The Transport Layer Security (TLS) Protocol v1.2

TLS

The Transport Layer Security (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

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., AES [AES], 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-1, 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], DSA [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.

1.1. Requirements Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 [REQ].

1.2. Major Differences from TLS 1.1

This document is a revision of the TLS 1.1 [TLS1.1] protocol which contains improved flexibility, particularly for negotiation of cryptographic algorithms. The major changes are:

  • The MD5/SHA-1 combination in the pseudorandom function (PRF) has been replaced with cipher-suite-specified PRFs. All cipher suites in this document use P_SHA256.
  • The MD5/SHA-1 combination in the digitally-signed element has been replaced with a single hash. Signed elements now include a field that explicitly specifies the hash algorithm used.
  • Substantial cleanup to the client’s and server’s ability to specify which hash and signature algorithms they will accept. Note that this also relaxes some of the constraints on signature
  • and hash algorithms from previous versions of TLS.
  • Addition of support for authenticated encryption with additional data modes.
  • TLS Extensions definition and AES Cipher Suites were merged in from external [TLSEXT] and [TLSAES].
  • Tighter checking of EncryptedPreMasterSecret version numbers.
  • Tightened up a number of requirements.
  • Verify_data length now depends on the cipher suite (default is still 12).
  • Cleaned up description of Bleichenbacher/Klima attack defenses.
  • Alerts MUST now be sent in many cases.
  • After a certificate_request, if no certificates are available, clients now MUST send an empty certificate list.
  • TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement cipher suite.
  • Added HMAC-SHA256 cipher suites.
  • Removed IDEA and DES cipher suites. They are now deprecated and will be documented in a separate document.
  • Support for the SSLv2 backward-compatible hello is now a MAY, not a SHOULD, with sending it a SHOULD NOT. Support will probably become a SHOULD NOT in the future.
  • Added limited “fall-through” to the presentation language to allow multiple case arms to have the same encoding.
  • Added an Implementation Pitfalls sections
  • The usual clarifications and editorial work.

2. Goals

The goals of the TLS protocol, in order of priority, are as follows:

  1. Cryptographic security: TLS should be used to establish a secure connection between two parties.
  2. Interoperability: Independent programmers should be able to develop applications utilizing TLS that can successfully exchange cryptographic parameters without knowledge of one another’s code.
  3. Extensibility: TLS seeks to provide a framework into which new public key and bulk encryption methods can be incorporated as necessary. This will also accomplish two sub-goals: preventing the need to create a new protocol (and risking the introduction of possible new weaknesses) and avoiding the need to implement an entire new security library.
  4. Relative efficiency: Cryptographic operations tend to be highly CPU intensive, particularly public key operations. For this reason, the TLS protocol has incorporated an optional session caching scheme to reduce the number of connections that need to be established from scratch. Additionally, care has been taken to reduce network activity.

3. Goals of This Document

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 the various versions of TLS and SSL 3.0 do not interoperate (although each protocol incorporates a mechanism by which an implementation can back down to 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.

4. Presentation Language

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.

4.1. Basic Block Size

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 byte stream, 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.

4.2. Miscellaneous

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.

4.3. Vectors

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, which is 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 */

4.4. Numbers

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 byte (big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060.

Note that in some cases (e.g., DH parameters) it is necessary to represent integers as opaque vectors. In such cases, they are represented as unsigned integers (i.e., leading zero octets are not required even if the most significant bit is set).

4.5. Enumerateds

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;

An enumerated occupies 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;

4.6. Constructed Types

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.

4.6.1. Variants

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. Case arms have limited fall-through: if two case arms follow in immediate succession with no fields in between, then they both contain the same fields. Thus, in the example below, “orange” and “banana” both contain V2. Note that this is a new piece of syntax in TLS 1.2.

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;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
case e3: case e4: Te3;
....
case en: Ten;
} [[fv]];
} [[Tv]];

For example:

enum { apple, orange, banana } 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:
case banana:
V2; /* VariantBody, tag = orange or banana */
} variant_body; /* optional label on variant */
} VariantRecord;

4.7. Cryptographic Attributes

The five cryptographic operations — digital signing, stream cipher encryption, block cipher encryption, authenticated encryption with additional data (AEAD) encryption, and public key encryption — are designated digitally-signed, stream-ciphered, block-ciphered, aead-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).

A digitally-signed element is encoded as a struct DigitallySigned:

struct {
SignatureAndHashAlgorithm algorithm;
opaque signature<0..2^16-1>;
} DigitallySigned;

The algorithm field specifies the algorithm used (see Section 7.4.1.4.1 for the definition of this field). Note that the introduction of the algorithm field is a change from previous versions. The signature is a digital signature using those algorithms over the contents of the element. The contents themselves
do not appear on the wire but are simply calculated. The length of the signature is specified by the signing algorithm and key.

In RSA signing, the opaque vector contains the signature generated using the RSASSA-PKCS1-v1_5 signature scheme defined in [PKCS1]. As discussed in [PKCS1], the DigestInfo MUST be DER-encoded [X680] [X690]. For hash algorithms without parameters (which includes SHA-1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but implementations MUST accept both without parameters and with NULL parameters. Note that earlier versions of TLS used a different RSA signature scheme that did not include a DigestInfo encoding.

In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital Signing Algorithm with no additional hashing. This produces two values, r and s. The DSA signature is an opaque vector, as above, the contents of which are the DER encoding of:

Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}

Note: In current terminology, DSA refers to the Digital Signature Algorithm and DSS refers to the NIST standard. In the original SSL and TLS specs, “DSS” was used universally. This document uses “DSA”
to refer to the algorithm, “DSS” to refer to the standard, and it uses “DSS” in the code point definitions for historical continuity.

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 AEAD encryption, the plaintext is simultaneously encrypted and integrity protected. The input may be of any length, and aead-ciphered output is generally larger than the input in order to accommodate the integrity check value.

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 encryption algorithm and key.

RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme defined in [PKCS1].

In the following example

stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque {
uint8 field3<0..255>;
uint8 field4;
};
} UserType;

The contents of the inner struct (field3 and field4) are used as input for the signature/hash 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 signature and hash algorithm, plus two bytes for the length of the signature, plus the length of the output of the signing algorithm. The length of the signature is known because the algorithm and key used for the signing are known prior to encoding or decoding this structure.

4.8. Constants

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 */

5. HMAC and the Pseudorandom Function

The TLS record layer uses a keyed Message Authentication Code (MAC) to protect message integrity. The cipher suites defined in this document use a construction known as HMAC, described in [HMAC], which is based on a hash function. Other cipher suites MAY define their own MAC constructions, if needed.

In addition, a construction is required to do expansion of secrets into blocks of data for the purposes of key generation or validation. This pseudorandom function (PRF) takes as input a secret, a seed, and an identifying label and produces an output of arbitrary length.

In this section, we define one PRF, based on HMAC. This PRF with the SHA-256 hash function is used for all cipher suites defined in this document and in TLS documents published prior to this document when
TLS 1.2 is negotiated. New cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger standard hash function.

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 necessary to produce the required quantity of data. For example, if P_SHA256 is being used to create 80 bytes of data, it will have to be iterated three times
(through A(3)), creating 96 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 80 bytes of output data.

TLS’s PRF is created by applying P_hash to the secret as:

PRF(secret, label, seed) = P_<hash>(secret, 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

6. The TLS Record Protocol

The TLS Record Protocol is a layered protocol. At each layer, messages may include fields for length, description, and content. The Record Protocol takes messages to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, and transmits the result. Received data is decrypted, verified, decompressed, reassembled, and then delivered to higher-level clients.

Four protocols that use the record protocol are described in this document: the handshake protocol, the alert protocol, the change cipher spec protocol, and the application data protocol. In order to allow extension of the TLS protocol, additional record content types can be supported by the record protocol. New record content type values are assigned by IANA in the TLS Content Type Registry as described in Section 12. Implementations MUST NOT send record types not defined in this
document unless negotiated by some extension. If a TLS implementation receives an unexpected record type, it MUST send an unexpected_message alert.

Any protocol designed for use over TLS must be carefully designed to deal with all possible attacks against it. As a practical matter, this means that the protocol designer must be aware of what security
properties TLS does and does not provide and cannot safely rely on the latter.

Note in particular that type and length of a record are not protected by encryption. If this information is itself sensitive, application designers may wish to take steps (padding, cover traffic) to minimize information leakage.

6.1. Connection States

A TLS connection state is the operating environment of the TLS Record Protocol. It specifies a compression algorithm, an encryption algorithm, and a MAC algorithm. In addition, the parameters for these algorithms are known: the MAC key and the bulk encryption keys for the connection in both the read and the write directions. Logically, there are always four connection states outstanding: the
current read and write states, and the pending read and write states. All records are processed under the current read and write states. The security parameters for the pending states can be set by the TLS Handshake Protocol, and the ChangeCipherSpec can selectively make either of the pending states current, in which case the appropriate current state is disposed of and replaced with the pending state; the pending state is then reinitialized to an empty state. It is illegal to make a state that has not been initialized with security parameters a current state. The initial current state always specifies that no encryption, compression, or MAC will be used. The security parameters for a TLS Connection read and write state are set by providing the following values:

connection end

Whether this entity is considered the “client” or the “server” in this connection.

PRF algorithm

An algorithm used to generate keys from the master secret (see Sections 5 and 6.3).
bulk encryption algorithm An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, whether it is a block, stream, or AEAD cipher, the block size of the cipher (if appropriate), and the lengths of explicit and implicit initialization vectors (or nonces).

MAC algorithm

An algorithm to be used for message authentication. This specification includes the size of the value returned by the MAC algorithm.

Compression algorithm

An algorithm to be used for data compression. This specification must include all information the algorithm requires to do compression.

Master secret

A 48-byte secret shared between the two peers in the connection.

Client random

A 32-byte value provided by the client.

Server random

A 32-byte value provided by the server.

These parameters are defined in the presentation language as:

enum { server, client } ConnectionEnd;

enum { tls_prf_sha256 } PRFAlgorithm;

enum { null, rc4, 3des, aes }
BulkCipherAlgorithm;

enum { stream, block, aead } CipherType;

enum { null, hmac_md5, hmac_sha1, hmac_sha256,
hmac_sha384, hmac_sha512} MACAlgorithm;

enum { null(0), (255) } CompressionMethod;

/* The algorithms specified in CompressionMethod, PRFAlgorithm,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
MACAlgorithm mac_algorithm;
uint8 mac_length;
uint8 mac_key_length;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;

The record layer will use the security parameters to generate the following six items (some of which are not required by all ciphers, and are thus empty):

client write MAC key
server write MAC key
client write encryption key
server write encryption key
client write IV
server write IV

The client write parameters are used by the server when receiving and processing records and vice versa. The algorithm used for generating these items from the security parameters is described in Section 6.3.

Once the security parameters have been set and the keys have been generated, the connection states can be instantiated by making them the current states. These current states MUST be updated for each
record processed. Each connection state includes the following elements:

Compression state

The current state of the compression algorithm.

Cipher state

The current state of the encryption algorithm. This will consist of the scheduled key for that connection. For stream ciphers, this will also contain whatever state information is necessary to allow the stream to continue to encrypt or decrypt data.

MAC key

The MAC key for this connection, as generated above.

