6.  The TLS Record Protocol

   The TLS Record Protocol is a layered protocol.  At each layer,
   messages may include fields for length, description, and content.
   The Record Protocol 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, reassembled, and then delivered to
   higher-level clients.

   Four protocols that use the record protocol are described in this
   document: the handshake protocol, the alert protocol, the change
   cipher spec protocol, and the application data protocol.  In order to
   allow extension of the TLS protocol, additional record content types
   can be supported by the record protocol.  New record content type
   values are assigned by IANA in the TLS Content Type Registry as
   described in Section 12.

   Implementations MUST NOT send record types not defined in this
   document unless negotiated by some extension.  If a TLS
   implementation receives an unexpected record type, it MUST send an
   unexpected_message alert.

   Any protocol designed for use over TLS must be carefully designed to
   deal with all possible attacks against it.  As a practical matter,
   this means that the protocol designer must be aware of what security
   properties TLS does and does not provide and cannot safely rely on
   the latter.

   Note in particular that type and length of a record are not protected
   by encryption.  If this information is itself sensitive, application
   designers may wish to take steps (padding, cover traffic) to minimize
   information leakage.

6.1.  Connection States

   A TLS connection state is the operating environment of the TLS Record
   Protocol.  It specifies a compression algorithm, an encryption
   algorithm, and a MAC algorithm.  In addition, the parameters for
   these algorithms are known: the MAC key and the bulk encryption keys
   for the connection in both the read and the write directions.
   Logically, there are always four connection states outstanding: the
   current read and write states, and the pending read and write states.
   All records are processed under the current read and write states.
   The security parameters for the pending states can be set by the TLS
   Handshake Protocol, and the ChangeCipherSpec can selectively make
   either of the pending states current, in which case the appropriate
   current state is disposed of and replaced with the pending state; the
   pending state is then reinitialized to an empty state.  It is illegal
   to make a state that has not been initialized with security
   parameters a current state.  The initial current state always
   specifies that no encryption, compression, or MAC will be used.

   The security parameters for a TLS Connection read and write state are
   set by providing the following values:

   connection end
      Whether this entity is considered the "client" or the "server" in
      this connection.

   PRF algorithm
      An algorithm used to generate keys from the master secret (see
      Sections 5 and 6.3).

   bulk encryption algorithm
      An algorithm to be used for bulk encryption.  This specification
      includes the key size of this algorithm, whether it is a block,
      stream, or AEAD cipher, the block size of the cipher (if
      appropriate), and the lengths of explicit and implicit
      initialization vectors (or nonces).

   MAC algorithm
      An algorithm to be used for message authentication.  This
      specification includes the size of the value returned by the MAC
      algorithm.

   compression algorithm
      An algorithm to be used for data compression.  This specification
      must include all information the algorithm requires to do
      compression.

   master secret
      A 48-byte secret shared between the two peers in the connection.

   client random
      A 32-byte value provided by the client.

   server random
      A 32-byte value provided by the server.

      These parameters are defined in the presentation language as:

      enum { server, client } ConnectionEnd;

      enum { tls_prf_sha256 } PRFAlgorithm;

      enum { null, rc4, 3des, aes }
        BulkCipherAlgorithm;

      enum { stream, block, aead } CipherType;

      enum { null, hmac_md5, hmac_sha1, hmac_sha256,
           hmac_sha384, hmac_sha512} MACAlgorithm;

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

      /* The algorithms specified in CompressionMethod, PRFAlgorithm,
         BulkCipherAlgorithm, and MACAlgorithm may be added to. */

      struct {
          ConnectionEnd          entity;
          PRFAlgorithm           prf_algorithm;
          BulkCipherAlgorithm    bulk_cipher_algorithm;
          CipherType             cipher_type;
          uint8                  enc_key_length;
          uint8                  block_length;
          uint8                  fixed_iv_length;
          uint8                  record_iv_length;
          MACAlgorithm           mac_algorithm;
          uint8                  mac_length;
          uint8                  mac_key_length;
          CompressionMethod      compression_algorithm;
          opaque                 master_secret[48];
          opaque                 client_random[32];
          opaque                 server_random[32];
      } SecurityParameters;

   The record layer will use the security parameters to generate the
   following six items (some of which are not required by all ciphers,
   and are thus empty):

      client write MAC key
      server write MAC key
      client write encryption key
      server write encryption key
      client write IV
      server write IV

   The client write parameters are used by the server when receiving and
   processing records and vice versa.  The algorithm used for generating
   these items from the security parameters is described in Section 6.3.

