One document matched: draft-freier-ssl-version3-00.txt
Internet Draft December 1995 (Expires 6/96)
Alan O. Freier, Netscape Communications
Philip Karlton, Netscape Communications
Paul C. Kocher, Independent Consultant
SSL Version 3.0
12/5/95
Abstract
This document specifies Version 3.0 of the Secure Sockets
Layer (SSL V3.0) protocol, a security protocol that provides
privacy over the Internet. The protocol allows client/server
applications to communicate in a way that prevents
eavesdropping, tampering, or message forgery.
Table of Contents
1. Status of this memo 3
2. Abstract 3
3. Introduction 3
4. Presentation language 4
4.1 Basic block size 4
4.2 Miscellaneous 4
4.3 Vectors 4
4.4 Numbers 5
4.5 Enumerateds 5
4.6 Constructed types 6
4.6.1 Variants 7
4.7 Cryptographic attributes 8
4.8 Constants 8
5. SSL protocol 8
5.1 Session and connection states 8
5.2 Record layer 10
5.2.1 Fragmentation 10
5.2.2 Record compression and decompression 11
5.2.3 Record payload protection and the CipherSpec 12
5.2.3.1 Null or standard stream cipher 12
5.2.3.2 CBC block cipher 13
5.3 Change cipher spec protocol 14
Freier, Karlton, Kocher [ Page 1]
5.4 Alert protocol 14
5.4.1 Closure alerts 15
5.4.2 Error alerts 15
5.5 Handshake protocol overview 16
5.6 Handshake protocol 17
5.6.1 Hello messages 18
5.6.1.1 Hello request 18
5.6.1.2 Client hello 19
5.6.1.3 Server hello 21
5.6.2 Server certificate 22
5.6.3 Server key exchange message 22
5.6.4 Certificate request 24
5.6.5 Server hello done 24
5.6.6 Client certificate 25
5.6.7 Client key exchange message 25
5.6.7.1 RSA encrypted premaster secret message 25
5.6.7.2 Fortezza key exchange message 26
5.6.7.3 Client Diffie-Hellman public value 27
5.6.8 Certificate verify 28
5.6.9 Finished 28
5.7 Application data protocol 29
6. Cryptographic computations 29
6.1 Asymmetric cryptographic computations 29
6.1.1 RSA 29
6.1.2 Diffie-Hellman 30
6.1.3 Fortezza 30
6.2 Symmetric cryptographic calculations and the CipherSpec 30
6.2.1 The master secret 30
6.2.2 Converting master secret into keys and MAC secrets 30
6.2.2.1 Export key generation example 32
A. Protocol constant values 33
A.1 Reserved port assignments 33
A.1.1 Record layer 33
A.2 Change cipher specs message 34
A.3 Alert messages 34
A.4 Handshake protocol 34
A.4.1 Hello messages 35
A.4.2 Server authentication and key exchange messages 35
A.5 Client authentication and key exchange messages 37
A.5.1 Handshake finalization message 37
A.6 The CipherSuite 38
A.7 The CipherSpec 39
B. Glossary 40
C. Version 2.0 Backward Compatibility 41
C.1 Version 2 client hello 42
D. Security analysis 44
D.1 Handshake protocol 44
D.1.1 Authentication and key exchange 44
D.1.1.1 Anonymous key exchange 45
D.1.1.2 RSA key exchange and authentication 45
D.1.1.3 Diffie-Hellman key exchange with authentication 46
D.1.1.4 Fortezza 46
D.1.2 Version rollback attacks 47
D.1.3 Detecting attacks against the handshake protocol 47
D.1.4 Resuming sessions 47
D.1.5 MD5 and SHA 48
D.2 Protecting application data 48
D.3 Final notes 49
E. Patent Statement 49
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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
privacy over the Internet. The protocol allows client/server
applications to communicate in a way that prevents
eavesdropping, tampering, or message forgery.
3. Introduction
The 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. The 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:
o 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.)
o The connection can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.).
o The connection is reliable. Message transport includes a
message integrity check using a keyed MAC. Secure hash
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functions (e.g., SHA, MD5, etc.) are used for MAC
computations.
