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Network Working Group Request for Comments: 4346 Obsoletes: 2246 Category: Standards Track |
T. Dierks Independent E. Rescorla RTFM, Inc. April 2006 |
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2006).
This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.
1. Introduction
1.1. Differences from TLS 1.0
1.2. Requirements Terminology
2. Goals
3. Goals of This Document
4. Presentation Language
4.1. Basic Block Size
4.2. Miscellaneous
4.3. Vectors
4.4. Numbers
4.5. Enumerateds
4.6. Constructed Types
4.6.1. Variants
4.7. Cryptographic Attributes
4.8. Constants
5. HMAC and the Pseudorandom Function
6. The TLS Record Protocol
6.1. Connection States
6.2. Record layer
6.2.1. Fragmentation
6.2.2. Record Compression and Decompression
6.2.3. Record Payload Protection
6.2.3.1. Null or Standard Stream Cipher
6.2.3.2. CBC Block Cipher
6.3. Key Calculation
7. The TLS Handshaking Protocols
7.1. Change Cipher Spec Protocol
7.2. Alert Protocol
7.2.1. Closure Alerts
7.2.2. Error Alerts
7.3. Handshake Protocol Overview
7.4. Handshake Protocol
7.4.1. Hello Messages
7.4.1.1. Hello request
7.4.1.2. Client Hello
7.4.1.3. Server Hello
7.4.2. Server Certificate
7.4.3. Server Key Exchange Message
7.4.4. Certificate request
7.4.5. Server Hello Done
7.4.6. Client certificate
7.4.7. Client Key Exchange Message
7.4.7.1. RSA Encrypted Premaster Secret Message
7.4.7.2. Client Diffie-Hellman Public Value
7.4.8. Certificate verify
7.4.9. Finished
8. Cryptographic Computations
8.1. Computing the Master Secret
8.1.1. RSA
8.1.2. Diffie-Hellman
9. Mandatory Cipher Suites
10. Application Data Protocol
11. Security Considerations
12. IANA Considerations
A. Appendix - Protocol constant values
A.1. Record layer
A.2. Change cipher specs message
A.3. Alert messages
A.4. Handshake protocol
A.4.1. Hello messages
A.4.2. Server authentication and key exchange messages
A.4.3. Client authentication and key exchange messages
A.4.4.Handshake finalization message
A.5. The CipherSuite
A.6. The Security Parameters
B. Appendix - Glossary
C. Appendix - CipherSuite definitions
D. Appendix - Implementation Notes
D.1 Random Number Generation and Seeding
D.2 Certificates and authentication
D.3 CipherSuites
E. Appendix - Backward Compatibility With SSL
E.1. Version 2 client hello
E.2. Avoiding man-in-the-middle version rollback
F. Appendix - Security analysis
F.1. Handshake protocol
F.1.1. Authentication and key exchange
F.1.1.1. Anonymous key exchange
F.1.1.2. RSA key exchange and authentication
F.1.1.3. Diffie-Hellman key exchange with authentication ..76
F.1.2. Version rollback attacks
F.1.3. Detecting attacks against the handshake protocol ...77
F.1.4. Resuming sessions
F.1.5. MD5 and SHA
F.2. Protecting application data
F.3. Explicit IVs
F.4 Security of Composite Cipher Modes
F.5 Denial of Service
F.6. Final notes
Normative References
Informative References
The primary goal of the TLS Protocol is to provide privacy and data integrity between two communicating applications. The protocol is composed of two layers: the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level, layered on top of some reliable transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The TLS Record Protocol provides connection security that has two basic properties:
- The connection is private. Symmetric cryptography is used for
data encryption (e.g., DES [DES], RC4 [SCH] etc.). The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption.
- The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA, MD5, etc.) are used for MAC computations. The Record
Protocol can operate without a MAC, but is generally only used in
this mode while another protocol is using the Record Protocol as a
transport for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher- level protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first byte of data. The TLS Handshake Protocol provides connection security that has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required for
at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.
One advantage of TLS is that it is application protocol independent. Higher level protocols can layer on top of the TLS Protocol
transparently. The TLS standard, however, does not specify how protocols add security with TLS; the decisions on how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS.
This document is a revision of the TLS 1.0 [TLS1.0] protocol, and contains some small security improvements, clarifications, and editorial improvements. The major changes are:
- The implicit Initialization Vector (IV) is replaced with an
explicit IV to protect against CBC attacks [CBCATT].
- Handling of padding errors is changed to use the bad_record_mac
alert rather than the decryption_failed alert to protect against
CBC attacks.
- IANA registries are defined for protocol parameters.
- Premature closes no longer cause a session to be nonresumable.
- Additional informational notes were added for various new attacks
on TLS.
In addition, a number of minor clarifications and editorial improvements were made.
In this document, the keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and "MAY" are to be interpreted as described in RFC 2119 [REQ].
The goals of TLS Protocol, in order of their priority, are as follows:
This document and the TLS protocol itself are based on the SSL 3.0 Protocol Specification as published by Netscape. The differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough that TLS 1.1, TLS 1.0, and SSL 3.0 do not interoperate (although each protocol incorporates a mechanism by which an implementation can back down prior versions). This document is intended primarily for readers who will be implementing the protocol and for those doing cryptographic analysis of it. The specification has been written with this in mind, and it is intended to reflect the needs of those two groups. For that reason, many of the algorithm-dependent data structures and rules are included in the body of the text (as opposed to in an appendix), providing easier access to them.
This document is not intended to supply any details of service definition or of interface definition, although it does cover select areas of policy as they are required for the maintenance of solid security.
This document deals with the formatting of data in an external representation. The following very basic and somewhat casually defined presentation syntax will be used. The syntax draws from several sources in its structure. Although it resembles the programming language "C" in its syntax and XDR [XDR] in both its syntax and intent, it would be risky to draw too many parallels. The purpose of this presentation language is to document TLS only; it has no general application beyond that particular goal.
