6. The TLS Record Protocol The TLS Record Protocol is a layered protocol. At each layer, messages may include fields for length, description, and content. The Record Protocol takes messages to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, and transmits the result. Received data is decrypted, verified, decompressed, reassembled, and then delivered to higher-level clients. Four protocols that use the record protocol are described in this document: the handshake protocol, the alert protocol, the change cipher spec protocol, and the application data protocol. In order to allow extension of the TLS protocol, additional record content types can be supported by the record protocol. New record content type values are assigned by IANA in the TLS Content Type Registry as described in Section 12.
Implementations MUST NOT send record types not defined in this document unless negotiated by some extension. If a TLS implementation receives an unexpected record type, it MUST send an unexpected_message alert. Any protocol designed for use over TLS must be carefully designed to deal with all possible attacks against it. As a practical matter, this means that the protocol designer must be aware of what security properties TLS does and does not provide and cannot safely rely on the latter. Note in particular that type and length of a record are not protected by encryption. If this information is itself sensitive, application designers may wish to take steps (padding, cover traffic) to minimize information leakage. 6.1. Connection States A TLS connection state is the operating environment of the TLS Record Protocol. It specifies a compression algorithm, an encryption algorithm, and a MAC algorithm. In addition, the parameters for these algorithms are known: the MAC key and the bulk encryption keys for the connection in both the read and the write directions. Logically, there are always four connection states outstanding: the current read and write states, and the pending read and write states. All records are processed under the current read and write states. The security parameters for the pending states can be set by the TLS Handshake Protocol, and the ChangeCipherSpec can selectively make either of the pending states current, in which case the appropriate current state is disposed of and replaced with the pending state; the pending state is then reinitialized to an empty state. It is illegal to make a state that has not been initialized with security parameters a current state. The initial current state always specifies that no encryption, compression, or MAC will be used. The security parameters for a TLS Connection read and write state are set by providing the following values: connection end Whether this entity is considered the "client" or the "server" in this connection. PRF algorithm An algorithm used to generate keys from the master secret (see Sections 5 and 6.3).
bulk encryption algorithm
An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, whether it is a block,
stream, or AEAD cipher, the block size of the cipher (if
appropriate), and the lengths of explicit and implicit
initialization vectors (or nonces).
MAC algorithm
An algorithm to be used for message authentication. This
specification includes the size of the value returned by the MAC
algorithm.
compression algorithm
An algorithm to be used for data compression. This specification
must include all information the algorithm requires to do
compression.
master secret
A 48-byte secret shared between the two peers in the connection.
client random
A 32-byte value provided by the client.
server random
A 32-byte value provided by the server.
These parameters are defined in the presentation language as:
enum { server, client } ConnectionEnd;
enum { tls_prf_sha256 } PRFAlgorithm;
enum { null, rc4, 3des, aes }
BulkCipherAlgorithm;
enum { stream, block, aead } CipherType;
enum { null, hmac_md5, hmac_sha1, hmac_sha256,
hmac_sha384, hmac_sha512} MACAlgorithm;
enum { null(0), (255) } CompressionMethod;
/* The algorithms specified in CompressionMethod, PRFAlgorithm,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
MACAlgorithm mac_algorithm;
uint8 mac_length;
uint8 mac_key_length;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
The record layer will use the security parameters to generate the
following six items (some of which are not required by all ciphers,
and are thus empty):
client write MAC key
server write MAC key
client write encryption key
server write encryption key
client write IV
server write IV
The client write parameters are used by the server when receiving and
processing records and vice versa. The algorithm used for generating
these items from the security parameters is described in Section 6.3.
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
elements:
compression state
The current state of the compression algorithm.
cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. For stream ciphers,
this will also contain whatever state information is necessary to
allow the stream to continue to encrypt or decrypt data.
MAC key
The MAC key for this connection, as generated above.
sequence number
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may not
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it must
renegotiate instead. A sequence number is incremented after each
record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.
6.2. Record Layer
The TLS record layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
6.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Client
message boundaries are not preserved in the record layer (i.e.,
multiple client messages of the same ContentType MAY be coalesced
into a single TLSPlaintext record, or a single message MAY be
fragmented across several records).
struct {
uint8 major;
uint8 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[TLSPlaintext.length];
} TLSPlaintext;
type
The higher-level protocol used to process the enclosed fragment.
version
The version of the protocol being employed. This document
describes TLS Version 1.2, which uses the version { 3, 3 }. The
version value 3.3 is historical, deriving from the use of {3, 1}
for TLS 1.0. (See Appendix A.1.) Note that a client that
supports multiple versions of TLS may not know what version will
be employed before it receives the ServerHello. See Appendix E
for discussion about what record layer version number should be
employed for ClientHello.
length
The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14.
fragment
The application data. This data is transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.