4. Presentation language
This document deals with the formatting of data in an
external representation. The following very basic and
somewhat casually defined presentation syntax will be used.
The syntax draws from several sources in its structure.
Although it resembles the programming language 'C' in its
syntax and XDR [XDR] in both its syntax and intent, it would
be risky to draw too many parallels. The purpose of this
presentation language is to document SSL only, not to have
general application beyond that particular goal.
4.1 Basic block size
The representation of all data items is explicitly
specified. The basic data block size is one byte (i.e. 8
bits). Multiple byte data items are concatenations of the
bytes, from left to right, from top to bottom. From the
bytestream a multi-byte item (a numeric in the example) is
formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ...
| byte[n-1];
This byte ordering for multi-byte values is the commonplace
network byte order or big endian format.
4.2 Miscellaneous
Comments begin with /* and end with */.
Optional components are denoted by enclosing them in
(italicized) [ ] brackets.
Single byte entities containing uninterpreted data are of
type opaque.
4.3 Vectors
A vector (single dimensioned array) is a stream of
homogeneous data elements. The size of the vector may be
specified at documentation time or left unspecified until
runtime. In either case the length declares the number of
bytes, not the number of elements, in the vector.
The syntax for specifying a new type T' that is a fixed
length vector of type T is
T T'[n];
Here T' occupies n bytes in the data stream, where n is a
multiple of the size of T. The length of the vector is not
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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 elements of type opaque. It can
never be empty. The actual length field consumes two bytes,
a uint16, sufficient to represent the value 400 (see Section
4.4). On the other hand, longer can represent up to 800
bytes of data, or 400 uint16 elements, and it may be empty.
Its encoding will include a two byte actual length field
prepended to the vector.
opaque mandatory<300..400>; /* length field is 2 bytes,
cannot be empty */
uint16 longer<0..800>; /* zero to 400 16-bit
unsigned integers */
4.4 Numbers
The basic numeric data type is an unsigned byte (uint8). All
larger numeric data types are formed from fixed length
vectors concatenated as described in Section 4.1 and are
also unsigned. The following numeric types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
4.5 Enumerateds
An additional sparse data type is available called enum. A
field of type enum can only assume the values declared in
the definition. Each definition is a different type. Only
enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as
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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 example above, 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, but legal*
Color color = blue; /*correct, type is implicit*/
For enumerateds that are never converted to external
representation, the numerical information may be omitted.
enum { low, medium, high } Amount;
4.6 Constructed types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type.
The syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn f3;
} [T];
The fields within a structure may be qualified using the
type's name using a syntax much like that available for
enumerateds. For example, T.f2 refers to the second field of
the previous declaration. Structure definitions may be
embedded.
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4.6.1 Variants
Defined structures may have variants based on some knowledge
that is available within the environment. The selector must
be an enumerated type that defines the possible variants the
structure defines. There must be a case arm for every
element of the enumeration declared in the select. The body
of the variant structure may be given a label for reference.
The mechanism by which the variant is selected at runtime is
not prescribed by the presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
....
case en: Ten;
} [fv];
} [Tv];
For example
enum { apple, orange } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) {
case apple: V1;
case orange: V2;
} variant_body;
} VariantRecord;
Variant structures may be qualified (narrowed) by specifying
a value for the selector prior to the type. For example, a
orange VariantRecord
is a narrowed type of a VariantRecord containing a
variant_body of type V2.
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4.7 Cryptographic attributes
The four cryptographic operations digital signing, stream
cipher encryption, block cipher encryption, and public key
encryption are designated digitally-signed, stream-ciphered,
block-ciphered, and public-key-encrypted, respectively. A
field's cryptographic processing is specified by prepending
an appropriate key word designation before the field's type
specification. Cryptographic keys are implied by the current
session state (see Section 5.1).
In the following example:
stream-ciphered struct {
uint8 field1;
uint8 field2;
digitally-signed opaque field3[20];
} UserType;
The contents of field3 are signed, then the entire structure
is encrypted with a stream cipher.
4.8 Constants
Typed constants can be defined for purposes of specification
by declaring a symbol of the desired type and assigning
values to it. Under-specified types (opaque, variable length
vectors, and structures that contain opaque) cannot be
assigned values. No fields of a multi-element structure or
vector may be elided.
