INTERNET-DRAFT Tim Dierks
Obsoletes (if approved): RFC 3268, 4346, 4366 Independent
Intended status: Proposed Standard Eric Rescorla
Network Resonance, Inc.
<draft-ietf-tls-rfc4346-bis-06.txt> October
<draft-ietf-tls-rfc4346-bis-07.txt> November 2007 (Expires April May 2008)
The Transport Layer Security (TLS) Protocol
Version 1.2
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies Version 1.2 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.
Table of Contents
1. Introduction 3
1.1 4
1.1. Requirements Terminology 5
1.2
1.2. Major Differences from TLS 1.1 5
2. Goals 6
3. Goals of This Document 6
4. Presentation Language 7
4.1. Basic Block Size 7
4.2. Miscellaneous 7
4.3. Vectors 7 8
4.4. Numbers 8 9
4.5. Enumerateds 9
4.6. Constructed Types 10
4.6.1. Variants 10
4.7. Cryptographic Attributes 11
4.8. Constants 13
5. HMAC and the Pseudorandom Function 13
6. The TLS Record Protocol 14
6.1. Connection States 15
6.2. Record layer 17 18
6.2.1. Fragmentation 17 18
6.2.2. Record Compression and Decompression 19
6.2.3. Record Payload Protection 19 20
6.2.3.1. Null or Standard Stream Cipher 20 21
6.2.3.2. CBC Block Cipher 21
6.2.3.3. AEAD ciphers 23
6.3. Key Calculation 24
7. The TLS Handshaking Protocols 25
7.1. Change Cipher Spec Protocol 25 26
7.2. Alert Protocol 26 27
7.2.1. Closure Alerts 27 28
7.2.2. Error Alerts 28 29
7.3. Handshake Protocol Overview 31 32
7.4. Handshake Protocol 34 35
7.4.1. Hello Messages 35 36
7.4.1.1. Hello Request 36
7.4.1.2. Client Hello 36 37
7.4.1.3. Server Hello 39 40
7.4.1.4 Hello Extensions 41 42
7.4.1.4.1 Signature Hash Algorithms 42 43
7.4.2. Server Certificate 43 44
7.4.3. Server Key Exchange Message 46 47
7.4.4. Certificate Request 49
7.4.5 Server hello done 50 51
7.4.6. Client Certificate 51 52
7.4.7. Client Key Exchange Message 52 53
7.4.7.1. RSA Encrypted Premaster Secret Message 53 54
7.4.7.2. Client Diffie-Hellman Public Value 55 56
7.4.8. Certificate verify 56 57
7.4.9. Finished 57 58
8. Cryptographic Computations 58 59
8.1. Computing the Master Secret 58 60
8.1.1. RSA 59 60
8.1.2. Diffie-Hellman 59 60
9. Mandatory Cipher Suites 59 60
10. Application Data Protocol 59 60
11. Security Considerations 59 60
12. IANA Considerations 59 61
A. Protocol Constant Values 62 63
A.1. Record Layer 62 63
A.2. Change Cipher Specs Message 63 64
A.3. Alert Messages 63 64
A.4. Handshake Protocol 65
A.4.1. Hello Messages 65
A.4.2. Server Authentication and Key Exchange Messages 67
A.4.3. Client Authentication and Key Exchange Messages 68
A.4.4. Handshake Finalization Message 68 69
A.5. The CipherSuite 69
A.6. The Security Parameters 71 72
B. Glossary 73
C. CipherSuite Definitions 77
D. Implementation Notes 79
D.1 Random Number Generation and Seeding 79
D.2 Certificates and Authentication 79
D.3 CipherSuites 79
D.4 Implementation Pitfalls 79
E. Backward Compatibility 82
E.1 Compatibility with TLS 1.0/1.1 and SSL 3.0 82
E.2 Compatibility with SSL 2.0 83
E.3. Avoiding Man-in-the-Middle Version Rollback 85
F. Security Analysis 86
F.1. Handshake Protocol 86
F.1.1. Authentication and Key Exchange 86
F.1.1.1. Anonymous Key Exchange 86
F.1.1.2. RSA Key Exchange and Authentication 87
F.1.1.3. Diffie-Hellman Key Exchange with Authentication 87
F.1.2. Version Rollback Attacks 88
F.1.3. Detecting Attacks Against the Handshake Protocol 89
F.1.4. Resuming Sessions 89
F.2. Protecting Application Data 89
F.3. Explicit IVs 90
F.4. Security of Composite Cipher Modes 90
F.5 Denial of Service 91
F.6 Final Notes 92 91
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., DES [DES], RC4 [SCH] etc.). The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record Protocol
can also be used without encryption.
- The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA, MD5, etc.) are used for MAC computations. The Record Protocol
can operate without a MAC, but is generally only used in this mode
while another protocol is using the Record Protocol as a transport
for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher-
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other and
to negotiate an encryption algorithm and cryptographic keys before
the application protocol transmits or receives its first byte of
data. The TLS Handshake Protocol provides connection security that
has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required for
at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.
One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS Protocol
transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on how to initiate TLS
handshaking and how to interpret the authentication certificates
exchanged are left to the judgment of the designers and implementors
of protocols that run on top of TLS.
1.1
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
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:
- Merged in TLS Extensions definition and AES Cipher Suites from
external documents [TLSEXT] and [TLSAES].
- Replacement of The MD5/SHA-1 combination in the PRF. Addition
of cipher-suite PRF has been replaced with cipher
suite specified PRFs. All cipher suites in this document use
P_SHA256.
- Replacement of The MD5/SHA-1 combination in the digitally-signed
element. element has been
replaced with a single hash.
- Substantial cleanup to the clients and servers ability to specify
which digest hash and signature algorithms they will accept. Note that
this also relaxes some of the constraints on signature and digest 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.
- Added some guidance that DH groups should be checked for size. Verify_data length now depends on the cipher suite (default is
still 12).
- Cleaned up description of Bleichenbacher/Klima attack defenses.
- Tighter checking of EncryptedPreMasterSecret version numbers.
- Stronger language about when alerts Alerts MUST now be sent.
- Added an Implementation Pitfalls sections sent in many cases.
- Harmonized the requirement to After a certificate_request, if no certificates are available,
clients now MUST send an empty certificate list
after certificate_request even when no certs are available.
- Made the verify_data length depend on the cipher suite. list.
- TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement
cipher suite.
- IDEE and DES are now deprecated.
- Support for the SSLv2 backward-compatible hello is now a MAY, not
a SHOULD. This will probably become a SHOULD NOT in the future.
