Netscape Communications Corporation

Internet Draft

March 1996 (Expires 9/96)

Alan O. Freier, Netscape Communications
Philip Karlton, Netscape Communications
Paul C. Kocher, Independent Consultant

The SSL Protocol
Version 3.0

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Table of Contents
1. Status of this memo ..................................................................................................................................1
2. Abstract ......................................................................................................................................................1
3. Introduction...............................................................................................................................................1
4. Goals ...........................................................................................................................................................1
5. Goals of this document ............................................................................................................................2
6. Presentation language ..............................................................................................................................2
6.1 Basic block size....................................................................................................................................2
6.2 Miscellaneous ......................................................................................................................................3
6.3 Vectors..................................................................................................................................................3
6.4 Numbers...............................................................................................................................................4
6.5 Enumerateds........................................................................................................................................4
6.6 Constructed types ...............................................................................................................................4
6.6.1 Variants .........................................................................................................................................5
6.7 Cryptographic attributes....................................................................................................................6
6.8 Constants..............................................................................................................................................6
7. SSL protocol...............................................................................................................................................7
7.1 Session and connection states............................................................................................................7
7.2 Record layer.........................................................................................................................................8
7.2.1 Fragmentation ..............................................................................................................................8
7.2.2 Record compression and decompression .................................................................................97.2.3 Record payload protection and the CipherSpec ....................................................................10
7.3 Change cipher spec protocol ...........................................................................................................12
7.4 Alert protocol ....................................................................................................................................127.4.1 Closure alerts..............................................................................................................................127.4.2 Error alerts ..................................................................................................................................13

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7.5 Handshake protocol overview ........................................................................................................13
7.6 Handshake protocol .........................................................................................................................16
7.6.1 Hello messages...........................................................................................................................17
7.6.2 Server certificate.........................................................................................................................19
7.6.3 Server key exchange message ..................................................................................................20
7.6.4 Certificate request ......................................................................................................................21
7.6.5 Server hello done .......................................................................................................................22
7.6.6 Client certificate .........................................................................................................................22
7.6.7 Client key exchange message ...................................................................................................22
7.6.8 Certificate verify ........................................................................................................................24
7.6.9 Finished .......................................................................................................................................25
7.7 Application data protocol ................................................................................................................25
8. Cryptographic computations ................................................................................................................25
8.1 Asymmetric cryptographic computations.....................................................................................26
8.1.1 RSA ..............................................................................................................................................26
8.1.2 Diffie-Hellman............................................................................................................................26
8.1.3 Fortezza.......................................................................................................................................26
8.2 Symmetric cryptographic calculations and the CipherSpec .......................................................26
8.2.1 The master secret .......................................................................................................................27
8.2.2 Converting the master secret into keys and MAC secrets....................................................27
A. Protocol constant values .......................................................................................................................29
A.1 Reserved port assignments.............................................................................................................29
A.1.1 Record layer...............................................................................................................................29
A.2 Change cipher specs message ........................................................................................................30
A.3 Alert messages .................................................................................................................................30
A.4 Handshake protocol ........................................................................................................................30
A.4.1 Hello messages..........................................................................................................................31
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A.4.2 Server authentication and key exchange messages..............................................................31
A.5 Client authentication and key exchange messages .....................................................................33
A.5.1 Handshake finalization message ............................................................................................33
A.6 The CipherSuite ...............................................................................................................................33
A.7 The CipherSpec ................................................................................................................................35
B. Glossary ...................................................................................................................................................37
C. CipherSuite definitions .........................................................................................................................41
D. Implementation Notes...........................................................................................................................43
D.1 Temporary RSA keys.......................................................................................................................43
D.2 Random Number Generation and Seeding ..................................................................................43
D.3 Certificates and authentication ......................................................................................................43
D.4 CipherSuites .....................................................................................................................................44
E. Version 2.0 Backward Compatibility ...................................................................................................45
E.1 Version 2 client hello........................................................................................................................45
E.2 Avoiding man-in-the-middle version rollback.............................................................................46
F. Security analysis .....................................................................................................................................47
F.1 Handshake protocol .........................................................................................................................47
F.1.1 Authentication and key exchange ...........................................................................................47
F.1.2 Version rollback attacks............................................................................................................49
F.1.3 Detecting attacks against the handshake protocol ................................................................49
F.1.4 Resuming sessions .....................................................................................................................49
F.1.5 MD5 and SHA............................................................................................................................50
F.2 Protecting application data .............................................................................................................50
F.3 Final notes..........................................................................................................................................50
G. Patent Statement ....................................................................................................................................51
References ....................................................................................................................................................53
Authors.........................................................................................................................................................55

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Other contributors ......................................................................................................................................55
Early reviewers ...........................................................................................................................................55

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1. Status of this memoThis document is an Internet-Draft. Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated,
replaced, or made obsolete by other documents at any time. It is inappropriate to use InternetDrafts as reference material or to cite them other than as work in progress.
To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing containedin the Internet Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

2. Abstract
This document specifies Version 3.0 of the Secure Sockets Layer (SSL V3.0) protocol, a security
protocol that provides communications privacy over the Internet. The protocol allows
client/server applications to communicate in a way that is designed to prevent eavesdropping,tampering, or message forgery.

3. Introduction
The primary goal of the SSL Protocol is to provide privacy and reliability between two
communicating applications. The protocol is composed of two layers. At the lowest level, layeredon top of some reliable transport protocol (e.g., TCP[TCP]), is the SSL Record Protocol. The SSL
Record Protocol is used for encapsulation of various higher level protocols. One such
encapsulated protocol, the SSL Handshake Protocol, allows the server and client to authenticate
each other and to negotiate an encryption algorithm and cryptographic keys before theapplication protocol transmits or receives its first byte of data. One advantage of SSL is that it isapplication protocol independent. A higher level protocol can layer on top of the SSL Protocol
transparently.
The SSL protocol provides connection security that has three basic properties:•

The connection is private. Encryption is used after an initial handshake to define a secret key.
Symmetric cryptography is used for data encryption (e.g., DES[DES], RC4[RC4], etc.)•

The peer’s identity can be authenticated using asymmetric, or public key, cryptography (e.g.,
RSA[RSA], DSS[DSS], etc.).

•

The connection is reliable. Message transport includes a message integrity check using akeyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations.

4. Goals
The goals of SSL Protocol v3.0, in order of their priority, are:
1. Cryptographic security

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SSL should be used to establish a secure connection between two
parties.

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2. Interoperability

Note:

Independent programmers should be able to develop
applications utilizing SSL 3.0 that will then be able tosuccessfully exchange cryptographic parameters without
knowledge of one another’s code.
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 aparticular 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 tocreate 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 establishedfrom scratch. Additionally, care has been taken to reduce
network activity.

5. Goals of this document
The SSL Protocol Version 3.0 Specification is intended primarily for readers who will be
implementing the protocol and those doing cryptographic analysis of it. The spec has been
written with this in mind, and it is intended to reflect the needs of those two groups. For thatreason, many of the algorithm-dependent data structures and rules are included in the body ofthe text (as opposed to in an Appendix), providing easier access to them.
This document is not intended to supply any details of service definition nor interface definition,although it does cover select areas of policy as they are required for the maintenance of solid
security.

6. Presentation language
This document deals with the formatting of data in an external representation. The following very
basic and somewhat casually defined presentation syntax will be used. The syntax draws fromseveral sources in its structure. Although it resembles the programming language ‘C’ in its syntax
and XDR [XDR] in both its syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document SSL only, not to have general application
beyond that particular goal.

6.1 Basic block size
The representation of all data items is explicitly specified. The basic data block size is one byte (i.e.
8 bits). Multiple byte data items are concatenations of bytes, from left to right, from top to bottom.
From the bytestream a multi-byte item (a numeric in the example) is formed (using C notation)
by:

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value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ... | byte[n-1];This byte ordering for multi-byte values is the commonplace network byte order or big endian
format.

6.2 MiscellaneousComments begin with “/*” and end with “*/”.
Optional components are denoted by enclosing them in italicized “[ ]” brackets.
Single byte entities containing uninterpreted data are of type opaque.

