Internet Engineering Task Force (IETF) A. Freier
Request for Comments: 6101 P. Karlton
Category: Historic Netscape Communications
ISSN: 2070-1721 P. Kocher
Independent Consultant
August 2011
The Secure Sockets Layer (SSL) Protocol Version 3.0
Abstract
This document is published as a historical record of the SSL 3.0
protocol. The original Abstract follows.
This document specifies version 3.0 of the Secure Sockets Layer (SSL
3.0) 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.
Foreword
Although the SSL 3.0 protocol is a widely implemented protocol, a
pioneer in secure communications protocols, and the basis for
Transport Layer Security (TLS), it was never formally published by
the IETF, except in several expired Internet-Drafts. This allowed no
easy referencing to the protocol. We believe a stable reference to
the original document should exist and for that reason, this document
describes what is known as the last published version of the SSL 3.0
protocol, that is, the November 18, 1996, version of the protocol.
There were no changes to the original document other than trivial
editorial changes and the addition of a "Security Considerations"
section. However, portions of the original document that no longer
apply were not included. Such as the "Patent Statement" section, the
"Reserved Ports Assignment" section, and the cipher-suite registrator
note in the "The CipherSuite" section. The "US export rules"
discussed in the document do not apply today but are kept intact to
provide context for decisions taken in protocol design. The "Goals
of This Document" section indicates the goals for adopters of SSL
3.0, not goals of the IETF.
The authors and editors were retained as in the original document.
The editor of this document is Nikos Mavrogiannopoulos
(nikos.mavrogiannopoulos@esat.kuleuven.be). The editor would like to
thank Dan Harkins, Linda Dunbar, Sean Turner, and Geoffrey Keating
for reviewing this document and providing helpful comments.
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RFC 6101 The SSL Protocol Version 3.0 August 2011
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for the historical record.
This document defines a Historic Document for the Internet community.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6101.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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RFC 6101 The SSL Protocol Version 3.0 August 2011
Table of Contents
1. Introduction ....................................................5
2. Goals ...........................................................5
3. Goals of This Document ..........................................6
4. Presentation Language ...........................................6
4.1. Basic Block Size ...........................................7
4.2. Miscellaneous ..............................................7
4.3. Vectors ....................................................7
4.4. Numbers ....................................................8
4.5. Enumerateds ................................................8
4.6. Constructed Types ..........................................9
4.6.1. Variants ...........................................10
4.7. Cryptographic Attributes ..................................11
4.8. Constants .................................................12
5. SSL Protocol ...................................................12
5.1. Session and Connection States .............................12
5.2. Record Layer ..............................................14
5.2.1. Fragmentation ......................................14
5.2.2. Record Compression and Decompression ...............15
5.2.3. Record Payload Protection and the CipherSpec .......16
5.3. Change Cipher Spec Protocol ...............................18
5.4. Alert Protocol ............................................18
5.4.1. Closure Alerts .....................................19
5.4.2. Error Alerts .......................................20
5.5. Handshake Protocol Overview ...............................21
5.6. Handshake Protocol ........................................23
5.6.1. Hello messages .....................................24
5.6.2. Server Certificate .................................28
5.6.3. Server Key Exchange Message ........................28
5.6.4. Certificate Request ................................30
5.6.5. Server Hello Done ..................................31
5.6.6. Client Certificate .................................31
5.6.7. Client Key Exchange Message ........................31
5.6.8. Certificate Verify .................................34
5.6.9. Finished ...........................................35
5.7. Application Data Protocol .................................36
6. Cryptographic Computations .....................................36
6.1. Asymmetric Cryptographic Computations .....................36
6.1.1. RSA ................................................36
6.1.2. Diffie-Hellman .....................................37
6.1.3. FORTEZZA ...........................................37
6.2. Symmetric Cryptographic Calculations and the CipherSpec ...37
6.2.1. The Master Secret ..................................37
6.2.2. Converting the Master Secret into Keys and
MAC Secrets ........................................37
7. Security Considerations ........................................39
8. Informative References .........................................40