- Added an Implementation Pitfalls sections
- The usual clarifications and editorial work.
2. Goals
The goals of TLS Protocol, in order of their 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 bytestream, a multi-byte item (a numeric in the
example) is formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
... | byte[n-1];
This byte ordering for multi-byte values is the commonplace network
byte order or big endian format.
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, 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" or "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;
Enumerateds occupy as much space in the byte stream as would its
maximal defined ordinal value. The following definition would cause
one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to
force the width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2, or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to
the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is well
specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation,
the numerical information may be omitted.
enum { low, medium, high } Amount;
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. 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 en: Ten;
} [[fv]];
} [[Tv]];
For example:
enum { apple, orange } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple: V1; /* VariantBody, tag = apple */
case orange: V2; /* VariantBody, tag = orange */
} variant_body; /* optional label on variant */
} VariantRecord;
Variant structures may be qualified (narrowed) by specifying a value
for the selector prior to the type. For example, an
orange VariantRecord
is a narrowed type of a VariantRecord containing a variant_body of
type V2.
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).
In digital signing, one-way hash functions are used as input for a
signing algorithm. A digitally-signed element is encoded as an opaque
vector <0..2^16-1>, where the length is specified by the signing
algorithm and key.
In RSA signing, 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 and for
digest hash
algorithms without parameters (which include 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 which did not include a DigestInfo encoding.
In DSS, 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 DSS signature is an opaque vector, as above,
the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
Note: In current terminology, DSA refers to the Digital Signature
Algorithm and DSS refers to the NIST standard. For historical
reasons, this document uses DSS and DSA interchangeably
to refer to the DSA algorithm, as was done in SSLv3.
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 the aead-ciphered
output is generally larger than the input in order to accomodate 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 hash[20];
} UserType;
the contents of hash are used as input for the signing algorithm, and
then the entire structure is encrypted with a stream cipher. The
length of this structure, in bytes, would be equal to two bytes for
field1 and field2, plus two bytes for the length of the signature,
plus the length of the output of the signing algorithm. This is known
because the algorithm and key used for the signing are known prior to
encoding or decoding this structure.
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 pseudo-random 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 is necessary to produce the
required quantity of data. For example, if P_SHA-1 is being used to
create 64 bytes of data, it will have to be iterated 4 times (through
A(4)), creating 80 bytes of output data; the last 16 bytes of the
final iteration will then be discarded, leaving 64 bytes of output
data.
TLS's PRF is created by 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 record protocol clients are described in this document: the
handshake protocol, the alert protocol, the change cipher spec
protocol, and the application data protocol. In order to allow
extension of the TLS protocol, additional record types can be
supported by the record protocol. New record type values are assigned
by IANA 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. Note that because the
type and length of a record are not protected by encryption, care
SHOULD be taken to minimize the value of traffic analysis of these
values.
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 secret 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 Change Cipher Spec can selectively make either of
the pending states current, in which case the appropriate current
state is disposed of and replaced with the pending state; the pending
state is then reinitialized to an empty state. It is illegal to make
a state that has not been initialized with security parameters a
current state. The initial current state always specifies that no
encryption, compression, or MAC will be used.
The security parameters for a TLS Connection read and write state are
set by providing the following values:
connection end
Whether this entity is considered the "client" or the "server" in
this connection.
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, how much of that key is
secret, whether it is a block,
stream, or AEAD cipher, and the block size and fixed initialization vector size of the cipher (if
appropriate).
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, rc2, des, 3des, idea, aes }
BulkCipherAlgorithm;
enum { stream, block, aead } CipherType;
enum { null, hmac_md5, hmac_sha, hmac_sha256, hmac_sha384,
hmac_sha512} MACAlgorithm;
/* The use of "sha" above is historical and denotes SHA-1 */
enum { null(0), (255) } CompressionMethod;
/* The algorithms specified in CompressionMethod,
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;
uint8 verify_data_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: items (some of which are not required by all ciphers,
and are thus empty):
client write MAC secret key
server write MAC secret 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 secret key
The MAC secret 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, 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 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 Change Cipher Spec 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.
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_secret, 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 CipherSuite is TLS_NULL_WITH_NULL_NULL, encryption
consists of the identity operation (i.e., the data is not encrypted,
and the MAC size is zero, implying that no MAC is used).
TLSCiphertext.length is TLSCompressed.length plus
SecurityParameters.mac_length.
6.2.3.2. CBC Block Cipher
For block ciphers (such as RC2, DES, 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 has to 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]; in 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(key, IV, plaintext,
additional_data)
In order to decrypt and verify, the cipher takes as input the key,
IV, 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. I.e.,
TLSCompressed.fragment = AEAD-Decrypt(write_key, IV,
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, and MAC
secrets generates keys required
by the current connection state (see Appendix A.6) from the security
parameters provided by the handshake protocol.
The master secret is hashed expanded into a sequence of secure bytes, which
are assigned
is then split to the MAC secrets and keys required by the current
connection state (see Appendix A.6). CipherSpecs require a client write MAC secret, key, a server write MAC secret, key, a
client write encryption key, and a server write key, each encryption key. Each
of which these is generated from the master secret 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 secrets 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_secret[SecurityParameters.mac_key_length]
server_write_MAC_secret[SecurityParameters.mac_key_length]
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]
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_SHA. It requires 2 x 32
byte keys and 2 x 20 byte MAC secrets, keys, for a total 104 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 bulk data encryption algorithm (such as null, DES,
etc.) and a MAC algorithm (such as MD5 or SHA). It also defines
cryptographic attributes such as the 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 change cipher spec message is sent by both the client and the
server to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the Record Layer to
immediately copy the read pending state into the read current state.
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See Section 6.1.) The change cipher spec message is sent during the
handshake after the security parameters have been agreed upon, but
before the verifying finished message is sent.
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 and a
description of the alert. Alert messages with a level of fatal result
in the immediate termination of the connection. In this case, other
connections corresponding to the session may continue, but the
session identifier MUST be invalidated, preventing the failed session
from being used to establish new connections. Like other messages,
alert messages are encrypted and compressed, as specified by the
current connection state.
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
7.2.1. Closure Alerts
The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages.
close_notify
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.
If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding close_notify. No part of
this standard should be taken to dictate the manner in which a usage
profile for TLS manages its data transport, including when
connections are opened or closed.
Note: It is assumed that closing a connection reliably delivers
pending data before destroying the transport.
7.2.2. Error Alerts
Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other
party. Upon transmission or receipt of a fatal alert message, both
parties immediately close the connection. Servers and clients MUST
forget any session-identifiers, keys, and secrets associated with a
failed connection. Thus, any connection terminated with a fatal alert
MUST NOT be resumed.
