RFC 2246
The TLS Protocol Version 1.0
A. Protocol constant values This section describes protocol types and constants. A.1. Record layer struct { uint8 major, minor; } ProtocolVersion; ProtocolVersion version = { 3, 1 }; /* TLS v1.0 */ enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; struct { ContentType type; ProtocolVersion version; uint16 length; select (CipherSpec.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; } fragment; } TLSCiphertext; stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[CipherSpec.hash_size]; } GenericStreamCipher; block-ciphered struct { opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
A.2. Change cipher specs message
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
A.3. Alert messages
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
(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_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20), (255) } HandshakeType; struct { HandshakeType msg_type; uint24 length; 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; 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<2..2^16-1>; CompressionMethod compression_methods<1..2^8-1>;
} 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<2^24-1>;
struct {
ASN.1Cert certificate_list<1..2^24-1>;
} Certificate;
enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
struct {
opaque RSA_modulus<1..2^16-1>;
opaque RSA_exponent<1..2^16-1>;
} ServerRSAParams;
struct {
opaque DH_p<1..2^16-1>;
opaque DH_g<1..2^16-1>;
opaque DH_Ys<1..2^16-1>;
} ServerDHParams;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
};
} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;
select (SignatureAlgorithm)
{ case anonymous: struct { };
case rsa:
digitally-signed struct {
opaque md5_hash[16];
opaque sha_hash[20];
};
case dsa:
digitally-signed struct {
opaque sha_hash[20];
};
} Signature;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
A.4.3. Client authentication and key exchange messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: DiffieHellmanClientPublicValue;
} exchange_keys;
} ClientKeyExchange;
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
enum { implicit, explicit } PublicValueEncoding;
struct {
select (PublicValueEncoding) {
case implicit: struct {};
case explicit: opaque DH_Yc<1..2^16-1>;
} dh_public;
} ClientDiffieHellmanPublic;
struct {
Signature signature;
} CertificateVerify;
A.4.4. Handshake finalization message
struct {
opaque verify_data[12];
} Finished;
A.5. The CipherSuite
The following values define the CipherSuite codes used in the client
hello and server hello messages.
A CipherSuite defines a cipher specification supported in TLS Version
1.0.
TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
TLS connection during the first handshake on that channel, but must
not be negotiated, as it provides no more protection than an
unsecured connection.
CipherSuite TLS_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 TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
CipherSuite TLS_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. The server can request an RSA or DSS signature-
capable certificate from the client for client authentication or it
may request a Diffie-Hellman certificate. Any Diffie-Hellman
certificate provided by the client must use the parameters (group and
generator) described by the server.
CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C };
CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
CipherSuite TLS_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 deprecated.
CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
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 can be registered by publishing an RFC
which specifies the cipher suites, including the necessary TLS
protocol information, including message encoding, premaster
secret derivation, symmetric encryption and MAC calculation and
appropriate reference information for the algorithms involved.
The RFC editor's office may, at its discretion, choose to publish
specifications for cipher suites which are not completely
described (e.g., for classified algorithms) if it finds the
specification to be of technical interest and completely
specified.
Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in
SSL 3.
A.6. The Security Parameters
These security parameters are determined by the TLS Handshake
Protocol and provided as parameters to the TLS Record Layer in order
to initialize a connection state. SecurityParameters includes:
enum { null(0), (255) } CompressionMethod;
enum { server, client } ConnectionEnd;
enum { null, rc4, rc2, des, 3des, des40, idea }
BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { true, false } IsExportable;
enum { null, md5, sha } MACAlgorithm;
/* The algorithms specified in CompressionMethod,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 key_size;
uint8 key_material_length;
IsExportable is_exportable;
MACAlgorithm mac_algorithm;
uint8 hash_size;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
B. Glossary 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 common block size. bulk cipher A symmetric encryption algorithm used to encrypt large quantities of data. cipher block chaining (CBC) CBC is a mode in which every plaintext block encrypted with a block cipher is first exclusive-ORed with the previous ciphertext block (or, in the case of the first block, with the initialization vector). For decryption, every block is first decrypted, then exclusive-ORed with the previous ciphertext block (or IV). certificate As part of the X.509 protocol (a.k.a. ISO Authentication framework), certificates are assigned by a trusted Certificate Authority and provide a strong binding between a party's identity or some other attributes and its public key. client The application entity that initiates a TLS connection to a server. This may or may not imply that the client initiated the underlying transport connection. The primary operational difference between the server and client is that the server is generally authenticated, while the client is only optionally authenticated. 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 TLS,
such connections are peer to peer relationships. The connections
are transient. Every connection is associated with one session.
