One document matched: draft-freier-ssl-version3-01.txt
Differences from draft-freier-ssl-version3-00.txt
Internet Draft March 1996 (Expires 9/96)
Alan O. Freier, Netscape Communications
Philip Karlton, Netscape Communications
Paul C. Kocher, Independent Consultant
The SSL Protocol
Version 3.0
<draft-freier-ssl-version3-01.txt>
Table of Contents
1. Status of this memo 3
2. Abstract 3
3. Introduction 3
4. Goals 4
5. Goals of this document 4
6. Presentation language 5
6.1 Basic block size 5
6.2 Miscellaneous 5
6.3 Vectors 5
6.4 Numbers 6
6.5 Enumerateds 7
6.6 Constructed types 7
6.6.1 Variants 8
6.7 Cryptographic attributes 9
6.8 Constants 9
7. SSL protocol 10
7.1 Session and connection states 10
7.2 Record layer 11
7.2.1 Fragmentation 11
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7.2.2 Record compression and decompression 12
7.2.3 Record payload protection and the CipherSpec 13
7.2.3.1 Null or standard stream cipher 14
7.2.3.2 CBC block cipher 15
7.3 Change cipher spec protocol 15
7.4 Alert protocol 16
7.4.1 Closure alerts 16
7.4.2 Error alerts 17
7.5 Handshake protocol overview 18
7.6 Handshake protocol 19
7.6.1 Hello messages 20
7.6.1.1 Hello request 20
7.6.1.2 Client hello 21
7.6.1.3 Server hello 23
7.6.2 Server certificate 24
7.6.3 Server key exchange message 25
7.6.4 Certificate request 26
7.6.5 Server hello done 27
7.6.6 Client certificate 27
7.6.7 Client key exchange message 27
7.6.7.1 RSA encrypted premaster secret message 28
7.6.7.2 Fortezza key exchange message 28
7.6.7.3 Client Diffie-Hellman public value 29
7.6.8 Certificate verify 30
7.6.9 Finished 30
7.7 Application data protocol 31
8. Cryptographic computations 31
8.1 Asymmetric cryptographic computations 31
8.1.1 RSA 32
8.1.2 Diffie-Hellman 32
8.1.3 Fortezza 32
8.2 Symmetric cryptographic calculations and the CipherSpec 32
8.2.1 The master secret 32
8.2.2 Converting the master secret into keys and MAC 33
8.2.2.1 Export key generation example 34
A. Protocol constant values 35
A.1 Reserved port assignments 35
A.1.1 Record layer 35
A.2 Change cipher specs message 36
A.3 Alert messages 36
A.4 Handshake protocol 36
A.4.1 Hello messages 37
A.4.2 Server authentication and key exchange messages 37
A.5 Client authentication and key exchange messages 39
A.5.1 Handshake finalization message 39
A.6 The CipherSuite 40
A.7 The CipherSpec 42
B. Glossary 42
C. CipherSuite definitions 45
D. Implementation Notes 48
D.1 Temporary RSA keys 48
D.2 Random Number Generation and Seeding 48
D.3 Certificates and authentication 49
D.4 CipherSuites 49
E. Version 2.0 Backward Compatibility 49
E.1 Version 2 client hello 50
E.2 Avoiding man-in-the-middle version rollback 51
F. Security analysis 52
F.1 Handshake protocol 52
F.1.1 Authentication and key exchange 52
F.1.1.1 Anonymous key exchange 53
F.1.1.2 RSA key exchange and authentication 53
F.1.1.3 Diffie-Hellman key exchange with authentication 54
F.1.1.4 Fortezza 54
F.1.2 Version rollback attacks 54
F.1.3 Detecting attacks against the handshake protocol 55
F.1.4 Resuming sessions 55
F.1.5 MD5 and SHA 56
F.2 Protecting application data 56
F.3 Final notes 56
G. Patent Statement 57
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1. Status of this memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or made obsolete by
other documents at any time. It is inappropriate to use
Internet-Drafts as reference material or to cite them other
than as work in progress.
To learn the current status of any Internet-Draft, please
check the 1id-abstracts.txt listing contained in the
Internet Drafts Shadow Directories on ds.internic.net (US
East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West
Coast), or munnari.oz.au (Pacific Rim).
2. Abstract
This document specifies Version 3.0 of the Secure Sockets
Layer (SSL V3.0) protocol, a security protocol that 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.
3. Introduction
The primary goal of the SSL Protocol is to provide privacy
and reliability between two communicating applications. The
protocol is composed of two layers. At the lowest level,
layered on top of some reliable transport protocol (e.g.,
TCP[TCP]), is the SSL Record Protocol. The SSL Record
Protocol is used for encapsulation of various higher level
protocols. One such encapsulated protocol, the SSL 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. One advantage of SSL is
that it is application protocol independent. A higher level
protocol can layer on top of the SSL Protocol transparently.
The SSL protocol provides connection security that has three
basic properties:
- The connection is private. Encryption is used after an
initial handshake to define a secret key. Symmetric
cryptography is used for data encryption (e.g.,
DES[DES], RC4[RC4], etc.)
- The peer's identity can be authenticated using
asymmetric, or public key, cryptography (e.g., RSA[RSA],
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SSL 3.0 March 1996
DSS[DSS], etc.).
- 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.
4. Goals
The goals of SSL Protocol v3.0, in order of their priority,
are:
1. Cryptographic security
SSL should be used to establish a secure
connection between two parties.
2. Interoperability
Independent programmers should be able
to develop applications utilizing SSL
3.0 that will then be able to
successfully exchange cryptographic
parameters without knowledge of one
another's code.
Note: It is not the case that all instances of SSL
(even in the same application domain) will be
able to successfully connect. For instance,
if the server supports a particular hardware
token, and the client does not have access to
such a token, then the connection will not
succeed.
3. Extensibility SSL 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 SSL 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.
5. Goals of this document
The SSL Protocol Version 3.0 Specification is intended
primarily for readers who will be implementing the protocol
and those doing cryptographic analysis of it. The spec has
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SSL 3.0 March 1996
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.
6. 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 SSL only, not to have
general application beyond that particular goal.
6.1 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.
6.2 Miscellaneous
Comments begin with "/*" and end with "*/".
Optional components are denoted by enclosing them in
italicized "[ ]" brackets.
Single byte entities containing uninterpreted data are of
type opaque.
6.3 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
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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 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
6.4). 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.
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
6.4 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 6.1 and
are also unsigned. The following numeric types are
predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
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6.5 Enumerateds
An additional sparse data type is available called enum. A
field of type enum can only assume the values declared in
the definition. Each definition is a different type. Only
enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as
demonstrated in the following example. Since the elements of
the enumerated are not ordered, they can be assigned any
unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn), [(n)] } Te;
Enumerateds occupy as much space in the byte stream as would
its maximal defined ordinal value. The following definition
would cause one byte to be used to carry fields of type
Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated
tag to force the width definition without defining a
superfluous element. In the following example, Taste will
consume two bytes in the data stream but can only assume the
values 1, 2 or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped
within the defined type. In the first example, a fully
qualified reference to the second element of the enumeration
would be Color.blue. Such qualification is not required if
the target of the assignment is well specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external
representation, the numerical information may be omitted.
enum { low, medium, high } Amount;
6.6 Constructed types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type.
The syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [T];
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The fields within a structure may be qualified using the
type's name using a syntax much like that available for
enumerateds. For example, T.f2 refers to the second field of
the previous declaration. Structure definitions may be
embedded.