6.3 Vectors
A vector (single dimensioned array) is a stream of homogeneous data elements. The size of thevector may be specified at documentation time or left unspecified until runtime. In either case thelength 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 ofthe 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];Datum Data[9];/* three uninterpreted bytes of data *//* 3 consecutive 3 byte vectors */Variable length vectors are defined by specifying a subrange of legal lengths, inclusively, using
the notation <floor..ceiling>. When encoded, the actual length precedes the vector’s contents
in the byte stream. The length will be in the form of a number consuming as many bytes as
required to hold the vector’s specified maximum (ceiling) length. A variable length vector with anactual 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 oftype opaque. It can never be empty. The actual length field consumes two bytes, a uint16,
sufficient to represent the value 400 (see Section 6.4). On the other hand, longer can represent up
to 800 bytes of data, or 400 uint16 elements, and it may be empty. Its encoding will include a
two byte actual length field prepended to the vector.
opaque mandatory<300..400>;
uint16 longer<0..800>;

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6.4 Numbers
The basic numeric data type is an unsigned byte (uint8). All larger numeric data types areformed from fixed length series of bytes concatenated as described in Section 6.1 and are also
unsigned. The following numeric types are predefined.
uint8 uint16[2];uint8 uint24[3];uint8 uint32[4];uint8 uint64[8];6.5 Enumerateds
An additional sparse data type is available called enum. A field of type enum can only assume thevalues declared in the definition. Each definition is a different type. Only enumerateds of thesame type may be assigned or compared. Every element of an enumerated must be assigned avalue, as demonstrated in the following example. Since the elements of the enumerated are notordered, they can be assigned any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn), [(n)] } Te;

Enumerateds occupy as much space in the byte stream as would its maximal defined ordinalvalue. The following definition would cause one byte to be used to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;

One may optionally specify a value without its associated tag to force the width definition
without defining a superfluous element. In the following example, Taste will consume two bytes
in the data stream but can only assume the values 1, 2 or 4.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;

The names of the elements of an enumeration are scoped within the defined type. In the firstexample, 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;
Color color = blue;/* overspecified, but legal *//* correct, type is implicit */For enumerateds that are never converted to external representation, the numerical information
may be omitted.
enum { low, medium, high } Amount;

6.6 Constructed types
Structure types may be constructed from primitive types for convenience. Each specification
declares a new, unique type. The syntax for definition is much like that of C.
struct {T1 f1;
T2 f2;...
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The fields within a structure may be qualified using the type’s name using a syntax much likethat available for enumerateds. For example, T.f2 refers to the second field of the previous
declaration. Structure definitions may be embedded.

6.6.1 Variants
Defined structures may have variants based on some knowledge that is available within theenvironment. 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 theselect. The body of the variant structure may be given a label for reference. The mechanism bywhich 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>;} V1;/* variable length */struct {uint32 number;
opaque string[10];} V2;/* fixed length */struct {select (VariantTag) {case apple: V1;
case orange: V2;} variant_body;} VariantRecord;/* value of variant selector is implicit *//* definition of VariantBody, tag = apple *//* definition of VariantBody, tag = orange *//* optional label on the variant portion */Variant structures may be qualified (narrowed) by specifying a value for the selector prior to thetype. For example, aorange VariantRecord

is a narrowed type of a VariantRecord containing a variant_body of type V2.

6.7 Cryptographic attributes
The four cryptographic operations digital signing, stream cipher encryption, block cipher encryption,and public key encryption are designated digitally-signed, stream-ciphered, blockciphered, and public-key-encrypted, respectively. A field’s cryptographic processing is

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specified by prepending an appropriate key word designation before the field’s typespecification. Cryptographic keys are implied by the current session state (see Section 7.1).
In digital signing, one-way hash functions are used as input for a signing algorithm. In RSA
signing, a 36-byte structure of two hashes (one SHA and one MD5) is signed (encrypted with theprivate key). In DSS, the 20 bytes of the SHA hash are run directly through the Digital Signing
Algorithm with no additional hashing.
In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output
generated from a cryptographically-secure keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. Because it isunlikely 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 theoutgoing data. Data encrypted with the public key of a given key pair can only be decrypted withthe private key, and vice-versa.
In the following example:
stream-ciphered struct {uint8 field1;
uint8 field2;
digitally-signed opaque hash[20];} UserType;The contents of hash are used as input for the signing algorithm, then the entire structure is
encrypted with a stream cipher.

6.8 Constants
Typed constants can be defined for purposes of specification by declaring a symbol of the desired
type and assigning values to it. Under-specified types (opaque, variable length vectors, andstructures that contain opaque) cannot be assigned values. No fields of a multi-element structure
or vector may be elided.
For example,
struct {uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4};/* assigns f1 = 1, f2 = 4 */7. SSL protocol
SSL is a layered protocol. At each layer, messages may include fields for length, description, andcontent. 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.

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7.1 Session and connection states
An SSL session is stateful. It is the responsibility of the SSL Handshake protocol to coordinate thestates of the client and server, thereby allowing the protocol state machines of each to operateconsistently, 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 thepending state. Additionally, separate read and write states are maintained. When the client orserver 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 clientand server exchange change cipher spec messages (see Section 7.3), and they then communicate
using the newly agreed-upon cipher spec.
An SSL session may include multiple secure connections; in addition, parties may have multiple
simultaneous sessions.
The session state includes the following elements:
session identifier

An arbitrary byte sequence chosen by the server to identify anactive 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 secret48-byte secret shared between the client and server.

is resumable

A flag indicating whether the session can be used to initiate newconnections.

The connection state includes the following elements:
server and client random

Byte sequences that are chosen by the server and client for eachconnection.

server write MAC secret

The secret used in MAC operations on data written by theserver.

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 anddecrypted by the client.

client write key

The bulk cipher key for data encrypted by the client anddecrypted by the server.

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initialization vectors

When a block cipher in CBC mode is used, an initialization
vector (IV) is maintained for each key. This field is firstinitialized by the SSL handshake protocol. Thereafter the final
ciphertext block from each record is preserved for use with thefollowing record.

sequence numbers

Each party maintains separate sequence numbers for transmitted
and received messages for each connection. When a party sends
or receives a change cipher spec message, the appropriate
sequence number is set to zero. Sequence numbers are of type64
uint64 and may not exceed 2 -1.

7.2 Record layer
The SSL Record Layer receives uninterpreted data from higher layers in non-empty blocks ofarbitrary size.

7.2.1 Fragmentation
14

The record layer fragments information blocks into SSLPlaintext records of 2 bytes or less.
Client message boundaries are not preserved in the record layer (i.e., multiple client messages ofthe 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.

versionThe version of protocol being employed. This document
describes SSL Version 3.0 (See Appendix A.1.1).

lengthThe length (in bytes) of the following SSLPlaintext.fragment.
14
The length should not exceed 2 .

fragment

The application data. This data is transparent and treated as anindependent block to be dealt with by the higher level protocol
specified by the type field.

Note:

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Data of different SSL Record layer content types may be interleaved. Application
data is generally of lower precedence for transmission than other content types.

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7.2.2 Record compression and decompression
All records are compressed using the compression algorithm defined in the current session state.
There is always an active compression algorithm, however initially it is defined as
CompressionMethod.null. The compression algorithm translates an SSLPlaintext structure
into an SSLCompressed structure. Compression functions erase their state information whenever
the CipherSpec is replaced.
Note:

The CipherSpec is part of the session state described in Section 7.1. References tofields 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 to14
a length in excess of 2 bytes, it should issue a fatal decompression_failure alert (Section
7.4.2).
struct {ContentType type;/* same as SSLPlaintext.type */ProtocolVersion version;/* same as SSLPlaintext.version */uint16 length;
opaque fragment[SSLCompressed.length];} SSLCompressed;
lengthThe length (in bytes) of the following SSLCompressed.fragment.
14
The length should not exceed 2 + 1024.

fragment

The compressed form of SSLPlaintext.fragment.

Note:

A CompressionMethod.null operation is an identity operation; no fields arealtered. (See Appendix A.4.1)

Implementation note:

Decompression functions are responsible for ensuring that messages cannot
cause internal buffer overflows.

7.2.3 Record payload protection and the CipherSpec
All records are protected using the encryption and MAC algorithms defined in the current
CipherSpec. There is always an active CipherSpec, however initially it is
SSL_NULL_WITH_NULL_NULL, which does not provide any security.
Once the handshake is complete, the two parties have shared secrets which are used to encrypt
records and compute keyed message authentication codes (MACs) on their contents. The
techniques used to perform the encryption and MAC operations are defined by the CipherSpec
and constrained by CipherSpec.cipher_type. The encryption and MAC functions translate an
SSLCompressed structure into an SSLCiphertext. The decryption functions reverse the process.
Transmissions also include a sequence number so that missing, altered, or extra messages aredetectable.

