Tim Dierks
Independent
Eric Rescorla
INTERNET-DRAFT RTFM, Inc.
<draft-ietf-tls-rfc2246-bis-13.txt> June 2005 (Expires December 2005)
The TLS Protocol
Version 1.1
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
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.
Table of Contents
1. Introduction
5 1.1 Differences from TLS 1.0
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6 1.1 Requirements Terminology
7 2. Goals
7 3. Goals of this document
7 4. Presentation language
8 4.1. Basic block size
9 4.2. Miscellaneous
9 4.3. Vectors
9 4.4. Numbers
10 4.5. Enumerateds
10 4.6. Constructed types
11 4.6.1. Variants
12 4.7. Cryptographic attributes
13 4.8. Constants
14 5. HMAC and the pseudorandom function
14 6. The TLS Record Protocol
17 6.1. Connection states
18 6.2. Record layer
20 6.2.1. Fragmentation
20 6.2.2. Record compression and decompression
21 6.2.3. Record payload protection
22 6.2.3.1. Null or standard stream cipher
23 6.2.3.2. CBC block cipher
23 6.3. Key calculation
26 7. The TLS Handshaking Protocols
27 7.1. Change cipher spec protocol
28 7.2. Alert protocol
28 7.2.1. Closure alerts
29 7.2.2. Error alerts
30 7.3. Handshake Protocol overview
33 7.4. Handshake protocol
37 7.4.1. Hello messages
38 7.4.1.1. Hello request
38 7.4.1.2. Client hello
39 7.4.1.3. Server hello
41 7.4.2. Server certificate
42 7.4.3. Server key exchange message
44 7.4.4. Certificate request
46 7.4.5. Server hello done
48 7.4.6. Client certificate
48 7.4.7. Client key exchange message
48 7.4.7.1. RSA encrypted premaster secret message
49 7.4.7.2. Client Diffie-Hellman public value
51 7.4.8. Certificate verify
52 7.4.9. Finished
52 8. Cryptographic computations
53 8.1. Computing the master secret
54 8.1.1. RSA
55 8.1.2. Diffie-Hellman
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55 9. Mandatory Cipher Suites
55 A. Protocol constant values
57 A.1. Record layer
57 A.2. Change cipher specs message
58 A.3. Alert messages
58 A.4. Handshake protocol
59 A.4.1. Hello messages
59 A.4.2. Server authentication and key exchange messages
60 A.4.3. Client authentication and key exchange messages
61 A.4.4. Handshake finalization message
62 A.5. The CipherSuite
62 A.6. The Security Parameters
65 B. Glossary
67 C. CipherSuite definitions
71 D. Implementation Notes
73 D.1 Random Number Generation and Seeding
73 D.2 Certificates and authentication
73 D.3 CipherSuites
73 E. Backward Compatibility With SSL
74 E.1. Version 2 client hello
75 E.2. Avoiding man-in-the-middle version rollback
76 F. Security analysis
78 F.1. Handshake protocol
78 F.1.1. Authentication and key exchange
78 F.1.1.1. Anonymous key exchange
78 F.1.1.2. RSA key exchange and authentication
79 F.1.1.3. Diffie-Hellman key exchange with authentication
80 F.1.2. Version rollback attacks
80 F.1.3. Detecting attacks against the handshake protocol
81 F.1.4. Resuming sessions
81 F.1.5. MD5 and SHA
82 F.2. Protecting application data
82 F.3. Explicit IVs
82 F.4 Security of Composite Cipher Modes
83 F.5 Denial of Service
84 F.6. Final notes
84
Change history
22-Jun-05 ekr@rtfm.com
* IESG comments
* IANA comments
* Cleaned up some references
31-May-05 ekr@rtfm.com
* IETF Last Call comments (minor cleanups)
Dierks & Rescorla Standards Track [Page 3]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
03-Dec-04 ekr@rtfm.com
* Removed export cipher suites
26-Oct-04 ekr@rtfm.com
* Numerous cleanups from Last Call comments
10-Aug-04 ekr@rtfm.com
* Added clarifying material about interleaved application data.
27-Jul-04 ekr@rtfm.com
* Premature closes no longer cause a session to be nonresumable.
Response to WG consensus.
* Added IANA considerations and registry for cipher suites
and ClientCertificateTypes
26-Jun-03 ekr@rtfm.com
* Incorporated Last Call comments from Franke Marcus, Jack Lloyd,
Brad Wetmore, and others.
