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Versions: 00 01 02 03 04 05 RFC 2246

Transport Layer Security Working Group                  Alan O. Freier
INTERNET-DRAFT                                 Netscape Communications
Expires May 31, 1997                                    Philip Karlton
                                               Netscape Communications
                                                        Paul C. Kocher
                                                Independent Consultant
                                                            Tim Dierks
                                                 Consensus Development
                                                     November 26, 1996










                          The TLS Protocol
                             Version 1.0


                   <draft-ietf-tls-protocol-00.txt>

Status of this memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet- Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or made obsolete by other
   documents at any time. It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as work in progress.

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).


Abstract

   This document specifies Version 1.0 of the Transport Layer Security
   (TLS) protocol, which is at this stage is strictly based on the
   Secure Sockets Layer (SSL) version 3.0 protocol, and is to serve as
   a basis for future discussions. The TLS protocol 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.

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Table of Contents
           Status of this memo                                       1
           Abstract                                                  1
           Table of Contents                                         2
   1.      Introduction                                              4
   2.      Goals                                                     4
   3.      Goals of this document                                    5
   4.      Presentation language                                     6
   4.1     Basic block size                                          6
   4.2     Miscellaneous                                             6
   4.3     Vectors                                                   6
   4.4     Numbers                                                   7
   4.5     Enumerateds                                               7
   4.6     Constructed types                                         8
   4.6.1   Variants                                                  9
   4.7     Cryptographic attributes                                 10
   4.8     Constants                                                11
   5.      The TLS Record Protocol                                  11
   5.1     Connection states                                        11
   5.2     Record layer                                             14
   5.2.1   Fragmentation                                            14
   5.2.2   Record compression and decompression                     15
   5.2.3   Record payload protection                                16
   5.2.3.1 Null or standard stream cipher                           16
   5.2.3.2 CBC block cipher                                         17
   5.3     Key calculation                                          18
   5.3.1   Export key generation example                            19
   6.      The TLS Handshake Protocol                               20
   6.1     Change cipher spec protocol                              21
   6.2     Alert protocol                                           21
   6.2.1   Closure alerts                                           22
   6.2.2   Error alerts                                             22
   6.3     Handshake protocol overview                              23
   6.4     Handshake protocol                                       26
   6.4.1   Hello messages                                           27
   6.4.1.1 Hello request                                            27
   6.4.1.2 Client hello                                             28
   6.4.1.3 Server hello                                             30
   6.4.2   Server certificate                                       31
   6.4.3   Server key exchange message                              32
   6.4.4   Certificate request                                      35
   6.4.5   Server hello done                                        36
   6.4.6   Client certificate                                       36
   6.4.7   Client key exchange message                              36
   6.4.7.1 RSA encrypted premaster secret message                   37
   6.4.7.2 Client Diffie-Hellman public value                       38
   6.4.8   Certificate verify                                       38
   6.4.9   Finished                                                 39
   7.      Cryptographic computations                               40
   7.1     Computing the master secret                              41
   7.1.1   RSA                                                      41

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   7.1.2   Diffie-Hellman                                           41
   8.      Application data protocol                                41
   A.      Protocol constant values                                 42
   A.1     Reserved port assignments                                42
   A.1.1   Record layer                                             42
   A.2     Change cipher specs message                              43
   A.3     Alert messages                                           43
   A.4     Handshake protocol                                       44
   A.4.1   Hello messages                                           44
   A.4.2   Server authentication and key exchange messages          45
   A.5     Client authentication and key exchange messages          46
   A.5.1   Handshake finalization message                           47
   A.6     The CipherSuite                                          47
   A.7     The Security Parameters                                  48
   B.      Glossary                                                 49
   C.      CipherSuite definitions                                  52
   D.      Implementation Notes                                     54
   D.1     Temporary RSA keys                                       54
   D.2     Random Number Generation and Seeding                     55
   D.3     Certificates and authentication                          55
   D.4     CipherSuites                                             55
   E.      Version 2.0 Backward Compatibility                       56
   E.1     Version 2 client hello                                   56
   E.2     Avoiding man-in-the-middle version rollback              58
   F.      Security analysis                                        58
   F.1     Handshake protocol                                       58
   F.1.1   Authentication and key exchange                          58
   F.1.1.1 Anonymous key exchange                                   59
   F.1.1.2 RSA key exchange and authentication                      59
   F.1.1.3 Diffie-Hellman key exchange with authentication          60
   F.1.2   Version rollback attacks                                 60
   F.1.3   Detecting attacks against the handshake protocol         61
   F.1.4   Resuming sessions                                        61
   F.1.5   MD5 and SHA                                              62
   F.2     Protecting application data                              62
   F.3     Final notes                                              62
   G.      Patent Statement                                         63
           References                                               63
           Credits                                                  65

1. Introduction

   The primary goal of the TLS Protocol is to provide privacy and
   reliability between two communicating applications. The protocol is
   composed of two layers: the TLS Record Protocol and the TLS
   Handshake Protocol. At the lowest level, layered on top of some
   reliable transport protocol (e.g., TCP[TCP]), is the TLS Record
   Protocol. The TLS Record Protocol provides connection security that
   has two basic properties:

      -  The connection is private. Symmetric cryptography is used for
         data encryption (e.g., DES[DES], RC4[RC4], etc.) The keys for

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         this symmetric encryption are generated uniquely for each
         connection and are based on a secret negotiated by another
         protocol (such as the TLS Handshake Protocol). The Record
         Protocol can also be used with no encryption.

      -  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. The Record Protocol can operate without a MAC,
         but is generally only used in this mode while another protocol
         is using the Record Protocol as a transport for negotiating
         security parameters.

   The TLS Record Protocol is used for encapsulation of various higher
   level protocols. One such encapsulated protocol, the TLS Handshake
   Protocol, allows the server and client to authenticate each other
   and to negotiate an encryption algorithm and cryptographic keys
   before the application protocol transmits or receives its first byte
   of data. The TLS Handshake Protocol provides connection security
   that has three basic properties:

      -  The peer's identity can be authenticated using asymmetric, or
         public key, cryptography (e.g., RSA[RSA], DSS[DSS], etc.).
         This authentication can be made optional, but is generally
         required for at least one of the peers.

      -  The negotiation of a shared secret is secure: the negotiated
         secret is unavailable to eavesdroppers, and for all
         authenticated connections, cannot be obtained by an attacker
         who can place himself in the middle of the connection.

      -  The negotiation is reliable: no attacker can modify the
         negotiation communication without being detected by the peers.

   One advantage of TLS is that it is application protocol independent.
   A higher level protocol can layer on top of the TLS Protocol
   transparently.

2. Goals

   The goals of TLS Protocol, in order of their priority, are:

      1. Cryptographic security
         TLS should be used to establish a secure connection between
         two parties.

      2. Interoperability
         Independent programmers should be able to develop applications
         utilizing TLS that will then be able to successfully
         exchange cryptographic parameters without knowledge of one
         another's code.


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         Note:
         It is not the case that all instances of TLS (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
         TLS 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 TLS
         protocol has incorporated an optional session caching scheme
         to reduce the number of connections that need to be
         established from scratch. Additionally, care has been taken to
         reduce network activity.

