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Previous page | Next page | Contents SSL protocolSSL is a layered protocol. At each layer, messages may include fields for length, description, and content. SSL takes messages to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, and transmits the result. Received data is decrypted, verified, decompressed, and reassembled, then delivered to higher level clients.
An SSL session may include multiple secure connections; in addition, parties may have multiple simultaneous sessions. The session state includes the following elements:
The connection state includes the following elements:
7.2 Record layerThe SSL Record Layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size.
struct {
uint8 major, minor;
} ProtocolVersion;
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[SSLPlaintext.length];
} SSLPlaintext;
7.2.2 Record compression and decompressionAll records are compressed using the compression algorithm defined in the current session state. There is always an active compression algorithm, however initially it is defined as CompressionMethod.null. The compression algorithm translates an SSLPlaintext structure into an SSLCompressed structure. Compression functions erase their state information whenever the CipherSpec is replaced.Note: The CipherSpec is part of the session state described in Section 7.1. References to fields of the CipherSpec are made throughout this document using presentation syntax. A more complete description of the CipherSpec is shown in Appendix A.7. Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters an SSLCompressed.fragment that would decompress to a length in excess of 214 bytes, it should issue a fatal decompression_failure alert (Section 7.4.2).
struct {
ContentType type;
/* same as SSLPlaintext.type */
ProtocolVersion version;
/* same as SSLPlaintext.version */
uint16 length;
opaque fragment[SSLCompressed.length];
} SSLCompressed;
Note: A CompressionMethod.null operation is an identity operation; no fields are altered. (See Appendix A.4.1) Implementation note: Decompression functions are responsible for ensuring that messages cannot cause internal buffer overflows.
Once the handshake is complete, the two parties have shared secrets which are used to encrypt records and compute keyed message authentication codes (MACs) on their contents. The techniques used to perform the encryption and MAC operations are defined by the CipherSpec and constrained by CipherSpec.cipher_type. The encryption and MAC functions translate an SSLCompressed structure into an SSLCiphertext. The decryption functions reverse the process. Transmissions also include a sequence number so that missing, altered, or extra messages are detectable.
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
} SSLCiphertext;
stream-ciphered struct {
opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
} GenericStreamCipher;
The MAC is generated as:
hash(MAC_write_secret + pad_2 +
hash (MAC_write_secret + pad_1 + seq_num + length + content));
where "+" denotes concatenation.
Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet. If the CipherSuite is SSL_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not encrypted and the MAC size is zero implying that no MAC is used). SSLCiphertext.length is SSLCompressed.length plus CipherSpec.hash_size.
7.2.3.2 CBC block cipherFor block ciphers (such as RC2 or DES), the encryption and MAC functions convert SSLCompressed.fragment structures to and from block SSLCiphertext.fragment structures.
block-ciphered struct {
opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
The MAC is generated as described in Section 7.2.3.1.
The encrypted data length (SSLCiphertext.length) is one more than the sum of SSLCompressed.length, CipherSpec.hash_size, and padding_length. Note: With CBC block chaining the initialization vector (IV) for the first record is provided by the handshake protocol. The IV for subsequent records is the last ciphertext block from the previous record.
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
The change cipher spec message is sent by both the client and server to notify the receiving party that subsequent records will be protected under the just-negotiated CipherSpec and keys. Reception of this message causes the receiver to copy the read pending state into the read current state. Separate read and write states are maintained by both the SSL client and server. When the client or server receives a change cipher spec message, it copies the pending read state into the current read state. When the client or server writes a change cipher spec message, it copies the pending write state into the current write state. The client sends a change cipher spec message following handshake key exchange and certificate verify messages (if any), and the server sends one after successfully processing the key exchange message it received from the client. An unexpected change cipher spec message should generate an unexpected_message alert (Section 7.4.2). When resuming a previous session, the change cipher spec message is sent after the hello messages.