Whenever an implementation encounters a condition which is defined as
a fatal alert, it MUST send the appropriate alert prior to closing
the connection. In cases where an implementation chooses to send an
alert which may be a warning 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.
The following error alerts are defined:
unexpected_message
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn't an
even multiple of the block length, or its padding values, when
checked, weren't correct. This message is always fatal.
decryption_failed_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.
decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal.
handshake_failure
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not 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.
decrypt_error
A handshake cryptographic operation failed, including being unable
to correctly verify a signature, decrypt a key exchange, or
validate a finished message.
export_restriction_RESERVED
This alert was used in 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.
For all errors where an alert level is not explicitly specified, the
sending party MAY determine at its discretion whether this is a fatal
error or not; if an alert with a level of warning is received, the
receiving party MAY decide at its discretion whether to treat this as
a fatal error or not. However, all messages that are transmitted
with a level of fatal MUST be treated as fatal messages.
New Alert values 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 client hello message to
which the server must respond with a server hello message, or else a
fatal error will occur and the connection will fail. The client hello
and server hello are used to establish security enhancement
capabilities between client and server. The client hello and server
hello establish the following attributes: Protocol Version, Session
ID, Cipher Suite, and Compression Method. Additionally, two random
values are generated and exchanged: ClientHello.random and
ServerHello.random.
The actual key exchange uses up to four messages: the server
certificate, the server key exchange, the client certificate, and the
client key exchange. New key exchange methods can be created by
specifying a format for these messages and by defining the use of the
messages to allow the client and server to agree upon a shared
secret. This secret MUST be quite long; currently defined key
exchange methods exchange secrets that range from 46 bytes upwards.
Following the hello messages, the server will send its certificate,
if it is to be authenticated. Additionally, a server key exchange
message may be sent, if it is required (e.g., if their server has no
certificate, or if its certificate is for signing only). If the
server is authenticated, it may request a certificate from the
client, if that is appropriate to the cipher suite selected. Next,
the server will send the server hello done message, indicating that
the hello-message phase of the handshake is complete. The server will
then wait for a client response. If the server has sent a certificate
request message, the client MUST send the certificate message. The
client key exchange message is now sent, and the content of that
message will depend on the public key algorithm selected between the
client hello and the server hello. If the client has sent a
certificate with signing ability, a digitally-signed certificate
verify message is sent to explicitly verify possession of the private
key in the certificate.
At this point, a change cipher spec message is sent by the client,
and the client copies the pending Cipher Spec into the current Cipher
Spec. The client then immediately sends the finished message under
the new algorithms, keys, and secrets. In response, the server will
send its own change cipher spec message, transfer the pending to the
current Cipher Spec, and send its finished message under the new
Cipher Spec. At this point, the handshake is complete, and the client
and server may begin to exchange application layer data. (See flow
chart below.) Application data MUST NOT be sent prior to the
completion of the first handshake (before a cipher suite other 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
Fig. 1. Message flow for a full handshake
* Indicates optional or situation-dependent messages that are not
always sent.
Note: To help avoid pipeline stalls, ChangeCipherSpec is an
independent TLS Protocol content type, and is not actually a TLS
handshake message.
When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security
parameters), the message flow is as follows:
The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match. If
a match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both
client and server MUST send change cipher spec messages and proceed
directly to finished messages. Once the re-establishment is complete,
the client and server MAY begin to exchange application layer data.
(See flow chart below.) If a Session ID match is not found, the
server generates a new session ID and the TLS client and server
perform a full handshake.
Client Server
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Fig. 2. Message flow for an abbreviated handshake
The contents and significance of each message will be presented in
detail in the following sections.
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 Hello
Request message, which can be sent at any time, but which SHOULD be
ignored by the client if it arrives in the middle of a handshake.
New Handshake message 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 hello request message MAY be sent by the server at any time.
Meaning of this message:
Hello request is a simple notification that the client should
begin the negotiation process anew by sending a client hello
message when convenient. This message 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 hello request but
does not receive a client hello in response, it may close the
connection with a fatal alert.
After sending a hello request, servers SHOULD NOT repeat the
request until the subsequent handshake negotiation is complete.
Structure of this message:
struct { } HelloRequest;
Note: This message MUST NOT be included in the message hashes that
are maintained throughout the handshake and used in the finished
messages and the certificate verify message.
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 client hello as its first message. The client can also send a
client hello in response to a hello request or on its own
initiative in order to renegotiate the security parameters in an
existing connection.
Structure of this message:
The client hello message includes a random structure, which is
used later in the protocol.
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
gmt_unix_time
The current time and date in standard UNIX 32-bit format
(seconds since the midnight starting Jan 1, 1970, GMT, ignoring
leap seconds) according to the sender's internal clock. Clocks
are not required to be set correctly by the basic TLS Protocol; higher-
level
higher-level or application protocols may define additional
requirements.
random_bytes
28 bytes generated by a secure random number generator.
The client hello message includes a variable-length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes to
reuse. The session identifier MAY be from an earlier connection, this
connection, or from another currently active connection. The second
option is useful if the client only wishes to update the random
structures and derived values of a connection, and the third option
makes it possible to establish several independent secure connections
without repeating the full handshake protocol. These independent
connections may occur sequentially or simultaneously; a SessionID
becomes valid when the handshake negotiating it completes with the
exchange of Finished messages and persists until it is removed due to
aging or because a fatal error was encountered on a connection
associated with the session. The actual contents of the SessionID are
defined by the server.
opaque SessionID<0..32>;
Warning: Because the SessionID is transmitted without encryption or
immediate MAC protection, servers MUST NOT place confidential
information in session identifiers or let the contents of fake
session identifiers cause any breach of security. (Note that the
content of the handshake as a whole, including the SessionID, is
protected by the Finished messages exchanged at the end of the
handshake.)
The CipherSuite list, passed from the client to the server in the
client hello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client's
preference (favorite choice first). Each CipherSuite defines a key
exchange algorithm, a bulk encryption algorithm (including secret key
length), 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.
uint8 CipherSuite[2]; /* Cryptographic suite selector */
The client hello includes a list of compression algorithms supported
by the client, ordered according to the client's preference.
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
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 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 it 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.
client_hello_extension_list
Clients MAY request extended functionality from servers by sending
data in the client_hello_extension_list. Here the new
"client_hello_extension_list" field contains a list of extensions.
