SSL 3.0 Specification

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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.
Introduction
The primary goal of the SSL Protocol is to provide privacy and reliability between two communicating applications. The protocol is composed of two layers. At the lowest level, layered on top of some reliable transport protocol (e.g., TCP[TCP]), is the SSL Record Protocol. The SSL Record Protocol is used for encapsulation of various higher level protocols. One such encapsulated protocol, theSSL Handshake Protocol, allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first byte of data. One advantage of SSL is that it is application protocol independent. A higher level protocol can layer on top of the SSL Protocol transparently.
The SSL protocol provides connection security that has three basic properties:
- The connection is private. Encryption is used after an initial handshake to define a secret key. Symmetric cryptography is used for data encryption (e.g., DES[DES], RC4[RC4], etc.)
- The peer's identity can be authenticated using asymmetric, or public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.).
- The connection is reliable. Message transport includes a message integrity check using a keyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations.
Goals
The goals of SSL Protocol v3.0, in order of their priority, are:
1. Cryptographic security
SSL should be used to establish a secure connection between two parties.
2. Interoperability
Independent programmers should be able to develop applications utilizing SSL 3.0 that will then be able to successfully exchange cryptographic parameters without knowledge of one another's code.
Note:It is not the case that all instances of SSL (even in the same application domain) will be able to successfully connect. For instance, if the server supports a particular hardware token, and the client does not have access to such a token, then the connection will not succeed.
3. Extensibility
SSL seeks to provide a framework into which new public key and bulk encryption methods can be incorporated as necessary. This will also accomplish two sub-goals: to prevent the need to create a new protocol (and risking the introduction of possible new weaknesses) and to avoid the need to implement an entire new security library.
4. Relative efficiency
Cryptographic operations tend to be highly CPU intensive, particularly public key operations. For this reason, the SSL protocol has incorporated an optional session caching scheme to reduce the number of connections that need to be established from scratch. Additionally, care has been taken to reduce network activity.
Goals of this document
The SSL Protocol Version 3.0 Specification is intended primarily for readers who will be implementing the protocol and those doing cryptographic analysis of it. The spec has been written with this in mind, and it is intended to reflect the needs of those two groups. For that reason, many of the algorithm-dependent data structures and rules are included in the body of the text (as opposed to in an Appendix), providing easier access to them.
This document is not intended to supply any details of service definition nor interface definition, although it does cover select areas of policy as they are required for the maintenance of solid security.
Presentation language
This document deals with the formatting of data in an external representation. The following very basic and somewhat casually defined presentation syntax will be used. The syntax draws from several sources in its structure. Although it resembles the programming language 'C' in its syntax and XDR [XDR] in both its syntax and intent, it would be risky to draw too many parallels. The purpose of this presentation language is to document SSL only, not to have general application beyond that particular goal.
6.1 Basic block size
The representation of all data items is explicitly specified. The basic data block size is one byte (i.e. 8 bits). Multiple byte data items are concatenations of bytes, from left to right, from top to bottom. From the bytestream a multi-byte item (a numeric in the example) is formed (using C notation) by:
```
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ... | byte[n-1];
```
This byte ordering for multi-byte values is the commonplace network byte order or big endian format.
6.2 Miscellaneous
Comments begin with "/*" and end with "*/".
Optional components are denoted by enclosing them in italic "[ ]" brackets.
Single byte entities containing uninterpreted data are of type opaque.
6.3 Vectors
A vector (single dimensioned array) is a stream of homogeneous data elements. The size of the vector may be specified at documentation time or left unspecified until runtime. In either case the length declares the number of bytes, not the number of elements, in the vector.
The syntax for specifying a new type T' that is a fixed length vector of typeT is
```
T T'[n];
```
Here T' occupies n bytes in the data stream, where n is a multiple of the size ofT. The length of the vector is not included in the encoded stream.
In the following example, Datum is defined to be three consecutive bytes that the protocol does not interpret, whileData is three consecutiveDatum, consuming a total of nine bytes.
```
opaque Datum[3]; /* three uninterpreted bytes of data */Datum Data[9];   /* 3 consecutive 3 byte vectors */
```
Variable length vectors are defined by specifying a subrange of legallengths, inclusively, using the notation<floor..ceiling>.When encoded, theactual length precedes the vector's contents inthe byte stream. The length will be in the form of a number consuming asmany bytes as required to hold the vector's specified maximum(ceiling) length. A variable length vector with an actual lengthfield of zero is referred to as an empty vector.
```
T T'<floor..ceiling>;
```
In the following example, mandatory is a vector that must containbetween 300 and 400 bytes of typeopaque. It can never be empty.Theactual length field consumes two bytes, auint16,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 byteactual length field prepended to the vector.
```
opaque mandatory<300..400>; /* length field is 2 bytes, cannot be empty */uint16 longer<0..800>;      /* zero to 400 16-bit unsigned integers */
```
6.4 Numbers
The basic numeric data type is an unsigned byte (uint8). All larger numeric data types are formed from fixed length series of bytes concatenated as described inSection 6.1and 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 calledenum. A field of typeenum can only assume the values declared in the definition. Each definition is a different type. Only enumerateds of the same type may be assigned or compared. Every element of an enumerated must be assigned a value, as demonstrated in the following example. Since the elements of the enumerated are notordered, they can be assigned any unique value, in any order.
```
enum { e₁(v₁), e₂ (v_₁), ... , e_n (v_N), [(n)] } T_e;
```
Enumerateds occupy as much space in the byte stream as would its maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of typeColor.
```
enum { red(3), blue(5), white(7) } Color;
