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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5246
INTERNET-DRAFT Tim Dierks
Obsoletes (if approved): RFC 3268, 4346, 4366 Independent
Updates (if approved): RFC 4492 Eric Rescorla
Intended status: Proposed Standard Network Resonance, Inc.
<draft-ietf-tls-rfc4346-bis-10.txt> March 2008 (Expires September 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 (2008).
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.
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Table of Contents
1. Introduction 4
1.1. Requirements Terminology 5
1.2. Major Differences from TLS 1.1 5
2. Goals 6
3. Goals of This Document 7
4. Presentation Language 7
4.1. Basic Block Size 7
4.2. Miscellaneous 7
4.3. Vectors 8
4.4. Numbers 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 14
6. The TLS Record Protocol 15
6.1. Connection States 16
6.2. Record layer 18
6.2.1. Fragmentation 19
6.2.2. Record Compression and Decompression 20
6.2.3. Record Payload Protection 21
6.2.3.1. Null or Standard Stream Cipher 21
6.2.3.2. CBC Block Cipher 22
6.2.3.3. AEAD ciphers 24
6.3. Key Calculation 25
7. The TLS Handshaking Protocols 26
7.1. Change Cipher Spec Protocol 27
7.2. Alert Protocol 27
7.2.1. Closure Alerts 28
7.2.2. Error Alerts 29
7.3. Handshake Protocol Overview 33
7.4. Handshake Protocol 37
7.4.1. Hello Messages 38
7.4.1.1. Hello Request 38
7.4.1.2. Client Hello 39
7.4.1.3. Server Hello 42
7.4.1.4 Hello Extensions 43
7.4.1.4.1 Signature Algorithms 45
7.4.2. Server Certificate 46
7.4.3. Server Key Exchange Message 49
7.4.4. Certificate Request 51
7.4.5 Server Hello Done 53
7.4.6. Client Certificate 53
7.4.7. Client Key Exchange Message 55
7.4.7.1. RSA Encrypted Premaster Secret Message 56
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7.4.7.2. Client Diffie-Hellman Public Value 58
7.4.8. Certificate verify 59
7.4.9. Finished 60
8. Cryptographic Computations 62
8.1. Computing the Master Secret 62
8.1.1. RSA 62
8.1.2. Diffie-Hellman 62
9. Mandatory Cipher Suites 63
10. Application Data Protocol 63
11. Security Considerations 63
12. IANA Considerations 63
A. Protocol Data Structures and Constant Values 65
A.1. Record Layer 65
A.2. Change Cipher Specs Message 66
A.3. Alert Messages 66
A.4. Handshake Protocol 67
A.4.1. Hello Messages 67
A.4.2. Server Authentication and Key Exchange Messages 69
A.4.3. Client Authentication and Key Exchange Messages 70
A.4.4. Handshake Finalization Message 71
A.5. The Cipher Suite 71
A.6. The Security Parameters 73
A.7. Changes to RFC 4492 74
B. Glossary 74
C. Cipher Suite Definitions 79
D. Implementation Notes 81
D.1 Random Number Generation and Seeding 81
D.2 Certificates and Authentication 81
D.3 Cipher Suites 81
D.4 Implementation Pitfalls 81
E. Backward Compatibility 84
E.1 Compatibility with TLS 1.0/1.1 and SSL 3.0 84
E.2 Compatibility with SSL 2.0 85
E.3. Avoiding Man-in-the-Middle Version Rollback 87
F. Security Analysis 88
F.1. Handshake Protocol 88
F.1.1. Authentication and Key Exchange 88
F.1.1.1. Anonymous Key Exchange 88
F.1.1.2. RSA Key Exchange and Authentication 89
F.1.1.3. Diffie-Hellman Key Exchange with Authentication 89
F.1.2. Version Rollback Attacks 90
F.1.3. Detecting Attacks Against the Handshake Protocol 91
F.1.4. Resuming Sessions 91
F.2. Protecting Application Data 91
F.3. Explicit IVs 92
F.4. Security of Composite Cipher Modes 92
F.5 Denial of Service 93
F.6 Final Notes 93
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1. Introduction
The primary goal of the TLS Protocol is to provide privacy and data
integrity between two communicating applications. The protocol is
composed of two layers: the TLS Record Protocol and the TLS Handshake
Protocol. At the lowest level, layered on top of some reliable
transport protocol (e.g., TCP[TCP]), is the TLS Record Protocol. The
TLS Record Protocol provides connection security that has two basic
properties:
- The connection is private. Symmetric cryptography is used for
data encryption (e.g., AES [AES], RC4 [SCH] etc.). The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record Protocol
can also be used without encryption.
- The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA-1, etc.) are used for MAC computations. The Record Protocol
can operate without a MAC, but is generally only used in this mode
while another protocol is using the Record Protocol as a transport
for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher-
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other and
to negotiate an encryption algorithm and cryptographic keys before
the application protocol transmits or receives its first byte of
data. The TLS Handshake Protocol provides connection security that
has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSA [DSS], etc.). This
authentication can be made optional, but is generally required for
at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.
One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS Protocol
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transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on how to initiate TLS
handshaking and how to interpret the authentication certificates
exchanged are left to the judgment of the designers and implementors
of protocols that run on top of TLS.
1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [REQ].