The representation of all data items is explicitly specified. The basic data block size is one byte (i.e., 8 bits). Multiple byte data items are concatenations of bytes, from left to right, from top to bottom. From the bytestream, a multi-byte item (a numeric in the example) is formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
... | byte[n-1];
This byte ordering for multi-byte values is the commonplace network byte order or big endian format.
Comments begin with "/*" and end with "*/".
Optional components are denoted by enclosing them in "[[ ]]" double brackets.
Single-byte entities containing uninterpreted data are of type opaque.
A vector (single dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case, the length declares the number of bytes, not the number of elements, in the vector. The syntax for specifying a new type, T', that is a fixed- length vector of type T is
T T'[n];
Here, T' occupies n bytes in the data stream, where n is a multiple of the size of T. The length of the vector is not included in the encoded stream.
In the following example, Datum is defined to be three consecutive bytes that the protocol does not interpret, while Data is three consecutive Datum, consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3 byte vectors */
Variable-length vectors are defined by specifying a subrange of legal lengths, inclusively, using the notation <floor..ceiling>. When
these are encoded, the actual length precedes the vector's contents in the byte stream. The length will be in the form of a number consuming as many bytes as required to hold the vector's specified maximum (ceiling) length. A variable-length vector with an actual length field of zero is referred to as an empty vector.
T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain between 300 and 400 bytes of type opaque. It can never be empty. The actual length field consumes two bytes, a uint16, sufficient to represent the value 400 (see Section 4.4). On the other hand, longer can represent up to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a two-byte actual length field prepended to the vector. The length of an encoded vector must be an even multiple of the length of a single element (for example, a 17-byte vector of uint16 would be illegal).
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
The basic numeric data type is an unsigned byte (uint8). All larger numeric data types are formed from fixed-length series of bytes concatenated as described in Section 4.1 and are also unsigned. The following numeric types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
All values, here and elsewhere in the specification, are stored in "network" or "big-endian" order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060.
An additional sparse data type is available called enum. A field of type enum can only assume the values declared in the definition. Each definition is a different type. Only enumerateds of the same type may be assigned or compared. Every element of an enumerated must be assigned a value, as demonstrated in the following example. Since the elements of the enumerated are not ordered, they can be assigned any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
Enumerateds occupy as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element. In the following example, Taste will consume two bytes in the data stream but can only assume the values 1, 2, or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would be Color.blue. Such qualification is not required if the target of the assignment is well specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation, the numerical information may be omitted.
enum { low, medium, high } Amount;
Structure types may be constructed from primitive types for convenience. Each specification declares a new, unique type. The syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
The fields within a structure may be qualified using the type's name, with a syntax much like that available for enumerateds. For example, T.f2 refers to the second field of the previous declaration. Structure definitions may be embedded.
Defined structures may have variants based on some knowledge that is available within the environment. The selector must be an enumerated type that defines the possible variants the structure defines. There must be a case arm for every element of the enumeration declared in the select. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
select (E) {
case e1: Te1;
case e2: Te2;
For example:
enum { apple, orange } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple: V1; /* VariantBody, tag = apple */
case orange: V2; /* VariantBody, tag = orange */
} variant_body; /* optional label on variant */
} VariantRecord;
Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, an
orange VariantRecord
is a narrowed type of a VariantRecord containing a variant_body of type V2.
The four cryptographic operations digital signing, stream cipher encryption, block cipher encryption, and public key encryption are designated digitally-signed, stream-ciphered, block-ciphered, and public-key-encrypted, respectively. A field's cryptographic processing is specified by prepending an appropriate key word designation before the field's type specification. Cryptographic keys are implied by the current session state (see Section 6.1).
In digital signing, one-way hash functions are used as input for a signing algorithm. A digitally-signed element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the signing algorithm and key.
In RSA signing, a 36-byte structure of two hashes (one SHA and one MD5) is signed (encrypted with the private key). It is encoded with PKCS #1 block type 1, as described in [PKCS1A].
Note: The standard reference for PKCS#1 is now RFC 3447 [PKCS1B]. However, to minimize differences with TLS 1.0 text, we are using the terminology of RFC 2313 [PKCS1A].
In DSS, the 20 bytes of the SHA hash are run directly through the Digital Signing Algorithm with no additional hashing. This produces two values, r and s. The DSS signature is an opaque vector, as above, the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically secure keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. All block cipher encryption is done in CBC (Cipher Block Chaining) mode, and all items that are block-ciphered will be an exact multiple of the cipher block length.
In public key encryption, a public key algorithm is used to encrypt data in such a way that it can be decrypted only with the matching private key. A public-key-encrypted element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the signing algorithm and key.
An RSA-encrypted value is encoded with PKCS #1 block type 2, as described in [PKCS1A].
In the following example,
stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque hash[20];
} UserType;
the contents of hash are used as input for the signing algorithm, and then the entire structure is encrypted with a stream cipher. The length of this structure, in bytes, would be equal to two bytes for field1 and field2, plus two bytes for the length of the signature, plus the length of the output of the signing algorithm. This is known because the algorithm and key used for the signing are known prior to encoding or decoding this structure.
Typed constants can be defined for purposes of specification by declaring a symbol of the desired type and assigning values to it. Under-specified types (opaque, variable length vectors, and structures that contain opaque) cannot be assigned values. No fields of a multi-element structure or vector may be elided.
For example:
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
A number of operations in the TLS record and handshake layer require a keyed MAC; this is a secure digest of some data protected by a secret. Forging the MAC is infeasible without knowledge of the MAC secret. The construction we use for this operation is known as HMAC, and is described in [HMAC].