For example,
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /*assigns f1 = 1, f2 = 4*/
5. 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.
5.1 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
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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. When the handshake negotiation is complete, the
client and server exchange change cipher spec messages (see
Section 5.3). At that time, the pending state is copied into
the operating state. State information used only during the
handshake protocol is not listed.
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
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.
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
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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
specs message, the appropriate sequence
number is set to zero. Sequence numbers
are of type uint64 and may not exceed
2^64-1.
5.2 Record layer
The SSL Record Layer receives uninterpreted data from
clients in non-empty blocks of arbitrary size.
5.2.1 Fragmentation
The record layer fragments client 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;
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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.
5.2.2 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 5.1. References to
fields of the CipherSpec are made throughout
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 5.4.2).
struct {
ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[SSLCompressed.length];
} SSLCompressed;
length The length (in bytes) of the following
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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.
Implementation note: Decompression functions are
responsible for ensuring that messages cannot
cause internal buffer overflows.
5.2.3 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, 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 authenticity checks (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.
struct {
ContentType type;
ProtocolVersion version;
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.
5.2.3.1 Null or standard stream cipher
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Stream ciphers (including BulkCipherAlgorithm.null) 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 secret + hash
MAC secret + write sequence number +
SSLCompressed.type + SSLCompressed.length +
SSLCompressed.fragment));
where '+' denotes concatenation.
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, 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
CipherSpec.hash_size.
5.2.3.2 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 5.2.3.1.
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
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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.
5.3 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 pending state into the current state.
The client sends a change cipher spec message immediately
following a handshake key exchange and certificate verify
(if any) messages, 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 5.4.2).
5.4 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),
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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),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
5.4.1 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
terminated without proper close_notify
messages with level equal to warning.
5.4.2 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 unrecoverable error 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.
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handshake_failure Reception of a handshake_failure alert
message indicates that the sender was
unable to negotiate an acceptable set of
security parameters given the options
available. This is a fatal error.
no_certificate 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.
5.5 Handshake protocol overview
The cryptographic parameters of the session state are
produced by the SSL Handshake Protocol, a client of the
Record Layer. When a 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 consist of the following
messages:
Client Server
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
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CertificateVerify*
ChangeCipherSpec
[Begin new CipherSpec]
Finished -------->
ChangeCipherSpec
[Begin new CipherSpec]
<-------- Finished
Application Data <-------> Application Data
Notes: Messages marked with an asterisk (*) are not
always sent. 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:
Client Server
ClientHello -------->
ServerHello
ChangeCipherSpec
[Resume CipherSpec]
<-------- Finished
ChangeCipherSpec
[Resume CipherSpec]
Finished -------->
Application Data <-------> Application Data
The contents and significance of each message will be
presented in the subsequent sections.
5.6 Handshake protocol
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_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), (255)
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SSL 3.0 December 1995
} 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;
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.
5.6.1 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.
5.6.1.1 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.
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:
Freier, Karlton, Kocher [ Page 18 ]
SSL 3.0 December 1995
struct { } HelloRequest;
5.6.1.2 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 identifer 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
by the server.
opaque SessionID<0..32>;
Warning: Servers must not place confidential
information in session identifers or let the
contents of fake session identifers 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
Freier, Karlton, Kocher [ Page 19 ]
SSL 3.0 December 1995
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: 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..2^216-1>;
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 (See Appendix C
for details about backward
compatibility).
random A client-generated random structure.
session_id The ID of a session the client wishes to
use for this connection. This field
should be empty if no session_id is
available or the client wishes to
generate new security parameters.
cipher_suites This is a list of the cryptographic
options supported by the client, 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
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SSL 3.0 December 1995
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. The
security of transmitted application data is
unknown until a valid finished message has
been received.
5.6.1.3 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 C for details about
backward compatibility).
random This structure is generated by the
server and must be different from (and
independent of) ClientHello.random.
session_id This is the identity of the session
corresponding to this connection. If the
ClientHello.session_id was non-empty,
the server will look in its session
cache for a match. If a match is found
and the server is willing to establish
the new connection using the specified
session state, the server will respond
with the same value as was supplied by
the client. This indicates a resumed
session and dictates that the parties
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SSL 3.0 December 1995
must proceed directly to the finished
messages. Otherwise this field will
contain a different value identifying
the new session, or 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.
