   algorithm; however, initially it is defined as
   CompressionMethod.null.  The compression algorithm translates an
   SSLPlaintext structure into an SSLCompressed structure.  Compression
   functions erase their state information whenever the CipherSpec is
   replaced.

   Note: The CipherSpec is part of the session state described in
   Section 5.1.  References to fields of the CipherSpec are made
   throughout this document using presentation syntax.  A more complete
   description of the CipherSpec is shown in Appendix A.7.

   Compression must be lossless and may not increase the content length
   by more than 1024 bytes.  If the decompression function encounters an
   SSLCompressed.fragment that would decompress to a length in excess of
   2^14 bytes, it should issue a fatal decompression_failure alert
   (Section 5.4.2).

        struct {
            ContentType type;       /* same as SSLPlaintext.type */
            ProtocolVersion version;/* same as SSLPlaintext.version */
            uint16 length;
            opaque fragment[SSLCompressed.length];
        } SSLCompressed;

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

   fragment:  The compressed form of SSLPlaintext.fragment.

   Note: A CompressionMethod.null operation is an identity operation; no
   fields are altered (see Appendix A.4.1.)

   Implementation note: Decompression functions are responsible for
   ensuring that messages cannot cause internal buffer overflows.

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5.2.3.  Record Payload Protection and the CipherSpec

   All records are protected using the encryption and MAC algorithms
   defined in the current CipherSpec.  There is always an active
   CipherSpec; however, initially it is SSL_NULL_WITH_NULL_NULL, which
   does not provide any security.

   Once the handshake is complete, the two parties have shared secrets
   that are used to encrypt records and compute keyed Message
   Authentication Codes (MACs) on their contents.  The techniques used
   to perform the encryption and MAC operations are defined by the
   CipherSpec and constrained by CipherSpec.cipher_type.  The encryption
   and MAC functions translate an SSLCompressed structure into an
   SSLCiphertext.  The decryption functions reverse the process.
   Transmissions also include a sequence number so that missing,
   altered, or extra messages are detectable.

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

   type:  The type field is identical to SSLCompressed.type.

   version:  The version field is identical to SSLCompressed.version.

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

   fragment:  The encrypted form of SSLCompressed.fragment, including
      the MAC.

5.2.3.1.  Null or Standard Stream Cipher

   Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.7)
   convert SSLCompressed.fragment structures to and from stream
   SSLCiphertext.fragment structures.

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

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   The MAC is generated as:

        hash(MAC_write_secret + pad_2 +
             hash(MAC_write_secret + pad_1 + seq_num +
                  SSLCompressed.type + SSLCompressed.length +
                  SSLCompressed.fragment));

   where "+" denotes concatenation.

   pad_1:  The character 0x36 repeated 48 times for MD5 or 40 times for
      SHA.

   pad_2:  The character 0x5c repeated 48 times for MD5 or 40 times for
      SHA.

   seq_num:  The sequence number for this message.

   hash:  Hashing algorithm derived from the cipher suite.

   Note that the MAC is computed before encryption.  The stream cipher
   encrypts the entire block, including the MAC.  For stream ciphers
   that do not use a synchronization vector (such as RC4), the stream
   cipher state from the end of one record is simply used on the
   subsequent packet.  If the CipherSuite is SSL_NULL_WITH_NULL_NULL,
   encryption consists of the identity operation (i.e., the data is not
   encrypted and the MAC size is zero implying that no MAC is used).
   SSLCiphertext.length is SSLCompressed.length plus
   CipherSpec.hash_size.

5.2.3.2.  CBC Block Cipher

   For block ciphers (such as RC2 or DES), the encryption and MAC
   functions convert SSLCompressed.fragment structures to and from block
   SSLCiphertext.fragment structures.

        block-ciphered struct {
            opaque content[SSLCompressed.length];
            opaque MAC[CipherSpec.hash_size];
            uint8 padding[GenericBlockCipher.padding_length];
            uint8 padding_length;
        } GenericBlockCipher;

   The MAC is generated as described in Section 5.2.3.1.

   padding:  Padding that is added to force the length of the plaintext
      to be a multiple of the block cipher's block length.