HMAC can be used with a variety of different hash algorithms. TLS uses it in the handshake with two different algorithms, MD5 and SHA- 1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret, data). Additional hash algorithms can be defined by cipher suites
and used to protect record data, but MD5 and SHA-1 are hard coded into the description of the handshaking for this version of the protocol.
In addition, a construction is required to do expansion of secrets into blocks of data for the purposes of key generation or validation. This pseudo-random function (PRF) takes as input a secret, a seed, and an identifying label and produces an output of arbitrary length.
In order to make the PRF as secure as possible, it uses two hash algorithms in a way that should guarantee its security if either algorithm remains secure.
First, we define a data expansion function, P_hash(secret, data) that uses a single hash function to expand a secret and seed into an arbitrary quantity of output:
P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
HMAC_hash(secret, A(2) + seed) +
HMAC_hash(secret, A(3) + seed) + ...
Where + indicates concatenation.
A() is defined as:
A(0) = seed
A(i) = HMAC_hash(secret, A(i-1))
P_hash can be iterated as many times as is necessary to produce the required quantity of data. For example, if P_SHA-1 is being used to create 64 bytes of data, it will have to be iterated 4 times (through A(4)), creating 80 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 64 bytes of output data.
TLS's PRF is created by splitting the secret into two halves and using one half to generate data with P_MD5 and the other half to generate data with P_SHA-1, then exclusive-ORing the outputs of these two expansion functions together.
S1 and S2 are the two halves of the secret, and each is the same length. S1 is taken from the first half of the secret, S2 from the second half. Their length is created by rounding up the length of the overall secret, divided by two; thus, if the original secret is an odd number of bytes long, the last byte of S1 will be the same as the first byte of S2.
L_S = length in bytes of secret;
L_S1 = L_S2 = ceil(L_S / 2);
The secret is partitioned into two halves (with the possibility of one shared byte) as described above, S1 taking the first L_S1 bytes, and S2 the last L_S2 bytes.
The PRF is then defined as the result of mixing the two pseudorandom streams by exclusive-ORing them together.
PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
P_SHA-1(S2, label + seed);
The label is an ASCII string. It should be included in the exact form it is given without a length byte or trailing null character. For example, the label "slithy toves" would be processed by hashing the following bytes:
73 6C 69 74 68 79 20 74 6F 76 65 73
Note that because MD5 produces 16-byte outputs and SHA-1 produces 20-byte outputs, the boundaries of their internal iterations will not be aligned. Generating an 80-byte output will require that P_MD5 iterate through A(5), while P_SHA-1 will only iterate through A(4).
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 record protocol clients 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 types can be supported by the record protocol. Any new record types SHOULD allocate type values immediately beyond the ContentType values for the four record types described here (see Appendix A.1). All such values must be defined by RFC 2434 Standards Action. See Section 11 for IANA Considerations for ContentType values.
If a TLS implementation receives a record type it does not understand, it SHOULD just ignore it. Any protocol designed for use
over TLS MUST be carefully designed to deal with all possible attacks against it. Note that because the type and length of a record are not protected by encryption, care SHOULD be taken to minimize the value of traffic analysis of these values.
A TLS connection state is the operating environment of the TLS Record Protocol. It specifies a compression algorithm, and encryption algorithm, and a MAC algorithm. In addition, the parameters for these algorithms are known: the MAC secret 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 Change Cipher Spec 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.
bulk encryption algorithm
An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, how much of that key is
secret, whether it is a block or stream cipher, and the block size
of the cipher (if appropriate).
MAC algorithm
An algorithm to be used for message authentication. This
specification includes the size of the hash returned by the MAC
algorithm.
compression algorithm
An algorithm to be used for data compression. This specification
must include all information the algorithm requires 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 { null, rc4, rc2, des, 3des, des40, idea, aes }
BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { null, md5, sha } MACAlgorithm;
enum { null(0), (255) } CompressionMethod;
/* 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;
MACAlgorithm mac_algorithm;
uint8 hash_size;
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 four items:
client write MAC secret
server write MAC secret
client write key
server write key
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 secret
The MAC secret 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.
The TLS Record Layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size.
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, 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 1.1, which uses the version { 3, 2 }. The
version value 3.2 is historical: TLS version 1.1 is a minor
modification to the TLS 1.0 protocol, which was itself a minor
modification to the SSL 3.0 protocol, which bears the version
value 3.0. (See Appendix A.1.)
length
The length (in bytes) of the following TLSPlaintext.fragment. The
length should not exceed 2^14.
fragment
The application data. This data is transparent and is treated as
an independent block to be dealt with by the higher-level protocol
specified by the type field.
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.
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.
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 should 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 should 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.
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 (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} 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 may not exceed 2^14 + 2048.
fragment
The encrypted form of TLSCompressed.fragment, with the MAC.
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[CipherSpec.hash_size];
} GenericStreamCipher;
The MAC is generated as:
HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length + TLSCompressed.fragment));
where "+" denotes concatenation.
seq_num
The sequence number for this record.
hash
The hashing 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 CipherSuite 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).
TLSCiphertext.length is TLSCompressed.length plus
CipherSpec.hash_size.
For block ciphers (such as RC2, DES, or AES), the encryption and MAC functions convert TLSCompressed.fragment structures to and from block TLSCiphertext.fragment structures.
block-ciphered struct {
opaque IV[CipherSpec.block_length];
opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
The MAC is generated as described in Section 6.2.3.1.
IV
Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
IV in order to prevent the attacks described by [CBCATT]. We
recommend the following equivalently strong procedures. For
clarity we use the following notation.
IV
The transmitted value of the IV field in the GenericBlockCipher
structure.
CBC residue
The last ciphertext block of the previous record.
mask
The actual value that the cipher XORs with the plaintext prior
to encryption of the first cipher block of the record.