```
One may optionally specify a value without its associated tag to force the width definition without defining a superfluous element. In the following example,Taste will consume two bytes in the data stream but can only assume the values 1, 2 or 4.
```
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
```
The names of the elements of an enumeration are scoped within the defined type. In the first example, a fully qualified reference to the second element of the enumeration would beColor.blue. Such qualification is not required if the target of the assignment is well specified.
```
Color color = Color.blue;       /* overspecified, but legal */Color color = blue;     /* correct, type is implicit */
```
For enumerateds that are never converted to external representation, the numerical information may be omitted.
```
enum { low, medium, high } Amount;
```
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;        ...        T_n  f_n;} [T];
```
The fields within a structure may be qualified using the type's name using a syntax much like that available for enumerateds. For example, T.f₂ refers to the second field of the previous declaration. Structure definitions may be embedded.
6.6.1 Variants
Defined structures may have variants based on some knowledge that is available within the environment. The selector must be an enumerated type that defines the possible variants the structure defines. There must be a case arm for every element of the enumeration declared in the select. The body of the variant structure may be given a label for reference. The mechanism by which the variant is selected at runtime is not prescribed by the presentation language.
```
struct {        T₁ f₁;        T₂ f₂;            ....        T_n f_n;        select (E) {            case e₁: T_e1;            case e₂: T_e2;                ....            case e_n: T_en;        } [f_v];} [T_v];
```
For example
```
enum { apple, orange } VariantTag;struct {        uint16 number;        opaque string<0..10<;     /* variable length */} V1;struct {        uint32 number;        opaque string[10];      /* fixed length */} V2;struct {        select (VariantTag) {   /* value of variant selector is implicit */            case apple: V1;     /* definition of VariantBody, tag = apple */            case orange: V2;    /* definition of VariantBody, tag = orange */        } variant_body; /* optional label on the variant portion */} VariantRecord;
```
Variant structures may be qualified (narrowed) by specifying a value for the selector prior to the type. For example, a
```
orange VariantRecord
```
is a narrowed type of a VariantRecord containing a variant_body of type V2.
6.7 Cryptographic attributes
The four cryptographic operations digital signing, stream cipher encryption, block cipher encryption, andpublic key encryption are designateddigitally-signed, stream-ciphered, block-ciphered, andpublic-key-encrypted, respectively. A field's cryptographic processing is specified by prepending an appropriate key word designation before the field's type specification. Cryptographic keys are implied by thecurrent session state (seeSection 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) issigned (encrypted with the private key). In DSS, the 20 bytes of the SHA hash are run directly through the Digital Signing Algorithm with no additional hashing.
In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically-secure keyed pseudorandom number generator.
In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. Because it is unlikely that the plaintext (whatever data is to be sent) will break neatly into the necessary block size (usually 64 bits), it is necessary to pad out the end of short blocks with some regular pattern, usually all zeroes.
In public key encryption, one-way functions with secret "trapdoors" are used to encrypt the outgoing data. Data encrypted with the public key of a given key pair can only be decrypted with the private key, and vice-versa.
In the following example:
```
stream-ciphered struct {        uint8 field1;        uint8 field2;        digitally-signed opaque hash[20];} UserType;
```
The contents of hash are used as input for a 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, and structures that contain opaque) cannot be assigned values. No fields of a multi-element structure or vector may be elided.
For example,
```
struct {        uint8 f1;        uint8 f2;} Example1;Example1 ex1 = {1, 4};  /* assigns f1 = 1, f2 = 4 */
```
SSL protocol
SSL is a layered protocol. At each layer, messages may include fields for length, description, and content. SSL takes messages to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, and transmits the result. Received data is decrypted, verified, decompressed, and reassembled, then delivered to higher level clients.
7.1 Session and connection states
An SSL session is stateful. It is the responsibility of the SSL Handshake protocol to coordinate the states of the client and server, thereby allowing the protocol state machines of each to operate consistently, despite the fact that the state is not exactly parallel. Logically the state is represented twice, once as the currentoperating state, and (during the handshake protocol) again as thepending state. Additionally, separate read and write states are maintained. When the client or server receives achange cipher spec message, it copies the pending read state into the current read state. When the client or server sends achange cipher spec message, it copies the pending write state into the current write state. When the handshake negotiation is complete, the client and server exchangechange cipher spec messages (seeSection 7.3), and then communicate using the newly agreed-upon cipher spec.
An SSL session may include multiple secure connections; in addition, parties may have multiple simultaneous sessions.
The session state includes the following elements:
session identifier
An arbitrary byte sequence chosen by the server to identify an active or resumable session state
peer certificate
X509.v3[X509] certificate of the peer. This element of the state may be null.
compression method
The algorithm used to compress data 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 thehash_size. (SeeAppendix A.7 for formal definition.)
master secret
48-byte secret shared between the client and server.
is resumable
A flag indicating whether the session can be used to initiate new connections.
The connection state includes the following elements:
server and client random
Byte sequences that are chosen by the server and client for each connection.
server write MAC secret
The secret used in MAC operations on data written by the server.
client write MAC secret
The secret used in MAC operations on data written by the client.
server write key
The bulk cipher key for data encrypted by the server and decrypted by the client.
client write key
The bulk cipher key for data encrypted by the client and decrypted by the server.
initialization vectors
When a block cipher in CBC mode is used, an initialization vector (IV) is maintained for each key. This field is first initialized by the SSL handshake protocol. Thereafter the final ciphertext block from each record is preserved for use with the following record.