6.6.1 Variants
Defined structures may have variants based on some knowledge
that is available within the environment. The selector must
be an enumerated type that defines the possible variants the
structure defines. There must be a case arm for every
element of the enumeration declared in the select. The body
of the variant structure may be given a label for reference.
The mechanism by which the variant is selected at runtime is
not prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
....
case en: Ten;
} [fv];
} [Tv];
For example
enum { apple, orange } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple: V1; /* VariantBody, tag = apple */
case orange: V2; /* VariantBody, tag = orange */
} variant_body; /* optional label on variant */
} VariantRecord;
Variant structures may be qualified (narrowed) by specifying
a value for the selector prior to the type. For example, a
orange VariantRecord
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is a narrowed type of a VariantRecord containing a
variant_body of type V2.
6.7 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 7.1).
In digital signing, one-way hash functions are used as input
for a signing algorithm. In RSA signing, a 36-byte structure
of two hashes (one SHA and one MD5) is signed (encrypted
with the private key). In DSS, the 20 bytes of the SHA hash
are run directly through the Digital Signing Algorithm with
no additional hashing.
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. Because it is unlikely
that the plaintext (whatever data is to be sent) will break
neatly into the necessary block size (usually 64 bits), it
is necessary to pad out the end of short blocks with some
regular pattern, usually all zeroes.
In public key encryption, one-way functions with secret
"trapdoors" are used to encrypt the outgoing data. Data
encrypted with the public key of a given key pair can only
be decrypted with the private key, and vice-versa.
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.
6.8 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
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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 */
7. SSL protocol
SSL is a layered protocol. At each layer, messages may
include fields for length, description, and content. SSL
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.
7.1 Session and connection states
An SSL session is stateful. It is the responsibility of the
SSL Handshake protocol to coordinate the states of the
client and server, thereby allowing the protocol state
machines of each to operate consistently, despite the fact
that the state is not exactly parallel. Logically the state
is represented twice, once as the current operating state,
and (during the handshake protocol) again as the pending
state. Additionally, separate read and write states are
maintained. When the client or server receives a change
cipher spec message, it copies the pending read state into
the current read state. When the client or server sends a
change cipher spec message, it copies the pending write
state into the current write state. When the handshake
negotiation is complete, the client and server exchange
change cipher spec messages (see Section 7.3), and they then
communicate using the newly agreed-upon cipher spec.
An SSL session may include multiple secure connections; in
addition, parties may have multiple simultaneous sessions.
The session state includes the following elements:
session identifier
An arbitrary byte sequence chosen by the
server to identify an active or
resumable session state.
peer certificate X509.v3[X509] certificate of the peer.
This element of the state may be null.
compression method
The algorithm used to compress data
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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.7
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.
The connection state includes the following elements:
server and client random
Byte sequences that are chosen by the
server and client for each connection.
server write MAC secret
The secret used in MAC operations
on data written by the server
client write MAC secret
The secret used in MAC operations
on data written by the client.
server write key The bulk cipher key for data encrypted
by the server and decrypted by the
client.
client write key The bulk cipher key for data encrypted
by the client and decrypted by the
server.
initialization vectors
When a block cipher in CBC mode is used,
an initialization vector (IV) is
maintained for each key. This field is
first initialized by the SSL handshake
protocol. Thereafter the final
ciphertext block from each record is
preserved for use with the following
record.
sequence numbers Each party maintains separate sequence
numbers for transmitted and received
messages for each connection. When a
party sends or receives a change cipher
spec message, the appropriate sequence
number is set to zero. Sequence numbers
are of type uint64 and may not exceed
2^64-1.
7.2 Record layer
The SSL Record Layer receives uninterpreted data from higher
layers in non-empty blocks of arbitrary size.
7.2.1 Fragmentation
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The record layer fragments information blocks into
SSLPlaintext records 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 SSLPlaintext record).
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[SSLPlaintext.length];
} SSLPlaintext;
type The higher level protocol used to
process the enclosed fragment.
version The version of protocol being employed.
This document describes SSL Version 3.0
(See Appendix A.1.1).
length The length (in bytes) of the following
SSLPlaintext.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 SSL Record layer content
types may be interleaved. Application data is
generally of lower precedence for
transmission than other content types.
7.2.2 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 an SSLPlaintext structure into an SSLCompressed
structure. Compression functions erase their state
information whenever the CipherSpec is replaced.
Note: The CipherSpec is part of the session state
described in Section 7.1. References to
fields of the CipherSpec are made throughout
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this document using presentation syntax. A
more complete description of the CipherSpec
is shown in Appendix A.7.
Compression must be lossless and may not increase the
content length by more than 1024 bytes. If the decompression
function encounters an SSLCompressed.fragment that would
decompress to a length in excess of 2^14 bytes, it should
issue a fatal decompression_failure alert (Section 7.4.2).
struct {
ContentType type; /* same as SSLPlaintext.type */
ProtocolVersion version;/* same as SSLPlaintext.version */
uint16 length;
opaque fragment[SSLCompressed.length];
} SSLCompressed;
length The length (in bytes) of the following
SSLCompressed.fragment. The length
should not exceed 2^14 + 1024.
fragment The compressed form of
SSLPlaintext.fragment.
Note: A CompressionMethod.null operation is an
identity operation; no fields are altered.
(See Appendix A.4.1)
Implementation note:
Decompression functions are responsible for
ensuring that messages cannot cause internal
buffer overflows.
7.2.3 Record payload protection and the CipherSpec
''
All records are protected using the encryption and MAC
algorithms defined in the current CipherSpec. There is
always an active CipherSpec, however initially it is
SSL_NULL_WITH_NULL_NULL, which does not provide any
security.
Once the handshake is complete, the two parties have shared
secrets which are used to encrypt records and compute keyed
message authentication codes (MACs) on their contents. The
techniques used to perform the encryption and MAC operations
are defined by the CipherSpec and constrained by
CipherSpec.cipher_type. The encryption and MAC functions
translate an SSLCompressed structure into an SSLCiphertext.
The decryption functions reverse the process. Transmissions
also include a sequence number so that missing, altered, or
extra messages are detectable.
struct {
ContentType type;
ProtocolVersion version;
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uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
} SSLCiphertext;
type The type field is identical to
SSLCompressed.type.
version The version field is identical to
SSLCompressed.version.
length The length (in bytes) of the following
SSLCiphertext.fragment. The length may
not exceed 2^14 + 2048.
fragment The encrypted form of
SSLCompressed.fragment, including the
MAC.
7.2.3.1 Null or standard stream cipher
Stream ciphers (including BulkCipherAlgorithm.null - see
Appendix A.7) convert SSLCompressed.fragment structures to
and from stream SSLCiphertext.fragment structures.
stream-ciphered struct {
opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
} GenericStreamCipher;
The MAC is generated as:
hash(MAC_write_secret + pad_2 +
hash (MAC_write_secret + pad_1 + seq_num + length +
content));
where "+" denotes concatenation.
pad_1 The character 0x36 repeated 48 times for
MD5 this or 40 times for SHA.
pad_2 The character 0x5c repeated the same
number of times.
seq_num The sequence number for this message.
hash Hashing algorithm derived from the
cipher suite.
