7.4.  Handshake Protocol

   The TLS Handshake Protocol is one of the defined higher-level clients
   of the TLS Record Protocol.  This protocol is used to negotiate the
   secure attributes of a session.  Handshake messages are supplied to
   the TLS record layer, where they are encapsulated within one or more
   TLSPlaintext 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;

   The handshake protocol messages are presented below in the order they
   MUST be sent; sending handshake messages in an unexpected order
   results in a fatal error.  Unneeded handshake messages can be
   omitted, however.  Note one exception to the ordering: the
   Certificate message is used twice in the handshake (from server to
   client, then from client to server), but described only in its first
   position.  The one message that is not bound by these ordering rules
   is the HelloRequest message, which can be sent at any time, but which
   SHOULD be ignored by the client if it arrives in the middle of a
   handshake.

   New handshake message types are assigned by IANA as described in
   Section 12.

7.4.1.  Hello Messages

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

7.4.1.1.  Hello Request

   When this message will be sent:

      The HelloRequest message MAY be sent by the server at any time.

   Meaning of this message:

      HelloRequest is a simple notification that the client should begin
      the negotiation process anew.  In response, the client should send
      a ClientHello message when convenient.  This message is not
      intended to establish which side is the client or server but
      merely to initiate a new negotiation.  Servers SHOULD NOT send a
      HelloRequest immediately upon the client's initial connection.  It
      is the client's job to send a ClientHello at that time.

      This message will be ignored by the client if the client is
      currently negotiating a session.  This message MAY be ignored by
      the client if it does not wish to renegotiate a session, or the
      client may, if it wishes, respond with a no_renegotiation alert.
      Since handshake messages are intended to have transmission
      precedence over application data, it is expected that the
      negotiation will begin before no more than a few records are
      received from the client.  If the server sends a HelloRequest but
      does not receive a ClientHello in response, it may close the
      connection with a fatal alert.

      After sending a HelloRequest, servers SHOULD NOT repeat the
      request until the subsequent handshake negotiation is complete.

   Structure of this message:

      struct { } HelloRequest;

   This message MUST NOT be included in the message hashes that are
   maintained throughout the handshake and used in the Finished messages
   and the certificate verify message.

7.4.1.2.  Client Hello

   When this message will be sent:

      When a client first connects to a server, it is required to send
      the ClientHello as its first message.  The client can also send a
      ClientHello in response to a HelloRequest or on its own initiative
      in order to renegotiate the security parameters in an existing
      connection.

   Structure of this message:

      The ClientHello message includes a random structure, which is used
      later in the protocol.

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

      gmt_unix_time
         The current time and date in standard UNIX 32-bit format
         (seconds since the midnight starting Jan 1, 1970, UTC, ignoring
         leap seconds) according to the sender's internal clock.  Clocks
         are not required to be set correctly by the basic TLS protocol;
         higher-level or application protocols may define additional
         requirements.  Note that, for historical reasons, the data
         element is named using GMT, the predecessor of the current
         worldwide time base, UTC.

      random_bytes
         28 bytes generated by a secure random number generator.

   The ClientHello 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 from 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, and the third
   option makes it possible to establish several independent secure
   connections without repeating the full handshake protocol.  These
   independent connections may occur sequentially or simultaneously; a
   SessionID becomes valid when the handshake negotiating it completes
   with the exchange of Finished messages and persists until it is
   removed due to aging or because a fatal error was encountered on a
   connection associated with the session.  The actual contents of the
   SessionID are defined by the server.

      opaque SessionID<0..32>;

   Warning: Because the SessionID is transmitted without encryption or
   immediate MAC protection, servers MUST NOT place confidential
   information in session identifiers or let the contents of fake
   session identifiers cause any breach of security.  (Note that the
   content of the handshake as a whole, including the SessionID, is
   protected by the Finished messages exchanged at the end of the
   handshake.)

   The cipher suite list, passed from the client to the server in the
   ClientHello message, contains the combinations of cryptographic
   algorithms supported by the client in order of the client's
   preference (favorite choice first).  Each cipher suite defines a key
   exchange algorithm, a bulk encryption algorithm (including secret key
   length), a MAC algorithm, and a PRF.  The server will select a cipher
   suite or, if no acceptable choices are presented, return a handshake
   failure alert and close the connection.  If the list contains cipher
   suites the server does not recognize, support, or wish to use, the
   server MUST ignore those cipher suites, and process the remaining
   ones as usual.

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

   The ClientHello includes a list of compression algorithms supported
   by the client, ordered according to the client's preference.