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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.

versionThe version field is identical to SSLCompressed.version.

lengthThe length (in bytes) of the following SSLCiphertext.fragment.
14
The length may not exceed 2 + 2048.

fragment

The encrypted form of SSLCompressed.fragment, including theMAC.

7.2.3.1 Null or standard stream cipher
Stream ciphers (including BulkCipherAlgorithm.null - see Appendix A.7) convert
SSLCompressed.fragment structures to and from stream SSLCiphertext.fragment structures.
stream-ciphered struct {opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];} GenericStreamCipher;

The MAC is generated as:hash(MAC_write_secret + pad_2 +hash (MAC_write_secret + pad_1 + seq_num + length + content));

where “+” denotes concatenation.
pad_1

The character 0x36 repeated 48 times for MD5 this or 40 times for
SHA.

pad_2

The character 0x5c repeated the same number of times.

seq_num

The sequence number for this message.

hashHashing 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., thedata is not encrypted and the MAC size is zero implying that no MAC is used).
SSLCiphertext.length is SSLCompressed.length plus CipherSpec.hash_size.
7.2.3.2 CBC block cipher
For block ciphers (such as RC2 or DES), the encryption and MAC functions convert
SSLCompressed.fragment structures to and from block SSLCiphertext.fragment structures.
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block-ciphered struct {opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;} GenericBlockCipher;

The MAC is generated as described in Section 7.2.3.1.
padding

Padding that is added to force the length of the plaintext to be amultiple of the block cipher’s block length.

padding_length

The length of the padding must be less than the cipher’s block
length and may be zero. The padding length should be such thatthe total size of the GenericBlockCipher structure is a multipleof the cipher’s block length.

The encrypted data length (SSLCiphertext.length) is one more
SSLCompressed.length, CipherSpec.hash_size, and padding_length.
Note:than the sum ofWith CBC block chaining the initialization vector (IV) for the first record isprovided by the handshake protocol. The IV for subsequent records is the last
ciphertext block from the previous record.

7.3 Change cipher spec protocol
The change cipher spec protocol exists to signal transitions in ciphering strategies. The protocol
consists of a single message, which is encrypted and compressed under the current (not thepending) 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 andcertificate verify messages (if any), and the server sends one after successfully processing the key
exchange message it received from the client. An unexpected change cipher spec message should
generate an unexpected_message alert (Section 7.4.2). When resuming a previous session, thechange cipher spec message is sent after the hello messages.

7.4 Alert protocol
One of the content types supported by the SSL Record layer is the alert type. Alert messages
convey the severity of the message and a description of the alert. Alert messages with a level offatal result in the immediate termination of the connection. In this case, other connections
corresponding to the session may continue, but the session identifier must be invalidated,preventing the failed session from being used to establish new connections. Like other messages,
alert messages are encrypted and compressed, as specified by the current connection state.
enum { warning(1), fatal(2), (255) } AlertLevel;

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close_notify(0),unexpected_message(10),bad_record_mac(20),decompression_failure(30),
handshake_failure(40), no_certificate(41), bad_certificate(42),unsupported_certificate(43), certificate_revoked(44),certificate_expired(45), certificate_unknown(46),
illegal_parameter (47)(255)
} AlertDescription;
struct {AlertLevel level;
AlertDescription description;} Alert;7.4.1 Closure alerts
The client and the server must share knowledge that the connection is ending in order to avoid atruncation 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.

7.4.2 Error alerts
Error handling in the SSL Handshake protocol is very simple. When an error is detected, thedetecting party sends a message to the other party. Upon transmission or receipt of an fatal alert
message, both parties immediately close the connection. Servers and clients are required to forget
any session-identifiers, keys, and secrets associated with a failed connection. The following error
alerts are defined:
unexpected_message

An inappropriate message was received. This alert is always
fatal and should never be observed in communication between
proper implementations.

bad_record_mac

This alert is returned if a record is received with an incorrect
MAC. This message is always fatal.

decompression_failure

The decompression function received improper input (e.g. datathat would expand to excessive length). This message is always
fatal.

handshake_failure

Reception of a handshake_failure alert message indicates thatthe 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 acertification request if no appropriate certificate is available.

bad_certificate

A certificate was corrupt, contained signatures that did notverify correctly, etc.

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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 withother fields. This is always fatal.

7.5 Handshake protocol overview
The cryptographic parameters of the session state are produced by the SSL Handshake Protocol,
which operates on top of the SSL Record Layer. When a SSL client and server first start
communicating, they agree on a protocol version, select cryptographic algorithms, optionallyauthenticate 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 hellomessage, 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 andexchanged: 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 acertificate 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 phaseof 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 ofthat message will depend on the public key algorithm selected between the client hello and theserver hello. If the client has sent a certificate with signing ability, a digitally-signed certificate
verify message is sent to explicitly verify the certificate.
At this point, a change cipher spec message is sent by the client, and the client copies the pending
Cipher Spec into the current Cipher Spec. The client then immediately sends the finished message
under the new algorithms, keys, and secrets. In response, the server will send its own change
cipher spec message, transfer the pending to the current Cipher Spec, and send its finishedmessage under the new Cipher Spec. At this point, the handshake is complete and the client andserver may begin to exchange application layer data. (See flow chart below.)

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CLIENTSERVER

ClientHello
ServerHello
Certificate*
CertificateRequest*
ServerKeyExchange*
Certificate*
ClientKeyExchange
CertificateVerify*
cha nge cipher spec
Finishedcha nge cipher spec
Finished

Application Data

Application Data

* Indicates optional or situation-dependent messages that are notalways 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 serverthen checks its session cache for a match. If a match is found, and the server is willing to reestablish the connection under the specified session state, it will send a ServerHello with thesame 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, theclient 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 andserver perform a full handshake.

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CLIENTSERVER

ClientHello
ServerHellocha nge cipher spec
Finishedcha nge cipher spec
Finished

Application Data

Application Data

The contents and significance of each message will be presented in detail in the following
sections.

7.6 Handshake protocol
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 aresupplied 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;/* type of handshake message */uint24 length;/* # bytes in handshake message body */select (HandshakeType) {case hello_request: HelloRequest;
case client_hello: ClientHello;case server_hello: ServerHello;case certificate: Certificate;case server_key_exchange: ServerKeyExchange;case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;case certificate_verify: CertificateVerify;case client_key_exchange: ClientKeyExchange;case finished: Finished;} body;} Handshake;

The handshake protocol messages are presented in the order they must be sent; sending
handshake messages in an unexpected order results in a fatal error.

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7.6.1 Hello messages
The hello phase messages are used to exchange security enhancement capabilities between theclient and server. When a new session begins, the CipherSpec encryption, hash, and compression
algorithms are initialized to null. The current CipherSpec is used for renegotiation messages.
7.6.1.1 Hello request
The hello request message may be sent by the server at any time, but will be ignored by the client
if the handshake protocol is already underway. It is a simple notification that the client should
begin the negotiation process anew by sending a client hello message when convenient.
Note:

Since handshake messages are intended to have transmission precedence overapplication 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 handshakenegotiation state should simply ignore the message.
The structure of a hello request message is as follows:
struct { } HelloRequest;

7.6.1.2 Client hello
When a client first connects to a server it is required to send the client hello as its first message.
The client can also send a client hello in response to a hello request or on its own initiative inorder to renegotiate the security parameters in an existing connection.
The client hello message includes a random structure, which is used later in the protocol.
struct {uint32 gmt_unix_time;
opaque random_bytes[28];} Random;
gmt_unix_time

The current time and date in standard UNIX 32-bit format
according to the sender’s internal clock. Clocks are not required
to be set correctly by the basic SSL Protocol; higher level orapplication 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 thefull handshake protocol. The actual contents of the SessionID are defined by the server.
opaque SessionID<0..32>;

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Warning:Servers must not place confidential information in session identifiers or let thecontents 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, returna 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, orderedaccording to the client’s preference. If the server supports none of those specified by the client, thesession must fail.
enum { null(0), (255) } CompressionMethod;
Issue:

Which compression methods to support is under investigation.