Sequence number

Each connection state contains a sequence number, which is maintained separately for read and write states. The sequence number MUST be set to zero whenever a connection state is made the active state. Sequence numbers are of type uint64 and may not exceed 2^64-1. Sequence numbers do not wrap. If a TLS implementation would need to wrap a sequence number, it must renegotiate instead. A sequence number is incremented after each record: specifically, the first record transmitted under a particular connection state MUST use sequence number 0.

6.2. Record Layer

The TLS record layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size.

6.2.1. Fragmentation

The record layer fragments information blocks into TLSPlaintext records carrying data in chunks of 2^14 bytes or less. Client message boundaries are not preserved in the record layer (i.e., multiple client messages of the same ContentType MAY be coalesced into a single TLSPlaintext record, or a single message MAY be fragmented across several records).

struct {
uint8 major;
uint8 minor;
} ProtocolVersion;

enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;

struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;

type

The higher-level protocol used to process the enclosed fragment.

version

The version of the protocol being employed. This document describes TLS Version 1.2, which uses the version { 3, 3 }. The version value 3.3 is historical, deriving from the use of {3, 1} for TLS 1.0. (See Appendix A.1.) Note that a client that supports multiple versions of TLS may not know what version will
be employed before it receives the ServerHello. See Appendix E for discussion about what record layer version number should be employed for ClientHello.

length

The length (in bytes) of the following TLSPlaintext.fragment. The length MUST NOT exceed 2^14.

fragment

The application data. This data is transparent and treated as an independent block to be dealt with by the higher-level protocol specified by the type field.

Implementations MUST NOT send zero-length fragments of Handshake, Alert, or ChangeCipherSpec content types. Zero-length fragments of Application data MAY be sent as they are potentially useful as a
traffic analysis countermeasure.

Note: Data of different TLS record layer content types MAY be interleaved. Application data is generally of lower precedence for transmission than other content types. However, records MUST be delivered to the network in the same order as they are protected by the record layer. Recipients MUST receive and process interleaved application layer traffic during handshakes subsequent to the first one on a connection.

6.2.2. Record Compression and Decompression

All records are compressed using the compression algorithm defined in the current session state. There is always an active compression algorithm; however, initially it is defined as CompressionMethod.null. The compression algorithm translates a TLSPlaintext structure into a TLSCompressed structure. Compression functions are initialized with default state information whenever a connection state is made active. [RFC3749] describes compression algorithms for TLS.

Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters a TLSCompressed.fragment that would decompress to a length in excess of 2^14 bytes, it MUST report a fatal decompression failure error.

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

length

The length (in bytes) of the following TLSCompressed.fragment. The length MUST NOT exceed 2^14 + 1024.

fragment

The compressed form of TLSPlaintext.fragment.

Note: A CompressionMethod.null operation is an identity operation; no fields are altered.

Implementation note: Decompression functions are responsible for ensuring that messages cannot cause internal buffer overflows.

6.2.3. Record Payload Protection

The encryption and MAC functions translate a TLSCompressed structure into a TLSCiphertext. The decryption functions reverse the process. The MAC of the record also includes a sequence number so that missing, extra, or repeated messages are detectable.

struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (SecurityParameters.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
case aead: GenericAEADCipher;
} fragment;
} TLSCiphertext;

type

The type field is identical to TLSCompressed.type.

version

The version field is identical to TLSCompressed.version.

length

The length (in bytes) of the following TLSCiphertext.fragment. The length MUST NOT exceed 2^14 + 2048.

fragment

The encrypted form of TLSCompressed.fragment, with the MAC.

6.2.3.1. Null or Standard Stream Cipher

Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6) convert TLSCompressed.fragment structures to and from stream TLSCiphertext.fragment structures.

stream-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
} GenericStreamCipher;

The MAC is generated as:

MAC(MAC_write_key, seq_num +
TLSCompressed.type +
TLSCompressed.version +
TLSCompressed.length +
TLSCompressed.fragment);

where “+” denotes concatenation.

seq_num

The sequence number for this record.

MAC

The MAC algorithm specified by SecurityParameters.mac_algorithm.

Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream
cipher state from the end of one record is simply used on the subsequent packet. If the cipher suite is TLS_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not
encrypted, and the MAC size is zero, implying that no MAC is used). For both null and stream ciphers, TLSCiphertext.length is TLSCompressed.length plus SecurityParameters.mac_length.

6.2.3.2. CBC Block Cipher

For block ciphers (such as 3DES or AES), the encryption and MAC functions convert TLSCompressed.fragment structures to and from block TLSCiphertext.fragment structures.

struct {
opaque IV[SecurityParameters.record_iv_length];
block-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
};
} GenericBlockCipher;

The MAC is generated as described in Section 6.2.3.1.

IV

The Initialization Vector (IV) SHOULD be chosen at random, and MUST be unpredictable. Note that in versions of TLS prior to 1.1, there was no IV field, and the last ciphertext block of the previous record (the “CBC residue”) was used as the IV. This was changed to prevent the attacks described in [CBCATT]. For block ciphers, the IV length is of length SecurityParameters.record_iv_length, which is equal to the
SecurityParameters.block_size.

padding

Padding that is added to force the length of the plaintext to be an integral multiple of the block cipher’s block length. The padding MAY be any length up to 255 bytes, as long as it results
in the TLSCiphertext.length being an integral multiple of the block length. Lengths longer than necessary might be desirable to frustrate attacks on a protocol that are based on analysis of the lengths of exchanged messages. Each uint8 in the padding data vector MUST be filled with the padding length value. The receiver MUST check this padding and MUST use the bad_record_mac alert to
indicate padding errors.

padding_length

The padding length MUST be such that the total size of the GenericBlockCipher structure is a multiple of the cipher’s block length. Legal values range from zero to 255, inclusive. This length specifies the length of the padding field exclusive of the padding_length field itself.

The encrypted data length (TLSCiphertext.length) is one more than the sum of SecurityParameters.block_length, TLSCompressed.length, SecurityParameters.mac_length, and padding_length.

Example: If the block length is 8 bytes, the content length (TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes, then the length before padding is 82 bytes (this does not include the IV. Thus, the padding length modulo 8 must be equal to 6 in order to make the total length an even multiple of 8 bytes (the block length). The padding length can be 6, 14, 22, and so on, through 254. If the padding length were the minimum necessary, 6, the padding would be 6 bytes, each containing the value 6. Thus, the last 8 octets of the GenericBlockCipher before block encryption would be xx 06 06 06 06 06 06 06, where xx is the last octet of the MAC.

Note: With block ciphers in CBC mode (Cipher Block Chaining), it is critical that the entire plaintext of the record be known before any ciphertext is transmitted. Otherwise, it is possible for the attacker to mount the attack described in [CBCATT].

Implementation note: Canvel et al. [CBCTIME] have demonstrated a timing attack on CBC padding based on the time required to compute the MAC. In order to defend against this attack, implementations MUST ensure that record processing time is essentially the same whether or not the padding is correct. In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC. This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment,
but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal.

6.2.3.3. AEAD Ciphers

For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function converts TLSCompressed.fragment structures to and from AEAD TLSCiphertext.fragment structures.

struct {
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSCompressed.length];
};
} GenericAEADCipher;

AEAD ciphers take as input a single key, a nonce, a plaintext, and “additional data” to be included in the authentication check, as described in Section 2.1 of [AEAD]. The key is either the client_write_key or the server_write_key. No MAC key is used.

Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD operation is constructed, and what is the length of the GenericAEADCipher.nonce_explicit part. In many cases, it is appropriate to use the partially implicit nonce technique described in Section 3.2.1 of [AEAD]; with record_iv_length being the length of the explicit part. In this case, the implicit part SHOULD be derived from key_block as client_write_iv and server_write_iv (as described in Section 6.3), and the explicit part is included in GenericAEAEDCipher.nonce_explicit.

The plaintext is the TLSCompressed.fragment.

The additional authenticated data, which we denote as additional_data, is defined as follows:

additional_data = seq_num + TLSCompressed.type +
TLSCompressed.version + TLSCompressed.length;

where “+” denotes concatenation.

The aead_output consists of the ciphertext output by the AEAD encryption operation. The length will generally be larger than TLSCompressed.length, but by an amount that varies with the AEAD cipher. Since the ciphers might incorporate padding, the amount of overhead could vary with different TLSCompressed.length values. Each AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes. Symbolically,

AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext, additional_data)

In order to decrypt and verify, the cipher takes as input the key, nonce, the “additional_data”, and the AEADEncrypted value. The output is either the plaintext or an error indicating that the decryption failed. There is no separate integrity check. That is:

TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce,
AEADEncrypted,
additional_data)

If the decryption fails, a fatal bad_record_mac alert MUST be generated.

6.3. Key Calculation

The Record Protocol requires an algorithm to generate keys required
by the current connection state (see Appendix A.6) from the security
parameters provided by the handshake protocol.

 

The master secret is expanded into a sequence of secure bytes, which
is then split to a client write MAC key, a server write MAC key, a
client write encryption key, and a server write encryption key. Each
of these is generated from the byte sequence in that order. Unused
values are empty. Some AEAD ciphers may additionally require a
client write IV and a server write IV (see Section 6.2.3.3).

When keys and MAC keys are generated, the master secret is used as an
entropy source.

To generate the key material, compute

key_block = PRF(SecurityParameters.master_secret,
“key expansion”,
SecurityParameters.server_random +
SecurityParameters.client_random);

until enough output has been generated. Then, the key_block is
partitioned as follows:

client_write_MAC_key[SecurityParameters.mac_key_length]
server_write_MAC_key[SecurityParameters.mac_key_length]
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.fixed_iv_length]
server_write_IV[SecurityParameters.fixed_iv_length]

Currently, the client_write_IV and server_write_IV are only generated
for implicit nonce techniques as described in Section 3.2.1 of
[AEAD].

Implementation note: The currently defined cipher suite which
requires the most material is AES_256_CBC_SHA256. It requires 2 x 32
byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key
material.

7. The TLS Handshaking Protocols

TLS has three subprotocols that are used to allow peers to agree upon
security parameters for the record layer, to authenticate themselves,
to instantiate negotiated security parameters, and to report error
conditions to each other.

The Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.

peer certificate
X509v3 [PKIX] certificate of the peer. This element of the state
may be null.

compression method
The algorithm used to compress data prior to encryption.

cipher spec
Specifies the pseudorandom function (PRF) used to generate keying
material, the bulk data encryption algorithm (such as null, AES,
etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines
cryptographic attributes such as the mac_length. (See Appendix
A.6 for formal definition.)

master secret
48-byte secret shared between the client and server.

is resumable
A flag indicating whether the session can be used to initiate new
connections.

These items are then used to create security parameters for use by
the record layer when protecting application data. Many connections
can be instantiated using the same session through the resumption
feature of the TLS Handshake Protocol.

7.1. Change Cipher Spec Protocol

The change cipher spec protocol exists to signal transitions in
ciphering strategies. The protocol consists of a single message,
which is encrypted and compressed under the current (not the pending)
connection state. The message consists of a single byte of value 1.

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

The ChangeCipherSpec message is sent by both the client and the
server to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the record layer to
immediately copy the read pending state into the read current state.
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See Section 6.1.) The ChangeCipherSpec message is sent during the
handshake after the security parameters have been agreed upon, but
before the verifying Finished message is sent.

