Network Working Group T. Dierks
Request for Comments: 4346 Independent
Obsoletes: 2246 E. Rescorla
Category: Standards Track RTFM, Inc.
April 2006
The Transport Layer Security (TLS) Protocol
Version 1.1
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies Version 1.1 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.
Dierks & Rescorla Standards Track [Page 1]
RFC 4346 The TLS Protocol April 2006
Table of Contents
1. Introduction ....................................................4
1.1. Differences from TLS 1.0 ...................................5
1.2. Requirements Terminology ...................................5
2. Goals ...........................................................5
3. Goals of This Document ..........................................6
4. Presentation Language ...........................................6
4.1. Basic Block Size ...........................................7
4.2. Miscellaneous ..............................................7
4.3. Vectors ....................................................7
4.4. Numbers ....................................................8
4.5. Enumerateds ................................................8
4.6. Constructed Types ..........................................9
4.6.1. Variants ...........................................10
4.7. Cryptographic Attributes ..................................11
4.8. Constants .................................................12
5. HMAC and the Pseudorandom Function .............................12
6. The TLS Record Protocol ........................................14
6.1. Connection States .........................................15
6.2. Record layer ..............................................17
6.2.1. Fragmentation ......................................17
6.2.2. Record Compression and Decompression ...............19
6.2.3. Record Payload Protection ..........................19
6.2.3.1. Null or Standard Stream Cipher ............20
6.2.3.2. CBC Block Cipher ..........................21
6.3. Key Calculation ...........................................24
7. The TLS Handshaking Protocols ..................................24
7.1. Change Cipher Spec Protocol ...............................25
7.2. Alert Protocol ............................................26
7.2.1. Closure Alerts .....................................27
7.2.2. Error Alerts .......................................28
7.3. Handshake Protocol Overview ...............................31
7.4. Handshake Protocol ........................................34
7.4.1. Hello Messages .....................................35
7.4.1.1. Hello request .............................35
7.4.1.2. Client Hello ..............................36
7.4.1.3. Server Hello ..............................39
7.4.2. Server Certificate .................................40
7.4.3. Server Key Exchange Message ........................42
7.4.4. Certificate request ................................44
7.4.5. Server Hello Done ..................................46
7.4.6. Client certificate .................................46
7.4.7. Client Key Exchange Message ........................47
7.4.7.1. RSA Encrypted Premaster Secret Message ....47
7.4.7.2. Client Diffie-Hellman Public Value ........50
7.4.8. Certificate verify .................................50
7.4.9. Finished ...........................................51
Dierks & Rescorla Standards Track [Page 2]
RFC 4346 The TLS Protocol April 2006
8. Cryptographic Computations .....................................52
8.1. Computing the Master Secret ...............................52
8.1.1. RSA ................................................53
8.1.2. Diffie-Hellman .....................................53
9. Mandatory Cipher Suites ........................................53
10. Application Data Protocol .....................................53
11. Security Considerations .......................................53
12. IANA Considerations ...........................................54
A. Appendix - Protocol constant values ............................55
A.1. Record layer .........................................55
A.2. Change cipher specs message ..........................56
A.3. Alert messages .......................................56
A.4. Handshake protocol ...................................57
A.4.1. Hello messages .....................................57
A.4.2. Server authentication and key exchange messages ....58
A.4.3. Client authentication and key exchange messages ....59
A.4.4.Handshake finalization message ......................60
A.5. The CipherSuite ......................................60
A.6. The Security Parameters ..............................63
B. Appendix - Glossary ............................................64
C. Appendix - CipherSuite definitions .............................68
D. Appendix - Implementation Notes ................................69
D.1 Random Number Generation and Seeding ..................70
D.2 Certificates and authentication .......................70
D.3 CipherSuites ..........................................70
E. Appendix - Backward Compatibility With SSL .....................71
E.1. Version 2 client hello ...............................72
E.2. Avoiding man-in-the-middle version rollback ..........74
F. Appendix - Security analysis ...................................74
F.1. Handshake protocol ...................................74
F.1.1. Authentication and key exchange ....................74
F.1.1.1. Anonymous key exchange ...........................75
F.1.1.2. RSA key exchange and authentication ..............75
F.1.1.3. Diffie-Hellman key exchange with authentication ..76
F.1.2. Version rollback attacks ...........................77
F.1.3. Detecting attacks against the handshake protocol ...77
F.1.4. Resuming sessions ..................................78
F.1.5. MD5 and SHA ........................................78
F.2. Protecting application data ..........................78
F.3. Explicit IVs .........................................79
F.4 Security of Composite Cipher Modes ...................79
F.5 Denial of Service ....................................80
F.6. Final notes ..........................................80
Normative References ..............................................81
Informative References ............................................82
Dierks & Rescorla Standards Track [Page 3]
RFC 4346 The TLS Protocol April 2006
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
Dierks & Rescorla Standards Track [Page 4]
RFC 4346 The TLS Protocol April 2006
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. Differences from TLS 1.0
This document is a revision of the TLS 1.0 [TLS1.0] protocol, and
contains some small security improvements, clarifications, and
editorial improvements. The major changes are:
- The implicit Initialization Vector (IV) is replaced with an
explicit IV to protect against CBC attacks [CBCATT].
- Handling of padding errors is changed to use the bad_record_mac
alert rather than the decryption_failed alert to protect against
CBC attacks.
- IANA registries are defined for protocol parameters.
- Premature closes no longer cause a session to be nonresumable.
- Additional informational notes were added for various new attacks
on TLS.
In addition, a number of minor clarifications and editorial
improvements were made.
1.2. Requirements Terminology
In this document, the keywords "MUST", "MUST NOT", "REQUIRED",
"SHOULD", "SHOULD NOT" and "MAY" are to be interpreted as described
in RFC 2119 [REQ].
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.
Dierks & Rescorla Standards Track [Page 5]
RFC 4346 The TLS Protocol April 2006
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 TLS 1.1, TLS 1.0, and SSL 3.0 do not
interoperate (although each protocol incorporates a mechanism by
which an implementation can back down prior versions). This document
is intended primarily for readers who will be implementing the
protocol and for those doing cryptographic analysis of it. The
specification has been written with this in mind, and it is intended
to reflect the needs of those two groups. For that reason, many of
the algorithm-dependent data structures and rules are included in the
body of the text (as opposed to in an appendix), providing easier
access to them.
This document is not intended to supply any details of service
definition or of interface definition, although it does cover select
areas of policy as they are required for the maintenance of solid
security.
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.
Dierks & Rescorla Standards Track [Page 6]
RFC 4346 The TLS Protocol April 2006
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
Dierks & Rescorla Standards Track [Page 7]
RFC 4346 The TLS Protocol April 2006
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.
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.
Dierks & Rescorla Standards Track [Page 8]
RFC 4346 The TLS Protocol April 2006
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.
Dierks & Rescorla Standards Track [Page 9]
RFC 4346 The TLS Protocol April 2006
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.
Dierks & Rescorla Standards Track [Page 10]
RFC 4346 The TLS Protocol April 2006
4.7. Cryptographic Attributes
The four cryptographic operations digital signing, stream cipher
encryption, block cipher encryption, and public key encryption are
designated digitally-signed, stream-ciphered, block-ciphered, and
public-key-encrypted, respectively. A field's cryptographic
processing is specified by prepending an appropriate key word
designation before the field's type specification. Cryptographic
keys are implied by the current session state (see Section 6.1).
In digital signing, one-way hash functions are used as input for a
signing algorithm. A digitally-signed element is encoded as an
opaque vector <0..2^16-1>, where the length is specified by the
signing algorithm and key.
In RSA signing, a 36-byte structure of two hashes (one SHA and one
MD5) is signed (encrypted with the private key). It is encoded with
PKCS #1 block type 1, as described in [PKCS1A].
Note: The standard reference for PKCS#1 is now RFC 3447 [PKCS1B].
However, to minimize differences with TLS 1.0 text, we are
using the terminology of RFC 2313 [PKCS1A].
In DSS, the 20 bytes of the SHA hash are run directly through the
Digital Signing Algorithm with no additional hashing. This produces
two values, r and s. The DSS signature is an opaque vector, as
above, the contents of which are the DER encoding of:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
In stream cipher encryption, the plaintext is exclusive-ORed with an
identical amount of output generated from a cryptographically secure
keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a
block of ciphertext. All block cipher encryption is done in CBC
(Cipher Block Chaining) mode, and all items that are block-ciphered
will be an exact multiple of the cipher block length.
In public key encryption, a public key algorithm is used to encrypt
data in such a way that it can be decrypted only with the matching
private key. A public-key-encrypted element is encoded as an opaque
vector <0..2^16-1>, where the length is specified by the signing
algorithm and key.