   Once the security parameters have been set and the keys have been
   generated, the connection states can be instantiated by making them
   the current states.  These current states MUST be updated for each
   record processed.  Each connection state includes the following
   elements:

   compression state
      The current state of the compression algorithm.

   cipher state
      The current state of the encryption algorithm.  This will consist
      of the scheduled key for that connection.  For stream ciphers,
      this will also contain whatever state information is necessary to
      allow the stream to continue to encrypt or decrypt data.

   MAC key
      The MAC key for this connection, as generated above.

   sequence number
      Each connection state contains a sequence number, which is
      maintained separately for read and write states.  The sequence
      number MUST be set to zero whenever a connection state is made the
      active state.  Sequence numbers are of type uint64 and may not
      exceed 2^64-1.  Sequence numbers do not wrap.  If a TLS
      implementation would need to wrap a sequence number, it must
      renegotiate instead.  A sequence number is incremented after each
      record: specifically, the first record transmitted under a
      particular connection state MUST use sequence number 0.

6.2.  Record Layer

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

6.2.1.  Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks 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 TLSPlaintext record, or a single message MAY be
   fragmented across several records).

      struct {
          uint8 major;
          uint8 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[TLSPlaintext.length];
      } TLSPlaintext;

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

   version
      The version of the protocol being employed.  This document
      describes TLS Version 1.2, which uses the version { 3, 3 }.  The
      version value 3.3 is historical, deriving from the use of {3, 1}
      for TLS 1.0.  (See Appendix A.1.)  Note that a client that
      supports multiple versions of TLS may not know what version will
      be employed before it receives the ServerHello.  See Appendix E
      for discussion about what record layer version number should be
      employed for ClientHello.

   length
      The length (in bytes) of the following TLSPlaintext.fragment.  The
      length MUST 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.

   Implementations MUST NOT send zero-length fragments of Handshake,
   Alert, or ChangeCipherSpec content types.  Zero-length fragments of
   Application data MAY be sent as they are potentially useful as a
   traffic analysis countermeasure.

   Note: Data of different TLS record layer content types MAY be
   interleaved.  Application data is generally of lower precedence for
   transmission than other content types.  However, records MUST be
   delivered to the network in the same order as they are protected by
   the record layer.  Recipients MUST receive and process interleaved
   application layer traffic during handshakes subsequent to the first
   one on a connection.

6.2.2.  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 a
   TLSPlaintext structure into a TLSCompressed structure.  Compression
   functions are initialized with default state information whenever a
   connection state is made active.  [RFC3749] describes compression
   algorithms for TLS.

   Compression must be lossless and may not increase the content length
   by more than 1024 bytes.  If the decompression function encounters a
   TLSCompressed.fragment that would decompress to a length in excess of
   2^14 bytes, it MUST report a fatal decompression failure error.

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

   length
      The length (in bytes) of the following TLSCompressed.fragment.
      The length MUST NOT exceed 2^14 + 1024.

   fragment
      The compressed form of TLSPlaintext.fragment.

      Note: A CompressionMethod.null operation is an identity operation;
      no fields are altered.

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

6.2.3.  Record Payload Protection

      The encryption and MAC functions translate a TLSCompressed
      structure into a TLSCiphertext.  The decryption functions reverse
      the process.  The MAC of the record also includes a sequence
      number so that missing, extra, or repeated messages are
      detectable.

      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          select (SecurityParameters.cipher_type) {
              case stream: GenericStreamCipher;
              case block:  GenericBlockCipher;
              case aead:   GenericAEADCipher;
          } fragment;
      } TLSCiphertext;

   type
      The type field is identical to TLSCompressed.type.

   version
      The version field is identical to TLSCompressed.version.

   length
      The length (in bytes) of the following TLSCiphertext.fragment.
      The length MUST NOT exceed 2^14 + 2048.

   fragment
      The encrypted form of TLSCompressed.fragment, with the MAC.