The structure of the client hello is as follows.
struct {ProtocolVersion client_version;
Random random;
SessionID session_id;
16
CipherSuite cipher_suites<2..2 -1>;
8
CompressionMethod compression_methods<1..2 -1>;
} ClientHello;client_version

The version of the SSL protocol by which the client wishes tocommunicate during this session. This should be the most recent(highest valued) version supported by the client. For this versionof 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 theclient 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) thisvector 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 otherhandshake message returned by the server except for a hello request is treated as a fatal error.

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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 finishedmessage has been received. This absolute restriction is relaxed if there is acurrent, non-null encryption on this connection.

7.6.1.3 Server hello
The server processes the client hello message and responds with either a handshake_failure
alert or server hello message.
struct {ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;} ServerHello;server_version

This field will contain the lower of that suggested by the client in
the client hello and the highest supported by the server. For thisversion of the specification, the version will be 3.0 (SeeAppendix E for details about backward compatibility).

randomThis 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 thisconnection. If the ClientHello.session_id was non-empty, theserver will look in its session cache for a match. If a match isfound and the server is willing to establish the new connection
using the specified session state, the server will respond with thesame 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 willcontain 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 isthe value from the state of the session being resumed.

compression_method

The single compression algorithm selected by the server from thelist in ClientHello.compression_methods. For resumed
sessions this field is the value from the resumed session state.

7.6.2 Server certificate
If the server is to be authenticated (which is generally the case), the server sends its certificate
immediately following the server hello message. The certificate type must be appropriate for theselected cipher suite’s key exchange algorithm, and is generally an X.509.v3 certificate (or amodified X.509 certificate in the case of Fortezza [FOR]). The same message type will be used forthe client’s response to a certificate request message.
24

opaque ASN.1Cert<1..2 -1>;

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struct {24
ASN.1Cert certificate_list<1..2 -1>;
} Certificate;certificate_list

Note:This is a sequence (chain) of X.509.v3 certificates, ordered withthe sender’s certificate first and the root certificate authority last.
PKCS #7 [PKCS7] is not used as the format for the certificate vector because
PKCS #6 [PKCS6] extended certificates are not used. Also PKCS #7 defines a SET
rather than a SEQUENCE, making the task of parsing the list more difficult.

7.6.3 Server key exchange message
The server key exchange message is sent by the server if it has no certificate, has a certificate onlyused for signing (e.g., DSS [DSS] certificates, signing-only RSA [RSA] certificates), or
Fortezza/DMS key exchange is used. This message is not used if the server certificate contains
Diffie-Hellman [DH1] parameters.
Note:

According to current US export law, RSA moduli larger than 512 bits may not be
used for key exchange in software exported from the US. With this message,larger RSA keys may be used as signature-only certificates to sign temporary
shorter RSA keys for key exchange.

enum { rsa, diffie_hellman, fortezza_dms } KeyExchangeAlgorithm;
struct {16-1>;
opaque rsa_modulus<1..2 16
opaque rsa_exponent<1..2 -1>;
} ServerRSAParams;
rsa_modulus

The modulus of the server’s temporary RSA key.

rsa_exponent

The public exponent of the server’s temporary RSA key.

struct {16
opaque dh_p<1..216-1>;
opaque dh_g<1..2 16-1>;
opaque dh_Ys<1..2 -1>;
} ServerDHParams;/* Ephemeral DH parameters */dh_p

The prime modulus used for the Diffie-Hellman operation.

dh_g

The generator used for the Diffie-Hellman operation.

dh_Ys

The server’s Diffie-Hellman public value (g mod p).

Xstruct {opaque r_s [128];} ServerFortezzaParams;

r_s

SSL Version 3.0

Server random number for Fortezza KEA (Key Exchange
Algorithm).

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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;
struct {select (KeyExchangeAlgorithm) {case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_dms:
ServerFortezzaParams params;};} ServerKeyExchange;

params

The server’s key exchange parameters.

signed_params

A hash of the corresponding params value, with the signatureappropriate to that hash applied.

md5_hash

MD5(ClientHello.random + ServerHello.random +ServerParams);

sha_hash

SHA(ClientHello.random + ServerHello.random +ServerParams);

enum { anonymous, rsa, dsa } SignatureAlgorithm;

7.6.4 Certificate request
A non-anonymous server can optionally request a certificate from the client, if appropriate for theselected cipher suite.
enum {
rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza_dms(20), (255)} ClientCertificateType;16

opaque DistinguishedName<3..2 -1>;struct {8
ClientCertificateType certificate_types<1..2 -1>;
16
DistinguishedName certificate_authorities<3..2 -1>;
} CertificateRequest;
certificate_types

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This field is a list of the types of certificates requested, sorted inorder of the server’s preference.

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certificate_authorities A list of the distinguished names of acceptable certificate

authorities.
Note:

DistinguishedName is derived from [X509].

Note:

It is a fatal handshake_failure alert for an anonymous server to request
client identification.

7.6.5 Server hello done
The server hello done message is sent by the server to indicate the end of the server hello andassociated 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 avalid certificate if required and check that the server hello parameters are acceptable.

7.6.6 Client certificate
This is the first message the client can send after receiving a server hello done message. Thismessage is only sent if the server requests a certificate. If no suitable certificate is available, theclient should send a no certificate alert instead. This error is only a warning, however the server
may respond with a fatal handshake failure alert if client authentication is required.
Client certificates are sent using the Certificate defined in Section 7.6.2.
Note:

Client Diffie-Hellman certificates must match the server specified Diffie-Hellman
parameters.

7.6.7 Client key exchange message
The choice of messages depends on which public key algorithm(s) has (have) been selected. See
Section 7.6.3 for the KeyExchangeAlgorithm definition.
struct {select (KeyExchangeAlgorithm) {case rsa: EncryptedPreMasterSecret;case diffie_hellman: ClientDiffieHellmanPublic;case fortezza_dms: FortezzaKeys;} exchange_keys;} ClientKeyExchange;

The information to select the appropriate record structure is in the pending session state (seeSection 7.1).
7.6.7.1 RSA encrypted premaster secret message
If RSA is being used for key agreement and authentication, the client generates a 48-byte 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 secretmessage.

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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.

random46 securely-generated random bytes.

struct {public-key-encrypted PreMasterSecret pre_master_secret;} EncryptedPreMasterSecret;pre_master_secretThis random value is generated by the client and is used togenerate the master secret, as specified in Section 8.1.

7.6.7.2 Fortezza key exchange message
Under Fortezza DMS, the client derives a Token Encryption Key (TEK) using the Fortezza Key
Exchange Algorithm (KEA). The client’s KEA calculation uses the public key in the server’s
certificate along with private parameters in the client’s token. The client sends public parameters
needed for the server to generate the TEK, using its own private parameters. The client generates
session keys, wraps them using the TEK, and sends the results to the server. The client generates
IV’s for the session keys and TEK and sends them also. The client generates a random 48-bytepremaster secret, encrypts it using the TEK, and sends the result:struct {opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[20];opaque wrapped_client_write_key[12];opaque wrapped_server_write_key[12];opaque client_write_IV[24];opaque server_write_IV[24];opaque master_secret_IV[24];block-ciphered opaque encrypted_pre_master_secret[48];} FortezzaKeys;y_signaturey_signature is the signature of the KEA public key, signed withthe client’s DSS private key.

y_c

The client’s Yc value (public key) for the KEA calculation. If theclient 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 signedwith 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.

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This is the IV for the client write key.

server_write_IV

This is the IV for the server write key.

master_secret_IV

This is the IV for the TEK used to encrypt the pre-master secret.

pre_master_secretThis is a random value, generated by the client and used togenerate the master secret, as specified in Section 8.1. In the above
structure, it is encrypted using the TEK.

7.6.7.3 Client Diffie-Hellman public value
This structure conveys the client’s Diffie-Hellman public value (Yc) if it was not already includedin 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 { };16case explicit: opaque dh_Yc<1..2 -1>;
} dh_public;} ClientDiffieHellmanPublic;dh_Yc

The client’s Diffie-Hellman public value (Yc).

7.6.8 Certificate verify
This message is used to provide explicit verification of a client certificate. This message is only
sent following any client certificate that has signing capability (i.e. all certificates except thosecontaining fixed Diffie-Hellman parameters).
struct {Signature signature;} CertificateVerify;
CertificateVerify.signature.md5_hash
MD5(master_secret + pad2 +MD5(handshake_messages + master_secret + pad1));
Certificate.signature.sha_hash
SHA(master_secret + pad2 +SHA(handshake_messages + master_secret + pad1));

Here handshake_messages refers to all handshake messages starting at client hello up to but
not including this message.