3. Goals of this document

   This document describing the TLS Protocol Version 1.0 Specification
   is strictly based on Secure Sockets Layer (SSL) Version 3.0 [SSL3],
   incorporating only errata as well as several clarifications to the
   SSL 3.0 draft, but will have no substantive changes to the "bits on
   the wire" of the SSL 3.0 protocol. This draft will be the starting
   point for future discussions, and from its base the TLS working
   group will work together to agree on what changes need to be made.

   Note that in all cases TLS has been substituted for the word SSL in
   the presentation language examples. In no way is the presentation
   language of this document any different then with SSL 3.0. This was
   done this way to ease the transition to TLS.

   This document 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.



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4. Presentation language

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


4.1 Basic block size

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

       value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | ...
       | byte[n-1];

   This byte ordering for multi-byte values is the commonplace network
   byte order or big endian format.


4.2 Miscellaneous

   Comments begin with "/*" and end with "*/".

   Optional components are denoted by enclosing them in "[[ ]]" double
   brackets.

   Single byte entities containing uninterpreted data are of type
   opaque.


4.3 Vectors

   A vector (single dimensioned array) is a stream of homogeneous data
   elements. The size of the vector may be specified at documentation
   time or left unspecified until runtime. In either case the length
   declares the number of bytes, not the number of elements, in the
   vector. The syntax for specifying a new type T' that is a fixed
   length vector of type T is

       T T'[n];

   Here T' occupies n bytes in the data stream, where n is a multiple
   of the size of T. The length of the vector is not included in the
   encoded stream.

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   In the following example, Datum is defined to be three consecutive
   bytes that the protocol does not interpret, while Data is three
   consecutive Datum, consuming a total of nine bytes.

       opaque Datum[3];      /* three uninterpreted bytes */
       Datum Data[9];        /* 3 consecutive 3 byte vectors */

   Variable length vectors are defined by specifying a subrange of
   legal lengths, inclusively, using the notation <floor..ceiling>.
   When encoded, the actual length precedes the vector's contents in
   the byte stream. The length will be in the form of a number
   consuming as many bytes as required to hold the vector's specified
   maximum (ceiling) length. A variable length vector with an actual
   length field of zero is referred to as an empty vector.

       T T'<floor..ceiling>;

   In the following example, mandatory is a vector that must contain
   between 300 and 400 bytes of type opaque. It can never be empty. The
   actual length field consumes two bytes, a uint16, sufficient to
   represent the value 400 (see Section 4.4). On the other hand, longer
   can represent up to 800 bytes of data, or 400 uint16 elements, and
   it may be empty. Its encoding will include a two byte actual length
   field prepended to the vector.

       opaque mandatory<300..400>;
             /* length field is 2 bytes, cannot be empty */
       uint16 longer<0..800>;
             /* zero to 400 16-bit unsigned integers */


4.4 Numbers

   The basic numeric data type is an unsigned byte (uint8). All larger
   numeric data types are formed from fixed length series of bytes
   concatenated as described in Section 4.1 and are also unsigned. The
   following numeric types are predefined.

       uint8 uint16[2];
       uint8 uint24[3];
       uint8 uint32[4];
       uint8 uint64[8];


4.5 Enumerateds

   An additional sparse data type is available called enum. A field of
   type enum can only assume the values declared in the definition.
   Each definition is a different type. Only enumerateds of the same
   type may be assigned or compared. Every element of an enumerated
   must be assigned a value, as demonstrated in the following example.
   Since the elements of the enumerated are not ordered, they can be

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   assigned any unique value, in any order.

       enum { e1(v1), e2(v2), ... , en(vn), [[(n)]] } Te;

   Enumerateds occupy as much space in the byte stream as would its
   maximal defined ordinal value. The following definition would cause
   one byte to be used to carry fields of type Color.

       enum { red(3), blue(5), white(7) } Color;

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

       enum { sweet(1), sour(2), bitter(4), (32000) } Taste;

   The names of the elements of an enumeration are scoped within the
   defined type. In the first example, a fully qualified reference to
   the second element of the enumeration would be Color.blue. Such
   qualification is not required if the target of the assignment is
   well specified.

       Color color = Color.blue;     /* overspecified, legal */
       Color color = blue;           /* correct, type implicit */

   For enumerateds that are never converted to external representation,
   the numerical information may be omitted.

       enum { low, medium, high } Amount;


4.6 Constructed types

   Structure types may be constructed from primitive types for
   convenience. Each specification declares a new, unique type. The
   syntax for definition is much like that of C.

       struct {
         T1 f1;
         T2 f2;
         ...
         Tn fn;
       } [[T]];

   The fields within a structure may be qualified using the type's name
   using a syntax much like that available for enumerateds. For
   example, T.f2 refers to the second field of the previous
   declaration. Structure definitions may be embedded.




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4.6.1 Variants

   Defined structures may have variants based on some knowledge that is
   available within the environment. The selector must be an enumerated
   type that defines the possible variants the structure defines. There
   must be a case arm for every element of the enumeration declared in
   the select. The body of the variant structure may be given a label
   for reference. The mechanism by which the variant is selected at
   runtime is not prescribed by the presentation language.

       struct {
           T1 f1;
           T2 f2;
            ....
           Tn fn;
           select (E) {
               case e1: Te1;
               case e2: Te2;
                   ....
               case en: Ten;
           } [[fv]];
       } [[Tv]];

   For example

       enum { apple, orange } VariantTag;
       struct {
           uint16 number;
           opaque string<0..10>; /* variable length */
       } V1;
       struct {
           uint32 number;
           opaque string[10];    /* fixed length */
       } V2;
       struct {
           select (VariantTag) { /* value of selector is implicit */
               case apple: V1;   /* VariantBody, tag = apple */
               case orange: V2;  /* VariantBody, tag = orange */
           } variant_body;       /* optional label on variant */
       } VariantRecord;

   Variant structures may be qualified (narrowed) by specifying a value
   for the selector prior to the type. For example, a

       orange VariantRecord

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





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4.7 Cryptographic attributes

   The four cryptographic operations digital signing, stream cipher
   encryption, block cipher encryption, and public key encryption are
   designated digitally-signed, stream-ciphered, block-ciphered, and
   public-key-encrypted, respectively. A field's cryptographic
   processing is specified by prepending an appropriate key word
   designation before the field's type specification. Cryptographic
   keys are implied by the current session state (see Section 5.1).

   In digital signing, one-way hash functions are used as input for a
   signing algorithm. In RSA signing, a 36-byte structure of two hashes
   (one SHA and one MD5) is signed (encrypted with the private key). In
   DSS, the 20 bytes of the SHA hash are run directly through the
   Digital Signing Algorithm with no additional hashing. A
   digitally-signed element is encoded as an opaque vector <0..2^16-1>,
   where the length is specified by the signing algorithm and key.

   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. All block cipher encryption is done in CBC
   (Cipher Block Chaining) mode, and all items which are block-ciphered
   will be an exact multiple of the cipher block length.

   In public key encryption, a public key algorithm is used to encrypt
   data in such a way that it can be decrypted only with the matching
   private key. A public-key-encrypted element is encoded as an opaque
   vector <0..2^16-1>, where the length is specified by the signing
   algorithm and key.

   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. The
   length of this structure, in bytes would be equal to 2 bytes for
   field1 and field2, plus two bytes for the length of the signature,
   plus the length of the output of the signing algorithm. This is
   known due to the fact that the algorithm and key used for the
   signing are known prior to encoding or decoding this structure.





