Transport Layer Security Working Group Tim Dierks
INTERNET-DRAFT Certicom
Expires May 12, 1999 Christopher Allen
Certicom
November 12, 1998
The TLS Protocol
Version 1.0
<draft-ietf-tls-protocol-06.txt>
Status of this memo
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Rim).
Abstract
This document specifies Version 1.0 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications privacy
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.
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Table of Contents
Status of this memo 1
Abstract 1
Table of Contents 2
1. Introduction 3
2. Goals 4
3. Goals of this document 5
4. Presentation language 5
4.1. Basic block size 6
4.2. Miscellaneous 6
4.3. Vectors 6
4.4. Numbers 7
4.5. Enumerateds 7
4.6. Constructed types 8
4.6.1. Variants 8
4.7. Cryptographic attributes 9
4.8. Constants 11
5. HMAC and the pseudorandom function 11
6. The TLS Record Protocol 13
6.1. Connection states 13
6.2. Record layer 16
6.2.1. Fragmentation 16
6.2.2. Record compression and decompression 17
6.2.3. Record payload protection 17
6.2.3.1. Null or standard stream cipher 18
6.2.3.2. CBC block cipher 18
6.3. Key calculation 20
6.3.1. Export key generation example 21
7. The TLS Handshake Protocol 22
7.1. Change cipher spec protocol 22
7.2. Alert protocol 23
7.2.1. Closure alerts 24
7.2.2. Error alerts 24
7.3. Handshake Protocol overview 27
7.4. Handshake protocol 30
7.4.1. Hello messages 31
7.4.1.1. Hello request 31
7.4.1.2. Client hello 31
7.4.1.3. Server hello 34
7.4.2. Server certificate 35
7.4.3. Server key exchange message 36
7.4.4. Certificate request 39
7.4.5. Server hello done 39
7.4.6. Client certificate 40
7.4.7. Client key exchange message 40
7.4.7.1. RSA encrypted premaster secret message 41
7.4.7.2. Client Diffie-Hellman public value 42
7.4.8. Certificate verify 42
7.4.9. Finished 43
8. Cryptographic computations 44
8.1. Computing the master secret 44
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8.1.1. RSA 44
8.1.2. Diffie-Hellman 45
9. Mandatory Cipher Suites 45
10. Application data protocol 45
A. Protocol constant values 45
A.1. Record layer 45
A.2. Change cipher specs message 46
A.3. Alert messages 46
A.4. Handshake protocol 47
A.4.1. Hello messages 48
A.4.2. Server authentication and key exchange messages 48
A.4.3. Client authentication and key exchange messages 49
A.4.4. Handshake finalization message 50
A.5. The CipherSuite 50
A.6. The Security Parameters 52
B. Glossary 52
C. CipherSuite definitions 56
D. Implementation Notes 58
D.1. Temporary RSA keys 58
D.2. Random Number Generation and Seeding 59
D.3. Certificates and authentication 59
D.4. CipherSuites 59
E. Backward Compatibility With SSL 60
E.1. Version 2 client hello 61
E.2. Avoiding man-in-the-middle version rollback 62
F. Security analysis 62
F.1. Handshake protocol 63
F.1.1. Authentication and key exchange 63
F.1.1.1. Anonymous key exchange 63
F.1.1.2. RSA key exchange and authentication 64
F.1.1.3. Diffie-Hellman key exchange with authentication 64
F.1.2. Version rollback attacks 65
F.1.3. Detecting attacks against the handshake protocol 65
F.1.4. Resuming sessions 65
F.1.5. MD5 and SHA 66
F.2. Protecting application data 66
F.3. Final notes 66
G. Patent Statement 67
References 68
Credits 71
Comments 72
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:
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- The connection is private. Symmetric cryptography is used for
data encryption (e.g., DES[DES], RC4[RC4], etc.) The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption.
- The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA, MD5, etc.) are used for MAC computations. The Record
Protocol can operate without a MAC, but is generally only used
in this mode while another protocol is using the Record Protocol
as a transport for negotiating security parameters.
The TLS Record Protocol is used for encapsulation of various higher
level protocols. One such encapsulated protocol, the TLS Handshake
Protocol, allows the server and client to authenticate each other
and to negotiate an encryption algorithm and cryptographic keys
before the application protocol transmits or receives its first byte
of data. The TLS Handshake Protocol provides connection security
that has three basic properties:
- The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.). This
authentication can be made optional, but is generally required
for at least one of the peers.