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7.6.9 FinishedA finished message is always sent immediately after a change cipher specs message to verify
that the key exchange and authentication processes were successful. The finished message is thefirst protected with the just-negotiated algorithms, keys, and secrets. No acknowledgment of thefinished message is required; parties may begin sending confidential data immediately after
sending the finished message. Recipients of finished messages must verify that the contents arecorrect.
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));

The hash contained in finished messages sent by the server incorporate Sender.server; thosesent by the client incorporate Sender.client. The value handshake_messages includes all
handshake messages starting at client hello up to, but not including, the finished messages. Thismay be different from handshake_messages in Section 7.6.8 because it would include thecertificate verify message (if sent).
Note:

Change cipher spec messages are not handshake messages and are not includedin the hash computations.

7.7 Application data protocol
Application data messages are carried by the Record Layer and are fragmented, compressed andencrypted based on the current connection state. The messages are treated as transparent data to
the record layer.

8. Cryptographic computations
The key exchange, authentication, encryption, and MAC algorithms are determined by the
cipher_suite selected by the server and revealed in the server hello message.

8.1 Asymmetric cryptographic computations
The asymmetric algorithms are used in the handshake protocol to authenticate parties and to
generate shared keys and secrets.
For Diffie-Hellman, RSA, and Fortezza, the same algorithm is used to convert the
pre_master_secret into the master_secret. The pre_master_secret should be deleted frommemory once the master_secret has been computed.

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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));

8.1.1 RSA
When RSA is used for server authentication and key exchange, a 48-byte pre_master_secret isgenerated by the client, encrypted under the server’s public key, and sent to the server. The
server uses its private key to decrypt the pre_master_secret. Both parties then convert the
pre_master_secret into the master_secret, as specified above.
RSA digital signatures are performed using PKCS #1 [PKCS1] block type 1. RSA public key
encryption is performed using PKCS #1 block type 2.

8.1.2 Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The negotiated key (Z) is used as the
pre_master_secret, and is converted into the master_secret, as specified above.
Note:

Diffie-Hellman parameters are specified by the server, and may be either
ephemeral or contained within the server’s certificate.

8.1.3 Fortezza
A random 48-byte pre_master_secret is sent encrypted under the TEK and its IV. The server
decrypts the pre_master_secret and converts it into a master_secret, as specified above. Bulk
cipher keys and IVs for encryption are generated by the client’s token and exchanged in the key
exchange message; the master_secret is only used for MAC computations.

8.2 Symmetric cryptographic calculations and the CipherSpec
The technique used to encrypt and verify the integrity of SSL records is specified by the currently
active CipherSpec. A typical example would be to encrypt data using DES and generateauthentication 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 amore secure CipherSpec and to generate cryptographic keys.

8.2.1 The master secret
Before secure encryption or integrity verification can be performed on records, the client andserver need to generate shared secret information known only to themselves. This value is a 48byte quantity called the master secret. The master secret is used to generate keys and secrets forencryption 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).

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8.2.2 Converting the master secret into keys and MAC secrets
The master secret is hashed into a sequence of secure bytes, which are assigned to the MAC
secrets, keys, and non-export IVs required by the current CipherSpec (see Appendix A.7).
CipherSpecs require a client write MAC secret, a server write MAC secret, a client write key, a server
write key, a client write IV, and a server write IV, which are generated from the master secret in thatorder. Unused values, such as Fortezza keys communicated in the KeyExchange message, areempty. 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 therandom 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]
server_write_IV[CipherSpec.IV_size]/* non-export ciphers *//* 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);

Exportable encryption algorithms derive their IVs from the random messages:client_write_IV = MD5(ClientHello.random + ServerHello.random);server_write_IV = MD5(ServerHello.random + ClientHello.random);

MD5 outputs are trimmed to the appropriate size by discarding the least-significant bytes.
8.2.2.1 Export key generation example
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

SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5

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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_secretserver_write_MAC_secretclient_write_key
server_write_key= key_block0..15= key_block16..31= key_block32..36= key_block37..41

final_client_write_key = MD5 (client_write_key +ClientHello.random + ServerHello.random)0..15;
final_server_write_key = MD5 (server_write_key +ServerHello.random + ClientHello.random)0..15;
client_write_IV = MD5(ClientHello.random + ServerHello.random)0..7;
server_write_IV = MD5(ServerHello.random + ClientHello.random)0..7;

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Appendix A
A. Protocol constant values
This section describes protocol types and constants.

A.1 Reserved port assignments
At the present time SSL is implemented using TCP/IP as the base networking technology. The
IANA reserved the following Internet Protocol [IP] port numbers for use in conjunction with SSL.
443

Reserved for use by Hypertext Transfer Protocol with SSL (https).

465

Reserved (pending) for use by Simple Mail Transfer Protocol with SSL (ssmtp).

563

Reserved (pending) for use by Network News Transfer Protocol (snntp).

A.1.1 Record layer
struct {uint8 major, minor;} ProtocolVersion;
ProtocolVersion version = { 3,0 };/* Define SSL version 3.0 */enum {
change_cipher_spec(20), alert(21), handshake(22),application_data(23), (255)} ContentType;
struct {ContentType type;
ProtocolVersion version;
uint16 length;
opaque fragment[SSLPlaintext.length];} SSLPlaintext;
struct {ContentType type;/* same as SSLPlaintext.type */ProtocolVersion version;/* same as SSLPlaintext.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;

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block-ciphered struct {opaque content[SSLCompressed.length];
opaque MAC[CipherSpec.hash_size];
uint8 padding[GenericBlockCipher.padding_length];
uint8 padding_length;} GenericBlockCipher;

A.2 Change cipher specs message
struct {enum { change_cipher_spec(1), (255) } type;} ChangeCipherSpec;

A.3 Alert messages
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),unexpected_message(10),bad_record_mac(20),decompression_failure(30),
handshake_failure(40), no_certificate(41), bad_certificate(42),unsupported_certificate(43), certificate_revoked(44),certificate_expired(45), certificate_unknown(46),
illegal_parameter (47),(255)} AlertDescription;
struct {AlertLevel level;
AlertDescription description;} Alert;

A.4 Handshake protocol
enum {
hello_request(0), client_hello(1), server_hello(2),certificate(11), server_key_exchange (12), certificate_request(13),server_done(14), certificate_verify(15), client_key_exchange(16),finished(20), (255)} HandshakeType;
struct {HandshakeType msg_type;/* type of handshake message */uint24 length;/* # bytes in handshake msg body */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;

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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;
16
CipherSuite cipher_suites<0..2 -1>;
8
CompressionMethod compression_methods<0..2 -1>;
} ClientHello;
struct {ProtocolVersion server_version;
Random random;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;} ServerHello;

A.4.2 Server authentication and key exchange messages
24

opaque ASN.1Cert<2 -1>;struct {24
ASN.1Cert certificate_list<1..2 -1>;
} Certificate;enum { rsa, diffie_hellman, fortezza_dms } KeyExchangeAlgorithm;
struct {16
opaque RSA_modulus<1..2 16-1>;opaque RSA_exponent<1..2 -1>;
} ServerRSAParams;
struct {16
opaque DH_p<1..216-1>;
opaque DH_g<1..2 16-1>;
opaque DH_Ys<1..2 -1>;
} ServerDHParams;
struct {opaque r_s [128]} ServerFortezzaParams
struct {select (KeyExchangeAlgorithm) {case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case fortezza_dms:
ServerFortezzaParams params;};} ServerKeyExchange;
enum { anonymous, rsa, dsa } SignatureAlgorithm;

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digitally-signed struct {select(SignatureAlgorithm) {case anonymous: struct { };case rsa:
opaque md5_hash[16];opaque sha_hash[20];case dsa:
opaque sha_hash[20];};} Signature;enum {
RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3), DSS_fixed_DH(4),RSA_ephemeral_DH(5), DSS_ephemeral_DH(6), Fortezza_dms(20), (255)} CertificateType;16

opaque DistinguishedName<3..2 -1>;struct {8
CertificateType certificate_types<1..2 -1>;
16
DistinguishedName certificate_authorities<3..2 -1>;
} CertificateRequest;
struct { } ServerHelloDone;