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

      struct {
          ProtocolVersion client_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suites<2..2^16-2>;
          CompressionMethod compression_methods<1..2^8-1>;
          select (extensions_present) {
              case false:
                  struct {};
              case true:
                  Extension extensions<0..2^16-1>;
          };
      } ClientHello;

   TLS allows extensions to follow the compression_methods field in an
   extensions block.  The presence of extensions can be detected by
   determining whether there are bytes following the compression_methods
   at the end of the ClientHello.  Note that this method of detecting
   optional data differs from the normal TLS method of having a
   variable-length field, but it is used for compatibility with TLS
   before extensions were defined.

   client_version
      The version of the TLS protocol by which the client wishes to
      communicate during this session.  This SHOULD be the latest
      (highest valued) version supported by the client.  For this
      version of the specification, the version will be 3.3 (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 is empty if no session_id is available, or if the
      client wishes to generate new security parameters.

   cipher_suites
      This is a list of the cryptographic options supported by the
      client, 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.5.

   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), it MUST include the

      compression_method from that session.  This vector MUST contain,
      and all implementations MUST support, CompressionMethod.null.
      Thus, a client and server will always be able to agree on a
      compression method.

   extensions
      Clients MAY request extended functionality from servers by sending
      data in the extensions field.  The actual "Extension" format is
      defined in Section 7.4.1.4.

   In the event that a client requests additional functionality using
   extensions, and this functionality is not supplied by the server, the
   client MAY abort the handshake.  A server MUST accept ClientHello
   messages both with and without the extensions field, and (as for all
   other messages) it MUST check that the amount of data in the message
   precisely matches one of these formats; if not, then it MUST send a
   fatal "decode_error" alert.

   After sending the ClientHello message, the client waits for a
   ServerHello message.  Any handshake message returned by the server,
   except for a HelloRequest, is treated as a fatal error.

7.4.1.3.  Server Hello

   When this message will be sent:

      The server will send this message in response to a ClientHello
      message when it was able to find an acceptable set of algorithms.
      If it cannot find such a match, it will respond with a handshake
      failure alert.

   Structure of this message:

      struct {
          ProtocolVersion server_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suite;
          CompressionMethod compression_method;
          select (extensions_present) {
              case false:
                  struct {};
              case true:
                  Extension extensions<0..2^16-1>;
          };
      } ServerHello;

   The presence of extensions can be detected by determining whether
   there are bytes following the compression_method field at the end of
   the 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 is 3.3.  (See
      Appendix E for details about backward compatibility.)

   random
      This structure is generated by the server and MUST be
      independently generated from the 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.  If a session is
      resumed, it must be resumed using the same cipher suite it was
      originally negotiated with.  Note that there is no requirement
      that the server resume any session even if it had formerly
      provided a session_id.  Clients MUST be prepared to do a full
      negotiation -- including negotiating new cipher suites -- during
      any handshake.

   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.

   extensions
      A list of extensions.  Note that only extensions offered by the
      client can appear in the server's list.

7.4.1.4.  Hello Extensions

   The extension format is:

      struct {
          ExtensionType extension_type;
          opaque extension_data<0..2^16-1>;
      } Extension;

      enum {
          signature_algorithms(13), (65535)
      } ExtensionType;

   Here:

   -  "extension_type" identifies the particular extension type.

   -  "extension_data" contains information specific to the particular
      extension type.

   The initial set of extensions is defined in a companion document
   [TLSEXT].  The list of extension types is maintained by IANA as
   described in Section 12.

   An extension type MUST NOT appear in the ServerHello unless the same
   extension type appeared in the corresponding ClientHello.  If a
   client receives an extension type in ServerHello that it did not
   request in the associated ClientHello, it MUST abort the handshake
   with an unsupported_extension fatal alert.

   Nonetheless, "server-oriented" extensions may be provided in the
   future within this framework.  Such an extension (say, of type x)
   would require the client to first send an extension of type x in a
   ClientHello with empty extension_data to indicate that it supports
   the extension type.  In this case, the client is offering the
   capability to understand the extension type, and the server is taking
   the client up on its offer.

   When multiple extensions of different types are present in the
   ClientHello or ServerHello messages, the extensions MAY appear in any
   order.  There MUST NOT be more than one extension of the same type.

   Finally, note that extensions can be sent both when starting a new
   session and when requesting session resumption.  Indeed, a client
   that requests session resumption does not in general know whether the
   server will accept this request, and therefore it SHOULD send the
   same extensions as it would send if it were not attempting
   resumption.

   In general, the specification of each extension type needs to
   describe the effect of the extension both during full handshake and
   session resumption.  Most current TLS extensions are relevant only
   when a session is initiated: when an older session is resumed, the
   server does not process these extensions in Client Hello, and does
   not include them in Server Hello.  However, some extensions may
   specify different behavior during session resumption.