- The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any
authenticated connection the secret cannot be obtained, even by
an attacker who can place himself in the middle of the
connection.
- The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties
to the communication.
One advantage of TLS is that it is application protocol independent.
Higher level protocols can layer on top of the TLS Protocol
transparently. The TLS standard, however, does not specify how
protocols add security with TLS; the decisions on how to initiate
TLS handshaking and how to interpret the authentication certificates
exchanged are left up to the judgment of the designers and
implementors of protocols which run on top of TLS.
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.
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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 1.0 and SSL 3.0 do not interoperate
(although TLS 1.0 does incorporate a mechanism by which a TLS
implementation can back down to SSL 3.0). This document 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 */
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
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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.
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.
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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
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.
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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
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
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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 [PKCS1].
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
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 [PKCS1].
In the following example:
stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque hash[20];
} UserType;
The contents of hash are used as input for the signing algorithm,
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.
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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].
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
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) + ...
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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);
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).
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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.2). 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 take 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, encryption
algorithm, and MAC algorithm. In addition, the parameters for these
algorithms are known: the MAC secret and the bulk encryption keys
and IVs 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 Handshake Protocol 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.
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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), and whether it is considered an
"export" cipher.
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.
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 } BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { true, false } IsExportable;
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;
IsExportable is_exportable;
MACAlgorithm mac_algorithm;
uint8 hash_size;
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CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
The record layer will use the security parameters to generate the
following six items:
client write MAC secret
server write MAC secret
client write key
server write key
client write IV (for block ciphers only)
server write IV (for block ciphers only)
The client write parameters are used by the server when receiving
and processing records and vice-versa. The algorithm used for
generating these items from the security parameters is described in
section 6.3.
Once the security parameters have been set and the keys have been
generated, the connection states can be instantiated by making them
the current states. These current states must be updated for each
record processed. Each connection state includes the following
elements:
compression state
The current state of the compression algorithm.
cipher state
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. In addition, for block
ciphers running in CBC mode (the only mode specified for TLS),
this will initially contain the IV for that connection state and
be updated to contain the ciphertext of the last block encrypted
or decrypted as records are processed. For stream ciphers, this
will 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. A sequence number is incremented after each
record: specifically, the first record which is transmitted
under a particular connection state should use sequence number
0.
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6.2. Record layer
The TLS Record Layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
6.2.1. Fragmentation
The record layer fragments information blocks into TLSPlaintext
records carrying data in chunks of 2^14 bytes or less. Client
message boundaries are not preserved in the record layer (i.e.,
multiple client messages of the same ContentType may be coalesced
into a single TLSPlaintext record, or a single message may be
fragmented across several records).
struct {
uint8 major, minor;
} ProtocolVersion;
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
The higher level protocol used to process the enclosed fragment.
version
The version of the protocol being employed. This document
describes TLS Version 1.0, which uses the version { 3, 1 }. The
version value 3.1 is historical: TLS version 1.0 is 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 lower precedence
for transmission than other content types.
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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.
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
} TLSCiphertext;
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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.
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 or DES), the encryption and MAC
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functions convert TLSCompressed.fragment structures to and from
block TLSCiphertext.fragment structures.
block-ciphered struct {
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.
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.
padding_length
The padding length should 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. 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) the
initialization vector (IV) for the first record is generated
with the other keys and secrets when the security parameters are
set. The IV for subsequent records is the last ciphertext block
from the previous record.
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6.3. Key calculation
The Record Protocol requires an algorithm to generate keys, IVs, 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, keys, and non-export IVs 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, a server write key, a client write IV, and a
server write IV, 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, and the random values provide unencrypted salt
material and IVs for exportable ciphers.
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]
server_write_key[SecurityParameters.key_material]
client_write_IV[SecurityParameters.IV_size]
server_write_IV[SecurityParameters.IV_size]
The client_write_IV and server_write_IV are only generated for
non-export block ciphers. For exportable block ciphers, the
initialization vectors are generated later, as described below. Any
extra key_block material is discarded.
Implementation note:
The cipher spec which is defined in this document which requires
the most material is 3DES_EDE_CBC_SHA: it requires 2 x 24 byte
keys, 2 x 20 byte MAC secrets, and 2 x 8 byte IVs, for a total
of 104 bytes of key material.