1.2. Major Differences from TLS 1.1
This document is a revision of the TLS 1.1 [TLS1.1] protocol which
contains improved flexibility, particularly for negotiation of
cryptographic algorithms. The major changes are:
- The MD5/SHA-1 combination in the pseudorandom function (PRF) has
been replaced with cipher suite specified PRFs. All cipher suites
in this document use P_SHA256.
- The MD5/SHA-1 combination in the digitally-signed element has been
replaced with a single hash. Signed elements now include a field
that explicitly specifies the hash algorithm used.
- Substantial cleanup to the client's and server's ability to
specify which hash and signature algorithms they will accept. Note
that this also relaxes some of the constraints on signature and
hash algorithms from previous versions of TLS.
- Addition of support for authenticated encryption with additional
data modes.
- TLS Extensions definition and AES Cipher Suites were merged in
from external [TLSEXT] and [TLSAES].
- Tighter checking of EncryptedPreMasterSecret version numbers.
- Tightened up a number of requirements.
- Verify_data length now depends on the cipher suite (default is
still 12).
- Cleaned up description of Bleichenbacher/Klima attack defenses.
- Alerts MUST now be sent in many cases.
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- After a certificate_request, if no certificates are available,
clients now MUST send an empty certificate list.
- TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement
cipher suite.
- Added HMAC-SHA256 cipher suites
- Removed IDEA and DES cipher suites. They are now deprecated and
will be documented in a separate document.
- Support for the SSLv2 backward-compatible hello is now a MAY, not
a SHOULD, with sending it a SHOULD NOT. Support will probably
become a SHOULD NOT in the future.
- Added limited "fall-through" to the presentation language to allow
multiple case arms to have the same encoding.
- Added an Implementation Pitfalls sections
- The usual clarifications and editorial work.
2. Goals
The goals of 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.
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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
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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
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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.
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In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2, or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to
the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is well
specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation,
the numerical information may be omitted.
enum { low, medium, high } Amount;
4.6. Constructed Types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type. The
syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
The fields within a structure may be qualified using the type's name,
with a syntax much like that available for enumerateds. For example,
T.f2 refers to the second field of the previous declaration.
Structure definitions may be embedded.
4.6.1. Variants
Defined structures may have variants based on some knowledge that is
available within the environment. The selector must be an enumerated
type that defines the possible variants the structure defines. There
must be a case arm for every element of the enumeration declared in
the select. Case arms have limited fall-through: if two case arms
follow in immediate succession with no fields in between, then they
both contain the same fields. Thus, in the example below, "orange"
and "banana" both contain V2. Note that this is a new piece of syntax
in TLS 1.2.
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The body of the variant structure may be given a label for reference.
The mechanism by which the variant is selected at runtime is not
prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
case e3: case e4: Te3;
....
case en: Ten;
} [[fv]];
} [[Tv]];
For example:
enum { apple, orange, banana } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple:
V1; /* VariantBody, tag = apple */
case orange:
case banana:
V2; /* VariantBody, tag = orange or banana */
} variant_body; /* optional label on variant */
} VariantRecord;
4.7. Cryptographic Attributes
The five cryptographic operations digital signing, stream cipher
encryption, block cipher encryption, authenticated encryption with
additional data (AEAD) encryption and public key encryption are
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designated digitally-signed, stream-ciphered, block-ciphered, aead-
ciphered, and public-key-encrypted, respectively. A field's
cryptographic processing is specified by prepending an appropriate
key word designation before the field's type specification.
Cryptographic keys are implied by the current session state (see
Section 6.1).
A digitally-signed element is encoded as a struct DigitallySigned:
struct {
SignatureAndHashAlgorithm algorithm;
opaque signature<0..2^16-1>;
} DigitallySigned;
The algorithm field specifies the algorithm used (see Section
7.4.1.4.1 for the definition of this field.) Note that the
introduction of the algorithm field is a change from previous
versions. The signature is a digital signature using those
algorithms over the contents of the element. The contents themselves
do not appear on the wire but are simply calculated. The length of
the signature is specified by the signing algorithm and key.
In RSA signing, the opaque vector contains the signature generated
using the RSASSA-PKCS1-v1_5 signature scheme defined in [PKCS1]. As
discussed in [PKCS1], the DigestInfo MUST be DER [X680] [X690]
encoded and for 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 DSA, the 20 bytes of the SHA-1 hash are run directly through the
Digital Signing Algorithm with no additional hashing. This produces
two values, r and s. The DSA signature is an opaque vector, as above,
the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
Note: In current terminology, DSA refers to the Digital Signature
Algorithm and DSS refers to the NIST standard. In the original
SSL and TLS specs, "DSS" was used universally. This document
uses "DSA" to refer to the algorithm, "DSS" to refer to the
standard, and uses "DSS" in the code point definitions for
historical continuity.
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In stream cipher encryption, the plaintext is exclusive-ORed with an
identical amount of output generated from a cryptographically secure
keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a
block of ciphertext. All block cipher encryption is done in CBC
(Cipher Block Chaining) mode, and all items that are block-ciphered
will be an exact multiple of the cipher block length.