A.5 Client authentication and key exchange messages
struct {select (KeyExchangeAlgorithm) {case rsa: EncryptedPreMasterSecret;case diffie_hellman: DiffieHellmanClientPublicValue;case fortezza_dms: FortezzaKeys;} exchange_keys;} ClientKeyExchange;
struct {ProtocolVersion client_version;
opaque random[46];} PreMasterSecret;struct {public-key-encrypted PreMasterSecret pre_master_secret;} EncryptedPreMasterSecret;struct {opaque y_c<0..128>;
opaque r_c[128];
opaque y_signature[20];opaque wrapped_client_write_key[12];opaque wrapped_server_write_key[12];opaque client_write_iv[24];opaque server_write_iv[24];opaque master_secret_iv[24];opaque encrypted_preMasterSecret[48];} FortezzaKeys;
enum { implicit, explicit } PublicValueEncoding;
struct {select (PublicValueEncoding) {case implicit: struct {};16
case explicit: opaque DH_Yc<1..2 -1>;
} dh_public;} ClientDiffieHellmanPublic;

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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 hellomessages.
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 canbe used for key exchange. The server may request either an RSA or a DSS signature-capable
certificate in the certificate request message.
CipherSuite SSL_RSA_WITH_NULL_MD5
CipherSuite SSL_RSA_WITH_NULL_SHA
CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5
CipherSuite SSL_RSA_WITH_RC4_128_MD5
CipherSuite SSL_RSA_WITH_RC4_128_SHA
CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5
CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA
CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_RSA_WITH_DES_CBC_SHA
CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA= { 0x00,0x01 };= { 0x00,0x02 };= { 0x00,0x03 };= { 0x00,0x04 };= { 0x00,0x05 };= { 0x00,0x06 };= { 0x00,0x07 };= { 0x00,0x08 };= { 0x00,0x09 };= { 0x00,0x0A };The following CipherSuite definitions are used for server-authenticated (and optionally clientauthenticated) 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 DiffieHellman parameters chosen by the server.
CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA
CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA
CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA
CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA= { 0x00,0x0B };= { 0x00,0x0C };= { 0x00,0x0D };= { 0x00,0x0E };= { 0x00,0x0F };= { 0x00,0x10 };CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA
CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA
CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA= { 0x00,0x11 };= { 0x00,0x12 };= { 0x00,0x13 };= { 0x00,0x14 };= { 0x00,0x15 };= { 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.

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CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5
CipherSuite SSL_DH_anon_WITH_RC4_128_MD5
CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA
CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA
CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA= { 0x00,0x17 };= { 0x00,0x18 };= { 0x00,0x19 };= { 0x00,0x1A };= { 0x00,0x1B };The final cipher suite is for the Fortezza token.
CipherSuite SSL_FORTEZZA_DMS_WITH_NULL_SHA
CipherSuite SSL_FORTEZZA_DMS_WITH_FORTEZZA_CBC_SHA= { 0X00,0X1C };= { 0x00,0x1D };Note:

All cipher suites whose first byte is 0xFF are considered private and can be used
for defining local/experimental algorithms. Interoperability of such types is alocal matter.

Note:

Additional cipher suites will be considered for implementation only withsubmission of notarized letters from two independent entities. NetscapeCommunications Corp. will act as an interim registration office, until a publicstandards body assumes control of SSL.

A.7 The CipherSpec
A cipher suite identifies a CipherSpec. These structures are part of the SSL session state. The
CipherSpec includes:
enum { stream, block } CipherType;enum { true, false } IsExportable;
enum { null, rc4, rc2, des, 3des, des40, fortezza } BulkCipherAlgorithm;
enum { null, md5, sha } MACAlgorithm;
struct {BulkCipherAlgorithm bulk_cipher_algorithm;
MACAlgorithm mac_algorithm;
CipherType cipher_type;
IsExportable is_exportable
uint8 hash_size;
uint8 key_material;
uint8 IV_size;} CipherSpec;SSL Version 3.0

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Appendix B
B. Glossary
application protocol

An application protocol is a protocol that normally layers
directly on top of the transport layer (e.g., TCP/IP).
Examples include HTTP, TELNET, FTP, and SMTP.

asymmetric cipher

See public key cryptography.

authentication

Authentication is the ability of one entity to determine theidentity of another entity.

block cipher

A block cipher is an algorithm that operates on plaintext ingroups of bits, called blocks. 64 bits is a typical block size.

bulk cipher

A symmetric encryption algorithm used to encrypt large
quantities of data.

Cipher Block Chaining Mode

(CBC) CBC is a mode in which every plaintext block
encrypted with the block cipher is first exclusive-ORed withthe 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 theclient.

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 associatedwith one session.

Data Encryption Standard(DES) DES is a very widely used symmetric encryption
algorithm. DES is a block cipher.

Digital Signature Standard(DSS) A standard for digital signing, including the Digital
Signing Algorithm, approved by the National Institute ofStandards and Technology, defined in NIST FIPS PUB 186,
“Digital Signature Standard,” published May, 1994 by the
U.S. Dept. of Commerce.

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digital signatures

Digital signatures utilize public key cryptography and oneway hash functions to produce a signature of the data thatcan be authenticated, and is difficult to forge or repudiate.

Fortezza

A PCMCIA card that provides both encryption and digitalsigning.

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, theinitialization vector is exclusive-ORed with the firstplaintext block prior to encryption.

IDEA

A 64-bit block cipher designed by Xuejia Lai and James
Massey.

Message Authentication Code(MAC) A Message Authentication Code is a one-way hashcomputed from a message and some secret data. Its
purpose is to detect if the message has been altered.

master secretSecure secret data used for generating encryption keys,
MAC secrets, and IVs.

MD5

MD5 [7] is a secure hashing function that converts anarbitrarily 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 withthe public key.

one-way hash function

A one-way transformation that converts an arbitraryamount of data into a fixed-length hash. It is
computationally hard to reverse the transformation or tofind collisions. MD5 and SHA are examples of one-way
hash functions.

RC2, RC4

Proprietary bulk ciphers from RSA Data Security, Inc.
(There is no good reference to these as they are unpublishedworks; however, see [RSADSI]). RC2 is block cipher andRC4 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 torequests for connections from clients. The server is passive,
waiting for requests from clients.

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sessionA SSL session is an association between a client and aserver. 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 theserver.

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 thenexclusive-ORed with the plaintext.

symmetric cipher

See bulk cipher.

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Appendix C
C. CipherSuite definitions
CipherSuite
SSL_NULL_WITH_NULL_NULL
SSL_RSA_WITH_NULL_MD5
SSL_RSA_WITH_NULL_SHA
SSL_RSA_EXPORT_WITH_RC4_40_MD5
SSL_RSA_WITH_RC4_128_MD5
SSL_RSA_WITH_RC4_128_SHA
SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5
SSL_RSA_WITH_IDEA_CBC_SHA
SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
SSL_RSA_WITH_DES_CBC_SHA
SSL_RSA_WITH_3DES_EDE_CBC_SHA
SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
SSL_DH_DSS_WITH_DES_CBC_SHA
SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA
SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
SSL_DH_RSA_WITH_DES_CBC_SHA
SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA
SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
SSL_DHE_DSS_WITH_DES_CBC_SHA
SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
SSL_DHE_RSA_WITH_DES_CBC_SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
SSL_DH_anon_EXPORT_WITH_RC4_40_MD5
SSL_DH_anon_WITH_RC4_128_MD5
SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA
SSL_DH_anon_WITH_DES_CBC_SHA
SSL_DH_anon_WITH_3DES_EDE_CBC_SHA
SSL_FORTEZZA_DMS_WITH_NULL_SHA
SSL_FORTEZZA_DMS_WITH_FORTEZZA_CBC_SHA

IsExpor Key Exchange-table
*
NULL
*
RSA
*
RSA
*
RSA_EXPORT
RSA
RSA
*
RSA_EXPORTRSA*RSA_EXPORT
RSA
RSA
*
DH_DSS_EXPORTDH_DSS
DH_DSS*DH_RSA_EXPORTDH_RSADH_RSA*DHE_DSS_EXPORTDHE_DSS
DHE_DSS*DHE_RSA_EXPORTDHE_RSADHE_RSA*
DH_anon_EXPORTDH_anonDH_anonDH_anonDH_anon
FORTEZZA_DMS
FORTEZZA_DMS