sequence numbers
Each party maintains separate sequence numbers for transmitted and received messages for each connection. When a party sends or receives achange cipher spec message, the appropriate sequence number is set to zero. Sequence numbers are of type uint64 and may not exceed 2⁶⁴-1.
7.2 Record layer
The SSL Record Layer receives uninterpreted data from higher layers in non-empty blocks of arbitrary size.
7.2.1Fragmentation
The record layer fragments information blocks into SSLPlaintext records of 2¹⁴ bytes or less. Client message boundaries are not preserved in the record layer (i.e., multiple client messages of the same ContentType may be coalesced into a single SSLPlaintext record).
```
struct {        uint8 major, minor;} ProtocolVersion;enum {        change_cipher_spec(20), alert(21), handshake(22),        application_data(23), (255)} ContentType;struct {        ContentType type;        ProtocolVersion version;        uint16 length;        opaque fragment[SSLPlaintext.length];} SSLPlaintext;
```
type
The higher level protocol used to process the enclosed fragment.
version
The version of protocol being employed. This document describes SSL Version 3.0 (SeeAppendix A.1.1).
length
The length (in bytes) of the following SSLPlaintext.fragment. The length should not exceed 2¹⁴.
fragment
The application data. This data is transparent and treated as an independent block to be dealt with by the higher level protocol specified by the type field.
Note: Data of different SSL Record layer content types may be interleaved. Application data is generally of lower precedence for transmission than other content types.
7.2.2 Record compression and decompression
All records are compressed using the compression algorithm defined in the current session state. There is always an active compression algorithm, however initially it is defined asCompressionMethod.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 inSection 7.1. References to fields of the CipherSpec are made throughout this document using presentation syntax. A more complete description of the CipherSpec is shown inAppendix A.7.
Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters anSSLCompressed.fragment that would decompress to a length in excess of 2¹⁴ bytes, it should issue a fataldecompression_failure alert (Section 7.4.2).
```
struct {        ContentType type;               /* same as SSLPlaintext.type */        ProtocolVersion version;        /* same as SSLPlaintext.version */        uint16 length;        opaque fragment[SSLCompressed.length];} SSLCompressed;
```
length
The length (in bytes) of the following SSLCompressed.fragment. The length should not exceed 2¹⁴ + 1024.
fragment
The compressed form of SSLPlaintext.fragment.
Note: A CompressionMethod.null operation is an identity operation; no fields are altered. (See Appendix A.4.1)
Implementation note: Decompression functions are responsible for ensuring that messages cannot cause internal buffer overflows.
7.2.3 Record payload protection and the CipherSpec
All records are protected using the encryption and MAC algorithms defined in the current CipherSpec. There is always an activeCipherSpec, however initially it isSSL_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 anSSLCiphertext. The decryption functions reverse the process. Transmissions also include a sequence number so that missing, altered, or extra messages are detectable.
```
struct {        ContentType type;        ProtocolVersion version;        uint16 length;        select (CipherSpec.cipher_type) {            case stream: GenericStreamCipher;            case block: GenericBlockCipher;        } fragment;} SSLCiphertext;
```
type
The type field is identical to SSLCompressed.type.
version
The version field is identical to SSLCompressed.version.
length
The length (in bytes) of the following SSLCiphertext.fragment. The length may not exceed 2¹⁴ + 2048.
fragment
The encrypted form of SSLCompressed.fragment, including the MAC.
7.2.3.1 Null or standard stream cipher
Stream ciphers (including BulkCipherAlgorithm.null - See Appendix A.7 ) convert SSLCompressed.fragment structures to and from streamSSLCiphertext.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 time for MD5 or 40 times for SHA.
pad_2
The character 0x5c repeated the same number of times.
seq_num
The sequence number for this message.
hash
The hashing algorithm derived from the cipher suite.
Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet. If the CipherSuite is SSL_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not encrypted and the MAC size is zero implying that no MAC is used).SSLCiphertext.length isSSLCompressed.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.
```
block-ciphered struct {        opaque content[SSLCompressed.length];        opaque MAC[CipherSpec.hash_size];        uint8 padding[GenericBlockCipher.padding_length];        uint8 padding_length;} GenericBlockCipher;
```
The MAC is generated as described in Section 7.2.3.1.
padding
Padding that is added to force the length of the plaintext to be a multiple of the block cipher's block length.
padding_length
The length of the padding must be less than the cipher's block length and may be zero. The padding length should be such that the total size of the GenericBlockCipher structure is a multiple of the cipher's block length.
The encrypted data length (SSLCiphertext.length) is one more than the sum ofSSLCompressed.length, CipherSpec.hash_size, andpadding_length.
Note: With CBC block chaining the initialization vector (IV) for the first record is provided by the handshake protocol. The IV for subsequent records is the last ciphertext block from the previous record.
7.3 Change cipher spec protocol
The change cipher spec protocol exists to signal transitions in ciphering strategies. The protocol consists of a single message, which is encrypted and compressed under thecurrent (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-negotiatedCipherSpec and keys. Reception of this message causes the receiver to copy theread pending state into the read current state. Separate read and write states are maintained by both the SSL client and server. When the client or server receives achange cipher spec message, it copies the pending read state into the current read state. When the client or server writes achange cipher spec message, it copies the pending write state into the current write state. The client sends achange cipher spec message following handshakekey exchange andcertificate verify messages (if any), and the server sends one after successfully processing thekey exchange message it received from the client. An unexpectedchange cipher specmessage should generate an unexpected_messagealert (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 thealert type.Alert messages convey the severity of the message and a description of the alert.Alert messages with a level of fatal result in the immediate termination of the connection. In this case, other connections corresponding to the session may continue, but the session identifier must be invalidated, preventing the failed session from being used to establish new connections. Like other messages,Alert messages are encrypted and compressed, as specified by the current connection state.