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 SSL_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).
SSLCiphertext.length is SSLCompressed.length plus
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CipherSpec.hash_size.
7.2.3.2 CBC block cipher
For block ciphers (such as RC2 or DES), the encryption and
MAC functions convert SSLCompressed.fragment structures to
and from block SSLCiphertext.fragment structures.
block-ciphered struct {
opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;
} GenericBlockCipher;
The MAC is generated as described in Section 7.2.3.1.
padding Padding that is added to force the
length of the plaintext to be a multiple
of the block cipher's block length.
padding_length The length of the padding must be less
than the cipher's block length and may
be zero. The padding length should be
such that the total size of the
GenericBlockCipher structure is a
multiple of the cipher's block length.
The encrypted data length (SSLCiphertext.length) is one more
than the sum of SSLCompressed.length, CipherSpec.hash_size,
and padding_length.
Note: With CBC block chaining the initialization
vector (IV) for the first record is provided
by the handshake protocol. The IV for
subsequent records is the last ciphertext
block from the previous record.
7.3 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) CipherSpec. The message consists of a
single byte of value 1.
struct {
enum { change_cipher_spec(1), (255) } type;
} ChangeCipherSpec;
The change cipher spec message is sent by both the client
and server to notify the receiving party that subsequent
records will be protected under the just-negotiated
CipherSpec and keys. Reception of this message causes the
receiver to copy the read pending state into the read
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current state. The client sends a change cipher spec message
following handshake key exchange and certificate verify
messages (if any), and the server sends one after
successfully processing the key exchange message it received
from the client. An unexpected change cipher spec message
should generate an unexpected_message alert (Section 7.4.2).
When resuming a previous session, the change cipher spec
message is sent after the hello messages.
7.4 Alert protocol
One of the content types supported by the SSL 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),
decompression_failure(30),
handshake_failure(40), no_certificate(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45), certificate_unknown(46),
illegal_parameter (47)
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
7.4.1 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
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terminated without proper close_notify
messages with level equal to warning.
7.4.2 Error alerts
Error handling in the SSL Handshake protocol is very simple.
When an error is detected, the detecting party sends a
message to the other party. Upon transmission or receipt of
an fatal alert message, both parties immediately close the
connection. Servers and clients 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.
decompression_failure
The decompression function received
improper input (e.g. data that would
expand to excessive length). This
message is always fatal.
handshake_failure Reception of a handshake_failure alert
message indicates that the sender was
unable to negotiate an acceptable set of
security parameters given the options
available. This is a fatal error.
no_certificate A no_certificate alert message may be
sent in response to a certification
request if no appropriate certificate is
available.
bad_certificate A certificate was corrupt, contained
signatures that did not verify
correctly, etc.
unsupported_certificate
A certificate was of an unsupported
type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not
currently valid.
certificate_unknown
Some other (unspecified) issue arose in
processing the certificate, rendering it
unacceptable.
illegal_parameter A field in the handshake was out of
range or inconsistent with other fields.
This is always fatal.
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7.5 Handshake protocol overview
The cryptographic parameters of the session state are
produced by the SSL Handshake Protocol, which operates on
top of the SSL Record Layer. When a SSL 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. These processes are performed in
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.
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 either the certificate message or a no
certificate alert. The client key exchange message is now
sent, and the content of that message will depend on the
public key algorithm selected between the client hello and
the server hello. If the client has sent a certificate with
signing ability, a digitally-signed certificate verify
message is sent to explicitly verify the certificate.
At this point, a change cipher spec message is sent by the
client, and the client copies the pending Cipher Spec into
the current Cipher Spec. The client then immediately sends
the finished message under the new algorithms, keys, and
secrets. In response, the server will send its own change
cipher spec message, transfer the pending to the current
Cipher Spec, and send its finished message under the new
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 -------->
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SSL 3.0 March 1996
ServerHello
Certificate*
CertificateRequest*
<-------- ServerKeyExchange*
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
* Indicates optional or situation-dependent messages that
are not always sent.
Note: To help avoid pipeline stalls,
ChangeCipherSpec is an independent SSL
Protocol content type, and is not actually an
SSL handshake message.
When the client and server decide to resume a previous
session or duplicate an existing session (instead of
negotiating new security parameters) the message flow is as
follows:
The client sends a ClientHello using the Session ID of the
session to be resumed. The server then checks its session
cache for a match. If a match is found, and the server is
willing to re-establish the connection under the specified
session state, it will send a ServerHello with the same
Session ID value. At this point, both client and server must
send change cipher spec messages and proceed directly to
finished messages. Once the re-establishment is complete,
the client and server may begin to exchange application
layer data. (See flow chart below.) If a Session ID match
is not found, the server generates a new session ID and the
SSL client and server perform a full handshake.
Client Server
ClientHello -------->
ServerHello
[change cipher spec]
<-------- Finished
change cipher spec
Finished -------->
Application Data <-------> Application Data
The contents and significance of each message will be
presented in detail in the following sections.
7.6 Handshake protocol
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The SSL Handshake Protocol is one of the defined higher
level clients of the SSL Record Protocol. This protocol is
used to negotiate the secure attributes of a session.
Handshake messages are supplied to the SSL Record Layer,
where they are encapsulated within one or more SSLPlaintext
structures, which are processed and transmitted as specified
by the current active session state.
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13),
server_hello_done(14), certificate_verify(15),
client_key_exchange(16),
finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
The handshake protocol messages are presented in the order
they must be sent; sending handshake messages in an
unexpected order results in a fatal error.
7.6.1 Hello messages
The hello phase messages are used to exchange security
enhancement capabilities between the client and server. When
a new session begins, the CipherSpec encryption, hash, and
compression algorithms are initialized to null. The current
CipherSpec is used for renegotiation messages.
7.6.1.1 Hello request
The hello request message may be sent by the server at any
time, but will be ignored by the client if the handshake
protocol is already underway. It is a simple notification
that the client should begin the negotiation process anew by
sending a client hello message when convenient.
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Note: Since handshake messages are intended to have
transmission precedence over application
data, it is expected that the negotiation
begin in no more than one or two times the
transmission time of a maximum length
application data message.
After sending a hello request, servers should not repeat the
request until the subsequent handshake negotiation is
complete. A client that receives a hello request while in a
handshake negotiation state should simply ignore the
message.
The structure of a hello request message is as follows:
struct { } HelloRequest;
7.6.1.2 Client hello
When a client first connects to a server it is required to
send the client hello as its first message. The client can
also send a client hello in response to a hello request or
on its own initiative in order to renegotiate the security
parameters in an existing connection.
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 according to the
sender's internal clock. Clocks are not
required to be set correctly by the
basic SSL 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 simultaneous independent
secure connections without repeating the full handshake
protocol. The actual contents of the SessionID are defined
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SSL 3.0 March 1996
by the server.
opaque SessionID<0..32>;
Warning: Servers must not place confidential
information in session identifiers or let the
contents of fake session identifiers cause
any breach of security.