Dierks & Rescorla Standards Track [Page 11]
RFC 4346 The TLS Protocol April 2006
An RSA-encrypted value is encoded with PKCS #1 block type 2, as
described in [PKCS1A].
In the following example,
stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque hash[20];
} UserType;
the contents of hash are used as input for the signing algorithm, and
then the entire structure is encrypted with a stream cipher. The
length of this structure, in bytes, would be equal to two bytes for
field1 and field2, plus two bytes for the length of the signature,
plus the length of the output of the signing algorithm. This is
known because the algorithm and key used for the signing are known
prior to encoding or decoding this structure.
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
A number of operations in the TLS record and handshake layer require
a keyed MAC; this is a secure digest of some data protected by a
secret. Forging the MAC is infeasible without knowledge of the MAC
secret. The construction we use for this operation is known as HMAC,
and is described in [HMAC].
HMAC can be used with a variety of different hash algorithms. TLS
uses it in the handshake with two different algorithms, MD5 and SHA-
1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
data). Additional hash algorithms can be defined by cipher suites
Dierks & Rescorla Standards Track [Page 12]
RFC 4346 The TLS Protocol April 2006
and used to protect record data, but MD5 and SHA-1 are hard coded
into the description of the handshaking for this version of the
protocol.
In addition, a construction is required to do expansion of secrets
into blocks of data for the purposes of key generation or validation.
This pseudo-random function (PRF) takes as input a secret, a seed,
and an identifying label and produces an output of arbitrary length.
In order to make the PRF as secure as possible, it uses two hash
algorithms in a way that should guarantee its security if either
algorithm remains secure.
First, we define a data expansion function, P_hash(secret, data) that
uses a single hash function to expand a secret and seed into an
arbitrary quantity of output:
P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
HMAC_hash(secret, A(2) + seed) +
HMAC_hash(secret, A(3) + seed) + ...
Where + indicates concatenation.
A() is defined as:
A(0) = seed
A(i) = HMAC_hash(secret, A(i-1))
P_hash can be iterated as many times as is necessary to produce the
required quantity of data. For example, if P_SHA-1 is being used to
create 64 bytes of data, it will have to be iterated 4 times (through
A(4)), creating 80 bytes of output data; the last 16 bytes of the
final iteration will then be discarded, leaving 64 bytes of output
data.
TLS's PRF is created by splitting the secret into two halves and
using one half to generate data with P_MD5 and the other half to
generate data with P_SHA-1, then exclusive-ORing the outputs of these
two expansion functions together.
S1 and S2 are the two halves of the secret, and each is the same
length. S1 is taken from the first half of the secret, S2 from the
second half. Their length is created by rounding up the length of
the overall secret, divided by two; thus, if the original secret is
an odd number of bytes long, the last byte of S1 will be the same as
the first byte of S2.
Dierks & Rescorla Standards Track [Page 13]
RFC 4346 The TLS Protocol April 2006
L_S = length in bytes of secret;
L_S1 = L_S2 = ceil(L_S / 2);
The secret is partitioned into two halves (with the possibility of
one shared byte) as described above, S1 taking the first L_S1 bytes,
and S2 the last L_S2 bytes.
The PRF is then defined as the result of mixing the two pseudorandom
streams by exclusive-ORing them together.
PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
P_SHA-1(S2, label + seed);
The label is an ASCII string. It should be included in the exact
form it is given without a length byte or trailing null character.
For example, the label "slithy toves" would be processed by hashing
the following bytes:
73 6C 69 74 68 79 20 74 6F 76 65 73
Note that because MD5 produces 16-byte outputs and SHA-1 produces
20-byte outputs, the boundaries of their internal iterations will not
be aligned. Generating an 80-byte output will require that P_MD5
iterate through A(5), while P_SHA-1 will only iterate through A(4).
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. Any new record types SHOULD
allocate type values immediately beyond the ContentType values for
the four record types described here (see Appendix A.1). All such
values must be defined by RFC 2434 Standards Action. See Section 11
for IANA Considerations for ContentType values.
If a TLS implementation receives a record type it does not
understand, it SHOULD just ignore it. Any protocol designed for use
Dierks & Rescorla Standards Track [Page 14]
RFC 4346 The TLS Protocol April 2006
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, and encryption
algorithm, and a MAC algorithm. In addition, the parameters for
these algorithms are known: the MAC secret and the bulk encryption
keys for the connection in both the read and the write directions.
Logically, there are always four connection states outstanding: the
current read and write states, and the pending read and write states.
All records are processed under the current read and write states.
The security parameters for the pending states can be set by the TLS
Handshake Protocol, and the Change Cipher Spec can selectively make
either of the pending states current, in which case the appropriate
current state is disposed of and replaced with the pending state; the
pending state is then reinitialized to an empty state. It is illegal
to make a state that has not been initialized with security
parameters a current state. The initial current state always
specifies that no encryption, compression, or MAC will be used.
The security parameters for a TLS Connection read and write state are
set by providing the following values:
connection end
Whether this entity is considered the "client" or the "server" in
this connection.
bulk encryption algorithm
An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, how much of that key is
secret, whether it is a block or stream cipher, and the block size
of the cipher (if appropriate).
MAC algorithm
An algorithm to be used for message authentication. This
specification includes the size of the hash returned by the MAC
algorithm.
compression algorithm
An algorithm to be used for data compression. This specification
must include all information the algorithm requires compression.
master secret
A 48-byte secret shared between the two peers in the connection.
Dierks & Rescorla Standards Track [Page 15]
RFC 4346 The TLS Protocol April 2006
client random
A 32-byte value provided by the client.
server random
A 32-byte value provided by the server.
These parameters are defined in the presentation language as:
enum { server, client } ConnectionEnd;
enum { null, rc4, rc2, des, 3des, des40, idea, aes }
BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { null, md5, sha } MACAlgorithm;
enum { null(0), (255) } CompressionMethod;
/* The algorithms specified in CompressionMethod,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 key_size;
uint8 key_material_length;
MACAlgorithm mac_algorithm;
uint8 hash_size;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
The record layer will use the security parameters to generate the
following four items:
client write MAC secret
server write MAC secret
client write key
server write key
The client write parameters are used by the server when receiving and
processing records and vice-versa. The algorithm used for generating
these items from the security parameters is described in Section 6.3.
Dierks & Rescorla Standards Track [Page 16]
RFC 4346 The TLS Protocol April 2006
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
elements:
compression state
The current state of the compression algorithm.
cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. For stream ciphers,
this will also contain whatever state information is necessary to
allow the stream to continue to encrypt or decrypt data.
MAC secret
The MAC secret for this connection, as generated above.
sequence number
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may not
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it must
renegotiate instead. A sequence number is incremented after each
record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.
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).
Dierks & Rescorla Standards Track [Page 17]
RFC 4346 The TLS Protocol April 2006
struct {
uint8 major, minor;
} ProtocolVersion;
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
The higher-level protocol used to process the enclosed fragment.
version
The version of the protocol being employed. This document
describes TLS Version 1.1, which uses the version { 3, 2 }. The
version value 3.2 is historical: TLS version 1.1 is a minor
modification to the TLS 1.0 protocol, which was itself a minor
modification to the SSL 3.0 protocol, which bears the version
value 3.0. (See Appendix A.1.)
length
The length (in bytes) of the following TLSPlaintext.fragment. The
length should not exceed 2^14.
fragment
The application data. This data is transparent and is treated as
an independent block to be dealt with by the higher-level protocol
specified by the type field.
Note: Data of different TLS Record layer content types MAY be
interleaved. Application data is generally of lower precedence for
transmission than other content types. However, records MUST be
delivered to the network in the same order as they are protected by
the record layer. Recipients MUST receive and process interleaved
application layer traffic during handshakes subsequent to the first
one on a connection.
Dierks & Rescorla Standards Track [Page 18]
RFC 4346 The TLS Protocol April 2006
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 should report a fatal decompression failure error.
struct {
ContentType type; /* same as TLSPlaintext.type */
ProtocolVersion version;/* same as TLSPlaintext.version */
uint16 length;
opaque fragment[TLSCompressed.length];
} TLSCompressed;
length
The length (in bytes) of the following TLSCompressed.fragment.
The length should not exceed 2^14 + 1024.
fragment
The compressed form of TLSPlaintext.fragment.
Note: A CompressionMethod.null operation is an identity operation; no
fields are altered.
Implementation note: Decompression functions are responsible for
ensuring that messages cannot cause internal
buffer overflows.
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.
Dierks & Rescorla Standards Track [Page 19]
RFC 4346 The TLS Protocol April 2006
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
} TLSCiphertext;
type
The type field is identical to TLSCompressed.type.
version
The version field is identical to TLSCompressed.version.
length
The length (in bytes) of the following TLSCiphertext.fragment.