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4.8 Constants

   Typed constants can be defined for purposes of specification by
   declaring a symbol of the desired type and assigning values to it.
   Under-specified types (opaque, variable length vectors, and
   structures that contain opaque) cannot be assigned values. No fields
   of a multi-element structure or vector may be elided.

   For example,

       struct {
           uint8 f1;
           uint8 f2;
       } Example1;

   Example1
          ex1 = {1, 4};/* assigns f1 = 1, f2 = 4 */


5. The TLS Record Protocol

   The TLS Record Protocol is a layered protocol. At each layer,
   messages may include fields for length, description, and content.
   The Record Protocol takes messages to be transmitted, fragments the
   data into manageable blocks, optionally compresses the data, applies
   a MAC, encrypts, and transmits the result. Received data is
   decrypted, verified, decompressed, and reassembled, then delivered
   to higher level clients.


5.1 Connection states

   An TLS connection state is the operating environment of the TLS
   Record Protocol. It specifies a compression algorithm, encryption
   algorithm, and MAC algorithm. In addition, the parameters for these
   algorithms are known: the MAC secret and the bulk encryption keys
   and IVs for the connection in both the read and the write
   directions. Logically, there are always four connection states
   outstanding: the current read and write states, and the pending read
   and write states. All records are processed under the current read
   and write states. The security parameters for the pending states can
   be set by the TLS Handshake Protocol, and the Handshake Protocol can
   selectively make either of the pending states current, in which case
   the appropriate current state is disposed of and replaced with the
   pending state; the pending state is then reinitialized to an empty
   state. It is illegal to make a state which has not been initialized
   with security parameters a current state (although those security
   parameters may specify that no compression, encryption or MAC
   algorithm is to be used). The initial current state always specifies
   that no encryption, compression, or MAC will be used.



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   The security parameters for a TLS Connection read and write state
   are set by providing the following values:

   connection end
         Whether this entity is considered the "client" or the "server"
         in this connection.

   bulk encryption algorithm
         An algorithm to be used for bulk encryption. This
         specification includes the key size of this algorithm, how
         much of that key is secret, whether it is a block or stream
         cipher, the block size of the cipher (if appropriate), and
         whether it is considered an "export" cipher.

   MAC algorithm
         An algorithm to be used for message authentication. This
         specification includes the size of the hash to be returned by
         the MAC algorithm, and a pad size which is to be used when
         whitening the hash.

   compression algorithm
         An algorithm to be used for data compression. This
         specification must include all information the algorithm
         requires to do compression.

   master secret
         A 48 byte secret shared between the two peers in the
         connection.

   client random
         A 32 byte value provided by the client.

   server random
         A 32 byte value provided by the server.

   These parameters are defined in the presentation language as:

       enum { null(0), (255) } CompressionMethod;

       enum { server, client } ConnectionEnd;

       enum { null, rc4, rc2, des, 3des, des40 } BulkCipherAlgorithm;

       enum { stream, block } CipherType;

       enum { true, false } IsExportable;

       enum { null, md5, sha } MACAlgorithm;

   /* The algorithms specified in CompressionMethod,
      BulkCipherAlgorithm, and MACAlgorithm may be added to. */


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       struct {
           ConnectionEnd entity;
           BulkCipherAlgorithm bulk_cipher_algorithm;
           CipherType cipher_type;
           uint8 key_size;
           uint8 key_material_length;
           IsExportable is_exportable;
           MACAlgorithm mac_algorithm;
           uint8 hash_size;
           uint8 whitener_length;
           CompressionMethod compression_algorithm;
           opaque master_secret[48];
           opaque client_random[32];
           opaque server_random[32];
       } SecurityParameters;

   The record layer will use the security parameters to generate the
   following six items:

          client write MAC secret
          server write MAC secret
          client write key
          server write key
          client write IV (for block ciphers only)
          server write IV (for block ciphers only)

   The client write parameters are used by the server when receiving
   and processing records and vice-versa. The algorithm used for
   generating these items from the security parameters is described in
   section 5.3.

   Once the security parameters have been set and the keys have been
   generated, the connection states can be instantiated by making them
   the current states. These current states must be updated for each
   record processed. Each connection state includes the following
   elements:

   compression state
         The current state of the compression algorithm.

   cipher state
         The current state of the encryption algorithm. This will
         consist of the scheduled key for that connection. In addition,
         for block ciphers, this will initially contain the IV for that
         connection state and be updated to contain the ciphertext of
         the last block encrypted or decrypted as records are
         processed. For stream ciphers, this will contain whatever the
         necessary state information is to allow the stream to continue
         to encrypt or decrypt data.

   MAC secret
         The MAC secret for this connection as generated above.

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   sequence number
         Each connection state contains a sequence number, which is
         maintained seperately for read and write states. The sequence
         number must be set to zero whenever a connection state is made
         the active state. Sequence numbers are of type uint64 and may
         not exceed 2^64-1. A sequence number is incremented after each
         record: specifically, the first record which is transmitted
         under a particular connection state should use sequence number
         0.


5.2 Record layer

   The TLS Record Layer receives uninterpreted data from higher layers
   in non-empty blocks of arbitrary size.


5.2.1 Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records of 2^14 bytes or less. Client message boundaries are not
   preserved in the record layer (i.e., multiple client messages of the
   same ContentType may be coalesced into a single TLSPlaintext record,
   or may be fragmented across several records).

       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[TLSPlaintext.length];
       } TLSPlaintext;

   type
         The higher level protocol used to process the enclosed
         fragment.

   version
         The version of the protocol being employed. This document
         describes TLS Version 1.0, which uses the version { 3, 0 },
         as it is identical to SSL Version 3.0 (See Appendix A.1.1).




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

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

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


5.2.2 Record compression and decompression

   All records are compressed using the compression algorithm defined
   in the current session state. There is always an active compression
   algorithm, however initially it is defined as
   CompressionMethod.null. The compression algorithm translates an
   TLSPlaintext structure into an TLSCompressed structure. Compression
   functions are initialized with default state information whenever a
   connection state is made active.

   Compression must be lossless and may not increase the content length
   by more than 1024 bytes. If the decompression function encounters an
   TLSCompressed.fragment that would decompress to a length in excess
   of 2^14 bytes, it should report a fatal decompression failure error.

       struct {
           ContentType type;       /* same as TLSPlaintext.type */
           ProtocolVersion version;/* same as TLSPlaintext.version */
           uint16 length;
           opaque fragment[TLSCompressed.length];
       } TLSCompressed;

   length
         The length (in bytes) of the following TLSCompressed.fragment.
         The length should not exceed 2^14 + 1024.

   fragment
         The compressed form of TLSPlaintext.fragment.

   Note:
         A CompressionMethod.null operation is an identity operation;
         no fields are altered. (See Appendix A.4.1)

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


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5.2.3 Record payload protection

   The encryption and MAC functions translate an TLSCompressed
   structure into an TLSCiphertext. 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;
       } TLSCiphertext;

   type
         The type field is identical to TLSCompressed.type.

   version
         The version field is identical to TLSCompressed.version.

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

   fragment
         The encrypted form of TLSCompressed.fragment, with the MAC.


5.2.3.1 Null or standard stream cipher

   Stream ciphers (including BulkCipherAlgorithm.null - see Appendix
   A.7) convert TLSCompressed.fragment structures to and from stream
   TLSCiphertext.fragment structures.

       stream-ciphered struct {
           opaque content[TLSCompressed.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 +
                 TLSCompressed.type + TLSCompressed.length +
                 TLSCompressed.fragment));

   where "+" denotes concatenation.