```
enum { warning(1), fatal(2), (255) } AlertLevel;enum {        close_notify(0),        unexpected_message(10),        bad_record_mac(20),        decompression_failure(30),        handshake_failure(40), no_certificate(41), bad_certificate(42),            unsupported_certificate(43), certificate_revoked(44),            certificate_expired(45), certificate_unknown(46),            illegal_parameter (47)        (255)} AlertDescription;struct {        AlertLevel level;        AlertDescription description;} Alert;
```
7.4.1 Closure alerts
The client and the server must share knowledge that the connection is ending in order to avoid a truncation attack. Either party may initiate the exchange of closing messages.
close_notify
This message notifies the recipient that the sender will not send any more messages on this connection. The session becomes unresumable if any connection is terminated without properclose_notify messages with level equal towarning.
7.4.2 Error alerts
Error handling in the SSL Handshake protocol is very simple. When an error is detected, the detecting party sends a message to the other party. Upon transmission or receipt of anfatal alert message, both parties immediately close the connection. Servers and clients are required to forget any session-identifiers, keys, and secrets associated with a failed connection. The following error alerts are defined:
unexpected_message
An inappropriate message was received. This alert is always fatal and should never be observed in communication between proper implementations.
bad_record_mac
This alert is returned if a record is received with an incorrect MAC. This message is always fatal.
decompression_failure
The decompression function received improper input (e.g. data that would expand to excessive length). This message is always fatal.
handshake_failure
Reception of a handshake_failure alert message indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available. This is a fatal error.
no_certificate
A no_certificate alert message may be sent in response to acertification request if no appropriate certificate is available.
bad_certificate
A certificate was corrupt, contained signatures that did not verify correctly, etc.
unsupported_certificate
A certificate was of an unsupported type.
certificate_revoked
A certificate was revoked by its signer.
certificate_expired
A certificate has expired or is not currently valid.
certificate_unknown
Some other (unspecified) issue arose in processing the certificate, rendering it unacceptable.
illegal_parameter
A field in the handshake was out of range or inconsistent with other fields. This is always fatal.
7.5 Handshake protocol overview
The cryptographic parameters of the session state are produced by theSSL Handshake Protocol, which operates on top of theSSL Record Layer. When a SSL client and server first start communicating, they agree on a protocol version, select cryptographic algorithms, optionally authenticate each other, and use public-key encryption techniques to generate shared secrets. These processes are performed in the handshake protocol, which can be summarized as follows:
The client sends a client hello message to which the server must respond with aserver hello message, or else a fatal error will occur and the connection will fail. Theclient hello andserver hello are used to establish security enhancement capabilities between client and server. Theclient hello andserver hello establish the following attributes: protocol version, session ID, cipher suite, and compression method. Additionally, two random values are generated and exchanged:ClientHello.random andServerHello.random.
Following the hello messages, the server will send its certificate, if it is to be authenticated. Additionally, aserver key exchange message may be sent, if it is required (e.g. if their server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected.
Now the server will send the server hello done message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response.
If the server has sent a certificate request message, the client must send either thecertificate message or ano certificate alert. Theclient key exchange message is now sent, and the content of that message will depend on the public key algorithm selected between theclient hello and theserver hello. If the client has sent a certificate with signing ability, a digitally-signedcertificate 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 thepending Cipher Spec into thecurrent Cipher Spec. The client then immediately sends thefinished message under the new algorithms, keys, and secrets. In response, the server will send its ownchange cipher spec message, transfer thepending to thecurrent Cipher Spec, and send itsFinished message under the new Cipher Spec. At this point, the handshake is complete and the client and server may begin to exchange application layer data. (See flow chart below.)
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 client hello using the Session ID of the session to be resumed. The Server then checks its session cache for a match. If a match is found, and the server is willing to re-establish the connection under the specified session state, it will send a server hello with the same Session ID value. At this point, both client and server must sendchange cipher spec messages and proceed directly tofinished messages. Once the re-establishment is complete, the client and server may begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID and the SSL client and server perform a full handshake.
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 asession. Handshake messages are supplied to the SSL Record Layer, where they are encapsulated within one or more SSLPlaintext structures, which are processed and transmitted as specified by the current active session state.
```
enum {        hello_request(0), client_hello(1), server_hello(2),        certificate(11), server_key_exchange (12), certificate_request(13),        server_hello_done(14), certificate_verify(15), client_key_exchange(16),        finished(20), (255)} HandshakeType;struct {        HandshakeType msg_type;         /* 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.
7.6.1 Hello messages
The hello phase messages are used to exchange security enhancement capabilities between the client and server. When a new session begins, theCipherSpec encryption, hash, and compression algorithms are initialized to null. The currentCipherSpec 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 over application data, it is expected that the negotiation begin in no more than one or two times the transmission time of a maximum length application data message.
After sending a hello request, servers should not repeat the request until the subsequent handshake negotiation is complete. A client that receives ahello request while in a handshake negotiation state should simply ignore the message.
The structure of a hello request message is as follows:
```
struct { } HelloRequest;