   There are subtle (and not so subtle) interactions that may occur in
   this protocol between new features and existing features which may
   result in a significant reduction in overall security.  The following
   considerations should be taken into account when designing new
   extensions:

   -  Some cases where a server does not agree to an extension are error
      conditions, and some are simply refusals to support particular
      features.  In general, error alerts should be used for the former,
      and a field in the server extension response for the latter.

   -  Extensions should, as far as possible, be designed to prevent any
      attack that forces use (or non-use) of a particular feature by
      manipulation of handshake messages.  This principle should be
      followed regardless of whether the feature is believed to cause a
      security problem.

      Often the fact that the extension fields are included in the
      inputs to the Finished message hashes will be sufficient, but
      extreme care is needed when the extension changes the meaning of
      messages sent in the handshake phase.  Designers and implementors
      should be aware of the fact that until the handshake has been
      authenticated, active attackers can modify messages and insert,
      remove, or replace extensions.

   -  It would be technically possible to use extensions to change major
      aspects of the design of TLS; for example the design of cipher
      suite negotiation.  This is not recommended; it would be more
      appropriate to define a new version of TLS -- particularly since
      the TLS handshake algorithms have specific protection against
      version rollback attacks based on the version number, and the
      possibility of version rollback should be a significant
      consideration in any major design change.

7.4.1.4.1.  Signature Algorithms

   The client uses the "signature_algorithms" extension to indicate to
   the server which signature/hash algorithm pairs may be used in
   digital signatures.  The "extension_data" field of this extension
   contains a "supported_signature_algorithms" value.

      enum {
          none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
          sha512(6), (255)
      } HashAlgorithm;

      enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
        SignatureAlgorithm;

      struct {
            HashAlgorithm hash;
            SignatureAlgorithm signature;
      } SignatureAndHashAlgorithm;

      SignatureAndHashAlgorithm
        supported_signature_algorithms<2..2^16-2>;

   Each SignatureAndHashAlgorithm value lists a single hash/signature
   pair that the client is willing to verify.  The values are indicated
   in descending order of preference.

   Note: Because not all signature algorithms and hash algorithms may be
   accepted by an implementation (e.g., DSA with SHA-1, but not
   SHA-256), algorithms here are listed in pairs.

   hash
      This field indicates the hash algorithm which may be used.  The
      values indicate support for unhashed data, MD5 [MD5], SHA-1,
      SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively.  The
      "none" value is provided for future extensibility, in case of a
      signature algorithm which does not require hashing before signing.

   signature
      This field indicates the signature algorithm that may be used.
      The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
      [PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively.  The
      "anonymous" value is meaningless in this context but used in
      Section 7.4.3.  It MUST NOT appear in this extension.

   The semantics of this extension are somewhat complicated because the
   cipher suite indicates permissible signature algorithms but not hash
   algorithms.  Sections 7.4.2 and 7.4.3 describe the appropriate rules.

   If the client supports only the default hash and signature algorithms
   (listed in this section), it MAY omit the signature_algorithms
   extension.  If the client does not support the default algorithms, or
   supports other hash and signature algorithms (and it is willing to
   use them for verifying messages sent by the server, i.e., server
   certificates and server key exchange), it MUST send the

   signature_algorithms extension, listing the algorithms it is willing
   to accept.

   If the client does not send the signature_algorithms extension, the
   server MUST do the following:

   -  If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
      DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had
      sent the value {sha1,rsa}.

   -  If the negotiated key exchange algorithm is one of (DHE_DSS,
      DH_DSS), behave as if the client had sent the value {sha1,dsa}.

   -  If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
      ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}.

   Note: this is a change from TLS 1.1 where there are no explicit
   rules, but as a practical matter one can assume that the peer
   supports MD5 and SHA-1.

   Note: this extension is not meaningful for TLS versions prior to 1.2.
   Clients MUST NOT offer it if they are offering prior versions.
   However, even if clients do offer it, the rules specified in [TLSEXT]
   require servers to ignore extensions they do not understand.

   Servers MUST NOT send this extension.  TLS servers MUST support
   receiving this extension.

   When performing session resumption, this extension is not included in
   Server Hello, and the server ignores the extension in Client Hello
   (if present).

7.4.2.  Server Certificate

   When this message will be sent:

      The server MUST send a Certificate message whenever the agreed-
      upon key exchange method uses certificates for authentication
      (this includes all key exchange methods defined in this document
      except DH_anon).  This message will always immediately follow the
      ServerHello message.

   Meaning of this message:

      This message conveys the server's certificate chain to the client.

      The certificate MUST be appropriate for the negotiated cipher
      suite's key exchange algorithm and any negotiated extensions.