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Appendix A. Protocol Constant Values ..............................42
A.1. Record Layer ...............................................42
A.2. Change Cipher Specs Message ................................43
A.3. Alert Messages .............................................43
A.4. Handshake Protocol .........................................44
A.4.1. Hello Messages .........................................44
A.4.2. Server Authentication and Key Exchange Messages ........45
A.5. Client Authentication and Key Exchange Messages ............46
A.5.1. Handshake Finalization Message .........................47
A.6. The CipherSuite ............................................47
A.7. The CipherSpec .............................................49
Appendix B. Glossary ..............................................50
Appendix C. CipherSuite Definitions ...............................53
Appendix D. Implementation Notes ..................................56
D.1. Temporary RSA Keys .........................................56
D.2. Random Number Generation and Seeding .......................56
D.3. Certificates and Authentication ............................57
D.4. CipherSuites ...............................................57
D.5. FORTEZZA ...................................................57
D.5.1. Notes on Use of FORTEZZA Hardware ......................57
D.5.2. FORTEZZA Cipher Suites .................................58
D.5.3. FORTEZZA Session Resumption ............................58
Appendix E. Version 2.0 Backward Compatibility ....................59
E.1. Version 2 Client Hello .....................................59
E.2. Avoiding Man-in-the-Middle Version Rollback ..............61
Appendix F. Security Analysis .....................................61
F.1. Handshake Protocol .........................................61
F.1.1. Authentication and Key Exchange ........................61
F.1.2. Version Rollback Attacks ...............................64
F.1.3. Detecting Attacks against the Handshake Protocol .......64
F.1.4. Resuming Sessions ......................................65
F.1.5. MD5 and SHA ............................................65
F.2. Protecting Application Data ................................65
F.3. Final Notes ................................................66
Appendix G. Acknowledgements ......................................66
G.1. Other Contributors .........................................66
G.2. Early Reviewers ............................................67
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1. 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 [RFC0793]), 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:
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], 3DES [3DES], RC4 [SCH]).
o The peer's identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA [RSA], DSS [DSS]).
o The connection is reliable. Message transport includes a message
integrity check using a keyed Message Authentication Code (MAC)
[RFC2104]. Secure hash functions (e.g., SHA, MD5) are used for
MAC computations.
2. Goals
The goals of SSL protocol version 3.0, 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.
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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.
3. 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 been written with this in
mind, and it is intended to reflect the needs of those two groups.
For that reason, many of the algorithm-dependent data structures and
rules are included in the body of the text (as opposed to in an
appendix), providing easier access to them.
This document is not intended to supply any details of service
definition or interface definition, although it does cover select
areas of policy as they are required for the maintenance of solid
security.
4. Presentation Language
This document deals with the formatting of data in an external
representation. The following very basic and somewhat casually
defined presentation syntax will be used. The syntax draws from
several sources in its structure. Although it resembles the
programming language "C" in its syntax and External Data
Representation (XDR) [RFC1832] in both its syntax and intent, it
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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 bytes, from left to right, from top to
bottom. From the byte stream, a multi-byte item (a numeric in the
example) is formed (using C notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ...
| byte[n-1];
This byte ordering for multi-byte values is the commonplace network
byte order or big-endian format.
4.2. Miscellaneous
Comments begin with "/*" and end with "*/". Optional components are
denoted by enclosing them in "[[ ]]" double brackets. Single-byte
entities containing uninterpreted data are of type opaque.
4.3. Vectors
A vector (single dimensioned array) is a stream of homogeneous data
elements. The size of the vector may be specified at documentation
time or left unspecified until runtime. In either case, the length
declares the number of bytes, not the number of elements, in the
vector. The syntax for specifying a new type T' that is a fixed-
length vector of type T is
T T'[n];
Here, T' occupies n bytes in the data stream, where n is a multiple
of the size of T. The length of the vector is not included in the
encoded stream.