Exportable encryption algorithms (for which CipherSpec.is_exportable
is true) require additional processing as follows to derive their
final write keys:
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final_client_write_key =
PRF(SecurityParameters.client_write_key,
"client write key",
SecurityParameters.client_random +
SecurityParameters.server_random);
final_server_write_key =
PRF(SecurityParameters.server_write_key,
"server write key",
SecurityParameters.client_random +
SecurityParameters.server_random);
Exportable encryption algorithms derive their IVs solely from the
random values from the hello messages:
iv_block = PRF("", "IV block", SecurityParameters.client_random
+
SecurityParameters.server_random);
The iv_block is partitioned into two initialization vectors as the
key_block was above:
client_write_IV[SecurityParameters.IV_size]
server_write_IV[SecurityParameters.IV_size]
Note that the PRF is used without a secret in this case: this just
means that the secret has a length of zero bytes and contributes
nothing to the hashing in the PRF.
6.3.1. Export key generation example
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for
each of the two encryption keys and 16 bytes for each of the MAC
keys, for a total of 42 bytes of key material. The PRF output is
stored in the key_block. The key_block is partitioned, and the write
keys are salted because this is an exportable encryption algorithm.
key_block = PRF(master_secret,
"key expansion",
server_random +
client_random)[0..41]
client_write_MAC_secret = key_block[0..15]
server_write_MAC_secret = key_block[16..31]
client_write_key = key_block[32..36]
server_write_key = key_block[37..41]
final_client_write_key = PRF(client_write_key,
"client write key",
client_random +
server_random)[0..15]
final_server_write_key = PRF(server_write_key,
"server write key",
client_random +
server_random)[0..15]
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iv_block = PRF("", "IV block", client_random +
server_random)[0..15]
client_write_IV = iv_block[0..7]
server_write_IV = iv_block[8..15]
7. The TLS Handshake Protocol
The TLS Handshake Protocol consists of a suite of three
sub-protocols 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.
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.
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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 should 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).
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),
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(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
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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. The session becomes
unresumable if any connection is terminated without proper
close_notify messages with level equal to warning.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Each party is required to send a close_notify alert before closing
the write side of the connection. It is required that the other
party 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.
NB: 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
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parties immediately close the connection. Servers and clients are
required to forget any session-identifiers, keys, and secrets
associated with a failed connection. The following error alerts are
defined:
unexpected_message
An inappropriate message was received. This alert is always
fatal and should never be observed in communication between
proper implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect
MAC. This message is always fatal.
decryption_failed
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.
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.
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.
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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
A negotiation not in compliance with export restrictions was
detected; for example, attempting to transfer a 1024 bit
ephemeral RSA key for the RSA_EXPORT handshake method. This
message is always fatal.
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.
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
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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.
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.
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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. 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 should 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
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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 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.)
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
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found, the server generates a new session ID and the TLS client and
server perform a full handshake.
Client Server
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Fig. 2 - Message flow for an abbreviated handshake
The contents and significance of each message will be presented in
detail in the following sections.
7.4. Handshake protocol
The TLS Handshake Protocol is one of the defined higher level
clients of the TLS Record Protocol. This protocol is used to
negotiate the secure attributes of a session. Handshake messages are
supplied to the TLS Record Layer, where they are encapsulated within
one or more TLSPlaintext structures, which are processed and
transmitted as specified by the current active session state.
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
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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
in 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.
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;
Note: This message should never be included in the message hashes
which 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
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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) 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.
The client hello message includes a variable length session
identifier. If not empty, the value identifies a session between the
same client and server whose security parameters the client wishes
to reuse. The session identifier may be from an earlier connection,
this connection, or 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, while 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 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.)
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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.
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 3.1 (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 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
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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 for a
client hello message to include extra data after the compression
methods. This data must be included 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.
7.4.1.3. Server hello
When this message will be sent:
The server will send this message in response to a client hello
message when it was able to find an acceptable set of
algorithms. If it cannot find such a match, it will respond with
a handshake failure alert.
Structure of this message:
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} 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
3.1 (See Appendix E for details about backward compatibility).
random
This structure is generated by the server and must be different
from (and independent of) ClientHello.random.
session_id
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
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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 which 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.