Cipher

Hash

NULL
NULL
NULL
RC4_40
RC4_128
RC4_128
RC2_CBC_40
IDEA_CBC
DES40_CBC
DES_CBC
3DES_EDE_CBC
DES40_CBC
DES_CBC
3DES_EDE_CBC
DES40_CBC
DES_CBC
3DES_EDE_CBC
DES40_CBC
DES_CBC
3DES_EDE_CBC
DES40_CBC
DES_CBC
3DES_EDE_CBC
RC4_40
RC4_128
DES40_CBC
DES_CBC
3DES_EDE_CBC
NULL
FORTEZZA_CBC

NULL
MD5
SHA
MD5
MD5
SHA
MD5
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
SHA
MD5
MD5
SHA
SHA
SHA
SHA
SHA

* Indicates IsExportable is True
Key Exchange
Algorithm
DHE_DSS
DHE_DSS_EXPORTDHE_RSADHE_RSA_EXPORTDH_anonDH_anon_EXPORTDH_DSS
DH_DSS_EXPORTDH_RSADH_RSA_EXPORTFORTEZZA_DMS
NULL
RSA
RSA_EXPORTKey size limit

SSL Version 3.0

Description

Key size limit

Ephemeral DH with DSS signatures
Ephemeral DH with DSS signatures
Ephemeral DH with RSA signatures
Ephemeral DH with RSA signatures
Anonymous DH, no signatures
Anonymous DH, no signatures
DH with DSS-based certificates
DH with DSS-based certificates
DH with RSA-based certificates
DH with RSA-based certificates
Fortezza DMS. Details unpublished.
No key exchange.
RSA key exchange.
RSA key exchange.

None
DH = 512 bits
None
DH = 512 bits, RSA = none
None
DH = 512 bits
None
DH = 512 bits
None
DH = 512 bits, RSA = none.
N/A
N/A
None
RSA = 512 bits.

The key size limit gives the size of the largest public key thatcan be legally used for encryption in cipher suites that areexportable.

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CipherCipherType

IsExportable

Key
Material

Exp.
Key
Material

Effective
Key Bits

IV_
Size

Block
Size

NULL
FORTEZZA_CBC
IDEA_CBC
RC2_CBC_40
RC4_40
RC4_128
DES40_CBC
DES_CBC
3DES_EDE_CBC

Stream
Block
Block
Block
StreamStream
Block
Block
Block

*

0
NA (**)
16
5
5
16
5
8
24

0
12 (**)
16
16
16
16
8
8
24

0
96 (**)
128
40
40
128
40
56
168

0
20 (**)
8
8
0
0
8
8
8

N/A
8
8
8
N/A
N/A
8
8
8

*
*
*

* Indicates IsExportable is true.
** Fortezza uses its own key and IV generation algorithms.

Key Material

The number of bytes from the key_block that are used forgenerating 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

HashSize

Padding
Size

NULL

0

0

MD5

16

48

SHA

20

40

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Appendix D
D. Implementation Notes
The SSL protocol cannot prevent many common security mistakes. This section provides several
recommendations to assist implementers.

D.1 Temporary RSA keys
US Export restrictions limit RSA keys used for encryption to 512 bits, but do not place any limit
on lengths of RSA keys used for signing operations. Certificates often need to be larger than 512
bits, since 512-bit RSA keys are not secure enough for high-value transactions or for applications
requiring long-term security. Some certificates are also designated signing-only, in which casethey 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 thatkeys be changed daily or every 500 transactions, and more often if possible. Note that while it isacceptable 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 PCcompatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of thecounter value is 16 bits or more. To seed a 128-bit PRNG, one would thus require approximately
100 such timer values.
Note:

The seeding functions in RSAREF and versions of BSAFE prior to 3.0 are orderindependent. For example, if 1000 seed bits are supplied, one at a time, in 1000
separate calls to the seed function, the PRNG will end up in a state which
depends only on the number of 0 or 1 seed bits in the seed data (i.e., there are1001 possible final states). Applications using BSAFE or RSAREF must takeextra 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

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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 rootCA.

D.4 CipherSuites
SSL supports a range of key sizes and security levels, including some which provide no or
minimal security. A proper implementation will probably not support many cipher suites. For
example, 40-bit encryption is easily broken, so implementations requiring strong security should
not allow 40-bit keys. Similarly, anonymous Diffie-Hellman is strongly discouraged because it
cannot prevent man-in-the-middle attacks. Applications should also enforce minimum andmaximum key sizes. For example, certificate chains containing 512-bit RSA keys or signatures are
not appropriate for high-security applications.

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Appendix E
E. Version 2.0 Backward Compatibility
Version 3.0 clients that support Version 2.0 servers must send Version 2.0 client hello messages[SSL-2]. Version 3.0 servers should accept either client hello format. The only deviations from the
Version 2.0 specification are the ability to specify a version with a value of three and the support
for more ciphering types in the CipherSpec.
Warning:

The ability to send Version 2.0 client hello messages will be phased out with all
due haste. Implementers should make every effort to move forward as quickly as
possible. Version 3.0 provides better mechanisms for transitioning to newer
versions.

The following cipher specifications are carryovers from SSL Version 2.0. These are assumed touse 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 hellomessages using the syntax below. Any V2CipherSpec element with its first byte equal to zero willbe ignored by Version 2.0 servers. Clients sending any of the above V2CipherSpecs should also
include the Version 3.0 equivalent (see Appendix A.6):
V2CipherSpec (see Version 3.0 name) = { 0x00, CipherSuite };E.1 Version 2 client hello
The Version 2.0 client hello message is presented below using this document’s presentation
model. The true definition is still assumed to be the SSL Version 2.0 specification.
uint8 V2CipherSpec[3];struct {unit8 msg_type;
Version version;
uint16 cipher_spec_length;
uint16 session_id_length;
uint16 challenge_length;
V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
opaque session_id[V2ClientHello.session_id_length];
Random challenge;} V2ClientHello;
msg_typeThis field, in conjunction with the version field, identifies aversion 2 client hello message. The value should equal one (1).

versionThe highest version of the protocol supported by the client(equals ProtocolVersion.version, see Appendix A.1.1).

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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, theclient is creating a new session. If 16, the session_id field willcontain the 16 bytes of session identification.

challenge_length

The length in bytes of the client’s challenge to the server toauthenticate itself. This value must be 32.

cipher_specs

This is a list of all CipherSpecs the client is willing and able touse. There must be at least one CipherSpec acceptable to theserver.

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 willright 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 lengthof 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 ofchallenge data.

Note:

Requests to resume an SSL 3.0 session should use an SSL 3.0 client hello.

E.2 Avoiding man-in-the-middle version rollback
When SSL Version 3.0 clients fall back to Version 2.0 compatibility mode, they use special PKCS
#1 block formatting. This is done so that Version 3.0 servers will reject Version 2.0 sessions with
Version 3.0-capable clients.
When Version 3.0 clients are in Version 2.0 compatibility mode, they set the right-hand (leastsignificant) 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 to0x03 (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.

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Appendix F
F. Security analysis
The SSL protocol is designed to establish a secure connection between a client and a server
communicating over an insecure channel. This document makes several traditional assumptions,
including that attackers have substantial computational resources and cannot obtain secret
information from sources outside the protocol. Attackers are assumed to have the ability tocapture, modify, delete, replay, and otherwise tamper with messages sent over thecommunication 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 asecure session. The handshake protocol can also optionally authenticate parties who havecertificates 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, thechannel should be secure against man-in-the-middle attacks, but completely anonymous sessions
are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients, since
the client signature in the certificate verify message may require a server certificate to bind thesignature 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 thecommunicating parties and not to attackers. The pre_master_secret will be used to generate themaster_secret (see Section 8.1). The master_secret is required to generate the finishedmessages, encryption keys, and MAC secrets (see Sections 7.6.9 and 8.2.2). By sending a correctfinished 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
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who do not know the private values should not be able to find the Diffie-Hellman result (i.e. thepre_master_secret) or the Fortezza token encryption key (TEK).
Warning:

Completely anonymous connections only provide protection against passive
eavesdropping. Unless an independent tamper-proof channel is used to verify
that the finished messages were not replaced by an attacker, server
authentication is required in environments where active man-in-the-middle
attacks are a concern.