Implementations MUST NOT send zero-length fragments of Handshake,
Alert, or ChangeCipherSpec content types. Zero-length fragments of
Application data MAY be sent as they are potentially useful as a
traffic analysis countermeasure.
Note: Data of different TLS record layer content types MAY be
interleaved. Application data is generally of lower precedence for
transmission than other content types. However, records MUST be
delivered to the network in the same order as they are protected by
the record layer. Recipients MUST receive and process interleaved
application layer traffic during handshakes subsequent to the first
one on a connection.
6.2.2. Record Compression and Decompression
All records are compressed using the compression algorithm defined in
the current session state. There is always an active compression
algorithm; however, initially it is defined as
CompressionMethod.null. The compression algorithm translates a
TLSPlaintext structure into a TLSCompressed structure. Compression
functions are initialized with default state information whenever a
connection state is made active. [RFC3749] describes compression
algorithms for TLS.
Compression must be lossless and may not increase the content length
by more than 1024 bytes. If the decompression function encounters a
TLSCompressed.fragment that would decompress to a length in excess of
2^14 bytes, it MUST report a fatal decompression failure error.
struct {
ContentType type; /* same as TLSPlaintext.type */
ProtocolVersion version;/* same as TLSPlaintext.version */
uint16 length;
opaque fragment[TLSCompressed.length];
} TLSCompressed;
length
The length (in bytes) of the following TLSCompressed.fragment.
The length MUST NOT exceed 2^14 + 1024.
fragment
The compressed form of TLSPlaintext.fragment.
Note: A CompressionMethod.null operation is an identity operation;
no fields are altered.
Implementation note: Decompression functions are responsible for
ensuring that messages cannot cause internal buffer overflows.
6.2.3. Record Payload Protection
The encryption and MAC functions translate a TLSCompressed
structure into a TLSCiphertext. The decryption functions reverse
the process. The MAC of the record also includes a sequence
number so that missing, extra, or repeated messages are
detectable.
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (SecurityParameters.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
case aead: GenericAEADCipher;
} fragment;
} TLSCiphertext;
type
The type field is identical to TLSCompressed.type.
version
The version field is identical to TLSCompressed.version.
length
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.
fragment
The encrypted form of TLSCompressed.fragment, with the MAC.
6.2.3.1. Null or Standard Stream Cipher
Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6)
convert TLSCompressed.fragment structures to and from stream
TLSCiphertext.fragment structures.
stream-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
} GenericStreamCipher;
The MAC is generated as:
MAC(MAC_write_key, seq_num +
TLSCompressed.type +
TLSCompressed.version +
TLSCompressed.length +
TLSCompressed.fragment);
where "+" denotes concatenation.
seq_num
The sequence number for this record.
MAC
The MAC algorithm specified by SecurityParameters.mac_algorithm.
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 cipher suite is TLS_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).
For both null and stream ciphers, TLSCiphertext.length is
TLSCompressed.length plus SecurityParameters.mac_length.
6.2.3.2. CBC Block Cipher
For block ciphers (such as 3DES or AES), the encryption and MAC
functions convert TLSCompressed.fragment structures to and from block
TLSCiphertext.fragment structures.
struct {
opaque IV[SecurityParameters.record_iv_length];
block-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
};
} GenericBlockCipher;
The MAC is generated as described in Section 6.2.3.1.
IV
The Initialization Vector (IV) SHOULD be chosen at random, and
MUST be unpredictable. Note that in versions of TLS prior to 1.1,
there was no IV field, and the last ciphertext block of the
previous record (the "CBC residue") was used as the IV. This was
changed to prevent the attacks described in [CBCATT]. For block
ciphers, the IV length is of length
SecurityParameters.record_iv_length, which is equal to the
SecurityParameters.block_size.
padding
Padding that is added to force the length of the plaintext to be
an integral multiple of the block cipher's block length. The
padding MAY be any length up to 255 bytes, as long as it results
in the TLSCiphertext.length being an integral multiple of the
block length. Lengths longer than necessary might be desirable to
frustrate attacks on a protocol that are based on analysis of the
lengths of exchanged messages. Each uint8 in the padding data
vector MUST be filled with the padding length value. The receiver
MUST check this padding and MUST use the bad_record_mac alert to
indicate padding errors.
padding_length
The padding length MUST be such that the total size of the
GenericBlockCipher structure is a multiple of the cipher's block
length. Legal values range from zero to 255, inclusive. This
length specifies the length of the padding field exclusive of the
padding_length field itself.