22-Apr-03 ekr@rtfm.com
* coverage of the Vaudenay, Boneh-Brumley, and KPR attacks
* cleaned up IV text a bit.
* Added discussion of Denial of Service attacks.
11-Feb-02 ekr@rtfm.com
* Clarified the behavior of empty certificate lists [Nelson Bolyard]
* Added text explaining the security implications of authenticate
then encrypt.
* Cleaned up the explicit IV text.
* Added some more acknowledgement names
02-Nov-02 ekr@rtfm.com
* Changed this to be TLS 1.1.
* Added fixes for the Rogaway and Vaudenay CBC attacks
* Separated references into normative and informative
01-Mar-02 ekr@rtfm.com
* Tightened up the language in F.1.1.2 [Peter Watkins]
* Fixed smart quotes [Bodo Moeller]
* Changed handling of padding errors to prevent CBC-based attack
[Bodo Moeller]
* Fixed certificate_list spec in the appendix [Aman Sawrup]
* Fixed a bug in the V2 definitions [Aman Sawrup]
* Fixed S 7.2.1 to point out that you don't need a close notify
if you just sent some other fatal alert [Andreas Sterbenz]
* Marked alert 41 reserved [Andreas Sterbenz]
* Changed S 7.4.2 to point out that 512-bit keys cannot be used for
Dierks & Rescorla Standards Track [Page 4]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
signing [Andreas Sterbenz]
* Added reserved client key types from SSLv3 [Andreas Sterbenz]
* Changed EXPORT40 to "40-bit EXPORT" in S 9 [Andreas Sterbenz]
* Removed RSA patent statement [Andreas Sterbenz]
* Removed references to BSAFE and RSAREF [Andreas Sterbenz]
14-Feb-02 ekr@rtfm.com
* Re-converted to I-D from RFC
* Made RSA/3DES the mandatory cipher suite.
* Added discussion of the EncryptedPMS encoding and PMS version number
issues to 7.4.7.1
* Removed the requirement in 7.4.1.3 that the Server random must be
different from the Client random, since these are randomly generated
and we don't expect servers to reject Server random values which
coincidentally are the same as the Client random.
* Replaced may/should/must with MAY/SHOULD/MUST where appropriate.
In many cases, shoulds became MUSTs, where I believed that was the
actual sense of the text. Added an RFC 2119 bulletin.
* Clarified the meaning of "empty certificate" message. [Peter Gutmann]
* Redid the CertificateRequest grammar to allow no distinguished names.
[Peter Gutmann]
* Removed the reference to requiring the master secret to generate
the CertificateVerify in F.1.1 [Bodo Moeller]
* Deprecated EXPORT40.
* Fixed a bunch of errors in the SSLv2 backward compatible client hello.
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
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a transport for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other and
to negotiate an encryption algorithm and cryptographic keys before
the application protocol transmits or receives its first byte of
data. The TLS Handshake Protocol provides connection security that
has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This
authentication can be made optional, but is generally required
for at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties
to the communication.
One advantage of TLS is that it is application protocol independent.
Higher level protocols can layer on top of the TLS Protocol
transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on how to initiate TLS
handshaking and how to interpret the authentication certificates
exchanged are left up to the judgment of the designers and
implementors of protocols which 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 which
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.
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- Additional informational notes were added for various new attacks
on TLS.
In addition, a number of minor clarifications and editorial
improvements were made.
1.1 Requirements Terminology
Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
"MAY" that appear in this document are to be interpreted as described
in RFC 2119 [REQ].
2. Goals
The goals of TLS Protocol, in order of their priority, are:
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 will then be able to
successfully exchange cryptographic parameters without knowledge
of one another's code.
3. Extensibility: TLS seeks to provide a framework into which new
public key and bulk encryption methods can be incorporated as
necessary. This will also accomplish two sub-goals: to prevent
the need to create a new protocol (and risking the introduction
of possible new weaknesses) and to avoid 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
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is intended primarily for readers who will be implementing the
protocol and 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 nor 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, not to
have general application beyond that particular goal.