   Structure of this message:

      opaque ASN.1Cert<1..2^24-1>;

      struct {
          ASN.1Cert certificate_list<0..2^24-1>;
      } Certificate;

   certificate_list
      This is a sequence (chain) of certificates.  The sender's
      certificate MUST come first in the list.  Each following
      certificate MUST directly certify the one preceding it.  Because
      certificate validation requires that root keys be distributed
      independently, the self-signed certificate that specifies the root
      certificate authority MAY be omitted from the chain, under the
      assumption that the remote end must already possess it in order to
      validate it in any case.

   The same message type and structure will be used for the client's
   response to a certificate request message.  Note that a client MAY
   send no certificates if it does not have an appropriate certificate
   to send in response to the server's authentication request.

   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.

   The following rules apply to the certificates sent by the server:

   -  The certificate type MUST be X.509v3, unless explicitly negotiated
      otherwise (e.g., [TLSPGP]).

   -  The end entity certificate's public key (and associated
      restrictions) MUST be compatible with the selected key exchange
      algorithm.

      Key Exchange Alg.  Certificate Key Type

      RSA                RSA public key; the certificate MUST allow the
      RSA_PSK            key to be used for encryption (the
                         keyEncipherment bit MUST be set if the key
                         usage extension is present).
                         Note: RSA_PSK is defined in [TLSPSK].

      DHE_RSA            RSA public key; the certificate MUST allow the
      ECDHE_RSA          key to be used for signing (the
                         digitalSignature bit MUST be set if the key
                         usage extension is present) with the signature
                         scheme and hash algorithm that will be employed
                         in the server key exchange message.
                         Note: ECDHE_RSA is defined in [TLSECC].

      DHE_DSS            DSA public key; the certificate MUST allow the
                         key to be used for signing with the hash
                         algorithm that will be employed in the server
                         key exchange message.

      DH_DSS             Diffie-Hellman public key; the keyAgreement bit
      DH_RSA             MUST be set if the key usage extension is
                         present.

      ECDH_ECDSA         ECDH-capable public key; the public key MUST
      ECDH_RSA           use a curve and point format supported by the
                         client, as described in [TLSECC].

      ECDHE_ECDSA        ECDSA-capable public key; the certificate MUST
                         allow the key to be used for signing with the
                         hash algorithm that will be employed in the
                         server key exchange message.  The public key
                         MUST use a curve and point format supported by
                         the client, as described in  [TLSECC].

   -  The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are
      used to guide certificate selection.

   If the client provided a "signature_algorithms" extension, then all
   certificates provided by the server MUST be signed by a
   hash/signature algorithm pair that appears in that extension.  Note
   that this implies that a certificate containing a key for one
   signature algorithm MAY be signed using a different signature
   algorithm (for instance, an RSA key signed with a DSA key).  This is
   a departure from TLS 1.1, which required that the algorithms be the
   same.  Note that this also implies that the DH_DSS, DH_RSA,
   ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the
   algorithm used to sign the certificate.  Fixed DH certificates MAY be
   signed with any hash/signature algorithm pair appearing in the
   extension.  The names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are
   historical.

   If the server has multiple certificates, it chooses one of them based
   on the above-mentioned criteria (in addition to other criteria, such
   as transport layer endpoint, local configuration and preferences,
   etc.).  If the server has a single certificate, it SHOULD attempt to
   validate that it meets these criteria.

   Note that there are certificates that use algorithms and/or algorithm
   combinations that cannot be currently used with TLS.  For example, a
   certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in
   SubjectPublicKeyInfo) cannot be used because TLS defines no
   corresponding signature algorithm.

   As cipher suites that specify new key exchange methods are specified
   for the TLS protocol, they will imply the certificate format and the
   required encoded keying information.

7.4.3.  Server Key Exchange Message

   When this message will be sent:

      This message will be sent immediately after the server Certificate
      message (or the ServerHello message, if this is an anonymous
      negotiation).

      The ServerKeyExchange message is sent by the server only when the
      server Certificate message (if sent) does not contain enough data
      to allow the client to exchange a premaster secret.  This is true
      for the following key exchange methods:

         DHE_DSS
         DHE_RSA
         DH_anon

      It is not legal to send the ServerKeyExchange message for the
      following key exchange methods:

         RSA
         DH_DSS
         DH_RSA

      Other key exchange algorithms, such as those defined in [TLSECC],
      MUST specify whether the ServerKeyExchange message is sent or not;
      and if the message is sent, its contents.

   Meaning of this message:

      This message conveys cryptographic information to allow the client
      to communicate the premaster secret: a Diffie-Hellman public key
      with which the client can complete a key exchange (with the result
      being the premaster secret) or a public key for some other
      algorithm.