5.6.2 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
agreement 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 server certificate request.
opaque ASN.1Cert<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
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.
5.6.3 Server key exchange message
The ServerKeyExchange message is sent by the server if it
has no certificate or has a certificate only used for
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SSL 3.0 December 1995
signing (e.g., DSS [DSS] certificates, signing-only RSA
[RSA] certificates) This message is not used if the server
certificate contains Diffie-Hellman [DH1] parameters.
Note: RSA moduli larger than 512 bits may not be
used for key exchange in export software, but
with this message larger RSA keys may be used
as signature-only certificates to sign
temporary shorter RSA keys for key exchange.
enum { anonymous, certified } ServerIdentity;
enum { rsa, diffie_hellman, fortezza }
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 {
select (KeyExchangeAlgorithm) {
case diffie_hellman: ServerDHParams;
case rsa: ServerRSAParams;
case fortezza: struct { }; /*will not occur*/
}
} ServerParams;
struct {
ServerParams params;
select (ServerIdentity) {
case anonymous: struct { };
case certified: digitally-signed struct {
opaque md5_hash[16];
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SSL 3.0 December 1995
opaque sha_hash[20];
};
};
} ServerKeyExchange;
params The server's key exchange parameters.
md5_hash MD5(ClientHello.random +
ServerHello.random +
ServerParams);
sha_hash SHA(ClientHello.random +
ServerHello.random +
ServerParams);
5.6.4 Certificate request
A non-anonymous server can optionally request a certificate
from the client, if appropriate for the selected cipher
suite.
opaque CertificateAuthority <0..2^24-1>;
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3),
dss_fixed_dh(4), rsa_ephemeral_dh(5),
dss_ephemeral_dh(6), fortezza(20), (255)
} ClientCertificateType;
opaque DistinquishedName<1..2^8-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinquishedName 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_authortie
A list of the distinguished names
of acceptable certificate authorities.
Note: DistinquishedName is derived from [X509].
Note: It is a fatal handshake_failure alert for an
anonymous server to request client
identification.
5.6.5 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
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SSL 3.0 December 1995
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.
5.6.6 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 client
certificate alert instead. This error is only a warning,
however the server may respond with a fatal handshake
failure if client authentication is required.
Client certificates are sent using the Certificate message
type defined in Section 5.6.2.
Note: Client Diffie-Hellman certificates must match
the server specified Diffie-Hellman
parameters.
5.6.7 Client key exchange message
The choice of messages depends on which public key
algorithm(s) has (have) been selected. See Section 5.6.3 for
the KeyExchangeAlgorithm.
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case fortezza: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
The information to select the appropriate record structure
is in the pending session state (see Section 5.1).
5.6.7.1 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];
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SSL 3.0 December 1995
} 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
6.1.
5.6.7.2 Fortezza key exchange message
Under Fortezza, the client derives a Token Encryption Key
(TEK) using the 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 also generates a master secret key, wraps it using
the TEK, and sends it to the server. The client generates
IV's for the session keys and master secret key and sends
them also. The client generates a random 48-byte master
secret, encrypts it using the master secret key, and sends
the result:
struct {
opaque y_c<0..128>;
opaque r_c[128];
opaque wrapped_client_write_key[12];
opaque wrapped_server_write_key[12];
opaque wrapped_master_secret_key[12];
opaque client_write_iv[24];
opaque server_write_iv[24];
opaque master_secret_iv[24];
EncryptedPreMasterSecret
encrypted_pre_master_secret[48];
} FortezzaKeys;
y_c The client's Yc value (public key) for
the KEA calculation. If the client sent
a certificate, this value must be empty
since the certificate already contains
this value. However, for anonymous
Fortezza, this value is required and
must be between 64 and 128 bytes,
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SSL 3.0 December 1995
inclusively.