The length may not exceed 2^14 + 2048.
fragment
The encrypted form of TLSCompressed.fragment, with the MAC.
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[CipherSpec.hash_size];
} GenericStreamCipher;
The MAC is generated as:
HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type +
TLSCompressed.version + TLSCompressed.length +
TLSCompressed.fragment));
where "+" denotes concatenation.
seq_num
The sequence number for this record.
hash
The hashing algorithm specified by
SecurityParameters.mac_algorithm.
Dierks & Rescorla Standards Track [Page 20]
RFC 4346 The TLS Protocol April 2006
Note that the MAC is computed before encryption. The stream cipher
encrypts the entire block, including the MAC. For stream ciphers
that do not use a synchronization vector (such as RC4), the stream
cipher state from the end of one record is simply used on the
subsequent packet. If the CipherSuite is TLS_NULL_WITH_NULL_NULL,
encryption consists of the identity operation (i.e., the data is not
encrypted, and the MAC size is zero, implying that no MAC is used).
TLSCiphertext.length is TLSCompressed.length plus
CipherSpec.hash_size.
6.2.3.2. CBC Block Cipher
For block ciphers (such as RC2, DES, or AES), the encryption and MAC
functions convert TLSCompressed.fragment structures to and from block
TLSCiphertext.fragment structures.
block-ciphered struct {
opaque IV[CipherSpec.block_length];
opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
The MAC is generated as described in Section 6.2.3.1.
IV
Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
IV in order to prevent the attacks described by [CBCATT]. We
recommend the following equivalently strong procedures. For
clarity we use the following notation.
IV
The transmitted value of the IV field in the GenericBlockCipher
structure.
CBC residue
The last ciphertext block of the previous record.
mask
The actual value that the cipher XORs with the plaintext prior
to encryption of the first cipher block of the record.
In prior versions of TLS, there was no IV field and the CBC
residue and mask were one and the same. See Sections 6.1,
6.2.3.2, and 6.3, of [TLS1.0] for details of TLS 1.0 IV handling.
Dierks & Rescorla Standards Track [Page 21]
RFC 4346 The TLS Protocol April 2006
One of the following two algorithms SHOULD be used to generate the
per-record IV:
(1) Generate a cryptographically strong random string R of length
CipherSpec.block_length. Place R in the IV field. Set the
mask to R. Thus, the first cipher block will be encrypted as
E(R XOR Data).
(2) Generate a cryptographically strong random number R of length
CipherSpec.block_length and prepend it to the plaintext prior
to encryption. In this case either:
(a) The cipher may use a fixed mask such as zero.
(b) The CBC residue from the previous record may be used as
the mask. This preserves maximum code compatibility with
TLS 1.0 and SSL 3. It also has the advantage that it does
not require the ability to quickly reset the IV, which is
known to be a problem on some systems.
In either (2)(a) or (2)(b) the data (R || data) is fed into
the encryption process. The first cipher block (containing
E(mask XOR R) is placed in the IV field. The first block of
content contains E(IV XOR data).
The following alternative procedure MAY be used; however, it has
not been demonstrated to be as cryptographically strong as the
above procedures. The sender prepends a fixed block F to the
plaintext (or, alternatively, a block generated with a weak PRNG).
He then encrypts as in (2), above, using the CBC residue from the
previous block as the mask for the prepended block. Note that in
this case the mask for the first record transmitted by the
application (the Finished) MUST be generated using a
cryptographically strong PRNG.
The decryption operation for all three alternatives is the same.
The receiver decrypts the entire GenericBlockCipher structure and
then discards the first cipher block, corresponding to the IV
component.
padding
Padding that is added to force the length of the plaintext to be
an integral multiple of the block cipher's block length. The
padding MAY be any length up to 255 bytes, as long as it results
in the TLSCiphertext.length being an integral multiple of the
block length. Lengths longer than necessary might be desirable to
frustrate attacks on a protocol that are based on analysis of the
lengths of exchanged messages. Each uint8 in the padding data
vector MUST be filled with the padding length value. The receiver
Dierks & Rescorla Standards Track [Page 22]
RFC 4346 The TLS Protocol April 2006
MUST check this padding and SHOULD use the bad_record_mac alert to
indicate padding errors.
padding_length
The padding length MUST be such that the total size of the
GenericBlockCipher structure is a multiple of the cipher's block
length. Legal values range from zero to 255, inclusive. This
length specifies the length of the padding field exclusive of the
padding_length field itself.
The encrypted data length (TLSCiphertext.length) is one more than the
sum of CipherSpec.block_length, TLSCompressed.length,
CipherSpec.hash_size, and padding_length.
Example: If the block length is 8 bytes, the content length
(TLSCompressed.length) is 61 bytes, and the MAC length is 20
bytes, then the length before padding is 82 bytes (this does
not include the IV, which may or may not be encrypted, as
discussed above). Thus, the padding length modulo 8 must be
equal to 6 in order to make the total length an even
multiple of 8 bytes (the block length). The padding length
can be 6, 14, 22, and so on, through 254. If the padding
length were the minimum necessary, 6, the padding would be 6
bytes, each containing the value 6. Thus, the last 8 octets
of the GenericBlockCipher before block encryption would be
xx 06 06 06 06 06 06 06, where xx is the last octet of the
MAC.
Note: With block ciphers in CBC mode (Cipher Block Chaining), it is
critical that the entire plaintext of the record be known
before any ciphertext is transmitted. Otherwise, it is
possible for the attacker to mount the attack described in
[CBCATT].
Implementation Note: Canvel et al. [CBCTIME] have demonstrated a
timing attack on CBC padding based on the time
required to compute the MAC. In order to defend
against this attack, implementations MUST ensure
that record processing time is essentially the
same whether or not the padding is correct. In
general, the best way to do this is to compute
the MAC even if the padding is incorrect, and
only then reject the packet. For instance, if
the pad appears to be incorrect, the
implementation might assume a zero-length pad
and then compute the MAC. This leaves a small
timing channel, since MAC performance depends to
some extent on the size of the data fragment,
Dierks & Rescorla Standards Track [Page 23]
RFC 4346 The TLS Protocol April 2006
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.
6.3. Key Calculation
The Record Protocol requires an algorithm to generate keys, and MAC
secrets from the security parameters provided by the handshake
protocol.
The master secret is hashed into a sequence of secure bytes, which
are assigned to the MAC secrets and keys required by the current
connection state (see Appendix A.6). CipherSpecs require a client
write MAC secret, a server write MAC secret, a client write key, and
a server write key, each of which is generated from the master secret
in that order. Unused values are empty.
When keys and MAC secrets are generated, the master secret is used as
an entropy source.
To generate the key material, compute
key_block = PRF(SecurityParameters.master_secret,
"key expansion",
SecurityParameters.server_random +
SecurityParameters.client_random);
until enough output has been generated. Then the key_block is
partitioned as follows:
client_write_MAC_secret[SecurityParameters.hash_size]
server_write_MAC_secret[SecurityParameters.hash_size]
client_write_key[SecurityParameters.key_material_length]
server_write_key[SecurityParameters.key_material_length]
Implementation note: The currently defined cipher suite that requires
the most material is AES_256_CBC_SHA, defined in [TLSAES]. It
requires 2 x 32 byte keys, 2 x 20 byte MAC secrets, and 2 x 16 byte
Initialization Vectors, for a total of 136 bytes of key material.
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.
Dierks & Rescorla Standards Track [Page 24]
RFC 4346 The TLS Protocol April 2006
The Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
peer certificate
X509v3 [X509] certificate of the peer. This element of the state
may be null.
compression method
The algorithm used to compress data prior to encryption.
cipher spec
Specifies the bulk data encryption algorithm (such as null, DES,
etc.) and a MAC algorithm (such as MD5 or SHA). It also defines
cryptographic attributes such as the hash_size. (See Appendix A.6
for formal definition.)
master secret
48-byte secret shared between the client and server.
is resumable
A flag indicating whether the session can be used to initiate new
connections.
These items are then used to create security parameters for use by
the Record Layer when protecting application data. Many connections
can be instantiated using the same session through the resumption
feature of the TLS Handshake Protocol.
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.
Dierks & Rescorla Standards Track [Page 25]
RFC 4346 The TLS Protocol April 2006
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See Section 6.1.) The change cipher spec message is sent during the
handshake after the security parameters have been agreed upon, but
before the verifying finished message is sent (see Section 7.4.9).
Note: If a rehandshake occurs while data is flowing on a connection,
the communicating parties may continue to send data using the
old CipherSpec. However, once the ChangeCipherSpec has been
sent, the new CipherSpec MUST be used. The first side to send
the ChangeCipherSpec does not know that the other side has
finished computing the new keying material (e.g., if it has to
perform a time consuming public key operation). Thus, a small
window of time, during which the recipient must buffer the
data, MAY exist. In practice, with modern machines this
interval is likely to be fairly short.