The encrypted data length (TLSCiphertext.length) is one more than the
sum of SecurityParameters.block_length, TLSCompressed.length,
SecurityParameters.mac_length, and padding_length.
Example: If the block length is 8 bytes, the content length
(TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes,
then the length before padding is 82 bytes (this does not include the
IV. Thus, the padding length modulo 8 must be equal to 6 in order to make the total length an even multiple of 8 bytes (the block length). The padding length can be 6, 14, 22, and so on, through 254. If the padding length were the minimum necessary, 6, the padding would be 6 bytes, each containing the value 6. Thus, the last 8 octets of the GenericBlockCipher before block encryption would be xx 06 06 06 06 06 06 06, where xx is the last octet of the MAC. Note: With block ciphers in CBC mode (Cipher Block Chaining), it is critical that the entire plaintext of the record be known before any ciphertext is transmitted. Otherwise, it is possible for the attacker to mount the attack described in [CBCATT]. Implementation note: Canvel et al. [CBCTIME] have demonstrated a timing attack on CBC padding based on the time required to compute the MAC. In order to defend against this attack, implementations MUST ensure that record processing time is essentially the same whether or not the padding is correct. In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC. This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment, but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal. 6.2.3.3. AEAD Ciphers For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function converts TLSCompressed.fragment structures to and from AEAD TLSCiphertext.fragment structures. struct { opaque nonce_explicit[SecurityParameters.record_iv_length]; aead-ciphered struct { opaque content[TLSCompressed.length]; }; } GenericAEADCipher; AEAD ciphers take as input a single key, a nonce, a plaintext, and "additional data" to be included in the authentication check, as described in Section 2.1 of [AEAD]. The key is either the client_write_key or the server_write_key. No MAC key is used. Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD operation is constructed, and what is the length of the GenericAEADCipher.nonce_explicit part. In many cases, it is
appropriate to use the partially implicit nonce technique described in Section 3.2.1 of [AEAD]; with record_iv_length being the length of the explicit part. In this case, the implicit part SHOULD be derived from key_block as client_write_iv and server_write_iv (as described in Section 6.3), and the explicit part is included in GenericAEAEDCipher.nonce_explicit. The plaintext is the TLSCompressed.fragment. The additional authenticated data, which we denote as additional_data, is defined as follows: additional_data = seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length; where "+" denotes concatenation. The aead_output consists of the ciphertext output by the AEAD encryption operation. The length will generally be larger than TLSCompressed.length, but by an amount that varies with the AEAD cipher. Since the ciphers might incorporate padding, the amount of overhead could vary with different TLSCompressed.length values. Each AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes. Symbolically, AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext, additional_data) In order to decrypt and verify, the cipher takes as input the key, nonce, the "additional_data", and the AEADEncrypted value. The output is either the plaintext or an error indicating that the decryption failed. There is no separate integrity check. That is: TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce, AEADEncrypted, additional_data) If the decryption fails, a fatal bad_record_mac alert MUST be generated. 6.3. Key Calculation The Record Protocol requires an algorithm to generate keys required by the current connection state (see Appendix A.6) from the security parameters provided by the handshake protocol.
The master secret is expanded into a sequence of secure bytes, which is then split to a client write MAC key, a server write MAC key, a client write encryption key, and a server write encryption key. Each of these is generated from the byte sequence in that order. Unused values are empty. Some AEAD ciphers may additionally require a client write IV and a server write IV (see Section 6.2.3.3). When keys and MAC keys are generated, the master secret is used as an entropy source. To generate the key material, compute key_block = PRF(SecurityParameters.master_secret, "key expansion", SecurityParameters.server_random + SecurityParameters.client_random); until enough output has been generated. Then, the key_block is partitioned as follows: client_write_MAC_key[SecurityParameters.mac_key_length] server_write_MAC_key[SecurityParameters.mac_key_length] client_write_key[SecurityParameters.enc_key_length] server_write_key[SecurityParameters.enc_key_length] client_write_IV[SecurityParameters.fixed_iv_length] server_write_IV[SecurityParameters.fixed_iv_length] Currently, the client_write_IV and server_write_IV are only generated for implicit nonce techniques as described in Section 3.2.1 of [AEAD]. Implementation note: The currently defined cipher suite which requires the most material is AES_256_CBC_SHA256. It requires 2 x 32 byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key material. 7. The TLS Handshaking Protocols TLS has three subprotocols that are used to allow peers to agree upon security parameters for the record layer, to authenticate themselves, to instantiate negotiated security parameters, and to report error conditions to each other. The Handshake Protocol is responsible for negotiating a session, which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
peer certificate
X509v3 [PKIX] certificate of the peer. This element of the state
may be null.