Key Exchange Algorithm Certificate Key Type
RSA RSA public key; the certificate must
allow the key to be used for encryption.
RSA_EXPORT RSA public key of length greater than
512 bits which can be used for signing,
or a key of 512 bits or shorter which
can be used for either encryption or
signing.
DHE_DSS DSS public key.
DHE_DSS_EXPORT DSS public key.
DHE_RSA RSA public key which can be used for
signing.
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DHE_RSA_EXPORT RSA public key which can be used for
signing.
DH_DSS Diffie-Hellman key. The algorithm used
to sign the certificate should be DSS.
DH_RSA Diffie-Hellman key. The algorithm used
to sign the certificate should be RSA.
All certificate profiles, 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 which 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 which specifies the
root certificate authority may optionally be omitted from the
chain, under the assumption that the remote end must already
possess it in order to validate it in any case.
The same message type and structure will be used for the client's
response to a certificate request message. Note that a client may
send no certificates if it does not have an appropriate certificate
to send in response to the server's authentication request.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate
vector because PKCS #6 [PKCS6] extended certificates are not
used. Also PKCS #7 defines a SET rather than a SEQUENCE, making
the task of parsing the list more difficult.
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
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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:
RSA_EXPORT (if the public key in the server certificate is
longer than 512 bits)
DHE_DSS
DHE_DSS_EXPORT
DHE_RSA
DHE_RSA_EXPORT
DH_anon
It is not legal to send the server key exchange message for the
following key exchange methods:
RSA
RSA_EXPORT (when the public key in the server certificate is
less than or equal to 512 bits in length)
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 to encrypt the premaster secret with, 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 which 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.
Note: According to current US export law, RSA moduli larger than 512
bits may not be used for key exchange in software exported from
the US. With this message, the larger RSA keys encoded in
certificates may be used to sign temporary shorter RSA keys for
the RSA_EXPORT key exchange method.
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;
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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;
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;
select (SignatureAlgorithm)
{ case anonymous: struct { };
case rsa:
digitally-signed struct {
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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 appropriate for the selected cipher suite. This
message, if sent, will immediately follow the Server Key
Exchange message (if it is sent; otherwise, the Server
Certificate message).
Structure of this message:
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..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 both to describe known roots
and a desired authorization space.
Note: DistinguishedName is derived from [X509].
Note: It is a fatal handshake_failure alert for an anonymous server to
request client identification.
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
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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. 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.
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 will immediately
follow the client certificate message, if it is sent. Otherwise
it will 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 which 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 which contained a Diffie-Hellman public key whose
parameters (group and generator) matched those specified by the
server in its certificate, this message will not contain any
data.
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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 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, 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;
Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used
to attack a TLS server which is using PKCS#1 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 formatted
or not.
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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 it receives an
incorrectly formatted RSA block, a server 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.
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.
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, 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 will 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 will immediately follow the client key exchange
message.
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Structure of this message:
struct {
Signature signature;
} CertificateVerify;
The Signature type is defined in 6.4.3.
CertificateVerify.signature.md5_hash
MD5(handshake_messages);
Certificate.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];
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 handshake messages up to but not
including this message. This is only data visible at the
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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 hash contained in finished messages sent by the server
incorporate Sender.server; those sent by the client incorporate
Sender.client. 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 which is sent
second will include the prior one.
Note: Change cipher spec messages, alerts and any other record types
are not handshake messages and are not included in the hash
computations. Also, Hello Request messages are omitted from
handshake hashes.
8. Cryptographic computations
In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret,
and the client and server random values. The authentication,
encryption, and MAC algorithms are determined by the cipher_suite
selected by the server and revealed in the server hello message. The
compression algorithm is negotiated in the hello messages, and the
random values are exchanged in the hello messages. All that remains
is to calculate the master secret.
8.1. Computing the master secret
For all key exchange methods, the same algorithm is used to convert
the pre_master_secret into the master_secret. The pre_master_secret
should be deleted from memory once the master_secret has been
computed.
master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random + ServerHello.random)
[0..47];
The master secret is always exactly 48 bytes in length. The length
of the premaster secret will vary depending on key exchange method.
8.1.1. RSA
When RSA is used for server authentication and key exchange, a
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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.
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_DHE_DSS_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.
A. Protocol constant values
This section describes protocol types and constants.