F.1.1.2 RSA key exchange and authentication
With RSA, key exchange and server authentication are combined. The public key may be either
contained in the server’s certificate or may be a temporary RSA key sent in a server key exchange
message. When temporary RSA keys are used, they are signed by the server’s RSA or DSS
certificate. The signature includes the current ClientHello.random, so old signatures andtemporary 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 finishedmessage, the server demonstrates that it knows the private key corresponding to the server
certificate.
When RSA is used for key exchange, clients are authenticated using the certificate verify message(see Section 7.6.8). The client signs a value derived from the master_secret and all precedinghandshake messages. These handshake messages include the server certificate, which binds thesignature to the server, and ServerHello.random, which binds the signature to the current
handshake process.

F.1.1.3 Diffie-Hellman key exchange with authentication
When Diffie-Hellman key exchange is used, the server can either supply a certificate containing
fixed Diffie-Hellman parameters or can use the client key exchange message to send a set of
temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary
parameters are hashed with the hello.random values before signing to ensure that attackers do
not replay old parameters. In either case, the client can verify the certificate or signature to ensurethat 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 thannecessary, it should be converted into the master_secret as soon as possible. Client DiffieHellman 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 acertificate 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 fixedpublic 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 thepre_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 notsecure against attackers who can brute force the key and substitute a new ENCRYPTED-KEY-DATA
message containing the same key (but with normal padding) before the application specified wait
threshold has expired. Parties concerned about attacks of this scale should not be using 40-bit
encryption keys anyway. Altering the padding of the least-significant 8 bytes of the PKCS
padding does not impact security, since this is essentially equivalent to increasing the input block
size by 8 bytes.

F.1.3 Detecting attacks against the handshake protocol
An attacker might try to influence the handshake exchange to make the parties select different
encryption algorithms than they would normally choose. Because many implementations will
support 40-bit exportable encryption and some may even support null encryption or MAC
algorithms, this attack is of particular concern.
For this attack, an attacker must actively change one or more handshake messages. If this occurs,
the client and server will compute different values for the handshake message hashes. As a result,the parties will not accept each others’ finished messages. Without the master_secret, theattacker cannot repair the finished messages, so the attack will be discovered.

F.1.4 Resuming sessions
When a connection is established by resuming a session, new ClientHello.random andServerHello.random values are hashed with the session’s master_secret. Provided that themaster_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 thatthe session may have been compromised, or that certificates may have expired or been revoked, itshould 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.

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F.1.5 MD5 and SHA
SSL uses hash functions very conservatively. Where possible, both MD5 and SHA are used intandem 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, themessage length, the message contents, and two fixed character strings. The message type field isnecessary to ensure that messages intended for one SSL Record Layer client are not redirected toanother. 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 fromone party cannot be inserted into the other’s output, since they use independent MAC secrets.
Similarly, the server-write and client-write keys are independent so stream cipher keys are used
only once.
If an attacker does break an encryption key, all messages encrypted with it can be read. Similarly,
compromise of a MAC key can make message modification attacks possible. Because MACs arealso encrypted, message-alteration attacks generally require breaking the encryption algorithm aswell 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, andapplications 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.

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Appendix G
G. Patent Statement
This version of the SSL protocol relies on the use of patented public key encryption technology forauthentication and encryption. The Internet Standards Process as defined in RFC 1310 requires awritten statement from the Patent holder that a license will be made available to applicants underreasonable terms and conditions prior to approving a specification as a Proposed, Draft orInternet Standard.
The Massachusetts Institute of Technology has granted RSA Data Security, Inc., exclusive sublicensing rights to the following patent issued in the United States:
Cryptographic Communications System and Method (“RSA”), No. 4,405,829
The Board of Trustees of the Leland Stanford Junior University have granted Caro-Kann
Corporation, a wholly owned subsidiary corporation, exclusive sub-licensing rights to thefollowing patents issued in the United States, and all of their corresponding foreign patents:
Cryptographic Apparatus and Method (“Diffie-Hellman”), No. 4,200,770
Public Key Cryptographic Apparatus and Method (“Hellman-Merkle”), No. 4,218,582
The Internet Society, Internet Architecture Board, Internet Engineering Steering Group and the
Corporation for National Research Initiatives take no position on the validity or scope of thepatents and patent applications, nor on the appropriateness of the terms of the assurance. The
Internet Society and other groups mentioned above have not made any determination as to any
other intellectual property rights which may apply to the practice of this standard. Any further
consideration of these matters is the user’s own responsibility.

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References[DH1] W. Diffie and M. E. Hellman, “New Directions in Cryptography,” IEEE Transactions on
Information Theory, V. IT-22, n. 6, Jun 1977, pp. 74-84.
[3DES] W. Tuchman, “Hellman Presents No Shortcut Solutions To DES,” IEEE Spectrum, v. 16, n.
7, July 1979, pp40-41.
[DES] ANSI X3.106, “American National Standard for Information Systems-Data Link
Encryption,” American National Standards Institute, 1983.
[DSS] NIST FIPS PUB 186, “Digital Signature Standard,” National Institute of Standards andTechnology, U.S. Department of Commerce, 18 May 1994.
[FOR] NSA X22, Document # PD4002103-1.01, “Fortezza: Application Implementers Guide,”
April 6, 1995.
[FTP] J. Postel and J. Reynolds, RFC 959: File Transfer Protocol, October 1985.
[HTTP] T. Berners-Lee, R. Fielding, H. Frystyk, Hypertext Transfer Protocol -- HTTP/1.0,
October, 1995.
[IDEA] X. Lai, “On the Design and Security of Block Ciphers,” ETH Series in Information
Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992.
[KRAWCZYK] H. Krawczyk, IETF Draft: Keyed-MD5 for Message Authentication, November
1995.
[MD2] R. Rivest. RFC 1319: The MD2 Message Digest Algorithm, April 1992.
[MD5] R. Rivest. RFC 1321: The MD5 Message Digest Algorithm, April 1992.
[PKCS1] RSA Laboratories, “PKCS #1: RSA Encryption Standard,” version 1.5, November 1993.
[PKCS6] RSA Laboratories, “PKCS #6: RSA Extended Certificate Syntax Standard,” version 1.5,
November 1993.
[PKCS7] RSA Laboratories, “PKCS #7: RSA Cryptographic Message Syntax Standard,” version
1.5, November 1993.
[RSA] R. Rivest, A. Shamir, and L. M. Adleman, “A Method for Obtaining Digital Signatures andPublic-Key Cryptosystems,” Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120-126.
[RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782[SCH] B. Schneier. Applied Cryptography: Protocols, Algorithms, and Source Code in C,
Published by John Wiley & Sons, Inc. 1994.
[SHA] NIST FIPS PUB 180-1, “Secure Hash Standard,” National Institute of Standards andTechnology, U.S. Department of Commerce, DRAFT, 31 May 1994.
[TCP] ISI for DARPA, RFC 793: Transport Control Protocol, September 1981.

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Netscape Communications Corporation[TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993.
[X509] CCITT. Recommendation X.509: “The Directory - Authentication Framework”, 1988.
[XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data Representation
Standard, August 1995.

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Authors
Alan O. Freier
Netscape Communications
freier@netscape.com

Paul C. Kocher
Independent Consultant
pck@netcom.com

Philip L. Karlton
Netscape Communications
karlton@netscape.com

Other contributors
Martin Abadi
Digital Equipment Corporation
ma@pa.dec.com

Robert Relyea
Netscape Communications
relyea@netscape.com

Taher Elgamal
Netscape Communications
elgamal@netscape.com

Jim Roskind
Netscape Communications
jar@netscape.com

Anil Gangolli
Netscape Communications
gangolli@netscape.com

Micheal J. Sabin, Ph. D.
Consulting Engineer
msabin@netcom.com

Kipp E.B. Hickman
Netscape Communications
kipp@netscape.com

Tom Weinstein
Netscape Communications
tomw@netscape.com

Early reviewers
Robert Baldwin
RSA Data Security, Inc.
baldwin@rsa.com

Clyde Monma
Bellcoreclyde@bellcore.com

George CoxIntel Corporation
cox@ibeam.jf.intel.com

Eric Murrayericm@lne.com

Cheri Dowell
Sun Microsystems
cheri@eng.sun.com

Avi Rubin
Bellcorerubin@bellcore.com

Stuart Haber
Bellcorestuart@bellcore.com

Don Stephenson
Sun Microsystems
don.stephenson@eng.sun.com

Burt Kaliski
RSA Data Security, Inc.
burt@rsa.com

Joe Tardo
General Magictardo@genmagic.com

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Send all written communication about this document to:
Netscape Communications
501 East Middlefield Rd.
Mountain View, CA 94043
Attn: Alan Freier

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