In the following example, Datum is defined to be three consecutive
bytes that the protocol does not interpret, while Data is three
consecutive Datum, consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3 byte vectors */
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Variable-length vectors are defined by specifying a subrange of legal
lengths, inclusively, using the notation <floor..ceiling>. When
encoded, the actual length precedes the vector's contents in the byte
stream. The length will be in the form of a number consuming as many
bytes as required to hold the vector's specified maximum (ceiling)
length. A variable-length vector with an actual length field of zero
is referred to as an empty vector.
T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain
between 300 and 400 bytes of type opaque. It can never be empty.
The actual length field consumes two bytes, a uint16, sufficient to
represent the value 400 (see Section 4.4). On the other hand, longer
can represent up to 800 bytes of data, or 400 uint16 elements, and it
may be empty. Its encoding will include a two-byte actual length
field prepended to the vector.
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
4.4. Numbers
The basic numeric data type is an unsigned byte (uint8). All larger
numeric data types are formed from fixed-length series of bytes
concatenated as described in Section 4.1 and are also unsigned. The
following numeric types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
4.5. Enumerateds
An additional sparse data type is available called enum. A field of
type enum can only assume the values declared in the definition.
Each definition is a different type. Only enumerateds of the same
type may be assigned or compared. Every element of an enumerated
must be assigned a value, as demonstrated in the following example.
Since the elements of the enumerated are not ordered, they can be
assigned any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn), [[(n)]] } Te;
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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;
Optionally, one may specify a value without its associated tag to
force the width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data
stream but can only assume the values 1, 2, or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the
defined type. In the first example, a fully qualified reference to
the second element of the enumeration would be Color.blue. Such
qualification is not required if the target of the assignment is well
specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation,
the numerical information may be omitted.
enum { low, medium, high } Amount;
4.6. Constructed Types
Structure types may be constructed from primitive types for
convenience. Each specification declares a new, unique type. The
syntax for definition is much like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
The fields within a structure may be qualified using the type's name
using a syntax much like that available for enumerateds. For
example, T.f2 refers to the second field of the previous declaration.
Structure definitions may be embedded.
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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) { /* value of selector is implicit */
case apple: V1; /* VariantBody, tag = apple */
case orange: V2; /* VariantBody, tag = orange */
} variant_body; /* optional label on variant */
} VariantRecord;
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Variant structures may be qualified (narrowed) by specifying a value
for the selector prior to the type. For example, an
orange VariantRecord
is a narrowed type of a VariantRecord containing a variant_body of
type V2.
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 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 Signature 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.
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4.8. Constants
Typed constants can be defined for purposes of specification by
declaring a symbol of the desired type and assigning values to it.
Under-specified types (opaque, variable-length vectors, and
structures that contain opaque) cannot be assigned values. No fields
of a multi-element structure or vector may be elided.
For example,
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4};/* assigns f1 = 1, f2 = 4 */
5. 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 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 5.3), 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.
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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. (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 Cipher Block Chaining
(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.
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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.
5.2. Record Layer
The SSL record layer receives uninterpreted data from higher layers
in non-empty blocks of arbitrary size.
5.2.1. Fragmentation
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).
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.
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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; /* 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.
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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_NULL, which
does not provide any security.
Once the handshake is complete, the two parties have shared secrets
that 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;
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
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;
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The MAC is generated as:
hash(MAC_write_secret + pad_2 +
hash(MAC_write_secret + pad_1 + seq_num +
SSLCompressed.type + SSLCompressed.length +
SSLCompressed.fragment));
where "+" denotes concatenation.
pad_1: The character 0x36 repeated 48 times for MD5 or 40 times for
SHA.
pad_2: The character 0x5c repeated 48 times for MD5 or 40 times for
SHA.
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
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.
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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, 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 read pending state
into the read 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 5.4.2). When resuming a previous
session, the change cipher spec message is sent after the hello
messages.
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.