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(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),
Dierks & Rescorla Standards Track [Page 26]
RFC 4346 The TLS Protocol April 2006
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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
Dierks & Rescorla Standards Track [Page 27]
RFC 4346 The TLS Protocol April 2006
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. The following error alerts are defined:
unexpected_message
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn't an
even multiple of the block length, or its padding values, when
checked, weren't correct. This message is always fatal.
decryption_failed
This alert MAY be returned if a TLSCiphertext decrypted in an
invalid way: either it wasn't an even multiple of the block
length, or its padding values, when checked, weren't correct.
This message is always fatal.
Note: Differentiating between bad_record_mac and decryption_failed
alerts may permit certain attacks against CBC mode as used in
TLS [CBCATT]. It is preferable to uniformly use the
bad_record_mac alert to hide the specific type of the error.
record_overflow
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed
record with more than 2^14+1024 bytes. This message is always
fatal.
Dierks & Rescorla Standards Track [Page 28]
RFC 4346 The TLS Protocol April 2006
decompression_failure
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal.
handshake_failure
Reception of a handshake_failure alert message indicates that
the sender was unable to negotiate an acceptable set of
security parameters given the options available. This is a
fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not in TLS. It should not be
sent by compliant implementations.
bad_certificate
A certificate was corrupt, contained signatures that did not
verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
certificate_unknown
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was out of range or inconsistent with
other fields. This is always fatal.
unknown_ca
A valid certificate chain or partial chain was received, but
the certificate was not accepted because the CA certificate
could not be located or couldn't be matched with a known,
trusted CA. This message is always fatal.
access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation.
This message is always fatal.
Dierks & Rescorla Standards Track [Page 29]
RFC 4346 The TLS Protocol April 2006
decode_error
A message could not be decoded because some field was out of
the specified range or the length of the message was incorrect.
This message is always fatal.
decrypt_error
A handshake cryptographic operation failed, including being
unable to correctly verify a signature, decrypt a key exchange,
or validate a finished message.
export_restriction_RESERVED
This alert was used in TLS 1.0 but not TLS 1.1.
protocol_version
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol
versions might be avoided for security reasons). This message
is always fatal.
insufficient_security
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is
always fatal.
internal_error
An internal error unrelated to the peer or the correctness of
the protocol (such as a memory allocation failure) makes it
impossible to continue. This message is always fatal.
user_canceled
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be
followed by a close_notify. This message is generally a
warning.
no_renegotiation
Sent by the client in response to a hello request or by the
server in response to a client hello after initial handshaking.
Either of these would normally lead to renegotiation; when that
is not appropriate, the recipient should respond with this
alert. At that point, the original requester can decide
whether to proceed with the connection. One case where this
would be appropriate is where a server has spawned a process to
satisfy a request; the process might receive security
parameters (key length, authentication, etc.) at startup and it
Dierks & Rescorla Standards Track [Page 30]
RFC 4346 The TLS Protocol April 2006
might be difficult to communicate changes to these parameters
after that point. This message is always a warning.
For all errors where an alert level is not explicitly specified, the
sending party MAY determine at its discretion whether this is a fatal
error or not; if an alert with a level of warning is received, the
receiving party MAY decide at its discretion whether to treat this as
a fatal error or not. However, all messages that are transmitted
with a level of fatal MUST be treated as fatal messages.
New alert values MUST be defined by RFC 2434 Standards Action. See
Section 11 for IANA Considerations for alert values.
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
Dierks & Rescorla Standards Track [Page 31]
RFC 4346 The TLS Protocol April 2006
example, an attacker could block access to the port a secure service
runs on, or attempt to get the peers to negotiate an unauthenticated
connection. The fundamental rule is that higher levels must be
cognizant of what their security requirements are and never transmit
information over a channel less secure than what they require. The
TLS protocol is secure in that any cipher suite offers its promised
level of security: if you negotiate 3DES with a 1024 bit RSA key
exchange with a host whose certificate you have verified, you can
expect to be that secure.
However, one SHOULD never send data over a link encrypted with 40-bit
security unless one feels that data is worth no more than the effort
required to break that encryption.
These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a client hello message to
which the server must respond with a server hello message, or else a
fatal error will occur and the connection will fail. The client
hello and server hello are used to establish security enhancement
capabilities between client and server. The client hello and server
hello establish the following attributes: Protocol Version, Session
ID, Cipher Suite, and Compression Method. Additionally, two random
values are generated and exchanged: ClientHello.random and
ServerHello.random.
The actual key exchange uses up to four messages: the server
certificate, the server key exchange, the client certificate, and the
client key exchange. New key exchange methods can be created by
specifying a format for these messages and by defining the use of the
messages to allow the client and server to agree upon a shared
secret. This secret MUST be quite long; currently defined key
exchange methods exchange secrets that range from 48 to 128 bytes in
length.
Following the hello messages, the server will send its certificate,
if it is to be authenticated. Additionally, a server key exchange
message may be sent, if it is required (e.g., if the server has no
certificate, or if its certificate is for signing only). If the
server is authenticated, it may request a certificate from the
client, if that is appropriate to the cipher suite selected. Next,
the server will send the server hello done message, indicating that
the hello-message phase of the handshake is complete. The server
will then wait for a client response. If the server has sent a
certificate request message, the client must send the certificate
message. The client key exchange message is now sent, and the
content of that message will depend on the public key algorithm
selected between the client hello and the server hello. If the
client has sent a certificate with signing ability, a digitally-
Dierks & Rescorla Standards Track [Page 32]
RFC 4346 The TLS Protocol April 2006
signed certificate verify message is sent to explicitly verify the
certificate.
At this point, a change cipher spec message is sent by the client,
and the client copies the pending Cipher Spec into the current Cipher
Spec. The client then immediately sends the finished message under
the new algorithms, keys, and secrets. In response, the server will
send its own change cipher spec message, transfer the pending to the
current Cipher Spec, and send its finished message under the new
Cipher Spec. At this point, the handshake is complete, and the
client and server may begin to exchange application layer data. (See
flow chart below.) Application data MUST NOT be sent prior to the
completion of the first handshake (before a cipher suite other
TLS_NULL_WITH_NULL_NULL is established).
Client Server
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Fig. 1. Message flow for a full handshake
* Indicates optional or situation-dependent messages that are not
always sent.
Note: To help avoid pipeline stalls, ChangeCipherSpec is an
independent TLS Protocol content type, and is not actually a
TLS handshake message.
When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security
parameters), the message flow is as follows:
The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match.
Dierks & Rescorla Standards Track [Page 33]
RFC 4346 The TLS Protocol April 2006
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;
Dierks & Rescorla Standards Track [Page 34]
RFC 4346 The TLS Protocol April 2006
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be
omitted, however. Note one exception to the ordering: the
Certificate message is used twice in the handshake (from server to
client, then from client to server), but is described only in its
first position. The one message that is not bound by these ordering
rules is the Hello Request message, which can be sent at any time,
but which should be ignored by the client if it arrives in the middle
of a handshake.
New Handshake message type values MUST be defined via RFC 2434
Standards Action. See Section 11 for IANA Considerations for these
values.
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 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
Dierks & Rescorla Standards Track [Page 35]
RFC 4346 The TLS Protocol April 2006
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 or application protocols may define
additional requirements.
random_bytes
28 bytes generated by a secure random number generator.
Dierks & Rescorla Standards Track [Page 36]
RFC 4346 The TLS Protocol April 2006
The client hello message includes a variable-length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes to
reuse. The session identifier MAY be from an earlier connection,
from this connection, or from another currently active connection.
The second option is useful if the client only wishes to update the
random structures and derived values of a connection, and the third
option makes it possible to establish several independent secure
connections without repeating the full handshake protocol. These
independent connections may occur sequentially or simultaneously; a
SessionID becomes valid when the handshake negotiating it completes
with the exchange of Finished messages and persists until it is
removed due to aging or because a fatal error was encountered on a
connection associated with the session. The actual contents of the
SessionID are defined by the server.
opaque SessionID<0..32>;
Warning: Because the SessionID is transmitted without encryption or
immediate MAC protection, servers MUST not place
confidential information in session identifiers or let the
contents of fake session identifiers cause any breach of
security. (Note that the content of the handshake as a
whole, including the SessionID, is protected by the Finished
messages exchanged at the end of the handshake.)