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   padding_length:  The length of the padding must be less than the
      cipher's block length and may be zero.  The padding length should
      be such that the total size of the GenericBlockCipher structure is
      a multiple of the cipher's block length.

   The encrypted data length (SSLCiphertext.length) is one more than the
   sum of SSLCompressed.length, CipherSpec.hash_size, and
   padding_length.

   Note: With CBC, the initialization vector (IV) for the first record
   is provided by the handshake protocol.  The IV for subsequent records
   is the last ciphertext block from the previous record.

5.3.  Change Cipher Spec Protocol

   The change cipher spec protocol exists to signal transitions in
   ciphering strategies.  The protocol consists of a single message,
   which is encrypted and compressed under the current (not the pending)
   CipherSpec.  The message consists of a single byte of value 1.

        struct {
            enum { change_cipher_spec(1), (255) } type;
        } ChangeCipherSpec;

   The change cipher spec message is sent by both the client and server
   to notify the receiving party that subsequent records will be
   protected under the just-negotiated CipherSpec and keys.  Reception
   of this message causes the receiver to copy the read pending state
   into the read current state.  The client sends a change cipher spec
   message following handshake key exchange and certificate verify
   messages (if any), and the server sends one after successfully
   processing the key exchange message it received from the client.  An
   unexpected change cipher spec message should generate an
   unexpected_message alert (Section 5.4.2).  When resuming a previous
   session, the change cipher spec message is sent after the hello
   messages.

5.4.  Alert Protocol

   One of the content types supported by the SSL record layer is the
   alert type.  Alert messages convey the severity of the message and a
   description of the alert.  Alert messages with a level of fatal
   result in the immediate termination of the connection.  In this case,
   other connections corresponding to the session may continue, but the
   session identifier must be invalidated, preventing the failed session
   from being used to establish new connections.  Like other messages,
   alert messages are encrypted and compressed, as specified by the
   current connection state.

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        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;

5.4.1.  Closure Alerts

   The client and the server must share knowledge that the connection is
   ending in order to avoid a truncation attack.  Either party may
   initiate the exchange of closing messages.

   close_notify:  This message notifies the recipient that the sender
      will not send any more messages on this connection.  The session
      becomes unresumable if any connection is terminated without proper
      close_notify messages with level equal to warning.

   Either party may initiate a close by sending a close_notify alert.
   Any data received after a closure alert is ignored.

   Each party is required to send a close_notify alert before closing
   the write side of the connection.  It is required that the other
   party respond with a close_notify alert of its own and close down the
   connection immediately, discarding any pending writes.  It is not
   required for the initiator of the close to wait for the responding
   close_notify alert before closing the read side of the connection.

   NB: It is assumed that closing a connection reliably delivers pending
   data before destroying the transport.

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5.4.2.  Error Alerts

   Error handling in the SSL handshake protocol is very simple.  When an
   error is detected, the detecting party sends a message to the other
   party.  Upon transmission or receipt of a fatal alert message, both
   parties immediately close the connection.  Servers and clients are
   required to forget any session identifiers, keys, and secrets
   associated with a failed connection.  The following error alerts are
   defined:

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

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

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

   handshake_failure:  Reception of a handshake_failure alert message
      indicates that the sender was unable to negotiate an acceptable
      set of security parameters given the options available.  This is a
      fatal error.

   no_certificate:  A no_certificate alert message may be sent in
      response to a certification request if no appropriate certificate
      is available.

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

   unsupported_certificate:  A certificate was of an unsupported type.

   certificate_revoked:  A certificate was revoked by its signer.

   certificate_expired:  A certificate has expired or is not currently
      valid.