A.1. Record layer
struct {
uint8 major, minor;
} ProtocolVersion;
ProtocolVersion version = { 3, 1 }; /* TLS v1.0 */
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
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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 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),
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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(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
A.4. Handshake protocol
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type;
uint24 length;
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
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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>;
} 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<1..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;
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struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
};
} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;
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),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
A.4.3. Client authentication and key exchange messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: DiffieHellmanClientPublicValue;
} exchange_keys;
} ClientKeyExchange;
struct {
ProtocolVersion client_version;
opaque random[46];
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} 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 1.0.
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.
CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
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CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
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, which has been signed by the CA. The signing algorithm
used is specified after the DH or DHE parameter. The server can
request an RSA or DSS signature-capable 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_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
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_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
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_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
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_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
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_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
Note: All cipher suites whose first byte is 0xFF are considered
private and can be used for defining local/experimental
algorithms. Interoperability of such types is a local matter.
Note: Additional cipher suites can be registered by publishing an RFC
which specifies the cipher suites, including the necessary TLS
protocol information, including message encoding, premaster
secret derivation, symmetric encryption and MAC calculation and
appropriate reference information for the algorithms involved.
The RFC editor's office may, at its discretion, choose to
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publish specifications for cipher suites which are not
completely described (e.g., for classified algorithms) if it
finds the specification to be of technical interest and
completely specified.
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, idea }
BulkCipherAlgorithm;
enum { stream, block } CipherType;
enum { true, false } IsExportable;
enum { null, md5, sha } MACAlgorithm;
/* 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;
IsExportable is_exportable;
MACAlgorithm mac_algorithm;
uint8 hash_size;
CompressionMethod compression_algorithm;
opaque master_secret[48];
opaque client_random[32];
opaque server_random[32];
} SecurityParameters;
B. Glossary
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.
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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).
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
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that in TLS, for key generation purposes, DES is treated as
having an 8 byte key length (64 bits), but it still only
provides 56 bits of protection. (The low bit of each key byte is
presumed to be set to produce odd parity in that key byte.) DES
can also be operated in a mode where three independent keys and
three encryptions are used for each block of data; this uses 168
bits of key (24 bytes in the TLS key generation method) and
provides the equivalent of 112 bits of security. [DES], [3DES]
Digital Signature Standard (DSS)
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186, "Digital Signature
Standard," published May, 1994 by the U.S. Dept. of Commerce.
[DSS]
digital signatures
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.
handshake
An initial negotiation between client and server that
establishes the parameters of their transactions.
Initialization Vector (IV)
When a block cipher is used in CBC mode, the initialization
vector is exclusive-ORed with the first plaintext block prior to
encryption.
IDEA
A 64-bit block cipher designed by Xuejia Lai and James Massey.
[IDEA]
Message Authentication Code (MAC)
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.
master secret
Secure secret data used for generating encryption keys, MAC
secrets, and IVs.
MD5
MD5 is a secure hashing function that converts an arbitrarily
long data stream into a digest 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
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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].
RC4
A stream cipher licensed by RSA Data Security [RSADSI]. A
compatible cipher is described in [RC4].
RSA
A very widely used public-key algorithm that can be used for
either encryption or digital signing. [RSA]
salt
Non-secret random data used to make export encryption keys
resist precomputation attacks.
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, which 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 180-1. It
produces a 20-byte output. Note that all references to SHA
actually use the modified SHA-1 algorithm. [SHA]
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SSL
Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on
SSL Version 3.0
stream cipher
An encryption algorithm that converts a key into a
cryptographically-strong keystream, which is then exclusive-ORed
with the plaintext.
symmetric cipher
See bulk cipher.
Transport Layer Security (TLS)
This protocol; also, the Transport Layer Security working group
of the Internet Engineering Task Force (IETF). See "Comments" at
the end of this document.
