Appendices

443 - Reserved for use by Hypertext Transfer Protocol with SSL (https).

465 - Reserved (pending) for use by Simple Mail Transfer Protocol with SSL (ssmtp).

563 - Reserved (pending) for use by Network News Transfer Protocol (snntp).

A.1.1 Record layer

struct {
        uint8 major, minor;
} ProtocolVersion;

ProtocolVersion version = { 3,0 };      /* Define SSL version 3.0 */

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;

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

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;

block-ciphered struct {
        opaque content[SSLCompressed.length];
        opaque MAC[CipherSpec.hash_size];
        uint8 padding[GenericBlockCipher.padding_length];
        uint8 padding_length;
} GenericBlockCipher;

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

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;

A.4 Handshake protocol

enum {
        hello_request(0), client_hello(1), server_hello(2),
        certificate(11), server_key_exchange (12), certificate_request(13),
        server_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 msg 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_done: ServerHelloDone;
            case certificate_verify: CertificateVerify;
            case client_key_exchange: ClientKeyExchange;
            case finished: Finished;
        } body;
} Handshake;

A.4.1 Hello messages

struct { } HelloRequest;

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

opaque SessionID<0..32>;
uint8 CipherSuite[2];
enum { null(0), (255) } CompressionMethod;

struct {
        ProtocolVersion client_version;
        Random random;
        SessionID session_id;
        CipherSuite cipher_suites<0..216-1>;
        CompressionMethod compression_methods<0..28-1>;
} ClientHello;

struct {
        ProtocolVersion server_version;
        Random random;
        SessionID session_id;
        CipherSuite cipher_suite;
        CompressionMethod compression_method;
} ServerHello;

A.4.2 Server authentication and key exchange messages

opaque ASN.1Cert<224-1>;

struct {
        ASN.1Cert certificate_list<1..224-1>;
} Certificate;

enum { rsa, diffie_hellman, fortezza_dms } KeyExchangeAlgorithm;

struct {
        opaque RSA_modulus<1..216-1>;
        opaque RSA_exponent<1..216-1>;
} ServerRSAParams;

struct {
        opaque DH_p<1..216-1>;
        opaque DH_g<1..216-1>;
        opaque DH_Ys<1..216-1>;
} ServerDHParams;

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;

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)
} CertificateType;

opaque DistinguishedName<3..216-1>;

struct {
        CertificateType certificate_types<1..28-1>;
        DistinguishedName certificate_authorities<3..216-1>;
} CertificateRequest;
struct { } ServerHelloDone;

A.5 Client authentication and key exchange messages

struct {
        select (KeyExchangeAlgorithm) {
            case rsa: EncryptedPreMasterSecret;
            case diffie_hellman: DiffieHellmanClientPublicValue;
            case fortezza_dms: FortezzaKeys;
        } exchange_keys;
} ClientKeyExchange;

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];
        opaque encrypted_preMasterSecret[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;

A.5.1 Handshake finalization message

struct {
        opaque md5_hash[16];
        opaque sha_hash[20];
} Finished;

A.6 The CipherSuite

A CipherSuite defines a cipher specifications supported in SSL Version 3.0.

CipherSuite SSL_NULL_WITH_NULL_NULL                     = { 0x00,0x00 };

The following CipherSuite definitions require that the server provide an RSA certificate that can be used for key exchange. The server may request either an RSA or a DSS signature-capable certificate in the certificate request message.

CipherSuite SSL_RSA_WITH_NULL_MD5                       = { 0x00,0x01 };
CipherSuite SSL_RSA_WITH_NULL_SHA                       = { 0x00,0x02 };
CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5              = { 0x00,0x03 };
CipherSuite SSL_RSA_WITH_RC4_128_MD5                    = { 0x00,0x04 };
CipherSuite SSL_RSA_WITH_RC4_128_SHA                    = { 0x00,0x05 };
CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5          = { 0x00,0x06 };
CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA                   = { 0x00,0x07 };
CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA           = { 0x00,0x08 };
CipherSuite SSL_RSA_WITH_DES_CBC_SHA                    = { 0x00,0x09 };
CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA               = { 0x00,0x0A };

The following CipherSuite definitions are used for server-authenticated (and optionally client-authenticated) Diffie-Hellman. DH denotes cipher suites in which the server's certificate contains the Diffie-Hellman parameters signed by the certificate authority (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman parameters are signed by a DSS or RSA certificate, which has been signed by the CA. The signing algorithm used is specified after the DH or DHE parameter. In all cases, the client must have the same type of certificate, and must use the Diffie-Hellman parameters chosen by the server.

CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA        = { 0x00,0x0B };
CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA                 = { 0x00,0x0C };
CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA            = { 0x00,0x0D };
CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA        = { 0x00,0x0E };
CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA                 = { 0x00,0x0F };
CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA            = { 0x00,0x10 };
CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA       = { 0x00,0x11 };
CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA                = { 0x00,0x12 };
CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA           = { 0x00,0x13 };
CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA       = { 0x00,0x14 };
CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA                = { 0x00,0x15 };
CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA           = { 0x00,0x16 };

The following cipher suites are used for completely anonymous Diffie-Hellman communications in which neither party is authenticated. Note that this mode is vulnerable to man-in-the-middle attacks and is therefore strongly discouraged.

CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5          = { 0x00,0x17 };
CipherSuite SSL_DH_anon_WITH_RC4_128_MD5                = { 0x00,0x18 };
CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA       = { 0x00,0x19 };
CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA                = { 0x00,0x1A };
CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA           = { 0x00,0x1B };

The final cipher suite is for the Fortezza token.

CipherSuite SSL_FORTEZZA_DMS_WITH_NULL_SHA              = { 0X00,0X1C };
CipherSuite SSL_FORTEZZA_DMS_WITH_FORTEZZA_CBC_SHA      = { 0x00,0x1D };

Note: All cipher suites whose first byte is 0xFF are considered private and can be used for defining local/experimental algorithms. Interoperability of such types is a local matter.

Note: Additional cipher suites will be considered for implementation only with submission of notarized letters from two independent entities. Netscape Communications Corp. will act as an interim registration office, until a public standards body assumes control of SSL.

A.7 The CipherSpec

enum { stream, block } CipherType;
enum { true, false } IsExportable;
enum { null, rc4, rc2, des, 3des, des40, fortezza } BulkCipherAlgorithm;
enum { null, md5, sha } MACAlgorithm;

struct {
        BulkCipherAlgorithm bulk_cipher_algorithm;
        MACAlgorithm mac_algorithm;
        CipherType cipher_type;
        IsExportable is_exportable
        uint8 hash_size;
        uint8 key_material;
        uint8 IV_size;
} CipherSpec;

application protocol

An application protocol is a protocol that normally layers directly on top of the transport layer (e.g., TCP/IP). Examples include HTTP, TELNET, FTP, and SMTP.

asymmetric cipher

See public key cryptography.

authentication

Authentication is the ability of one entity to determine the identity of another entity.

block cipher

A block cipher is an algorithm that operates on plaintext in groups of bits, called blocks. 64 bits is a typical block size.

bulk cipher

A symmetric encryption algorithm used to encrypt large quantities of data.

Cipher Block Chaining Mode (CBC)

CBC is a mode in which every plaintext block encrypted with the block cipher is first exclusive-Ored with the previous ciphertext block (or, in the case of the first block, with the initialization vector).

certificate

As part of the X.509 protocol (a.k.a. ISO Authentication framework), certificates are assigned by a trusted Certificate Authority and provide verification of a party's identity and may also supply its public key.

client

The application entity that initiates a connection to a server.

client write key

The key used to encrypt data written by the client.

client write MAC secret

The secret data used to authenticate data written by the client.

connection

A connection is a transport (in the OSI layering model definition) that provides a suitable type of service. For SSL, such connections are peer to peer relationships. The connections are transient. Every connection is associated with one session.

Data Encryption Standard (DES)

DES is a very widely used symmetric encryption algorithm. DES is a block cipher.

Digital Signature Standard (DSS)

A standard for digital signing, including the Digital Signing Algorithm, approved by the National Institute of Standards and Technology, defined in NIST FIPS PUB 186, "Digital Signature Standard," published May, 1994 by the U.S. Dept. of Commerce.

digital signatures

Digital signatures utilize public key cryptography and one-way hash functions to produce a signature of the data that can be authenticated, and is difficult to forge or repudiate.

Fortezza

A PCMCIA card that provides both encryption and digital.

handshake

An initial negotiation between client and server that establishes the parameters of their transactions.

Initialization Vector (IV)

When a block cipher is used in CBC mode, the initialization vector is exclusive-ORed with the first plaintext block prior to encryption.