compression method
The algorithm used to compress data prior to encryption.
cipher spec
Specifies the pseudorandom function (PRF) used to generate keying
material, the bulk data encryption algorithm (such as null, AES,
etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines
cryptographic attributes such as the mac_length. (See Appendix
A.6 for formal definition.)
master secret
48-byte secret shared between the client and server.
is resumable
A flag indicating whether the session can be used to initiate new
connections.
These items are then used to create security parameters for use by
the record layer when protecting application data. Many connections
can be instantiated using the same session through the resumption
feature of the TLS Handshake Protocol.
7.1. Change Cipher Spec Protocol
The change cipher spec protocol exists to signal transitions in
ciphering strategies. The protocol consists of a single message,
which is encrypted and compressed under the current (not the pending)
connection state. The message consists of a single byte of value 1.
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
The ChangeCipherSpec message is sent by both the client and the
server to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the record layer to
immediately copy the read pending state into the read current state.
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See Section 6.1.) The ChangeCipherSpec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying Finished message is sent. Note: If a rehandshake occurs while data is flowing on a connection, the communicating parties may continue to send data using the old CipherSpec. However, once the ChangeCipherSpec has been sent, the new CipherSpec MUST be used. The first side to send the ChangeCipherSpec does not know that the other side has finished computing the new keying material (e.g., if it has to perform a time-consuming public key operation). Thus, a small window of time, during which the recipient must buffer the data, MAY exist. In practice, with modern machines this interval is likely to be fairly short. 7.2. Alert Protocol One of the content types supported by the TLS record layer is the alert type. Alert messages convey the severity of the message (warning or fatal) and a description of the alert. Alert messages with a level of fatal result in the immediate termination of the connection. In this case, other connections corresponding to the session may continue, but the session identifier MUST be invalidated, preventing the failed session from being used to establish new connections. Like other messages, alert messages are encrypted and compressed, as specified by the current connection state. enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed_RESERVED(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED(41), 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_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
7.2.1. Closure Alerts
The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages.
close_notify
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.
If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding close_notify. No part
of this standard should be taken to dictate the manner in which a usage profile for TLS manages its data transport, including when connections are opened or closed. Note: It is assumed that closing a connection reliably delivers pending data before destroying the transport. 7.2.2. Error Alerts Error handling in the TLS Handshake protocol is very simple. When an error is detected, the detecting party sends a message to the other party. Upon transmission or receipt of a fatal alert message, both parties immediately close the connection. Servers and clients MUST forget any session-identifiers, keys, and secrets associated with a failed connection. Thus, any connection terminated with a fatal alert MUST NOT be resumed. Whenever an implementation encounters a condition which is defined as a fatal alert, it MUST send the appropriate alert prior to closing the connection. For all errors where an alert level is not explicitly specified, the sending party MAY determine at its discretion whether to treat this as a fatal error or not. If the implementation chooses to send an alert but intends to close the connection immediately afterwards, it MUST send that alert at the fatal alert level. If an alert with a level of warning is sent and received, generally the connection can continue normally. If the receiving party decides not to proceed with the connection (e.g., after having received a no_renegotiation alert that it is not willing to accept), it SHOULD send a fatal alert to terminate the connection. Given this, the sending party cannot, in general, know how the receiving party will behave. Therefore, warning alerts are not very useful when the sending party wants to continue the connection, and thus are sometimes omitted. For example, if a peer decides to accept an expired certificate (perhaps after confirming this with the user) and wants to continue the connection, it would not generally send a certificate_expired alert. The following error alerts are defined: unexpected_message An inappropriate message was received. This alert is always fatal and should never be observed in communication between proper implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn't an
even multiple of the block length, or its padding values, when
checked, weren't correct. This message is always fatal and should
never be observed in communication between proper implementations
(except when messages were corrupted in the network).