6.2.3.1.  Null or Standard Stream Cipher

   Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6)
   convert TLSCompressed.fragment structures to and from stream
   TLSCiphertext.fragment structures.

      stream-ciphered struct {
          opaque content[TLSCompressed.length];
          opaque MAC[SecurityParameters.mac_length];
      } GenericStreamCipher;

   The MAC is generated as:

      MAC(MAC_write_key, seq_num +
                            TLSCompressed.type +
                            TLSCompressed.version +
                            TLSCompressed.length +
                            TLSCompressed.fragment);

   where "+" denotes concatenation.

   seq_num
      The sequence number for this record.

   MAC
      The MAC algorithm specified by SecurityParameters.mac_algorithm.

   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 cipher suite is TLS_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).
   For both null and stream ciphers, TLSCiphertext.length is
   TLSCompressed.length plus SecurityParameters.mac_length.

6.2.3.2.  CBC Block Cipher

   For block ciphers (such as 3DES or AES), the encryption and MAC
   functions convert TLSCompressed.fragment structures to and from block
   TLSCiphertext.fragment structures.

      struct {
          opaque IV[SecurityParameters.record_iv_length];
          block-ciphered struct {
              opaque content[TLSCompressed.length];
              opaque MAC[SecurityParameters.mac_length];
              uint8 padding[GenericBlockCipher.padding_length];
              uint8 padding_length;
          };
      } GenericBlockCipher;

   The MAC is generated as described in Section 6.2.3.1.

   IV
      The Initialization Vector (IV) SHOULD be chosen at random, and
      MUST be unpredictable.  Note that in versions of TLS prior to 1.1,
      there was no IV field, and the last ciphertext block of the
      previous record (the "CBC residue") was used as the IV.  This was
      changed to prevent the attacks described in [CBCATT].  For block
      ciphers, the IV length is of length
      SecurityParameters.record_iv_length, which is equal to the
      SecurityParameters.block_size.

   padding
      Padding that is added to force the length of the plaintext to be
      an integral multiple of the block cipher's block length.  The
      padding MAY be any length up to 255 bytes, as long as it results
      in the TLSCiphertext.length being an integral multiple of the
      block length.  Lengths longer than necessary might be desirable to
      frustrate attacks on a protocol that are based on analysis of the
      lengths of exchanged messages.  Each uint8 in the padding data
      vector MUST be filled with the padding length value.  The receiver
      MUST check this padding and MUST use the bad_record_mac alert to
      indicate padding errors.

   padding_length
      The padding length MUST be such that the total size of the
      GenericBlockCipher structure is a multiple of the cipher's block
      length.  Legal values range from zero to 255, inclusive.  This
      length specifies the length of the padding field exclusive of the
      padding_length field itself.

   The encrypted data length (TLSCiphertext.length) is one more than the
   sum of SecurityParameters.block_length, TLSCompressed.length,
   SecurityParameters.mac_length, and padding_length.

   Example: If the block length is 8 bytes, the content length
   (TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes,
   then the length before padding is 82 bytes (this does not include the

   IV.  Thus, the padding length modulo 8 must be equal to 6 in order to
   make the total length an even multiple of 8 bytes (the block length).
   The padding length can be 6, 14, 22, and so on, through 254.  If the
   padding length were the minimum necessary, 6, the padding would be 6
   bytes, each containing the value 6.  Thus, the last 8 octets of the
   GenericBlockCipher before block encryption would be xx 06 06 06 06 06
   06 06, where xx is the last octet of the MAC.

   Note: With block ciphers in CBC mode (Cipher Block Chaining), it is
   critical that the entire plaintext of the record be known before any
   ciphertext is transmitted.  Otherwise, it is possible for the
   attacker to mount the attack described in [CBCATT].