The CipherSuite list, passed from the client to the server in the
client hello message, contains the combinations of cryptographic
algorithms supported by the client in order of the client's
preference (favorite choice first). Each CipherSuite defines a key
exchange algorithm, a bulk encryption algorithm (including secret key
length), and a MAC algorithm. The server will select a cipher suite
or, if no acceptable choices are presented, return a handshake
failure alert and close the connection.
uint8 CipherSuite[2]; /* Cryptographic suite selector */
The client hello includes a list of compression algorithms supported
by the client, ordered according to the client's preference.
Dierks & Rescorla Standards Track [Page 37]
RFC 4346 The TLS Protocol April 2006
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<2..2^16-1>;
CompressionMethod compression_methods<1..2^8-1>;
} ClientHello;
client_version
The version of the TLS protocol by which the client wishes to
communicate during this session. This SHOULD be the latest
(highest valued) version supported by the client. For this
version of the specification, the version will be 3.2. (See
Appendix E for details about backward compatibility.)
random
A client-generated random structure.
session_id
The ID of a session the client wishes to use for this connection.
This field should be empty if no session_id is available or if the
client wishes to generate new security parameters.
cipher_suites
This is a list of the cryptographic options supported by the
client, with the client's first preference first. If the
session_id field is not empty (implying a session resumption
request) this vector MUST include at least the cipher_suite from
that session. Values are defined in Appendix A.5.
compression_methods
This is a list of the compression methods supported by the client,
sorted by client preference. If the session_id field is not empty
(implying a session resumption request) it MUST include the
compression_method from that session. This vector MUST contain,
and all implementations MUST support, CompressionMethod.null.
Thus, a client and server will always be able to agree on a
compression method.
After sending the client hello message, the client waits for a server
hello message. Any other handshake message returned by the server
except for a hello request is treated as a fatal error.
Forward compatibility note: In the interests of forward
compatibility, it is permitted that a client hello message include
extra data after the compression methods. This data MUST be included
Dierks & Rescorla Standards Track [Page 38]
RFC 4346 The TLS Protocol April 2006
in the handshake hashes, but must otherwise be ignored. This is the
only handshake message for which this is legal; for all other
messages, the amount of data in the message MUST match the
description of the message precisely.
Note: For the intended use of trailing data in the ClientHello,
see RFC 3546 [TLSEXT].
7.4.1.3. Server Hello
The server will send this message in response to a client hello
message when it was able to find an acceptable set of algorithms. If
it cannot find such a match, it will respond with a handshake failure
alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} ServerHello;
server_version
This field will contain the lower of that suggested by the client
in the client hello and the highest supported by the server. For
this version of the specification, the version is 3.2. (See
Appendix E for details about backward compatibility.)
random
This structure is generated by the server and MUST be
independently generated from the ClientHello.random.
Dierks & Rescorla Standards Track [Page 39]
RFC 4346 The TLS Protocol April 2006
session_id
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the finished messages. Otherwise this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with.
cipher_suite
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is
the value from the state of the session being resumed.
compression_method The single compression algorithm selected by the
server from the list in ClientHello.compression_methods. For
resumed sessions this field is the value from the resumed session
state.
7.4.2. Server Certificate
When this message will be sent:
The server MUST send a certificate whenever the agreed-upon key
exchange method is not an anonymous one. This message will always
immediately follow the server hello message.
Meaning of this message:
The certificate type MUST be appropriate for the selected cipher
suite's key exchange algorithm, and is generally an X.509v3
certificate. It MUST contain a key that matches the key exchange
method, as follows. Unless otherwise specified, the signing
algorithm for the certificate MUST be the same as the algorithm
for the certificate key. Unless otherwise specified, the public
key MAY be of any length.
Dierks & Rescorla Standards Track [Page 40]
RFC 4346 The TLS Protocol April 2006
Key Exchange Algorithm Certificate Key Type
RSA RSA public key; the certificate MUST
allow the key to be used for encryption.
DHE_DSS DSS public key.
DHE_RSA RSA public key that can be used for
signing.
DH_DSS Diffie-Hellman key. The algorithm used
to sign the certificate MUST be DSS.
DH_RSA Diffie-Hellman key. The algorithm used
to sign the certificate MUST be RSA.
All certificate profiles and key and cryptographic formats are
defined by the IETF PKIX working group [PKIX]. When a key usage
extension is present, the digitalSignature bit MUST be set for the
key to be eligible for signing, as described above, and the
keyEncipherment bit MUST be present to allow encryption, as described
above. The keyAgreement bit must be set on Diffie-Hellman
certificates.
As CipherSuites that specify new key exchange methods are specified
for the TLS Protocol, they will imply certificate format and the
required encoded keying information.
Structure of this message:
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
certificate_list
This is a sequence (chain) of X.509v3 certificates. The sender's
certificate must come first in the list. Each following
certificate must directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority may optionally be omitted from the chain,
under the assumption that the remote end must already possess it
in order to validate it in any case.
The same message type and structure will be used for the client's
response to a certificate request message. Note that a client MAY
Dierks & Rescorla Standards Track [Page 41]
RFC 4346 The TLS Protocol April 2006
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.
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: either an RSA public key with
which to encrypt the premaster secret, or a Diffie-Hellman public
key with which the client can complete a key exchange (with the
result being the premaster secret).
As additional CipherSuites are defined for TLS that include new key
exchange algorithms, the server key exchange message will be sent if
and only if the certificate type associated with the key exchange
algorithm does not provide enough information for the client to
exchange a premaster secret.
Dierks & Rescorla Standards Track [Page 42]
RFC 4346 The TLS Protocol April 2006
Structure of this message:
enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
struct {
opaque rsa_modulus<1..2^16-1>;
opaque rsa_exponent<1..2^16-1>;
} ServerRSAParams;
rsa_modulus
The modulus of the server's temporary RSA key.
rsa_exponent
The public exponent of the server's temporary RSA key.
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams; /* Ephemeral DH parameters */
dh_p
The prime modulus used for the Diffie-Hellman operation.
dh_g
The generator used for the Diffie-Hellman operation.
dh_Ys
The server's Diffie-Hellman public value (g^X mod p).
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
};
} ServerKeyExchange;
Dierks & Rescorla Standards Track [Page 43]
RFC 4346 The TLS Protocol April 2006
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
case rsa:
ServerRSAParams params;
};
} ServerParams;
params
The server's key exchange parameters.
signed_params
For non-anonymous key exchanges, a hash of the corresponding
params value, with the signature appropriate to that hash
applied.
md5_hash
MD5(ClientHello.random + ServerHello.random + ServerParams);
sha_hash
SHA(ClientHello.random + ServerHello.random + ServerParams);
enum { anonymous, rsa, dsa } SignatureAlgorithm;
struct {
select (SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
digitally-signed struct {
opaque md5_hash[16];
opaque sha_hash[20];
};
case dsa:
digitally-signed struct {
opaque sha_hash[20];
};
};
};
} Signature;
7.4.4. Certificate request
When this message will be sent:
A non-anonymous server can optionally request a certificate from
the client, if it is appropriate for the selected cipher suite.
Dierks & Rescorla Standards Track [Page 44]
RFC 4346 The TLS Protocol April 2006
This message, if sent, will immediately follow the Server Key
Exchange message (if it is sent; otherwise, the Server Certificate
message).
Structure of this message:
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
certificate_types
This field is a list of the types of certificates requested,
sorted in order of the server's preference.
certificate_authorities
A list of the distinguished names of acceptable certificate
authorities. These distinguished names may specify a desired
distinguished name for a root CA or for a subordinate CA; thus,
this message can be used to describe both known roots and a
desired authorization space. If the certificate_authorities
list is empty then the client MAY send any certificate of the
appropriate ClientCertificateType, unless there is some
external arrangement to the contrary.
ClientCertificateType values are divided into three groups:
1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
reserved for IETF Standards Track protocols.
2. Values from 64 decimal (0x40) through 223 decimal (0xDF)
inclusive are reserved for assignment for non-Standards Track
methods.
3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
inclusive are reserved for private use.
Dierks & Rescorla Standards Track [Page 45]
RFC 4346 The TLS Protocol April 2006
Additional information describing the role of IANA in the allocation
of ClientCertificateType code points is described in Section 11.
Note: Values listed as RESERVED may not be used. They were used in
SSLv3.
Note: DistinguishedName is derived from [X501]. DistinguishedNames
are represented in DER-encoded format.
Note: It is a fatal handshake_failure alert for an anonymous server
to request client authentication.
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 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.
Dierks & Rescorla Standards Track [Page 46]
RFC 4346 The TLS Protocol April 2006
Note: When using a static Diffie-Hellman based key exchange method
(DH_DSS or DH_RSA), if client authentication is requested, the
Diffie-Hellman group and generator encoded in the client's
certificate MUST match the server specified Diffie-Hellman
parameters if the client's parameters are to be used for the key
exchange.
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 the client was able to respond with a certificate
that contained a Diffie-Hellman public key whose parameters (group
and generator) matched those specified by the server in its
certificate, this message MUST not contain any data.