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 (first choice first). Each
CipherSuite defines both a key exchange algorithm and a
CipherSpec. 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. If the server supports none of those specified
by the client, the session must fail.
enum { null(0), (255) } CompressionMethod;
Issue: Which compression methods to support is under
investigation.
The structure of the client hello is as follows.
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 SSL protocol by which
the client wishes to communicate during
this session. This should be the most
recent (highest valued) version
supported by the client. For this
version of the specification, the
version will be 3.0 (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
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options supported by the client, sorted
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.6.
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) this vector must
include at least the compression_method
from that session. All implementations
must support CompressionMethod.null.
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.
Implementation note:
Application data may not be sent before a
finished message has been sent. Transmitted
application data is known to be insecure until
a valid finished message has been received.
This absolute restriction is relaxed if there
is a current, non-null encryption on this
connection.
7.6.1.3 Server hello
The server processes the client hello message and responds
with either a handshake_failure alert or server hello
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 will be 3.0
(See Appendix E for details about
backward compatibility).
random This structure is generated by the
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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 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.
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.
7.6.2 Server certificate
If the server is to be authenticated (which is generally the
case), the server sends its certificate immediately
following the server hello message. The certificate type
must be appropriate for the selected cipher suite's key
exchange algorithm, and is generally an X.509.v3 certificate
(or a modified X.509 certificate in the case of Fortezza
[FOR]). The same message type will be used for the client's
response to a certificate request message.
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<1..2^24-1>;
} Certificate;
certificate_list This is a sequence (chain) of X.509.v3
certificates, ordered with the sender's
certificate first and the root
certificate authority last.
Note: PKCS #7 [PKCS7] is not used as the format for
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SSL 3.0 March 1996
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.
7.6.3 Server key exchange message
The server key exchange message is sent by the server if it
has no certificate, has a certificate only used for signing
(e.g., DSS [DSS] certificates, signing-only RSA [RSA]
certificates), or Fortezza/DMS key exchange is used. This
message is not used if the server certificate contains
Diffie-Hellman [DH1] parameters.
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, larger RSA keys
may be used as signature-only certificates to
sign temporary shorter RSA keys for key
exchange.
enum { rsa, diffie_hellman, fortezza_dms }
KeyExchangeAlgorithm;
struct {
opaque rsa_modulus<1..2^16-1>;
opaque rsa_exponent<1..2^16-1>;
} ServerRSAParams;
rsa_modulus The modulus of the server's temporary
RSA key.
rsa_exponent The public exponent of the server's
temporary RSA key.
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams; /* Ephemeral DH parameters */
dh_p The prime modulus used for the Diffie-
Hellman operation.
dh_g The generator used for the Diffie-
Hellman operation.
dh_Ys The server's Diffie-Hellman public value
(gX mod p).
struct {
opaque r_s [128];
} ServerFortezzaParams;
r_s Server random number for Fortezza KEA
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SSL 3.0 March 1996
(Key Exchange Algorithm).
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_dms:
ServerFortezzaParams params;
};
} ServerKeyExchange;
params The server's key exchange parameters.
signed_params 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;
digitally-signed struct {
select(SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
opaque md5_hash[16];
opaque sha_hash[20];
case dsa:
opaque sha_hash[20];
};
} Signature;
7.6.4 Certificate request
A non-anonymous server can optionally request a certificate
from the client, if appropriate for the selected cipher
suite.
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3),
dss_fixed_dh(4),
rsa_ephemeral_dh(5), dss_ephemeral_dh(6),
fortezza_dms(20), (255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
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SSL 3.0 March 1996
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.
Note: DistinguishedName is derived from [X509].
Note: It is a fatal handshake_failure alert for an
anonymous server to request client
identification.
7.6.5 Server hello done
The server hello done message is sent by the server to
indicate the end of the server hello and associated
messages. After sending this message the server will wait
for a client response.
struct { } ServerHelloDone;
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.
7.6.6 Client certificate
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 no
certificate alert instead. This error is only a warning,
however the server may respond with a fatal handshake
failure alert if client authentication is required.
Client certificates are sent using the Certificate defined
in Section 7.6.2.
Note: Client Diffie-Hellman certificates must match
the server specified Diffie-Hellman
parameters.
7.6.7 Client key exchange message
The choice of messages depends on which public key
algorithm(s) has (have) been selected. See Section 7.6.3 for
the KeyExchangeAlgorithm definition.
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SSL 3.0 March 1996
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case fortezza_dms: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
The information to select the appropriate record structure
is in the pending session state (see Section 7.1).
7.6.7.1 RSA encrypted premaster secret message
If RSA is being used for key agreement and authentication,
the client generates a 48-byte pre-master secret, encrypts
it under the public key from the server's certificate or
temporary RSA key from a server key exchange message, and
sends the result in an encrypted premaster secret message.
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
client_version The latest (newest) version supported by
the client. This is used to detect
version roll-back attacks.
random 46 securely-generated random bytes.
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
pre_master_secret This random value is generated by the
client and is used to generate the
master secret, as specified in Section
8.1.
7.6.7.2 Fortezza key exchange message
Under Fortezza DMS, the client derives a Token Encryption
Key (TEK) using the Fortezza Key Exchange Algorithm (KEA).
The client's KEA calculation uses the public key in the
server's certificate along with private parameters in the
client's token. The client sends public parameters needed
for the server to generate the TEK, using its own private
parameters. The client generates session keys, wraps them
using the TEK, and sends the results to the server. The
client generates IV's for the session keys and TEK and sends
them also. The client generates a random 48-byte premaster
secret, encrypts it using the TEK, and sends the result:
struct {
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SSL 3.0 March 1996
opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[20];
opaque wrapped_client_write_key[12];
opaque wrapped_server_write_key[12];
opaque client_write_iv[24];
opaque server_write_iv[24];
opaque master_secret_iv[24];
block-ciphered opaque encrypted_pre_master_secret[48];
} FortezzaKeys;
y_signature y_signature is the signature of the KEA
public key, signed with the client's DSS
private key.
y_c The client's Yc value (public key) for
the KEA calculation. If the client has
sent a certificate, and its KEA public
key is suitable, this value must be
empty since the certificate already
contains this value. If the client sent
a certificate without a suitable public
key, y_c is used and y_signature is the
KEA public key signed with the client's
DSS private key. For this value to be
used, it must be between 64 and 128
bytes.
r_c The client's Rc value for the KEA
calculation.
wrapped_client_write_key
This is the client's write key, wrapped
by the TEK.
wrapped_server_write_key
This is the server's write key, wrapped
by the TEK.
client_write_iv The IV for the client write key.
server_write_iv The IV for the server write key.
master_secret_iv This is the IV for the TEK used to
encrypt the pre-master secret.
pre_master_secret A random value, generated by the client
and used to generate the master secret,
as specified in Section 8.1. In the
the above structure, it is encrypted
using the TEK.
7.6.7.3 Client Diffie-Hellman public value
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.
enum { implicit, explicit } PublicValueEncoding;
implicit If the client certificate already
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SSL 3.0 March 1996
contains the public value, then it is
implicit and Yc does not need to be sent
again.
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).