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.
wrapped_master_secret_key
This is the master secret key, wrapped
by the TEK. This key is only used for
the encryption of the pre_master_secret.
client_write_iv This is the IV for the client write key.
server_write_iv This is the IV for the server write key.
master_secret_iv This is the IV for the master secret
key.
encrypted_pre_master_secret
This is a random value, generated by the
client, and encrypted using the master
secret key. It is used to generate the
master secret, as specified in Section
6.1.
Note: The server's R value for KEA is the integer
1. It is fixed and not sent in any message.
5.6.7.3 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
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 { };
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SSL 3.0 December 1995
case explicit: opaque dh_Yc<1..2^16-1>;
} dh_public;
} ClientDiffieHellmanPublic;
dh_Yc The client's Diffie-Hellman public value
(Yc).
5.6.8 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).
digitally-signed struct {
opaque md5_hash[16];
opaque sha_hash[20];
} CertificateVerify;
md5_hash MD5(master_secret +
SHA(handshake_messages +
master_secret));
sha_hash SHA(master_secret +
MD5(handshake_messages +
master_secret));
Here handshake_messages refers to all handshake messages
starting at client hello up to but not including this
message.
5.6.9 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;
struct {
opaque md5_hash[16];
opaque sha_hash[20];
} Finished;
md5_hash MD5(master_secret +
SHA(handshake_messages +
Sender + master_secret));
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SSL 3.0 December 1995
sha_hash SHA(master_secret +
MD5(handshake_messages +
Sender + master_secret));
The hash containd in finished messages sent by the server
incorporate Sender.server; those sent by the client
incoporate Sender.client. The value handshake_messages
includes to all handshake messages starting at client hello
up to but not including the finished messages. This may be
different from handshake_messages in Section 5.6.8 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.
5.7 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.
6. 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.
6.1 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 +
ClientHello.random + ServerHello.random));
6.1.1 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
Freier, Karlton, Kocher [ Page 29 ]
SSL 3.0 December 1995
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.
6.1.2 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.
6.1.3 Fortezza
A random 48-byte pre_master_secret is sent encrypted under
an additional key and an IV reserved for this task. 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.
6.2 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 default to SSL_NULL_WITH_NULL at the
beginning of the SSL Handshake Protocol, indicating that no
authentication or encryption is performed. The handshake
protocol is used to negotiate a more secure CipherSpec and
to generate cryptographic keys.
6.2.1 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
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).
6.2.2 Converting master secret into keys and MAC secrets
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SSL 3.0 December 1995
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(master_secret +
ServerHello.random +
ClientHello.random + 'A')) +
MD5(master_secret + SHA(master_secret +
ServerHello.random +
ClientHello.random + 'BB')) +
MD5(master_secret + SHA(master_secret +
ServerHello.random +
ClientHello.random + 'CCC')) + [...];
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_size]
server_write_key[CipherSPec.key_size]
client_write_IV[CipherSpec.IV_size]
server_write_IV[CipherSpec.IV_size]
Any extra key_block material is discarded.
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
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SSL 3.0 December 1995
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.
6.2.2.1 Export key generation example
SSL_RSA_WITH_RC2_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 December 1995
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 }; /*SSL 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 December 1995
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), (255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
A.4 Handshake protocol
One of the content types predefined to be carried by the
SSL Record Protocol is the 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)
} HandshakeType;
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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[2^8];
} Random;
opaque SessionID<0..32>;
uint8 CipherSuite[2]; /* Cryptographic suite selector */
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>;
struct {
ASN.1Cert certificate_list<1..2^24-1>;
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SSL 3.0 December 1995
} Certificate;
enum { anonymous, certified } ServerIdentity;
enum { rsa, diffie_hellman, fortezza }
KeyExchangeAlgorithm;
struct {
opaque RSA_modulus<1..2^16-1>;
opaque RSA_exponent<1..2^16-1>;
} ServerRSAParams;
RSA_modulus The modulus of the servers temporary
RSA key.