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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 */
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Variable length vectors are defined by specifying a subrange of legal
lengths, inclusively, using the notation <floor..ceiling>. When
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
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be assigned a value, as demonstrated in the following example. Since
the elements of the enumerated are not ordered, they can be assigned
any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
Enumerateds occupy as much space in the byte stream as would its
maximal defined ordinal value. The following definition would cause
one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to
force the width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2 or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to
the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is well
specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation,
the numerical information may be omitted.
enum { low, medium, high } Amount;
4.6. Constructed types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type. The
syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
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The fields within a structure may be qualified using the type's name
using a syntax much like that available for enumerateds. For example,
T.f2 refers to the second field of the previous declaration.
Structure definitions may be embedded.
4.6.1. Variants
Defined structures may have variants based on some knowledge that is
available within the environment. The selector must be an enumerated
type that defines the possible variants the structure defines. There
must be a case arm for every element of the enumeration declared in
the select. The body of the variant structure may be given a label
for reference. The mechanism by which the variant is selected at
runtime is not prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
....
case en: Ten;
} [[fv]];
} [[Tv]];
For example:
enum { apple, orange } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple: V1; /* VariantBody, tag = apple */
case orange: V2; /* VariantBody, tag = orange */
} variant_body; /* optional label on variant */
} VariantRecord;
Variant structures may be qualified (narrowed) by specifying a value
for the selector prior to the type. For example, a
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orange VariantRecord
is a narrowed type of a VariantRecord containing a variant_body of
type V2.
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 0 or 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 which 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
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data in such a way that it can be decrypted only with the matching
private key. A public-key-encrypted element is encoded as an opaque
vector <0..2^16-1>, where the length is specified by the signing
algorithm and key.
An RSA encrypted value is encoded with PKCS #1 block type 2 as
described in [PKCS1A].
In the following example:
stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque hash[20];
} UserType;
The contents of hash are used as input for the signing algorithm,
then the entire structure is encrypted with a stream cipher. The
length of this structure, in bytes would be equal to 2 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
due to the fact that 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 required
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,
described in [HMAC].
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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,
Dierks & Rescorla Standards Track [Page 15]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
data). Additional hash algorithms can be defined by cipher suites and
used to protect record data, but MD5 and SHA-1 are hard coded into
the description of the handshaking for this version of the protocol.
In addition, a construction is required to do expansion of secrets
into blocks of data for the purposes of key generation or validation.
This pseudo-random function (PRF) takes as input a secret, a seed,
and an identifying label and produces an output of arbitrary length.
In order to make the PRF as secure as possible, it uses two hash
algorithms in a way which should guarantee its security if either
algorithm remains secure.
First, we define a data expansion function, P_hash(secret, data)
which 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 was being used to
create 64 bytes of data, it would have to be iterated 4 times
(through A(4)), creating 80 bytes of output data; the last 16 bytes
of the final iteration would 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-or'ing the outputs of
these two expansion functions together.
S1 and S2 are the two halves of the secret and each is the same
length. S1 is taken from the first half of the secret, S2 from the
second half. Their length is created by rounding up the length of the
overall secret divided by two; thus, if the original secret is an odd
number of bytes long, the last byte of S1 will be the same as the
first byte of S2.
L_S = length in bytes of secret;
L_S1 = L_S2 = ceil(L_S / 2);
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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-or'ing 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; to generate a 80 byte output will involve P_MD5 being
iterated 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, and reassembled, then delivered to
higher level clients.
Four record protocol clients are described in this document: the
handshake protocol, the alert protocol, the change cipher spec
protocol, and the application data protocol. In order to allow
extension of the TLS protocol, additional record types can be
supported by the record protocol. Any new record types SHOULD
allocate type values immediately beyond the ContentType values for
the four record types described here (see Appendix A.1). All such
values must be defined by RFC 2434 Standards Action. See section 11
for IANA Considerations for ContentType values.
If a TLS implementation receives a record type it does not
understand, it SHOULD just ignore it. Any protocol designed for use
over TLS MUST be carefully designed to deal with all possible attacks
against it. Note that because the type and length of a record are
not protected by encryption, care SHOULD be taken to minimize the
value of traffic analysis of these values.
Dierks & Rescorla Standards Track [Page 17]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
6.1. Connection states
A TLS connection state is the operating environment of the TLS Record
Protocol. It specifies a compression algorithm, encryption algorithm,
and 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 which 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, 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 which is returned by
the MAC algorithm.
compression algorithm
An algorithm to be used for data compression. This specification
must include all information the algorithm requires to do
compression.
master secret
A 48 byte secret shared between the two peers in the connection.
client random
A 32 byte value provided by the client.