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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;
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.
Either party may initiate a close by sending a close_notify alert.
Any data received after a closure alert is ignored.
Each party is required to send a close_notify alert before closing
the write side of the connection. It is required that the other
party respond with a close_notify alert of its own and close down the
connection immediately, discarding any pending writes. It is not
required for the initiator of the close to wait for the responding
close_notify alert before closing the read side of the connection.
NB: It is assumed that closing a connection reliably delivers pending
data before destroying the transport.
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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 a 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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5.5. 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 an 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 CipherSpec into the current
CipherSpec. 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 CipherSpec, and send its finished message under the new
CipherSpec. At this point, the handshake is complete and the client
and server may begin to exchange application layer data. (See flow
chart below.)
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RFC 6101 The SSL Protocol Version 3.0 August 2011
Client Server
ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
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.
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RFC 6101 The SSL Protocol Version 3.0 August 2011
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.
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_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
} body;
} Handshake;
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The handshake protocol messages are presented 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:
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.
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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 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.
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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 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.
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Forward compatibility note: In the interests of forward
compatibility, it is permitted for a client hello message to include
extra data after the compression methods. This data must be included
in the handshake hashes, but must otherwise be ignored.
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 E for details about backward compatibility).
random: This structure is generated by the server and must be
different from (and independent of) ClientHello.random.
session_id: This is the identity of the session corresponding to
this connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
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.
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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 exchange algorithm, and is generally an
X.509.v3 certificate (or a modified X.509 certificate in the case of
FORTEZZA(tm) [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 followed
by any certificate authority certificates proceeding sequentially
upward.
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 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 KEA
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_kea }
KeyExchangeAlgorithm;
struct {
opaque rsa_modulus<1..2^16-1>;
opaque rsa_exponent<1..2^16-1>;
} ServerRSAParams;
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rsa_modulus: The modulus of the server's temporary RSA key.
rsa_exponent: The public exponent of the server's temporary RSA key.
struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} ServerDHParams; /* Ephemeral DH parameters */
dh_p: The prime modulus used for the Diffie-Hellman operation.
dh_g: The generator used for the Diffie-Hellman operation.
dh_Ys: The server's Diffie-Hellman public value (gX mod p).
struct {
opaque r_s [128];
} ServerFortezzaParams;
r_s: Server random number for FORTEZZA KEA (Key Exchange Algorithm).
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_kea:
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);
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RFC 6101 The SSL Protocol Version 3.0 August 2011
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;
5.6.4. 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_kea(20),
(255)
} ClientCertificateType;
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
DistinguishedName certificate_authorities<3..2^16-1>;
} CertificateRequest;
certificate_types: This field is a list of the types of certificates
requested, sorted in order of the server's preference.
certificate_authorities: A list of the distinguished names of
acceptable certificate authorities.
Note: DistinguishedName is derived from [X509].
Note: It is a fatal handshake_failure alert for an anonymous server
to request client identification.
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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 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_certificate alert instead. This alert 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 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
definition.
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case fortezza_kea: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
The information to select the appropriate record structure is in the
pending session state (see Section 5.1).
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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 premaster 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 6.1.
5.6.7.2. FORTEZZA Key Exchange Message
Under FORTEZZA, 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 IVs 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:
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RFC 6101 The SSL Protocol Version 3.0 August 2011
struct {
opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[40];
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
premaster secret.
pre_master_secret: A random value, generated by the client and used
to generate the master secret, as specified in Section 6.1. In
the above structure, it is encrypted using the TEK.
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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 { };
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).
struct {
Signature signature;
} CertificateVerify;
CertificateVerify.signature.md5_hash
MD5(master_secret + pad_2 +
MD5(handshake_messages + master_secret + pad_1));
Certificate.signature.sha_hash
SHA(master_secret + pad_2 +
SHA(handshake_messages + master_secret + pad_1));
pad_1: This is identical to the pad_1 defined in Section 5.2.3.1.
pad_2: This is identical to the pad_2 defined in Section 5.2.3.1.