Note: If a rehandshake occurs while data is flowing on a connection,
the communicating parties may continue to send data using the old
CipherSpec. However, once the ChangeCipherSpec has been sent, the
new CipherSpec MUST be used. The first side to send the
ChangeCipherSpec does not know that the other side has finished
computing the new keying material (e.g., if it has to perform a
time-consuming public key operation). Thus, a small window of time,
during which the recipient must buffer the data, MAY exist. In
practice, with modern machines this interval is likely to be fairly
short.

7.2. Alert Protocol

One of the content types supported by the TLS record layer is the
alert type. Alert messages convey the severity of the message
(warning or fatal) and a description of the alert. Alert messages
with a level of fatal result in the immediate termination of the
connection. In this case, other connections corresponding to the
session may continue, but the session identifier MUST be invalidated,
preventing the failed session from being used to establish new
connections. Like other messages, alert messages are encrypted and
compressed, as specified by the current connection state.

enum { warning(1), fatal(2), (255) } AlertLevel;

enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),

 

Dierks & Rescorla Standards Track [Page 28]

RFC 5246 TLS August 2008
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;

struct {
AlertLevel level;
AlertDescription description;
} Alert;

7.2.1. Closure Alerts

The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages.

close_notify
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.

Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.

Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.

If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding close_notify. No part
of this standard should be taken to dictate the manner in which a
usage profile for TLS manages its data transport, including when
connections are opened or closed.

Note: It is assumed that closing a connection reliably delivers
pending data before destroying the transport.

7.2.2. Error Alerts

Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other
party. Upon transmission or receipt of a fatal alert message, both
parties immediately close the connection. Servers and clients MUST
forget any session-identifiers, keys, and secrets associated with a
failed connection. Thus, any connection terminated with a fatal
alert MUST NOT be resumed.

Whenever an implementation encounters a condition which is defined as
a fatal alert, it MUST send the appropriate alert prior to closing
the connection. For all errors where an alert level is not
explicitly specified, the sending party MAY determine at its
discretion whether to treat this as a fatal error or not. If the
implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the
fatal alert level.

If an alert with a level of warning is sent and received, generally
the connection can continue normally. If the receiving party decides
not to proceed with the connection (e.g., after having received a
no_renegotiation alert that it is not willing to accept), it SHOULD
send a fatal alert to terminate the connection. Given this, the
sending party cannot, in general, know how the receiving party will
behave. Therefore, warning alerts are not very useful when the
sending party wants to continue the connection, and thus are
sometimes omitted. For example, if a peer decides to accept an
expired certificate (perhaps after confirming this with the user) and
wants to continue the connection, it would not generally send a
certificate_expired alert.

The following error alerts are defined:

unexpected_message
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn’t an
even multiple of the block length, or its padding values, when
checked, weren’t correct. This message is always fatal and should
never be observed in communication between proper implementations
(except when messages were corrupted in the network).

decryption_failed_RESERVED
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode [CBCATT]. It MUST
NOT be sent by compliant implementations.

record_overflow
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).

decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal and should never be observed in communication between proper
implementations.

handshake_failure
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.

no_certificate_RESERVED
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.

bad_certificate
A certificate was corrupt, contained signatures that did not
verify correctly, etc.

unsupported_certificate
A certificate was of an unsupported type.

certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.

certificate_unknown
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.

illegal_parameter
A field in the handshake was out of range or inconsistent with
other fields. This message is always fatal.

unknown_ca
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn’t be matched with a known, trusted CA. This
message is always fatal.

access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.

decode_error
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).

decrypt_error
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This message is always fatal.

export_restriction_RESERVED
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.

protocol_version
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This message is always
fatal.
insufficient_security
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.

internal_error
An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue. This message is always fatal.

user_canceled
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed
by a close_notify. This message is generally a warning.

no_renegotiation
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Either
of these would normally lead to renegotiation; when that is not
appropriate, the recipient should respond with this alert. At
that point, the original requester can decide whether to proceed
with the connection. One case where this would be appropriate is
where a server has spawned a process to satisfy a request; the
process might receive security parameters (key length,
authentication, etc.) at startup, and it might be difficult to
communicate changes to these parameters after that point. This
message is always a warning.

unsupported_extension
sent by clients that receive an extended server hello containing
an extension that they did not put in the corresponding client
hello. This message is always fatal.

New Alert values are assigned by IANA as described in Section 12.

7.3. Handshake Protocol Overview

The cryptographic parameters of the session state are produced by the
TLS Handshake Protocol, which operates on top of the TLS record
layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and use public-key encryption
techniques to generate shared secrets.
The TLS Handshake Protocol involves the following steps:

– Exchange hello messages to agree on algorithms, exchange random
values, and check for session resumption.

– Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.

– Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.

– Generate a master secret from the premaster secret and exchanged
random values.

– Provide security parameters to the record layer.

– Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.

Note that higher layers should not be overly reliant on whether TLS
always negotiates the strongest possible connection between two
peers. There are a number of ways in which a man-in-the-middle
attacker can attempt to make two entities drop down to the least
secure method they support. The protocol has been designed to
minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service
runs on, or attempt to get the peers to negotiate an unauthenticated
connection. The fundamental rule is that higher levels must be
cognizant of what their security requirements are and never transmit
information over a channel less secure than what they require. The
TLS protocol is secure in that any cipher suite offers its promised
level of security: if you negotiate 3DES with a 1024-bit RSA key
exchange with a host whose certificate you have verified, you can
expect to be that secure.

These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a ClientHello message to
which the server must respond with a ServerHello message, or else a
fatal error will occur and the connection will fail. The ClientHello
and ServerHello are used to establish security enhancement
capabilities between client and server. The ClientHello and
ServerHello establish the following attributes: Protocol Version,
Session ID, Cipher Suite, and Compression Method. Additionally, two
random values are generated and exchanged: ClientHello.random and
ServerHello.random.
The actual key exchange uses up to four messages: the server
Certificate, the ServerKeyExchange, the client Certificate, and the
ClientKeyExchange. New key exchange methods can be created by
specifying a format for these messages and by defining the use of the
messages to allow the client and server to agree upon a shared
secret. This secret MUST be quite long; currently defined key
exchange methods exchange secrets that range from 46 bytes upwards.

Following the hello messages, the server will send its certificate in
a Certificate message if it is to be authenticated. Additionally, a
ServerKeyExchange message may be sent, if it is required (e.g., if
the server has no certificate, or if its certificate is for signing
only). If the server is authenticated, it may request a certificate
from the client, if that is appropriate to the cipher suite selected.
Next, the server will send the ServerHelloDone message, indicating
that the hello-message phase of the handshake is complete. The
server will then wait for a client response. If the server has sent
a CertificateRequest message, the client MUST send the Certificate
message. The ClientKeyExchange message is now sent, and the content
of that message will depend on the public key algorithm selected
between the ClientHello and the ServerHello. If the client has sent
a certificate with signing ability, a digitally-signed
CertificateVerify message is sent to explicitly verify possession of
the private key in the certificate.

At this point, a ChangeCipherSpec message is sent by the client, and
the client copies the pending Cipher Spec into the current Cipher
Spec. The client then immediately sends the Finished message under
the new algorithms, keys, and secrets. In response, the server will
send its own ChangeCipherSpec message, transfer the pending to the
current Cipher Spec, and send its Finished message under the new
Cipher Spec. At this point, the handshake is complete, and the
client and server may begin to exchange application layer data. (See
flow chart below.) Application data MUST NOT be sent prior to the
completion of the first handshake (before a cipher suite other than
TLS_NULL_WITH_NULL_NULL is established).
Client Server

ClientHello ——–>
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<——– ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ——–>
[ChangeCipherSpec]
<——– Finished
Application Data <——-> Application Data

Figure 1. Message flow for a full handshake

* Indicates optional or situation-dependent messages that are not
always sent.

Note: To help avoid pipeline stalls, ChangeCipherSpec is an
independent TLS protocol content type, and is not actually a TLS
handshake message.

When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security
parameters), the message flow is as follows:

The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match.
If a match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both
client and server MUST send ChangeCipherSpec messages and proceed
directly to Finished messages. Once the re-establishment is
complete, the client and server MAY begin to exchange application
layer data. (See flow chart below.) If a Session ID match is not
found, the server generates a new session ID, and the TLS client and
server perform a full handshake.
Client Server

ClientHello ——–>
ServerHello
[ChangeCipherSpec]
<——– Finished
[ChangeCipherSpec]
Finished ——–>
Application Data <——-> Application Data

Figure 2. Message flow for an abbreviated handshake

The contents and significance of each message will be presented in
detail in the following sections.

7.4. Handshake Protocol

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 described only in its first
position. The one message that is not bound by these ordering rules
is the HelloRequest 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 types are assigned by IANA as described in
Section 12.

7.4.1. Hello Messages

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.

7.4.1.1. Hello Request

When this message will be sent:

The HelloRequest message MAY be sent by the server at any time.

Meaning of this message:

HelloRequest is a simple notification that the client should begin
the negotiation process anew. In response, the client should send
a ClientHello message when convenient. This message is not
intended to establish which side is the client or server but
merely to initiate a new negotiation. Servers SHOULD NOT send a
HelloRequest immediately upon the client’s initial connection. It
is the client’s job to send a ClientHello at that time.

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 HelloRequest but
does not receive a ClientHello in response, it may close the
connection with a fatal alert.
After sending a HelloRequest, servers SHOULD NOT repeat the
request until the subsequent handshake negotiation is complete.

Structure of this message:

struct { } HelloRequest;

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.

7.4.1.2. Client Hello

When this message will be sent:

When a client first connects to a server, it is required to send
the ClientHello as its first message. The client can also send a
ClientHello in response to a HelloRequest or on its own initiative
in order to renegotiate the security parameters in an existing
connection.

Structure of this message:

The ClientHello 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, UTC, 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. Note that, for historical reasons, the data
element is named using GMT, the predecessor of the current
worldwide time base, UTC.

random_bytes
28 bytes generated by a secure random number generator.

The ClientHello message includes a variable-length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes to
reuse. The session identifier MAY be from an earlier connection,
this connection, or 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 cipher suite list, passed from the client to the server in the
ClientHello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client’s
preference (favorite choice first). Each cipher suite defines a key
exchange algorithm, a bulk encryption algorithm (including secret key
length), a MAC algorithm, and a PRF. The server will select a cipher
suite or, if no acceptable choices are presented, return a handshake
failure alert and close the connection. If the list contains cipher
suites the server does not recognize, support, or wish to use, the
server MUST ignore those cipher suites, and process the remaining
ones as usual.

uint8 CipherSuite[2]; /* Cryptographic suite selector */

The ClientHello 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..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ClientHello;

TLS allows extensions to follow the compression_methods field in an
extensions block. The presence of extensions can be detected by
determining whether there are bytes following the compression_methods
at the end of the ClientHello. Note that this method of detecting
optional data differs from the normal TLS method of having a
variable-length field, but it is used for compatibility with TLS
before extensions were defined.

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 3.3 (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 is 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.

extensions
Clients MAY request extended functionality from servers by sending
data in the extensions field. The actual “Extension” format is
defined in Section 7.4.1.4.

In the event that a client requests additional functionality using
extensions, and this functionality is not supplied by the server, the
client MAY abort the handshake. A server MUST accept ClientHello
messages both with and without the extensions field, and (as for all
other messages) it MUST check that the amount of data in the message
precisely matches one of these formats; if not, then it MUST send a
fatal “decode_error” alert.