Dierks & Rescorla Standards Track [Page 18]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
server random
A 32 byte value provided by the server.
These parameters are defined in the presentation language as:
enum { server, client } ConnectionEnd;
enum { null, rc4, rc2, des, 3des, des40, idea, aes } BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { null, md5, sha } MACAlgorithm;
enum { null(0), (255) } CompressionMethod;
/* The algorithms specified in CompressionMethod,
BulkCipherAlgorithm, and MACAlgorithm may be added to. */
struct {
ConnectionEnd entity;
BulkCipherAlgorithm bulk_cipher_algorithm;
CipherType cipher_type;
uint8 key_size;
uint8 key_material_length;
MACAlgorithm mac_algorithm;
uint8 hash_size;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
The record layer will use the security parameters to generate the
following four items:
client write MAC secret
server write MAC secret
client write key
server write key
The client write parameters are used by the server when receiving and
processing records and vice-versa. The algorithm used for generating
these items from the security parameters is described in section 6.3.
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states MUST be updated for each
record processed. Each connection state includes the following
Dierks & Rescorla Standards Track [Page 19]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 the necessary state information
is 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 which is transmitted under
a particular connection state MUST use sequence number 0.
6.2. Record layer
The TLS Record Layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
6.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Client message
boundaries are not preserved in the record layer (i.e., multiple
client messages of the same ContentType MAY be coalesced into a
single TLSPlaintext record, or a single message MAY be fragmented
across several records).
struct {
uint8 major, minor;
} ProtocolVersion;
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
Dierks & Rescorla Standards Track [Page 20]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 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 higher precedence
for transmission than other content types and therefore handshake
records may be held if application data is pending. However,
records MUST be delivered to the network in the same order as
they are protected by the record layer. Recipients MUST receive
and process interleaved application layer traffic during
handshakes subsequent to the first one on a connection.
6.2.2. Record compression and decompression
All records are compressed using the compression algorithm defined in
the current session state. There is always an active compression
algorithm; however, initially it is defined as
CompressionMethod.null. The compression algorithm translates a
TLSPlaintext structure into a TLSCompressed structure. Compression
functions are initialized with default state information whenever a
connection state is made active.
Dierks & Rescorla Standards Track [Page 21]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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.
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.
Dierks & Rescorla Standards Track [Page 22]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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.
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.
Dierks & Rescorla Standards Track [Page 23]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 which the cipher XORs with the
plaintext prior to encryption of the first cipher block
of the record.
In prior versions of TLS, there was no IV field and the CBC residue
and mask were one and the same. See Sections 6.1, 6.2.3.2 and 6.3,
of [TLS1.0] for details of TLS 1.0 IV handling.
One of the following two algorithms SHOULD be used to generate the
per-record IV:
(1) Generate a cryptographically strong random string R of
length CipherSpec.block_length. Place R
in the IV field. Set the mask to R. Thus, the first
cipher block will be encrypted as E(R XOR Data).
(2) Generate a cryptographically strong random number R of
length CipherSpec.block_length and prepend it to the plaintext
prior to encryption. In
this case either:
(a) The cipher may use a fixed mask such as zero.
(b) The CBC residue from the previous record may be used
as the mask. This preserves maximum code compatibility
with TLS 1.0 and SSL 3. It also has the advantage that
it does not require the ability to quickly reset the IV,
which is known to be a problem on some systems.
In either 2(a) or 2(b) the data (R || data) is fed into the
Dierks & Rescorla Standards Track [Page 24]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 equivalently cryptographically strong
to 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 long, 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 based on analysis of
the lengths of exchanged messages. Each uint8 in the padding data
vector MUST be filled with the padding length value. The receiver
MUST check this padding and SHOULD use the bad_record_mac alert
to indicate padding errors.
padding_length
The padding length MUST be such that the total size of the
GenericBlockCipher structure is a multiple of the cipher's block
length. Legal values range from zero to 255, inclusive. This
length specifies the length of the padding field exclusive of the
padding_length field itself.
The encrypted data length (TLSCiphertext.length) is one more than the
sum of 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, 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
Dierks & Rescorla Standards Track [Page 25]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 to do this is to compute the MAC even if
the padding is incorrect, and only then reject the packet. For
instance, if the pad appears to be incorrect the implementation
might assume a zero-length pad and then compute the MAC. This
leaves a small timing channel, since MAC performance depends to
some extent on the size of the data fragment, but it is not
believed to be large enough to be exploitable due to the large
block size of existing MACs and the small size of the timing
signal.