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

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

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5.5.  Handshake Protocol Overview

   The cryptographic parameters of the session state are produced by the
   SSL handshake protocol, which operates on top of the SSL record
   layer.  When an SSL client and server first start communicating, they
   agree on a protocol version, select cryptographic algorithms,
   optionally authenticate each other, and use public key encryption
   techniques to generate shared secrets.  These processes are performed
   in the handshake protocol, which can be summarized as follows: the
   client sends a client hello message to which the server must respond
   with a server hello message, or else a fatal error will occur and the
   connection will fail.  The client hello and server hello are used to
   establish security enhancement capabilities between client and
   server.  The client hello and server hello establish the following
   attributes: Protocol Version, Session ID, Cipher Suite, and
   Compression Method.  Additionally, two random values are generated
   and exchanged: ClientHello.random and ServerHello.random.

   Following the hello messages, the server will send its certificate,
   if it is to be authenticated.  Additionally, a server key exchange
   message may be sent, if it is required (e.g., if their server has no
   certificate, or if its certificate is for signing only).  If the
   server is authenticated, it may request a certificate from the
   client, if that is appropriate to the cipher suite selected.  Now the
   server will send the server hello done message, indicating that the
   hello-message phase of the handshake is complete.  The server will
   then wait for a client response.  If the server has sent a
   certificate request message, the client must send either the
   certificate message or a no_certificate alert.  The client key
   exchange message is now sent, and the content of that message will
   depend on the public key algorithm selected between the client hello
   and the server hello.  If the client has sent a certificate with
   signing ability, a digitally-signed certificate verify message is
   sent to explicitly verify the certificate.

   At this point, a change cipher spec message is sent by the client,
   and the client copies the pending CipherSpec into the current
   CipherSpec.  The client then immediately sends the finished message
   under the new algorithms, keys, and secrets.  In response, the server
   will send its own change cipher spec message, transfer the pending to
   the current CipherSpec, and send its finished message under the new
   CipherSpec.  At this point, the handshake is complete and the client
   and server may begin to exchange application layer data.  (See flow
   chart below.)

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      Client                                                Server

      ClientHello                   -------->
                                                       ServerHello
                                                      Certificate*
                                                ServerKeyExchange*
                                               CertificateRequest*
                                    <--------      ServerHelloDone
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                      -------->
                                                [ChangeCipherSpec]
                                    <--------             Finished
      Application Data              <------->     Application Data

      * Indicates optional or situation-dependent messages that are not
        always sent.

   Note: To help avoid pipeline stalls, ChangeCipherSpec is an
   independent SSL protocol content type, and is not actually an SSL
   handshake message.

   When the client and server decide to resume a previous session or
   duplicate an existing session (instead of negotiating new security
   parameters) the message flow is as follows:

   The client sends a ClientHello using the session ID of the session to
   be resumed.  The server then checks its session cache for a match.
   If a match is found, and the server is willing to re-establish the
   connection under the specified session state, it will send a
   ServerHello with the same session ID value.  At this point, both
   client and server must send change cipher spec messages and proceed
   directly to finished messages.  Once the re-establishment is
   complete, the client and server may begin to exchange application
   layer data.  (See flow chart below.)  If a session ID match is not
   found, the server generates a new session ID and the SSL client and
   server perform a full handshake.

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      Client                                                Server

      ClientHello                   -------->
                                                       ServerHello
                                              [change cipher spec]
                                    <--------             Finished
      change cipher spec
      Finished                      -------->
      Application Data              <------->     Application Data


   The contents and significance of each message will be presented in
   detail in the following sections.

5.6.  Handshake Protocol

   The SSL handshake protocol is one of the defined higher level clients
   of the SSL record protocol.  This protocol is used to negotiate the
   secure attributes of a session.  Handshake messages are supplied to
   the SSL record layer, where they are encapsulated within one or more
   SSLPlaintext structures, which are processed and transmitted as
   specified by the current active session state.

        enum {
            hello_request(0), client_hello(1), server_hello(2),
            certificate(11), server_key_exchange (12),
            certificate_request(13), server_hello_done(14),
            certificate_verify(15), client_key_exchange(16),
            finished(20), (255)
        } HandshakeType;

        struct {
            HandshakeType msg_type;    /* handshake type */
            uint24 length;             /* bytes in message */
            select (HandshakeType) {
                case hello_request: HelloRequest;
                case client_hello: ClientHello;
                case server_hello: ServerHello;
                case certificate: Certificate;
                case server_key_exchange: ServerKeyExchange;
                case certificate_request: CertificateRequest;
                case server_hello_done: ServerHelloDone;
                case certificate_verify: CertificateVerify;
                case client_key_exchange: ClientKeyExchange;
                case finished: Finished;
            } body;
        } Handshake;

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   The handshake protocol messages are presented in the order they must
   be sent; sending handshake messages in an unexpected order results in
   a fatal error.