Structure of this message:
The choice of messages depends on which key exchange method has
been selected. See Section 7.4.3 for the KeyExchangeAlgorithm
definition.
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
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 or the temporary RSA key
Dierks & Rescorla Standards Track [Page 47]
RFC 4346 The TLS Protocol April 2006
provided in a server key exchange message, and sends the result in
an encrypted premaster secret message. This structure is a
variant of the client key exchange message and is not a message in
itself.
Structure of this message:
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
client_version The latest (newest) version supported by the
client. This is used to detect version roll-back attacks.
Upon receiving the premaster secret, the server SHOULD check
that this value matches the value transmitted by the client in
the client hello message.
random
46 securely-generated random bytes.
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
pre_master_secret
This random value is generated by the client and is used to
generate the master secret, as specified in Section 8.1.
Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be
used to attack a TLS server that is using PKCS#1 v 1.5 encoded
RSA. The attack takes advantage of the fact that, by failing
in different ways, a TLS server can be coerced into revealing
whether a particular message, when decrypted, is properly
PKCS#1 v1.5 formatted or not.
The best way to avoid vulnerability to this attack is to treat
incorrectly formatted messages in a manner indistinguishable
from correctly formatted RSA blocks. Thus, when a server
receives an incorrectly formatted RSA block, it should generate
a random 48-byte value and proceed using it as the premaster
secret. Thus, the server will act identically whether the
received RSA block is correctly encoded or not.
[PKCS1B] defines a newer version of PKCS#1 encoding that is
more secure against the Bleichenbacher attack. However, for
maximal compatibility with TLS 1.0, TLS 1.1 retains the
original encoding. No variants of the Bleichenbacher attack
Dierks & Rescorla Standards Track [Page 48]
RFC 4346 The TLS Protocol April 2006
are known to exist provided that the above recommendations are
followed.
Implementation Note: Public-key-encrypted data is represented as an
opaque vector <0..2^16-1> (see Section 4.7).
Thus, the RSA-encrypted PreMasterSecret in a
ClientKeyExchange is preceded by two length
bytes. These bytes are redundant in the case of
RSA because the EncryptedPreMasterSecret is the
only data in the ClientKeyExchange and its
length can therefore be unambiguously
determined. The SSLv3 specification was not
clear about the encoding of public-key-encrypted
data, and therefore many SSLv3 implementations
do not include the length bytes, encoding the
RSA encrypted data directly in the
ClientKeyExchange message.
This specification requires correct encoding of
the EncryptedPreMasterSecret complete with
length bytes. The resulting PDU is incompatible
with many SSLv3 implementations. Implementors
upgrading from SSLv3 must modify their
implementations to generate and accept the
correct encoding. Implementors who wish to be
compatible with both SSLv3 and TLS should make
their implementation's behavior dependent on the
protocol version.
Implementation Note: It is now known that remote timing-based attacks
on SSL are possible, at least when the client
and server are on the same LAN. Accordingly,
implementations that use static RSA keys SHOULD
use RSA blinding or some other anti-timing
technique, as described in [TIMING].
Note: The version number in the PreMasterSecret MUST be the version
offered by the client in the ClientHello, not the version
negotiated for the connection. This feature is designed to
prevent rollback attacks. Unfortunately, many implementations
use the negotiated version instead, and therefore checking the
version number may lead to failure to interoperate with such
incorrect client implementations. Client implementations, MUST
and Server implementations MAY, check the version number. In
practice, since the TLS handshake MACs prevent downgrade and no
good attacks are known on those MACs, ambiguity is not
considered a serious security risk. Note that if servers
choose to check the version number, they should randomize the
Dierks & Rescorla Standards Track [Page 49]
RFC 4346 The TLS Protocol April 2006
PreMasterSecret in case of error, rather than generate an
alert, in order to avoid variants on the Bleichenbacher attack.
[KPR03]
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 certificate already contains a suitable Diffie-
Hellman key, then Yc is implicit and does not need to be sent
again. In this case, the client key exchange message will be
sent, but it MUST be empty.
explicit
Yc needs to be sent.
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: opaque dh_Yc<1..2^16-1>;
} dh_public;
} ClientDiffieHellmanPublic;
dh_Yc
The client's Diffie-Hellman public value (Yc).
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.
Dierks & Rescorla Standards Track [Page 50]
RFC 4346 The TLS Protocol April 2006
Structure of this message:
struct {
Signature signature;
} CertificateVerify;
The Signature type is defined in 7.4.3.
CertificateVerify.signature.md5_hash
MD5(handshake_messages);
CertificateVerify.signature.sha_hash
SHA(handshake_messages);
Here handshake_messages refers to all handshake messages sent or
received starting at client hello up to but not including this
message, including the type and length fields of the handshake
messages. This is the concatenation of all the Handshake structures,
as defined in 7.4, exchanged thus far.
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.
struct {
opaque verify_data[12];
} Finished;
verify_data
PRF(master_secret, finished_label, MD5(handshake_messages) +
SHA-1(handshake_messages)) [0..11];
Dierks & Rescorla Standards Track [Page 51]
RFC 4346 The TLS Protocol April 2006
finished_label
For Finished messages sent by the client, the string "client
finished". For Finished messages sent by the server, the
string "server finished".
handshake_messages
All of the data from all messages in this handshake (not
including any HelloRequest messages) up to but not including
this message. This is only data visible at the handshake
layer and does not include record layer headers. This is the
concatenation of all the Handshake structures, as defined in
7.4, exchanged thus far.
It is a fatal error if a finished message is not preceded by a change
cipher spec message at the appropriate point in the handshake.
The value handshake_messages includes all handshake messages starting
at client hello up to, but not including, this finished message.
This may be different from handshake_messages in Section 7.4.8
because it would include the certificate verify message (if sent).
Also, the handshake_messages for the finished message sent by the
client will be different from that for the finished message sent by
the server, because the one that is sent second will include the
prior one.
Note: Change cipher spec messages, alerts, and any other record types
are not handshake messages and are not included in the hash
computations. Also, Hello Request messages are omitted from
handshake hashes.
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.
Dierks & Rescorla Standards Track [Page 52]
RFC 4346 The TLS Protocol April 2006
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.
RSA digital signatures are performed using PKCS #1 [PKCS1] block type
1. RSA public key encryption is performed using PKCS #1 block type 2.
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_3DES_EDE_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.
Dierks & Rescorla Standards Track [Page 53]
RFC 4346 The TLS Protocol April 2006
12. IANA Considerations
This document describes a number of new registries that have been
created by IANA. We recommended that they be placed as individual
registries items under a common TLS category.
Section 7.4.3 describes a TLS ClientCertificateType Registry to be
maintained by the IANA, defining a number of such code point
identifiers. ClientCertificateType identifiers with values in the
range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards
Action. Values from the range 64-223 (decimal) inclusive are
assigned via [RFC2434] Specification Required. Identifier values
from 224-255 (decimal) inclusive are reserved for RFC 2434 Private
Use. The registry will initially be populated with the values in
this document, Section 7.4.4.
Section A.5 describes a TLS Cipher Suite Registry to be maintained by
the IANA, and it defines a number of such cipher suite identifiers.
Cipher suite values with the first byte in the range 0-191 (decimal)
inclusive are assigned via RFC 2434 Standards Action. Values with
the first byte in the range 192-254 (decimal) are assigned via RFC
2434 Specification Required. Values with the first byte 255
(decimal) are reserved for RFC 2434 Private Use. The registry will
initially be populated with the values from Section A.5 of this
document, [TLSAES], and from Section 3 of [TLSKRB].
Section 6 requires that all ContentType values be defined by RFC 2434
Standards Action. IANA has created a TLS ContentType registry,
initially populated with values from Section 6.2.1 of this document.
Future values MUST be allocated via Standards Action as described in
[RFC2434].
Section 7.2.2 requires that all Alert values be defined by RFC 2434
Standards Action. IANA has created a TLS Alert registry, initially
populated with values from Section 7.2 of this document and from
Section 4 of [TLSEXT]. Future values MUST be allocated via Standards
Action as described in [RFC2434].
Section 7.4 requires that all HandshakeType values be defined by RFC
2434 Standards Action. IANA has created a TLS HandshakeType
registry, initially populated with values from Section 7.4 of this
document and from Section 2.4 of [TLSEXT]. Future values MUST be
allocated via Standards Action as described in [RFC2434].
Dierks & Rescorla Standards Track [Page 54]
RFC 4346 The TLS Protocol April 2006
Appendix A. Protocol Constant Values
This section describes protocol types and constants.