6.3. 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, which are generated from the master secret in
that order. Unused values are empty.
When generating keys and MAC secrets, 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 +
Dierks & Rescorla Standards Track [Page 26]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 which requires the most material is
AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte
keys and 2 x 20 byte MAC secrets, for a total 104 bytes of key
material.
7. The TLS Handshaking Protocols
TLS has three subprotocols which are used to allow peers to agree
upon security parameters for the record layer, authenticate
themselves, instantiate negotiated security parameters, and
report error conditions to each other.
The Handshake Protocol is responsible for negotiating a session,
which consists of the following items:
session identifier
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
peer certificate
X509v3 [X509] certificate of the peer. This element of the
state may be null.
compression method
The algorithm used to compress data prior to encryption.
cipher spec
Specifies the bulk data encryption algorithm (such as null,
DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also
defines cryptographic attributes such as the hash_size. (See
Appendix A.6 for formal definition)
master secret
48-byte secret shared between the client and server.
Dierks & Rescorla Standards Track [Page 27]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 server
to notify the receiving party that subsequent records will be
protected under the newly negotiated CipherSpec and keys. Reception
of this message causes the receiver to instruct the Record Layer to
immediately copy the read pending state into the read current state.
Immediately after sending this message, the sender MUST instruct the
record layer to make the write pending state the write active state.
(See section 6.1.) The change cipher spec message is sent during the
handshake after the security parameters have been agreed upon, but
before the verifying finished message is sent (see section 7.4.9).
Note: if a rehandshake occurs while data is flowing on a connection,
the communicating parties may continue to send data using the old
CipherSpec. However, once the ChangeCipherSpec has been sent, the new
CipherSpec MUST be used. The first side to send the ChangeCipherSpec
does not know that the other side has finished computing the new
keying material (e.g. if it has to perform a time consuming public
key operation). Thus, a small window of time during which the
recipient must buffer the data MAY exist. In practice, with modern
machines this interval is likely to be fairly short.
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
Dierks & Rescorla Standards Track [Page 28]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
session identifier MUST be invalidated, preventing the failed session
from being used to establish new connections. Like other messages,
alert messages are encrypted and compressed, as specified by the
current connection state.
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED (41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
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
Dierks & Rescorla Standards Track [Page 29]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write side
of the connection. The other party MUST respond with a close_notify
alert of its own and close down the connection immediately,
discarding any pending writes. It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.
If the application protocol using TLS provides that any data may be
carried over the underlying transport after the TLS connection is
closed, the TLS implementation must receive the responding
close_notify alert before indicating to the application layer that
the TLS connection has ended. If the application protocol will not
transfer any additional data, but will only close the underlying
transport connection, then the implementation MAY choose to close the
transport without waiting for the responding close_notify. No part of
this standard should be taken to dictate the manner in which a usage
profile for TLS manages its data transport, including when
connections are opened or closed.
Note: It is assumed that closing a connection reliably delivers
pending data before destroying the transport.
7.2.2. Error alerts
Error handling in the TLS Handshake protocol is very simple. When an
error is detected, the detecting party sends a message to the other
party. Upon transmission or receipt of an 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
Dierks & Rescorla Standards Track [Page 30]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 which had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal.
decompression_failure
The decompression function received improper input (e.g. data
that would expand to excessive length). This message is always
fatal.
handshake_failure
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.
no_certificate_RESERVED
This alert was used in SSLv3 but not in TLS. It should not be
sent by compliant implementations.
bad_certificate
A certificate was corrupt, contained signatures that did not
verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
Dierks & Rescorla Standards Track [Page 31]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
certificate_unknown
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was out of range or inconsistent with
other fields. This is always fatal.
unknown_ca
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn't be matched with a known, trusted CA. This
message is always fatal.
access_denied
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation.
This message is always fatal.
decode_error
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal.
decrypt_error
A handshake cryptographic operation failed, including being
unable to correctly verify a signature, decrypt a key exchange,
or validate a finished message.
export_restriction_RESERVED
This alert was used in TLS 1.0 but not TLS 1.1.
protocol_version
The protocol version the client has attempted to negotiate is
recognized, but not supported. (For example, old protocol
versions might be avoided for security reasons). This message is
always fatal.
insufficient_security
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.
internal_error
An internal error unrelated to the peer or the correctness of the
protocol makes it impossible to continue (such as a memory
allocation failure). This message is always fatal.