5.6.1.  Hello messages

   The hello phase messages are used to exchange security enhancement
   capabilities between the client and server.  When a new session
   begins, the CipherSpec encryption, hash, and compression algorithms
   are initialized to null.  The current CipherSpec is used for
   renegotiation messages.

5.6.1.1.  Hello Request

   The hello request message may be sent by the server at any time, but
   will be ignored by the client if the handshake protocol is already
   underway.  It is a simple notification that the client should begin
   the negotiation process anew by sending a client hello message when
   convenient.

   Note: Since handshake messages are intended to have transmission
   precedence over application data, it is expected that the negotiation
   begin in no more than one or two times the transmission time of a
   maximum-length application data message.

   After sending a hello request, servers should not repeat the request
   until the subsequent handshake negotiation is complete.  A client
   that receives a hello request while in a handshake negotiation state
   should simply ignore the message.

   The structure of a hello request message is as follows:

        struct { } HelloRequest;

5.6.1.2.  Client Hello

   When a client first connects to a server it is required to send the
   client hello as its first message.  The client can also send a client
   hello in response to a hello request or on its own initiative in
   order to renegotiate the security parameters in an existing
   connection.  The client hello message includes a random structure,
   which is used later in the protocol.

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      struct {
          uint32 gmt_unix_time;
          opaque random_bytes[28];
      } Random;

   gmt_unix_time:  The current time and date in standard UNIX 32-bit
      format according to the sender's internal clock.  Clocks are not
      required to be set correctly by the basic SSL protocol; higher
      level or application protocols may define additional requirements.

   random_bytes:  28 bytes generated by a secure random number
      generator.

   The client hello message includes a variable-length session
   identifier.  If not empty, the value identifies a session between the
   same client and server whose security parameters the client wishes to
   reuse.  The session identifier may be from an earlier connection,
   this connection, or another currently active connection.  The second
   option is useful if the client only wishes to update the random
   structures and derived values of a connection, while the third option
   makes it possible to establish several simultaneous independent
   secure connections without repeating the full handshake protocol.
   The actual contents of the SessionID are defined by the server.

        opaque SessionID<0..32>;

   Warning: Servers must not place confidential information in session
   identifiers or let the contents of fake session identifiers cause any
   breach of security.

   The CipherSuite list, passed from the client to the server in the
   client hello message, contains the combinations of cryptographic
   algorithms supported by the client in order of the client's
   preference (first choice first).  Each CipherSuite defines both a key
   exchange algorithm and a CipherSpec.  The server will select a cipher
   suite or, if no acceptable choices are presented, return a handshake
   failure alert and close the connection.

        uint8 CipherSuite[2];  /* Cryptographic suite selector */

   The client hello includes a list of compression algorithms supported
   by the client, ordered according to the client's preference.  If the
   server supports none of those specified by the client, the session
   must fail.

        enum { null(0), (255) } CompressionMethod;

   Issue: Which compression methods to support is under investigation.

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   The structure of the client hello is as follows.

        struct {
            ProtocolVersion client_version;
            Random random;
            SessionID session_id;
            CipherSuite cipher_suites<2..2^16-1>;
            CompressionMethod compression_methods<1..2^8-1>;
        } ClientHello;

   client_version:  The version of the SSL protocol by which the client
      wishes to communicate during this session.  This should be the
      most recent (highest valued) version supported by the client.  For
      this version of the specification, the version will be 3.0 (see
      Appendix E for details about backward compatibility).

   random:  A client-generated random structure.

   session_id:  The ID of a session the client wishes to use for this
      connection.  This field should be empty if no session_id is
      available or the client wishes to generate new security
      parameters.

   cipher_suites:  This is a list of the cryptographic options supported
      by the client, sorted with the client's first preference first.
      If the session_id field is not empty (implying a session
      resumption request), this vector must include at least the
      cipher_suite from that session.  Values are defined in
      Appendix A.6.

   compression_methods:  This is a list of the compression methods
      supported by the client, sorted by client preference.  If the
      session_id field is not empty (implying a session resumption
      request), this vector must include at least the compression_method
      from that session.  All implementations must support
      CompressionMethod.null.