   Structure of this message:

      enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa
            /* may be extended, e.g., for ECDH -- see [TLSECC] */
           } KeyExchangeAlgorithm;

      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 (g^X mod p).

      struct {
          select (KeyExchangeAlgorithm) {
              case dh_anon:
                  ServerDHParams params;
              case dhe_dss:
              case dhe_rsa:
                  ServerDHParams params;
                  digitally-signed struct {
                      opaque client_random[32];
                      opaque server_random[32];
                      ServerDHParams params;
                  } signed_params;
              case rsa:
              case dh_dss:
              case dh_rsa:
                  struct {} ;
                 /* message is omitted for rsa, dh_dss, and dh_rsa */
              /* may be extended, e.g., for ECDH -- see [TLSECC] */
          };
      } ServerKeyExchange;

      params
         The server's key exchange parameters.

      signed_params
         For non-anonymous key exchanges, a signature over the server's
         key exchange parameters.

   If the client has offered the "signature_algorithms" extension, the
   signature algorithm and hash algorithm MUST be a pair listed in that
   extension.  Note that there is a possibility for inconsistencies
   here.  For instance, the client might offer DHE_DSS key exchange but
   omit any DSA pairs from its "signature_algorithms" extension.  In
   order to negotiate correctly, the server MUST check any candidate
   cipher suites against the "signature_algorithms" extension before
   selecting them.  This is somewhat inelegant but is a compromise
   designed to minimize changes to the original cipher suite design.

   In addition, the hash and signature algorithms MUST be compatible
   with the key in the server's end-entity certificate.  RSA keys MAY be
   used with any permitted hash algorithm, subject to restrictions in
   the certificate, if any.

   Because DSA signatures do not contain any secure indication of hash
   algorithm, there is a risk of hash substitution if multiple hashes
   may be used with any key.  Currently, DSA [DSS] may only be used with
   SHA-1.  Future revisions of DSS [DSS-3] are expected to allow the use
   of other digest algorithms with DSA, as well as guidance as to which

   digest algorithms should be used with each key size.  In addition,
   future revisions of [PKIX] may specify mechanisms for certificates to
   indicate which digest algorithms are to be used with DSA.

   As additional cipher suites are defined for TLS that include new key
   exchange algorithms, the server key exchange message will be sent if
   and only if the certificate type associated with the key exchange
   algorithm does not provide enough information for the client to
   exchange a premaster secret.

7.4.4.  Certificate Request

   When this message will be sent:

       A non-anonymous server can optionally request a certificate from
       the client, if appropriate for the selected cipher suite.  This
       message, if sent, will immediately follow the ServerKeyExchange
       message (if it is sent; otherwise, this message follows the
       server's Certificate message).

   Structure of this message:

      enum {
          rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
          rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
          fortezza_dms_RESERVED(20), (255)
      } ClientCertificateType;

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

      struct {
          ClientCertificateType certificate_types<1..2^8-1>;
          SignatureAndHashAlgorithm
            supported_signature_algorithms<2^16-1>;
          DistinguishedName certificate_authorities<0..2^16-1>;
      } CertificateRequest;

   certificate_types
      A list of the types of certificate types that the client may
      offer.

         rsa_sign        a certificate containing an RSA key
         dss_sign        a certificate containing a DSA key
         rsa_fixed_dh    a certificate containing a static DH key.
         dss_fixed_dh    a certificate containing a static DH key

   supported_signature_algorithms
      A list of the hash/signature algorithm pairs that the server is
      able to verify, listed in descending order of preference.

   certificate_authorities
      A list of the distinguished names [X501] of acceptable
      certificate_authorities, represented in DER-encoded format.  These
      distinguished names may specify a desired distinguished name for a
      root CA or for a subordinate CA; thus, this message can be used to
      describe known roots as well as a desired authorization space.  If
      the certificate_authorities list is empty, then the client MAY
      send any certificate of the appropriate ClientCertificateType,
      unless there is some external arrangement to the contrary.

   The interaction of the certificate_types and
   supported_signature_algorithms fields is somewhat complicated.
   certificate_types has been present in TLS since SSLv3, but was
   somewhat underspecified.  Much of its functionality is superseded by
   supported_signature_algorithms.  The following rules apply:

   -  Any certificates provided by the client MUST be signed using a
      hash/signature algorithm pair found in
      supported_signature_algorithms.

   -  The end-entity certificate provided by the client MUST contain a
      key that is compatible with certificate_types.  If the key is a
      signature key, it MUST be usable with some hash/signature
      algorithm pair in supported_signature_algorithms.

   -  For historical reasons, the names of some client certificate types
      include the algorithm used to sign the certificate.  For example,
      in earlier versions of TLS, rsa_fixed_dh meant a certificate
      signed with RSA and containing a static DH key.  In TLS 1.2, this
      functionality has been obsoleted by the
      supported_signature_algorithms, and the certificate type no longer
      restricts the algorithm used to sign the certificate.  For
      example, if the server sends dss_fixed_dh certificate type and
      {{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
      with a certificate containing a static DH key, signed with RSA-
      SHA1.