Dierks & Rescorla Standards Track [Page 32]draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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 would be where a server has spawned a process to
satisfy a request; the process might receive security parameters
(key length, authentication, etc.) at startup and it might be
difficult to communicate changes to these parameters after that
point. This message is always a warning.
For all errors where an alert level is not explicitly specified, the
sending party MAY determine at its discretion whether this is a fatal
error or not; if an alert with a level of warning is received, the
receiving party MAY decide at its discretion whether to treat this as
a fatal error or not. However, all messages which are transmitted
with a level of fatal MUST be treated as fatal messages.
New alerts 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
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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 TLS always
negotiating the strongest possible connection between two peers:
there are a number of ways a man in the middle attacker can attempt
to make two entities drop down to the least secure method they
support. The protocol has been designed to minimize this risk, but
there are still attacks available: for example, an attacker could
block access to the port a secure service runs on, or attempt to get
the peers to negotiate an unauthenticated connection. The fundamental
rule is that higher levels must be cognizant of what their security
requirements are and never transmit information over a channel less
secure than what they require. The TLS protocol is secure, in that
any cipher suite offers its promised level of security: if you
negotiate 3DES with a 1024 bit RSA key exchange with a host whose
certificate you have verified, you can expect to be that secure.
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However, you SHOULD never send data over a link encrypted with 40 bit
security unless you feel 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 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 which 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 their server has no
certificate, or if its certificate is for signing only). If the
server is authenticated, it may request a certificate from the
client, if that is appropriate to the cipher suite selected. Now the
server will send the server hello done message, indicating that the
hello-message phase of the handshake is complete. The server will
then wait for a client response. If the server has sent a certificate
request message, the client must send the certificate message. The
client key exchange message is now sent, and the content of that
message will depend on the public key algorithm selected between the
client hello and the server hello. If the client has sent a
certificate with signing ability, a digitally-signed certificate
verify message is sent to explicitly verify the certificate.
At this point, a change cipher spec message is sent by the client,
and the client copies the pending Cipher Spec into the current Cipher
Spec. The client then immediately sends the finished message under
the new algorithms, keys, and secrets. In response, the server will
send its own change cipher spec message, transfer the pending to the
current Cipher Spec, and send its finished message under the new
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Cipher Spec. At this point, the handshake is complete and the client
and server may begin to exchange application layer data. (See flow
chart below.) Application data MUST NOT be sent prior to the
completion of the first handshake (before a cipher suite other
TLS_NULL_WITH_NULL_NULL is established).
Client Server
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Fig. 1 - Message flow for a full handshake
* Indicates optional or situation-dependent messages that are not
always sent.
Note: To help avoid pipeline stalls, ChangeCipherSpec is an
independent TLS Protocol content type, and is not actually a TLS
handshake message.
When the client and server decide to resume a previous session or
duplicate an existing session (instead of negotiating new security
parameters) the message flow is as follows:
The client sends a ClientHello using the Session ID of the session to
be resumed. The server then checks its session cache for a match. If
a match is found, and the server is willing to re-establish the
connection under the specified session state, it will send a
ServerHello with the same Session ID value. At this point, both
client and server MUST send change cipher spec messages and proceed
directly to finished messages. Once the re-establishment is complete,
the client and server MAY begin to exchange application layer data.
(See flow chart below.) If a Session ID match is not found, the
server generates a new session ID and the TLS client and server
perform a full handshake.
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Client Server
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Fig. 2 - Message flow for an abbreviated handshake
The contents and significance of each message will be presented in
detail in the following sections.
7.4. Handshake protocol
The TLS Handshake Protocol is one of the defined higher level clients
of the TLS Record Protocol. This protocol is used to negotiate the
secure attributes of a session. Handshake messages are supplied to
the TLS Record Layer, where they are encapsulated within one or more
TLSPlaintext structures, which are processed and transmitted as
specified by the current active session state.
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
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The handshake protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in a fatal error. Unneeded handshake messages can be omitted,
however. Note one exception to the ordering: the Certificate message
is used twice in the handshake (from server to client, then from
client to server), but described only in its first position. The one
message which 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
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;
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