RSA_exponent The public exponent of the servers
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;
struct
select (KeyExchangeAlgorithm) {
case diffie_hellman: ServerDHParams;
case rsa: ServerRSAParams;
case fortezza: { }; /* illegal, will not occur */
}
} ServerParams;
struct {
ServerParams params;
select (ServerIdentity) {
case anonymous: empty;
case certified: digitally-signed struct {
uint8 MD5_hash[16];
uint8 SHA_hash[20];
};
};
} ServerKeyExchange;
opaque CertificateAuthority <0..224-1>;
enum {
RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3),
DSS_fixed_DH(4), RSA_ephemeral_DH(5), DSS_ephemeral_DH(6)
Fortezza(20), (255)
} CertificateType;
opaque DistinquishedName<1..2^8-1>;
struct {
CertificateType certificate_types<1..2^8-1>;
DistinquishedName certificate_authorities<3..2^16-1>;
} CertificateRequest;
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struct { } ServerHelloDone;
A.5 Client authentication and key exchange messages
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: DiffieHellmanClientPublicValue;
case fortezza: 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 wrapped_client_write_key[12];
opaque wrapped_server_write_key[12];
opaque wrapped_master_secret_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;
digitally-signed struct {
uint8 MD5_hash[16];
uint8 SHA_hash[20];
} CertificateVerify;
A.5.1 Handshake finalization message
struct {
opaque md5_hash[16];
opaque sha_hash[20];
} Finished;
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A.6 The CipherSuite
The following values define the CipherKind 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_WITH_RC4_40_MD5 = { 0x01,0x01 };
CipherSuite SSL_RSA_WITH_RC4_40_SHA = { 0x01,0x02 };
CipherSuite SSL_RSA_WITH_RC4_128_MD5 = { 0x01,0x03 };
CipherSuite SSL_RSA_WITH_RC4_128_SHA = { 0x01,0x04 };
CipherSuite SSL_RSA_WITH_RC2_40_MD5 = { 0x01,0x05 };
CipherSuite SSL_RSA_WITH_RC2_40_SHA = { 0x01,0x06 };
CipherSuite SSL_RSA_WITH_RC2_128_MD5 = { 0x01,0x07 };
CipherSuite SSL_RSA_WITH_RC2_128_SHA = { 0x01,0x08 };
CipherSuite SSL_RSA_WITH_IDEA_CBC_MD5= { 0x01,0x09 };
CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA= { 0x01,0x0A };
CipherSuite SSL_RSA_WITH_DES40_CBC_MD5= { 0x01,0x0B };
CipherSuite SSL_RSA_WITH_DES40_CBC_SHA= { 0x01,0x0C };
CipherSuite SSL_RSA_WITH_DES_CBC_MD5 = { 0x01,0x0D };
CipherSuite SSL_RSA_WITH_DES_CBC_SHA = { 0x01,0x0E };
CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_MD5= { 0x01,0x0F };
CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA= { 0x01,0x10 };
The following CipherSuite definitions are used for server-
authenticated (and optionally client-authenticated) Diffie-
Hellman. "DH" denotes cipher suites in which the servers
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_WITH_DES40_CBC_SHA = { 0x02,0x01 };
CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA = { 0x02,0x02 };
CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA= { 0x02,0x03 };
CipherSuite SSL_DH_RSA_WITH_DES40_CBC_SHA = { 0x02,0x04 };
CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA = { 0x02,0x05 };
CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA= { 0x02,0x06 };
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CipherSuite SSL_DHE_DSS_WITH_DES40_CBC_SHA = { 0x03,0x01 };
CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA = { 0x03,0x02 };
CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA= { 0x03,0x03 };
CipherSuite SSL_DHE_RSA_WITH_DES40_CBC_SHA = { 0x03,0x04 };
CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA = { 0x03,0x05 };
CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA= { 0x03,0x06 };
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_WITH_RC4_40_MD5 = { 0x04,0x01 };
CipherSuite SSL_DH_anon_WITH_RC4_40_SHA = { 0x04,0x02 };
CipherSuite SSL_DH_anon_WITH_RC4_128_MD5 = { 0x04,0x03 };
CipherSuite SSL_DH_anon_WITH_RC4_128_SHA = { 0x04,0x04 };
CipherSuite SSL_DH_anon_WITH_RC2_40_MD5 = { 0x04,0x05 };
CipherSuite SSL_DH_anon_WITH_RC2_40_SHA = { 0x04,0x06 };
CipherSuite SSL_DH_anon_WITH_RC2_128_MD5 = { 0x04,0x07 };
CipherSuite SSL_DH_anon_WITH_RC2_128_SHA = { 0x04,0x08 };
CipherSuite SSL_DH_anon_WITH_IDEA_CBC_MD5 = { 0x04,0x09 };
CipherSuite SSL_DH_anon_WITH_IDEA_CBC_SHA = { 0x04,0x0A };
CipherSuite SSL_DH_anon_WITH_DES40_CBC_MD5= { 0x04,0x0B };
CipherSuite SSL_DH_anon_WITH_DES40_CBC_SHA= { 0x04,0x0C };
CipherSuite SSL_DH_anon_WITH_DES_CBC_MD5 = { 0x04,0x0D };
CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA = { 0x04,0x0E };
CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_MD5={ 0x04,0x0F };
CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA={ 0x04,0x10 };
The final cipher suite is for the Fortezza token.