In prior versions of TLS, there was no IV field and the CBC residue and mask were one and the same. See Sections 6.1, 6.2.3.2, and 6.3, of [TLS1.0] for details of TLS 1.0 IV handling.
One of the following two algorithms SHOULD be used to generate the per-record IV:
(1) Generate a cryptographically strong random string R of length CipherSpec.block_length. Place R in the IV field. Set the mask to R. Thus, the first cipher block will be encrypted as E(R XOR Data).
(2) Generate a cryptographically strong random number R of length CipherSpec.block_length and prepend it to the plaintext prior to encryption. In this case either:
(a) The cipher may use a fixed mask such as zero.
(b) The CBC residue from the previous record may be used as
the mask. This preserves maximum code compatibility with
TLS 1.0 and SSL 3. It also has the advantage that it does
not require the ability to quickly reset the IV, which is
known to be a problem on some systems.
In either (2)(a) or (2)(b) the data (R || data) is fed into the encryption process. The first cipher block (containing E(mask XOR R) is placed in the IV field. The first block of content contains E(IV XOR data).
The following alternative procedure MAY be used; however, it has
not been demonstrated to be as cryptographically strong as the
above procedures. The sender prepends a fixed block F to the
plaintext (or, alternatively, a block generated with a weak PRNG).
He then encrypts as in (2), above, using the CBC residue from the
previous block as the mask for the prepended block. Note that in
this case the mask for the first record transmitted by the
application (the Finished) MUST be generated using a
cryptographically strong PRNG.
The decryption operation for all three alternatives is the same. The receiver decrypts the entire GenericBlockCipher structure and then discards the first cipher block, corresponding to the IV component.
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 SHOULD 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 CipherSpec.block_length, TLSCompressed.length,
CipherSpec.hash_size, 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, which may or may not be encrypted, as discussed above). 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.
The Record Protocol requires an algorithm to generate keys, and MAC secrets from the security parameters provided by the handshake protocol.
The master secret is hashed into a sequence of secure bytes, which are assigned to the MAC secrets and keys required by the current connection state (see Appendix A.6). CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, and a server write key, each of which is generated from the master secret in that order. Unused values are empty.
When keys and MAC secrets 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_secret[SecurityParameters.hash_size] server_write_MAC_secret[SecurityParameters.hash_size] client_write_key[SecurityParameters.key_material_length] server_write_key[SecurityParameters.key_material_length]
Implementation note: The currently defined cipher suite that requires the most material is AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte keys, 2 x 20 byte MAC secrets, and 2 x 16 byte Initialization Vectors, for a total of 136 bytes of key material.
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 [X509] 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 bulk data encryption algorithm (such as null, DES,
etc.) and a MAC algorithm (such as MD5 or SHA). It also defines
cryptographic attributes such as the hash_size. (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.
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 change cipher spec 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 change cipher spec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying finished message is sent (see Section 7.4.9).
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.
One of the content types supported by the TLS Record layer is the alert type. Alert messages convey the severity of the message 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(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),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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.
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. 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.
decryption_failed
This alert MAY be returned if 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.
Note: Differentiating between bad_record_mac and decryption_failed alerts may permit certain attacks against CBC mode as used in TLS [CBCATT]. It is preferable to uniformly use the bad_record_mac alert to hide the specific type of the error.
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.
decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal.
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 in TLS. It should 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 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.
decrypt_error
A handshake cryptographic operation failed, including being
unable to correctly verify a signature, decrypt a key exchange,
or validate a finished message.
export_restriction_RESERVED
This alert was used in TLS 1.0 but not TLS 1.1.
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.
For all errors where an alert level is not explicitly specified, the sending party MAY determine at its discretion whether this is a fatal error or not; if an alert with a level of warning is received, the receiving party MAY decide at its discretion whether to treat this as a fatal error or not. However, all messages that are transmitted with a level of fatal MUST be treated as fatal messages.
New alert values MUST be defined by RFC 2434 Standards Action. See Section 11 for IANA Considerations for alert values.
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.
However, one SHOULD never send data over a link encrypted with 40-bit security unless one feels that data is worth no more than the effort required to break that encryption.
These goals are achieved by the handshake protocol, which can be summarized as follows: The client sends a client hello message to which the server must respond with a server hello message, or else a fatal error will occur and the connection will fail. The client hello and server hello are used to establish security enhancement capabilities between client and server. The client hello and server hello establish the following attributes: Protocol Version, Session ID, Cipher Suite, and Compression Method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random.
The actual key exchange uses up to four messages: the server certificate, the server key exchange, the client certificate, and the client key exchange. 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 48 to 128 bytes in length.
Following the hello messages, the server will send its certificate, if it is to be authenticated. Additionally, a server key exchange message may be sent, if it is required (e.g., if 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 server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. If the server has sent a certificate request message, the client must send the certificate message. The client key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between the client hello and the server hello. If the client has sent a certificate with signing ability, a digitally-
signed certificate verify message is sent to explicitly verify the certificate.
At this point, a change cipher spec message is sent by the client, and the client copies the pending Cipher Spec into the current Cipher Spec. The client then immediately sends the finished message under the new algorithms, keys, and secrets. In response, the server will send its own change cipher spec message, transfer the pending to the current Cipher Spec, and send its finished message under the new Cipher Spec. At this point, the handshake is complete, and the client and server may begin to exchange application layer data. (See flow chart below.) Application data MUST NOT be sent prior to the completion of the first handshake (before a cipher suite other 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
Fig. 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 change cipher spec messages and proceed directly to finished messages. Once the re-establishment is complete, the client and server MAY begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID and the TLS client and server perform a full handshake.
Client Server
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Fig. 2. Message flow for an abbreviated handshake
The contents and significance of each message will be presented in detail in the following sections.