C. CipherSuite definitions
CipherSuite Is Key Cipher Hash
Exportable Exchange
TLS_NULL_WITH_NULL_NULL * NULL NULL NULL
TLS_RSA_WITH_NULL_MD5 * RSA NULL MD5
TLS_RSA_WITH_NULL_SHA * RSA NULL SHA
TLS_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 MD5
TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5
TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5
TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_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_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_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_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5
TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA
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
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* Indicates IsExportable is True
Key
Exchange
Algorithm Description Key size limit
DHE_DSS Ephemeral DH with DSS signatures None
DHE_DSS_EXPORT Ephemeral DH with DSS signatures DH = 512 bits
DHE_RSA Ephemeral DH with RSA signatures None
DHE_RSA_EXPORT Ephemeral DH with RSA signatures DH = 512 bits,
RSA = none
DH_anon Anonymous DH, no signatures None
DH_anon_EXPORT Anonymous DH, no signatures DH = 512 bits
DH_DSS DH with DSS-based certificates None
DH_DSS_EXPORT DH with DSS-based certificates DH = 512 bits
DH_RSA DH with RSA-based certificates None
DH_RSA_EXPORT DH with RSA-based certificates DH = 512 bits,
RSA = none
NULL No key exchange N/A
RSA RSA key exchange None
RSA_EXPORT RSA key exchange RSA = 512 bits
Key size limit
The key size limit gives the size of the largest public key that
can be legally used for encryption in cipher suites that are
exportable.
Key Expanded Effective IV Block
Cipher Type Material Key Material Key Bits Size Size
NULL * Stream 0 0 0 0 N/A
IDEA_CBC Block 16 16 128 8 8
RC2_CBC_40 * Block 5 16 40 8 8
RC4_40 * Stream 5 16 40 0 N/A
RC4_128 Stream 16 16 128 0 N/A
DES40_CBC * Block 5 8 40 8 8
DES_CBC Block 8 8 56 8 8
3DES_EDE_CBC Block 24 24 168 8 8
* Indicates IsExportable is true.
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
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Effective Key Bits
How much entropy material is in the key material being fed into
the encryption routines.
IV Size
How much data needs 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
D. Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This
section provides several recommendations to assist implementors.
D.1. Temporary RSA keys
US Export restrictions limit RSA keys used for encryption to 512
bits, but do not place any limit on lengths of RSA keys used for
signing operations. Certificates often need to be larger than 512
bits, since 512-bit RSA keys are not secure enough for high-value
transactions or for applications requiring long-term security. Some
certificates are also designated signing-only, in which case they
cannot be used for key exchange.
When the public key in the certificate cannot be used for
encryption, the server signs a temporary RSA key, which is then
exchanged. In exportable applications, the temporary RSA key should
be the maximum allowable length (i.e., 512 bits). Because 512-bit
RSA keys are relatively insecure, they should be changed often. For
typical electronic commerce applications, it is suggested that keys
be changed daily or every 500 transactions, and more often if
possible. Note that while it is acceptable to use the same temporary
key for multiple transactions, it must be signed each time it is
used.
RSA key generation is a time-consuming process. In many cases, a
low-priority process can be assigned the task of key generation.
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Whenever a new key is completed, the existing temporary key can be
replaced with the new one.
D.2. 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. To seed a 128-bit PRNG, one
would thus require approximately 100 such timer values.
Warning: The seeding functions in RSAREF and versions of BSAFE prior to
3.0 are order-independent. For example, if 1000 seed bits are
supplied, one at a time, in 1000 separate calls to the seed
function, the PRNG will end up in a state which depends only
on the number of 0 or 1 seed bits in the seed data (i.e.,
there are 1001 possible final states). Applications using
BSAFE or RSAREF must take extra care to ensure proper seeding.
This may be accomplished by accumulating seed bits into a
buffer and processing them all at once or by processing an
incrementing counter with every seed bit; either method will
reintroduce order dependence into the seeding process.
D.3. 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.4. CipherSuites
TLS supports a range of key sizes and security levels, including
some which 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.
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E. Backward Compatibility With SSL
For historical reasons and in order to avoid a profligate
consumption of reserved port numbers, application protocols which
are secured by 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 version 1.0 and SSL 3.0 are very similar; thus, supporting both
is easy. TLS clients who wish to negotiate with SSL 3.0 servers
should send client hello messages using the SSL 3.0 record format
and client hello structure, sending {3, 1} for the version field to
note that they support TLS 1.0. If the server supports only SSL 3.0,
it will respond with an SSL 3.0 server hello; if it supports TLS,
with a TLS server hello. The negotiation then proceeds as
appropriate for the negotiated protocol.
Similarly, a TLS server which wishes to interoperate with SSL 3.0
clients should accept SSL 3.0 client hello messages and respond with
an SSL 3.0 server hello if an SSL 3.0 client hello is received which
has a version field of {3, 0}, denoting that this client does not
support TLS.