A.1. Record Layer
struct {
uint8 major, minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 2 }; /* TLS v1.1 */
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSCompressed.length];
} TLSCompressed;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
} TLSCiphertext;
stream-ciphered struct {
opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size];
} GenericStreamCipher;
block-ciphered struct {
opaque IV[CipherSpec.block_length];
Dierks & Rescorla Standards Track [Page 55]
RFC 4346 The TLS Protocol April 2006
opaque content[TLSCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
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(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED (41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
Dierks & Rescorla Standards Track [Page 56]
RFC 4346 The TLS Protocol April 2006
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>;
CompressionMethod compression_methods<1..2^8-1>;
Dierks & Rescorla Standards Track [Page 57]
RFC 4346 The TLS Protocol April 2006
} ClientHello;
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} ServerHello;
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 rsa_modulus<1..2^16-1>;
opaque rsa_exponent<1..2^16-1>;
} ServerRSAParams;
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
};
} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Dierks & Rescorla Standards Track [Page 58]
RFC 4346 The TLS Protocol April 2006
case rsa:
ServerRSAParams params;
};
} ServerParams;
struct {
select (SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
digitally-signed struct {
opaque md5_hash[16];
opaque sha_hash[20];
};
case dsa:
digitally-signed struct {
opaque sha_hash[20];
};
};
};
} Signature;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
fortezza_dms_RESERVED(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
A.4.3. Client Authentication and Key Exchange Messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
} exchange_keys;
} ClientKeyExchange;
Dierks & Rescorla Standards Track [Page 59]
RFC 4346 The TLS Protocol April 2006
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[12];
} 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
1.1.
TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
TLS connection during the first handshake on that channel, but must
not be negotiated, as it provides no more protection than an
unsecured connection.
CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 };
The following CipherSuite definitions require that the server provide
an RSA certificate that can be used for key exchange. The server may
request either an RSA or a DSS signature-capable certificate in the
certificate request message.
Dierks & Rescorla Standards Track [Page 60]
RFC 4346 The TLS Protocol April 2006
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 };
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 certificate that 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 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 };
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 and is therefore deprecated.
CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
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 1.1 implementations MUST NOT negotiate
these cipher suites in TLS 1.1 mode. However, for backward
compatibility they may be offered in the ClientHello for use with TLS
Dierks & Rescorla Standards Track [Page 61]
RFC 4346 The TLS Protocol April 2006
1.0 or SSLv3-only servers. TLS 1.1 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 };
The following cipher suites were defined in [TLSKRB] and are included
here for completeness. See [TLSKRB] for details:
CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x1E }:
CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1F };
CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x20 };
CipherSuite TLS_KRB5_WITH_IDEA_CBC_SHA = { 0x00,0x21 };
CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x22 };
CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x23 };
CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x24 };
CipherSuite TLS_KRB5_WITH_IDEA_CBC_MD5 = { 0x00,0x25 };
The following exportable cipher suites were defined in [TLSKRB] and
are included here for completeness. TLS 1.1 implementations MUST NOT
negotiate these cipher suites.
CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA = { 0x00,0x26};
CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA = { 0x00,0x27};
CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_SHA = { 0x00,0x28};
CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 = { 0x00,0x29};
CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x2A};
CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x2B};
The following cipher suites were defined in [TLSAES] and are included
here for completeness. See [TLSAES] for details:
CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F };
CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 };
CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 };
CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x32 };
CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x33 };
CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 };
Dierks & Rescorla Standards Track [Page 62]
RFC 4346 The TLS Protocol April 2006
CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 };
CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x36 };
CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x37 };
CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00, 0x38 };
CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x39 };
CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A };
The cipher suite space is divided into three regions:
1. Cipher suite values with first byte 0x00 (zero) through decimal
191 (0xBF) inclusive are reserved for the IETF Standards Track
protocols.
2. Cipher suite values with first byte decimal 192 (0xC0) through
decimal 254 (0xFE) inclusive are reserved for assignment for
non-Standards Track methods.
3. Cipher suite values with first byte 0xFF are reserved for
private use.
Additional information describing the role of IANA in the allocation
of cipher suite code points is described in Section 11.
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 { null, rc4, rc2, des, 3des, des40, aes, idea }
BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { null, md5, sha } MACAlgorithm;
/* The algorithms specified in CompressionMethod,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
Dierks & Rescorla Standards Track [Page 63]
RFC 4346 The TLS Protocol April 2006
struct {
ConnectionEnd entity;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 key_size;
uint8 key_material_length;
MACAlgorithm mac_algorithm;
uint8 hash_size;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
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.
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).
Dierks & Rescorla Standards Track [Page 64]
RFC 4346 The TLS Protocol April 2006
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
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]
Dierks & Rescorla Standards Track [Page 65]
RFC 4346 The TLS Protocol April 2006
digital signatures
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.
handshake
An initial negotiation between client and server that establishes
the parameters of their transactions.
Initialization Vector (IV)
When a block cipher is used in CBC mode, the initialization vector
is exclusive-ORed with the first plaintext block prior to
encryption.
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 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 at RSA Data Security, Inc.
[RSADSI] described in [RC2].
Dierks & Rescorla Standards Track [Page 66]
RFC 4346 The TLS Protocol April 2006
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
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.
Dierks & Rescorla Standards Track [Page 67]
RFC 4346 The TLS Protocol April 2006
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 Exchange Cipher Hash
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_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
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
RSA = none
NULL No key exchange N/A
RSA RSA key exchange None
Dierks & Rescorla Standards Track [Page 68]
RFC 4346 The TLS Protocol April 2006
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
RC2_CBC_40 Block 5 16 8 8
RC4_40 Stream 5 16 0 N/A
RC4_128 Stream 16 16 0 N/A
DES40_CBC Block 5 8 8 8
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.
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.
Dierks & Rescorla Standards Track [Page 69]
RFC 4346 The TLS Protocol April 2006
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 MD5 and/or SHA, are
acceptable, but cannot provide more security than the size of the
random number generator state. (For example, MD5-based PRNGs usually
provide 128 bits of 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 example, 40-bit
encryption is easily broken, so implementations requiring strong
security should not allow 40-bit keys. Similarly, 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.
Dierks & Rescorla Standards Track [Page 70]
RFC 4346 The TLS Protocol April 2006
Appendix E. Backward Compatibility with SSL
For historical reasons and in order to avoid a profligate consumption
of reserved port numbers, application protocols that are secured by
TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same
connection port. For example, the https protocol (HTTP secured by
SSL or TLS) uses port 443 regardless of which security protocol it is
using. Thus, some mechanism must be determined to distinguish and
negotiate among the various protocols.
TLS versions 1.1 and 1.0, and SSL 3.0 are very similar; thus,
supporting both is easy. TLS clients who wish to negotiate with such
older servers SHOULD send client hello messages using the SSL 3.0
record format and client hello structure, sending {3, 2} for the
version field to note that they support TLS 1.1. If the server
supports only TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0
server hello; if it supports TLS 1.1 it will respond with a TLS 1.1
server hello. The negotiation then proceeds as appropriate for the
negotiated protocol.
Similarly, a TLS 1.1 server that wishes to interoperate with TLS 1.0
or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages and
respond with a SSL 3.0 server hello if an SSL 3.0 client hello with a
version field of {3, 0} is received, denoting that this client does
not support TLS. Similarly, if a SSL 3.0 or TLS 1.0 hello with a
version field of {3, 1} is received, the server SHOULD respond with a
TLS 1.0 hello with a version field of {3, 1}.
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.
TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL
Version 2.0 client hello messages [SSL2]. TLS servers SHOULD accept
either client hello format if they wish to support SSL 2.0 clients on
the same connection port. The only deviations from the Version 2.0
specification are the ability to specify a version with a value of
three and the support for more ciphering types in the CipherSpec.
Warning: The ability to send Version 2.0 client hello messages will be
phased out with all due haste. Implementors SHOULD make every
effort to move forward as quickly as possible. Version 3.0
provides better mechanisms for moving to newer versions.
Dierks & Rescorla Standards Track [Page 71]
RFC 4346 The TLS Protocol April 2006
The following cipher specifications are carryovers from SSL
Version 2.0. These are assumed to use RSA for key exchange and
authentication.
V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 };
V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 };
V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
= { 0x04,0x00,0x80 };
V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 };
V2CipherSpec TLS_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 };
V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
Cipher specifications native to TLS can be included in Version
2.0 client hello messages using the syntax below. Any
V2CipherSpec element with its first byte equal to zero will be
ignored by Version 2.0 servers. Clients sending any of the above
V2CipherSpecs SHOULD also include the TLS equivalent (see
Appendix A.5):
V2CipherSpec (see TLS name) = { 0x00, CipherSuite };
Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
handshakes for backward compatibility but MUST NOT negotiate them
in TLS 1.1 mode.