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 that supports the
extensions mechanism MUST accept client hello messages in either the
original (TLS 1.0/TLS 1.1) ClientHello or the extended ClientHello
format defined in this document, and (as for all other messages) 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 client hello message, the client waits for a server
hello message. Any other handshake message returned by the server
except for a hello request is treated as a fatal error.
7.4.1.3. Server Hello
When this message will be sent:
The server will send this message in response to a client hello
message when it was able to find an acceptable set of algorithms.
If it cannot find such a match, it will respond with a handshake
failure alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
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. Client 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.
server_hello_extension_list
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_hash_algorithms(TBD-BY-IANA),
signature_algorithms(TBD-BY-IANA), (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.
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 simply a refusal to support a particular
feature. 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 Hash Algorithms
The client MAY use the "signature_hash_algorithms" "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{
enum {
none(0), md5(1), sha1(2), sha256(3), sha384(4),
sha512(5), (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>;
Each SignatureAndHashAlgorithm value lists a single digest/signature hash/signature
pair which 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 undigested unhashed data, MD5 [MD5], SHA-1,
SHA-256, SHA-384, and SHA-512 [SHA] 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 which may be used.
The values indicate anonymous signatures, RSA [PKCS1] and DSA
[DSS] respectively. The "anonymous" value is meaningless in this
context but used later in the specification. 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
digest hash
algorithm. Sections 7.4.2 and 7.4.3 describe the appropriate rules.
Clients SHOULD send this extension if they support any digest hash algorithm
other than SHA-1.
If this extension is the client does not used, servers send the signature_algorithms extension, the
server SHOULD assume that 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 supports only SHA-1. 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 exchnage 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.
Servers MUST NOT send this extension.
7.4.2. Server Certificate
When this message will be sent:
The server MUST send a certificate 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 server
hello message.
Meaning of this message:
This message conveys the server's certificate to the client. The
certificate MUST be appropriate for the negotiated cipher suite's
key exchange algorithm, and any negotiated extensions.
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 optionally be omitted from the chain,
under the assumption that the remote end must already possess it
in order to validate it in any case.
The same message type and structure will be used for the client's
response to a certificate request message. Note that a client MAY
send no certificates if it does not have an appropriate certificate
to send in response to the server's authentication request.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate
vector because PKCS #6 [PKCS6] extended certificates are not used.
Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task
of parsing the list more difficult.
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 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
RSA_PSK allow the 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
ECDHE_RSA allow the 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.
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
DH_RSA keyAgreement bit MUST be set if the
key usage extension is present.
ECDH_ECDSA ECDH-capable public key; the public key
ECDH_RSA MUST 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 [4366bis] 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
digest/signature
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_DS, 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 digest/signature hash/signature algorithm pair appearing in the
extension. The naming is historical.
If no "signature_algorithms" extension is present, the end-entity
certificate MUST be signed as follows:
Key Exchange Alg. Signature Algorithm Used by Issuer
RSA RSA (RSASSA-PKCS1-v1_5)
DHE_RSA
DH_RSA
RSA_PSK
ECDH_RSA
ECDHE_RSA
DHE_DSS DSA
DH_DSS
ECDH_ECDSA ECDSA
ECDHE_ECDSA
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.).
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 CipherSuites that specify new key exchange methods are specified
for the TLS Protocol, they will imply 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 server hello message, if this is an anonymous
negotiation).
The server key exchange message is sent by the server only when
the server certificate message (if sent) does not contain enough
data to allow the client to exchange a premaster secret. This is
true for the following key exchange methods:
DHE_DSS
DHE_RSA
DH_anon
It is not legal to send the server key exchange message for the
following key exchange methods:
RSA
DH_DSS
DH_RSA
Meaning of this message:
This message conveys cryptographic information to allow the client
to communicate the premaster secret: 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 { diffie_hellman, rsa} rsa } 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 diffie_hellman:
ServerDHParams params;
Signature signed_params;
};
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
};
} ServerParams;
params
The server's key exchange parameters.
signed_params
For non-anonymous key exchanges, a hash of the corresponding
params value, with the signature appropriate to that hash
applied.
hash
Hash(ClientHello.random + ServerHello.random + ServerParams)
where Hash is the chosen hash value and Hash.length is
its output.
struct {
select (SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
SignatureAndHashAlgorithm signature_algorithm; /*NEW*/
digitally-signed struct {
opaque hash[Hash.length];
};
case dsa:
SignatureAndHashAlgorithm signature_algorithm; /*NEW*/
digitally-signed struct {
opaque hash[Hash.length];
};
};
};
} Signature;
If the client has offered the "signature_algorithms" extension, the
signature algorithm and digest 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 DSS 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.
If no "signature_algorithms" extension is present, the server MUST
use SHA-1 as the hash algorithm.
In addition, the digest hash and signature algorithms MUST be compatible
with the key in the client's server's end-entity certificate. RSA keys MAY be
used with any permitted digest algorithm. hash algorithm, subject to restrictions in
the certificate, if any.
Because DSA signatures do not contain any secure indication of digest hash
algorithm, it must be unambiguous which digest algorithm there is to a risk of hash substitution if multiple hashes
may be used with any key. DSA keys specified with Object Identifier
1 2 840 10040 4 1 MUST Currently, DSS [DSS] may only be used with
SHA-1. Future revisions of
[PKIX] MAY define new object identifiers for DSA with DSS [DSS-3] are expected to allow other
digest
algorithms.
The hash algorithm is denoted Hash below. Hash.length is the length
of the output algorithms, as well as guidance as to which digest algorithms
should be used with each key size. In addition, future revisions of that algorithm.
[PKIX] may specify mechanisms for certificates to indicate which
digest algorithms are to be used with DSA.
As additional CipherSuites are defined for TLS that include new key
exchange algorithms, the server key exchange message will be sent if
and only if the certificate type associated with the key exchange
algorithm does not provide enough information for the client to
exchange a premaster secret.
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 Server Key Exchange
message (if it is sent; otherwise, the Server Certificate
message).
Structure of this message:
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
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 which the client may
offer.
rsa_sign a certificate containing an RSA key
dss_sign a certificate containing a DSS 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 digest/signature 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
both to describe known roots and a desired authorization space. If
the certificate_authorities list is empty then the client MAY send
any certificate of the appropriate ClientCertificateType, unless
there is some external arrangement to the contrary.
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 digest/signature
hash/signature algorithm pair found in supported_signature_types.
- The end-entity certificate provided by the client MUST contain a
key which is compatible with certificate_types. If the key is a
signature key, it MUST be usable with some digest/signature hash/signature
algorithm pair in supported_signature_types.