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 1.0 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.
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 };
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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 };
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.
uint8 V2CipherSpec[3];
struct {
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];
Random challenge;
} V2ClientHello;
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 either zero or 16. If zero, the
client is creating a new session. If 16, the session_id field
will contain the 16 bytes of session identification.
challenge_length
The length in bytes of the client's challenge to the server to
authenticate itself. This value must be 32.
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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
If this field's length is not zero, it will contain the
identification for a session that the client wishes to resume.
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 should use a TLS client hello.
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
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.
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F.1. Handshake protocol
The handshake protocol is responsible for selecting a CipherSpec and
generating a Master Secret, which together comprise the primary
cryptographic parameters associated with a secure session. The
handshake protocol can also optionally authenticate parties who have
certificates signed by a trusted certificate authority.
F.1.1. Authentication and key exchange
TLS supports three authentication modes: authentication of both
parties, server authentication with an unauthenticated client, and
total anonymity. Whenever the server is authenticated, the channel
is secure against man-in-the-middle attacks, but completely
anonymous sessions are inherently vulnerable to such attacks.
Anonymous servers cannot authenticate clients. If the server is
authenticated, its certificate message must provide a valid
certificate chain leading to an acceptable certificate authority.
Similarly, authenticated clients must supply an acceptable
certificate to the server. Each party is responsible for verifying
that the other's certificate is valid and has not expired or been
revoked.
The general goal of the key exchange process is to create a
pre_master_secret known to the communicating parties and not to
attackers. The pre_master_secret will be used to generate the
master_secret (see Section 8.1). The master_secret is required to
generate the certificate verify and 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 that 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
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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 may be either 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 or DSS 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
certificates but must comply with government-imposed size limits
on keys used for key exchange.
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
can 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.
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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.
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 be
using 40-bit encryption keys anyway. Altering the padding of the
least-significant 8 bytes of the PKCS padding does not impact
security for the size of the signed hashes and RSA key lengths used
in the protocol, since this is essentially equivalent to increasing
the input block size by 8 bytes.
F.1.3. Detecting attacks against the handshake protocol
An attacker might try to influence the handshake exchange to make
the parties select different encryption algorithms than they would
normally choose. Because many implementations will support 40-bit
exportable encryption and some may even support null encryption or
MAC algorithms, this attack is of particular concern.
For this attack, an attacker must actively change one or more
handshake messages. If this occurs, the client and server will
compute different values for the handshake message hashes. As a
result, the parties will not accept each others' finished messages.
Without the master_secret, the attacker cannot repair the finished
messages, so the attack will be discovered.
F.1.4. Resuming sessions
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not
been compromised and that the secure hash operations used to produce
the encryption keys and MAC secrets 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).
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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.
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. 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.
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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.
Appendix G
G. Patent Statement
Some of the cryptographic algorithms proposed for use in this
protocol have patent claims on them. In addition Netscape
Communications Corporation has a patent claim on the Secure Sockets
Layer (SSL) work that this standard is based on. The Internet
Standards Process as defined in RFC 1310 requires a written
statement from the Patent holder that a license will be made
available to applicants under reasonable terms and conditions prior
to approving a specification as a Proposed, Draft or Internet
Standard.
The Massachusetts Institute of Technology has granted RSA Data
Security, Inc., exclusive sub-licensing rights to the following
patent issued in the United States:
Cryptographic Communications System and Method ("RSA"), No.
4,405,829
Netscape Communications Corporation has been issued the following
patent in the United States:
Secure Socket Layer Application Program Apparatus And Method
("SSL"), No. 5,657,390
Netscape Communications has issued the following statement:
Intellectual Property Rights
Secure Sockets Layer
The United States Patent and Trademark Office ("the PTO")
recently issued U.S. Patent No. 5,657,390 ("the SSL Patent") to
Netscape for inventions described as Secure Sockets Layers
("SSL"). The IETF is currently considering adopting SSL as a
transport protocol with security features. Netscape encourages
the royalty-free adoption and use of the SSL protocol upon the
following terms and conditions:
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* If you already have a valid SSL Ref license today which
includes source code from Netscape, an additional patent
license under the SSL patent is not required.