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

   Note: Values listed as RESERVED may not be used.  They were used in
   SSLv3.

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

7.4.5.  Server Hello Done

   When this message will be sent:

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

   Meaning of this message:

      This message means that the server is done sending messages to
      support the key exchange, and the client can proceed with its
      phase of the key exchange.

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

   Structure of this message:

      struct { } ServerHelloDone;

7.4.6.  Client Certificate

   When this message will be sent:

      This is the first message the client can send after receiving a
      ServerHelloDone message.  This message is only sent if the server
      requests a certificate.  If no suitable certificate is available,
      the client MUST send a certificate message containing no
      certificates.  That is, the certificate_list structure has a
      length of zero.  If the client does not send any certificates, the
      server MAY at its discretion either continue the handshake without
      client authentication, or respond with a fatal handshake_failure
      alert.  Also, if some aspect of the certificate chain was
      unacceptable (e.g., it was not signed by a known, trusted CA), the
      server MAY at its discretion either continue the handshake
      (considering the client unauthenticated) or send a fatal alert.

      Client certificates are sent using the Certificate structure
      defined in Section 7.4.2.

   Meaning of this message:

      This message conveys the client's certificate chain to the server;
      the server will use it when verifying the CertificateVerify
      message (when the client authentication is based on signing) or
      calculating the premaster secret (for non-ephemeral Diffie-
      Hellman).  The certificate MUST be appropriate for the negotiated
      cipher suite's key exchange algorithm, and any negotiated
      extensions.

   In particular:

   -  The certificate type MUST be X.509v3, unless explicitly negotiated
      otherwise (e.g., [TLSPGP]).

   -  The end-entity certificate's public key (and associated
      restrictions) has to be compatible with the certificate types
      listed in CertificateRequest:

      Client Cert. Type   Certificate Key Type

      rsa_sign            RSA public key; the certificate MUST allow the
                          key to be used for signing with the signature
                          scheme and hash algorithm that will be
                          employed in the certificate verify message.

      dss_sign            DSA public key; the certificate MUST allow the
                          key to be used for signing with the hash
                          algorithm that will be employed in the
                          certificate verify message.

      ecdsa_sign          ECDSA-capable public key; the certificate MUST
                          allow the key to be used for signing with the
                          hash algorithm that will be employed in the
                          certificate verify message; the public key
                          MUST use a curve and point format supported by
                          the server.

      rsa_fixed_dh        Diffie-Hellman public key; MUST use the same
      dss_fixed_dh        parameters as server's key.

      rsa_fixed_ecdh      ECDH-capable public key; MUST use the
      ecdsa_fixed_ecdh    same curve as the server's key, and MUST use a
                          point format supported by the server.

   -  If the certificate_authorities list in the certificate request
      message was non-empty, one of the certificates in the certificate
      chain SHOULD be issued by one of the listed CAs.

   -  The certificates MUST be signed using an acceptable hash/
      signature algorithm pair, as described in Section 7.4.4.  Note
      that this relaxes the constraints on certificate-signing
      algorithms found in prior versions of TLS.

   Note that, as with the server certificate, there are certificates
   that use algorithms/algorithm combinations that cannot be currently
   used with TLS.

7.4.7.  Client Key Exchange Message

   When this message will be sent:

      This message is always sent by the client.  It MUST immediately
      follow the client certificate message, if it is sent.  Otherwise,
      it MUST be the first message sent by the client after it receives
      the ServerHelloDone message.

   Meaning of this message:

      With this message, the premaster secret is set, either by direct
      transmission of the RSA-encrypted secret or by the transmission of
      Diffie-Hellman parameters that will allow each side to agree upon
      the same premaster secret.

      When the client is using an ephemeral Diffie-Hellman exponent,
      then this message contains the client's Diffie-Hellman public
      value.  If the client is sending a certificate containing a static
      DH exponent (i.e., it is doing fixed_dh client authentication),
      then this message MUST be sent but MUST be empty.

   Structure of this message:

      The choice of messages depends on which key exchange method has
      been selected.  See Section 7.4.3 for the KeyExchangeAlgorithm
      definition.

      struct {
          select (KeyExchangeAlgorithm) {
              case rsa:
                  EncryptedPreMasterSecret;
              case dhe_dss:
              case dhe_rsa:
              case dh_dss:
              case dh_rsa:
              case dh_anon:
                  ClientDiffieHellmanPublic;
          } exchange_keys;
      } ClientKeyExchange;

7.4.7.1.  RSA-Encrypted Premaster Secret Message

   Meaning of this message:

      If RSA is being used for key agreement and authentication, the
      client generates a 48-byte premaster secret, encrypts it using the
      public key from the server's certificate, and sends the result in
      an encrypted premaster secret message.  This structure is a
      variant of the ClientKeyExchange message and is not a message in
      itself.