CipherSuite SSL_FORTEZZA_WITH_FORTEZZA_CBC_SHA
{ 0x10,0x00 };
Note: Any cipher types 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.
A.7 The CipherSpec
A cipher suite defines 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;
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SSL 3.0 December 1995
struct {
BulkCipherAlgorithm bulk_cipher_algorithm;
MACAlgorithm mac_algorithm;
CipherType cipher_type;
IsExportable is_exportable
uint8 hash_size;
uint8 key_size;
uint8 IV_size;
opaque IV[CipherSpec.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.
bulk cipher A symmetric encryption algorithm used to
encrypt large quantities of data.
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.
MAC A Message Authentication Code (MAC) 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
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SSL 3.0 December 1995
encrypted with the public key can only
be decrypted with the associated private
key. Conversely, messages signed with
the private key canbe verified with the
public key.
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.
salt Non-secret random data used to make
export encryption keys resist
precomputation attacks.
secure hash function
A one-way transformation that converts
an arbitrary amount of data into a fixed
length hash. It is hard to reverse the
transformation or to find collisions.
MD5 and SHA are examples of secure hash
functions.
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
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].
symmetric cipher See bulk cipher.
Appendix C
C. Version 2.0 Backward Compatibility
Version 3.0 clients that support Version 2.0 servers must
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SSL 3.0 December 1995
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. Implementors should make every effort
to move forward as quickly as possible.
Version 3.0 provides better mechanisms for
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_128_CBC_WITH_MD5 =
{ 0x03,0x00,0x80 };
V2CipherSpec SSL_RC2_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 };
C.1 Version 2 client hello
The Version 2.0 client hello message is presented below
using this documents 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;
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SSL 3.0 December 1995
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 highers version of protocol
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 clients
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. The
CipherSpecs are given first. There must
be at least one CipherSpec acceptable to
the server.
session_id If this fields length is not zero, it
will contain the identification for a
session that the client wishes to
resume.
challenge The clients challenge to the server
for the server to identify itself. The
Version 3.0 server will use the
challenge data as the client random data
as specified in this Version 3.0
protocol.
Note: Requests to resume an SSL 3.0 session should
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SSL 3.0 December 1995
use an SSL 3.0 client hello.
C.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 #7 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 bytes of
the random PKCS padding for the RSA encryption of the
ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03
(the other padding bytes are random). After decrypting the
ENCRYPTED-KEY-DATA field, servers that support 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 D
D. 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.
D.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.
D.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
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SSL 3.0 December 1995
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 others 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 6.1). The master_secret is
required to generate the finished messages, encryption keys,
and MAC secrets (see Sections 5.6.9 and 6.2.2). By sending a
correct finished message, parties thus prove that they know
the correct pre_master_secret.
D.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 servers 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
servers private key, it will be infeasible for them to
decode the pre_master_secret.
With Diffie-Hellman or Fortezza, the servers public
parameters are contained in the server key exchange message
and the clients 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.
D.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
servers 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 servers RSA or DSS
certificate. The signature includes the current
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SSL 3.0 December 1995
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 servers certificate, the client
encrypts a pre_master_secret with the servers public key.
By successfully decoding the pre_master_secret and producing
a correct finished message, the server demonstrates that it
knows the private key corresponding to the server
certificate.