7.6.8 Certificate verify
This message is used to provide explicit verification of a
client certificate. This message is only sent following any
client certificate that has signing capability (i.e. all
certificates except those containing fixed Diffie-Hellman
parameters).
struct {
Signature signature;
} CertificateVerify;
CertificateVerify.signature.md5_hash
MD5(master_secret + pad2 +
MD5(handshake_messages + master_secret +
pad1));
Certificate.signature.sha_hash
SHA(master_secret + pad2 +
SHA(handshake_messages + master_secret +
pad1));
Here handshake_messages refers to all handshake messages
starting at client hello up to but not including this
message.
7.6.9 Finished
A finished message is always sent immediately after a change
cipher specs message to verify that the key exchange and
authentication processes were successful. The finished
message is the first protected with the just-negotiated
algorithms, keys, and secrets. No acknowledgment of the
finished message is required; parties may begin sending
confidential data immediately after sending the finished
message. Recipients of finished messages must verify that
the contents are correct.
enum { client(0x434C4E54), server(0x53525652) } Sender;
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SSL 3.0 March 1996
struct {
opaque md5_hash[16];
opaque sha_hash[20];
} Finished;
md5_hash MD5(master_secret + pad2 +
MD5(handshake_messages +
Sender + master_secret + pad1));
sha_hash SHA(master_secret + pad2 +
SHA(handshake_messages +
Sender + master_secret + pad1));
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, the finished messages. This may be
different from handshake_messages in Section 7.6.8 because
it would include the certificate verify message (if sent).
Note: Change cipher spec messages are not handshake
messages and are not included in the hash
computations.
7.7 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.
8. Cryptographic computations
The key exchange, authentication, encryption, and MAC
algorithms are determined by the cipher_suite selected by
the server and revealed in the server hello message.
8.1 Asymmetric cryptographic computations
The asymmetric algorithms are used in the handshake protocol
to authenticate parties and to generate shared keys and
secrets.
For Diffie-Hellman, RSA, and Fortezza, 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 =
MD5(pre_master_secret + SHA('A' + pre_master_secret +
ClientHello.random + ServerHello.random)) +
MD5(pre_master_secret + SHA('BB' + pre_master_secret +
ClientHello.random + ServerHello.random)) +
MD5(pre_master_secret + SHA('CCC' + pre_master_secret +
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SSL 3.0 March 1996
ClientHello.random + ServerHello.random));
8.1.1 RSA
When RSA is used for server authentication and key exchange,
a 48-byte pre_master_secret is generated by the client,
encrypted under the server's public key, and sent to the
server. The server uses its private key to decrypt the
pre_master_secret. Both parties then convert the
pre_master_secret into the master_secret, as specified
above.
RSA digital signatures are performed using PKCS #1 [PKCS1]
block type 1. RSA public key encryption is performed using
PKCS #1 block type 2.
8.1.2 Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is used as the pre_master_secret, and is
converted into the master_secret, as specified above.
Note: Diffie-Hellman parameters are specified by
the server, and may be either ephemeral or
contained within the server's certificate.
8.1.3 Fortezza
A random 48-byte pre_master_secret is sent encrypted under
the TEK and its IV. The server decrypts the
pre_master_secret and converts it into a master_secret, as
specified above. Bulk cipher keys and IVs for encryption are
generated by the client's token and exchanged in the key
exchange message; the master_secret is only used for MAC
computations.
8.2 Symmetric cryptographic calculations and the CipherSpec
The technique used to encrypt and verify the integrity of
SSL records is specified by the currently active CipherSpec.
A typical example would be to encrypt data using DES and
generate authentication codes using MD5. The encryption and
MAC algorithms are set to SSL_NULL_WITH_NULL_NULL at the
beginning of the SSL Handshake Protocol, indicating that no
message authentication or encryption is performed. The
handshake protocol is used to negotiate a more secure
CipherSpec and to generate cryptographic keys.
8.2.1 The master secret
Before secure encryption or integrity verification can be
performed on records, the client and server need to generate
shared secret information known only to themselves. This
value is a 48-byte quantity called the master secret. The
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SSL 3.0 March 1996
master secret is used to generate keys and secrets for
encryption and MAC computations. Some algorithms, such as
Fortezza, may have their own procedure for generating
encryption keys (the master secret is used only for MAC
computations in Fortezza).
8.2.2 Converting the master secret into keys and MAC
secrets
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 CipherSpec (see Appendix A.7).
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, such as
Fortezza keys communicated in the KeyExchange message, are
empty. The following inputs are available to the key
definition process:
opaque MasterSecret[48]
ClientHello.random
ServerHello.random
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 =
MD5(master_secret + SHA(`A' + master_secret +
ServerHello.random +
ClientHello.random)) +
MD5(master_secret + SHA(`BB' + master_secret +
ServerHello.random +
ClientHello.random)) +
MD5(master_secret + SHA(`CCC' + master_secret +
ServerHello.random +
ClientHello.random)) + [...];
until enough output has been generated. Then the key_block
is partitioned as follows.
client_write_MAC_secret[CipherSpec.hash_size]
server_write_MAC_secret[CipherSpec.hash_size]
client_write_key[CipherSpec.key_material]
server_write_key[CipherSpec.key_material]
client_write_IV[CipherSpec.IV_size]
/* non-export ciphers */
server_write_IV[CipherSpec.IV_size]
/* non-export ciphers */
Any extra key_block material is discarded.
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SSL 3.0 March 1996
Exportable encryption algorithms (for which
CipherSpec.is_exportable is true) require additional
processing as follows to derive their final write keys:
final_client_write_key = MD5(client_write_key +
ClientHello.random + ServerHello.random);
final_server_write_key = MD5(server_write_key +
ServerHello.random + ClientHello.random);
Exportable encryption algorithms derive their IVs from the
random messages:
client_write_IV = MD5(ClientHello.random +
ServerHello.random);
server_write_IV = MD5(ServerHello.random +
ClientHello.random);
MD5 outputs are trimmed to the appropriate size by
discarding the least-significant bytes.
8.2.2.1 Export key generation example
SSL_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. MD5 produces 16 bytes of output per call, so three
calls to MD5 are required. The MD5 outputs are concatenated
into a 48-byte key_block with the first MD5 call providing
bytes zero through 15, the second providing bytes 16 through
31, etc. The key_block is partitioned, and the write keys
are salted because this is an exportable encryption
algorithm.
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 = MD5 (client_write_key +
ClientHello.random + ServerHello.random)[0..15;
final_server_write_key = MD5 (server_write_key +
ServerHello.random + ClientHello.random)[0..15];
client_write_IV = MD5(ClientHello.random +
ServerHello.random)[0..7];
server_write_IV = MD5(ServerHello.random +
ClientHello.random)[0..7];
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SSL 3.0 March 1996
Appendix A
A. Protocol constant values
This section describes protocol types and constants.
A.1 Reserved port assignments
At the present time SSL is implemented using TCP/IP as the
base networking technology. The IANA reserved the following
Internet Protocol [IP] port numbers for use in conjunction
with SSL.
443 Reserved for use by Hypertext Transfer Protocol with
SSL (https).
465 Reserved (pending) for use by Simple Mail Transfer
Protocol with SSL (ssmtp).