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   pad_1
         The character 0x36 repeated SecurityParameters.whitener_length
         times.

   pad_2
         The character 0x5c repeated SecurityParameters.whitener_length
         times.

   seq_num
         The sequence number for this record.

   hash
         The hashing algorithm specified by
         SecurityParameters.mac_algorithm.

   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 TLS_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).
   TLSCiphertext.length is TLSCompressed.length plus
   CipherSpec.hash_size.


5.2.3.2 CBC block cipher

   For block ciphers (such as RC2 or DES), the encryption and MAC
   functions convert TLSCompressed.fragment structures to and from
   block TLSCiphertext.fragment structures.

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

   The MAC is generated as described in Section 5.2.3.1.

   padding
         Padding that is added to force the length of the plaintext to
         be a multiple of the block cipher's block length.

   padding_length
         The length of the padding must be less than the cipher's block
         length and may be zero. The padding length should be such that
         the total size of the GenericBlockCipher structure is a
         multiple of the cipher's block length.

   The encrypted data length (TLSCiphertext.length) is one more than

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   the sum of TLSCompressed.length, CipherSpec.hash_size, and
   padding_length.

   Note:
         With CBC block chaining the initialization vector (IV) for the
         first record is generated with the other keys and secrets when
         the security parameters are set. The IV for subsequent records
         is the last ciphertext block from the previous record.


5.3 Key calculation

   The Record Protocol requires an algorithm to generate keys, IVs, and
   MAC secrets from the security parameters provided by the handshake
   protocol.

   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 connection state (see Appendix A.7). CipherSpecs
   require a client write MAC secret, a server write MAC secret, a
   client write key, a server write key, a client write IV, and a
   server write IV, which are generated from the master secret in that
   order. Unused values are empty.

   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' + SecurityParameters.master_secret +
                            SecurityParameters.server_random +
                            SecurityParameters.client_random)) +
     MD5(master_secret + SHA('BB' + SecurityParameters.master_secret +
                            SecurityParameters.server_random +
                            SecurityParameters.client_random)) +
     MD5(master_secret + SHA('CCC' + SecurityParameters.master_secret +
                            SecurityParameters.server_random +
                            SecurityParameters.client_random)) + [...];

   until enough output has been generated. Then the key_block is
   partitioned as follows.

   client_write_MAC_secret[SecurityParameters.hash_size]
   server_write_MAC_secret[SecurityParameters.hash_size]
   client_write_key[SecurityParameters.key_material]
   server_write_key[SecurityParameters.key_material]
   client_write_IV[SecurityParameters.IV_size] /* non-export ciphers */
   server_write_IV[SecurityParameters.IV_size] /* non-export ciphers */

   Any extra key_block material is discarded.

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   Implementation note:
         The cipher spec which is defined in this document which
         requires the most material is 3DES_EDE_CBC_SHA: it requires 2
         x 24 byte keys, 2 x 20 byte MAC secrets, and 2 x 8 byte IVs,
         for a total of 104 bytes of key material. This will require
         iterating the key generation algorithm seven times, through
         'GGGGGGG'.

   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 +
                                    SecurityParameters.client_random +
                                    SecurityParameters.server_random);
       final_server_write_key = MD5(server_write_key +
                                    SecurityParameters.server_random +
                                    SecurityParameters.client_random);

   Note that this implies that exportable algorithms cannot have final
   write keys larger than the output of MD5 (16 bytes).

   Exportable encryption algorithms derive their IVs from the random
   messages:

       client_write_IV = MD5(SecurityParameters.client_random +
                             SecurityParameters.server_random);
       server_write_IV = MD5(SecurityParameters.server_random +
                             SecurityParameters.client_random);

   MD5 outputs are trimmed to the appropriate size by discarding the
   trailing bytes. (The key or IV is taken from the first bytes of the
   MD5 output.)


5.3.1 Export key generation example

   TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for
   each of the two encryption keys and 16 bytes for each of the MAC
   keys, for a total of 42 bytes of key material. MD5 produces 16 bytes
   of output per call, so three calls to MD5 are required. The MD5
   outputs are concatenated into a 48-byte key_block with the first MD5
   call providing bytes zero through 15, the second providing bytes 16
   through 31, etc. The key_block is partitioned, and the write keys
   are salted because this is an exportable encryption algorithm.

       client_write_MAC_secret = key_block[0..15]
       server_write_MAC_secret = key_block[16..31]
       client_write_key        = key_block[32..36]
       server_write_key        = key_block[37..41]
       final_client_write_key  = MD5(client_write_key +
                                     ClientHello.random +

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                                     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];


6. The TLS Handshake Protocol

   The TLS Handshake Protocol consists of a suite of three
   sub-protocols which are used to allow peers to agree upon security
   parameters for the record layer, authenticate themselves,
   instantiate negotiated security parameters, and report error
   conditions to each other.

   The Handshake Protocol is responsible for negotiating a session,
   which consists of the following items:

   session identifier
         An arbitrary byte sequence chosen by the server to identify an
         active or resumable session state.

   peer certificate
         X509.v3[X509] certificate of the peer. This element of the
         state may be null.

   compression method
         The algorithm used to compress data prior to encryption.

   cipher spec
         Specifies the bulk data encryption algorithm (such as null,
         DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also
         defines cryptographic attributes such as the hash_size. (See
         Appendix A.7 for formal definition)

   master secret
         48-byte secret shared between the client and server.

   is resumable
         A flag indicating whether the session can be used to initiate
         new connections.

   These items are then used to create security parameters for use by
   the Record Layer when protecting application data. Many connections
   can be instantiated using the same session through the resumption
   feature of the TLS Handshake Protocol.




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6.1 Change cipher spec protocol

   The change cipher spec protocol exists to signal transitions in
   ciphering strategies. The protocol consists of a single message,
   which is encrypted and compressed under the current (not the
   pending) connection state. 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 newly negotiated CipherSpec and keys. Reception
   of this message causes the receiver to instruct the Record Layer to
   immediately copy the read pending state into the read current state.
   Immediately after sending this message, the sender should instruct
   the record layer to make the write pending state the write active
   state. (See section 5.1.) The change cipher spec message is sent
   during the handshake after the security parameters have been agreed
   upon, but before the verifying finished message is sent (see section
   6.4.9).


6.2 Alert protocol

   One of the content types supported by the TLS Record layer is the
   alert type. Alert messages convey the severity of the message and a
   description of the alert. Alert messages with a level of fatal
   result in the immediate termination of the connection. In this case,
   other connections corresponding to the session may continue, but the
   session identifier must be invalidated, preventing the failed
   session from being used to establish new connections. Like other
   messages, alert messages are encrypted and compressed, as specified
   by the current connection state.

       enum { warning(1), fatal(2), (255) } AlertLevel;

       enum {
           close_notify(0),
           unexpected_message(10),
           bad_record_mac(20),
           decompression_failure(30),
           handshake_failure(40),
           no_certificate(41),
           bad_certificate(42),
           unsupported_certificate(43),
           certificate_revoked(44),
           certificate_expired(45),
           certificate_unknown(46),
           illegal_parameter (47),

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           (255)
       } AlertDescription;

       struct {
           AlertLevel level;
           AlertDescription description;
       } Alert;


6.2.1 Closure alerts

   The client and the server must share knowledge that the connection
   is ending in order to avoid a truncation attack. Either party may
   initiate the exchange of closing messages.

   close_notify
         This message notifies the recipient that the sender will not
         send any more messages on this connection. The session becomes
         unresumable if any connection is terminated without proper
         close_notify messages with level equal to warning.