   Implementation note: Canvel et al. [CBCTIME] have demonstrated a
   timing attack on CBC padding based on the time required to compute
   the MAC.  In order to defend against this attack, implementations
   MUST ensure that record processing time is essentially the same
   whether or not the padding is correct.  In general, the best way to
   do this is to compute the MAC even if the padding is incorrect, and
   only then reject the packet.  For instance, if the pad appears to be
   incorrect, the implementation might assume a zero-length pad and then
   compute the MAC.  This leaves a small timing channel, since MAC
   performance depends to some extent on the size of the data fragment,
   but it is not believed to be large enough to be exploitable, due to
   the large block size of existing MACs and the small size of the
   timing signal.

6.2.3.3.  AEAD Ciphers

   For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function
   converts TLSCompressed.fragment structures to and from AEAD
   TLSCiphertext.fragment structures.

      struct {
         opaque nonce_explicit[SecurityParameters.record_iv_length];
         aead-ciphered struct {
             opaque content[TLSCompressed.length];
         };
      } GenericAEADCipher;

   AEAD ciphers take as input a single key, a nonce, a plaintext, and
   "additional data" to be included in the authentication check, as
   described in Section 2.1 of [AEAD].  The key is either the
   client_write_key or the server_write_key.  No MAC key is used.

   Each AEAD cipher suite MUST specify how the nonce supplied to the
   AEAD operation is constructed, and what is the length of the
   GenericAEADCipher.nonce_explicit part.  In many cases, it is

   appropriate to use the partially implicit nonce technique described
   in Section 3.2.1 of [AEAD]; with record_iv_length being the length of
   the explicit part.  In this case, the implicit part SHOULD be derived
   from key_block as client_write_iv and server_write_iv (as described
   in Section 6.3), and the explicit part is included in
   GenericAEAEDCipher.nonce_explicit.

   The plaintext is the TLSCompressed.fragment.

   The additional authenticated data, which we denote as
   additional_data, is defined as follows:

      additional_data = seq_num + TLSCompressed.type +
                        TLSCompressed.version + TLSCompressed.length;

   where "+" denotes concatenation.

   The aead_output consists of the ciphertext output by the AEAD
   encryption operation.  The length will generally be larger than
   TLSCompressed.length, but by an amount that varies with the AEAD
   cipher.  Since the ciphers might incorporate padding, the amount of
   overhead could vary with different TLSCompressed.length values.  Each
   AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes.
   Symbolically,

      AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext,
                                   additional_data)

   In order to decrypt and verify, the cipher takes as input the key,
   nonce, the "additional_data", and the AEADEncrypted value.  The
   output is either the plaintext or an error indicating that the
   decryption failed.  There is no separate integrity check.  That is:

      TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce,
                                            AEADEncrypted,
                                            additional_data)

   If the decryption fails, a fatal bad_record_mac alert MUST be
   generated.

6.3.  Key Calculation

   The Record Protocol requires an algorithm to generate keys required
   by the current connection state (see Appendix A.6) from the security
   parameters provided by the handshake protocol.

   The master secret is expanded into a sequence of secure bytes, which
   is then split to a client write MAC key, a server write MAC key, a
   client write encryption key, and a server write encryption key.  Each
   of these is generated from the byte sequence in that order.  Unused
   values are empty.  Some AEAD ciphers may additionally require a
   client write IV and a server write IV (see Section 6.2.3.3).

   When keys and MAC keys are generated, the master secret is used as an
   entropy source.

   To generate the key material, compute

      key_block = PRF(SecurityParameters.master_secret,
                      "key expansion",
                      SecurityParameters.server_random +
                      SecurityParameters.client_random);

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

      client_write_MAC_key[SecurityParameters.mac_key_length]
      server_write_MAC_key[SecurityParameters.mac_key_length]
      client_write_key[SecurityParameters.enc_key_length]
      server_write_key[SecurityParameters.enc_key_length]
      client_write_IV[SecurityParameters.fixed_iv_length]
      server_write_IV[SecurityParameters.fixed_iv_length]

   Currently, the client_write_IV and server_write_IV are only generated
   for implicit nonce techniques as described in Section 3.2.1 of
   [AEAD].

   Implementation note: The currently defined cipher suite which
   requires the most material is AES_256_CBC_SHA256.  It requires 2 x 32
   byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key
   material.