After sending the ClientHello message, the client waits for a
ServerHello message. Any handshake message returned by the server,
except for a HelloRequest, is treated as a fatal error.

7.4.1.3. Server Hello

When this message will be sent:

The server will send this message in response to a ClientHello
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;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
The presence of extensions can be detected by determining whether
there are bytes following the compression_method field at the end of
the 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 3.3. (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. Note that there is no requirement
that the server resume any session even if it had formerly
provided a session_id. Clients MUST be prepared to do a full
negotiation — including negotiating new cipher suites — during
any handshake.

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.

extensions
A list of extensions. Note that only extensions offered by the
client can appear in the server’s list.
7.4.1.4. Hello Extensions

The extension format is:

struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;

enum {
signature_algorithms(13), (65535)
} ExtensionType;

Here:

– “extension_type” identifies the particular extension type.

– “extension_data” contains information specific to the particular
extension type.

The initial set of extensions is defined in a companion document
[TLSEXT]. The list of extension types is maintained by IANA as
described in Section 12.

An extension type MUST NOT appear in the ServerHello unless the same
extension type appeared in the corresponding ClientHello. If a
client receives an extension type in ServerHello that it did not
request in the associated ClientHello, it MUST abort the handshake
with an unsupported_extension fatal alert.

Nonetheless, “server-oriented” extensions may be provided in the
future within this framework. Such an extension (say, of type x)
would require the client to first send an extension of type x in a
ClientHello with empty extension_data to indicate that it supports
the extension type. In this case, the client is offering the
capability to understand the extension type, and the server is taking
the client up on its offer.

When multiple extensions of different types are present in the
ClientHello or ServerHello messages, the extensions MAY appear in any
order. There MUST NOT be more than one extension of the same type.

Finally, note that extensions can be sent both when starting a new
session and when requesting session resumption. Indeed, a client
that requests session resumption does not in general know whether the
server will accept this request, and therefore it SHOULD send the
same extensions as it would send if it were not attempting
resumption.
In general, the specification of each extension type needs to
describe the effect of the extension both during full handshake and
session resumption. Most current TLS extensions are relevant only
when a session is initiated: when an older session is resumed, the
server does not process these extensions in Client Hello, and does
not include them in Server Hello. However, some extensions may
specify different behavior during session resumption.

There are subtle (and not so subtle) interactions that may occur in
this protocol between new features and existing features which may
result in a significant reduction in overall security. The following
considerations should be taken into account when designing new
extensions:

– Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular
features. In general, error alerts should be used for the former,
and a field in the server extension response for the latter.

– Extensions should, as far as possible, be designed to prevent any
attack that forces use (or non-use) of a particular feature by
manipulation of handshake messages. This principle should be
followed regardless of whether the feature is believed to cause a
security problem.

Often the fact that the extension fields are included in the
inputs to the Finished message hashes will be sufficient, but
extreme care is needed when the extension changes the meaning of
messages sent in the handshake phase. Designers and implementors
should be aware of the fact that until the handshake has been
authenticated, active attackers can modify messages and insert,
remove, or replace extensions.

– It would be technically possible to use extensions to change major
aspects of the design of TLS; for example the design of cipher
suite negotiation. This is not recommended; it would be more
appropriate to define a new version of TLS — particularly since
the TLS handshake algorithms have specific protection against
version rollback attacks based on the version number, and the
possibility of version rollback should be a significant
consideration in any major design change.

7.4.1.4.1. Signature Algorithms

The client uses the “signature_algorithms” extension to indicate to
the server which signature/hash algorithm pairs may be used in
digital signatures. The “extension_data” field of this extension
contains a “supported_signature_algorithms” value.
enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;

enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
SignatureAlgorithm;

struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;

SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;

Each SignatureAndHashAlgorithm value lists a single hash/signature
pair that the client is willing to verify. The values are indicated
in descending order of preference.

Note: Because not all signature algorithms and hash algorithms may be
accepted by an implementation (e.g., DSA with SHA-1, but not
SHA-256), algorithms here are listed in pairs.

hash
This field indicates the hash algorithm which may be used. The
values indicate support for unhashed data, MD5 [MD5], SHA-1,
SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively. The
“none” value is provided for future extensibility, in case of a
signature algorithm which does not require hashing before signing.

signature
This field indicates the signature algorithm that may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
[PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively. The
“anonymous” value is meaningless in this context but used in
Section 7.4.3. It MUST NOT appear in this extension.

The semantics of this extension are somewhat complicated because the
cipher suite indicates permissible signature algorithms but not hash
algorithms. Sections 7.4.2 and 7.4.3 describe the appropriate rules.

If the client supports only the default hash and signature algorithms
(listed in this section), it MAY omit the signature_algorithms
extension. If the client does not support the default algorithms, or
supports other hash and signature algorithms (and it is willing to
use them for verifying messages sent by the server, i.e., server
certificates and server key exchange), it MUST send the
signature_algorithms extension, listing the algorithms it is willing
to accept.

If the client does not send the signature_algorithms extension, the
server MUST do the following:

– If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had
sent the value {sha1,rsa}.

– If the negotiated key exchange algorithm is one of (DHE_DSS,
DH_DSS), behave as if the client had sent the value {sha1,dsa}.

– If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}.

Note: this is a change from TLS 1.1 where there are no explicit
rules, but as a practical matter one can assume that the peer
supports MD5 and SHA-1.

Note: this extension is not meaningful for TLS versions prior to 1.2.
Clients MUST NOT offer it if they are offering prior versions.
However, even if clients do offer it, the rules specified in [TLSEXT]
require servers to ignore extensions they do not understand.

Servers MUST NOT send this extension. TLS servers MUST support
receiving this extension.

When performing session resumption, this extension is not included in
Server Hello, and the server ignores the extension in Client Hello
(if present).

7.4.2. Server Certificate

When this message will be sent:

The server MUST send a Certificate message whenever the agreed-
upon key exchange method uses certificates for authentication
(this includes all key exchange methods defined in this document
except DH_anon). This message will always immediately follow the
ServerHello message.

Meaning of this message:

This message conveys the server’s certificate chain to the client.

The certificate MUST be appropriate for the negotiated cipher
suite’s key exchange algorithm and any negotiated extensions.

 

Dierks & Rescorla Standards Track [Page 47]

RFC 5246 TLS August 2008
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 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 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.

The following rules apply to the certificates sent by the server:

– The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., [TLSPGP]).

– The end entity certificate’s public key (and associated
restrictions) MUST be compatible with the selected key exchange
algorithm.

Key Exchange Alg. Certificate Key Type

RSA RSA public key; the certificate MUST allow the
RSA_PSK key to be used for encryption (the
keyEncipherment bit MUST be set if the key
usage extension is present).
Note: RSA_PSK is defined in [TLSPSK].
DHE_RSA RSA public key; the certificate MUST allow the
ECDHE_RSA key to be used for signing (the
digitalSignature bit MUST be set if the key
usage extension is present) with the signature
scheme and hash algorithm that will be employed
in the server key exchange message.
Note: ECDHE_RSA is defined in [TLSECC].

DHE_DSS DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the server
key exchange message.

DH_DSS Diffie-Hellman public key; the keyAgreement bit
DH_RSA MUST be set if the key usage extension is
present.

ECDH_ECDSA ECDH-capable public key; the public key MUST
ECDH_RSA use a curve and point format supported by the
client, as described in [TLSECC].

ECDHE_ECDSA ECDSA-capable public key; the certificate MUST
allow the key to be used for signing with the
hash algorithm that will be employed in the
server key exchange message. The public key
MUST use a curve and point format supported by
the client, as described in [TLSECC].

– The “server_name” and “trusted_ca_keys” extensions [TLSEXT] are
used to guide certificate selection.

If the client provided a “signature_algorithms” extension, then all
certificates provided by the server MUST be signed by a
hash/signature algorithm pair that appears in that extension. Note
that this implies that a certificate containing a key for one
signature algorithm MAY be signed using a different signature
algorithm (for instance, an RSA key signed with a DSA key). This is
a departure from TLS 1.1, which required that the algorithms be the
same. Note that this also implies that the DH_DSS, DH_RSA,
ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the
algorithm used to sign the certificate. Fixed DH certificates MAY be
signed with any hash/signature algorithm pair appearing in the
extension. The names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are
historical.
If the server has multiple certificates, it chooses one of them based
on the above-mentioned criteria (in addition to other criteria, such
as transport layer endpoint, local configuration and preferences,
etc.). If the server has a single certificate, it SHOULD attempt to
validate that it meets these criteria.

Note that there are certificates that use algorithms and/or algorithm
combinations that cannot be currently used with TLS. For example, a
certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in
SubjectPublicKeyInfo) cannot be used because TLS defines no
corresponding signature algorithm.

As cipher suites that specify new key exchange methods are specified
for the TLS protocol, they will imply the certificate format and the
required encoded keying information.

7.4.3. Server Key Exchange Message

When this message will be sent:

This message will be sent immediately after the server Certificate
message (or the ServerHello message, if this is an anonymous
negotiation).

The ServerKeyExchange 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 ServerKeyExchange message for the
following key exchange methods:

RSA
DH_DSS
DH_RSA

Other key exchange algorithms, such as those defined in [TLSECC],
MUST specify whether the ServerKeyExchange message is sent or not;
and if the message is sent, its contents.
Meaning of this message:

This message conveys cryptographic information to allow the client
to communicate the premaster secret: a Diffie-Hellman public key
with which the client can complete a key exchange (with the result
being the premaster secret) or a public key for some other
algorithm.

Structure of this message:

enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa
/* may be extended, e.g., for ECDH — see [TLSECC] */
} KeyExchangeAlgorithm;

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 dh_anon:
ServerDHParams params;
case dhe_dss:
case dhe_rsa:
ServerDHParams params;
digitally-signed struct {
opaque client_random[32];
opaque server_random[32];
ServerDHParams params;
} signed_params;
case rsa:
case dh_dss:
case dh_rsa:
struct {} ;
/* message is omitted for rsa, dh_dss, and dh_rsa */
/* may be extended, e.g., for ECDH — see [TLSECC] */
};
} ServerKeyExchange;

params
The server’s key exchange parameters.

signed_params
For non-anonymous key exchanges, a signature over the server’s
key exchange parameters.

If the client has offered the “signature_algorithms” extension, the
signature algorithm and hash algorithm MUST be a pair listed in that
extension. Note that there is a possibility for inconsistencies
here. For instance, the client might offer DHE_DSS key exchange but
omit any DSA pairs from its “signature_algorithms” extension. In
order to negotiate correctly, the server MUST check any candidate
cipher suites against the “signature_algorithms” extension before
selecting them. This is somewhat inelegant but is a compromise
designed to minimize changes to the original cipher suite design.

In addition, the hash and signature algorithms MUST be compatible
with the key in the server’s end-entity certificate. RSA keys MAY be
used with any permitted hash algorithm, subject to restrictions in
the certificate, if any.

Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes
may be used with any key. Currently, DSA [DSS] may only be used with
SHA-1. Future revisions of DSS [DSS-3] are expected to allow the use
of other digest algorithms with DSA, as well as guidance as to which

 

digest algorithms should be used with each key size. In addition,
future revisions of [PKIX] may specify mechanisms for certificates to
indicate which digest algorithms are to be used with DSA.

As additional cipher suites 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.