   Structure of this 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 rollback 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 8.1.

   Note: The version number in the PreMasterSecret is the version
   offered by the client in the ClientHello.client_version, not the
   version negotiated for the connection.  This feature is designed to
   prevent rollback attacks.  Unfortunately, some old implementations
   use the negotiated version instead, and therefore checking the
   version number may lead to failure to interoperate with such
   incorrect client implementations.

   Client implementations MUST always send the correct version number in
   PreMasterSecret.  If ClientHello.client_version is TLS 1.1 or higher,
   server implementations MUST check the version number as described in
   the note below.  If the version number is TLS 1.0 or earlier, server
   implementations SHOULD check the version number, but MAY have a
   configuration option to disable the check.  Note that if the check
   fails, the PreMasterSecret SHOULD be randomized as described below.

   Note: Attacks discovered by Bleichenbacher [BLEI] and Klima et al.
   [KPR03] can be used to attack a TLS server that reveals whether a
   particular message, when decrypted, is properly PKCS#1 formatted,
   contains a valid PreMasterSecret structure, or has the correct
   version number.

   As described by Klima [KPR03], these vulnerabilities can be avoided
   by treating incorrectly formatted message blocks and/or mismatched
   version numbers in a manner indistinguishable from correctly
   formatted RSA blocks.  In other words:

      1. Generate a string R of 46 random bytes

      2. Decrypt the message to recover the plaintext M

      3. If the PKCS#1 padding is not correct, or the length of message
         M is not exactly 48 bytes:
            pre_master_secret = ClientHello.client_version || R
         else If ClientHello.client_version <= TLS 1.0, and version
         number check is explicitly disabled:
            pre_master_secret = M
         else:
            pre_master_secret = ClientHello.client_version || M[2..47]

   Note that explicitly constructing the pre_master_secret with the
   ClientHello.client_version produces an invalid master_secret if the
   client has sent the wrong version in the original pre_master_secret.

   An alternative approach is to treat a version number mismatch as a
   PKCS-1 formatting error and randomize the premaster secret
   completely:

      1. Generate a string R of 48 random bytes

      2. Decrypt the message to recover the plaintext M

      3. If the PKCS#1 padding is not correct, or the length of message
         M is not exactly 48 bytes:
            pre_master_secret = R
         else If ClientHello.client_version <= TLS 1.0, and version
         number check is explicitly disabled:
            premaster secret = M
         else If M[0..1] != ClientHello.client_version:
            premaster secret = R
         else:
            premaster secret = M

   Although no practical attacks against this construction are known,
   Klima et al. [KPR03] describe some theoretical attacks, and therefore
   the first construction described is RECOMMENDED.

   In any case, a TLS server MUST NOT generate an alert if processing an
   RSA-encrypted premaster secret message fails, or the version number
   is not as expected.  Instead, it MUST continue the handshake with a
   randomly generated premaster secret.  It may be useful to log the
   real cause of failure for troubleshooting purposes; however, care
   must be taken to avoid leaking the information to an attacker
   (through, e.g., timing, log files, or other channels.)

   The RSAES-OAEP encryption scheme defined in [PKCS1] is more secure
   against the Bleichenbacher attack.  However, for maximal
   compatibility with earlier versions of TLS, this specification uses
   the RSAES-PKCS1-v1_5 scheme.  No variants of the Bleichenbacher
   attack are known to exist provided that the above recommendations are
   followed.

   Implementation note: Public-key-encrypted data is represented as an
   opaque vector <0..2^16-1> (see Section 4.7).  Thus, the RSA-encrypted
   PreMasterSecret in a ClientKeyExchange is preceded by two length
   bytes.  These bytes are redundant in the case of RSA because the
   EncryptedPreMasterSecret is the only data in the ClientKeyExchange
   and its length can therefore be unambiguously determined.  The SSLv3
   specification was not clear about the encoding of public-key-
   encrypted data, and therefore many SSLv3 implementations do not
   include the length bytes -- they encode the RSA-encrypted data
   directly in the ClientKeyExchange message.

   This specification requires correct encoding of the
   EncryptedPreMasterSecret complete with length bytes.  The resulting
   PDU is incompatible with many SSLv3 implementations.  Implementors

   upgrading from SSLv3 MUST modify their implementations to generate
   and accept the correct encoding.  Implementors who wish to be
   compatible with both SSLv3 and TLS should make their implementation's
   behavior dependent on the protocol version.

   Implementation note: It is now known that remote timing-based attacks
   on TLS are possible, at least when the client and server are on the
   same LAN.  Accordingly, implementations that use static RSA keys MUST
   use RSA blinding or some other anti-timing technique, as described in
   [TIMING].