The TLS Handshake Protocol is one of the defined higher-level clients of the TLS Record Protocol. This protocol is used to negotiate the secure attributes of a session. Handshake messages are supplied to the TLS Record Layer, where they are encapsulated within one or more TLSPlaintext structures, which are processed and transmitted as specified by the current active session state.
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; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
The handshake protocol messages are presented below in the order they MUST be sent; sending handshake messages in an unexpected order results in a fatal error. Unneeded handshake messages can be omitted, however. Note one exception to the ordering: the Certificate message is used twice in the handshake (from server to client, then from client to server), but is described only in its first position. The one message that is not bound by these ordering rules is the Hello Request message, which can be sent at any time, but which should be ignored by the client if it arrives in the middle of a handshake.
New Handshake message type values MUST be defined via RFC 2434 Standards Action. See Section 11 for IANA Considerations for these values.
The hello phase messages are used to exchange security enhancement capabilities between the client and server. When a new session begins, the Record Layer's connection state encryption, hash, and compression algorithms are initialized to null. The current connection state is used for renegotiation messages.
When this message will be sent:
The hello request message MAY be sent by the server at any time.
Meaning of this message:
Hello request is a simple notification that the client should begin the negotiation process anew by sending a client hello message when convenient. This message will be ignored by the client if the client is currently negotiating a session. This message may be ignored by the client if it does not wish to renegotiate a session, or the client may, if it wishes, respond
with a no_renegotiation alert. Since handshake messages are intended to have transmission precedence over application data, it is expected that the negotiation will begin before no more than a few records are received from the client. If the server sends a hello request but does not receive a client hello in response, it may close the connection with a fatal alert.
After sending a hello request, servers SHOULD not repeat the request until the subsequent handshake negotiation is complete.
Structure of this message:
struct { } HelloRequest;
Note: This message MUST NOT be included in the message hashes that are maintained throughout the handshake and used in the finished messages and the certificate verify message.
When this message will be sent:
When a client first connects to a server it is required to send the client hello as its first message. The client can also send a client hello in response to a hello request or on its own initiative in order to renegotiate the security parameters in an existing connection.
Structure of this message:
The client hello message includes a random structure, which is used later in the protocol.
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
gmt_unix_time The current time and date in standard UNIX 32-bit format (seconds since the midnight starting Jan 1, 1970, GMT, ignoring leap seconds) according to the sender's internal clock. Clocks are not required to be set correctly by the basic TLS Protocol; higher-level or application protocols may define additional requirements.
random_bytes
28 bytes generated by a secure random number generator.
The client hello message includes a variable-length session identifier. If not empty, the value identifies a session between the same client and server whose security parameters the client wishes to reuse. The session identifier MAY be from an earlier connection, from this connection, or from another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, and the third option makes it possible to establish several independent secure connections without repeating the full handshake protocol. These independent connections may occur sequentially or simultaneously; a SessionID becomes valid when the handshake negotiating it completes with the exchange of Finished messages and persists until it is removed due to aging or because a fatal error was encountered on a connection associated with the session. The actual contents of the SessionID are defined by the server.
opaque SessionID<0..32>;
Warning: Because the SessionID is transmitted without encryption or immediate MAC protection, servers MUST not place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. (Note that the content of the handshake as a whole, including the SessionID, is protected by the Finished messages exchanged at the end of the handshake.)
The CipherSuite list, passed from the client to the server in the client hello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (favorite choice first). Each CipherSuite defines a key exchange algorithm, a bulk encryption algorithm (including secret key length), and a MAC algorithm. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection.
uint8 CipherSuite[2]; /* Cryptographic suite selector */
The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference.
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;
client_version
The version of the TLS protocol by which the client wishes to
communicate during this session. This SHOULD be the latest
(highest valued) version supported by the client. For this
version of the specification, the version will be 3.2. (See
Appendix E for details about backward compatibility.)
random
A client-generated random structure.
session_id
The ID of a session the client wishes to use for this connection.
This field should be empty if no session_id is available or if the
client wishes to generate new security parameters.
cipher_suites
This is a list of the cryptographic options supported by the
client, with the client's first preference first. If the
session_id field is not empty (implying a session resumption
request) this vector MUST include at least the cipher_suite from
that session. Values are defined in Appendix A.5.
compression_methods
This is a list of the compression methods supported by the client,
sorted by client preference. If the session_id field is not empty
(implying a session resumption request) it MUST include the
compression_method from that session. This vector MUST contain,
and all implementations MUST support, CompressionMethod.null.
Thus, a client and server will always be able to agree on a
compression method.
After sending the client hello message, the client waits for a server hello message. Any other handshake message returned by the server except for a hello request is treated as a fatal error.
Forward compatibility note: In the interests of forward
compatibility, it is permitted that a client hello message include
extra data after the compression methods. This data MUST be included
in the handshake hashes, but must otherwise be ignored. This is the only handshake message for which this is legal; for all other messages, the amount of data in the message MUST match the description of the message precisely.
Note: For the intended use of trailing data in the ClientHello, see RFC 3546 [TLSEXT].
The server will send this message in response to a client hello message when it was able to find an acceptable set of algorithms. If it cannot find such a match, it will respond with a handshake failure alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} ServerHello;
server_version
This field will contain the lower of that suggested by the client
in the client hello and the highest supported by the server. For
this version of the specification, the version is 3.2. (See
Appendix E for details about backward compatibility.)
random
This structure is generated by the server and MUST be
independently generated from the ClientHello.random.
session_id
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the finished messages. Otherwise this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with.
cipher_suite
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is
the value from the state of the session being resumed.
compression_method The single compression algorithm selected by the server from the list in ClientHello.compression_methods. For resumed sessions this field is the value from the resumed session state.