```
7.6.1.2 Client hello
When a client first connects to a server it is required to send the client hello as its first message. The client can also send aclient hello in response to ahello request or on its own initiative in order to renegotiate the security parameters in an existing connection.
The client hello message includes a random structure, which is used later in the protocol.
```
struct {   uint32 gmt_unix_time;   opaque random_bytes[28];} Random;
```
gmt_unix_time
The current time and date in standard UNIX 32-bit format according to the sender's internal clock. Clocks are not required to be set correctly by the basic SSL Protocol; higher level or application protocols may define additional requirements.
random_bytes
28 bytes generated by a secure random number generator.
The client hello message includes a variable length session identifier. If not empty, the value identifies a session between the same client and server whose security parameters the client wishes to reuse. The session identifier may be from an earlier connection, this connection, or another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, while the third option makes it possible to establish several simultaneous independent secure connections without repeating the full handshake protocol. The actual contents of theSessionID are defined by the server.
```
opaque SessionID<0..32>;
```
Warning: Servers must not place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security.
The CipherSuite list, passed from the client to the server in the client hello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (first choice first). EachCipherSuite defines both a key exchange algorithm and aCipherSpec. The server will select a cipher suite or, if no acceptable choices are presented, return ahandshake failure alert and close the connection.
```
uint8 CipherSuite[2];   /* Cryptographic suite selector */
```
The client hello includes a list of compression algorithms supported by the client, ordered according to the client's preference. If the server supports none of those specified by the client, the session must fail.
```
enum { null(0), (255) } CompressionMethod;
```
Issue: Which compression methods to support is under investigation.
The structure of the client hello is as follows.
```
struct {        ProtocolVersion client_version;        Random random;        SessionID session_id;        CipherSuite cipher_suites<2..2¹⁶-1>;        CompressionMethod compression_methods<1..2⁸-1>;} ClientHello;
```
client_version
The version of the SSL protocol by which the client wishes to communicate during this session. This should be the most recent (highest valued) version supported by the client. For this version of the specification, the version will be 3.0 (SeeAppendix 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 beempty if no session_id is available or the client wishes to generate new security parameters.
cipher_suites
This is a list of the cryptographic options supported by the client, sorted with the client's first preference first. If the session_id field is notempty (implying a session resumption request) this vector must include at least the cipher_suite from that session. Values are defined inAppendix 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 notempty (implying a session resumption request) this vector must include at least thecompression_method from that session. All implementations must support CompressionMethod.null.
After sending the client hello message, the client waits for aserver hello message. Any other handshake message returned by the server except for ahello request is treated as a fatal error.
Implementation note: Application data may not be sent before afinished message has been sent. Transmitted application data is known to be insecure until a validfinished message has been received. This absolute restriction is relaxed if there is a current, non-null encryption on this connection.
7.6.1.3 Server hello
The server processes the client hello message and responds with either ahandshake_failurealert orserver hello message.
```
struct {        ProtocolVersion server_version;        Random random;        SessionID session_id;        CipherSuite cipher_suite;        CompressionMethod compression_method;} ServerHello;
```
server_version
This field will contain the lower of that suggested by the client in the client hello and the highest supported by the server. For this version of the specification, the version will be 3.0 (SeeAppendix E for details about backward compatibility).
random
This structure is generated by the server and must be different from (and independent of)ClientHello.random.
session_id
This is the identity of the session corresponding to this connection. If the ClientHello.session_id was non-empty, the server will look in its session cache for a match. If a match is found and the server is willing to establish the new connection using the specified session state, the server will respond with the same value as was supplied by the client. This indicates a resumed session and dictates that the partiesmust proceed directly to thefinished messages. Otherwise this field will contain a different value identifying the new session. The server may return anemptysession_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. Forresumed sessions this field is the value from the state of the session being resumed.
compression_method
The single compression algorithm selected by the server from the list in ClientHello.compression_methods. For resumed sessions this field is the value from the resumed session state.
7.6.2 Server certificate
If the server is to be authenticated (which is generally the case), the server sends its certificate immediately following theserver hello message. The certificate type must be appropriate for the selected cipher suite's key exchange algorithm, and is generally an X.509.v3 certificate (or a modified X.509 certificate in the case of Fortezza [FOR]). The same message type will be used for the client's response to aserver certificate requestmessage.
```
opaque ASN.1Cert<1..2²⁴-1>;struct {        ASN.1Cert certificate_list<1..2²⁴-1>;} Certificate;
```
certificate_list This is a sequence (chain) of X.509.v3 certificates, ordered with the sender's certificate first and the root certificate authority last.
Note: PKCS #7 [PKCS7] is not used as the format for the certificate vector because PKCS #6 [PKCS6] extended certificates are not used. Also PKCS #7 defines a SET rather than a SEQUENCE, making the task of parsing the list more difficult.
7.6.3 Server key exchange message
The server key exchange message is sent by the server if it has no certificate, has a certificate only used for signing (e.g., DSS [DSS] certificates, signing-only RSA [RSA] certificates), or fortezza/DMS key exchange is used. This message is not used if the server certificate contains Diffie-Hellman [DH1] parameters.
Note: According to current US export law, RSA moduli larger than 512 bits may not be used for key exchange in software exported from the US. With this message, larger RSA keys may be used as signature-only certificates to sign temporary shorter RSA keys for key exchange.