In AEAD encryption, the plaintext is simultaneously encrypted and
integrity protected. The input may be of any length and aead-ciphered
output is generally larger than the input in order to 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 {
uint8 field3<0..255>;
uint8 field4;
};
} UserType;
The contents of the inner struct (field3 and field4) are used as
input for the signature/hash algorithm, and then the entire structure
is encrypted with a stream cipher. The length of this structure, in
bytes, would be equal to two bytes for field1 and field2, plus two
bytes for the signature and hash algorithm, plus two bytes for the
length of the signature, plus the length of the output of the signing
algorithm. 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.
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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))
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P_hash can be iterated as many times as is necessary to produce the
required quantity of data. For example, if P_SHA256 is being used to
create 80 bytes of data, it will have to be iterated three times
(through A(3)), creating 96 bytes of output data; the last 16 bytes
of the final iteration will then be discarded, leaving 80 bytes of
output data.
TLS's PRF is created by applying P_hash to the secret as:
PRF(secret, label, seed) = P_<hash>(secret, label + seed)
The label is an ASCII string. It should be included in the exact form
it is given without a length byte or trailing null character. For
example, the label "slithy toves" would be processed by hashing the
following bytes:
73 6C 69 74 68 79 20 74 6F 76 65 73
6. The TLS Record Protocol
The TLS Record Protocol is a layered protocol. At each layer,
messages may include fields for length, description, and content.
The Record Protocol takes messages to be transmitted, fragments the
data into manageable blocks, optionally compresses the data, applies
a MAC, encrypts, and transmits the result. Received data is
decrypted, verified, decompressed, reassembled, and then delivered to
higher-level clients.
Four protocols that use the record protocol are described in this
document: the handshake protocol, the alert protocol, the change
cipher spec protocol, and the application data protocol. In order to
allow extension of the TLS protocol, additional record content types
can be supported by the record protocol. New record content type
values are assigned by IANA in the TLS Content Type Registry as
described in Section 12.
Implementations MUST NOT send record types not defined in this
document unless negotiated by some extension. If a TLS
implementation receives an unexpected record type, it MUST send an
unexpected_message alert.
Any protocol designed for use over TLS must be carefully designed to
deal with all possible attacks against it. As a practical matter,
this means that the protocol designer must be aware of what security
properties TLS does and does not provide and cannot safely rely on
the latter.
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Note in particular that type and length of a record are not protected
by encryption. If this information is itself sensitive, application
designers may wish to take steps (padding, cover traffic) to minimize
information leakage.
6.1. Connection States
A TLS connection state is the operating environment of the TLS Record
Protocol. It specifies a compression algorithm, an encryption
algorithm, and a MAC algorithm. In addition, the parameters for these
algorithms are known: the MAC key and the bulk encryption keys for
the connection in both the read and the write directions. Logically,
there are always four connection states outstanding: the current read
and write states, and the pending read and write states. All records
are processed under the current read and write states. The security
parameters for the pending states can be set by the TLS Handshake
Protocol, and the ChangeCipherSpec can selectively make either of the
pending states current, in which case the appropriate current state
is disposed of and replaced with the pending state; the pending state
is then reinitialized to an empty state. It is illegal to make a
state that has not been initialized with security parameters a
current state. The initial current state always specifies that no
encryption, compression, or MAC will be used.
The security parameters for a TLS Connection read and write state are
set by providing the following values:
connection end
Whether this entity is considered the "client" or the "server" in
this connection.
PRF algorithm
An algorithm used to generate keys from the master secret (see
Sections 5 and 6.3).
bulk encryption algorithm
An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, whether it is a block,
stream, or AEAD cipher, the block size of the cipher (if
appropriate), and the lengths of explicit and implicit
initialization vectors (or nonces).
MAC algorithm
An algorithm to be used for message authentication. This
specification includes the size of the value returned by the MAC
algorithm.
compression algorithm
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An algorithm to be used for data compression. This specification
must include all information the algorithm requires to do
compression.
master secret
A 48-byte secret shared between the two peers in the connection.
client random
A 32-byte value provided by the client.
server random
A 32-byte value provided by the server.
These parameters are defined in the presentation language as:
enum { server, client } ConnectionEnd;
enum { tls_prf_sha256 } PRFAlgorithm;
enum { null, rc4, 3des, aes }
BulkCipherAlgorithm;
enum { stream, block, aead } CipherType;
enum { null, hmac_md5, hmac_sha1, hmac_sha256,
hmac_sha384, hmac_sha512} MACAlgorithm;
enum { null(0), (255) } CompressionMethod;
/* The algorithms specified in CompressionMethod, PRFAlgorithm
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
PRFAlgorithm prf_algorithm;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 enc_key_length;
uint8 block_length;
uint8 fixed_iv_length;
uint8 record_iv_length;
MACAlgorithm mac_algorithm;
uint8 mac_length;
uint8 mac_key_length;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
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} SecurityParameters;
The record layer will use the security parameters to generate the
following six items (some of which are not required by all ciphers,
and are thus empty):
client write MAC key
server write MAC key
client write encryption key
server write encryption key
client write IV
server write IV
The client write parameters are used by the server when receiving and
processing records and vice-versa. The algorithm used for generating
these items from the security parameters is described in Section 6.3.
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
elements:
compression state
The current state of the compression algorithm.
cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. For stream ciphers, this
will also contain whatever state information is necessary to allow
the stream to continue to encrypt or decrypt data.
MAC key
The MAC key for this connection, as generated above.
sequence number
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may not
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it must
renegotiate instead. A sequence number is incremented after each
record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.