IDEA

A 64-bit block cipher designed by Xuejia Lai and James Massey.

Message Authentication Code (MAC)

A Message Authentication Code is a one-way hash computed from a message and some secret data. Its purpose is to detect if the message has been altered.

master secret

Secure secret data used for generating encryption keys, MAC secrets, and IVs.

MD5

MD5 [7] is a secure hashing function that converts an arbitrarily long data stream into a digest of fixed size.

public key cryptography

A class of cryptographic techniques employing two-key ciphers. Messages encrypted with the public key can only be decrypted with the associated private key. Conversely, messages signed with the private key can be verified with the public key.

one-way hash function

A one-way transformation that converts an arbitrary amount of data into a fixed-length hash. It is computationally hard to reverse the transformation or to find collisions. MD5 and SHA are examples of one-way hash functions.

RC2, RC4

Proprietary bulk ciphers from RSA Data Security, Inc. (There is no good reference to these as they are unpublished works; however, see [RSADSI]). RC2 is block cipher and RC4 is a stream cipher.

RSA

A very widely used public-key algorithm that can be used for either encryption or digital signing.

salt

Non-secret random data used to make export encryption keys resist precomputation attacks.

server

The server is the application entity that responds to requests for connections from clients. The server is passive, waiting for requests from clients.

session

A SSL session is an association between a client and a server. Sessions are created by the handshake protocol. Sessions define a set of cryptographic security parameters, which can be shared among multiple connections. Sessions are used to avoid the expensive negotiation of new security parameters for each connection.

session identifier

A session identifier is a value generated by a server that identifies a particular session.

server write key

The key used to encrypt data written by the server.

server write MAC secret

The secret data used to authenticate data written by the server.

SHA

The Secure Hash Algorithm is defined in FIPS PUB 180-1. It produces a 20-byte output [SHA].

stream cipher

An encryption algorithm that converts a key into a cryptographically-strong keystream, which is then exclusive-ORed with the plaintext..

symmetric cipher

See bulk cipher.

Key size limit

The key size limit gives the size of the largest public key that can be legally used for encryption in cipher suites that are exportable.

* Indicares IsExportable is true.
** Fortezza uses its own key and IV generation algorithms.

Key Material

The number of bytes from the key_block that are used for generating the write keys.

Expanded Key Material

The number of bytes actually fed into the encryption algorithm.

Effective Key Bits

How much entropy material is in the key material being fed into the encryption routines.

D.1 Temporary RSA keys

When the public key in the certificate cannot be used for encryption, the server signs a temporary RSA key, which is then exchanged. In exportable applications, the temporary RSA key should be the maximum allowable length (i.e., 512 bits). Because 512-bit RSA keys are relatively insecure, they should be changed often. For typical electronic commerce applications, it is suggested that keys be changed daily or every 500 transactions, and more often if possible. Note that while it is acceptable to use the same temporary key for multiple transactions, it must be signed each time it is used.

RSA key generation is a time-consuming process. In many cases, a low-priority process can be assigned the task of key generation. Whenever a new key is completed, the existing temporary key can be replaced with the new one.

D.2 Random Number Generation and Seeding

To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC-compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. To seed a 128-bit PRNG, one would thus require approximately 100 such timer values.

Note: The seeding functions in RSAREF and versions of BSAFE prior to 3.0 are order-independent. For example, if 1000 seed bits are supplied, one at a time, in 1000 separate calls to the seed function, the PRNG will end up in a state which depends only on the number of 0 or 1 seed bits in the seed data (i.e., there are 1001 possible final states). Applications using BSAFE or RSAREF must take extra care to ensure proper seeding.

D.3 Certificates and authentication

D.4 CipherSuites

Warning: The ability to send Version 2.0 client hello messages will be phased out with all due haste. Implementors should make every effort to move forward as quickly as possible. Version 3.0 provides better mechanisms for transitioning to newer versions.

The following cipher specifications are carryovers from SSL Version 2.0. These are assumed to use RSA for key exchange and authentication.