Here, handshake_messages refers to all handshake messages starting at
client hello up to but not including this message.
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5.6.9. Finished
A finished message is always sent immediately after a change cipher
spec 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 encrypted 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 + pad2 + MD5(handshake_messages + Sender
+ master_secret + pad1));
sha_hash: SHA(master_secret + pad2 + SHA(handshake_messages + Sender
+ master_secret + pad1));
handshake_messages: All of the data from all handshake messages up
to but not including this message. This is only data visible at
the handshake layer and does not include record layer headers.
It is a fatal error if a finished message is not preceeded by a
change cipher spec message at the appropriate point in the handshake.
The hash contained in finished messages sent by the server
incorporate Sender.server; those sent by the client incorporate
Sender.client. The value handshake_messages includes all handshake
messages starting at client hello up to but not including this
finished message. This may be different from handshake_messages in
Section 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.
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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 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.
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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 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.
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 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.
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 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
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RFC 6101 The SSL Protocol Version 3.0 August 2011
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.
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);
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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.
6.2.2.1. 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];
7. Security Considerations
See Appendix F.
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RFC 6101 The SSL Protocol Version 3.0 August 2011
8. Informative References
[DH1] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory V.
IT-22, n. 6, pp. 74-84, June 1977.
[SSL-2] Hickman, K., "The SSL Protocol", February 1995.
[3DES] Tuchman, W., "Hellman Presents No Shortcut Solutions To
DES", IEEE Spectrum, v. 16, n. 7, pp 40-41, July 1979.
[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, May 1994.
[FOR] NSA X22, "FORTEZZA: Application Implementers Guide",
Document # PD4002103-1.01, April 1995.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, October 1985.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC1832] Srinivasan, R., "XDR: External Data Representation
Standard", RFC 1832, August 1995.
Freier, et al. Historic [Page 40]
RFC 6101 The SSL Protocol Version 3.0 August 2011
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[IDEA] Lai, X., "On the Design and Security of Block Ciphers",
ETH Series in Information Processing, v. 1, Konstanz:
Hartung-Gorre Verlag, 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] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", Communications of the ACM v. 21, n. 2 pp.
120-126., February 1978.
[SCH] Schneier, B., "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", John Wiley & Sons,
1994.
[SHA] NIST FIPS PUB 180-1, "Secure Hash Standard", May 1994.
National Institute of Standards and Technology, U.S.
Department of Commerce, DRAFT
[X509] CCITT, "The Directory - Authentication Framework",
Recommendation X.509 , 1988.
[RSADSI] RSA Data Security, Inc., "Unpublished works".
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RFC 6101 The SSL Protocol Version 3.0 August 2011
Appendix A. Protocol Constant Values
This section describes protocol types and constants.
A.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;
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];
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RFC 6101 The SSL Protocol Version 3.0 August 2011
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;
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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)
} 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>;
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} ClientHello;
struct {
ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
} ServerHello;
A.4.2. Server Authentication and Key Exchange Messages
opaque ASN.1Cert<2^24-1>;
struct {
ASN.1Cert certificate_list<1..2^24-1>;
} Certificate;
enum { rsa, diffie_hellman, fortezza_kea } 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_kea:
ServerFortezzaParams params;
};
} ServerKeyExchange;
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RFC 6101 The SSL Protocol Version 3.0 August 2011
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_MISSI(20), (255)
} CertificateType;
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_kea: FortezzaKeys;
} exchange_keys;
} ClientKeyExchange;
struct {
ProtocolVersion client_version;
opaque random[46];
} PreMasterSecret;
struct {
public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecret;
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struct {
opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[40];
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
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.
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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 = { 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 };
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The final cipher suites are for the FORTEZZA token.
CipherSuite SSL_FORTEZZA_KEA_WITH_NULL_SHA = { 0X00,0X1C };
CipherSuite SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA = { 0x00,0x1D };
CipherSuite SSL_FORTEZZA_KEA_WITH_RC4_128_SHA = { 0x00,0x1E };
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.