* If you don't have an SSL Ref license, you may have a royalty
free license to build implementations covered by the SSL
Patent Claims or the IETF TLS specification provided that
you do not to assert any patent rights against Netscape or
other companies for the implementation of SSL or the IETF
TLS recommendation.
What are "Patent Claims":
Patent claims are claims in an issued foreign or domestic patent
that:
1) must be infringed in order to implement methods or build
products according to the IETF TLS specification; or
2) patent claims which require the elements of the SSL patent
claims and/or their equivalents to be infringed.
The Internet Society, Internet Architecture Board, Internet
Engineering Steering Group and the Corporation for National Research
Initiatives take no position on the validity or scope of the patents
and patent applications, nor on the appropriateness of the terms of
the assurance. The Internet Society and other groups mentioned above
have not made any determination as to any other intellectual
property rights which may apply to the practice of this standard.
Any further consideration of these matters is the user's own
responsibility.
References
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
[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.
[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards
Institute, 1983.
[DH1] W. Diffie and M. E. Hellman, "New Directions in Cryptography,"
IEEE Transactions on Information Theory, V. IT-22, n. 6, Jun 1977,
pp. 74-84.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard," National
Institute of Standards and Technology, U.S. Department of Commerce,
May 18, 1994.
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[FTP] J. Postel and J. Reynolds, RFC 959: File Transfer Protocol,
October 1985.
[HTTP] T. Berners-Lee, R. Fielding, H. Frystyk, Hypertext Transfer
Protocol -- HTTP/1.0, October, 1995.
[HMAC] H. Krawczyk, M. Bellare, and R. Canetti, RFC 2104, HMAC:
Keyed-Hashing for Message Authentication, 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.
[MD2] R. Rivest. RFC 1319: The MD2 Message Digest Algorithm. April
1992.
[MD5] R. Rivest. RFC 1321: The MD5 Message Digest Algorithm. April
1992.
[PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard,"
version 1.5, November 1993.
[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.
[PKIX] R. Housley, W. Ford, W. Polk, D. Solo, Internet Public Key
Infrastructure: Part I: X.509 Certificate and CRL Profile,
<draft-ietf-pkix-ipki-part1-06.txt>, October 1997.
[RC2] R. Rivest, A Description of the RC2(r) Encryption Algorithm
<draft-rivest-rc2desc-00.txt>
[RC4] R. Thayer and K. Kaukonen, A Stream Cipher Encryption
Algorithm, <draft-kaukonen-cipher-arcfour-01.txt>, July 1997.
[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.
[RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782 [SCH] B.
Schneier. Applied Cryptography: Protocols, Algorithms, and Source
Code in C, Published by John Wiley & Sons, Inc. 1994.
[SHA] NIST FIPS PUB 180-1, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of Commerce,
DRAFT, May 31, 1994.
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[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.
[TCP] ISI for DARPA, RFC 793: Transport Control Protocol, September
1981.
[TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993.
[X509] CCITT. Recommendation X.509: "The Directory - Authentication
Framework". 1988.
[XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data
Representation Standard, August 1995.
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Credits
Working Group Chair
Win Treese
Open Market
treese@openmarket.com
Editors
Christopher Allen Tim Dierks
Certicom Certicom
callen@certicom.com tdierks@certicom.com
Authors
Tim Dierks Philip L. Karlton
Certicom Netscape Communications
tdierks@certicom.com
Alan O. Freier Paul C. Kocher
Netscape Communications Independent Consultant
freier@netscape.com pck@netcom.com
Other contributors
Martin Abadi Robert Relyea
Digital Equipment Corporation Netscape Communications
ma@pa.dec.com relyea@netscape.com
Ran Canetti Jim Roskind
IBM Watson Research Center Netscape Communications
canetti@watson.ibm.com jar@netscape.com
Taher Elgamal Micheal J. Sabin, Ph. D.
Securify Consulting Engineer
taher@pacbell.net msabin@netcom.com
Anil R. Gangolli Dan Simon
Structured Arts Computing Corp. Microsoft
gangolli@structuredarts.com dansimon@microsoft.com
Kipp E.B. Hickman Tom Weinstein
Netscape Communications Netscape Communications
kipp@netscape.com tomw@netscape.com
Hugo Krawczyk
IBM Watson Research Center
hugo@watson.ibm.com
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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, T. Expires May 12, 1999 [Page 72]