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decompression_failure(30),
handshake_failure(40), no_certificate(41), bad_certificate(42),
unsupported_certificate(43), certificate_revoked(44),
certificate_expired(45), certificate_unknown(46),
illegal_parameter (47)
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
The client sends a client hello message to which the server must respond with a server hello message, or else a fatal error will occur and the connection will fail. The client hello and server hello are used to establish security enhancement capabilities between client and server. The client hello and server hello establish the following attributes: protocol version, session ID, cipher suite, and compression method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random. Following the hello messages, the server will send its certificate, if it is to be authenticated. Additionally, a server key exchange message may be sent, if it is required (e.g. if their server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected. Now the server will send the server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. If the server has sent a certificate request message, the client must send either the certificate message or a no certificate alert. The client key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between the client hello and the server hello. If the client has sent a certificate with signing ability, a digitally-signed certificate verify message is sent to explicitly verify the certificate. At this point, a change cipher spec message is sent by the client, and the client copies the pending 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 change cipher spec 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.)
Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent SSL Protocol content type, and is not actually an SSL handshake message. When the client and server decide to resume a previous session or duplicate an existing session (instead of negotiating new security parameters) the message flow is as follows: The client sends a client hello 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 server hello with the same Session ID value. At this point, both client and server must send change cipher spec messages and proceed directly to finished messages. Once the re-establishment is complete, the client and server may begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID and the SSL client and server perform a full handshake.
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; /* type of handshake message */
uint24 length; /* # bytes in handshake message body */
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 in the order they must be sent; sending handshake messages in an unexpected order results in a fatal error.
7.6.1.1 Hello requestThe hello request message may be sent by the server at any time, but will be ignored by the client if the handshake protocol is already underway. It is a simple notification that the client should begin the negotiation process anew by sending a client hello message when convenient.Note: Since handshake messages are intended to have transmission precedence over application data, it is expected that the negotiation begin in no more than one or two times the transmission time of a maximum length application data message. After sending a hello request, servers should not repeat the request until the subsequent handshake negotiation is complete. A client that receives a hello request while in a handshake negotiation state should simply ignore the message. The structure of a hello request message is as follows:
struct { } HelloRequest;
7.6.1.2 Client helloWhen a client first connects to a server it is required to send the client hello as its first message. The client can also send a client hello in response to a hello request or on its own initiative in order to renegotiate the security parameters in an existing connection.The client hello message includes a random structure, which is used later in the protocol.
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
The client hello message includes a variable length session identifier. If not empty, the value identifies a session between the same client and server whose security parameters the client wishes to reuse. The session identifier may be from an earlier connection, this connection, or another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, while the third option makes it possible to establish several simultaneous independent secure connections without repeating the full handshake protocol. The actual contents of the SessionID are defined by the server.
opaque SessionID<0..32>; Warning: Servers must not place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. The CipherSuite list, passed from the client to the server in the client hello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (first choice first). Each CipherSuite defines both a key exchange algorithm and a CipherSpec. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection.
uint8 CipherSuite[2]; /* Cryptographic suite selector */ The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference. If the server supports none of those specified by the client, the session must fail.
enum { null(0), (255) } CompressionMethod;
Issue: Which compression methods to support is under investigation. The structure of the client hello is as follows.
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..216-1>;
CompressionMethod compression_methods<1..28-1>;
} ClientHello;
After sending the client hello message, the client waits for a server hello message. Any other handshake message returned by the server except for a hello request is treated as a fatal error. Implementation note: Application data may not be sent before a finished message has been sent. Transmitted application data is known to be insecure until a valid finished message has been received. This absolute restriction is relaxed if there is a current, non-null encryption on this connection.
7.6.1.3 Server helloThe server processes the client hello message and responds with either a handshake_failure alert or server hello message.
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} ServerHello;
opaque ASN.1Cert<1..224-1>;
struct {
ASN.1Cert certificate_list<1..224-1>;
} Certificate;
certificate_list This is a sequence (chain) of X.509.v3 certificates, ordered with the sender's certificate first and the root certificate authority last. 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.