6.2. Record layer
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The TLS Record Layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
6.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Client message
boundaries are not preserved in the record layer (i.e., multiple
client messages of the same ContentType MAY be coalesced into a
single TLSPlaintext record, or a single message MAY be fragmented
across several records).
struct {
uint8 major;
uint8 minor;
} ProtocolVersion;
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
The higher-level protocol used to process the enclosed fragment.
version
The version of the protocol being employed. This document
describes TLS Version 1.2, which uses the version { 3, 3 }. The
version value 3.3 is historical, deriving from the use of {3, 1}
for TLS 1.0. (See Appendix A.1). Note that a client that supports
multiple versions of TLS may not know what version will be
employed before it receives the ServerHello. See Appendix E for
discussion about what record layer version number should be
employed for ClientHello.
length
The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14.
fragment
The application data. This data is transparent and treated as an
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independent block to be dealt with by the higher-level protocol
specified by the type field.
Implementations MUST NOT send zero-length fragments of Handshake,
Alert, or ChangeCipherSpec content types. Zero-length fragments of
Application data MAY be sent as they are potentially useful as a
traffic analysis countermeasure.
Note: Data of different TLS Record layer content types MAY be
interleaved. Application data is generally of lower precedence for
transmission than other content types. However, records MUST be
delivered to the network in the same order as they are protected by
the record layer. Recipients MUST receive and process interleaved
application layer traffic during handshakes subsequent to the first
one on a connection.
6.2.2. Record Compression and Decompression
All records are compressed using the compression algorithm defined in
the current session state. There is always an active compression
algorithm; however, initially it is defined as
CompressionMethod.null. The compression algorithm translates a
TLSPlaintext structure into a TLSCompressed structure. Compression
functions are initialized with default state information whenever a
connection state is made active. [RFC3749] describes compression
algorithms for TLS.
Compression must be lossless and may not increase the content length
by more than 1024 bytes. If the decompression function encounters a
TLSCompressed.fragment that would decompress to a length in excess of
2^14 bytes, it MUST report a fatal decompression failure error.
struct {
ContentType type; /* same as TLSPlaintext.type */
ProtocolVersion version;/* same as TLSPlaintext.version */
uint16 length;
opaque fragment[TLSCompressed.length];
} TLSCompressed;
length
The length (in bytes) of the following TLSCompressed.fragment.
The length MUST NOT exceed 2^14 + 1024.
fragment
The compressed form of TLSPlaintext.fragment.
Note: A CompressionMethod.null operation is an identity operation; no
fields are altered.
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Implementation note: Decompression functions are responsible for
ensuring that messages cannot cause internal buffer overflows.
6.2.3. Record Payload Protection
The encryption and MAC functions translate a TLSCompressed structure
into a TLSCiphertext. The decryption functions reverse the process.
The MAC of the record also includes a sequence number so that
missing, extra, or repeated messages are detectable.
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (SecurityParameters.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
case aead: GenericAEADCipher;
} fragment;
} TLSCiphertext;
type
The type field is identical to TLSCompressed.type.
version
The version field is identical to TLSCompressed.version.
length
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.
fragment
The encrypted form of TLSCompressed.fragment, with the MAC.
6.2.3.1. Null or Standard Stream Cipher
Stream ciphers (including BulkCipherAlgorithm.null, see Appendix A.6)
convert TLSCompressed.fragment structures to and from stream
TLSCiphertext.fragment structures.
stream-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
} GenericStreamCipher;
The MAC is generated as:
MAC(MAC_write_key, seq_num +
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TLSCompressed.type +
TLSCompressed.version +
TLSCompressed.length +
TLSCompressed.fragment);
where "+" denotes concatenation.
seq_num
The sequence number for this record.
MAC
The MAC algorithm specified by SecurityParameters.mac_algorithm.
Note that the MAC is computed before encryption. The stream cipher
encrypts the entire block, including the MAC. For stream ciphers that
do not use a synchronization vector (such as RC4), the stream cipher
state from the end of one record is simply used on the subsequent
packet. If the cipher suite is TLS_NULL_WITH_NULL_NULL, encryption
consists of the identity operation (i.e., the data is not encrypted,
and the MAC size is zero, implying that no MAC is used). For both
null and stream ciphers, TLSCiphertext.length is TLSCompressed.length
plus SecurityParameters.mac_length.
6.2.3.2. CBC Block Cipher
For block ciphers (such as 3DES, or AES), the encryption and MAC
functions convert TLSCompressed.fragment structures to and from block
TLSCiphertext.fragment structures.
struct {
opaque IV[SecurityParameters.record_iv_length];
block-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[SecurityParameters.mac_length];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
};
} GenericBlockCipher;
The MAC is generated as described in Section 6.2.3.1.
IV
The Initialization Vector (IV) SHOULD be chosen at random, and
MUST be unpredictable. Note that in versions of TLS prior to 1.1,
there was no IV field, and the last ciphertext block of the
previous record (the "CBC residue") was used as the IV. This was
changed to prevent the attacks described in [CBCATT]. For block
ciphers, the IV length is of length
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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
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reject the packet. For instance, if the pad appears to be incorrect,
the implementation might assume a zero-length pad and then compute
the MAC. This leaves a small timing channel, since MAC performance
depends to some extent on the size of the data fragment, but it is
not believed to be large enough to be exploitable, due to the large
block size of existing MACs and the small size of the timing signal.