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;
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Appendix B. Glossary
application protocol: An application protocol is a protocol that
normally layers directly on top of the transport layer (e.g.,
TCP/IP [RFC0793]/[RFC0791]). Examples include HTTP [RFC1945],
TELNET [RFC0959], FTP [RFC0854], 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 (CBC) mode: 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
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 [DES] [3DES].
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Digital Signature Standard: (DSS) A standard for digital signing,
including the Digital Signature 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.
IDEA: A 64-bit block cipher designed by Xuejia Lai and James Massey
[IDEA].
Message Authentication Code (MAC): A Message Authentication Code is
a one-way hash computed from a message and some secret data. 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 [RFC1321] 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
computationally hard to reverse the transformation or to find
collisions. MD5 and SHA are examples of one-way hash functions.
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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 a 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: An 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].
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.
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Appendix C. CipherSuite Definitions
CipherSuite Is Key Cipher Hash
Exportable Exchange
SSL_NULL_WITH_NULL_NULL * NULL NULL NULL
SSL_RSA_WITH_NULL_MD5 * RSA NULL MD5
SSL_RSA_WITH_NULL_SHA * RSA NULL SHA
SSL_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 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_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5
SSL_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
SSL_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_CBC SHA
SSL_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA
SSL_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA
SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_CBC SHA
SSL_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA
SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA
SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_CBC SHA
SSL_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA
SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA
SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC SHA
SSL_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA
SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA
SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC SHA
SSL_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA
SSL_DH_anon_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5
SSL_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5
SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA
SSL_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA
SSL_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA
SSL_FORTEZZA_KEA_WITH_NULL_SHA FORTEZZA_KEA NULL SHA
SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA FORTEZZA_KEA FORTEZZA_CBC SHA
SSL_FORTEZZA_KEA_WITH_RC4_128_SHA FORTEZZA_KEA RC4_128 SHA
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+----------------+------------------------------+-------------------+
| Key Exchange | Description | Key Size Limit |
| Algorithm | | |
+----------------+------------------------------+-------------------+
| DHE_DSS | Ephemeral DH with DSS | None |
| | signatures | |
| DHE_DSS_EXPORT | Ephemeral DH with DSS | DH = 512 bits |
| | signatures | |
| DHE_RSA | Ephemeral DH with RSA | None |
| | signatures | |
| DHE_RSA_EXPORT | Ephemeral DH with RSA | DH = 512 bits, |
| | signatures | RSA = none |
| DH_anon | Anonymous DH, no signatures | None |
| DH_anon_EXPORT | Anonymous DH, no signatures | DH = 512 bits |
| DH_DSS | DH with DSS-based | None |
| | certificates | |
| DH_DSS_EXPORT | DH with DSS-based | DH = 512 bits |
| | certificates | |
| DH_RSA | DH with RSA-based | None |
| | certificates | |
| DH_RSA_EXPORT | DH with RSA-based | DH = 512 bits, |
| | certificates | RSA = none |
| FORTEZZA_KEA | FORTEZZA KEA. Details | N/A |
| | unpublished | |
| NULL | No key exchange | N/A |
| RSA | RSA key exchange | None |
| RSA_EXPORT | RSA key exchange | RSA = 512 bits |
+----------------+------------------------------+-------------------+
Table 1
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.