563 Reserved (pending) for use by Network News Transfer
Protocol (snntp).
A.1.1 Record layer
struct {
uint8 major, minor;
} ProtocolVersion;
ProtocolVersion version = { 3,0 };
enum {
change_cipher_spec(20), alert(21), handshake(22),
application_data(23), (255)
} ContentType;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[SSLPlaintext.length];
} SSLPlaintext;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[SSLCompressed.length];
} SSLCompressed;
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
select (CipherSpec.cipher_type) {
case stream: GenericStreamCipher;
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SSL 3.0 March 1996
case block: GenericBlockCipher;
} fragment;
} SSLCiphertext;
stream-ciphered struct {
opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
} GenericStreamCipher;
block-ciphered struct {
opaque content[SSLCompressed.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),
decompression_failure(30),
handshake_failure(40), no_certificate(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45), certificate_unknown(46),
illegal_parameter (47),
(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_done(14), certificate_verify(15),
client_key_exchange(16),
finished(20), (255)
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SSL 3.0 March 1996
} 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_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
A.4.1 Hello messages
struct { } HelloRequest;
struct {
uint32 gmt_unix_time;
opaque random_bytes[28];
} Random;
opaque SessionID<0..32>;
uint8 CipherSuite[2];
enum { null(0), (255) } CompressionMethod;
struct {
ProtocolVersion client_version;
Random random;
SessionID session_id;
CipherSuite cipher_suites<0..2^16-1>;
CompressionMethod compression_methods<0..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>;
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SSL 3.0 March 1996
struct {
ASN.1Cert certificate_list<1..2^24-1>;
} Certificate;
enum { rsa, diffie_hellman, fortezza_dms }
KeyExchangeAlgorithm;
struct {
opaque RSA_modulus<1..2^16-1>;
opaque RSA_exponent<1..2^16-1>;
} ServerRSAParams;
struct {
opaque DH_p<1..2^16-1>;
opaque DH_g<1..2^16-1>;
opaque DH_Ys<1..2^16-1>;
} ServerDHParams;
struct {
opaque r_s [128]
} ServerFortezzaParams
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_dms:
ServerFortezzaParams params;
};
} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;
digitally-signed struct {
select(SignatureAlgorithm) {
case anonymous: struct { };
case rsa:
opaque md5_hash[16];
opaque sha_hash[20];
case dsa:
opaque sha_hash[20];
};
} Signature;
enum {
RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3),
DSS_fixed_DH(4), RSA_ephemeral_DH(5), DSS_ephemeral_DH(6),
Fortezza_dms(20), (255)
} CertificateType;
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SSL 3.0 March 1996
opaque DistinguishedName<1..2^16-1>;
struct {
CertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..2^16-1>;
} CertificateRequest;
struct { } ServerHelloDone;
A.5 Client authentication and key exchange messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: DiffieHellmanClientPublicValue;
case fortezza_dms: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
struct {
opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[20];
opaque wrapped_client_write_key[12];
opaque wrapped_server_write_key[12];
opaque client_write_iv[24];
opaque server_write_iv[24];
opaque master_secret_iv[24];
opaque encrypted_preMasterSecret[48];
} FortezzaKeys;
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.5.1 Handshake finalization message
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SSL 3.0 March 1996
struct {
opaque md5_hash[16];
opaque sha_hash[20];
} Finished;
A.6 The CipherSuite
The following values define the CipherSuite codes used in
the client hello and server hello messages.
A CipherSuite defines a cipher specifications supported in
SSL Version 3.0.
CipherSuite SSL_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 SSL_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
CipherSuite SSL_RSA_WITH_NULL_SHA = { 0x00,0x02 };
CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5
= { 0x00,0x03 };
CipherSuite SSL_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
CipherSuite SSL_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5
= { 0x00,0x06 };
CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x08 };
CipherSuite SSL_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
CipherSuite SSL_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. In all cases, the client must have the same type
of certificate, and must use the Diffie-Hellman parameters
chosen by the server.
CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x0B };
CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA
= { 0x00,0x0C };
CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA
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SSL 3.0 March 1996
= { 0x00,0x0D };
CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x0E };
CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA
= { 0x00,0x0F };
CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA
= { 0x00,0x10 };
CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x11 };
CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA
= { 0x00,0x12 };
CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
= { 0x00,0x13 };
CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x14 };
CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA
= { 0x00,0x15 };
CipherSuite SSL_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 strongly
discouraged.
CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5
= { 0x00,0x17 };
CipherSuite SSL_DH_anon_WITH_RC4_128_MD5
= { 0x00,0x18 };
CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA
= { 0x00,0x19 };
CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA
= { 0x00,0x1A };
CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA
= { 0x00,0x1B };
The final cipher suite is for the Fortezza token.
CipherSuite SSL_FORTEZZA_DMS_WITH_NULL_SHA
= { 0X00,0X1C };
CipherSuite SSL_FORTEZZA_DMS_WITH_FORTEZZA_CBC_SHA
= { 0x00,0x1D };
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 will be considered
for implementation only with submission of
notarized letters from two independent
entities. Netscape Communications Corp. will
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SSL 3.0 March 1996
act as an interim registration office, until
a public standards body assumes control of
SSL.
A.7 The CipherSpec
A cipher suite identifies a CipherSpec. These structures are
part of the SSL session state. The CipherSpec includes:
enum { stream, block } CipherType;
enum { true, false } IsExportable;
enum { null, rc4, rc2, des, 3des, des40, fortezza }
BulkCipherAlgorithm;
enum { null, md5, sha } MACAlgorithm;
struct {
BulkCipherAlgorithm bulk_cipher_algorithm;
MACAlgorithm mac_algorithm;
CipherType cipher_type;
IsExportable is_exportable
uint8 hash_size;
uint8 key_material;
uint8 IV_size;
} CipherSpec;
Appendix B
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.
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
typical block size.
bulk cipher A symmetric encryption algorithm
used to encrypt large quantities of
data.
cipher block chaining
Mode (CBC) CBC is a mode in which
every plaintext block encrypted
with the block cipher is first
exclusive-ORed with the previous
ciphertext block (or, in the case
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SSL 3.0 March 1996
of the first block, with the
initialization vector).
certificate As part of the X.509 protocol
(a.k.a. ISO Authentication
framework), certificates are
assigned by a trusted Certificate
Authority and provide verification
of a party's identity and may also
supply its public key.
client The application entity that
initiates a connection to a server.
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 SSL, such connections are peer
to peer relationships. The
connections are transient. Every
connection is associated with one
session.
Data Encryption Standard
(DES) DES is a very widely used
symmetric encryption algorithm. DES
is a block cipher.
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.
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.
Fortezza A PCMCIA card that provides both
encryption and digital signing.
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.
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SSL 3.0 March 1996
IDEA A 64-bit block cipher designed by
Xuejia Lai and James Massey.
Message Authentication Code
(MAC) A Message Authentication Code
is a one-way hash computed from a
message and some 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 [7] is a secure hashing function
that converts an arbitrarily long
data stream into a digest of fixed
size.
public key cryptography
A class of cryptographic techniques
employing two-key ciphers. Messages
encrypted with the public key can
only be decrypted with the associated
private key. Conversely, messages
signed with the private key can be
verified with the public key.
one-way hash function
A one-way transformation that converts
an arbitrary amount of data into a
fixed-length hash. It is computation-
ally hard to reverse the transformation
or to find collisions. MD5 and SHA are
examples of one-way hash functions.