V2CipherSpec SSL_RC4_128_WITH_MD5                       = { 0x01,0x00,0x80 };
V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5              = { 0x02,0x00,0x80 };
V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5               = { 0x03,0x00,0x80 };
V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5      = { 0x04,0x00,0x80 };
V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5                  = { 0x05,0x00,0x80 };
V2CipherSpec SSL_DES_64_CBC_WITH_MD5                    = { 0x06,0x00,0x40 };
V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5              = { 0x07,0x00,0xC0 };

Cipher specifications introduced in Version 3.0 can be included in Version 2.0 client hello messages using the syntax below. Any V2CipherSpec element with its first byte equal to zero will be ignored by Version 2.0 servers. Clients sending any of the above V2CipherSpecs should also include the Version 3.0 equivalent (see Appendix A.6):

V2CipherSpec (see Version 3.0 name)                  = { 0x00, CipherSuite };

E.1 Version 2 client hello

uint8 V2CipherSpec[3];
struct {
    uint8        msg_type;
    Version      version;
    uint16       cipher_spec_length;
    uint16       session_id_length;
    uint16       challenge_length;
    V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
    opaque       session_id[V2ClientHello.session_id_length];
    Random       challenge;
} V2ClientHello;

msg_type

This field, in conjunction with the version field, identifies a version 2 client hello message. The value should equal one (1).

version

The highest version of the protocol supported by the client (equals ProtocolVersion.version, see Appendix A.1.1 ).

cipher_spec_length

This field is the total length of the field cipher_specs. It cannot be zero and must be a multiple of the V2CipherSpec length (3).

session_id_length

This field must have a value of either zero or 16. If zero, the client is creating a new session. If 16, the session_id field will contain the 16 bytes of session identification.

challenge_length

The length in bytes of the client's challenge to the server to authenticate itself. This value must be 32.

cipher_specs

This is a list of all CipherSpecs the client is willing and able to use. There must be at least one CipherSpec acceptable to the server.

session_id

If this field's length is not zero, it will contain the identification for a session that the client wishes to resume.

challenge

The client's challenge to the server for the server to identify itself is a (nearly) arbitrary length random. The Version 3.0 server will right justify the challenge data to become the ClientHello.random data (padded with leading zeroes, if necessary), as specified in this Version 3.0 protocol. If the length of the challenge is greater than 32 bytes, then only the last 32 bytes are used. It is legitimate (but not necessary) for a V3 server to reject a V2 ClientHello that has fewer than 16 bytes of challenge data.

Note: Requests to resume an SSL 3.0 session should use an SSL 3.0 client hello.

E.2 Avoiding man-in-the-middle version rollback

When SSL Version 3.0 clients fall back to Version 2.0 compatibility mode, they use special PKCS #1 block formatting. This is done so that Version 3.0 servers will reject Version 2.0 sessions with Version 3.0-capable clients.

When Version 3.0 clients are in Version 2.0 compatibility mode, they set the right-hand (least-significant) 8 random bytes of the PKCS padding (not including the terminal null of the padding) for the RSA encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding bytes are random). After decrypting the ENCRYPTED-KEY-DATA field, servers that support SSL 3.0 should issue an error if these eight padding bytes are 0x03. Version 2.0 servers receiving blocks padded in this manner will proceed normally.

F.1 Handshake protocol

F.1.1 Authentication and key exchange

If the server is authenticated, its certificate message must provide a valid certificate chain leading to an acceptable certificate authority. Similarly, authenticated clients must supply an acceptable certificate to the server. Each party is responsible for verifying that the other's certificate is valid and has not expired or been revoked.

The general goal of the key exchange process is to create a pre_master_secret known to the communicating parties and not to attackers. The pre_master_secret will be used to generate the master_secret (see Section 8.1). The master_secret is required to generate the finished messages, encryption keys, and MAC secrets (see Sections 7.6.9 and 8.2.2). By sending a correct finished message, parties thus prove that they know the correct pre_master_secret.

F.1.1.1 Anonymous key exchange

With anonymous RSA, the client encrypts a pre_master_secret with the server's uncertified public key extracted from the server key exchange message. The result is sent in a client key exchange message. Since eavesdroppers do not know the server's private key, it will be infeasible for them to decode the pre_master_secret.

With Diffie-Hellman or Fortezza, the server's public parameters are contained in the server key exchange message and the client's are sent in the client key exchange message. Eavesdroppers who do not know the private values should not be able to find the Diffie-Hellman result (i.e. the pre_master_secret) or the Fortezza token encryption key (TEK).

Warning: Completely anonymous connections only provide protection against passive eavesdropping. Unless an independent tamper-proof channel is used to verify that the finished messages were not replaced by an attacker, server authentication is required in environments where active man-in-the-middle attacks are a concern.