When this message will be sent:
The server MUST send a certificate whenever the agreed-upon key exchange method is not an anonymous one. This message will always immediately follow the server hello message.
Meaning of this message:
The certificate type MUST be appropriate for the selected cipher suite's key exchange algorithm, and is generally an X.509v3 certificate. It MUST contain a key that matches the key exchange method, as follows. Unless otherwise specified, the signing algorithm for the certificate MUST be the same as the algorithm for the certificate key. Unless otherwise specified, the public key MAY be of any length.
Key Exchange Algorithm Certificate Key Type
RSA RSA public key; the certificate MUST
allow the key to be used for encryption.
DHE_DSS DSS public key.
DHE_RSA RSA public key that can be used for
signing.
DH_DSS Diffie-Hellman key. The algorithm used
to sign the certificate MUST be DSS.
DH_RSA Diffie-Hellman key. The algorithm used
to sign the certificate MUST be RSA.
All certificate profiles and key and cryptographic formats are defined by the IETF PKIX working group [PKIX]. When a key usage extension is present, the digitalSignature bit MUST be set for the key to be eligible for signing, as described above, and the keyEncipherment bit MUST be present to allow encryption, as described above. The keyAgreement bit must be set on Diffie-Hellman certificates.
As CipherSuites that specify new key exchange methods are specified for the TLS Protocol, they will imply certificate format and the required encoded keying information.
Structure of this message:
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
certificate_list
This is a sequence (chain) of X.509v3 certificates. The sender's
certificate must come first in the list. Each following
certificate must directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority may optionally be omitted from the chain,
under the assumption that the remote end must already possess it
in order to validate it in any case.
The same message type and structure will be used for the client's response to a certificate request message. Note that a client MAY
send no certificates if it does not have an appropriate certificate to send in response to the server's authentication request.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate vector because PKCS #6 [PKCS6] extended certificates are not used. Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task of parsing the list more difficult.
When this message will be sent:
This message will be sent immediately after the server certificate message (or the server hello message, if this is an anonymous negotiation).
The server key exchange message is sent by the server only when the server certificate message (if sent) does not contain enough data to allow the client to exchange a premaster secret. This is true for the following key exchange methods:
DHE_DSS
DHE_RSA
DH_anon
It is not legal to send the server key exchange message for the following key exchange methods:
RSA
DH_DSS
DH_RSA
Meaning of this message:
This message conveys cryptographic information to allow the client to communicate the premaster secret: either an RSA public key with which to encrypt the premaster secret, or a Diffie-Hellman public key with which the client can complete a key exchange (with the result being the premaster secret).
As additional CipherSuites are defined for TLS that include new key exchange algorithms, the server key exchange message will be sent if and only if the certificate type associated with the key exchange algorithm does not provide enough information for the client to exchange a premaster secret.
Structure of this message:
enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
struct {
opaque rsa_modulus<1..2^16-1>;
opaque rsa_exponent<1..2^16-1>;
} ServerRSAParams;
rsa_modulus
The modulus of the server's temporary RSA key.
rsa_exponent
The public exponent of the server's temporary RSA key.
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams; /* Ephemeral DH parameters */
dh_p
The prime modulus used for the Diffie-Hellman operation.
dh_g
The generator used for the Diffie-Hellman operation.
dh_Ys
The server's Diffie-Hellman public value (g^X mod p).
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
};
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
case rsa:
ServerRSAParams params;
};
} ServerParams;
params
The server's key exchange parameters.
signed_params
For non-anonymous key exchanges, a hash of the corresponding
params value, with the signature appropriate to that hash
applied.
md5_hash
MD5(ClientHello.random + ServerHello.random + ServerParams);
sha_hash
SHA(ClientHello.random + ServerHello.random + ServerParams);
enum { anonymous, rsa, dsa } SignatureAlgorithm;
struct {
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;
When this message will be sent:
A non-anonymous server can optionally request a certificate from the client, if it is appropriate for the selected cipher suite.
This message, if sent, will immediately follow the Server Key Exchange message (if it is sent; otherwise, the Server Certificate message).
Structure of this message:
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
certificate_types
This field is a list of the types of certificates requested,
sorted in order of the server's preference.
certificate_authorities
A list of the distinguished names of acceptable certificate
authorities. These distinguished names may specify a desired
distinguished name for a root CA or for a subordinate CA; thus,
this message can be used to describe both known roots and a
desired authorization space. If the certificate_authorities
list is empty then the client MAY send any certificate of the
appropriate ClientCertificateType, unless there is some
external arrangement to the contrary.
ClientCertificateType values are divided into three groups:
Additional information describing the role of IANA in the allocation of ClientCertificateType code points is described in Section 11.
Note: Values listed as RESERVED may not be used. They were used in SSLv3.
Note: DistinguishedName is derived from [X501]. DistinguishedNames are represented in DER-encoded format.
Note: It is a fatal handshake_failure alert for an anonymous server to request client authentication.
When this message will be sent:
The server hello done message is sent by the server to indicate the end of the server hello and associated messages. After sending this message, the server will wait for a client response.
Meaning of this message:
This message means that the server is done sending messages to support the key exchange, and the client can proceed with its phase of the key exchange.
Upon receipt of the server hello done message, the client SHOULD verify that the server provided a valid certificate, if required and check that the server hello parameters are acceptable.
Structure of this message:
struct { } ServerHelloDone;
When this message will be sent:
This is the first message the client can send after receiving a server hello done message. This message is only sent if the server requests a certificate. If no suitable certificate is available, the client SHOULD send a certificate message containing no certificates. That is, the certificate_list structure has a length of zero. If client authentication is required by the server for the handshake to continue, it may respond with a fatal handshake failure alert. Client certificates are sent using the Certificate structure defined in Section 7.4.2.