   Either party may initiate a close by sending a close_notify alert.
   Any data received after a closure alert is ignored.

   Each party is required to send a close_notify alert before closing
   the write side of the connection. It is required that the other
   party respond with a close_notify alert of its own and close down
   the connection immediately, discarding any pending writes. It is not
   required for the initiator of the close to wait for the responding
   close_notify alert before closing the read side of the connection.

   NB:
         It is assumed that closing a connection reliably delivers
         pending data before destroying the transport.


6.2.2 Error alerts

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

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



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

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

   handshake_failure
         Reception of a handshake_failure alert message indicates that
         the sender was unable to negotiate an acceptable set of
         security parameters given the options available. This is a
         fatal error.

   no_certificate
         A no_certificate alert message may be sent in response to a
         certification request if no appropriate certificate is
         available.

   bad_certificate
         A certificate was corrupt, contained signatures that did not
         verify correctly, etc.

   unsupported_certificate
         A certificate was of an unsupported type.

   certificate_revoked
         A certificate was revoked by its signer.

   certificate_expired
          A certificate has expired or is not currently valid.

   certificate_unknown
         Some other (unspecified) issue arose in processing the
         certificate, rendering it unacceptable.

   illegal_parameter
         A field in the handshake was out of range or inconsistent with
         other fields. This is always fatal.

   For all errors where an alert level is not explicitly specified, the
   sending party may determine at its discretion whether this is a
   fatal error or not; if an alert with a level of warning is received,
   the receiving party may decide at its discretion whether to treat
   this as a fatal error or not. However, all messages which are
   transmitted with a level of fatal must be treated as fatal messages.


6.3 Handshake Protocol overview

   The cryptographic parameters of the session state are produced by

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   the TLS Handshake Protocol, which operates on top of the TLS Record
   Layer. When a TLS 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.

   The TLS Handshake Protocol has three goals:

      -     Exchange hello messages to agree on algorithms, exchange
            random values, and check for session resumption.

      -     Exchange the necessary cryptographic parameters to allow
            the client and server to agree on a premaster secret.

      -     Exchange certificates and cryptographic information to
            allow the client and server to authenticate themselves.

      -     Generate a master secret from the premaster secret and
            exchanged random values.

      -     Provide security paramers to the record layer.

      -     Allow the client and server to verify that their peer has
            calculated the same security parameters and that the
            handshake occured without tampering by an attacker.

   These goals are achieved by the handshake protocol, which can be
   summarized as follows: The client sends a client hello message to
   which the server must respond with a server hello message, or else a
   fatal error will occur and the connection will fail. The client
   hello and server hello are used to establish security enhancement
   capabilities between client and server. The client hello and server
   hello establish the following attributes: Protocol Version, Session
   ID, Cipher Suite, and Compression Method. Additionally, two random
   values are generated and exchanged: ClientHello.random and
   ServerHello.random.

   The actual key exchange uses up to four messages: the server
   certificate, the server key exchange, the client certificate, and
   the client key exchange. New key exchange methods can be created by
   specifing a format for these messages and defining the use of the
   messages to allow the client and server to agree upon a shared
   secret. This secret should be quite long; currently defined key
   exchange methods exchange secrets which range from 48 to 128 bytes
   in length.

   Following the hello messages, the server will send its certificate,
   if it is to be authenticated. Additionally, a server key exchange
   message may be sent, if it is required (e.g. if their server has no
   certificate, or if its certificate is for signing only). If the
   server is authenticated, it may request a certificate from the
   client, if that is appropriate to the cipher suite selected. Now the

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   server will send the server hello done message, indicating that the
   hello-message phase of the handshake is complete. The server will
   then wait for a client response. If the server has sent a
   certificate request Message, the client must send either the
   certificate message or a no_certificate alert. The client key
   exchange message is now sent, and the content of that message will
   depend on the public key algorithm selected between the client hello
   and the server hello. If the client has sent a certificate with
   signing ability, a digitally-signed certificate verify message is
   sent to explicitly verify the certificate.

   At this point, a change cipher spec message is sent by the client,
   and the client copies the pending Cipher Spec into the current
   Cipher Spec. The client then immediately sends the finished message
   under the new algorithms, keys, and secrets. In response, the server
   will send its own change cipher spec message, transfer the pending
   to the current Cipher Spec, and send its finished message under the
   new Cipher Spec. At this point, the handshake is complete and the
   client and server may begin to exchange application layer data. (See
   flow chart below.)

      Client                                               Server

      ClientHello                   -------->
                                                      ServerHello
                                                     Certificate*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                   <--------      ServerHelloDone
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                      -------->
                                               [ChangeCipherSpec]
                                   <--------             Finished
      Application Data             <------->     Application Data


   * Indicates optional or situation-dependent messages that are not
   always sent.

   Note:
         To help avoid pipeline stalls, ChangeCipherSpec is an
         independent TLS Protocol content type, and is not actually an
         TLS 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

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   to be resumed. The server then checks its session cache for a match.
   If a match is found, and the server is willing to re-establish the
   connection under the specified session state, it will send a
   ServerHello with the same Session ID value. At this point, both
   client and server must send change cipher spec messages and proceed
   directly to finished messages. Once the re-establishment is
   complete, the client and server may begin to exchange application
   layer data. (See flow chart below.) If a Session ID match is not
   found, the server generates a new session ID and the TLS client and
   server perform a full handshake.

      Client                                                Server

      ClientHello                   -------->
                                                       ServerHello
                                              [change cipher spec]
                                    <--------             Finished
      change cipher spec
      Finished                      -------->
      Application Data              <------->     Application Data

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


6.4 Handshake protocol

   The TLS Handshake Protocol is one of the defined higher level
   clients of the TLS Record Protocol. This protocol is used to
   negotiate the secure attributes of a session. Handshake messages are
   supplied to the TLS Record Layer, where they are encapsulated within
   one or more TLSPlaintext structures, which are processed and
   transmitted as specified by the current active session state.

       enum {
           hello_request(0), client_hello(1), server_hello(2),
           certificate(11), server_key_exchange (12),
           certificate_request(13), server_hello_done(14),
           certificate_verify(15), client_key_exchange(16),
           finished(20), (255)
       } HandshakeType;

       struct {
           HandshakeType msg_type;    /* handshake type */
           uint24 length;             /* bytes in message */
           select (HandshakeType) {
               case hello_request: HelloRequest;
               case client_hello: ClientHello;
               case server_hello: ServerHello;
               case certificate: Certificate;
               case server_key_exchange: ServerKeyExchange;
               case certificate_request: CertificateRequest;

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               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.


6.4.1 Hello messages

   The hello phase messages are used to exchange security enhancement
   capabilities between the client and server. When a new session
   begins, the Record Layer's connection state encryption, hash, and
   compression algorithms are initialized to null. The current
   connection state is used for renegotiation messages.


6.4.1.1 Hello request

   When this message will be sent:

   The hello request message may be sent by the server at any time.