7.  The TLS Handshaking Protocols

   TLS has three subprotocols that are used to allow peers to agree upon
   security parameters for the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to report error
   conditions to each other.

   The Handshake Protocol is responsible for negotiating a session,
   which consists of the following items:

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

   peer certificate
      X509v3 [PKIX] 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 pseudorandom function (PRF) used to generate keying
      material, the bulk data encryption algorithm (such as null, AES,
      etc.) and the MAC algorithm (such as HMAC-SHA1).  It also defines
      cryptographic attributes such as the mac_length.  (See Appendix
      A.6 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.

   These items are then used to create security parameters for use by
   the record layer when protecting application data.  Many connections
   can be instantiated using the same session through the resumption
   feature of the TLS Handshake Protocol.

7.1.  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)
   connection state.  The message consists of a single byte of value 1.

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

   The ChangeCipherSpec message is sent by both the client and the
   server to notify the receiving party that subsequent records will be
   protected under the newly negotiated CipherSpec and keys.  Reception
   of this message causes the receiver to instruct the record layer to
   immediately copy the read pending state into the read current state.
   Immediately after sending this message, the sender MUST instruct the
   record layer to make the write pending state the write active state.

   (See Section 6.1.)  The ChangeCipherSpec message is sent during the
   handshake after the security parameters have been agreed upon, but
   before the verifying Finished message is sent.

   Note: If a rehandshake occurs while data is flowing on a connection,
   the communicating parties may continue to send data using the old
   CipherSpec.  However, once the ChangeCipherSpec has been sent, the
   new CipherSpec MUST be used.  The first side to send the
   ChangeCipherSpec does not know that the other side has finished
   computing the new keying material (e.g., if it has to perform a
   time-consuming public key operation).  Thus, a small window of time,
   during which the recipient must buffer the data, MAY exist.  In
   practice, with modern machines this interval is likely to be fairly
   short.

7.2.  Alert Protocol

   One of the content types supported by the TLS record layer is the
   alert type.  Alert messages convey the severity of the message
   (warning or fatal) 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.

      enum { warning(1), fatal(2), (255) } AlertLevel;

      enum {
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decryption_failed_RESERVED(21),
          record_overflow(22),
          decompression_failure(30),
          handshake_failure(40),
          no_certificate_RESERVED(41),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter(47),
          unknown_ca(48),
          access_denied(49),
          decode_error(50),
          decrypt_error(51),

          export_restriction_RESERVED(60),
          protocol_version(70),
          insufficient_security(71),
          internal_error(80),
          user_canceled(90),
          no_renegotiation(100),
          unsupported_extension(110),
          (255)
      } AlertDescription;

      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

7.2.1.  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.  Note that as of TLS 1.1,
      failure to properly close a connection no longer requires that a
      session not be resumed.  This is a change from TLS 1.0 to conform
      with widespread implementation practice.

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

   Unless some other fatal alert has been transmitted, each party is
   required to send a close_notify alert before closing the write side
   of the connection.  The other party MUST 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.

   If the application protocol using TLS provides that any data may be
   carried over the underlying transport after the TLS connection is
   closed, the TLS implementation must receive the responding
   close_notify alert before indicating to the application layer that
   the TLS connection has ended.  If the application protocol will not
   transfer any additional data, but will only close the underlying
   transport connection, then the implementation MAY choose to close the
   transport without waiting for the responding close_notify.  No part

   of this standard should be taken to dictate the manner in which a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

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

7.2.2.  Error Alerts

   Error handling in the TLS 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 MUST
   forget any session-identifiers, keys, and secrets associated with a
   failed connection.  Thus, any connection terminated with a fatal
   alert MUST NOT be resumed.

   Whenever an implementation encounters a condition which is defined as
   a fatal alert, it MUST send the appropriate alert prior to closing
   the connection.  For all errors where an alert level is not
   explicitly specified, the sending party MAY determine at its
   discretion whether to treat this as a fatal error or not.  If the
   implementation chooses to send an alert but intends to close the
   connection immediately afterwards, it MUST send that alert at the
   fatal alert level.