```
enum { rsa, diffie_hellman, fortezza_dms } KeyExchangeAlgorithm;struct {        opaque rsa_modulus<1..2¹⁶-1>;        opaque rsa_exponent<1..2¹⁶-1>;} ServerRSAParams;
```
rsa_modulus The modulus of the server's temporary RSA key.
rsa_exponent The public exponent of the server's temporary RSA key.
```
struct {        opaque dh_p<1..2¹⁶-1>;        opaque dh_g<1..2¹⁶-1>;        opaque dh_Y_s<1..2¹⁶-1>;} 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_Y_s
The server's Diffie-Hellman public value (g^X mod p).
```
struct {            opaque r_s [128];} ServerFortezzaParams;
```
r_s
Server random number for Fortezza KEA (Key Exchange Algorithm).
```
struct {        select (KeyExchangeAlgorithm) {            case diffie_hellman:                  ServerDHParams params;                  Signature signed_params;            case rsa:                  ServerRSAParams params;                  Signature signed_params;            case fortezza_dms:                  ServerFortezzaParams params;        };} ServerKeyExchange;
```
params
The server's key exchange parameters.
signed_params
A hash of the corresponding params value, with the signature appropriate to that hash applied.
md5_hash
MD5(ClientHello.random + ServerHello.random + ServerParams);
sha_hash
SHA(ClientHello.random + ServerHello.random + ServerParams);
```
enum { anonymous, rsa, dsa } SignatureAlgorithm;digitally-signed struct {        select(SignatureAlgorithm) {             case anonymous: struct { };             case rsa:                  opaque md5_hash[16];                  opaque sha_hash[20];             case dsa:                  opaque sha_hash[20];        };} Signature;
```
7.6.4 Certificate request
A non-anonymous server can optionally request a certificate from the client, if appropriate for the selected cipher suite.
```
opaque CertificateAuthority<0..2²⁴-1>;enum {        rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),        rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza_dms(20), (255)} ClientCertificateType;opaque DistinguishedName<1..2¹⁶-1>;struct {        ClientCertificateType certificate_types<1..2⁸-1>;        DistinguishedName certificate_authorities<3..2¹⁶-1>;} CertificateRequest;
```
certificate_types
This field is a list of the types of certificates requested, sorted in order of the server's preference.
certificate_authorities
A list of the distinguished names of acceptable certificate authorities.
Note: DistinguishedName is derived from [X509].
Note: It is a fatal handshake_failure alert for an anonymous server to request client identification.
7.6.5 Server hello done
The server hello done message is sent by the server to indicate the end of theserver hello and associated messages. After sending this message the server will wait for a client response.
```
struct { } ServerHelloDone;
```
Upon receipt of the server hello done message the client should verify that the server provided a valid certificate if required and check that theserver hello parameters are acceptable.
7.6.6 Client certificate
This is the first message the client can send after receiving aserver hello done message. This message is only sent if the server requests a certificate. If no suitable certificate is available, the client should send ano certificate alert instead. This error is only a warning, however the server may respond with afatal 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. SeeSection 7.6.3 for the KeyExchangeAlgorithm.
```
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 pre-master secret, encrypts it under the public key from the server's certificate or temporary RSA key from aserver key exchange message, and sends the result in an encrypted premaster secret message.
```
struct {        ProtocolVersion client_version;        opaque random[46];} PreMasterSecret;
```
client_version
The latest (newest) version supported by the client. This is used to detect version roll-back attacks.
random
46 securely-generated random bytes.
```
struct {        public-key-encrypted PreMasterSecret pre_master_secret;} EncryptedPreMasterSecret;
```
pre_master_secret
This random value is generated by the client and is used to generate the master secret, as specified in Section 8.1.
7.6.7.2 Fortezza key exchange message
Under Fortezza DMS, the client derives a Token Encryption Key (TEK) using Fortezza's Key Exchange Algorithm (KEA). The client's KEA calculation uses the public key in the server's certificate along with private parameters in the client's token. The client sends public parameters needed for the server to generate the TEK, using its own private parameters. The client generates session keys, wraps them using the TEK, and sends the results to the server. The client generates IV's for the session keys and TEK and sends them also. The client generates a random 48-byte premaster secret, encrypts it using the TEK, and sends the result:
```
struct {        opaque y_c<0..128>;        opaque r_c[128];        opaque y_signature[20];        opaque wrapped_client_write_key[12];        opaque wrapped_server_write_key[12];        opaque client_write_iv[24];        opaque server_write_iv[24];        opaque master_secret_iv[24];        block-ciphered opaque encrypted_pre_master_secret[48];} FortezzaKeys;
```
y_signature
y_singnature is the signature of the KEA public key, signed with the client's DSS private key.
y_c
The client's Y_c value (public key) for the KEA calculation. If the client has sent a certificate, and its KEA public key is suitable, this value must beempty since the certificate already contains this value. If the client sent a certificate without a suitable public key, y_c is used and y_singnature is the KEA public key signed with the client's DSS private key. For this value to be used, it must be between 64 and 128 bytes.
r_c
The client's R_c value for the KEA calculation.
wrapped_client_write_key
This is the client's write key, wrapped by the TEK.
wrapped_server_write_key
This is the server's write key, wrapped by the TEK.
client_write_iv
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_secret
This is a random value, generated by the client and used to generate 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 (Y_c) if it was not already included in the client's certificate. The encoding used for Y_c is determined by the enumeratedPublicValueEncoding.
```
enum { implicit, explicit } PublicValueEncoding;
```
implicit
If the client certificate already contains the public value, then it is implicit and Y_c does not need to be sent again.
explicit
Y_c needs to be sent.
```
struct {        select (PublicValueEncoding) {            case implicit: struct { };            case explicit: opaque dh_Yc<1..2¹⁶-1>;        } dh_public;} ClientDiffieHellmanPublic;
```
dh_Yc
The client's Diffie-Hellman public value (Y_c).
7.6.8 Certificate verify
This message is used to provide explicit verification of a client certificate. This message is only sent following any client certificate that has signing capability (i.e. all certificates except those containing fixed Diffie-Hellman parameters).
```
        struct {             Signature signature;        } CertificateVerify;
```
```