Data Encryption Standard
DES is a very widely used symmetric encryption algorithm. DES is
a block cipher with a 56 bit key and an 8 byte block size. Note
that in TLS, for key generation purposes, DES is treated as
having an 8 byte key length (64 bits), but it still only provides
56 bits of protection. (The low bit of each key byte is presumed
to be set to produce odd parity in that key byte.) DES can also
be operated in a mode where three independent keys and three
encryptions are used for each block of data; this uses 168 bits
of key (24 bytes in the TLS key generation method) and provides
the equivalent of 112 bits of security. [DES], [3DES]
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.
[DSS]
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.
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.
[IDEA]
Message Authentication Code (MAC)
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the 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 is a secure hashing function that converts an arbitrarily
long data stream into a digest of fixed size (16 bytes). [MD5]
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
A block cipher developed by Ron Rivest at RSA Data Security, Inc.
[RSADSI] described in [RC2].
RC4
A stream cipher licensed by RSA Data Security [RSADSI]. A
compatible cipher is described in [RC4].
RSA
A very widely used public-key algorithm that can be used for
either encryption or digital signing. [RSA]
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. See also under client.
session
A TLS 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. Note that all references to SHA
actually use the modified SHA-1 algorithm. [SHA]
SSL
Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on
SSL Version 3.0
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.
Transport Layer Security (TLS)
This protocol; also, the Transport Layer Security working group
of the Internet Engineering Task Force (IETF). See "Comments" at
the end of this document.
C. CipherSuite definitions CipherSuite Is Key Cipher Hash Exportable Exchange TLS_NULL_WITH_NULL_NULL * NULL NULL NULL TLS_RSA_WITH_NULL_MD5 * RSA NULL MD5 TLS_RSA_WITH_NULL_SHA * RSA NULL SHA TLS_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 MD5 TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5 TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_CBC SHA TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_CBC SHA TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_CBC SHA TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC SHA TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC SHA TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5 TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA * Indicates IsExportable is True Key Exchange Algorithm Description Key size limit DHE_DSS Ephemeral DH with DSS signatures None DHE_DSS_EXPORT Ephemeral DH with DSS signatures DH = 512 bits DHE_RSA Ephemeral DH with RSA signatures None DHE_RSA_EXPORT Ephemeral DH with RSA signatures DH = 512 bits, RSA = none DH_anon Anonymous DH, no signatures None DH_anon_EXPORT Anonymous DH, no signatures DH = 512 bits
DH_DSS DH with DSS-based certificates None
DH_DSS_EXPORT DH with DSS-based certificates DH = 512 bits
DH_RSA DH with RSA-based certificates None
DH_RSA_EXPORT DH with RSA-based certificates DH = 512 bits,
RSA = none
NULL No key exchange N/A
RSA RSA key exchange None
RSA_EXPORT RSA key exchange RSA = 512 bits
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.
Key Expanded Effective IV Block
Cipher Type Material Key Material Key Bits Size Size
NULL * Stream 0 0 0 0 N/A
IDEA_CBC Block 16 16 128 8 8
RC2_CBC_40 * Block 5 16 40 8 8
RC4_40 * Stream 5 16 40 0 N/A
RC4_128 Stream 16 16 128 0 N/A
DES40_CBC * Block 5 8 40 8 8
DES_CBC Block 8 8 56 8 8
3DES_EDE_CBC Block 24 24 168 8 8
* Indicates IsExportable is true.
Type
Indicates whether this is a stream cipher or a block cipher
running in CBC mode.
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.
IV Size
How much data needs to be generated for the initialization
vector. Zero for stream ciphers; equal to the block size for
block ciphers.
Block Size
The amount of data a block cipher enciphers in one chunk; a
block cipher running in CBC mode can only encrypt an even
multiple of its block size.
Hash Hash Padding
function Size Size
NULL 0 0
MD5 16 48
SHA 20 40
D. Implementation Notes The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors. D.1. Temporary RSA keys US Export restrictions limit RSA keys used for encryption to 512 bits, but do not place any limit on lengths of RSA keys used for signing operations. Certificates often need to be larger than 512 bits, since 512-bit RSA keys are not secure enough for high-value transactions or for applications requiring long-term security. Some certificates are also designated signing-only, in which case they cannot be used for key exchange. 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 TLS requires a cryptographically-secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably MD5 and/or SHA, are acceptable, but cannot provide more security than the size of the random number generator state. (For example, MD5-based PRNGs usually provide 128 bits of state.) 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.