7.4.4. Certificate Request

When this message will be sent:

A non-anonymous server can optionally request a certificate from
the client, if appropriate for the selected cipher suite. This
message, if sent, will immediately follow the ServerKeyExchange
message (if it is sent; otherwise, this message follows the
server’s 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>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;

certificate_types
A list of the types of certificate types that the client may
offer.

rsa_sign a certificate containing an RSA key
dss_sign a certificate containing a DSA key
rsa_fixed_dh a certificate containing a static DH key.
dss_fixed_dh a certificate containing a static DH key
supported_signature_algorithms
A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference.

certificate_authorities
A list of the distinguished names [X501] of acceptable
certificate_authorities, represented in DER-encoded format. 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 known roots as well as 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.

The interaction of the certificate_types and
supported_signature_algorithms fields is somewhat complicated.
certificate_types has been present in TLS since SSLv3, but was
somewhat underspecified. Much of its functionality is superseded by
supported_signature_algorithms. The following rules apply:

– Any certificates provided by the client MUST be signed using a
hash/signature algorithm pair found in
supported_signature_algorithms.

– The end-entity certificate provided by the client MUST contain a
key that is compatible with certificate_types. If the key is a
signature key, it MUST be usable with some hash/signature
algorithm pair in supported_signature_algorithms.

– For historical reasons, the names of some client certificate types
include the algorithm used to sign the certificate. For example,
in earlier versions of TLS, rsa_fixed_dh meant a certificate
signed with RSA and containing a static DH key. In TLS 1.2, this
functionality has been obsoleted by the
supported_signature_algorithms, and the certificate type no longer
restricts the algorithm used to sign the certificate. For
example, if the server sends dss_fixed_dh certificate type and
{{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
with a certificate containing a static DH key, signed with RSA-
SHA1.

New ClientCertificateType values are assigned by IANA as described in
Section 12.

Note: Values listed as RESERVED may not be used. They were used in
SSLv3.
Note: It is a fatal handshake_failure alert for an anonymous server
to request client authentication.

7.4.5. Server Hello Done

When this message will be sent:

The ServerHelloDone message is sent by the server to indicate the
end of the ServerHello 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 ServerHelloDone 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;

7.4.6. Client Certificate

When this message will be sent:

This is the first message the client can send after receiving a
ServerHelloDone message. This message is only sent if the server
requests a certificate. If no suitable certificate is available,
the client MUST send a certificate message containing no
certificates. That is, the certificate_list structure has a
length of zero. If the client does not send any certificates, the
server MAY at its discretion either continue the handshake without
client authentication, or respond with a fatal handshake_failure
alert. Also, if some aspect of the certificate chain was
unacceptable (e.g., it was not signed by a known, trusted CA), the
server MAY at its discretion either continue the handshake
(considering the client unauthenticated) or send a fatal alert.

Client certificates are sent using the Certificate structure
defined in Section 7.4.2.
Meaning of this message:

This message conveys the client’s certificate chain to the server;
the server will use it when verifying the CertificateVerify
message (when the client authentication is based on signing) or
calculating the premaster secret (for non-ephemeral Diffie-
Hellman). The certificate MUST be appropriate for the negotiated
cipher suite’s key exchange algorithm, and any negotiated
extensions.

In particular:

– The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., [TLSPGP]).

– The end-entity certificate’s public key (and associated
restrictions) has to be compatible with the certificate types
listed in CertificateRequest:

Client Cert. Type Certificate Key Type

rsa_sign RSA public key; the certificate MUST allow the
key to be used for signing with the signature
scheme and hash algorithm that will be
employed in the certificate verify message.

dss_sign DSA public key; the certificate MUST allow the
key to be used for signing with the hash
algorithm that will be employed in the
certificate verify message.

ecdsa_sign ECDSA-capable public key; the certificate MUST
allow the key to be used for signing with the
hash algorithm that will be employed in the
certificate verify message; the public key
MUST use a curve and point format supported by
the server.

rsa_fixed_dh Diffie-Hellman public key; MUST use the same
dss_fixed_dh parameters as server’s key.

rsa_fixed_ecdh ECDH-capable public key; MUST use the
ecdsa_fixed_ecdh same curve as the server’s key, and MUST use a
point format supported by the server.

– If the certificate_authorities list in the certificate request
message was non-empty, one of the certificates in the certificate
chain SHOULD be issued by one of the listed CAs.
– The certificates MUST be signed using an acceptable hash/
signature algorithm pair, as described in Section 7.4.4. Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.

Note that, as with the server certificate, there are certificates
that use algorithms/algorithm combinations that cannot be currently
used with TLS.

7.4.7. Client Key Exchange Message

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 ServerHelloDone message.

Meaning of this message:

With this message, the premaster secret is set, either by 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 client is using an ephemeral Diffie-Hellman exponent,
then this message contains the client’s Diffie-Hellman public
value. If the client is sending a certificate containing a static
DH exponent (i.e., it is doing fixed_dh client authentication),
then this message MUST be sent but MUST be empty.

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 dhe_dss:
case dhe_rsa:
case dh_dss:
case dh_rsa:
case dh_anon:
ClientDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;

7.4.7.1. RSA-Encrypted Premaster Secret Message

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, and sends the result in
an encrypted premaster secret message. This structure is a
variant of the ClientKeyExchange 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 rollback attacks.

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: The version number in the PreMasterSecret is the version
offered by the client in the ClientHello.client_version, not the
version negotiated for the connection. This feature is designed to
prevent rollback attacks. Unfortunately, some old 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 always send the correct version number in
PreMasterSecret. If ClientHello.client_version is TLS 1.1 or higher,
server implementations MUST check the version number as described in
the note below. If the version number is TLS 1.0 or earlier, server
implementations SHOULD check the version number, but MAY have a
configuration option to disable the check. Note that if the check
fails, the PreMasterSecret SHOULD be randomized as described below.

Note: Attacks discovered by Bleichenbacher [BLEI] and Klima et al.
[KPR03] can be used to attack a TLS server that reveals whether a
particular message, when decrypted, is properly PKCS#1 formatted,
contains a valid PreMasterSecret structure, or has the correct
version number.

As described by Klima [KPR03], these vulnerabilities can be avoided
by treating incorrectly formatted message blocks and/or mismatched
version numbers in a manner indistinguishable from correctly
formatted RSA blocks. In other words:

1. Generate a string R of 46 random bytes

2. Decrypt the message to recover the plaintext M

3. If the PKCS#1 padding is not correct, or the length of message
M is not exactly 48 bytes:
pre_master_secret = ClientHello.client_version || R
else If ClientHello.client_version <= TLS 1.0, and version
number check is explicitly disabled:
pre_master_secret = M
else:
pre_master_secret = ClientHello.client_version || M[2..47]

Note that explicitly constructing the pre_master_secret with the
ClientHello.client_version produces an invalid master_secret if the
client has sent the wrong version in the original pre_master_secret.

An alternative approach is to treat a version number mismatch as a
PKCS-1 formatting error and randomize the premaster secret
completely:
1. Generate a string R of 48 random bytes

2. Decrypt the message to recover the plaintext M

3. If the PKCS#1 padding is not correct, or the length of message
M is not exactly 48 bytes:
pre_master_secret = R
else If ClientHello.client_version <= TLS 1.0, and version
number check is explicitly disabled:
premaster secret = M
else If M[0..1] != ClientHello.client_version:
premaster secret = R
else:
premaster secret = M

Although no practical attacks against this construction are known,
Klima et al. [KPR03] describe some theoretical attacks, and therefore
the first construction described is RECOMMENDED.

In any case, a TLS server MUST NOT generate an alert if processing an
RSA-encrypted premaster secret message fails, or the version number
is not as expected. Instead, it MUST continue the handshake with a
randomly generated premaster secret. It may be useful to log the
real cause of failure for troubleshooting purposes; however, care
must be taken to avoid leaking the information to an attacker
(through, e.g., timing, log files, or other channels.)

The RSAES-OAEP encryption scheme defined in [PKCS1] is more secure
against the Bleichenbacher attack. However, for maximal
compatibility with earlier versions of TLS, this specification uses
the RSAES-PKCS1-v1_5 scheme. 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 — they encode 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 TLS are possible, at least when the client and server are on the
same LAN. Accordingly, implementations that use static RSA keys MUST
use RSA blinding or some other anti-timing technique, as described in
[TIMING].

7.4.7.2. Client Diffie-Hellman Public Value

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 has sent a certificate which contains a suitable
Diffie-Hellman key (for fixed_dh client authentication), 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).
7.4.8. Certificate Verify

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 {
digitally-signed struct {
opaque handshake_messages[handshake_messages_length];
}
} CertificateVerify;

Here handshake_messages refers to all handshake messages sent or
received, starting at client hello and 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 Section 7.4) exchanged thus
far. Note that this requires both sides to either buffer the
messages or compute running hashes for all potential hash
algorithms up to the time of the CertificateVerify computation.
Servers can minimize this computation cost by offering a
restricted set of digest algorithms in the CertificateRequest
message.

The hash and signature algorithms used in the signature MUST be
one of those present in the supported_signature_algorithms field
of the CertificateRequest message. In addition, the hash and
signature algorithms MUST be compatible with the key in the
client’s end-entity certificate. RSA keys MAY be used with any
permitted hash algorithm, subject to restrictions in the
certificate, if any.

Because DSA signatures do not contain any secure indication of
hash algorithm, there is a risk of hash substitution if multiple
hashes may be used with any key. Currently, DSA [DSS] may only be
used with SHA-1. Future revisions of DSS [DSS-3] are expected to
allow the use of other digest algorithms with DSA, as well as
guidance as to which digest algorithms should be used with each
key size. In addition, future revisions of [PKIX] may specify
mechanisms for certificates to indicate which digest algorithms
are to be used with DSA.
7.4.9. Finished

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 one 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.

Structure of this message:

struct {
opaque verify_data[verify_data_length];
} Finished;

verify_data
PRF(master_secret, finished_label, Hash(handshake_messages))
[0..verify_data_length-1];

finished_label
For Finished messages sent by the client, the string
“client finished”. For Finished messages sent by the server,
the string “server finished”.

Hash denotes a Hash of the handshake messages. For the PRF
defined in Section 5, the Hash MUST be the Hash used as the basis
for the PRF. Any cipher suite which defines a different PRF MUST
also define the Hash to use in the Finished computation.

In previous versions of TLS, the verify_data was always 12 octets
long. In the current version of TLS, it depends on the cipher
suite. Any cipher suite which does not explicitly specify
verify_data_length has a verify_data_length equal to 12. This
includes all existing cipher suites. Note that this
representation has the same encoding as with previous versions.
Future cipher suites MAY specify other lengths but such length
MUST be at least 12 bytes.
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
Section 7.4, exchanged thus far.

It is a fatal error if a Finished message is not preceded by a
ChangeCipherSpec message at the appropriate point in the handshake.

The value handshake_messages includes all handshake messages starting
at ClientHello 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 CertificateVerify 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: ChangeCipherSpec messages, alerts, and any other record types
are not handshake messages and are not included in the hash
computations. Also, HelloRequest messages are omitted from handshake
hashes.

8. Cryptographic Computations

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 ServerHello 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.

8.1. Computing 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.
8.1.1. RSA

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.

8.1.2. Diffie-Hellman

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.