7.4.7.2.  Client Diffie-Hellman Public Value

   Meaning of this message:

      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.  This structure is a variant of the client
      key exchange message, and not a message in itself.

   Structure of this message:

      enum { implicit, explicit } PublicValueEncoding;

      implicit
         If the client has sent a certificate which contains a suitable
         Diffie-Hellman key (for fixed_dh client authentication), then
         Yc is implicit and does not need to be sent again.  In this
         case, the client key exchange message will be sent, but it MUST
         be empty.

      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).

7.4.8.  Certificate Verify

   When this message will be sent:

      This message is used to provide explicit verification of a client
      certificate.  This message is only sent following a client
      certificate that has signing capability (i.e., all certificates
      except those containing fixed Diffie-Hellman parameters).  When
      sent, it MUST immediately follow the client key exchange message.

   Structure of this message:

      struct {
           digitally-signed struct {
               opaque handshake_messages[handshake_messages_length];
           }
      } CertificateVerify;

      Here handshake_messages refers to all handshake messages sent or
      received, starting at client hello and up to, but not including,
      this message, including the type and length fields of the
      handshake messages.  This is the concatenation of all the
      Handshake structures (as defined in Section 7.4) exchanged thus
      far.  Note that this requires both sides to either buffer the
      messages or compute running hashes for all potential hash
      algorithms up to the time of the CertificateVerify computation.
      Servers can minimize this computation cost by offering a
      restricted set of digest algorithms in the CertificateRequest
      message.

      The hash and signature algorithms used in the signature MUST be
      one of those present in the supported_signature_algorithms field
      of the CertificateRequest message.  In addition, the hash and
      signature algorithms MUST be compatible with the key in the
      client's end-entity certificate.  RSA keys MAY be used with any
      permitted hash algorithm, subject to restrictions in the
      certificate, if any.

      Because DSA signatures do not contain any secure indication of
      hash algorithm, there is a risk of hash substitution if multiple
      hashes may be used with any key.  Currently, DSA [DSS] may only be
      used with SHA-1.  Future revisions of DSS [DSS-3] are expected to
      allow the use of other digest algorithms with DSA, as well as
      guidance as to which digest algorithms should be used with each
      key size.  In addition, future revisions of [PKIX] may specify
      mechanisms for certificates to indicate which digest algorithms
      are to be used with DSA.

7.4.9.  Finished

   When this message will be sent:

      A Finished message is always sent immediately after a change
      cipher spec message to verify that the key exchange and
      authentication processes were successful.  It is essential that a
      change cipher spec message be received between the other handshake
      messages and the Finished message.

   Meaning of this message:

      The Finished message is the first one protected with the just
      negotiated algorithms, keys, and secrets.  Recipients of Finished
      messages MUST verify that the contents are correct.  Once a side
      has sent its Finished message and received and validated the
      Finished message from its peer, it may begin to send and receive
      application data over the connection.

   Structure of this message:

      struct {
          opaque verify_data[verify_data_length];
      } Finished;

      verify_data
         PRF(master_secret, finished_label, Hash(handshake_messages))
            [0..verify_data_length-1];

      finished_label
         For Finished messages sent by the client, the string
         "client finished".  For Finished messages sent by the server,
         the string "server finished".

      Hash denotes a Hash of the handshake messages.  For the PRF
      defined in Section 5, the Hash MUST be the Hash used as the basis
      for the PRF.  Any cipher suite which defines a different PRF MUST
      also define the Hash to use in the Finished computation.

      In previous versions of TLS, the verify_data was always 12 octets
      long.  In the current version of TLS, it depends on the cipher
      suite.  Any cipher suite which does not explicitly specify
      verify_data_length has a verify_data_length equal to 12.  This
      includes all existing cipher suites.  Note that this
      representation has the same encoding as with previous versions.
      Future cipher suites MAY specify other lengths but such length
      MUST be at least 12 bytes.

      handshake_messages
         All of the data from all messages in this handshake (not
         including any HelloRequest messages) up to, but not including,
         this message.  This is only data visible at the handshake layer
         and does not include record layer headers.  This is the
         concatenation of all the Handshake structures as defined in
         Section 7.4, exchanged thus far.

   It is a fatal error if a Finished message is not preceded by a
   ChangeCipherSpec message at the appropriate point in the handshake.

   The value handshake_messages includes all handshake messages starting
   at ClientHello up to, but not including, this Finished message.  This
   may be different from handshake_messages in Section 7.4.8 because it
   would include the CertificateVerify message (if sent).  Also, the
   handshake_messages for the Finished message sent by the client will
   be different from that for the Finished message sent by the server,
   because the one that is sent second will include the prior one.

   Note: ChangeCipherSpec messages, alerts, and any other record types
   are not handshake messages and are not included in the hash
   computations.  Also, HelloRequest messages are omitted from handshake
   hashes.