Warning: 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.
This may be accomplished by accumulating seed bits into a
buffer and processing them all at once or by processing an
incrementing counter with every seed bit; either method will
reintroduce order dependence into the seeding process.
D.3. Certificates and authentication
Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation
messages. Certificates should always be verified to ensure proper
signing by a trusted Certificate Authority (CA). The selection and
addition of trusted CAs should be done very carefully. Users should
be able to view information about the certificate and root CA.
D.4. CipherSuites
TLS supports a range of key sizes and security levels, including some
which provide no or minimal security. A proper implementation will
probably not support many cipher suites. For example, 40-bit
encryption is easily broken, so implementations requiring strong
security should not allow 40-bit keys. Similarly, anonymous Diffie-
Hellman is strongly discouraged because it cannot prevent man-in-
the-middle attacks. Applications should also enforce minimum and
maximum key sizes. For example, certificate chains containing 512-bit
RSA keys or signatures are not appropriate for high-security
applications.
E. Backward Compatibility With SSL For historical reasons and in order to avoid a profligate consumption of reserved port numbers, application protocols which are secured by TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same connection port: for example, the https protocol (HTTP secured by SSL or TLS) uses port 443 regardless of which security protocol it is using. Thus, some mechanism must be determined to distinguish and negotiate among the various protocols. TLS version 1.0 and SSL 3.0 are very similar; thus, supporting both is easy. TLS clients who wish to negotiate with SSL 3.0 servers should send client hello messages using the SSL 3.0 record format and client hello structure, sending {3, 1} for the version field to note that they support TLS 1.0. If the server supports only SSL 3.0, it will respond with an SSL 3.0 server hello; if it supports TLS, with a TLS server hello. The negotiation then proceeds as appropriate for the negotiated protocol. Similarly, a TLS server which wishes to interoperate with SSL 3.0 clients should accept SSL 3.0 client hello messages and respond with an SSL 3.0 server hello if an SSL 3.0 client hello is received which has a version field of {3, 0}, denoting that this client does not support TLS. Whenever a client already knows the highest protocol known to a server (for example, when resuming a session), it should initiate the connection in that native protocol. TLS 1.0 clients that support SSL Version 2.0 servers must send SSL Version 2.0 client hello messages [SSL2]. TLS servers should accept either client hello format if they wish to support SSL 2.0 clients on the same connection port. The only deviations from the Version 2.0 specification are the ability to specify a version with a value of three and the support for more ciphering types in the CipherSpec. 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 moving 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 TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 }; V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 }; V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 };
V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
= { 0x04,0x00,0x80 };
V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 };
V2CipherSpec TLS_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 };
V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
Cipher specifications native to TLS 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 TLS equivalent (see Appendix A.5):
V2CipherSpec (see TLS name) = { 0x00, CipherSuite };
E.1. Version 2 client hello
The Version 2.0 client hello message is presented below using this
document's presentation model. The true definition is still assumed
to be the SSL Version 2.0 specification.
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 be one (1).
version
The highest version of the protocol supported by the client
(equals ProtocolVersion.version, see Appendix A.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 challenge to the server for the server to identify
itself is a (nearly) arbitrary length random. The TLS server will
right justify the challenge data to become the ClientHello.random
data (padded with leading zeroes, if necessary), as specified in
this protocol specification. If the length of the challenge is
greater than 32 bytes, 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 a TLS session should use a TLS client hello.
E.2. Avoiding man-in-the-middle version rollback
When TLS clients fall back to Version 2.0 compatibility mode, they
should use special PKCS #1 block formatting. This is done so that TLS
servers will reject Version 2.0 sessions with TLS-capable clients.