- 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 signature_types field, 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 {dss_sha1, rsa_sha1} signature
types, the client MAY to reply with a certificate containing a
static DH key, signed with RSA-
SHA1. 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 server hello done message is sent by the server to indicate
the end of the server hello and associated messages. After sending
this message, the server will wait for a client response.
Meaning of this message:
This message means that the server is done sending messages to
support the key exchange, and the client can proceed with its
phase of the key exchange.
Upon receipt of the server hello done message, the client SHOULD
verify that the server provided a valid certificate, if required
and check that the server hello parameters are acceptable.
Structure of this message:
struct { } ServerHelloDone;
7.4.6. Client Certificate
When this message will be sent:
This is the first message the client can send after receiving a
server hello done message. This message is only sent if the server
requests a certificate. If no suitable certificate is available,
the client SHOULD MUST send a certificate message containing no
certificates. That is, the certificate_list structure has a length
of zero. If client authentication is required by the server for
the handshake to continue, it may respond with a fatal handshake
failure alert. Client certificates are sent using the Certificate
structure defined in Section 7.4.2.
Meaning of this message:
This message conveys the client's certificate to the server; the
server will use it when verifying the certificate verify message
(when the client authentication is based on signing), or calculate
the premaster secret (for non-ephemeral Diffie-
Hellman). 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 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
dss_fixed_dh the same parameters as server's key.
rsa_fixed_ecdh ECDH-capable public key; MUST use
ecdsa_fixed_ecdh the same curve as server's key, and
MUST use a point format supported by
- If the certificate_authorities list in the certificate request
message was non-empty, the certificate SHOULD be issued by one of
the listed CAs.
- The certificates MUST be signed using an acceptable digest/ 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
server hello done message.
Meaning of this message:
With this message, the premaster secret is set, either though
direct transmission of the RSA-encrypted secret, or by the
transmission of Diffie-Hellman parameters that will allow each
side to agree upon the same premaster secret.
When the key exchange method is DH_RSA or
DH_DSS, client certification has been requested, and is using an ephemeral Diffie-Hellman exponent,
then this message contains the client's Diffie-Hellman public
value. If the client was
able to respond with is sending a certificate that contained containing a Diffie-Hellman
public key whose parameters (group and generator) matched those
specified by the server in its certificate, static
DH exponent (i.e., it is doing fixed_dh client authentication)
then this message MUST NOT
contain any data. 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 diffie_hellman: 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 client key exchange message and is not a message in itself.
Structure of this message:
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
client_version
The latest (newest) version supported by the client. This is
used to detect version roll-back attacks.
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 earlier than 1.0, 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.
The best way to avoid these vulnerabilities is to treat incorrectly
formatted messages 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 M
3. If the PKCS#1 padding is not correct, or the length of
message M is not exactly 48 bytes:
premaster secret = ClientHello.client_version || R
else If ClientHello.client_version <= TLS 1.0, and
version number check is explicitly disabled:
premaster secret = M
else:
premaster secret = ClientHello.client_version || M[2..47]
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
(though, 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 the length bytes, encoding the RSA encrypted data
directly in the ClientKeyExchange message.
This specification requires correct encoding of the
EncryptedPreMasterSecret complete with length bytes. The resulting
PDU is incompatible with many SSLv3 implementations. Implementors
upgrading from SSLv3 MUST modify their implementations to generate
and accept the correct encoding. Implementors who wish to be
compatible with both SSLv3 and TLS should make their implementation's
behavior dependent on the protocol version.
Implementation Note: It is now known that remote timing-based attacks
on 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 already which contains a suitable Diffie-
Hellman key,
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 {
Signature signature;
} CertificateVerify;
The Signature type is defined in 7.4.3.
The hash algorithm is denoted Hash below.
CertificateVerify.signature.hash = Hash(handshake_messages);
The digest hash and signature algorithms MUST be one of those present in the
supported_signature_algorithms field of the CertificateRequest
message. In addition, the digest hash and signature algorithms MUST be
compatible with the key in the client's end-
entity end-entity certificate. RSA
keys MAY be used with any permitted
digest algorithm. hash algorith, subject to
restrictions in the certificate, if any.
Because DSA signatures do not contain any secure indication of
digest hash
algorithm, it must be unambiguous which digest algorithm there is to a risk of hash substitution if multiple hashes
may be used with any key. DSA keys specified with Object
Identifier 1 2 840 10040 4 1 MUST Currently, DSS [DSS] may only be used with
SHA-1. Future revisions of DSS [DSS-3] are expected to allow other
digest algorithms, as well as guidance as to which digest algorithms
should be used with each key size. In addition, future revisions of
[PKIX] MAY define new object identifiers may specify mechanisms for
DSA with other certificates to indicate which
digest algorithms. algorithms are to be used with DSA.
Here handshake_messages refers to all handshake messages sent or
received starting at client hello up to but not including this
message, including the type and length fields of the handshake
messages. This is the concatenation of all the Handshake structures
as defined in 7.4 exchanged thus far.
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 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[SecurityParameters.verify_data_length]; verify_data[verify_data_length];
} Finished;
verify_data
PRF(master_secret, finished_label, Hash(handshake_messages))
[0..SecurityParameters.verify_data_length-1];
[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 negotiated hash handshake messages. For the PRF defined
in Section 5, the Hash MUST be the Hash used as the basis for the
PRF. If Any cipher suite which defines a new different PRF is defined, then this hash MUST be specified. 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 SecurityParameters.verify_data_length
verify_data_length has a
SecurityParameters.verify_data_length 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
7.4, exchanged thus far.
It is a fatal error if a finished message is not preceded by a change
cipher spec message at the appropriate point in the handshake.
The value handshake_messages includes all handshake messages starting
at client hello up to, but not including, this finished message. This
may be different from handshake_messages in Section 7.4.8 because it
would include the certificate verify message (if sent). Also, the
handshake_messages for the finished message sent by the client will
be different from that for the finished message sent by the server,
because the one that is sent second will include the prior one.
Note: Change cipher spec messages, alerts, and any other record types
are not handshake messages and are not included in the hash
computations. Also, Hello Request messages are omitted from handshake
hashes.
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 server hello message. The compression
algorithm is negotiated in the hello messages, and the random values
are exchanged in the hello messages. All that remains is to calculate
the master secret.
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.
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 is requested to update (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 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].
- 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 is requested to update (has updated) it to reference this
document. The registry and its allocation policy (unchanged from
[RFC4366]) is listed below:. below:
- TLS ExtensionType Registry: Future values are allocated via IETF
Consensus [RFC2434]
In addition, this document defines one two new registry registries to be
maintained by IANA:
- TLS SignatureAlgorithm Registry: The registry will be 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 will be 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 defines one new TLS extension, cert_hash_type, signature_algorithms,
which is to be (has been) allocated value TBD-BY-IANA in the TLS
ExtensionType registry.