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+--------------+--------+-----+-------+-------+-------+------+------+
| Cipher | Cipher | IsE | Key | Exp. | Effec | IV | Bloc |
| | Type | xpo | Mater | Key | tive | Size | k |
| | | rta | ial | Mater | Key | | Size |
| | | ble | | ial | Bits | | |
+--------------+--------+-----+-------+-------+-------+------+------+
| NULL | Stream | * | 0 | 0 | 0 | 0 | N/A |
| FORTEZZA_CBC | Block | | NA | 12 | 96 | 20 | 8 |
| | | | (**) | (**) | (**) | (**) | |
| IDEA_CBC | Block | | 16 | 16 | 128 | 8 | 8 |
| RC2_CBC_40 | Block | * | 5 | 16 | 40 | 8 | 8 |
| RC4_40 | Stream | * | 5 | 16 | 40 | 0 | N/A |
| RC4_128 | Stream | | 16 | 16 | 128 | 0 | N/A |
| DES40_CBC | Block | * | 5 | 8 | 40 | 8 | 8 |
| DES_CBC | Block | | 8 | 8 | 56 | 8 | 8 |
| 3DES_EDE_CBC | Block | | 24 | 24 | 168 | 8 | 8 |
+--------------+--------+-----+-------+-------+-------+------+------+
* Indicates IsExportable is true.
** FORTEZZA uses its own key and IV generation algorithms.
Table 2
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 Function | Hash Size | Padding Size |
+---------------+-----------+--------------+
| NULL | 0 | 0 |
| MD5 | 16 | 48 |
| SHA | 20 | 40 |
+---------------+-----------+--------------+
Table 3
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Appendix 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 approximately 100 such timer values.
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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 that 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
that provide no or minimal security. A proper implementation will
probably not support many cipher suites. For example, 40-bit
encryption is easily broken, so implementations requiring strong
security should not allow 40-bit keys. Similarly, anonymous Diffie-
Hellman is strongly discouraged because it cannot prevent man-in-the-
middle attacks. Applications should also enforce minimum and maximum
key sizes. For example, certificate chains containing 512-bit RSA
keys or signatures are not appropriate for high-security
applications.
D.5. FORTEZZA
This section describes implementation details for cipher suites that
make use of the FORTEZZA hardware encryption system.
D.5.1. Notes on Use of FORTEZZA Hardware
A complete explanation of all issues regarding the use of FORTEZZA
hardware is outside the scope of this document. However, there are a
few special requirements of SSL that deserve mention.
Because SSL is a full duplex protocol, two crypto states must be
maintained, one for reading and one for writing. There are also a
number of circumstances that can result in the crypto state in the
FORTEZZA card being lost. For these reasons, it's recommended that
the current crypto state be saved after processing a record, and
loaded before processing the next.
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After the client generates the TEK, it also generates two message
encryption keys (MEKs), one for reading and one for writing. After
generating each of these keys, the client must generate a
corresponding IV and then save the crypto state. The client also
uses the TEK to generate an IV and encrypt the premaster secret. All
three IVs are sent to the server, along with the wrapped keys and the
encrypted premaster secret in the client key exchange message. At
this point, the TEK is no longer needed, and may be discarded.
On the server side, the server uses the master IV and the TEK to
decrypt the premaster secret. It also loads the wrapped MEKs into
the card. The server loads both IVs to verify that the IVs match the
keys. However, since the card is unable to encrypt after loading an
IV, the server must generate a new IV for the server write key. This
IV is discarded.
When encrypting the first encrypted record (and only that record),
the server adds 8 bytes of random data to the beginning of the
fragment. These 8 bytes are discarded by the client after
decryption. The purpose of this is to synchronize the state on the
client and server resulting from the different IVs.
D.5.2. FORTEZZA Cipher Suites
5) FORTEZZA_NULL_WITH_NULL_SHA: Uses the full FORTEZZA key exchange,
including sending server and client write keys and IVs.
D.5.3. FORTEZZA Session Resumption
There are two possibilities for FORTEZZA session restart: 1) Never
restart a FORTEZZA session. 2) Restart a session with the previously
negotiated keys and IVs.
Never restarting a FORTEZZA session:
Clients who never restart FORTEZZA sessions should never send session
IDs that were previously used in a FORTEZZA session as part of the
ClientHello. Servers who never restart FORTEZZA sessions should
never send a previous session id on the ServerHello if the negotiated
session is FORTEZZA.