E.1. Version 2 Client Hello
The Version 2.0 client hello message is presented below using this
document's presentation model. The true definition is still assumed
to be the SSL Version 2.0 specification. Note that this message MUST
be sent directly on the wire, not wrapped as an SSLv3 record
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;
Dierks & Rescorla Standards Track [Page 72]
RFC 4346 The TLS Protocol April 2006
msg_length
This field is the length of the following data in bytes. The high
bit MUST be 1 and is not part of the length.
msg_type
This field, in conjunction with the version field, identifies a
version 2 client hello message. The value SHOULD be one (1).
version
The highest version of the protocol supported by the client
(equals ProtocolVersion.version; see Appendix A.1).
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.
challenge_length
The length in bytes of the client's challenge to the server to
authenticate itself. When using the SSLv2 backward compatible
handshake the client MUST use a 32-byte challenge.
cipher_specs
This is a list of all CipherSpecs the client is willing and able
to use. There MUST be at least one CipherSpec acceptable to the
server.
session_id
This field MUST be empty.
challenge The client challenge to the server for the server to
identify itself is a (nearly) arbitrary-length random. The TLS
server will right-justify the challenge data to become the
ClientHello.random data (padded with leading zeroes, if
necessary), as specified in this protocol specification. If the
length of the challenge is greater than 32 bytes, only the last 32
bytes are used. It is legitimate (but not necessary) for a V3
server to reject a V2 ClientHello that has fewer than 16 bytes of
challenge data.
Note: Requests to resume a TLS session MUST use a TLS client
hello.
Dierks & Rescorla Standards Track [Page 73]
RFC 4346 The TLS Protocol April 2006
E.2. Avoiding Man-in-the-Middle Version Rollback
When TLS clients fall back to Version 2.0 compatibility mode, they
SHOULD use special PKCS #1 block formatting. This is done so that
TLS servers will reject Version 2.0 sessions with TLS-capable
clients.
When TLS clients are in Version 2.0 compatibility mode, they 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). After decrypting the
ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an
error if these eight padding bytes are 0x03. Version 2.0 servers
receiving blocks padded in this manner will proceed normally.
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
Dierks & Rescorla Standards Track [Page 74]
RFC 4346 The TLS Protocol April 2006
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 (see
Sections 7.4.8, 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 RSA or Diffie-
Hellman for key exchange. With anonymous RSA, the client encrypts a
pre_master_secret with the server's uncertified public key extracted
from the server key exchange message. The result is sent in a client
key exchange message. Since eavesdroppers do not know the server's
private key, it will be infeasible for them to decode the
pre_master_secret.
Note: No anonymous RSA Cipher Suites are defined in this document.
With Diffie-Hellman, the server's public parameters are contained in
the server key exchange message and the client's are sent in the
client key exchange message. Eavesdroppers who do not know the
private values should not be able to find the Diffie-Hellman result
(i.e., the pre_master_secret).
Warning: Completely anonymous connections only provide protection
against passive eavesdropping. Unless an independent
tamper-proof channel is used to verify that the finished
messages were not replaced by an attacker, server
authentication is required in environments where active
man-in-the-middle attacks are a concern.
F.1.1.2. RSA Key Exchange and Authentication
With RSA, key exchange and server authentication are combined. The
public key either may be contained in the server's certificate or may
be a temporary RSA key sent in a server key exchange message. When
temporary RSA keys are used, they are signed by the server's RSA
certificate. The signature includes the current ClientHello.random,
so old signatures and temporary keys cannot be replayed. Servers may
use a single temporary RSA key for multiple negotiation sessions.
Note: The temporary RSA key option is useful if servers need large
Dierks & Rescorla Standards Track [Page 75]
RFC 4346 The TLS Protocol April 2006
certificates but must comply with government-imposed size
limits on keys used for key exchange.
Note that if ephemeral RSA is not used, 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 the master_secret and 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.
Dierks & Rescorla Standards Track [Page 76]
RFC 4346 The TLS Protocol April 2006
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.
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.
Parties concerned about attacks of this scale should not use 40-bit
encryption keys. 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.
Dierks & Rescorla Standards Track [Page 77]
RFC 4346 The TLS Protocol April 2006
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 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 (which use both SHA and MD5).
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.1.5. MD5 and SHA
TLS uses hash functions very conservatively. Where possible, both
MD5 and SHA are used in tandem to ensure that non-catastrophic flaws
in one algorithm will not break the overall protocol.
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, 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. Similarly,
the server-write and client-write keys are independent, so stream
cipher keys are used only once.
Dierks & Rescorla Standards Track [Page 78]
RFC 4346 The TLS Protocol April 2006
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 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.
Dierks & Rescorla Standards Track [Page 79]
RFC 4346 The TLS Protocol April 2006
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 previous versions of SSL, 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-ESP] or ESP [AH-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, 40-bit
bulk encryption 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.
Dierks & Rescorla Standards Track [Page 80]
RFC 4346 The TLS Protocol April 2006
Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard (AES)"
FIPS 197. November 26, 2001.
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To
DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp. 40-41.
[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards
Institute, 1983.
[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.
[PKCS1A] B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1:
RSA Cryptography Specifications Version 1.5", RFC 2313,
March 1998.
[PKCS1B] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards
(PKCS) #1: RSA Cryptography Specifications Version 2.1",
RFC 3447, February 2003.
[PKIX] Housley, R., Polk, W., Ford, 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, 2ed", Published by John Wiley &
Sons, Inc. 1996.
Dierks & Rescorla Standards Track [Page 81]
RFC 4346 The TLS Protocol April 2006
[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 26, RFC 2434,
October 1998.
[TLSAES] Chown, P., "Advanced Encryption Standard (AES)
Ciphersuites for Transport Layer Security (TLS)", RFC
3268, June 2002.
[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[TLSKRB] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.
Informative References
[AH-ESP] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
Eastlake 3rd, D., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4305, 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., "Password Interception in a SSL/TLS Channel",
http://lasecwww.epfl.ch/memo_ssl.shtml, 2003.
[ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)",
Crypto 2001.
Dierks & Rescorla Standards Track [Page 82]
RFC 4346 The TLS Protocol April 2006
[KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
March 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.
[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. Frier, 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] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[TIMING] Boneh, D., Brumley, D., "Remote timing attacks are
practical", USENIX Security Symposium 2003.
[TLS1.0] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[X501] ITU-T Recommendation X.501: Information Technology - Open
Systems Interconnection - The Directory: Models, 1993.
[X509] ITU-T Recommendation X.509 (1997 E): Information
Technology - Open Systems Interconnection - "The Directory
- Authentication Framework". 1988.
Dierks & Rescorla Standards Track [Page 83]
RFC 4346 The TLS Protocol April 2006
[XDR] Srinivasan, R., "XDR: External Data Representation
Standard", RFC 1832, August 1995.
Authors' Addresses
Working Group Chairs
Win Treese
EMail: treese@acm.org
Eric Rescorla
EMail: ekr@rtfm.com
Editors
Tim Dierks
Independent
EMail: tim@dierks.org
Eric Rescorla
RTFM, Inc.
EMail: ekr@rtfm.com
Other Contributors
Christopher Allen (co-editor of TLS 1.0)
Alacrity Ventures
EMail: ChristopherA@AlacrityManagement.com
Martin Abadi
University of California, Santa Cruz
EMail: abadi@cs.ucsc.edu
Ran Canetti
IBM
EMail: canetti@watson.ibm.com
Dierks & Rescorla Standards Track [Page 84]
RFC 4346 The TLS Protocol April 2006
Taher Elgamal
Securify
EMail: taher@securify.com
Anil Gangolli
EMail: anil@busybuddha.org
Kipp Hickman
Phil Karlton (co-author of SSLv3)
Paul Kocher (co-author of SSLv3)
Cryptography Research
EMail: paul@cryptography.com
Hugo Krawczyk
Technion Israel Institute of Technology
EMail: hugo@ee.technion.ac.il
Robert Relyea
Netscape Communications
EMail: relyea@netscape.com
Jim Roskind
Netscape Communications
EMail: jar@netscape.com
Michael Sabin
Dan Simon
Microsoft, Inc.
EMail: dansimon@microsoft.com
Tom Weinstein
Dierks & Rescorla Standards Track [Page 85]
RFC 4346 The TLS Protocol April 2006
Comments
The discussion list for the IETF TLS working group is located at the
e-mail address <ietf-tls@lists.consensus.com>. Information on the
group and information on how to subscribe to the list is at
<http://lists.consensus.com/>.
Archives of the list can be found at:
<http://www.imc.org/ietf-tls/mail-archive/>
Dierks & Rescorla Standards Track [Page 86]
RFC 4346 The TLS Protocol April 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Dierks & Rescorla Standards Track [Page 87]