decryption_failed_RESERVED
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode [CBCATT]. It MUST
NOT be sent by compliant implementations.
record_overflow
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).
decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal and should never be observed in communication between proper
implementations.
handshake_failure
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.
bad_certificate
A certificate was corrupt, contained signatures that did not
verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
certificate_unknown
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was out of range or inconsistent with
other fields. This message is always fatal.
unknown_ca
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn't be matched with a known, trusted CA. This
message is always fatal.
access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.
decode_error
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
decrypt_error
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This message is always fatal.
export_restriction_RESERVED
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.
protocol_version
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This message is always
fatal.
insufficient_security
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.
internal_error
An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue. This message is always fatal.
user_canceled
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed
by a close_notify. This message is generally a warning.
no_renegotiation
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Either
of these would normally lead to renegotiation; when that is not
appropriate, the recipient should respond with this alert. At
that point, the original requester can decide whether to proceed
with the connection. One case where this would be appropriate is
where a server has spawned a process to satisfy a request; the
process might receive security parameters (key length,
authentication, etc.) at startup, and it might be difficult to
communicate changes to these parameters after that point. This
message is always a warning.
unsupported_extension
sent by clients that receive an extended server hello containing
an extension that they did not put in the corresponding client
hello. This message is always fatal.
New Alert values are assigned by IANA as described in Section 12.
7.3. Handshake Protocol Overview
The cryptographic parameters of the session state are produced by the
TLS Handshake Protocol, which operates on top of the TLS record
layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and use public-key encryption
techniques to generate shared secrets.
The TLS Handshake Protocol involves the following steps:
- Exchange hello messages to agree on algorithms, exchange random
values, and check for session resumption.
- Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.
- Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.
- Generate a master secret from the premaster secret and exchanged
random values.
- Provide security parameters to the record layer.
- Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.
Note that higher layers should not be overly reliant on whether TLS
always negotiates the strongest possible connection between two
peers. There are a number of ways in which a man-in-the-middle
attacker can attempt to make two entities drop down to the least
secure method they support. The protocol has been designed to
minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service
runs on, or attempt to get the peers to negotiate an unauthenticated
connection. The fundamental rule is that higher levels must be
cognizant of what their security requirements are and never transmit
information over a channel less secure than what they require. The
TLS protocol is secure in that any cipher suite offers its promised
level of security: if you negotiate 3DES with a 1024-bit RSA key
exchange with a host whose certificate you have verified, you can
expect to be that secure.
These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a ClientHello message to
which the server must respond with a ServerHello message, or else a
fatal error will occur and the connection will fail. The ClientHello
and ServerHello are used to establish security enhancement
capabilities between client and server. The ClientHello and
ServerHello 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.
The actual key exchange uses up to four messages: the server Certificate, the ServerKeyExchange, the client Certificate, and the ClientKeyExchange. New key exchange methods can be created by specifying a format for these messages and by defining the use of the messages to allow the client and server to agree upon a shared secret. This secret MUST be quite long; currently defined key exchange methods exchange secrets that range from 46 bytes upwards. Following the hello messages, the server will send its certificate in a Certificate message if it is to be authenticated. Additionally, a ServerKeyExchange message may be sent, if it is required (e.g., if the 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. Next, the server will send the ServerHelloDone 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 CertificateRequest message, the client MUST send the Certificate message. The ClientKeyExchange message is now sent, and the content of that message will depend on the public key algorithm selected between the ClientHello and the ServerHello. If the client has sent a certificate with signing ability, a digitally-signed CertificateVerify message is sent to explicitly verify possession of the private key in the certificate. At this point, a ChangeCipherSpec 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 ChangeCipherSpec 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.) Application data MUST NOT be sent prior to the completion of the first handshake (before a cipher suite other than TLS_NULL_WITH_NULL_NULL is established).
Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data Figure 1. Message flow for a full handshake * Indicates optional or situation-dependent messages that are not always sent. Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent TLS protocol content type, and is not actually a TLS 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 ClientHello 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 ServerHello with the same Session ID value. At this point, both client and server MUST send ChangeCipherSpec 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 TLS client and server perform a full handshake.
Client Server ClientHello --------> ServerHello [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Figure 2. Message flow for an abbreviated handshake The contents and significance of each message will be presented in detail in the following sections.
(page 37 continued on part 3)