Note: When using a static Diffie-Hellman based key exchange method (DH_DSS or DH_RSA), if client authentication is requested, the Diffie-Hellman group and generator encoded in the client's certificate MUST match the server specified Diffie-Hellman parameters if the client's parameters are to be used for the key exchange.
When this message will be sent:
This message is always sent by the client. It MUST immediately follow the client certificate message, if it is sent. Otherwise it MUST be the first message sent by the client after it receives the server hello done message.
Meaning of this message:
With this message, the premaster secret is set, either though direct transmission of the RSA-encrypted secret or by the transmission of Diffie-Hellman parameters that will allow each side to agree upon the same premaster secret. When the key exchange method is DH_RSA or DH_DSS, client certification has been requested, and the client was able to respond with a certificate that contained a Diffie-Hellman public key whose parameters (group and generator) matched those specified by the server in its certificate, this message MUST not contain any data.
Structure of this message:
The choice of messages depends on which key exchange method has been selected. See Section 7.4.3 for the KeyExchangeAlgorithm definition.
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
Meaning of this message:
If RSA is being used for key agreement and authentication, the client generates a 48-byte premaster secret, encrypts it using the public key from the server's certificate or the temporary RSA key
provided in a server key exchange message, and sends the result in an encrypted premaster secret message. This structure is a variant of the client key exchange message and is not a message in itself.
Structure of this message:
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
client_version The latest (newest) version supported by the client. This is used to detect version roll-back attacks. Upon receiving the premaster secret, the server SHOULD check that this value matches the value transmitted by the client in the client hello message.
random
46 securely-generated random bytes.
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
pre_master_secret
This random value is generated by the client and is used to
generate the master secret, as specified in Section 8.1.
Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used to attack a TLS server that is using PKCS#1 v 1.5 encoded RSA. The attack takes advantage of the fact that, by failing in different ways, a TLS server can be coerced into revealing whether a particular message, when decrypted, is properly PKCS#1 v1.5 formatted or not.
The best way to avoid vulnerability to this attack is to treat incorrectly formatted messages in a manner indistinguishable from correctly formatted RSA blocks. Thus, when a server receives an incorrectly formatted RSA block, it should generate a random 48-byte value and proceed using it as the premaster secret. Thus, the server will act identically whether the received RSA block is correctly encoded or not.
[PKCS1B] defines a newer version of PKCS#1 encoding that is more secure against the Bleichenbacher attack. However, for maximal compatibility with TLS 1.0, TLS 1.1 retains the original encoding. No variants of the Bleichenbacher attack
are known to exist provided that the above recommendations are followed.
Implementation Note: Public-key-encrypted data is represented as an
opaque vector <0..2^16-1> (see Section 4.7).
Thus, the RSA-encrypted PreMasterSecret in a
ClientKeyExchange is preceded by two length
bytes. These bytes are redundant in the case of
RSA because the EncryptedPreMasterSecret is the
only data in the ClientKeyExchange and its
length can therefore be unambiguously
determined. The SSLv3 specification was not
clear about the encoding of public-key-encrypted
data, and therefore many SSLv3 implementations
do not include the length bytes, encoding the
RSA encrypted data directly in the
ClientKeyExchange message.
This specification requires correct encoding of the EncryptedPreMasterSecret complete with length bytes. The resulting PDU is incompatible with many SSLv3 implementations. Implementors upgrading from SSLv3 must modify their implementations to generate and accept the correct encoding. Implementors who wish to be compatible with both SSLv3 and TLS should make their implementation's behavior dependent on the protocol version.
Implementation Note: It is now known that remote timing-based attacks on SSL are possible, at least when the client and server are on the same LAN. Accordingly, implementations that use static RSA keys SHOULD use RSA blinding or some other anti-timing technique, as described in [TIMING].
Note: The version number in the PreMasterSecret MUST be the version offered by the client in the ClientHello, not the version negotiated for the connection. This feature is designed to prevent rollback attacks. Unfortunately, many implementations use the negotiated version instead, and therefore checking the version number may lead to failure to interoperate with such incorrect client implementations. Client implementations, MUST and Server implementations MAY, check the version number. In practice, since the TLS handshake MACs prevent downgrade and no good attacks are known on those MACs, ambiguity is not considered a serious security risk. Note that if servers choose to check the version number, they should randomize the
PreMasterSecret in case of error, rather than generate an alert, in order to avoid variants on the Bleichenbacher attack. [KPR03]
Meaning of this message:
This structure conveys the client's Diffie-Hellman public value (Yc) if it was not already included in the client's certificate. The encoding used for Yc is determined by the enumerated PublicValueEncoding. This structure is a variant of the client key exchange message and not a message in itself.
Structure of this message:
enum { implicit, explicit } PublicValueEncoding;
implicit
If the client certificate already contains a suitable Diffie-
Hellman key, then Yc is implicit and does not need to be sent
again. In this case, the client key exchange message will be
sent, but it MUST be empty.
explicit
Yc needs to be sent.
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: opaque dh_Yc<1..2^16-1>;
} dh_public;
} ClientDiffieHellmanPublic;
dh_Yc
The client's Diffie-Hellman public value (Yc).
When this message will be sent:
This message is used to provide explicit verification of a client certificate. This message is only sent following a client certificate that has signing capability (i.e., all certificates except those containing fixed Diffie-Hellman parameters). When sent, it MUST immediately follow the client key exchange message.
Structure of this message:
struct {
Signature signature;
} CertificateVerify;
The Signature type is defined in 7.4.3.
CertificateVerify.signature.md5_hash
MD5(handshake_messages);
CertificateVerify.signature.sha_hash
SHA(handshake_messages);
Here handshake_messages refers to all handshake messages sent or received starting at client hello up to but not including this message, including the type and length fields of the handshake messages. This is the concatenation of all the Handshake structures, as defined in 7.4, exchanged thus far.