6.2.3.3. AEAD ciphers
For AEAD [AEAD] ciphers (such as [CCM] or [GCM]) the AEAD function
converts TLSCompressed.fragment structures to and from AEAD
TLSCiphertext.fragment structures.
struct {
opaque nonce_explicit[SecurityParameters.record_iv_length];
aead-ciphered struct {
opaque content[TLSCompressed.length];
};
} GenericAEADCipher;
AEAD ciphers take as input a single key, a nonce, a plaintext, and
"additional data" to be included in the authentication check, as
described in Section 2.1 of [AEAD]. The key is either the
client_write_key or the server_write_key. No MAC key is used.
Each AEAD cipher suite MUST specify how the nonce supplied to the
AEAD operation is constructed, and what is the length of the
GenericAEADCipher.nonce_explicit part. In many cases, it is
appropriate to use the partially implicit nonce technique described
in Section 3.2.1 of [AEAD]; with record_iv_length being the length of
the explicit part. In this case, the implicit part SHOULD be derived
from key_block as client_write_iv and server_write_iv (as described
in Section 6.3), and the explicit part is included in
GenericAEAEDCipher.nonce_explicit.
The plaintext is the TLSCompressed.fragment.
The additional authenticated data, which we denote as
additional_data, is defined as follows:
additional_data = seq_num + TLSCompressed.type +
TLSCompressed.version + TLSCompressed.length;
Where "+" denotes concatenation.
The aead_output consists of the ciphertext output by the AEAD
encryption operation. The length will generally be larger than
TLSCompressed.length, but by an amount that varies with the AEAD
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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, nonce, plaintext,
additional_data)
In order to decrypt and verify, the cipher takes as input the key,
nonce, the "additional_data", and the AEADEncrypted value. The output
is either the plaintext or an error indicating that the decryption
failed. There is no separate integrity check. I.e.,
TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce,
AEADEncrypted,
additional_data)
If the decryption fails, a fatal bad_record_mac alert MUST be
generated.
6.3. Key Calculation
The Record Protocol requires an algorithm to generate keys required
by the current connection state (see Appendix A.6) from the security
parameters provided by the handshake protocol.
The master secret is expanded into a sequence of secure bytes, which
is then split to a client write MAC key, a server write MAC key, a
client write encryption key, and a server write encryption key. Each
of these is generated from the byte sequence in that order. Unused
values are empty. Some AEAD ciphers may additionally require a
client write IV and a server write IV (see Section 6.2.3.3).
When keys and MAC keys are generated, the master secret is used as an
entropy source.
To generate the key material, compute
key_block = PRF(SecurityParameters.master_secret,
"key expansion",
SecurityParameters.server_random +
SecurityParameters.client_random);
until enough output has been generated. Then the key_block is
partitioned as follows:
client_write_MAC_key[SecurityParameters.mac_key_length]
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server_write_MAC_key[SecurityParameters.mac_key_length]
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.fixed_iv_length]
server_write_IV[SecurityParameters.fixed_iv_length]
Currently, the client_write_IV and server_write_IV are only generated
for implicit nonce techniques as described in Section 3.2.1 of
[AEAD].
Implementation note: The currently defined cipher suite which
requires the most material is AES_256_CBC_SHA256. It requires 2 x 32
byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key
material.
7. The TLS Handshaking Protocols
TLS has three subprotocols that are used to allow peers to agree upon
security parameters for the record layer, to authenticate themselves,
to instantiate negotiated security parameters, and to report error
conditions to each other.
The Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
peer certificate
X509v3 [ <a href="'>PKIX] certificate of the peer. This element of the state
may be null.
compression method
The algorithm used to compress data prior to encryption.
cipher spec
Specifies the pseudorandom function (PRF) used to generate keying
material, the bulk data encryption algorithm (such as null, AES,
etc.) and a MAC algorithm (such as HMAC-SHA1). It also defines
cryptographic attributes such as the mac_length. (See Appendix A.6
for formal definition.)
master secret
48-byte secret shared between the client and server.
is resumable
A flag indicating whether the session can be used to initiate new
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connections.
These items are then used to create security parameters for use by
the Record Layer when protecting application data. Many connections
can be instantiated using the same session through the resumption
feature of the TLS Handshake Protocol.
7.1. Change Cipher Spec Protocol
The change cipher spec protocol exists to signal transitions in
ciphering strategies. The protocol consists of a single message,
which is encrypted and compressed under the current (not the pending)
connection state. The message consists of a single byte of value 1.
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
The ChangeCipherSpec message is sent by both the client and the
server to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the Record Layer to
immediately copy the read pending state into the read current state.
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See Section 6.1.) The 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
(warning or fatal) and a description of the alert. Alert messages
with a level of fatal result in the immediate termination of the
connection. In this case, other connections corresponding to the
session may continue, but the session identifier MUST be invalidated,
preventing the failed session from being used to establish new
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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,
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failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.
If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding close_notify. No part of
this standard should be taken to dictate the manner in which a usage
profile for TLS manages its data transport, including when
connections are opened or closed.
Note: It is assumed that closing a connection reliably delivers
pending data before destroying the transport.