   Meaning of this message:

   Hello request is a simple notification that the client should begin
   the negotiation process anew by sending a client hello message when
   convenient. This message will be ignored by the client if the client
   is currently negotiating a session. This message may be ignored by
   the client if it does not wish to renegotiate a session. Since
   handshake messages are intended to have transmission precedence over
   application data, it is expected that the negotiation will begin
   before no more than a few records are received from the client. If
   the server sends a hello request but does not recieve a client hello
   in response, it may close the connection with a fatal alert.

   After sending a hello request, servers should not repeat the request
   until the subsequent handshake negotiation is complete.

   Structure of this message:

       struct { } HelloRequest;

   Note:
         This message should never be included in the message hashes
         which are maintained throughout the handshake and used in the
         finished messages and the certificate verify message.


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6.4.1.2 Client hello

   When this message will be sent:

   When a client first connects to a server it is required to send the
   client hello as its first message. The client can also send a client
   hello in response to a hello request or on its own initiative in
   order to renegotiate the security parameters in an existing
   connection.

   Structure of this message:

   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 TLS Protocol; higher
         level or application protocols may define additional
         requirements.

   random_bytes
         28 bytes generated by a secure random number generator.

   The client hello message includes a variable length session
   identifier. If not empty, the value identifies a session between the
   same client and server whose security parameters the client wishes
   to reuse. The session identifier may be from an earlier connection,
   this connection, or another currently active connection. The second
   option is useful if the client only wishes to update the random
   structures and derived values of a connection, while the third
   option makes it possible to establish several simultaneous
   independent secure connections without repeating the full handshake
   protocol. The actual contents of the SessionID are defined by the
   server.

         opaque SessionID<0..32>;

   Warning:
      Servers must not place confidential information in session
      identifiers or let the contents of fake session identifiers
      cause any breach of security.

   The CipherSuite list, passed from the client to the server in the
   client hello message, contains the combinations of cryptographic
   algorithms supported by the client in order of the client's

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   preference (first choice first). Each CipherSuite defines a key
   exchange algorithm, a bulk encryption algorithm (including secret
   key length) and a MAC algorithm. The server will select a cipher
   suite or, if no acceptable choices are presented, return a handshake
   failure alert and close the connection.

       uint8 CipherSuite[2];  /* Cryptographic suite selector */

   The client hello includes a list of compression algorithms supported
   by the client, ordered according to the client's preference.

   Issue:
         Which compression methods to support is under investigation.

       enum { null(0), (255) } CompressionMethod;

       struct {
           ProtocolVersion client_version;
           Random random;
           SessionID session_id;
           CipherSuite cipher_suites<2..2^16-1>;
           CompressionMethod compression_methods<1..2^8-1>;
       } ClientHello;

   client_version
         The version of the TLS protocol by which the client wishes to
         communicate during this session. This should be the latest
         (highest valued) version supported by the client. For this
         version of the specification, the version will be 3.0 (See
         Appendix E for details about backward compatibility).

   random
         A client-generated random structure.

   session_id
         The ID of a session the client wishes to use for this
         connection. This field should be empty if no session_id is
         available or the client wishes to generate new security
         parameters.

   cipher_suites
         This is a list of the cryptographic options supported by the
         client, with the client's first preference first. If the
         session_id field is not empty (implying a session resumption
         request) this vector must include at least the cipher_suite
         from that session. Values are defined in Appendix A.6.

   compression_methods
         This is a list of the compression methods supported by the
         client, sorted by client preference. If the session_id field
         is not empty (implying a session resumption request) it
         must include the compression_method from that session.

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         This vector must contain, and all implementations must
         support, CompressionMethod.null. Thus, a client and server
         will always be able to agree on a compression method.

   After sending the client hello message, the client waits for a
   server hello message. Any other handshake message returned by the
   server except for a hello request is treated as a fatal error.

   Forward compatibility note:
         In the interests of forward compatibility, it is permitted for
         a client hello message to include extra data after the
         compression methods. This data must be included in the
         handshake hashes, but must otherwise be ignored. This is the
         only handshake message for which this is legal; for all other
         messages, the amount of data in the message must match the
         description of the message precisely.


6.4.1.3 Server hello

   When this message will be sent:

   The server will send this message in response to a client hello
   message when it was able to find an acceptable set of algorithms. If
   it cannot find such a match, it will respond with a handshake
   failure alert.

   Structure of this 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 is
       be 3.0 (See Appendix E for details about backward
       compatibility).

   random
       This structure is generated by the server and must be
       different from (and independent of) ClientHello.random.

   session_id
       This is the identity of the session corresponding to this
       connection. If the ClientHello.session_id was non-empty, the
       server will look in its session cache for a match. If a match

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       is found and the server is willing to establish the new
       connection using the specified session state, the server will
       respond with the same value as was supplied by the client. This
       indicates a resumed session and dictates that the parties must
       proceed directly to the finished messages. Otherwise this field
       will contain a different value identifying the new session. The
       server may return an empty session_id to indicate that the
       session will not be cached and therefore cannot be resumed.

   cipher_suite
       The single cipher suite selected by the server from the list in
       ClientHello.cipher_suites. For resumed sessions this field is
       the value from the state of the session being resumed.

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

6.4.2 Server certificate

   When this message will be sent:

   The server must send a certificate whenever the agreed-upon key
   exchange method is not an anonymous one. This message will always
   immediately follow the server hello message.

   Meaning of this message:

   The certificate type must be appropriate for the selected cipher
   suite's key exchange algorithm, and is generally an X.509.v3
   certificate. It must contain a key which matches the key exchange
   method, as follows. Unless otherwise specified, the signing
   algorithm for the certificate must be the same as the algorithm for
   the certificate key. Unless otherwise specified, the public key may
   be of any length.

       Key Exchange Algorithm  Certificate Key Type

       RSA                     RSA public key; the certificate must
                               allow the key to be used for encryption.

       RSA_EXPORT              RSA public key of length greater than
                               512 bits which can be used for signing,
                               or a key of 512 bits or shorter which
                               Can be used for encryption or signing.

       DHE_DSS                 DSS public key.

       DHE_DSS_EXPORT          DSS public key.


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       DHE_RSA                 RSA public key which can be used for
                               signing.

       DHE_RSA_EXPORT          RSA public key which can be used for
                               signing.

       DH_DSS                  Diffie-Hellman key. The algorithm used
                               to sign the certificate should be DSS.

       DH_RSA                  Diffie-Hellman key. The algorithm used
                               to sign the certificate should be RSA.

   As CipherSuites which specify new key exchange methods are specified
   for the TLS Protocol, they will imply certificate format and the
   required encoded keying information.

   Structure of this message:

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

   certificate_list
       This is a sequence (chain) of X.509.v3 certificates, ordered
       with the sender's certificate first followed by any certificate
       authority certificates proceeding sequentially upward, with a
       self-signed certificate for the root CA coming last in the
       list.

   The same message type and structure will be used for the client's
   response to a certificate request message.

   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.


6.4.3 Server key exchange message

   When this message will be sent:

   This message will be sent after the server certificate message (or
   the server hello message, if the server certificate is not sent),
   but before the server hello done message. The server key exchange
   message may be sent before or after this message.