This document also uses the TLS Compression Method Identifiers
Registry, defined in [RFC3749]. IANA is requested to allocate value
0 for the "null" compression method.
Appendix A. Protocol Constant Values
This section describes protocol types and constants.
A.1. Record Layer
struct {
uint8 major, 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;
aead-ciphered struct {
opaque IV[SecurityParameters.record_iv_length];
opaque aead_output[AEADEncrypted.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..2^16-1>; 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_hash_algorithms(TBD-BY-IANA),
signature_algorithms(TBD-BY-IANA), (65535)
} ExtensionType;
enum{
none(0), md5(1), sha1(2), sha256(3), sha384(4),
sha512(5), (255)
} HashAlgorithm;
enum { anonymous(0), rsa(1), dsa(2), (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 { rsa, diffie_hellman } KeyExchangeAlgorithm;
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
}
} ServerKeyExchange;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
};
} ServerParams;
struct {
select (SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
SignatureAndHashAlgorithm signature_algorithm; /*NEW*/
digitally-signed struct {
opaque hash[Hash.length];
};
case dsa:
SignatureAndHashAlgorithm signature_algorithm; /*NEW*/
digitally-signed struct {
opaque hash[Hash.length];
};
};
};
} Signature;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
A.4.3. Client Authentication and Key Exchange Messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
enum { implicit, explicit } PublicValueEncoding;
struct {
select (PublicValueEncoding) {
case implicit: struct {};
case explicit: opaque DH_Yc<1..2^16-1>;
} dh_public;
} ClientDiffieHellmanPublic;
struct {
Signature signature;
} CertificateVerify;
A.4.4. Handshake Finalization Message
struct {
opaque verify_data[SecurityParameters.verify_data_length]; verify_data[verify_data_length];
} Finished;
A.5. The CipherSuite
The following values define the CipherSuite codes used in the client
hello and server hello messages.
A CipherSuite defines a cipher specification supported in TLS Version
1.2.
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 either an RSA or a DSS 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_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A };
CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F 0x00,0x2F };
CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 0x00,0x35 };
The following CipherSuite definitions are used for server-
authenticated (and optionally client-authenticated) Diffie-Hellman.
DH denotes cipher suites in which the server's certificate contains
the Diffie-Hellman parameters signed by the certificate authority
(CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
parameters are signed by a DSS or RSA a signature-capable certificate, which has
been signed by the CA. The signing algorithm used is specified after
the DH or DHE parameter. The server can request an RSA or DSS signature-
capable 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_DES_CBC_SHA = { 0x00,0x0C };
CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 };
CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 0x00,0x30 };
CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 0x00,0x31 };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 0x00,0x32 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 0x00,0x33 };
CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 0x00,0x36 };
CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 0x00,0x37 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 0x00,0x38 };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 0x00,0x39 };
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 ciphersuites 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_DES_CBC_SHA = { 0x00, 0x1A 0x00,0x18 };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x1B 0x00,0x1B };
CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 0x00,0x34 };
CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A 0x00,0x3A };
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.
SSLv3, TLS 1.0, and TLS 1.1 supported DES and IDEA. DES had a 56-bit
key which is too weak for modern use. Triple-DES (3DES) has an
effective key strength of 112 bits and is still acceptable. IDEA and
is no longer in wide use. Cipher suites using RC2, DES, and IDEA are
hereby deprecated for TLS 1.2. TLS 1.2 implementations MUST NOT
negotiate these cipher suites in TLS 1.2 mode. However, for backward
compatibility they may be offered in the ClientHello for use with TLS
1.0 or SSLv3 only servers. TLS 1.2 clients MUST check that the server
did not choose one of these cipher suites during the handshake. These
ciphersuites are listed below for informational purposes and to
reserve the numbers.
CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C };
CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
When SSLv3 and TLS 1.0 were designed, the United States restricted
the export of cryptographic software containing certain strong
encryption algorithms. A series of cipher suites were designed to
operate at reduced key lengths in order to comply with those
regulations. Due to advances in computer performance, these
algorithms are now unacceptably weak and export restrictions have
since been loosened. TLS 1.2 implementations MUST NOT negotiate these
cipher suites in TLS 1.2 mode. However, for backward compatibility
they may be offered in the ClientHello for use with TLS 1.0 or SSLv3
only servers. TLS 1.2 clients MUST check that the server did not
choose one of these cipher suites during the handshake. These
ciphersuites are listed below for informational purposes and to
reserve the numbers.
CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
New cipher suite values are 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, rc2, des, 3des, des40, aes, idea aes }
BulkCipherAlgorithm;
enum { stream, block, aead } CipherType;
enum { null, hmac_md5, hmac_sha, hmac_sha256, hmac_sha384,
hmac_sha512} MACAlgorithm;
/* The algorithms specified in CompressionMethod,
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;
uint8 verify_data_length;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
Appendix B. Glossary
Advanced Encryption Standard (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. [AES] 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 is a common block size.
bulk cipher
A symmetric encryption algorithm used to encrypt large quantities
of data.
cipher block chaining (CBC)
CBC is a mode in which every plaintext block encrypted with a
block cipher is first exclusive-ORed with the previous ciphertext
block (or, in the case of the first block, with the initialization
vector). For decryption, every block is first decrypted, then
exclusive-ORed with the previous ciphertext block (or IV).
certificate
As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party's identity
or some other attributes and its public key.
client
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.
client write key
The key used to encrypt data written by the client.
client write MAC secret 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 is a very widely used symmetric encryption algorithm. DES is a
block cipher with a 56 bit key and an 8 byte block size. Note that
in TLS, for key generation purposes, DES is treated as having an 8
byte key length (64 bits), but it still only provides 56 bits of
protection. (The low bit of each key byte is presumed to be set to
produce odd parity in that key byte.) DES can also be operated in
a mode where three independent keys and three encryptions are used
for each block of data; this uses 168 bits of key (24 bytes in the
TLS key generation method) and provides the equivalent of 112 bits
of security. [DES], [3DES]
Digital Signature Standard (DSS)
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186, "Digital Signature
Standard", published May, 1994 by the U.S. Dept. of Commerce.
[DSS]
digital signatures
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.
handshake
An initial negotiation between client and server that establishes
the parameters of their transactions.
Initialization Vector (IV)
When a block cipher is used in CBC mode, the initialization vector
is exclusive-ORed with the first plaintext block prior to
encryption.