   After sending the client hello message, the client waits for a server
   hello message.  Any other handshake message returned by the server
   except for a hello request is treated as a fatal error.

   Implementation note: Application data may not be sent before a
   finished message has been sent.  Transmitted application data is
   known to be insecure until a valid finished message has been
   received.  This absolute restriction is relaxed if there is a
   current, non-null encryption on this connection.

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   Forward compatibility note: In the interests of forward
   compatibility, it is permitted for a client hello message to include
   extra data after the compression methods.  This data must be included
   in the handshake hashes, but must otherwise be ignored.

5.6.1.3.  Server Hello

   The server processes the client hello message and responds with
   either a handshake_failure alert or server hello message.

        struct {
            ProtocolVersion server_version;
            Random random;
            SessionID session_id;
            CipherSuite cipher_suite;
            CompressionMethod compression_method;
        } ServerHello;


   server_version:  This field will contain the lower of that suggested
      by the client in the client hello and the highest supported by the
      server.  For this version of the specification, the version will
      be 3.0 (see Appendix E for details about backward compatibility).

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

   session_id:  This is the identity of the session corresponding to
      this connection.  If the ClientHello.session_id was non-empty, the
      server will look in its session cache for a match.  If a match is
      found and the server is willing to establish the new connection
      using the specified session state, the server will respond with
      the same value as was supplied by the client.  This indicates a
      resumed session and dictates that the parties must proceed
      directly to the finished messages.  Otherwise, this field will
      contain a different value identifying the new session.  The server
      may return an empty session_id to indicate that the session will
      not be cached and therefore cannot be resumed.

   cipher_suite:  The single cipher suite selected by the server from
      the list in ClientHello.cipher_suites.  For resumed sessions, this
      field is the value from the state of the session being resumed.

   compression_method:  The single compression algorithm selected by the
      server from the list in ClientHello.compression_methods.  For
      resumed sessions, this field is the value from the resumed session
      state.

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5.6.2.  Server Certificate

   If the server is to be authenticated (which is generally the case),
   the server sends its certificate immediately following the server
   hello message.  The certificate type must be appropriate for the
   selected cipher suite's key exchange algorithm, and is generally an
   X.509.v3 certificate (or a modified X.509 certificate in the case of
   FORTEZZA(tm) [FOR]).  The same message type will be used for the
   client's response to a certificate request message.

        opaque ASN.1Cert<1..2^24-1>;
        struct {
            ASN.1Cert certificate_list<1..2^24-1>;
        } Certificate;

   certificate_list:  This is a sequence (chain) of X.509.v3
      certificates, ordered with the sender's certificate first followed
      by any certificate authority certificates proceeding sequentially
      upward.

   Note: PKCS #7 [PKCS7] is not used as the format for the certificate
   vector because PKCS #6 [PKCS6] extended certificates are not used.
   Also, PKCS #7 defines a Set rather than a Sequence, making the task
   of parsing the list more difficult.

5.6.3.  Server Key Exchange Message

   The server key exchange message is sent by the server if it has no
   certificate, has a certificate only used for signing (e.g., DSS [DSS]
   certificates, signing-only RSA [RSA] certificates), or FORTEZZA KEA
   key exchange is used.  This message is not used if the server
   certificate contains Diffie-Hellman [DH1] parameters.