When TLS 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 TLS 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. Security analysis The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel. This document makes several traditional assumptions, including that attackers have substantial computational resources and cannot obtain secret information from sources outside the protocol. Attackers are assumed to have the ability to capture, modify, delete, replay, and otherwise tamper with messages sent over the communication channel. This appendix outlines how TLS has been designed to resist a variety of attacks. F.1. Handshake protocol The handshake protocol is responsible for selecting a CipherSpec and generating a Master Secret, which together comprise the primary cryptographic parameters associated with a secure session. The handshake protocol can also optionally authenticate parties who have certificates signed by a trusted certificate authority. F.1.1. Authentication and key exchange TLS supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel is secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients. 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 certificate verify and finished messages, encryption keys, and MAC secrets (see Sections 7.4.8, 7.4.9 and 6.3). By sending a correct finished message, parties thus prove that they know the correct pre_master_secret. F.1.1.1. Anonymous key exchange Completely anonymous sessions can be established using RSA or Diffie-Hellman for 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. (Note that no anonymous RSA Cipher Suites are
defined in this document).
With Diffie-Hellman, 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).
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
With RSA, key exchange and server authentication are combined. The
public key may be either contained in the server's certificate or may
be a temporary RSA key sent in a server key exchange message. When
temporary RSA keys are used, they are signed by the server's RSA or
DSS certificate. The signature includes the current
ClientHello.random, so old signatures and temporary keys cannot be
replayed. Servers may use a single temporary RSA key for multiple
negotiation sessions.
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 7.4.8). 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 When Diffie-Hellman key exchange is used, the server can either supply a certificate containing fixed Diffie-Hellman parameters or can use the server key exchange message to send a set of temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary parameters are hashed with the hello.random values before signing to ensure that attackers do not replay old parameters. In either case, the client can verify the certificate or signature to ensure that the parameters belong to the server. 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.1.2. Version rollback attacks Because TLS includes substantial improvements over SSL Version 2.0, attackers may try to make TLS-capable clients and servers fall back to Version 2.0. This attack can occur if (and only if) two TLS- capable parties use an SSL 2.0 handshake. 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 for the size of the signed hashes and RSA key lengths used in the protocol, since this is essentially equivalent to increasing the input block size by 8 bytes.
F.1.3. Detecting attacks against the handshake protocol An attacker might try to influence the handshake exchange to make the parties select different encryption algorithms than they would normally choose. Because many implementations will support 40-bit exportable encryption and some may even support null encryption or MAC algorithms, this attack is of particular concern. 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 When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed with the session's master_secret. Provided that the master_secret has not been compromised and that the secure hash operations used to produce the encryption keys and MAC secrets are secure, the connection should be secure and effectively independent from previous connections. Attackers cannot use known encryption keys or MAC secrets to compromise the master_secret without breaking the secure hash operations (which use both SHA and MD5). 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 TLS uses hash functions very conservatively. Where possible, both MD5 and SHA are used in tandem to ensure that non-catastrophic flaws in one algorithm will not break the overall protocol. F.2. Protecting application data The master_secret is hashed with the ClientHello.random and ServerHello.random to produce unique data encryption keys and MAC secrets for each connection.
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 TLS 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
For TLS to be able to provide a secure connection, both the client
and server systems, keys, and applications must be secure. In
addition, the implementation must be free of security errors.
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.
G. Patent Statement Some of the cryptographic algorithms proposed for use in this protocol have patent claims on them. In addition Netscape Communications Corporation has a patent claim on the Secure Sockets Layer (SSL) work that this standard is based on. The Internet Standards Process as defined in RFC 2026 requests that a statement be obtained from a Patent holder indicating that a license will be made available to applicants under reasonable terms and conditions. The Massachusetts Institute of Technology has granted RSA Data Security, Inc., exclusive sub-licensing rights to the following patent issued in the United States: Cryptographic Communications System and Method ("RSA"), No. 4,405,829 Netscape Communications Corporation has been issued the following patent in the United States: Secure Socket Layer Application Program Apparatus And Method ("SSL"), No. 5,657,390 Netscape Communications has issued the following statement: Intellectual Property Rights Secure Sockets Layer The United States Patent and Trademark Office ("the PTO") recently issued U.S. Patent No. 5,657,390 ("the SSL Patent") to Netscape for inventions described as Secure Sockets Layers ("SSL"). The IETF is currently considering adopting SSL as a transport protocol with security features. Netscape encourages the royalty-free adoption and use of the SSL protocol upon the following terms and conditions: * If you already have a valid SSL Ref license today which includes source code from Netscape, an additional patent license under the SSL patent is not required. * If you don't have an SSL Ref license, you may have a royalty free license to build implementations covered by the SSL Patent Claims or the IETF TLS specification provided that you do not to assert any patent rights against Netscape or other companies for the implementation of SSL or the IETF TLS recommendation.