IDEA
A 64-bit block cipher designed by Xuejia Lai and James Massey.
[IDEA]
Message Authentication Code (MAC)
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.
master secret
Secure secret data used for generating encryption keys, MAC
secrets, and IVs.
MD5
MD5 is a secure hashing function that converts an arbitrarily long
data stream into a digest hash of fixed size (16 bytes). [MD5]
public key cryptography
A class of cryptographic techniques employing two-key ciphers.
Messages encrypted with the public key can only be decrypted with
the associated private key. Conversely, messages signed with the
private key can be verified with the public key.
one-way hash function
A one-way transformation that converts an arbitrary amount of data
into a fixed-length hash. It is computationally hard to reverse
the transformation or to find collisions. MD5 and SHA are examples
of one-way hash functions.
RC2
A block cipher developed by Ron Rivest, described in [RC2].
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 under client.
session
A TLS session is an association between a client and a server.
Sessions are created by the handshake protocol. Sessions define a
set of cryptographic security parameters 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 secret key
The secret data used to authenticate data written by the server.
SHA
The Secure Hash Algorithm is defined in FIPS PUB 180-2. It
produces a 20-byte output. Note that all references to SHA
actually use the modified SHA-1 algorithm. [SHA]
SSL
Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on
SSL Version 3.0
stream cipher
An encryption algorithm that converts a key into a
cryptographically strong keystream, which is then exclusive-ORed
with the plaintext.
symmetric cipher
See bulk cipher.
Transport Layer Security (TLS)
This protocol; also, the Transport Layer Security working group of
the Internet Engineering Task Force (IETF). See "Comments" at the
end of this document.
Appendix C. CipherSuite Definitions
CipherSuite Key Cipher Hash
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_RC4_128_MD5 RSA RC4_128 MD5
TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA
TLS_RSA_WITH_AES_128_CBC_SHA RSA AES_128_CBC SHA
TLS_RSA_WITH_AES_256_CBC_SHA RSA AES_256_CBC SHA
TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA
TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA
TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA
TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA
TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5
TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA
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
Key
Exchange
Algorithm Description Key size limit
DHE_DSS Ephemeral DH with DSS signatures None
DHE_RSA Ephemeral DH with RSA signatures None
DH_anon Anonymous DH, no signatures None
DH_DSS DH with DSS-based certificates None
DH_RSA DH with RSA-based certificates None
NULL No key exchange N/A
RSA RSA key exchange None
Key Expanded IV Block
Cipher Type Material Key Material Size Size
NULL Stream 0 0 0 N/A
IDEA_CBC Block 16 16 8 8
RC4_128 Stream 16 16 0 N/A
DES_CBC Block 8 8 8 8
3DES_EDE_CBC Block 24 24 8 8
Type
Indicates whether this is a stream cipher or a block cipher
running in CBC mode.
Key Material
The number of bytes from the key_block that are used for
generating the write keys.
Expanded Key Material
The number of bytes actually fed into the encryption algorithm.
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.
Hash Hash Padding
function Size Size
NULL 0 0
MD5 16 48
SHA 20 40
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 CipherSuites
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 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 this 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 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)?
- 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 support 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 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 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 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
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 ciphersuites 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 SHOULD 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 client hello message. The value MUST be one (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, each cipher suite prefixed by a zero
byte. For example, TLS ciphersuite {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 eight
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 CipherSpec and
generating a Master Secret, which together comprise the primary
cryptographic parameters associated with a secure session. The
handshake protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority.
F.1.1. Authentication and Key Exchange
TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and
total anonymity. Whenever the server is authenticated, the channel is
secure against man-in-the-middle attacks, but completely anonymous
sessions are inherently vulnerable to such attacks. Anonymous
servers cannot authenticate clients. If the server is authenticated,
its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly,
authenticated clients must supply an acceptable certificate to the
server. Each party is responsible for verifying that the other's
certificate is valid and has not expired or been revoked.
The general goal of the key exchange process is to create a
pre_master_secret known to the communicating parties and not to
attackers. The pre_master_secret will be used to generate the
master_secret (see Section 8.1). The master_secret is required to
generate the finished messages, encryption keys, and MAC secrets 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 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 DSS or RSA certificate.
Temporary parameters are hashed with the hello.random values before
signing to ensure that attackers do not replay old parameters. In
either case, the client can verify the certificate or signature to
ensure that the parameters belong to the server.
If the client has a certificate containing fixed Diffie-Hellman
parameters, its certificate contains the information required to
complete the key exchange. Note that in this case the client and
server will generate the same Diffie-Hellman result (i.e.,
pre_master_secret) every time they communicate. To prevent the
pre_master_secret from staying in memory any longer than necessary,
it should be converted into the master_secret as soon as possible.
Client Diffie-Hellman parameters must be compatible with those
supplied by the server for the key exchange to work.
If the client has a standard DSS or RSA certificate or is
unauthenticated, it sends a set of temporary parameters to the server
in the client key exchange message, then optionally uses a
certificate verify message to authenticate itself.
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 ciphersuites 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 ciphersuites.
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 chooses.
For this attack, an attacker must actively change one or more
handshake messages. If this occurs, the client and server will
compute different values for the handshake message hashes. As a
result, the parties will not accept each others' finished messages.
Without the master_secret, the attacker cannot repair the finished
messages, so the attack will be discovered.
F.1.4. Resuming Sessions
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not been
compromised and that the secure hash operations used to produce the
encryption keys and MAC secrets 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 secret, 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 secrets. 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 secrets 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
ciphersuite. 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
ciphersuites 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
pseudo-random generator and this pad is xor-ed 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 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
denial of service 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.
Changes in This Version
[RFC Editor: Please delete this]
- Redid the hash function advertisements for CertificateRequest
and the client-side extension. They are now pairs of
hash/signature and the semantics are clearly defined for
all uses of signatures (hopefully). [Issue 41]
- Clarified the DH group checking per list discussion [Issue 35] SSLv2 backward compatibility downgraded to MAY
- Added a note about DSS vs. Altered DSA [Issue 58] hash rules to more closely match FIPS186-3 and
PKIX, plus remove OID restriction.
- Editorial issues [Issue 59] verify_length no longer in SecurityParameters
- Cleaned Moved/cleaned up certificate cert selection text in 7.4.2 and 7.4.6 [Issue 57] for server cert
when signature_algorithms is not specified.
- Other editorial changes.
Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard (AES)"
FIPS 197. November 26, 2001.