   The server key exchange message is sent by the server only when the
   server certificate message (if sent) does not contain enough data to
   allow the client to exchange a premaster secret. This is true for

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   the following key exchange methods:

          RSA_EXPORT (if the public key in the server certificate is
                      longer than 512 bits)
          DHE_DSS
          DHE_DSS_EXPORT
          DHE_RSA
          DHE_RSA_EXPORT
          DH_anon

   It is not legal to send the server key exchange message for the
   following key exchange methods:

          RSA
          RSA_EXPORT (when the public key in the server certificate
                      is less than or equal to 512 bits in length)
          DH_DSS
          DH_RSA

   Meaning of this message:

   This message conveys cryptographic information to allow the client
   to communicate the premaster secret: either an RSA public key to
   encrypt the premaster secret with, or a Diffie-Hellman public key
   with which the client can complete a key exchange (with the result
   being the premaster secret.)

   As additional CipherSuites are defined for TLS which include new key
   exchange algorithms, the server key exchange message will be sent if
   and only if the certificate type associated with the key exchange
   algorithm does not provide enough information for the client to
   exchange a premaster secret.

   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, the larger RSA keys encoded in
         certificates may be used to sign temporary shorter RSA keys
         for the RSA_EXPORT key exchange method.

   Structure of this message:

       enum { rsa, diffie_hellman }
              KeyExchangeAlgorithm;

       struct {
           opaque rsa_modulus<1..2^16-1>;
           opaque rsa_exponent<1..2^16-1>;
       } ServerRSAParams;

   rsa_modulus
         The modulus of the server's temporary RSA key.

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   rsa_exponent
         The public exponent of the server's temporary RSA key.

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

   dh_p
         The prime modulus used for the Diffie-Hellman operation.

   dh_g
         The generator used for the Diffie-Hellman operation.

   dh_Ys
         The server's Diffie-Hellman public value (g^X mod p).

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

   params
         The server's key exchange parameters.

   signed_params
         For non-anonymous key exchanges, 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;

       select (SignatureAlgorithm)
       {   case anonymous: struct { };
           case rsa:
               digitally-signed struct {
                   opaque md5_hash[16];
                   opaque sha_hash[20];
               };

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           case dsa:
               digitally-signed struct {
                   opaque sha_hash[20];
               };
       } Signature;


6.4.4 Certificate request

   When this message will be sent:

   A non-anonymous server can optionally request a certificate from the
   client, if appropriate for the selected cipher suite.

   This message may be sent between the server certificate message and
   the server hello done message. It may legally precede or follow the
   server key exchange message. It is sent at the discretion of the
   server, when legal.

   Structure of this message:

       enum {
           rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
           rsa_ephemeral_dh(5), dss_ephemeral_dh(6),
           (255)
       } ClientCertificateType;

       opaque DistinguishedName<1..2^16-1>;

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


   certificate_types
         This field is a list of the types of certificates requested,
         sorted in order of the server's preference.

   certificate_authorities
         A list of the distinguished names of acceptable certificate
         authorities.

   Note:
         DistinguishedName is derived from [X509].

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




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6.4.5 Server hello done

   When this message will be sent:

   The server hello done message is sent by the server to indicate the
   end of the server hello and associated messages. After sending this
   message the server will wait for a client response.

   Meaning of this message:

   This message means that the server is done sending messages to
   support the key exchange, and the client can proceed with its phase
   of the key exchange.

   Upon receipt of the server hello done message the client should
   verify that the server provided a valid certificate if required and
   check that the server hello parameters are acceptable.

   Structure of this message:

       struct { } ServerHelloDone;


6.4.6 Client certificate

   When this message will be sent:

   This is the first message the client can send after receiving a
   server hello done message. This message is only sent if the server
   requests a certificate. If no suitable certificate is available, the
   client should send a no_certificate alert instead. This alert is
   only a warning, however the server may respond with a fatal
   handshake failure alert if client authentication is required. Client
   certificates are sent using the Certificate structure defined in
   Section 5.6.2.

   Note:
         When using a static Diffie-Hellman based key exchange method
         (DH_DSS or DH_RSA), if client authentication is requested, the
         Diffie-Hellman group and generator encoded in the client's
         certificate must match the server specified Diffie-Hellman
         parameters if the client's parameters are to be used for the
         key exchange.


6.4.7 Client key exchange message

   When this message will be sent:

   This message is always sent by the client. It will immediately
   follow the client certificate message, if it is sent, or the
   no_certificate alert, if a certificate was requested but an

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   appropriate one was not available. Otherwise it will be the first
   message sent by the client after it receives the server hello done
   message.

   Meaning of this message:

   With this message, the premaster secret is set, either though direct
   transmisson of the RSA-encrypted secret, or by the transmission of
   Diffie-Hellman parameters which will allow each side to agree upon
   the same premaster secret. When the key exchange method is DH_RSA or
   DH_DSS, client certification has been requested, and the client was
   able to respond with a certificate which contained a Diffie-Hellman
   public key whose parameters (group and generator) matched those
   specified by the server in its certificate, this message will not
   contain any data.

   Structure of this message:

   The choice of messages depends on which key exchange method has been
   selected. See Section 6.4.3 for the KeyExchangeAlgorithm definition.

       struct {
           select (KeyExchangeAlgorithm) {
               case rsa: EncryptedPreMasterSecret;
               case diffie_hellman: ClientDiffieHellmanPublic;
           } exchange_keys;
       } ClientKeyExchange;


6.4.7.1 RSA encrypted premaster secret message

   Meaning of this message:

   If RSA is being used for key agreement and authentication, the
   client generates a 48-byte premaster secret, encrypts it using the
   public key from the server's certificate or the temporary RSA key
   provided in a server key exchange message, and sends the result in
   an encrypted premaster secret message. This structure is a variant
   of the client key exchange message, not a message in itself.

   Structure of this 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.



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   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 7.1.


6.4.7.2 Client Diffie-Hellman public value

   Meaning of this message:

   This structure conveys the client's Diffie-Hellman public value (Yc)
   if it was not already included in the client's certificate. The
   encoding used for Yc is determined by the enumerated
   PublicValueEncoding. This structure is a variant of the client key
   exchange message, not a message in itself.

   Structure of this message:

       enum { implicit, explicit } PublicValueEncoding;

   implicit
         If the client certificate already contains a suitable
         Diffie-Hellman key, then Yc is implicit and does not need to
         be sent again.

   explicit
         Yc needs to be sent.

       struct {
           select (PublicValueEncoding) {
               case implicit: struct { };
               case explicit: opaque dh_Yc<1..2^16-1>;
           } dh_public;
       } ClientDiffieHellmanPublic;

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


6.4.8 Certificate verify

   When this message will be sent:

   This message is used to provide explicit verification of a client
   certificate. This message is only sent following a client
   certificate that has signing capability (i.e. all certificates

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   except those containing fixed Diffie-Hellman parameters). When sent,
   it will immediately follow the client key exchange message.

   Structure of this message:

       struct {
            Signature signature;
       } CertificateVerify;

   The Signature type is defined in 6.4.3.

   CertificateVerify.signature.md5_hash
      MD5(master_secret + pad_2 + MD5(handshake_messages +
      master_secret + pad_1));

   Certificate.signature.sha_hash
      SHA(master_secret + pad_2 + SHA(handshake_messages +
      master_secret + pad_1));

   pad_1
      The character 0x36 repeated 48 times for MD5 or 40 times for SHA.

   pad_2
      The character 0x5c repeated 48 times for MD5 or 40 times for SHA.

