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RFC 6101

The Secure Sockets Layer (SSL) Protocol Version 3.0

Pages: 67
Part 2 of 3 – Pages 12 to 36
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Top   ToC   RFC6101 - Page 12   prevText

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

   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. 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;
Top   ToC   RFC6101 - Page 17
   The MAC is generated as:

        hash(MAC_write_secret + pad_2 +
             hash(MAC_write_secret + pad_1 + seq_num +
                  SSLCompressed.type + SSLCompressed.length +

   where "+" denotes concatenation.

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

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

   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. 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 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

   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 {
            illegal_parameter (47)
        } 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                   -------->
                                    <--------      ServerHelloDone
      Finished                      -------->
                                    <--------             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.
Top   ToC   RFC6101 - Page 23
      Client                                                Server

      ClientHello                   -------->
                                              [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. 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; 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.
Top   ToC   RFC6101 - Page 25
      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

   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.
Top   ToC   RFC6101 - Page 26
   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

   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

   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. 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 +
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   sha_hash:  SHA(ClientHello.random + ServerHello.random +

        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).
Top   ToC   RFC6101 - Page 32 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. 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:
Top   ToC   RFC6101 - Page 33
        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.
Top   ToC   RFC6101 - Page 34 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 pad_2: This is identical to the pad_2 defined in Section Here, handshake_messages refers to all handshake messages starting at client hello up to but not including this message.
Top   ToC   RFC6101 - Page 35

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.
Top   ToC   RFC6101 - Page 36

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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