7.2.2. Error Alerts
Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other
party. Upon transmission or receipt of a fatal alert message, both
parties immediately close the connection. Servers and clients MUST
forget any session-identifiers, keys, and secrets associated with a
failed connection. Thus, any connection terminated with a fatal alert
MUST NOT be resumed.
Whenever an implementation encounters a condition which is defined as
a fatal alert, it MUST send the appropriate alert prior to closing
the connection. For all errors where an alert level is not explicitly
specified, the sending party MAY determine at its discretion whether
to treat this as a fatal error or not. If the implementation chooses
to send an alert but intends to close the connection immediately
afterwards, it MUST send that alert at the fatal alert level.
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If an alert with a level of warning is sent and received, generally
the connection can continue normally. If the receiving party decides
not to proceed with the connection (e.g., after having received a
no_renegotiation alert that it is not willing to accept), it SHOULD
send a fatal alert to terminate the connection. Given this, the
sending party cannot, in general, know how the receiving party will
behave. Therefore, warning alerts are not very useful when the
sending party wants to continue the connection, and thus are
sometimes omitted. For example, if a peer decides to accept an
expired certificate (perhaps after confirming this with the user) and
wants to continue the connection, it would not generally send a
certificate_expired alert.
The following error alerts are defined:
unexpected_message
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn't an
even multiple of the block length, or its padding values, when
checked, weren't correct. This message is always fatal and should
never be observed in communication between proper implementations
(except when messages were corrupted in the network).
decryption_failed_RESERVED
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode [CBCATT]. It MUST
NOT be sent by compliant implementations.
record_overflow
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).
decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal and should never be observed in communication between proper
implementations.
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handshake_failure
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.
bad_certificate
A certificate was corrupt, contained signatures that did not
verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
certificate_unknown
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was out of range or inconsistent with
other fields. This message is always fatal.
unknown_ca
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn't be matched with a known, trusted CA. This
message is always fatal.
access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.
decode_error
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
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decrypt_error
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a finished message.
This message is always fatal.
export_restriction_RESERVED
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.
protocol_version
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons). This message is always
fatal.
insufficient_security
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.
internal_error
An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue. This message is always fatal.
user_canceled
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed by
a close_notify. This message is generally a warning.
no_renegotiation
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Either
of these would normally lead to renegotiation; when that is not
appropriate, the recipient should respond with this alert. At
that point, the original requester can decide whether to proceed
with the connection. One case where this would be appropriate is
where a server has spawned a process to satisfy a request; the
process might receive security parameters (key length,
authentication, etc.) at startup and it might be difficult to
communicate changes to these parameters after that point. This
message is always a warning.
unsupported_extension
sent by clients that receive an extended server hello containing
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an extension that they did not put in the corresponding client
hello. This message is always fatal.
New Alert values are assigned by IANA as described in Section 12.
7.3. Handshake Protocol Overview
The cryptographic parameters of the session state are produced by the
TLS Handshake Protocol, which operates on top of the TLS Record
Layer. When a TLS client and server first start communicating, they
agree on a protocol version, select cryptographic algorithms,
optionally authenticate each other, and use public-key encryption
techniques to generate shared secrets.
The TLS Handshake Protocol involves the following steps:
- Exchange hello messages to agree on algorithms, exchange random
values, and check for session resumption.
- Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.
- Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.
- Generate a master secret from the premaster secret and exchanged
random values.
- Provide security parameters to the record layer.
- Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.
Note that higher layers should not be overly reliant on whether TLS
always negotiates the strongest possible connection between two
peers. There are a number of ways in which a man in the middle
attacker can attempt to make two entities drop down to the least
secure method they support. The protocol has been designed to
minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service
runs on, or attempt to get the peers to negotiate an unauthenticated
connection. The fundamental rule is that higher levels must be
cognizant of what their security requirements are and never transmit
information over a channel less secure than what they require. The
TLS protocol is secure in that any cipher suite offers its promised
level of security: if you negotiate 3DES with a 1024 bit RSA key
exchange with a host whose certificate you have verified, you can
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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 ServerKeyExchange, the client Certificate, and the
ClientKeyExchange. New key exchange methods can be created by
specifying a format for these messages and by defining the use of the
messages to allow the client and server to agree upon a shared
secret. This secret MUST be quite long; currently defined key
exchange methods exchange secrets that range from 46 bytes upwards.
Following the hello messages, the server will send its certificate in
a Certificate message if it is to be authenticated. Additionally, a
ServerKeyExchange message may be sent, if it is required (e.g., if
the server has no certificate, or if its certificate is for signing
only). If the server is authenticated, it may request a certificate
from the client, if that is appropriate to the cipher suite selected.
Next, the server will send the ServerHelloDone message, indicating
that the hello-message phase of the handshake is complete. The server
will then wait for a client response. If the server has sent a
CertificateRequest message, the client MUST send the Certificate
message. The ClientKeyExchange message is now sent, and the content
of that message will depend on the public key algorithm selected
between the client hello and the server hello. If the client has sent
a certificate with signing ability, a digitally-signed
CertificateVerify message is sent to explicitly verify possession of
the private key in the certificate.