   Note: According to current US export law, RSA moduli larger than 512
   bits may not be used for key exchange in software exported from the
   US.  With this message, larger RSA keys may be used as signature-only
   certificates to sign temporary shorter RSA keys for key exchange.

        enum { rsa, diffie_hellman, fortezza_kea }
               KeyExchangeAlgorithm;

        struct {
            opaque rsa_modulus<1..2^16-1>;
            opaque rsa_exponent<1..2^16-1>;
        } ServerRSAParams;

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   rsa_modulus:  The modulus of the server's temporary RSA key.

   rsa_exponent:  The public exponent of the server's temporary RSA key.

        struct {
            opaque dh_p<1..2^16-1>;
            opaque dh_g<1..2^16-1>;
            opaque dh_Ys<1..2^16-1>;
        } ServerDHParams;     /* Ephemeral DH parameters */

   dh_p:  The prime modulus used for the Diffie-Hellman operation.

   dh_g:  The generator used for the Diffie-Hellman operation.

   dh_Ys:  The server's Diffie-Hellman public value (gX mod p).

        struct {
            opaque r_s [128];
        } ServerFortezzaParams;

   r_s:  Server random number for FORTEZZA KEA (Key Exchange Algorithm).

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

   params:  The server's key exchange parameters.

   signed_params:  A hash of the corresponding params value, with the
      signature appropriate to that hash applied.

   md5_hash:  MD5(ClientHello.random + ServerHello.random +
      ServerParams);

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   sha_hash:  SHA(ClientHello.random + ServerHello.random +
      ServerParams);

        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;

5.6.4.  Certificate Request

   A non-anonymous server can optionally request a certificate from the
   client, if appropriate for the selected cipher suite.

        enum {
            rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
            rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza_kea(20),
            (255)
        } ClientCertificateType;

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

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

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

   certificate_authorities:  A list of the distinguished names of
      acceptable certificate authorities.

   Note: DistinguishedName is derived from [X509].

   Note: It is a fatal handshake_failure alert for an anonymous server
   to request client identification.

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5.6.5.  Server Hello Done

   The server hello done message is sent by the server to indicate the
   end of the server hello and associated messages.  After sending this
   message, the server will wait for a client response.

        struct { } ServerHelloDone;

   Upon receipt of the server hello done message the client should
   verify that the server provided a valid certificate if required and
   check that the server hello parameters are acceptable.

5.6.6.  Client Certificate

   This is the first message the client can send after receiving a
   server hello done message.  This message is only sent if the server
   requests a certificate.  If no suitable certificate is available, the
   client should send a no_certificate alert instead.  This alert is
   only a warning; however, the server may respond with a fatal
   handshake failure alert if client authentication is required.  Client
   certificates are sent using the certificate defined in Section 5.6.2.

   Note: Client Diffie-Hellman certificates must match the server
   specified Diffie-Hellman parameters.

5.6.7.  Client Key Exchange Message

   The choice of messages depends on which public key algorithm(s) has
   (have) been selected.  See Section 5.6.3 for the KeyExchangeAlgorithm
   definition.

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

   The information to select the appropriate record structure is in the
   pending session state (see Section 5.1).

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5.6.7.1.  RSA Encrypted Premaster Secret Message

   If RSA is being used for key agreement and authentication, the client
   generates a 48-byte premaster secret, encrypts it under the public
   key from the server's certificate or temporary RSA key from a server
   key exchange message, and sends the result in an encrypted premaster
   secret message.

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

   client_version:  The latest (newest) version supported by the client.
      This is used to detect version roll-back attacks.

   random:  46 securely-generated random bytes.

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

   pre_master_secret:  This random value is generated by the client and
      is used to generate the master secret, as specified in
      Section 6.1.

5.6.7.2.  FORTEZZA Key Exchange Message

   Under FORTEZZA, the client derives a token encryption key (TEK) using
   the FORTEZZA Key Exchange Algorithm (KEA).  The client's KEA
   calculation uses the public key in the server's certificate along
   with private parameters in the client's token.  The client sends
   public parameters needed for the server to generate the TEK, using
   its own private parameters.  The client generates session keys, wraps
   them using the TEK, and sends the results to the server.  The client
   generates IVs for the session keys and TEK and sends them also.  The
   client generates a random 48-byte premaster secret, encrypts it using
   the TEK, and sends the result:

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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];
            block-ciphered opaque encrypted_pre_master_secret[48];
        } FortezzaKeys;


   y_signature:  y_signature is the signature of the KEA public key,
      signed with the client's DSS private key.

   y_c:  The client's Yc value (public key) for the KEA calculation.  If
      the client has sent a certificate, and its KEA public key is
      suitable, this value must be empty since the certificate already
      contains this value.  If the client sent a certificate without a
      suitable public key, y_c is used and y_signature is the KEA public
      key signed with the client's DSS private key.  For this value to
      be used, it must be between 64 and 128 bytes.

   r_c:  The client's Rc value for the KEA calculation.

   wrapped_client_write_key:  This is the client's write key, wrapped by
      the TEK.

   wrapped_server_write_key:  This is the server's write key, wrapped by
      the TEK.

   client_write_iv:  The IV for the client write key.

   server_write_iv:  The IV for the server write key.

   master_secret_iv:  This is the IV for the TEK used to encrypt the
      premaster secret.

   pre_master_secret:  A random value, generated by the client and used
      to generate the master secret, as specified in Section 6.1.  In
      the above structure, it is encrypted using the TEK.

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5.6.7.3.  Client Diffie-Hellman Public Value

   This structure conveys the client's Diffie-Hellman public value (Yc)
   if it was not already included in the client's certificate.  The
   encoding used for Yc is determined by the enumerated
   PublicValueEncoding.

        enum { implicit, explicit } PublicValueEncoding;

   implicit:  If the client certificate already contains the public
      value, then it is implicit and Yc does not need to be sent again.

   explicit:  Yc needs to be sent.

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

   dh_Yc:  The client's Diffie-Hellman public value (Yc).

5.6.8.  Certificate Verify

   This message is used to provide explicit verification of a client
   certificate.  This message is only sent following any client
   certificate that has signing capability (i.e., all certificates
   except those containing fixed Diffie-Hellman parameters).

          struct {
               Signature signature;
          } CertificateVerify;

        CertificateVerify.signature.md5_hash
                   MD5(master_secret + pad_2 +
                       MD5(handshake_messages + master_secret + pad_1));
        Certificate.signature.sha_hash
                   SHA(master_secret + pad_2 +
                       SHA(handshake_messages + master_secret + pad_1));

   pad_1:  This is identical to the pad_1 defined in Section 5.2.3.1.

   pad_2:  This is identical to the pad_2 defined in Section 5.2.3.1.

   Here, handshake_messages refers to all handshake messages starting at
   client hello up to but not including this message.

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5.6.9.  Finished

   A finished message is always sent immediately after a change cipher
   spec message to verify that the key exchange and authentication
   processes were successful.  The finished message is the first
   protected with the just-negotiated algorithms, keys, and secrets.  No
   acknowledgment of the finished message is required; parties may begin
   sending encrypted data immediately after sending the finished
   message.  Recipients of finished messages must verify that the
   contents are correct.

        enum { client(0x434C4E54), server(0x53525652) } Sender;

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

   md5_hash:  MD5(master_secret + pad2 + MD5(handshake_messages + Sender
      + master_secret + pad1));

   sha_hash:  SHA(master_secret + pad2 + SHA(handshake_messages + Sender
      + master_secret + pad1));

   handshake_messages:  All of the data from all handshake messages up
      to but not including this message.  This is only data visible at
      the handshake layer and does not include record layer headers.

   It is a fatal error if a finished message is not preceeded by a
   change cipher spec message at the appropriate point in the handshake.

   The hash contained in finished messages sent by the server
   incorporate Sender.server; those sent by the client incorporate
   Sender.client.  The value handshake_messages includes all handshake
   messages starting at client hello up to but not including this
   finished message.  This may be different from handshake_messages in
   Section 5.6.8 because it would include the certificate verify message
   (if sent).

   Note: Change cipher spec messages are not handshake messages and are
   not included in the hash computations.

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5.7.  Application Data Protocol

   Application data messages are carried by the record layer and are
   fragmented, compressed, and encrypted based on the current connection
   state.  The messages are treated as transparent data to the record
   layer.



(page 36 continued on part 3)

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