RC2, RC4 Proprietary bulk ciphers from RSA
Data Security, Inc. (There is no
good reference to these as they are
unpublished works; however, see
[RSADSI]). RC2 is block cipher and
RC4 is a stream cipher.
RSA A very widely used public-key
algorithm that can be used for
either encryption or digital
signing.
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. The server
is passive, waiting for requests
from clients.
session A SSL 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
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SSL 3.0 March 1996
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 [SHA].
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.
Appendix C
C. CipherSuite definitions
CipherSuite IsEx Key Cipher Has
por- Exchange h
tabl
e
SSL_NULL_WITH_NULL_NULL * NULL NULL NUL
L
SSL_RSA_WITH_NULL_MD5 * RSA NULL MD5
SSL_RSA_WITH_NULL_SHA * RSA NULL SHA
SSL_RSA_EXPORT_WITH_RC4_4 * RSA_EXPORT RC4_40 MD5
0_MD5
SSL_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5
SSL_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
SSL_RSA_EXPORT_WITH_RC2_C * RSA_EXPORT RC2_CBC_ MD5
BC_40_MD5 40
SSL_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
SSL_RSA_EXPORT_WITH_DES40 * RSA_EXPORT DES40_CB SHA
_CBC_SHA C
SSL_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA
SSL_RSA_WITH_3DES_EDE_CBC RSA 3DES_EDE SHA
_SHA _CBC
SSL_DH_DSS_EXPORT_WITH_DE * DH_DSS_EXP DES40_CB SHA
S40_CBC_SHA ORT C
SSL_DH_DSS_WITH_DES_CBC_S DH_DSS DES_CBC SHA
HA
SSL_DH_DSS_WITH_3DES_EDE_ DH_DSS 3DES_EDE SHA
CBC_SHA _CBC
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SSL 3.0 March 1996
SSL_DH_RSA_EXPORT_WITH_DE * DH_RSA_EXP DES40_CB SHA
S40_CBC_SHA ORT C
SSL_DH_RSA_WITH_DES_CBC_S DH_RSA DES_CBC SHA
HA
SSL_DH_RSA_WITH_3DES_EDE_ DH_RSA 3DES_EDE SHA
CBC_SHA _CBC
SSL_DHE_DSS_EXPORT_WITH_D * DHE_DSS_EX DES40_CB SHA
ES40_CBC_SHA PORT C
SSL_DHE_DSS_WITH_DES_CBC_ DHE_DSS DES_CBC SHA
SHA
SSL_DHE_DSS_WITH_3DES_EDE DHE_DSS 3DES_EDE SHA
_CBC_SHA _CBC
SSL_DHE_RSA_EXPORT_WITH_D * DHE_RSA_EX DES40_CB SHA
ES40_CBC_SHA PORT C
SSL_DHE_RSA_WITH_DES_CBC_ DHE_RSA DES_CBC SHA
SHA
SSL_DHE_RSA_WITH_3DES_EDE DHE_RSA 3DES_EDE SHA
_CBC_SHA _CBC
SSL_DH_anon_EXPORT_WITH_R * DH_anon_EX RC4_40 MD5
C4_40_MD5 PORT
SSL_DH_anon_WITH_RC4_128_ DH_anon RC4_128 MD5
MD5
SSL_DH_anon_EXPORT_WITH_D DH_anon DES40_CB SHA
ES40_CBC_SHA C
SSL_DH_anon_WITH_DES_CBC_ DH_anon DES_CBC SHA
SHA
SSL_DH_anon_WITH_3DES_EDE DH_anon 3DES_EDE SHA
_CBC_SHA _CBC
SSL_FORTEZZA_DMS_WITH_NUL FORTEZZA_D NULL SHA
L_SHA MS
SSL_FORTEZZA_DMS_WITH_FOR FORTEZZA_D FORTEZZA SHA
TEZZA_CBC_SHA MS _CBC
* Indicates IsExportable is True
Key Description Key size limit
Exchange
Algorithm
DHE_DSS Ephemeral DH with DSS None
signatures
DHE_DSS_EXP Ephemeral DH with DSS DH = 512 bits
ORT signatures
DHE_RSA Ephemeral DH with RSA None
signatures
DHE_RSA_EXP Ephemeral DH with RSA DH = 512 bits, RSA =
ORT none
DH_anon Anonymous DH, no None
signatures
DH_anon_EXP Anonymous DH, no DH = 512 bits
ORT signatures
DH_DSS DH with DSS-based None
certificates
DH_DSS_EXPO DH with DSS-based DH = 512 bits
RT certificates
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SSL 3.0 March 1996
DH_RSA DH with RSA-based None
certificates
DH_RSA_EXPO DH with RSA-based DH = 512 bits, RSA =
RT certificates none.
FORTEZZA_DM Fortezza DMS. Details N/A
S unpublished.
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.
Cipher Ciphe IsExp Key Exp. Effect IV_ Block
r or- Mater Key ive Size Size
Type table ial Materi Key
al Bits
NULL Strea * 0 0 0 0 N/A
m
FORTEZZA_CB Block NA 12 96 20 8
C (**) (**) (**) (**)
IDEA_CBC Block 16 16 128 8 8
RC2_CBC_40 Block * 5 16 40 8 8
RC4_40 Strea * 5 16 40 0 N/A
m
RC4_128 Strea 16 16 128 0 N/A
m
DES40_CBC Block * 5 8 40 8 8
DES_CBC Block 8 8 56 8 8
3DES_EDE_CB 24 24 168 8 8
C Block
* Indicates IsExportable is true.
** Fortezza uses its own key and IV generation algorithms.
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.
Effective Key Bits
How much entropy material is in the key
material being fed into the encryption
routines.
Hash HashSize Padding
function Size
NULL 0 0
MD5 16 48
SHA 20 40
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Appendix D
D. Implementation Notes
The SSL protocol cannot prevent many common security
mistakes. This section provides several recommendations to
assist implementers.
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. Whenever a new key is completed, the
existing temporary key can be replaced with the new one.
D.2 Random Number Generation and Seeding
SSL 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
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SSL 3.0 March 1996
approximately 100 such timer values.
Note: 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.
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
SSL 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.
Appendix E
E. Version 2.0 Backward Compatibility
Version 3.0 clients that support Version 2.0 servers must
send Version 2.0 client hello messages [SSL-2]. Version 3.0
servers should accept either client hello format. 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. Implementers should make every effort
to move forward as quickly as possible.
Version 3.0 provides better mechanisms for
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SSL 3.0 March 1996
transitioning 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 SSL_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 };
V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5
= { 0x02,0x00,0x80 };
V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5
= { 0x03,0x00,0x80 };
V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
= { 0x04,0x00,0x80 };
V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5
= { 0x05,0x00,0x80 };
V2CipherSpec SSL_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 };
V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5
= { 0x07,0x00,0xC0 };
Cipher specifications introduced in Version 3.0 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 Version 3.0 equivalent (see Appendix A.6):
V2CipherSpec (see Version 3.0 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 {
unit8 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
equal one (1).
version The highest version of the protocol
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SSL 3.0 March 1996
supported by the client (equals
ProtocolVersion.version, see Appendix
A.1.1).