[3DES] National Institute of Standards and Technology,
"Recommendation for the Triple Data Encryption Algorithm
(TDEA) Block Cipher", NIST Special Publication 800-67, May
2004.
[DES] National Institute of Standards and Technology, "Data
Encryption Standard (DES)", FIPS PUB 46-3, October 1999.
[DSS] NIST FIPS PUB 186-2, "Digital Signature Standard," National
Institute of Standards and Technology, U.S. Department of
Commerce, 2000.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH
Series in Information Processing, v. 1, Konstanz: Hartung-
Gorre Verlag, 1992.
[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
[PKCS1] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards
(PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC
3447, February 2003.
[PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[RC2] Rivest, R., "A Description of the RC2(r) Encryption
Algorithm", RFC 2268, March 1998.
[SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms,
and Source Code in C, 2nd ed.", Published by John Wiley &
Sons, Inc. 1996.
[SHA] NIST FIPS PUB 180-2, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of
Commerce., August 2001.
[REQ] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 25, RFC 2434,
October 1998.
Informative References
[AEAD] Mcgrew, D., "Authenticated Encryption", February 2007,
draft-mcgrew-auth-enc-02.txt.
[AH] Kent, S., and Atkinson, R., "IP Authentication Header", RFC
4302, December 2005.
[BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against
Protocols Based on RSA Encryption Standard PKCS #1" in
Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages:
1-12, 1998.
[CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
Problems and Countermeasures",
http://www.openssl.org/~bodo/tls-cbc.txt.
[CBCTIME] Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux,
"Password Interception in a SSL/TLS Channel", Advances in
Cryptology -- CRYPTO 2003, LNCS vol. 2729, 2003.
[CCM] "NIST Special Publication 800-38C: The CCM Mode for
Authentication and Confidentiality",
http://csrc.nist.gov/publications/nistpubs/800-38C/
SP800-38C.pdf
[DSS-3] NIST FIPS PUB 186-3 Draft, "Digital Signature Standard,"
National Institute of Standards and Technology, U.S.
Department of Commerce, 2006.
[ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)",
Crypto 2001.
[ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security
Payload (ESP)", RFC 4303, December 2005.
[FI06] Hal Finney, "Bleichenbacher's RSA signature forgery based on
implementation error", ietf-openpgp@imc.org mailing list, 27
August 2006, http://www.imc.org/ietf-openpgp/mail-
archive/msg14307.html.
[GCM] "NIST Special Publication 800-38D DRAFT (June, 2007):
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC"
[IKEALG] Schiller, J., "Cryptographic Algorithms for Use in the
Internet Key Exchange Version 2 (IKEv2)", RFC 4307, December
2005.
[KEYSIZ] Orman, H., and Hoffman, P., "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys" RFC 3766,
April 2004.
[KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
March 2003.
[MODP] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC
3526, May 2003.
[PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax
Standard," version 1.5, November 1993.
[PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax
Standard," version 1.5, November 1993.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, May 2004.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
Wright, T., "Transport Layer Security (TLS) Extensions", RFC
4366, April 2006.
[RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key Cryptosystems,"
Communications of the ACM, v. 21, n. 2, Feb 1978, pp.
120-126.
[SEQNUM] Bellovin. S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications
Corp., Feb 9, 1995.
[SSL3] A. Freier, P. Karlton, and P. Kocher, "The SSL 3.0
Protocol", Netscape Communications Corp., Nov 18, 1996.
[SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
Attacks on the Diffie-Hellman Key Agreement Method for
S/MIME", RFC 2785, March 2000.
[TCP] Postel, J., "Transmission Control Protocol," STD 7, RFC 793,
September 1981.
[TIMING] Boneh, D., Brumley, D., "Remote timing attacks are
practical", USENIX Security Symposium 2003.
[TLSAES] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
[TLSECC] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and
Moeller, B., "Elliptic Curve Cryptography (ECC) Cipher
Suites for Transport Layer Security (TLS)", RFC 4492, May
2006.
[TLSEXT] Eastlake, D.E., "Transport Layer Security (TLS) Extensions:
Extension Definitions", July 2007, draft-ietf-tls-
rfc4366-bis-00.txt.
[TLSPGP] Mavrogiannopoulos, N., "Using OpenPGP keys for TLS
authentication", draft-ietf-tls-openpgp-keys-11, July 2006.
[TLSPSK] Eronen, P., Tschofenig, H., "Pre-Shared Key Ciphersuites for
Transport Layer Security (TLS)", RFC 4279, December 2005.
[TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol, Version 1.0",
RFC 2246, January 1999.
[TLS1.1] Dierks, T., and E. Rescorla, "The TLS Protocol, Version
1.1", RFC 4346, April, 2006.
[X501] ITU-T Recommendation X.501: Information Technology - Open
Systems Interconnection - The Directory: Models, 1993.
[XDR] Eisler, M., "External Data Representation Standard", RFC
4506, May 2006.
Credits
Working Group Chairs
Eric Rescorla
EMail: ekr@networkresonance.com
Pasi Eronen
pasi.eronen@nokia.com
Editors
Tim Dierks Eric Rescorla
Independent Network Resonance, Inc.
EMail: tim@dierks.org EMail: ekr@networkresonance.com
Other contributors
Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
ChristopherA@AlacrityManagement.com
Martin Abadi
University of California, Santa Cruz
abadi@cs.ucsc.edu
Steven M. Bellovin
Columbia University
smb@cs.columbia.edu
Simon Blake-Wilson
BCI
EMail: sblakewilson@bcisse.com
Ran Canetti
IBM
canetti@watson.ibm.com
Pete Chown
Skygate Technology Ltd
pc@skygate.co.uk
Taher Elgamal
taher@securify.com
Securify
Anil Gangolli
anil@busybuddha.org
Kipp Hickman
Alfred Hoenes
David Hopwood
Independent Consultant
EMail: david.hopwood@blueyonder.co.uk
Phil Karlton (co-author of SSLv3)
Paul Kocher (co-author of SSLv3)
Cryptography Research
paul@cryptography.com
Hugo Krawczyk
Technion Israel Institute of Technology
IBM
hugo@ee.technion.ac.il
Jan Mikkelsen
Transactionware
EMail: janm@transactionware.com
Magnus Nystrom
RSA Security
EMail: magnus@rsasecurity.com
Robert Relyea
Netscape Communications
relyea@netscape.com
Jim Roskind
Netscape Communications
jar@netscape.com
Michael Sabin
Dan Simon
Microsoft, Inc.
dansimon@microsoft.com
Tom Weinstein
Tim Wright
Vodafone
EMail: timothy.wright@vodafone.com
Comments
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