   If an alert with a level of warning is sent and received, generally
   the connection can continue normally.  If the receiving party decides
   not to proceed with the connection (e.g., after having received a
   no_renegotiation alert that it is not willing to accept), it SHOULD
   send a fatal alert to terminate the connection.  Given this, the
   sending party cannot, in general, know how the receiving party will
   behave.  Therefore, warning alerts are not very useful when the
   sending party wants to continue the connection, and thus are
   sometimes omitted.  For example, if a peer decides to accept an
   expired certificate (perhaps after confirming this with the user) and
   wants to continue the connection, it would not generally send a
   certificate_expired alert.

   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 alert also MUST be returned if an alert is sent because
      a TLSCiphertext decrypted in an invalid way: either it wasn't an
      even multiple of the block length, or its padding values, when
      checked, weren't correct.  This message is always fatal and should
      never be observed in communication between proper implementations
      (except when messages were corrupted in the network).

   decryption_failed_RESERVED
      This alert was used in some earlier versions of TLS, and may have
      permitted certain attacks against the CBC mode [CBCATT].  It MUST
      NOT be sent by compliant implementations.

   record_overflow
      A TLSCiphertext record was received that had a length more than
      2^14+2048 bytes, or a record decrypted to a TLSCompressed record
      with more than 2^14+1024 bytes.  This message is always fatal and
      should never be observed in communication between proper
      implementations (except when messages were corrupted in the
      network).

   decompression_failure
      The decompression function received improper input (e.g., data
      that would expand to excessive length).  This message is always
      fatal and should never be observed in communication between proper
      implementations.

   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_RESERVED
      This alert was used in SSLv3 but not any version of TLS.  It MUST
      NOT be sent by compliant implementations.

   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 message is always fatal.

   unknown_ca
      A valid certificate chain or partial chain was received, but the
      certificate was not accepted because the CA certificate could not
      be located or couldn't be matched with a known, trusted CA.  This
      message is always fatal.

   access_denied
      A valid certificate was received, but when access control was
      applied, the sender decided not to proceed with negotiation.  This
      message is always fatal.

   decode_error
      A message could not be decoded because some field was out of the
      specified range or the length of the message was incorrect.  This
      message is always fatal and should never be observed in
      communication between proper implementations (except when messages
      were corrupted in the network).

   decrypt_error
      A handshake cryptographic operation failed, including being unable
      to correctly verify a signature or validate a Finished message.
      This message is always fatal.

   export_restriction_RESERVED
      This alert was used in some earlier versions of TLS.  It MUST NOT
      be sent by compliant implementations.

   protocol_version
      The protocol version the client has attempted to negotiate is
      recognized but not supported.  (For example, old protocol versions
      might be avoided for security reasons.)  This message is always
      fatal.

   insufficient_security
      Returned instead of handshake_failure when a negotiation has
      failed specifically because the server requires ciphers more
      secure than those supported by the client.  This message is always
      fatal.

   internal_error
      An internal error unrelated to the peer or the correctness of the
      protocol (such as a memory allocation failure) makes it impossible
      to continue.  This message is always fatal.

   user_canceled
      This handshake is being canceled for some reason unrelated to a
      protocol failure.  If the user cancels an operation after the
      handshake is complete, just closing the connection by sending a
      close_notify is more appropriate.  This alert should be followed
      by a close_notify.  This message is generally a warning.

   no_renegotiation
      Sent by the client in response to a hello request or by the server
      in response to a client hello after initial handshaking.  Either
      of these would normally lead to renegotiation; when that is not
      appropriate, the recipient should respond with this alert.  At
      that point, the original requester can decide whether to proceed
      with the connection.  One case where this would be appropriate is
      where a server has spawned a process to satisfy a request; the
      process might receive security parameters (key length,
      authentication, etc.) at startup, and it might be difficult to
      communicate changes to these parameters after that point.  This
      message is always a warning.

   unsupported_extension
      sent by clients that receive an extended server hello containing
      an extension that they did not put in the corresponding client
      hello.  This message is always fatal.

   New Alert values are assigned by IANA as described in Section 12.