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.
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's challenge to the server for
the server to identify itself is a
(nearly) arbitrary length random. The
Version 3.0 server will right justify
the challenge data to become the
ClientHello.random data (padded with
leading zeroes, if necessary), as
specified in this Version 3.0 protocol.
If the length of the challenge is
greater than 32 bytes, then 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 an SSL 3.0 session should
use an SSL 3.0 client hello.
E.2 Avoiding man-in-the-middle version rollback
When SSL Version 3.0 clients fall back to Version 2.0
compatibility mode, they use special PKCS #1 block
formatting. This is done so that Version 3.0 servers will
reject Version 2.0 sessions with Version 3.0-capable
clients.
When Version 3.0 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-
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SSL 3.0 March 1996
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 SSL 3.0 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 SSL 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 SSL has been designed to
resist a variety of attacks.
F.1 Handshake protocol
The handshake protocol is responsible for selecting a
CipherSpec and generating a MasterSecret, 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
SSL supports three authentication modes: authentication of
both parties, server authentication with an unauthenticated
client, and total anonymity. Whenever the server is
authenticated, the channel should be secure against man-in-
the-middle attacks, but completely anonymous sessions are
inherently vulnerable to such attacks. Anonymous servers
cannot authenticate clients, since the client signature in
the certificate verify message may require a server
certificate to bind the signature to a particular server.
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
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SSL 3.0 March 1996
required to generate the finished messages, encryption keys,
and MAC secrets (see Sections 7.6.9 and 8.2.2). 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,
Diffie-Hellman, or Fortezza 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.
With Diffie-Hellman or Fortezza, 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) or the Fortezza token encryption key
(TEK).
Warning: Completely anonymous connections only provide
protection against passive eavesdropping.
Unless an independent tamper-proof channel is
used to verify that the finished messages
were not replaced by an attacker, server
authentication is required in environments
where active man-in-the-middle attacks are a
concern.
F.1.1.2 RSA key exchange and authentication
With RSA, key exchange and server authentication are
combined. The public key 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
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SSL 3.0 March 1996
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 7.6.8).
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 client key exchange message to
send a set of temporary Diffie-Hellman parameters signed
with a DSS or RSA certificate. Temporary parameters are
hashed with the hello.random values before signing to ensure
that attackers do not replay old parameters. In either case,
the client can verify the certificate or signature to ensure
that the parameters belong to the server.
If the client has a certificate containing fixed Diffie-
Hellman parameters, its certificate contains the information
required to complete the key exchange. Note that in this
case the client and server will generate the same Diffie-
Hellman result (i.e., pre_master_secret) every time they
communicate. To prevent the pre_master_secret from staying
in memory any longer than necessary, it should be converted
into the master_secret as soon as possible. Client Diffie-
Hellman parameters must be compatible with those supplied by
the server for the key exchange to work.
If the client has a standard DSS or RSA certificate or is
unauthenticated, it sends a set of temporary parameters to
the server in the client key exchange message, then
optionally uses a certificate verify message to authenticate
itself.
F.1.1.4 Fortezza
Fortezza's design is classified, but at the protocol level
it is similar to Diffie-Hellman with fixed public values
contained in certificates. The result of the key exchange
process is the token encryption key (TEK), which is used to
wrap data encryption keys, client write key, server write
key, and master secret encryption key. The data encryption
keys are not derived from the pre_master_secret because
unwrapped keys are not accessible outside the token. The
encrypted pre_master_secret is sent to the server in a
client key exchange message.
F.1.2 Version rollback attacks
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SSL 3.0 March 1996
Because SSL Version 3.0 includes substantial improvements
over SSL Version 2.0, attackers may try to make Version 3.0-
capable clients and servers fall back to Version 2.0. This
attack is occurring if (and only if) two Version 3.0-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, 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).
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
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SSL 3.0 March 1996
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
SSL 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. Fortezza encryption
keys are generated by the token, and are not derived from
the master_secret.
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 SSL 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 SSL 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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SSL 3.0 March 1996
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
This version of the SSL protocol relies on the use of
patented public key encryption technology for authentication
and encryption. 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
The Board of Trustees of the Leland Stanford Junior
University have granted Caro-Kann Corporation, a wholly
owned subsidiary corporation, exclusive sub-licensing rights
to the following patents issued in the United States, and
all of their corresponding foreign patents:
Cryptographic Apparatus and Method ("Diffie-Hellman"),
No. 4,200,770
Public Key Cryptographic Apparatus and Method ("Hellman-
Merkle"), No. 4,218,582
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.
Freier, Karlton, Kocher [ Page 57 ]
SSL 3.0 March 1996
References
[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.
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions
To DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
[DES] ANSI X3.106, "American National Standard for
Information Systems-Data Link Encryption," American National
Standards Institute, 1983.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard,"
National Institute of Standards and Technology, U.S.
Department of Commerce, 18 May 1994.
[FOR] NSA X22, Document # PD4002103-1.01, "Fortezza:
Application Implementers Guide," April 6, 1995.
[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.
[IDEA] X. Lai, "On the Design and Security of Block
Ciphers," ETH Series in Information Processing, v. 1,
Konstanz: Hartung-Gorre Verlag, 1992.
[KRAW] H. Krawczyk, IETF Draft: Keyed-MD5 for Message
Authentication, November 1995.
[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.
[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.
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[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, 31 May 1994.
[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.
Authors
Alan O. Freier Paul C. Kocher
Netscape Communications Independent Consultant
freier@netscape.com pck@netcom.com
Philip L. Karlton
Netscape Communications
karlton@netscape.com
Other contributors
Martin Abadi Robert Relyea
Digital Equipment Corporation Netscape Communications
ma@pa.dec.com relyea@netscape.com
Taher Elgamal Jim Roskind
Netscape Communications Netscape Communications
elgamal@netscape.com jar@netscape.com
Anil Gangolli Micheal J. Sabin, Ph. D.
Netscape Communications Consulting Engineer
gangolli@netscape.com msabin@netcom.com
Kipp E.B. Hickman Tom Weinstein
Netscape Communications Netscape Communications
kipp@netscape.com tomw@netscape.com
Early reviewers
Robert Baldwin Clyde Monma
RSA Data Security, Inc. Bellcore
baldwin@rsa.com clyde@bellcore.com
Freier, Karlton, Kocher [ Page 59 ]
SSL 3.0 March 1996
George Cox Eric Murray
Intel Corporation ericm@lne.com
cox@ibeam.jf.intel.com
Cheri Dowell Avi Rubin
Sun Microsystems Bellcore
cheri@eng.sun.com rubin@bellcore.com
Stuart Haber Don Stephenson
Bellcore Sun Microsystems
stuart@bellcore.com don.stephenson@eng.sun.com
Burt Kaliski Joe Tardo
RSA Data Security, Inc. General Magic
burt@rsa.com tardo@genmagic.com
Send all written communication about this document to:
Netscape Communications
501 East Middlefield Rd.
Mountain View, CA 94043
Attn: Alan Freier
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