9. Mandatory Cipher Suites

In the absence of an application profile standard specifying
otherwise, a TLS-compliant application MUST implement the cipher
suite TLS_RSA_WITH_AES_128_CBC_SHA (see Appendix A.5 for the
definition).

10. Application Data Protocol

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.

11. Security Considerations

Security issues are discussed throughout this memo, especially in
Appendices D, E, and F.

12. IANA Considerations

This document uses several registries that were originally created in
[TLS1.1]. IANA has updated these to reference this document. The
registries and their allocation policies (unchanged from [TLS1.1])
are listed below.
– TLS ClientCertificateType Identifiers Registry: Future values in
the range 0-63 (decimal) inclusive are assigned via Standards
Action [RFC2434]. Values in the range 64-223 (decimal) inclusive
are assigned via Specification Required [RFC2434]. Values from
224-255 (decimal) inclusive are reserved for Private Use
[RFC2434].

– TLS Cipher Suite Registry: Future values with the first byte in
the range 0-191 (decimal) inclusive are assigned via Standards
Action [RFC2434]. Values with the first byte in the range 192-254
(decimal) are assigned via Specification Required [RFC2434].
Values with the first byte 255 (decimal) are reserved for Private
Use [RFC2434].

– This document defines several new HMAC-SHA256-based cipher suites,
whose values (in Appendix A.5) have been allocated from the TLS
Cipher Suite registry.

– TLS ContentType Registry: Future values are allocated via
Standards Action [RFC2434].

– TLS Alert Registry: Future values are allocated via Standards
Action [RFC2434].

– TLS HandshakeType Registry: Future values are allocated via
Standards Action [RFC2434].

This document also uses a registry originally created in [RFC4366].
IANA has updated it to reference this document. The registry and its
allocation policy (unchanged from [RFC4366]) is listed below:

– TLS ExtensionType Registry: Future values are allocated via IETF
Consensus [RFC2434]. IANA has updated this registry to include
the signature_algorithms extension and its corresponding value
(see Section 7.4.1.4).

In addition, this document defines two new registries to be
maintained by IANA:

– TLS SignatureAlgorithm Registry: The registry has been initially
populated with the values described in Section 7.4.1.4.1. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action [RFC2434]. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required [RFC2434].
Values from 224-255 (decimal) inclusive are reserved for Private
Use [RFC2434].
– TLS HashAlgorithm Registry: The registry has been initially
populated with the values described in Section 7.4.1.4.1. Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action [RFC2434]. Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required [RFC2434].
Values from 224-255 (decimal) inclusive are reserved for Private
Use [RFC2434].

This document also uses the TLS Compression Method Identifiers
Registry, defined in [RFC3749]. IANA has allocated value 0 for
the “null” compression method.
Appendix A. Protocol Data Structures and Constant Values

This section describes protocol types and constants.

A.1. Record Layer

struct {
uint8 major;
uint8 minor;
} ProtocolVersion;

ProtocolVersion version = { 3, 3 }; /* TLS v1.2*/

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 (SecurityParameters.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
case aead: GenericAEADCipher;
} fragment;
} TLSCiphertext;

stream-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
} GenericStreamCipher;
struct {
opaque IV[SecurityParameters.record_iv_length];
block-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
};
} GenericBlockCipher;

struct {
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSCompressed.length];
};
} GenericAEADCipher;

A.2. Change Cipher Specs Message

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

A.3. Alert Messages

enum { warning(1), fatal(2), (255) } AlertLevel;

enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110), /* new */
(255)
} AlertDescription;

struct {
AlertLevel level;
AlertDescription description;
} Alert;

A.4. Handshake Protocol

enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20)
(255)
} HandshakeType;

struct {
HandshakeType msg_type;
uint24 length;
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
A.4.1. Hello Messages

struct { } HelloRequest;

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

opaque SessionID<0..32>;

uint8 CipherSuite[2];

enum { null(0), (255) } CompressionMethod;

struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ClientHello;

struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;

struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
signature_algorithms(13), (65535)
} ExtensionType;

enum{
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum {
anonymous(0), rsa(1), dsa(2), ecdsa(3), (255)
} SignatureAlgorithm;

struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;

SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-1>;

A.4.2. Server Authentication and Key Exchange Messages

opaque ASN.1Cert<2^24-1>;

struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;

enum { dhe_dss, dhe_rsa, dh_anon, rsa,dh_dss, dh_rsa
/* may be extended, e.g., for ECDH — see [TLSECC] */
} KeyExchangeAlgorithm;

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 */
struct {
select (KeyExchangeAlgorithm) {
case dh_anon:
ServerDHParams params;
case dhe_dss:
case dhe_rsa:
ServerDHParams params;
digitally-signed struct {
opaque client_random[32];
opaque server_random[32];
ServerDHParams params;
} signed_params;
case rsa:
case dh_dss:
case dh_rsa:
struct {} ;
/* message is omitted for rsa, dh_dss, and dh_rsa */
/* may be extended, e.g., for ECDH — see [TLSECC] */
} ServerKeyExchange;

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;
A.4.3. Client Authentication and Key Exchange Messages

struct {
select (KeyExchangeAlgorithm) {
case rsa:
EncryptedPreMasterSecret;
case dhe_dss:
case dhe_rsa:
case dh_dss:
case dh_rsa:
case dh_anon:
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 {
digitally-signed struct {
opaque handshake_messages[handshake_messages_length];
}
} CertificateVerify;

A.4.4. Handshake Finalization Message

struct {
opaque verify_data[verify_data_length];
} Finished;
A.5. The Cipher Suite

The following values define the cipher suite codes used in the
ClientHello and ServerHello messages.

A cipher suite defines a cipher specification supported in TLS
Version 1.2.

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 any signature-capable certificate in the certificate request
message.

CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
CipherSuite TLS_RSA_WITH_NULL_SHA256 = { 0x00,0x3B };
CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A };
CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x2F };
CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x35 };
CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x3C };
CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x3D };

The following cipher suite definitions are used for server-
authenticated (and optionally client-authenticated) Diffie-Hellman.
DH denotes cipher suites in which the server’s certificate contains
the Diffie-Hellman parameters signed by the certificate authority
(CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
parameters are signed by a signature-capable certificate, which has
been signed by the CA. The signing algorithm used by the server is
specified after the DHE component of the CipherSuite name. The
server can request any signature-capable certificate from the client
for client authentication, or it may request a Diffie-Hellman
certificate. Any Diffie-Hellman certificate provided by the client
must use the parameters (group and generator) described by the
server.
CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 };
CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x30 };
CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x31 };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x32 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x33 };
CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x36 };
CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x37 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x38 };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x39 };
CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA256 = { 0x00,0x3E };
CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x3F };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 = { 0x00,0x40 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x67 };
CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA256 = { 0x00,0x68 };
CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x69 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 = { 0x00,0x6A };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x6B };

The following cipher suites are used for completely anonymous
Diffie-Hellman communications in which neither party is
authenticated. Note that this mode is vulnerable to man-in-the-
middle attacks. Using this mode therefore is of limited use: These
cipher suites MUST NOT be used by TLS 1.2 implementations unless the
application layer has specifically requested to allow anonymous key
exchange. (Anonymous key exchange may sometimes be acceptable, for
example, to support opportunistic encryption when no set-up for
authentication is in place, or when TLS is used as part of more
complex security protocols that have other means to ensure
authentication.)

CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00,0x34 };
CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00,0x3A };
CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA256 = { 0x00,0x6C };
CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA256 = { 0x00,0x6D };

Note that using non-anonymous key exchange without actually verifying
the key exchange is essentially equivalent to anonymous key exchange,
and the same precautions apply. While non-anonymous key exchange
will generally involve a higher computational and communicational
cost than anonymous key exchange, it may be in the interest of
interoperability not to disable non-anonymous key exchange when the
application layer is allowing anonymous key exchange.
New cipher suite values have been assigned by IANA as described in
Section 12.

Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in
SSL 3.

A.6. The Security Parameters

These security parameters are determined by the TLS Handshake
Protocol and provided as parameters to the TLS record layer in order
to initialize a connection state. SecurityParameters includes:

enum { null(0), (255) } CompressionMethod;

enum { server, client } ConnectionEnd;

enum { tls_prf_sha256 } PRFAlgorithm;

enum { null, rc4, 3des, aes } BulkCipherAlgorithm;

enum { stream, block, aead } CipherType;

enum { null, hmac_md5, hmac_sha1, hmac_sha256, hmac_sha384,
hmac_sha512} MACAlgorithm;

/* Other values may be added to the algorithms specified in
CompressionMethod, PRFAlgorithm, BulkCipherAlgorithm, and
MACAlgorithm. */

struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
MACAlgorithm mac_algorithm;
uint8 mac_length;
uint8 mac_key_length;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
A.7. Changes to RFC 4492

RFC 4492 [TLSECC] adds Elliptic Curve cipher suites to TLS. This
document changes some of the structures used in that document. This
section details the required changes for implementors of both RFC
4492 and TLS 1.2. Implementors of TLS 1.2 who are not implementing
RFC 4492 do not need to read this section.

This document adds a “signature_algorithm” field to the digitally-
signed element in order to identify the signature and digest
algorithms used to create a signature. This change applies to
digital signatures formed using ECDSA as well, thus allowing ECDSA
signatures to be used with digest algorithms other than SHA-1,
provided such use is compatible with the certificate and any
restrictions imposed by future revisions of [PKIX].

As described in Sections 7.4.2 and 7.4.6, the restrictions on the
signature algorithms used to sign certificates are no longer tied to
the cipher suite (when used by the server) or the
ClientCertificateType (when used by the client). Thus, the
restrictions on the algorithm used to sign certificates specified in
Sections 2 and 3 of RFC 4492 are also relaxed. As in this document,
the restrictions on the keys in the end-entity certificate remain.

Appendix B. Glossary

Advanced Encryption Standard (AES)
AES [AES] is a widely used symmetric encryption algorithm. AES is
a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte
block size. TLS currently only supports the 128- and 256-bit key
sizes.

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

asymmetric cipher
See public key cryptography.

authenticated encryption with additional data (AEAD)
A symmetric encryption algorithm that simultaneously provides
confidentiality and message integrity.

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

 

block cipher
A block cipher is an algorithm that operates on plaintext in
groups of bits, called blocks. 64 bits was, and 128 bits is, a
common block size.

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

cipher block chaining (CBC)
CBC is a mode in which every plaintext block encrypted with a
block cipher is first exclusive-ORed with the previous ciphertext
block (or, in the case of the first block, with the initialization
vector). For decryption, every block is first decrypted, then
exclusive-ORed with the previous ciphertext block (or IV).

certificate
As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party’s identity
or some other attributes and its public key.

client
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.

client write key
The key used to encrypt data written by the client.

client write MAC key
The secret data used to authenticate data written by the client.

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

Data Encryption Standard
DES [DES] still is a very widely used symmetric encryption
algorithm although it is considered as rather weak now. DES is a
block cipher with a 56-bit key and an 8-byte block size. Note
that in TLS, for key generation purposes, DES is treated as having
an 8-byte key length (64 bits), but it still only provides 56 bits
of protection. (The low bit of each key byte is presumed to be
set to produce odd parity in that key byte.) DES can also be
operated in a mode [3DES] where three independent keys and three
encryptions are used for each block of data; this uses 168 bits of
key (24 bytes in the TLS key generation method) and provides the
equivalent of 112 bits of security.

