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The SSL protocol provides connection security that has three basic properties:
This document is not intended to supply any details of service definition nor interface definition, although it does cover select areas of policy as they are required for the maintenance of solid security.
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ... | byte[n-1];
This byte ordering for multi-byte values is the commonplace network byte order or big endian format.
Optional components are denoted by enclosing them in italic "[ ]" brackets.
Single byte entities containing uninterpreted data are of type opaque.
The syntax for specifying a new type T' that is a fixed length vector of type T is
T T'[n];
Here T' occupies n bytes in the data stream, where n is a multiple of the size of T. The length of the vector is not included in the encoded stream.
In the following example, Datum is defined to be three consecutive bytes that the protocol does not interpret, while Data is three consecutive Datum, consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes of data */ Datum Data[9]; /* 3 consecutive 3 byte vectors */
Variable length vectors are defined by specifying a subrange of legal lengths, inclusively, using the notation <floor..ceiling>. When encoded, the actual length precedes the vector's contents in the byte stream. The length will be in the form of a number consuming as many bytes as required to hold the vector's specified maximum (ceiling) length. A variable length vector with an actual length field of zero is referred to as an empty vector.
T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain between 300 and 400 bytes of type opaque. It can never be empty. The actual length field consumes two bytes, a uint16, sufficient to represent the value 400 (see Section 6.4). On the other hand, longer can represent up to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a two byte actual length field prepended to the vector.
opaque mandatory<300..400>; /* length field is 2 bytes, cannot be empty */ uint16 longer<0..800>; /* zero to 400 16-bit unsigned integers */
uint8 uint16[2]; uint8 uint24[3]; uint8 uint32[4]; uint8 uint64[8];
enum { e1(v1), e2 (v1), ... , en (vN), [(n)] } Te;
Enumerateds occupy as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element. In the following example, Taste will consume two bytes in the data stream but can only assume the values 1, 2 or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would be Color.blue. Such qualification is not required if the target of the assignment is well specified.
Color color = Color.blue; /* overspecified, but legal */ Color color = blue; /* correct, type is implicit */
For enumerateds that are never converted to external representation, the numerical information may be omitted.
enum { low, medium, high } Amount;
struct { T1 f1; T2 f2; ... Tn fn; } [T];
The fields within a structure may be qualified using the type's name using a syntax much like that available for enumerateds. For example, T.f2 refers to the second field of the previous declaration. Structure definitions may be embedded.
struct { T1 f1; T2 f2; .... Tn fn; select (E) { case e1: Te1; case e2: Te2; .... case en: Ten; } [fv]; } [Tv];
For example
enum { apple, orange } VariantTag; struct { uint16 number; opaque string<0..10<; /* variable length */ } V1; struct { uint32 number; opaque string[10]; /* fixed length */ } V2; struct { select (VariantTag) { /* value of variant selector is implicit */ case apple: V1; /* definition of VariantBody, tag = apple */ case orange: V2; /* definition of VariantBody, tag = orange */ } variant_body; /* optional label on the variant portion */ } VariantRecord;
Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, a
orange VariantRecord
is a narrowed type of a VariantRecord containing a variant_body of type V2.
In digital signing, one-way hash functions are used as input for a signing algorithm. In RSA signing, a 36-byte structure of two hashes (one SHA and one MD5) is signed (encrypted with the private key). In DSS, the 20 bytes of the SHA hash are run directly through the Digital Signing Algorithm with no additional hashing.
In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically-secure keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. Because it is unlikely that the plaintext (whatever data is to be sent) will break neatly into the necessary block size (usually 64 bits), it is necessary to pad out the end of short blocks with some regular pattern, usually all zeroes.
In public key encryption, one-way functions with secret "trapdoors" are used to encrypt the outgoing data. Data encrypted with the public key of a given key pair can only be decrypted with the private key, and vice-versa.
In the following example:
stream-ciphered struct { uint8 field1; uint8 field2; digitally-signed opaque hash[20]; } UserType;
The contents of hash are used as input for a signing algorithm, then the entire structure is encrypted with a stream cipher.
For example,
struct { uint8 f1; uint8 f2; } Example1; Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
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:
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;
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.
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.
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;
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.
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).
struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret;
struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret;
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;
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));sha_hash SHA(master_secret + pad2 + SHA(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.
For Diffie-Hellman, RSA, and Fortezza, the same algorithm is used to convert the pre_master_secret into the master_secret. The pre_master_secret should be deleted from memory once the master_secret has been computed.
master_secret = MD5(pre_master_secret + SHA('A' + pre_master_secret + ClientHello.random + ServerHello.random)) + MD5(pre_master_secret + SHA('BB' + pre_master_secret + ClientHello.random + ServerHello.random)) + MD5(pre_master_secret + SHA('CCC' + pre_master_secret + ClientHello.random + ServerHello.random));
RSA digital signatures are performed using PKCS #1 [PKCS1] block type 1. RSA public key encryption is performed using PKCS #1 block type 2.
Note: Diffie-Hellman parameters are specified by the server, and may be either ephemeral or contained within the server's certificate.
CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, a server write key, a client write IV, and a server write IV, which are generated from the master secret in that order. Unused values, such as Fortezza keys communicated in the KeyExchange message, are empty. The following inputs are available to the key definition process:
opaque MasterSecret[48] ClientHello.random ServerHello.random
When generating keys and MAC secrets, the master secret is used as an entropy source, and the random values provide unencrypted salt material and IVs for exportable ciphers.
To generate the key material, compute
key_block = MD5(master_secret + SHA('A' + master_secret + ServerHello.random + ClientHello.random)) + MD5(master_secret + SHA('BB' + master_secret + ServerHello.random + ClientHello.random)) + MD5(master_secret + SHA('CCC' + master_secret + ServerHello.random + ClientHello.random)) + [...];
until enough output has been generated. Then the key_block is partitioned as follows.
client_write_MAC_secret[CipherSpec.hash_size] server_write_MAC_secret[CipherSpec.hash_size] client_write_key[CipherSpec.key_material] server_write_key[CipherSPec.key_material] client_write_IV[CipherSpec.IV_size] /* non-export ciphers */ server_write_IV[CipherSpec.IV_size] /* non-export ciphers */
Any extra key_block material is discarded.
Exportable encryption algorithms (for which CipherSpec.is_exportable is true) require additional processing as follows to derive their final write keys:
final_client_write_key = MD5(client_write_key + ClientHello.random + ServerHello.random); final_server_write_key = MD5(server_write_key + ServerHello.random + ClientHello.random);
Exportable encryption algorithms derive their IVs from the random messages:
client_write_IV = MD5(ClientHello.random + ServerHello.random); server_write_IV = MD5(ServerHello.random + ClientHello.random);
MD5 outputs are trimmed to the appropriate size by discarding the least-significant bytes.
client_write_MAC_secret = key_block0..15 server_write_MAC_secret = key_block16..31 client_write_key = key_block32..36 server_write_key = key_block37..41 final_client_write_key = MD5 (client_write_key + ClientHello.random + ServerHello.random) 0..15; final_server_write_key = MD5 (server_write_key + ServerHello.random + ClientHello.random) 0..15; client_write_IV = MD5(ClientHello.random + ServerHello.random) 0..7; server_write_IV = MD5(ServerHello.random + ClientHello.random) 0..7;