CertificateVerify.signature.md5_hash        MD5(master_secret + pad2 + MD5(handshake_messages +                master_secret + pad1));Certificate.signature.sha_hash        SHA(master_secret + pad2 + SHA(handshake_messages +                master_secret + pad1));
```
Here handshake_messages refers to all handshake messages starting at client hello up to but not including this message.
7.6.9 Finished
A finished message is always sent immediately after achange cipher specsmessage to verify that the key exchange and authentication processes were successful. Thefinished message is the first 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 thefinished message. Recipients offinished messages must verify that the contents are correct.
```
enum { client(0x434C4E54), server(0x53525652) } Sender;struct {        opaque md5_hash[16];        opaque sha_hash[20];} Finished;
```
```
md5_hash        MD5(master_secret + pad2 + MD5(handshake_messages +                Sender + master_secret + pad1));sha_hash        SHA(master_secret + pad2 + SHA(handshake_messages +                Sender + master_secret + pad1));
```
The hash contained in finished messages sent by the server incorporate Sender.server; those sent by the client incorporateSender.client. The value handshake_messages includes all handshake messages starting atclient hello up to, but not including, the finished messages. This may be different fromhandshake_messages inSection 7.6.8 because it would include thecertificate verify message (if sent).
Note: Change cipher spec messages are not handshake messages and are not included in the hash computations.
7.7 Application data protocol
Application data messages are carried by the Record Layer and are fragmented, compressed and encrypted based on the current connection state. The messages are treated astransparent data to the record layer.
Cryptographic computations
The key exchange, authentication, encryption, and MAC algorithms are determined by thecipher_suite selected by the server and revealed in theserver 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 thepre_master_secret into themaster_secret. Thepre_master_secret should be deleted from memory once themaster_secret has been computed.
```
master_secret =        MD5(pre_master_secret + SHA('A' + pre_master_secret +            ClientHello.random + ServerHello.random)) +        MD5(pre_master_secret + SHA('BB' + pre_master_secret +            ClientHello.random + ServerHello.random)) +        MD5(pre_master_secret + SHA('CCC' + pre_master_secret +            ClientHello.random + ServerHello.random));
```
8.1.1 RSA
When RSA is used for server authentication and key exchange, a 48-bytepre_master_secret is generated by the client, encrypted under the server's public key, and sent to the server. The server uses its private key to decrypt thepre_master_secret. Both parties then convert the pre_master_secret into themaster_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 thepre_master_secret, and is converted into themaster_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 thepre_master_secret and converts it into amaster_secret, as specified above. Bulk cipher keys and IVs for encryption are generated by the client's token and exchanged in thekey exchange message; themaster_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 activeCipherSpec. A typical example would be to encrypt data using DES and generate authentication codes using MD5. The encryption and MAC algorithms are set toSSL_NULL_WITH_NULL_NULL at the beginning of theSSL Handshake Protocol, indicating that no message authentication or encryption is performed. The handshake protocol is used to negotiate a more secureCipherSpec and to generate cryptographic keys.
8.2.1 The master secret
Before secure encryption or integrity verification can be performed on records, the client and server need to generate shared secret information known only to themselves. This value is a 48-byte quantity called themaster secret. The master secret is used to generate keys and secrets for encryption and MAC computations. Some algorithms, such as Fortezza, may have their own procedure for generating encryption keys (the master secret is used only for MAC computations in Fortezza).
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 currentCipherSpec (seeAppendix 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 aserver write IV, which are generated from the master secret in that order. Unused values, such as Fortezza keys communicated in theKeyExchange message, are empty. The following inputs are available to the key definition process:
```
        opaque MasterSecret[48]        ClientHello.random        ServerHello.random
```
When generating keys and MAC secrets, the master secret is used as an entropy source, and the random values provide unencrypted salt material and IVs for exportable ciphers.
To generate the key material, compute
```
key_block =        MD5(master_secret + SHA('A' + master_secret + ServerHello.random +            ClientHello.random)) +        MD5(master_secret + SHA('BB' + master_secret + ServerHello.random +            ClientHello.random)) +        MD5(master_secret + SHA('CCC' + master_secret + ServerHello.random +            ClientHello.random)) + [...];
```
until enough output has been generated. Then the key_block is partitioned as follows.
```
client_write_MAC_secret[CipherSpec.hash_size]server_write_MAC_secret[CipherSpec.hash_size]client_write_key[CipherSpec.key_material]server_write_key[CipherSPec.key_material]client_write_IV[CipherSpec.IV_size]                     /* non-export ciphers */server_write_IV[CipherSpec.IV_size]                     /* non-export ciphers */
```
Any extra key_block material is discarded.
Exportable encryption algorithms (for which CipherSpec.is_exportable is true) require additional processing as follows to derive their final write keys:
```
final_client_write_key = MD5(client_write_key +        ClientHello.random + ServerHello.random);final_server_write_key = MD5(server_write_key +        ServerHello.random + ClientHello.random);
```
Exportable encryption algorithms derive their IVs from the random messages:
```
client_write_IV = MD5(ClientHello.random + ServerHello.random);server_write_IV = MD5(ServerHello.random + ClientHello.random);
```
MD5 outputs are trimmed to the appropriate size by discarding the least-significant bytes.
8.2.2.1 Export key generation example
SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for each of the two encryption keys and 16 bytes for each of the MAC keys, for a total of 42 bytes of key material. MD5 produces 16 bytes of output per call, so three calls to MD5 are required. The MD5 outputs are concatenated into a 48-byte key_block with the first MD5 call providing bytes zero through 15, the second providing bytes 16 through 31, etc. Thekey_block is partitioned, and the write keys are salted because this is an exportable encryption algorithm.
```
client_write_MAC_secret = key_block_0..15server_write_MAC_secret = key_block_16..31client_write_key        = key_block_32..36server_write_key        = key_block_37..41final_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;
```