Digital Signature Standard (DSS)
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186-2, “Digital Signature
Standard”, published January 2000 by the U.S. Department of
Commerce [DSS]. A significant update [DSS-3] has been drafted and
was published in March 2006.

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

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

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

Message Authentication Code (MAC)
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.

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

MD5
MD5 [MD5] is a hashing function that converts an arbitrarily long
data stream into a hash of fixed size (16 bytes). Due to
significant progress in cryptanalysis, at the time of publication
of this document, MD5 no longer can be considered a ‘secure’
hashing function.

 

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

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

RC4
A stream cipher invented by Ron Rivest. A compatible cipher is
described in [SCH].

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

server
The server is the application entity that responds to requests for
connections from clients. See also “client”.

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

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

server write key
The key used to encrypt data written by the server.

server write MAC key
The secret data used to authenticate data written by the server.

SHA
The Secure Hash Algorithm [SHS] is defined in FIPS PUB 180-2. It
produces a 20-byte output. Note that all references to SHA
(without a numerical suffix) actually use the modified SHA-1
algorithm.

 

 

Dierks & Rescorla Standards Track [Page 81]

RFC 5246 TLS August 2008
SHA-256
The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2.
It produces a 32-byte output.

SSL
Netscape’s Secure Socket Layer protocol [SSL3]. TLS is based on
SSL Version 3.0.

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

symmetric cipher
See bulk cipher.

Transport Layer Security (TLS)
This protocol; also, the Transport Layer Security working group of
the Internet Engineering Task Force (IETF). See “Working Group
Information” at the end of this document (see page 99).
Appendix C. Cipher Suite Definitions

Cipher Suite Key Cipher Mac
Exchange

TLS_NULL_WITH_NULL_NULL NULL NULL NULL
TLS_RSA_WITH_NULL_MD5 RSA NULL MD5
TLS_RSA_WITH_NULL_SHA RSA NULL SHA
TLS_RSA_WITH_NULL_SHA256 RSA NULL SHA256
TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5
TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA
TLS_RSA_WITH_AES_128_CBC_SHA RSA AES_128_CBC SHA
TLS_RSA_WITH_AES_256_CBC_SHA RSA AES_256_CBC SHA
TLS_RSA_WITH_AES_128_CBC_SHA256 RSA AES_128_CBC SHA256
TLS_RSA_WITH_AES_256_CBC_SHA256 RSA AES_256_CBC SHA256
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA
TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA
TLS_DH_DSS_WITH_AES_128_CBC_SHA DH_DSS AES_128_CBC SHA
TLS_DH_RSA_WITH_AES_128_CBC_SHA DH_RSA AES_128_CBC SHA
TLS_DHE_DSS_WITH_AES_128_CBC_SHA DHE_DSS AES_128_CBC SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA DHE_RSA AES_128_CBC SHA
TLS_DH_anon_WITH_AES_128_CBC_SHA DH_anon AES_128_CBC SHA
TLS_DH_DSS_WITH_AES_256_CBC_SHA DH_DSS AES_256_CBC SHA
TLS_DH_RSA_WITH_AES_256_CBC_SHA DH_RSA AES_256_CBC SHA
TLS_DHE_DSS_WITH_AES_256_CBC_SHA DHE_DSS AES_256_CBC SHA
TLS_DHE_RSA_WITH_AES_256_CBC_SHA DHE_RSA AES_256_CBC SHA
TLS_DH_anon_WITH_AES_256_CBC_SHA DH_anon AES_256_CBC SHA
TLS_DH_DSS_WITH_AES_128_CBC_SHA256 DH_DSS AES_128_CBC SHA256
TLS_DH_RSA_WITH_AES_128_CBC_SHA256 DH_RSA AES_128_CBC SHA256
TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 DHE_DSS AES_128_CBC SHA256
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 DHE_RSA AES_128_CBC SHA256
TLS_DH_anon_WITH_AES_128_CBC_SHA256 DH_anon AES_128_CBC SHA256
TLS_DH_DSS_WITH_AES_256_CBC_SHA256 DH_DSS AES_256_CBC SHA256
TLS_DH_RSA_WITH_AES_256_CBC_SHA256 DH_RSA AES_256_CBC SHA256
TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 DHE_DSS AES_256_CBC SHA256
TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 DHE_RSA AES_256_CBC SHA256
TLS_DH_anon_WITH_AES_256_CBC_SHA256 DH_anon AES_256_CBC SHA256

 

Key IV Block
Cipher Type Material Size Size
———— —— ——– —- —–
NULL Stream 0 0 N/A
RC4_128 Stream 16 0 N/A
3DES_EDE_CBC Block 24 8 8
AES_128_CBC Block 16 16 16
AES_256_CBC Block 32 16 16
MAC Algorithm mac_length mac_key_length
——– ———– ———- ————–
NULL N/A 0 0
MD5 HMAC-MD5 16 16
SHA HMAC-SHA1 20 20
SHA256 HMAC-SHA256 32 32

Type
Indicates whether this is a stream cipher or a block cipher
running in CBC mode.

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

IV Size
The amount of data needed to be generated for the initialization
vector. Zero for stream ciphers; equal to the block size for
block ciphers (this is equal to
SecurityParameters.record_iv_length).

Block Size
The amount of data a block cipher enciphers in one chunk; a block
cipher running in CBC mode can only encrypt an even multiple of
its block size.
Appendix D. Implementation Notes

The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors.

D.1. Random Number Generation and Seeding

TLS requires a cryptographically secure pseudorandom number generator
(PRNG). Care must be taken in designing and seeding PRNGs. PRNGs
based on secure hash operations, most notably SHA-1, are acceptable,
but cannot provide more security than the size of the random number
generator state.

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

[RANDOM] provides guidance on the generation of random values.

D.2. Certificates and Authentication

Implementations are responsible for verifying the integrity of
certificates and should generally support certificate revocation
messages. Certificates should always be verified to ensure proper
signing by a trusted Certificate Authority (CA). The selection and
addition of trusted CAs should be done very carefully. Users should
be able to view information about the certificate and root CA.

D.3. Cipher Suites

TLS supports a range of key sizes and security levels, including some
that provide no or minimal security. A proper implementation will
probably not support many cipher suites. For instance, anonymous
Diffie-Hellman is strongly discouraged because it cannot prevent man-
in-the-middle attacks. Applications should also enforce minimum and
maximum key sizes. For example, certificate chains containing 512-
bit RSA keys or signatures are not appropriate for high-security
applications.

D.4. Implementation Pitfalls

Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of
interoperability and security problems. Many of these areas have
been clarified in this document, but this appendix contains a short
list of the most important things that require special attention from
implementors.

TLS protocol issues:

– Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see Section 6.2.1)? Including corner cases
like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment
size? In particular, the certificate and certificate request
handshake messages can be large enough to require fragmentation.

– Do you ignore the TLS record layer version number in all TLS
records before ServerHello (see Appendix E.1)?

– Do you handle TLS extensions in ClientHello correctly, including
omitting the extensions field completely?

– Do you support renegotiation, both client and server initiated?
While renegotiation is an optional feature, supporting it is
highly recommended.

– When the server has requested a client certificate, but no
suitable certificate is available, do you correctly send an empty
Certificate message, instead of omitting the whole message (see
Section 7.4.6)?

Cryptographic details:

– In the RSA-encrypted Premaster Secret, do you correctly send and
verify the version number? When an error is encountered, do you
continue the handshake to avoid the Bleichenbacher attack (see
Section 7.4.7.1)?

– What countermeasures do you use to prevent timing attacks against
RSA decryption and signing operations (see Section 7.4.7.1)?

– When verifying RSA signatures, do you accept both NULL and missing
parameters (see Section 4.7)? Do you verify that the RSA padding
doesn’t have additional data after the hash value? [FI06]

– When using Diffie-Hellman key exchange, do you correctly strip
leading zero bytes from the negotiated key (see Section 8.1.2)?

– Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable (see Section F.1.1.3)?
– How do you generate unpredictable IVs for CBC mode ciphers (see
Section 6.2.3.2)?

– Do you accept long CBC mode padding (up to 255 bytes; see Section
6.2.3.2)?

– How do you address CBC mode timing attacks (Section 6.2.3.2)?

– Do you use a strong and, most importantly, properly seeded random
number generator (see Appendix D.1) for generating the premaster
secret (for RSA key exchange), Diffie-Hellman private values, the
DSA “k” parameter, and other security-critical values?

Appendix E. Backward Compatibility

E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0

Since there are various versions of TLS (1.0, 1.1, 1.2, and any
future versions) and SSL (2.0 and 3.0), means are needed to negotiate
the specific protocol version to use. The TLS protocol provides a
built-in mechanism for version negotiation so as not to bother other
protocol components with the complexities of version selection.

TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
compatible ClientHello messages; thus, supporting all of them is
relatively easy. Similarly, servers can easily handle clients trying
to use future versions of TLS as long as the ClientHello format
remains compatible, and the client supports the highest protocol
version available in the server.

A TLS 1.2 client who wishes to negotiate with such older servers will
send a normal TLS 1.2 ClientHello, containing { 3, 3 } (TLS 1.2) in
ClientHello.client_version. If the server does not support this
version, it will respond with a ServerHello containing an older
version number. If the client agrees to use this version, the
negotiation will proceed as appropriate for the negotiated protocol.

If the version chosen by the server is not supported by the client
(or not acceptable), the client MUST send a “protocol_version” alert
message and close the connection.

If a TLS server receives a ClientHello containing a version number
greater than the highest version supported by the server, it MUST
reply according to the highest version supported by the server.

A TLS server can also receive a ClientHello containing a version
number smaller than the highest supported version. If the server
wishes to negotiate with old clients, it will proceed as appropriate
for the highest version supported by the server that is not greater
than ClientHello.client_version. For example, if the server supports
TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will
proceed with a TLS 1.0 ServerHello. If server supports (or is
willing to use) only versions greater than client_version, it MUST
send a “protocol_version” alert message and close the connection.

Whenever a client already knows the highest protocol version known to
a server (for example, when resuming a session), it SHOULD initiate
the connection in that native protocol.

Note: some server implementations are known to implement version
negotiation incorrectly. For example, there are buggy TLS 1.0
servers that simply close the connection when the client offers a
version newer than TLS 1.0. Also, it is known that some servers will
refuse the connection if any TLS extensions are included in
ClientHello. Interoperability with such buggy servers is a complex
topic beyond the scope of this document, and may require multiple
connection attempts by the client.

Earlier versions of the TLS specification were not fully clear on
what the record layer version number (TLSPlaintext.version) should
contain when sending ClientHello (i.e., before it is known which
version of the protocol will be employed). Thus, TLS servers
compliant with this specification MUST accept any value {03,XX} as
the record layer version number for ClientHello.

TLS clients that wish to negotiate with older servers MAY send any
value {03,XX} as the record layer version number. Typical values
would be {03,00}, the lowest version number supported by the client,
and the value of ClientHello.client_version. No single value will
guarantee interoperability with all old servers, but this is a
complex topic beyond the scope of this document.

E.2. Compatibility with SSL 2.0

TLS 1.2 clients that wish to support SSL 2.0 servers MUST send
version 2.0 CLIENT-HELLO messages defined in [SSL2]. The message
MUST contain the same version number as would be used for ordinary
ClientHello, and MUST encode the supported TLS cipher suites in the
CIPHER-SPECS-DATA field as described below.