When RSA is used for key exchange, clients are authenticated
using the certificate verify message (see Section 5.6.8).
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.
D.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.
D.1.1.4 Fortezza
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SSL 3.0 December 1995
Fortezzas 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 accessable outside the token. The
encrypted pre_master_secret is sent to the server in a
client key exchange message.
D.1.2 Version rollback attacks
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.
D.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.
D.1.4 Resuming sessions
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed
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SSL 3.0 December 1995
with the sessions master_secret. Provided that the
master_secret has not been compromised and that the secure
hash operations used to produce the encryption keys and MAC
secrets are secure, the connection should be secure and
effectively independent from previous connections. Attackers
cannot use known encryption keys or MAC secrets to
compromise the master_secret without breaking the secure
hash operations (which use both SHA and MD5).
Sessions cannot be resumed unless both the client and server
agree. If either party suspects that the session may have
been compromised, or that certificates may have expired or
been revoked, it should force a full handshake. An upper
limit of 24 hours is suggested for session ID lifetimes,
since an attacker who obtains a master_secret may be able to
impersonate the compromised party until the corresponding
session ID is retired. Applications that may be run in
relatively insecure environments should not write session
IDs to stable storage.
D.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.
D.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 type, the message length, and the message contents.
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 others
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
Freier, Karlton, Kocher [ Page 48 ]
SSL 3.0 December 1995
as the MAC.
Note: MAC secrets may be larger than encryption
keys, so messages can remain tamper resistant
even if encryption keys are broken.
D.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.
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 E
E. 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
Freier, Karlton, Kocher [ Page 49 ]
SSL 3.0 December 1995
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 users own
responsibility.
Freier, Karlton, Kocher [ Page 50 ]
SSL 3.0 December 1995
References
[DH1] W. Diffie and M. E. Hellman, "New Directons 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 XX, "Digital Signature Standard,"
National Institute of Standards and Technology, U.S.
Department of Commerce, DRAFT, 1 Feb 1993
[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.
[FOR] NSA X22, Document # PD4002103-1.01, "Fortezza:
Application Implementors Guide," April 6, 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.
[RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782
Freier, Karlton, Kocher [ Page 51 ]
SSL 3.0 December 1995
[SCH] B. Schneier. Applied Cryptography: Protocols,
Algorithms, and Source Code in C, Published by John Wiley &
Sons, Inc. 1994.
[SHA] NIST FIPS PUB XX, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of
Commerce, DRAFT, 1 Feb 1993
[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
501 East Middlefield Rd. Box 8243
Mountain View, CA 94043 Stanford, CA 94039
freier@netscape.com pck@netcom.com
Philip L. Karlton
Netscape Communications
501 East Middlefield Rd.
Mountain View, CA 94043
karlton@netscape.com
Other contributors
Martin Abadi Kipp E.B. Hickman
Digital Equipment Corporation Netscape Communications
ma@pa.dec.com 501 East Middlefield Rd.
Mountain View, CA 94043
kipp@netscape.com
Taher Elgamal Jim Roskind
Netscape Communications Netscape Communications
501 East Middlefield Rd. 501 East Middlefield Rd
Mountain View, CA 94043 Mountain View, CA 94043
elgamal@netscape.com jar@netscape.com
Anil Gangolli Micheal J. Sabin, Ph. D.
Netscape Communications Consulting Engineer
501 East Middlefield Rd 833 Mango Ave.
Mountain View, CA 94043 Sunnyvale, CA 94087
gangolli@netscape.com msabin@netcom.com
Freier, Karlton, Kocher [ Page 52 ]
SSL 3.0 December 1995
Early reviewers
Robert Baldwin Eric Murray
RSA Data Security, Inc. ericm@lne.com
baldwin@rsa.com
George Cox Don Stephenson
Intel Corporation Sun Microsystems
cox@ibeam.jf.intel.com don.stephenson@eng.sun.com
Cheri Dowell Joe Tardo
Sun Microsystems General Magic
cheri@eng.sun.com tardo@genmagic.com
Burt Kaliski
RSA Data Security, Inc.
burt@rsa.com
Freier, Karlton, Kocher [ Page 53 ]
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