7.3.  Handshake Protocol Overview

   The cryptographic parameters of the session state are produced by the
   TLS Handshake Protocol, which operates on top of the TLS record
   layer.  When a TLS 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.

   The TLS Handshake Protocol involves the following steps:

   -  Exchange hello messages to agree on algorithms, exchange random
      values, and check for session resumption.

   -  Exchange the necessary cryptographic parameters to allow the
      client and server to agree on a premaster secret.

   -  Exchange certificates and cryptographic information to allow the
      client and server to authenticate themselves.

   -  Generate a master secret from the premaster secret and exchanged
      random values.

   -  Provide security parameters to the record layer.

   -  Allow the client and server to verify that their peer has
      calculated the same security parameters and that the handshake
      occurred without tampering by an attacker.

   Note that higher layers should not be overly reliant on whether TLS
   always negotiates the strongest possible connection between two
   peers.  There are a number of ways in which a man-in-the-middle
   attacker can attempt to make two entities drop down to the least
   secure method they support.  The protocol has been designed to
   minimize this risk, but there are still attacks available: for
   example, an attacker could block access to the port a secure service
   runs on, or attempt to get the peers to negotiate an unauthenticated
   connection.  The fundamental rule is that higher levels must be
   cognizant of what their security requirements are and never transmit
   information over a channel less secure than what they require.  The
   TLS protocol is secure in that any cipher suite offers its promised
   level of security: if you negotiate 3DES with a 1024-bit RSA key
   exchange with a host whose certificate you have verified, you can
   expect to be that secure.

   These goals are achieved by the handshake protocol, which can be
   summarized as follows: The client sends a ClientHello message to
   which the server must respond with a ServerHello message, or else a
   fatal error will occur and the connection will fail.  The ClientHello
   and ServerHello are used to establish security enhancement
   capabilities between client and server.  The ClientHello and
   ServerHello 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.

   The actual key exchange uses up to four messages: the server
   Certificate, the ServerKeyExchange, the client Certificate, and the
   ClientKeyExchange.  New key exchange methods can be created by
   specifying a format for these messages and by defining the use of the
   messages to allow the client and server to agree upon a shared
   secret.  This secret MUST be quite long; currently defined key
   exchange methods exchange secrets that range from 46 bytes upwards.

   Following the hello messages, the server will send its certificate in
   a Certificate message if it is to be authenticated.  Additionally, a
   ServerKeyExchange message may be sent, if it is required (e.g., if
   the 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.
   Next, the server will send the ServerHelloDone 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 CertificateRequest message, the client MUST send the Certificate
   message.  The ClientKeyExchange message is now sent, and the content
   of that message will depend on the public key algorithm selected
   between the ClientHello and the ServerHello.  If the client has sent
   a certificate with signing ability, a digitally-signed
   CertificateVerify message is sent to explicitly verify possession of
   the private key in the certificate.

   At this point, a ChangeCipherSpec message is sent by the client, and
   the client copies the pending Cipher Spec into the current Cipher
   Spec.  The client then immediately sends the Finished message under
   the new algorithms, keys, and secrets.  In response, the server will
   send its own ChangeCipherSpec message, transfer the pending to the
   current Cipher Spec, and send its Finished message under the new
   Cipher Spec.  At this point, the handshake is complete, and the
   client and server may begin to exchange application layer data.  (See
   flow chart below.)  Application data MUST NOT be sent prior to the
   completion of the first handshake (before a cipher suite other than
   TLS_NULL_WITH_NULL_NULL is established).

      Client                                               Server

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

             Figure 1.  Message flow for a full handshake

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

   Note: To help avoid pipeline stalls, ChangeCipherSpec is an
   independent TLS protocol content type, and is not actually a TLS
   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 ChangeCipherSpec 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 TLS client and
   server perform a full handshake.

      Client                                                Server

      ClientHello                   -------->
                                                       ServerHello
                                                [ChangeCipherSpec]
                                    <--------             Finished
      [ChangeCipherSpec]
      Finished                      -------->
      Application Data              <------->     Application Data

          Figure 2.  Message flow for an abbreviated handshake

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