At this point, a ChangeCipherSpec message is sent by the client, and
the client copies the pending Cipher Spec into the current Cipher
Spec. The client then immediately sends the Finished message under
the new algorithms, keys, and secrets. In response, the server will
send its own ChangeCipherSpec message, transfer the pending to the
current Cipher Spec, and send its Finished message under the new
Cipher Spec. At this point, the handshake is complete, and the client
and server may begin to exchange application layer data. (See flow
chart below.) Application data MUST NOT be sent prior to the
completion of the first handshake (before a cipher suite other than
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TLS_NULL_WITH_NULL_NULL is established).
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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 ChangeCipherSpec messages and proceed
directly to Finished messages. Once the re-establishment is complete,
the client and server MAY begin to exchange application layer data.
(See flow chart below.) If a Session ID match is not found, the
server generates a new session ID and the TLS client and server
perform a full handshake.
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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;
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The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be omitted,
however. Note one exception to the ordering: the Certificate message
is used twice in the handshake (from server to client, then from
client to server), but described only in its first position. The one
message that is not bound by these ordering rules is the HelloRequest
message, which can be sent at any time, but which SHOULD be ignored
by the client if it arrives in the middle of a handshake.
New Handshake message types are assigned by IANA as described in
Section 12.
7.4.1. Hello Messages
The hello phase messages are used to exchange security enhancement
capabilities between the client and server. When a new session
begins, the Record Layer's connection state encryption, hash, and
compression algorithms are initialized to null. The current
connection state is used for renegotiation messages.
7.4.1.1. Hello Request
When this message will be sent:
The HelloRequest message MAY be sent by the server at any time.
Meaning of this message:
HelloRequest is a simple notification that the client should begin
the negotiation process anew. In response, the client should a
ClientHello message when convenient. This message is not intended
to establish which side is the client or server but merely to
initiate a new negotiation. Servers SHOULD NOT send a HelloRequest
immediately upon the client's initial connection. It is the
client's job to send a ClientHello at that time.
This message will be ignored by the client if the client is
currently negotiating a session. This message MAY be ignored by
the client if it does not wish to renegotiate a session, or the
client may, if it wishes, respond with a no_renegotiation alert.
Since handshake messages are intended to have transmission
precedence over application data, it is expected that the
negotiation will begin before no more than a few records are
received from the client. If the server sends a HelloRequest but
does not receive a ClientHello in response, it may close the
connection with a fatal alert.
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After sending a HelloRequest, servers SHOULD NOT repeat the
request until the subsequent handshake negotiation is complete.
Structure of this message:
struct { } HelloRequest;
This message MUST NOT be included in the message hashes that are
maintained throughout the handshake and used in the finished messages
and the certificate verify message.
7.4.1.2. Client Hello
When this message will be sent:
When a client first connects to a server it is required to send
the ClientHello as its first message. The client can also send a
ClientHello in response to a HelloRequest or on its own initiative
in order to renegotiate the security parameters in an existing
connection.
Structure of this message:
The ClientHello message includes a random structure, which is used
later in the protocol.
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
gmt_unix_time
The current time and date in standard UNIX 32-bit format
(seconds since the midnight starting Jan 1, 1970, UTC, ignoring
leap seconds) according to the sender's internal clock. Clocks
are not required to be set correctly by the basic TLS Protocol;
higher-level or application protocols may define additional
requirements. Note that, for historical reasons, the data
element is named using GMT, the predecessor of the current
worldwide time base, UTC.
random_bytes
28 bytes generated by a secure random number generator.
The ClientHello message includes a variable-length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes to
reuse. The session identifier MAY be from an earlier connection, this
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connection, or from another currently active connection. The second
option is useful if the client only wishes to update the random
structures and derived values of a connection, and the third option
makes it possible to establish several independent secure connections
without repeating the full handshake protocol. These independent
connections may occur sequentially or simultaneously; a SessionID
becomes valid when the handshake negotiating it completes with the
exchange of Finished messages and persists until it is removed due to
aging or because a fatal error was encountered on a connection
associated with the session. The actual contents of the SessionID are
defined by the server.
opaque SessionID<0..32>;
Warning: Because the SessionID is transmitted without encryption or
immediate MAC protection, servers MUST NOT place confidential
information in session identifiers or let the contents of fake
session identifiers cause any breach of security. (Note that the
content of the handshake as a whole, including the SessionID, is
protected by the Finished messages exchanged at the end of the
handshake.)
The cipher suite list, passed from the client to the server in the
ClientHello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client's
preference (favorite choice first). Each cipher suite defines a key
exchange algorithm, a bulk encryption algorithm (including secret key
length), a MAC algorithm, and a PRF. The server will select a cipher
suite or, if no acceptable choices are presented, return a handshake
failure alert and close the connection. If the list contains cipher
suites the server does not recognize, support, or wish to use, the
server MUST ignore those cipher suites, and process the remaining
ones as usual.
uint8 CipherSuite[2]; /* Cryptographic suite selector */
The ClientHello includes a list of compression algorithms supported
by the client, ordered according to the client's preference.
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-2>;
CompressionMethod compression_methods<1..2^8-1>;
select (extensions_present) {
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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 if the
client wishes to generate new security parameters.
cipher_suites
This is a list of the cryptographic options supported by the
client, with the client's first preference first. If the
session_id field is not empty (implying a session resumption
request), this vector MUST include at least the cipher_suite from
that session. Values are defined in Appendix A.5.
compression_methods
This is a list of the compression methods supported by the client,
sorted by client preference. If the session_id field is not empty
(implying a session resumption request), it MUST include the
compression_method from that session. This vector MUST contain,
and all implementations MUST support, CompressionMethod.null.