F.1.1.2 RSA key exchange and authentication

Note: The temporary RSA key option is useful if servers need large certificates but must comply with government-imposed size limits on keys used for key exchange.

After verifying the server's certificate, the client encrypts a pre_master_secret with the server's public key. By successfully decoding the pre_master_secret and producing a correct finished message, the server demonstrates that it knows the private key corresponding to the server certificate.

When RSA is used for key exchange, clients are authenticated using the certificate verify message (see Section 7.6.8). The client signs a value derived from the master_secret and all preceding handshake messages. These handshake messages include the server certificate, which binds the signature to the server, and ServerHello.random, which binds the signature to the current handshake process.

F.1.1.3 Diffie-Hellman key exchange with authentication

If the client has a certificate containing fixed Diffie-Hellman parameters, its certificate contains the information required to complete the key exchange. Note that in this case the client and server will generate the same Diffie-Hellman result (i.e., pre_master_secret)every time they communicate. To prevent the pre_master_secret from staying in memory any longer than necessary, it should be converted into the master_secret as soon as possible. Client Diffie-Hellman parameters must be compatible with those supplied by the server for the key exchange to work.

If the client has a standard DSS or RSA certificate or is unauthenticated, it sends a set of temporary parameters to the server in the client key exchange message, then optionally uses a certificate verify message to authenticate itself.

F.1.1.4 Fortezza

F.1.2 Version rollback attacks

Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for Version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute force the key and substitute a new ENCRYPTED-KEY-DATA message containing the same key (but with normal padding) before the application specified wait threshold has expired. Parties concerned about attacks of this scale should not be using 40-bit encryption keys anyway. Altering the padding of the least-significant 8 bytes of the PKCS padding does not impact security, since this is essentially equivalent to increasing the input block size by 8 bytes.

F.1.3 Detecting attacks against the handshake protocol

For this attack, an attacker must actively change one or more handshake messages. If this occurs, the client and server will compute different values for the handshake message hashes. As a result, the parties will not accept each others' finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered.

F.1.4 Resuming sessions

Sessions cannot be resumed unless both the client and server agree. If either party suspects that the session may have been compromised, or that certificates may have expired or been revoked, it should force a full handshake. An upper limit of 24 hours is suggested for session ID lifetimes, since an attacker who obtains a master_secret may be able to impersonate the compromised party until the corresponding session ID is retired. Applications that may be run in relatively insecure environments should not write session IDs to stable storage.

F.1.5 MD5 and SHA

Outgoing data is protected with a MAC before transmission. To prevent message replay or modification attacks, the MAC is computed from the MAC secret, the sequence number, the message length, the message contents, and two fixed character strings. The message type field is necessary to ensure that messages intended for one SSL Record Layer client are not redirected to another. The sequence number ensures that attempts to delete or reorder messages will be detected. Since sequence numbers are 64-bits long, they should never overflow. Messages from one party cannot be inserted into the other's output, since they use independent MAC secrets. Similarly, the server-write and client-write keys are independent so stream cipher keys are used only once.

If an attacker does break an encryption key, all messages encrypted with it can be read. Similarly, compromise of a MAC key can make message modification attacks possible. Because MACs are also encrypted, message-alteration attacks generally require breaking the encryption algorithm as well as the MAC.

Note: MAC secrets may be larger than encryption keys, so messages can remain tamper resistant even if encryption keys are broken.

F.3 Final notes

The system is only as strong as the weakest key exchange and authentication algorithm supported, and only trustworthy cryptographic functions should be used. Short public keys, 40-bit bulk encryption keys, and anonymous servers should be used with great caution. Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage.

The Massachusetts Institute of Technology has granted RSA Data Security, Inc., exclusive sub-licensing rights to the following patent issued in the United States:

The Board of Trustees of the Leland Stanford Junior University have granted Caro-Kann Corporation, a wholly owned subsidiary corporation, exclusive sub-licensing rights to the following patents issued in the United States, and all of their corresponding foreign patents:

The Internet Society, Internet Architecture Board, Internet Engineering Steering Group and the Corporation for National Research Initiatives take no position on the validity or scope of the patents and patent applications, nor on the appropriateness of the terms of the assurance. The Internet Society and other groups mentioned above have not made any determination as to any other intellectual property rights which may apply to the practice of this standard. Any further consideration of these matters is the user's own responsibility.

Netscape Communications Corporation
SSL Version 3.0
March 1996