What are "Patent Claims":
Patent claims are claims in an issued foreign or domestic patent
that:
1) must be infringed in order to implement methods or build
products according to the IETF TLS specification; or
2) patent claims which require the elements of the SSL patent
claims and/or their equivalents to be infringed.
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.
Security Considerations
Security issues are discussed throughout this memo.
References
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
[BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against
Protocols Based on RSA Encryption Standard PKCS #1" in
Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages:
1--12, 1998.
[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards
Institute, 1983.
[DH1] W. Diffie and M. E. Hellman, "New Directions in
Cryptography," IEEE Transactions on Information Theory, V.
IT-22, n. 6, Jun 1977, pp. 74-84.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard," National
Institute of Standards and Technology, U.S. Department of
Commerce, May 18, 1994.
[FTP] Postel J., and J. Reynolds, "File Transfer Protocol", STD 9,
RFC 959, October 1985.
[HTTP] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication," RFC 2104, February 1997. [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH Series in Information Processing, v. 1, Konstanz: Hartung- Gorre Verlag, 1992. [MD2] Kaliski, B., "The MD2 Message Digest Algorithm", RFC 1319, April 1992. [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April 1992. [PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard," version 1.5, November 1993. [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax Standard," version 1.5, November 1993. [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax Standard," version 1.5, November 1993. [PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet Public Key Infrastructure: Part I: X.509 Certificate and CRL Profile", RFC 2459, January 1999. [RC2] Rivest, R., "A Description of the RC2(r) Encryption Algorithm", RFC 2268, January 1998. [RC4] Thayer, R. and K. Kaukonen, A Stream Cipher Encryption Algorithm, Work in Progress. [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems," Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120- 126. [RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782 [SCH] B. Schneier. Applied Cryptography: Protocols, Algorithms, and Source Code in C, Published by John Wiley & Sons, Inc. 1994.
[SHA] NIST FIPS PUB 180-1, "Secure Hash Standard," National Institute of Standards and Technology, U.S. Department of Commerce, Work in Progress, May 31, 1994. [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications Corp., Feb 9, 1995. [SSL3] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol", Netscape Communications Corp., Nov 18, 1996. [TCP] Postel, J., "Transmission Control Protocol," STD 7, RFC 793, September 1981. [TEL] Postel J., and J. Reynolds, "Telnet Protocol Specifications", STD 8, RFC 854, May 1993. [TEL] Postel J., and J. Reynolds, "Telnet Option Specifications", STD 8, RFC 855, May 1993. [X509] CCITT. Recommendation X.509: "The Directory - Authentication Framework". 1988. [XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data Representation Standard, August 1995. Credits Win Treese Open Market EMail: treese@openmarket.com Editors Christopher Allen Tim Dierks Certicom Certicom EMail: callen@certicom.com EMail: tdierks@certicom.com Authors' Addresses Tim Dierks Philip L. Karlton Certicom Netscape Communications EMail: tdierks@certicom.com
Alan O. Freier Paul C. Kocher Netscape Communications Independent Consultant EMail: freier@netscape.com EMail: pck@netcom.com Other contributors Martin Abadi Robert Relyea Digital Equipment Corporation Netscape Communications EMail: ma@pa.dec.com EMail: relyea@netscape.com Ran Canetti Jim Roskind IBM Watson Research Center Netscape Communications EMail: canetti@watson.ibm.com EMail: jar@netscape.com Taher Elgamal Micheal J. Sabin, Ph. D. Securify Consulting Engineer EMail: elgamal@securify.com EMail: msabin@netcom.com Anil R. Gangolli Dan Simon Structured Arts Computing Corp. Microsoft EMail: gangolli@structuredarts.com EMail: dansimon@microsoft.com Kipp E.B. Hickman Tom Weinstein Netscape Communications Netscape Communications EMail: kipp@netscape.com EMail: tomw@netscape.com Hugo Krawczyk IBM Watson Research Center EMail: hugo@watson.ibm.com Comments The discussion list for the IETF TLS working group is located at the e-mail address <ietf-tls@lists.consensus.com>. Information on the group and information on how to subscribe to the list is at <http://lists.consensus.com/>.
Archives of the list can be found at:
<http://www.imc.org/ietf-tls/mail-archive/>
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