Thus, a client and server will always be able to agree on a
compression method.
extensions
Clients MAY request extended functionality from servers by sending
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data in the extensions field. The actual "Extension" format is
defined in Section 7.4.1.4.
In the event that a client requests additional functionality using
extensions, and this functionality is not supplied by the server, the
client MAY abort the handshake. A server MUST accept client hello
messages both with and without the extensions field, 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
ServerHello message. Any other handshake message returned by the
server except for a HelloRequest is treated as a fatal error.
7.4.1.3. Server Hello
When this message will be sent:
The server will send this message in response to a ClientHello
message when it was able to find an acceptable set of algorithms.
If it cannot find such a match, it will respond with a handshake
failure alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
select (extensions_present) {
case false:
struct {};
case true:
Extension extensions<0..2^16-1>;
};
} ServerHello;
The presence of extensions can be detected by determining whether
there are bytes following the compression_method field at the end of
the ServerHello.
server_version
This field will contain the lower of that suggested by the client
in the client hello and the highest supported by the server. For
this version of the specification, the version is 3.3. (See
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Appendix E for details about backward compatibility.)
random
This structure is generated by the server and MUST be
independently generated from the ClientHello.random.
session_id
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the finished messages. Otherwise this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with. Note that there is no requirement that
the server resume any session even if it had formerly provided a
session_id. Clients MUST be prepared to do a full negotiation --
including negotiating new cipher suites -- during any handshake.
cipher_suite
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is the
value from the state of the session being resumed.
compression_method
The single compression algorithm selected by the server from the
list in ClientHello.compression_methods. For resumed sessions this
field is the value from the resumed session state.
extensions
A list of extensions. Note that only extensions offered by the
client can appear in the server's list.
7.4.1.4 Hello Extensions
The extension format is:
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
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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.
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7.4.1.4.1 Signature Algorithms
The client uses the "signature_algorithms" extension to indicate to
the server which signature/hash algorithm pairs may be used in
digital signatures. The "extension_data" field of this extension
contains a "supported_signature_algorithms" value.
enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
SignatureAlgorithm;
struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
SignatureAndHashAlgorithm
supported_signature_algorithms<2..2^16-2>;
Each SignatureAndHashAlgorithm value lists a single hash/signature
pair 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 unhashed data, MD5 [MD5], SHA-1,
SHA-224, SHA-256, SHA-384, and SHA-512 [SHS] respectively. The
"none" value is provided for future extensibility, in case of a
signature algorithm which does not require hashing before signing.
signature
This field indicates the signature algorithm which may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
[PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively. The
"anonymous" value is meaningless in this context but used in
Section 7.4.3. It MUST NOT appear in this extension.
The semantics of this extension are somewhat complicated because the
cipher suite indicates permissible signature algorithms but not hash
algorithms. Sections 7.4.2 and 7.4.3 describe the appropriate rules.
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If the client supports only the default hash and signature algorithms
(listed in this section), it MAY omit the signature_algorithms
extension. If the client does not support the default algorithms, or
supports other hash and signature algorithms (and it is willing to
use them for verifying messages sent by the server, i.e., server
certificates and server key exchange), it MUST send the
signature_algorithms extension, listing the algorithms it is willing
to accept.
If the client does not send the signature_algorithms extension, the
server MUST assume the following:
- If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had sent
the value {sha1,rsa}.
- If the negotiated key exchange algorithm is one of (DHE_DSS,
DH_DSS), behave as if the client had sent the value {sha1,dsa}.
- If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}.
Note: this is a change from TLS 1.1 where there are no explicit rules
but as a practical matter one can assume that the peer supports MD5
and SHA-1.
Note: this extension is not meaningful for TLS versions prior to 1.2.
Clients MUST NOT offer it if they are offering prior versions.
However, even if clients do offer it, the rules specified in [TLSEXT]
require servers to ignore extensions they do not understand.
Servers MUST NOT send this extension. TLS servers MUST support
receiving this extension.
7.4.2. Server Certificate
When this message will be sent:
The server MUST send a Certificate message whenever the agreed-
upon key exchange method uses certificates for authentication
(this includes all key exchange methods defined in this document
except DH_anon). This message will always immediately follow the
server hello message.
Meaning of this message:
This message conveys the server's certificate chain to the client.
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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 be omitted from the chain, under the
assumption that the remote end must already possess it in order to
validate it in any case.
The same message type and structure will be used for the client's
response to a certificate request message. Note that a client MAY
send no certificates if it does not have an appropriate certificate
to send in response to the server's authentication request.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate
vector because PKCS #6 [PKCS6] extended certificates are not used.
Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task
of parsing the list more difficult.
The following rules apply to the certificates sent by the server:
- The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., [TLSPGP]).
- The end entity certificate's public key (and associated
restrictions) MUST be compatible with the selected key exchange
algorithm.
Key Exchange Alg. Certificate Key Type
RSA RSA public key; the certificate MUST
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].
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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.
Note: ECDHE_RSA is defined in [TLSECC].
DHE_DSS DSA public key; the certificate MUST
allow the key to be used for signing with
the hash algorithm that will be employ