Connected: An Internet Encyclopedia
SSL 3.0 Specification

SSL 3.0 Specification

Status of this memo

Abstract

Introduction

Goals

Goals of this document

Presentation language

6.1 Basic block size

6.2 Miscellaneous

6.3 Vectors

6.4 Numbers

6.5 Enumerateds

6.6 Constructed types

6.6.1 Variants

6.7 Cryptographic attributes

6.8 Constants

SSL protocol

7.1 Session and connection states

7.2 Record layer

7.2.1Fragmentation

7.2.2 Record compression and decompression

7.2.3 Record payload protection and the CipherSpec

7.2.3.1 Null or standard stream cipher

7.2.3.2 CBC block cipher

7.3 Change cipher spec protocol

7.4 Alert protocol

7.4.1 Closure alerts

7.4.2 Error alerts

7.5 Handshake protocol overview

7.6 Handshake protocol

7.6.1 Hello messages

7.6.1.1 Hello request

7.6.1.2 Client hello

7.6.1.3 Server hello

7.6.2 Server certificate

7.6.3 Server key exchange message

7.6.4 Certificate request

7.6.5 Server hello done

7.6.6 Client certificate

7.6.7 Client key exchange message

7.6.7.1 RSA encrypted premaster secret message

7.6.7.2 Fortezza key exchange message

7.6.7.3 Client Diffie-Hellman public value

7.6.8 Certificate verify

7.6.9 Finished

7.7 Application data protocol

Cryptographic computations

8.1 Asymmetric cryptographic computations

8.1.1 RSA

8.1.2 Diffie-Hellman

8.1.3 Fortezza

8.2 Symmetric cryptographic calculations and the CipherSpec

8.2.1 The master secret

8.2.2 Converting the master secret into keys and MAC secrets

8.2.2.1 Export key generation example