Note: According to current US export law, RSA moduli larger than 512 bits may not be used for key exchange in software exported from the US. With this message, larger RSA keys may be used as signature-only certificates to sign temporary shorter RSA keys for key exchange.
enum { rsa, diffie_hellman, fortezza_dms } KeyExchangeAlgorithm;
struct {
opaque rsa_modulus<1..216-1>;
opaque rsa_exponent<1..216-1>;
} ServerRSAParams;
rsa_modulus The modulus of the server's temporary RSA key. rsa_exponent The public exponent of the server's temporary RSA key.
struct {
opaque dh_p<1..216-1>;
opaque dh_g<1..216-1>;
opaque dh_Ys<1..216-1>;
} ServerDHParams; /* Ephemeral DH parameters */
struct {
opaque r_s [128];
} ServerFortezzaParams;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_dms:
ServerFortezzaParams params;
};
} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;
digitally-signed struct {
select(SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
opaque md5_hash[16];
opaque sha_hash[20];
case dsa:
opaque sha_hash[20];
};
} Signature;
opaque CertificateAuthority<0..224-1>;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza_dms(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..216-1>;
struct {
ClientCertificateType certificate_types<1..28-1>;
DistinguishedName certificate_authorities<3..216-1>;
} CertificateRequest;
Note: DistinguishedName is derived from [X509]. Note: It is a fatal handshake_failure alert for an anonymous server to request client identification.
struct { } ServerHelloDone;
Upon receipt of the server hello done message the client should verify that the server provided a valid certificate if required and check that the server hello parameters are acceptable.
Client certificates are sent using the Certificate defined in Section 7.6.2. Note: Client Diffie-Hellman certificates must match the server specified Diffie-Hellman parameters.
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case fortezza_dms: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
The information to select the appropriate record structure is in the pending session state (see Section 7.1).
7.6.7.1 RSA encrypted premaster secret messageIf RSA is being used for key agreement and authentication, the client generates a 48-byte pre-master secret, encrypts it under the public key from the server's certificate or temporary RSA key from a server key exchange message, and sends the result in an encrypted premaster secret message.
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
7.6.7.2 Fortezza key exchange messageUnder Fortezza DMS, the client derives a Token Encryption Key (TEK) using Fortezza's Key Exchange Algorithm (KEA). The client's KEA calculation uses the public key in the server's certificate along with private parameters in the client's token. The client sends public parameters needed for the server to generate the TEK, using its own private parameters. The client generates session keys, wraps them using the TEK, and sends the results to the server. The client generates IV's for the session keys and TEK and sends them also. The client generates a random 48-byte premaster secret, encrypts it using the TEK, and sends the result:
struct {
opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[20];
opaque wrapped_client_write_key[12];
opaque wrapped_server_write_key[12];
opaque client_write_iv[24];
opaque server_write_iv[24];
opaque master_secret_iv[24];
block-ciphered opaque encrypted_pre_master_secret[48];
} FortezzaKeys;
7.6.7.3 Client Diffie-Hellman public valueThis structure conveys the client's Diffie-Hellman public value (Yc) if it was not already included in the client's certificate. The encoding used for Yc is determined by the enumerated PublicValueEncoding.
enum { implicit, explicit } PublicValueEncoding;
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: opaque dh_Yc<1..216-1>;
} dh_public;
} ClientDiffieHellmanPublic;
struct {
Signature signature;
} CertificateVerify;
CertificateVerify.signature.md5_hash
MD5(master_secret + pad2 + MD5(handshake_messages +
master_secret + pad1));
Certificate.signature.sha_hash
SHA(master_secret + pad2 + SHA(handshake_messages +
master_secret + pad1));
Here handshake_messages refers to all handshake messages starting at client hello up to but not including this message.
enum { client(0x434C4E54), server(0x53525652) } Sender;
struct {
opaque md5_hash[16];
opaque sha_hash[20];
} Finished;
md5_hash
MD5(master_secret + pad2 + MD5(handshake_messages +
Sender + master_secret + pad1));
The hash contained in finished messages sent by the server incorporate Sender.server; those sent by the client incorporate Sender.client. The value handshake_messages includes all handshake messages starting at client hello up to, but not including, the finished messages. This may be different from handshake_messages in Section 7.6.8 because it would include the certificate verify message (if sent). Note: Change cipher spec messages are not handshake messages and are not included in the hash computations.
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