Restart a session:
You cannot restart FORTEZZA on a session that has never done a
complete FORTEZZA key exchange (that is, you cannot restart FORTEZZA
if the session was an RSA/RC4 session renegotiated for FORTEZZA). If
you wish to restart a FORTEZZA session, you must save the MEKs and
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IVs from the initial key exchange for this session and reuse them for
any new connections on that session. This is not recommended, but it
is possible.
Appendix 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 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.
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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;
session 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 supported by the client
(equals ProtocolVersion.version; see Appendix A.1).
cipher_spec_length: This field is the total length of the field
cipher_specs. It cannot be zero and must be a multiple of the
V2CipherSpec length (3).
session_id_length: This field must have a value of either zero or
16. If zero, the client is creating a new session. If 16, the
session_id field will contain the 16 bytes of session
identification.
challenge_length: The length in bytes of the client's challenge to
the server to authenticate itself. This value must be 32.
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.
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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-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. 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.
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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 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.
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 either may be contained in the server's certificate or may
be a temporary RSA key sent in a server key exchange message. When
temporary RSA keys are used, they are signed by the server's RSA or
DSS certificate. The signature includes the current
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ClientHello.random, so old signatures and temporary keys cannot be
replayed. Servers may use a single temporary RSA key for multiple
negotiation sessions.
Note: The temporary RSA key option is useful if servers need large
certificates but must comply with government-imposed size limits on
keys used for key exchange.
After verifying the server's certificate, the client encrypts a
pre_master_secret with the server's public key. By successfully
decoding the pre_master_secret and producing a correct finished
message, the server demonstrates that it knows the private key
corresponding to the server certificate.
When RSA is used for key exchange, clients are authenticated using
the certificate verify message (see Section 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.
F.1.1.3. Diffie-Hellman Key Exchange with Authentication
When Diffie-Hellman key exchange is used, the server either can
supply a certificate containing fixed Diffie-Hellman parameters or
can use the server key exchange message to send a set of temporary
Diffie-Hellman parameters signed with a DSS or RSA certificate.
Temporary parameters are hashed with the hello.random values before
signing to ensure that attackers do not replay old parameters. In
either case, the client can verify the certificate or signature to
ensure that the parameters belong to the server.
If the client has a certificate containing fixed Diffie-Hellman
parameters, its certificate contains the information required to
complete the key exchange. Note that in this case, the client and
server will generate the same Diffie-Hellman result (i.e.,
pre_master_secret) every time they communicate. To prevent the
pre_master_secret from staying in memory any longer than necessary,
it should be converted into the master_secret as soon as possible.
Client Diffie-Hellman parameters must be compatible with those
supplied by the server for the key exchange to work.
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.
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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
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 other's finished messages.
Without the master_secret, the attacker cannot repair the finished
messages, so the attack will be discovered.
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F.1.4. Resuming Sessions
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not
been compromised and that the secure hash operations used to produce
the encryption keys and MAC secrets are secure, the connection should
be secure and effectively independent from previous connections.
Attackers cannot use known encryption keys or MAC secrets to
compromise the master_secret without breaking the secure hash
operations (which use both SHA and MD5).
Sessions cannot be resumed unless both the client and server agree.
If either party suspects that the session may have been compromised,
or that certificates may have expired or been revoked, it should
force a full handshake. An upper limit of 24 hours is suggested for
session ID lifetimes, since an attacker who obtains a master_secret
may be able to impersonate the compromised party until the
corresponding session ID is retired. Applications that may be run in
relatively insecure environments should not write session IDs to
stable storage.
F.1.5. MD5 and SHA
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.
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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.
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. Acknowledgements
G.1. 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
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G.2. Early Reviewers
Robert Baldwin Clyde Monma
RSA Data Security, Inc. Bellcore
baldwin@rsa.com clyde@bellcore.com
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
Authors' Addresses
Alan O. Freier
Netscape Communications
Philip Karlton
Netscape Communications
Paul C. Kocher
Independent Consultant
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