Warning: The ability to send version 2.0 CLIENT-HELLO messages will
be phased out with all due haste, since the newer ClientHello format
provides better mechanisms for moving to newer versions and
negotiating extensions. TLS 1.2 clients SHOULD NOT support SSL 2.0.
However, even TLS servers that do not support SSL 2.0 MAY accept
version 2.0 CLIENT-HELLO messages. The message is presented below in
sufficient detail for TLS server implementors; the true definition is
still assumed to be [SSL2].

For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same
way as a ClientHello with a “null” compression method and no
extensions. Note that this message MUST be sent directly on the
wire, not wrapped as a TLS record. For the purposes of calculating
Finished and CertificateVerify, the msg_length field is not
considered to be a part of the handshake message.

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

msg_length
The highest bit MUST be 1; the remaining bits contain the length
of the following data in bytes.

msg_type
This field, in conjunction with the version field, identifies a
version 2 ClientHello message. The value MUST be 1.

version
Equal to ClientHello.client_version.

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

session_id_length
This field MUST have a value of zero for a client that claims to
support TLS 1.2.
challenge_length
The length in bytes of the client’s challenge to the server to
authenticate itself. Historically, permissible values are between
16 and 32 bytes inclusive. When using the SSLv2 backward-
compatible handshake the client SHOULD use a 32-byte challenge.

cipher_specs
This is a list of all CipherSpecs the client is willing and able
to use. In addition to the 2.0 cipher specs defined in [SSL2],
this includes the TLS cipher suites normally sent in
ClientHello.cipher_suites, with each cipher suite prefixed by a
zero byte. For example, the TLS cipher suite {0x00,0x0A} would be
sent as {0x00,0x00,0x0A}.

session_id
This field MUST be empty.

challenge
Corresponds to ClientHello.random. If the challenge length is
less than 32, the TLS server will pad the data with leading (note:
not trailing) zero bytes to make it 32 bytes long.

Note: Requests to resume a TLS session MUST use a TLS client hello.

E.3. Avoiding Man-in-the-Middle Version Rollback

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

When a client negotiates SSL 2.0 but also supports TLS, it MUST set
the right-hand (least-significant) 8 random bytes of the PKCS padding
(not including the terminal null of the padding) for the RSA
encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
to 0x03 (the other padding bytes are random).

When a TLS-capable server negotiates SSL 2.0 it SHOULD, after
decrypting the ENCRYPTED-KEY-DATA field, check that these 8 padding
bytes are 0x03. If they are not, the server SHOULD generate a random
value for SECRET-KEY-DATA, and continue the handshake (which will
eventually fail since the keys will not match). Note that reporting
the error situation to the client could make the server vulnerable to
attacks described in [BLEI].
Appendix F. Security Analysis

The TLS protocol is designed to establish a secure connection between
a client and a server communicating over an insecure channel. This
document makes several traditional assumptions, including that
attackers have substantial computational resources and cannot obtain
secret information from sources outside the protocol. Attackers are
assumed to have the ability to capture, modify, delete, replay, and
otherwise tamper with messages sent over the communication channel.
This appendix outlines how TLS has been designed to resist a variety
of attacks.

F.1. Handshake Protocol

The handshake protocol is responsible for selecting a cipher spec and
generating a master secret, which together comprise the primary
cryptographic parameters associated with a secure session. The
handshake protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority.

F.1.1. Authentication and Key Exchange

TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and
total anonymity. Whenever the server is authenticated, the channel
is secure against man-in-the-middle attacks, but completely anonymous
sessions are inherently vulnerable to such attacks. Anonymous
servers cannot authenticate clients. If the server is authenticated,
its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly,
authenticated clients must supply an acceptable certificate to the
server. Each party is responsible for verifying that the other’s
certificate is valid and has not expired or been revoked.

The general goal of the key exchange process is to create a
pre_master_secret known to the communicating parties and not to
attackers. The pre_master_secret will be used to generate the
master_secret (see Section 8.1). The master_secret is required to
generate the Finished messages, encryption keys, and MAC keys (see
Sections 7.4.9 and 6.3). By sending a correct Finished message,
parties thus prove that they know the correct pre_master_secret.

F.1.1.1. Anonymous Key Exchange

Completely anonymous sessions can be established using Diffie-Hellman
for key exchange. The server’s public parameters are contained in
the server key exchange message, and the client’s are sent in the
client key exchange message. Eavesdroppers who do not know the
private values should not be able to find the Diffie-Hellman result
(i.e., the pre_master_secret).

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

F.1.1.2. RSA Key Exchange and Authentication

With RSA, key exchange and server authentication are combined. The
public key is contained in the server’s certificate. Note that
compromise of the server’s static RSA key results in a loss of
confidentiality for all sessions protected under that static key.
TLS users desiring Perfect Forward Secrecy should use DHE cipher
suites. The damage done by exposure of a private key can be limited
by changing one’s private key (and certificate) frequently.

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

When RSA is used for key exchange, clients are authenticated using
the certificate verify message (see Section 7.4.8). The client signs
a value derived from all preceding handshake messages. These
handshake messages include the server certificate, which binds the
signature to the server, and ServerHello.random, which binds the
signature to the current handshake process.

F.1.1.3. Diffie-Hellman Key Exchange with Authentication

When Diffie-Hellman key exchange is used, the server can either
supply a certificate containing fixed Diffie-Hellman parameters or
use the server key exchange message to send a set of temporary
Diffie-Hellman parameters signed with a DSA or RSA certificate.
Temporary parameters are hashed with the hello.random values before
signing to ensure that attackers do not replay old parameters. In
either case, the client can verify the certificate or signature to
ensure that the parameters belong to the server.

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

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

If the same DH keypair is to be used for multiple handshakes, either
because the client or server has a certificate containing a fixed DH
keypair or because the server is reusing DH keys, care must be taken
to prevent small subgroup attacks. Implementations SHOULD follow the
guidelines found in [SUBGROUP].

Small subgroup attacks are most easily avoided by using one of the
DHE cipher suites and generating a fresh DH private key (X) for each
handshake. If a suitable base (such as 2) is chosen, g^X mod p can
be computed very quickly; therefore, the performance cost is
minimized. Additionally, using a fresh key for each handshake
provides Perfect Forward Secrecy. Implementations SHOULD generate a
new X for each handshake when using DHE cipher suites.

Because TLS allows the server to provide arbitrary DH groups, the
client should verify that the DH group is of suitable size as defined
by local policy. The client SHOULD also verify that the DH public
exponent appears to be of adequate size. [KEYSIZ] provides a useful
guide to the strength of various group sizes. The server MAY choose
to assist the client by providing a known group, such as those
defined in [IKEALG] or [MODP]. These can be verified by simple
comparison.

F.1.2. Version Rollback Attacks

Because TLS includes substantial improvements over SSL Version 2.0,
attackers may try to make TLS-capable clients and servers fall back
to Version 2.0. This attack can occur if (and only if) two TLS-
capable parties use an SSL 2.0 handshake.

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

F.1.3. Detecting Attacks Against the Handshake Protocol

An attacker might try to influence the handshake exchange to make the
parties select different encryption algorithms than they would
normally choose.

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

F.1.4. Resuming Sessions

When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session’s master_secret. Provided that the master_secret has not
been compromised and that the secure hash operations used to produce
the encryption keys and MAC keys are secure, the connection should be
secure and effectively independent from previous connections.
Attackers cannot use known encryption keys or MAC secrets to
compromise the master_secret without breaking the secure hash
operations.

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

F.2. Protecting Application Data

The master_secret is hashed with the ClientHello.random and
ServerHello.random to produce unique data encryption keys and MAC
secrets for each connection.

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

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

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

F.3. Explicit IVs

[CBCATT] describes a chosen plaintext attack on TLS that depends on
knowing the IV for a record. Previous versions of TLS [TLS1.0] used
the CBC residue of the previous record as the IV and therefore
enabled this attack. This version uses an explicit IV in order to
protect against this attack.

F.4. Security of Composite Cipher Modes

TLS secures transmitted application data via the use of symmetric
encryption and authentication functions defined in the negotiated
cipher suite. The objective is to protect both the integrity and
confidentiality of the transmitted data from malicious actions by
active attackers in the network. It turns out that the order in
which encryption and authentication functions are applied to the data
plays an important role for achieving this goal [ENCAUTH].

The most robust method, called encrypt-then-authenticate, first
applies encryption to the data and then applies a MAC to the
ciphertext. This method ensures that the integrity and
confidentiality goals are obtained with ANY pair of encryption and
MAC functions, provided that the former is secure against chosen
plaintext attacks and that the MAC is secure against chosen-message
attacks. TLS uses another method, called authenticate-then-encrypt,
in which first a MAC is computed on the plaintext and then the
concatenation of plaintext and MAC is encrypted. This method has
been proven secure for CERTAIN combinations of encryption functions
and MAC functions, but it is not guaranteed to be secure in general.
In particular, it has been shown that there exist perfectly secure
encryption functions (secure even in the information-theoretic sense)
that combined with any secure MAC function, fail to provide the
confidentiality goal against an active attack. Therefore, new cipher
suites and operation modes adopted into TLS need to be analyzed under
the authenticate-then-encrypt method to verify that they achieve the
stated integrity and confidentiality goals.

Currently, the security of the authenticate-then-encrypt method has
been proven for some important cases. One is the case of stream
ciphers in which a computationally unpredictable pad of the length of
the message, plus the length of the MAC tag, is produced using a
pseudorandom generator and this pad is exclusive-ORed with the
concatenation of plaintext and MAC tag. The other is the case of CBC
mode using a secure block cipher. In this case, security can be
shown if one applies one CBC encryption pass to the concatenation of
plaintext and MAC and uses a new, independent, and unpredictable IV
for each new pair of plaintext and MAC. In versions of TLS prior to
1.1, CBC mode was used properly EXCEPT that it used a predictable IV
in the form of the last block of the previous ciphertext. This made
TLS open to chosen plaintext attacks. This version of the protocol
is immune to those attacks. For exact details in the encryption
modes proven secure, see [ENCAUTH].

F.5. Denial of Service

TLS is susceptible to a number of denial-of-service (DoS) attacks.
In particular, an attacker who initiates a large number of TCP
connections can cause a server to consume large amounts of CPU for
doing RSA decryption. However, because TLS is generally used over
TCP, it is difficult for the attacker to hide his point of origin if
proper TCP SYN randomization is used [SEQNUM] by the TCP stack.

Because TLS runs over TCP, it is also susceptible to a number of DoS
attacks on individual connections. In particular, attackers can
forge RSTs, thereby terminating connections, or forge partial TLS
records, thereby causing the connection to stall. These attacks
cannot in general be defended against by a TCP-using protocol.
Implementors or users who are concerned with this class of attack
should use IPsec AH [AH] or ESP [ESP].

F.6. Final Notes

For TLS to be able to provide a secure connection, both the client
and server systems, keys, and applications must be secure. In
addition, the implementation must be free of security errors.
The system is only as strong as the weakest key exchange and
authentication algorithm supported, and only trustworthy
cryptographic functions should be used. Short public keys and
anonymous servers should be used with great caution. Implementations
and users must be careful when deciding which certificates and
certificate authorities are acceptable; a dishonest certificate
authority can do tremendous damage.
RFC 5246 TLS August 2008