When this message will be sent:
A finished message is always sent immediately after a change cipher spec message to verify that the key exchange and authentication processes were successful. It is essential that a change cipher spec message be received between the other handshake messages and the Finished message.
Meaning of this message:
The finished message is the first protected with the just- negotiated algorithms, keys, and secrets. Recipients of finished messages MUST verify that the contents are correct. Once a side has sent its Finished message and received and validated the Finished message from its peer, it may begin to send and receive application data over the connection.
struct {
opaque verify_data[12];
} Finished;
verify_data
PRF(master_secret, finished_label, MD5(handshake_messages) +
SHA-1(handshake_messages)) [0..11];
finished_label
For Finished messages sent by the client, the string "client
finished". For Finished messages sent by the server, the
string "server finished".
handshake_messages
All of the data from all messages in this handshake (not
including any HelloRequest messages) up to but not including
this message. This is only data visible at the handshake
layer and does not include record layer headers. This is the
concatenation of all the Handshake structures, as defined in
7.4, exchanged thus far.
It is a fatal error if a finished message is not preceded by a change cipher spec message at the appropriate point in the handshake.
The value handshake_messages includes all handshake messages starting at client hello up to, but not including, this finished message. This may be different from handshake_messages in Section 7.4.8 because it would include the certificate verify message (if sent). Also, the handshake_messages for the finished message sent by the client will be different from that for the finished message sent by the server, because the one that is sent second will include the prior one.
Note: Change cipher spec messages, alerts, and any other record types are not handshake messages and are not included in the hash computations. Also, Hello Request messages are omitted from handshake hashes.
In order to begin connection protection, the TLS Record Protocol requires specification of a suite of algorithms, a master secret, and the client and server random values. The authentication, encryption, and MAC algorithms are determined by the cipher_suite selected by the server and revealed in the server hello message. The compression algorithm is negotiated in the hello messages, and the random values are exchanged in the hello messages. All that remains is to calculate the master secret.
For all key exchange methods, the same algorithm is used to convert the pre_master_secret into the master_secret. The pre_master_secret should be deleted from memory once the master_secret has been computed.
master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random + ServerHello.random)
[0..47];
The master secret is always exactly 48 bytes in length. The length of the premaster secret will vary depending on key exchange method.
When RSA is used for server authentication and key exchange, a 48- byte pre_master_secret is generated by the client, encrypted under the server's public key, and sent to the server. The server uses its private key to decrypt the pre_master_secret. Both parties then convert the pre_master_secret into the master_secret, as specified above.
RSA digital signatures are performed using PKCS #1 [PKCS1] block type 1. RSA public key encryption is performed using PKCS #1 block type 2.
A conventional Diffie-Hellman computation is performed. The negotiated key (Z) is used as the pre_master_secret, and is converted into the master_secret, as specified above. Leading bytes of Z that contain all zero bits are stripped before it is used as the pre_master_secret.
Note: Diffie-Hellman parameters are specified by the server and may be either ephemeral or contained within the server's certificate.
In the absence of an application profile standard specifying otherwise, a TLS compliant application MUST implement the cipher suite TLS_RSA_WITH_3DES_EDE_CBC_SHA.
Application data messages are carried by the Record Layer and are fragmented, compressed, and encrypted based on the current connection state. The messages are treated as transparent data to the record layer.
Security issues are discussed throughout this memo, especially in Appendices D, E, and F.
This document describes a number of new registries that have been created by IANA. We recommended that they be placed as individual registries items under a common TLS category.
Section 7.4.3 describes a TLS ClientCertificateType Registry to be maintained by the IANA, defining a number of such code point identifiers. ClientCertificateType identifiers with values in the range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards Action. Values from the range 64-223 (decimal) inclusive are assigned via [RFC2434] Specification Required. Identifier values from 224-255 (decimal) inclusive are reserved for RFC 2434 Private Use. The registry will initially be populated with the values in this document, Section 7.4.4.
Section A.5 describes a TLS Cipher Suite Registry to be maintained by the IANA, and it defines a number of such cipher suite identifiers. Cipher suite values with the first byte in the range 0-191 (decimal) inclusive are assigned via RFC 2434 Standards Action. Values with the first byte in the range 192-254 (decimal) are assigned via RFC 2434 Specification Required. Values with the first byte 255 (decimal) are reserved for RFC 2434 Private Use. The registry will initially be populated with the values from Section A.5 of this document, [TLSAES], and from Section 3 of [TLSKRB].
Section 6 requires that all ContentType values be defined by RFC 2434 Standards Action. IANA has created a TLS ContentType registry, initially populated with values from Section 6.2.1 of this document. Future values MUST be allocated via Standards Action as described in [RFC2434].
Section 7.2.2 requires that all Alert values be defined by RFC 2434 Standards Action. IANA has created a TLS Alert registry, initially populated with values from Section 7.2 of this document and from Section 4 of [TLSEXT]. Future values MUST be allocated via Standards Action as described in [RFC2434].
Section 7.4 requires that all HandshakeType values be defined by RFC 2434 Standards Action. IANA has created a TLS HandshakeType registry, initially populated with values from Section 7.4 of this document and from Section 2.4 of [TLSEXT]. Future values MUST be allocated via Standards Action as described in [RFC2434].
This section describes protocol types and constants.
struct {
uint8 major, minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 2 }; /* TLS v1.1 */
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 IV[CipherSpec.block_length];
opaque content[TLSCompressed.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),
decryption_failed(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),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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;
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;
opaque ASN.1Cert<2^24-1>;
struct {
ASN.1Cert certificate_list<0..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;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
case rsa:
ServerRSAParams params;
};
} ServerParams;
struct {
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),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} 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;
struct {
opaque verify_data[12];
} Finished;
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 1.1.
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_