   Here handshake_messages refers to all handshake messages sent or
   received starting at client hello up to but not including this
   message, including the type and length fields of the handshake
   messages. This is the concatenation of all the Handshake structures
   as defined in 6.4 exchanged thus far.


6.4.9 Finished

   When this message will be sent:

   A finished message is always sent immediately after a change cipher
   spec message to verify that the key exchange and authentication
   processes were successful. It is essential that a change cipher spec
   message be received between the other handshake messages and the
   Finished message.

   Meaning of this message:

   The finished message is the first protected with the just-negotiated
   algorithms, keys, and secrets. No acknowledgment of the finished
   message is required; parties may begin sending encrypted data
   immediately after sending the finished message. Recipients of
   finished messages must verify that the contents are correct.

       enum { client(0x434C4E54), server(0x53525652) } Sender;


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       struct {
           opaque md5_hash[16];
           opaque sha_hash[20];
       } Finished;

   md5_hash
         MD5(master_secret + pad2 + MD5(handshake_messages + Sender +
         master_secret + pad1));

   sha_hash
         SHA(master_secret + pad2 + SHA(handshake_messages + Sender +
         master_secret + pad1));

   handshake_messages
         All of the data from all handshake messages up to but not
         including this message. This is only data visible at the
         handshake layer and does not include record layer headers.
         This is the concatenation of all the Handshake structures as
         defined in 6.4 exchanged thus far.

   It is a fatal error if a finished message is not preceeded by a
   change cipher spec message at the appropriate point in the
   handshake.

   The hash contained in finished messages sent by the server
   incorporate Sender.server; those sent by the client incorporate
   Sender.client. The value handshake_messages includes all handshake
   messages starting at client hello up to, but not including, this
   finished message. This may be different from handshake_messages in
   Section 5.6.8 because it would include the certificate verify
   message (if sent). Also, the handshake_messages for the finished
   message sent by the client will be different from that for the
   finished message sent by the server, because the one which is sent
   second will include the prior one.

   Note:
         Change cipher spec messages are not handshake messages and are
         not included in the hash computations.


7. Cryptographic computations

   In order to begin connection protection, the TLS Record Protocol
   requires specification of a suite of algorithms, a master secret,
   and the client and server random values. The authentication,
   encryption, and MAC algorithms are determined by the cipher_suite
   selected by the server and revealed in the server hello message. The
   compression algorithm is negotiated in the hello messages, and the
   random values are exchanged in the hello messages. All that remains
   is to calculate the master secret.



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7.1 Computing the master secret

   For all key exchange methods, the same algorithm is used to convert
   the pre_master_secret into the master_secret. The pre_master_secret
   should be deleted from memory once the master_secret has been
   computed.

       master_secret =
         MD5(pre_master_secret + SHA('A' + pre_master_secret +
             ClientHello.random + ServerHello.random)) +
         MD5(pre_master_secret + SHA('BB' + pre_master_secret +
             ClientHello.random + ServerHello.random)) +
         MD5(pre_master_secret + SHA('CCC' + pre_master_secret +
             ClientHello.random + ServerHello.random));

   The master secret is always exactly 48 bytes in length. The length
   of the premaster secret will vary depending on key exchange method.


7.1.1 RSA

   When RSA is used for server authentication and key exchange, a
   48-byte pre_master_secret is generated by the client, encrypted
   under the server's public key, and sent to the server. The server
   uses its private key to decrypt the pre_master_secret. Both parties
   then convert the pre_master_secret into the master_secret, as
   specified above.

   RSA digital signatures are performed using PKCS #1 [PKCS1] block
   type 1. RSA public key encryption is performed using PKCS #1 block
   type 2.


7.1.2 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. Application data protocol

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


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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 TLS 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 the SSL 3.0
   Protocol, which we presume will be used by TLS as well.

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

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

   563 Reserved for use by Network News Transfer Protocol with SSL
       (snntp).

   636 Reserved for Light Directory Access Protocol with SSL (ssl-ldap)

   990 Reserved (pending) for File Transfer Protocol with SSL (ftps)

   995 Reserved for Post Office Protocol with SSL (spop3)


A.1.1 Record layer

       struct {
           uint8 major, minor;
       } ProtocolVersion;

       ProtocolVersion version = { 3,0 };

       enum {
           change_cipher_spec(20), alert(21), handshake(22),
           application_data(23), (255)
       } ContentType;

       struct {
           ContentType type;
           ProtocolVersion version;
           uint16 length;
           opaque fragment[TLSPlaintext.length];
       } TLSPlaintext;

       struct {
           ContentType type;
           ProtocolVersion version;

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           uint16 length;
           opaque fragment[TLSCompressed.length];
       } TLSCompressed;

       struct {
           ContentType type;
           ProtocolVersion version;
           uint16 length;
           select (CipherSpec.cipher_type) {
               case stream: GenericStreamCipher;
               case block:  GenericBlockCipher;
           } fragment;
       } TLSCiphertext;

       stream-ciphered struct {
           opaque content[TLSCompressed.length];
           opaque MAC[CipherSpec.hash_size];
       } GenericStreamCipher;

       block-ciphered struct {
           opaque content[TLSCompressed.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),



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           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;
           uint24 length;
           select (HandshakeType) {
               case hello_request: HelloRequest;
               case client_hello: ClientHello;
               case server_hello: ServerHello;
               case certificate: Certificate;
               case server_key_exchange: ServerKeyExchange;
               case certificate_request: CertificateRequest;
               case server_done: ServerHelloDone;
               case certificate_verify: CertificateVerify;
               case client_key_exchange: ClientKeyExchange;
               case finished: Finished;
           } body;
       } Handshake;


A.4.1 Hello messages

       struct { } HelloRequest;

       struct {
           uint32 gmt_unix_time;
           opaque random_bytes[28];
       } Random;

       opaque SessionID<0..32>;

       uint8 CipherSuite[2];

       enum { null(0), (255) } CompressionMethod;

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       struct {
           ProtocolVersion client_version;
           Random random;
           SessionID session_id;
           CipherSuite cipher_suites<0..2^16-1>;
           CompressionMethod compression_methods<0..2^8-1>;
       } ClientHello;

       struct {
           ProtocolVersion server_version;
           Random random;
           SessionID session_id;
           CipherSuite cipher_suite;
           CompressionMethod compression_method;
       } ServerHello;


A.4.2 Server authentication and key exchange messages

       opaque ASN.1Cert<2^24-1>;

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

       enum { rsa, diffie_hellman } KeyExchangeAlgorithm;

       struct {
           opaque RSA_modulus<1..2^16-1>;
           opaque RSA_exponent<1..2^16-1>;
       } ServerRSAParams;

       struct {
           opaque DH_p<1..2^16-1>;
           opaque DH_g<1..2^16-1>;
           opaque DH_Ys<1..2^16-1>;
       } ServerDHParams;

       struct {
           select (KeyExchangeAlgorithm) {
               case diffie_hellman:
                   ServerDHParams params;
                   Signature signed_params;
               case rsa:
                   ServerRSAParams params;
                   Signature signed_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),
           (255)
       } CertificateType;

       opaque DistinguishedName<1..2^16-1>;

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

       struct { } ServerHelloDone;


A.5 Client authentication and key exchange messages

       struct {
           select (KeyExchangeAlgorithm) {
               case rsa: EncryptedPreMasterSecret;
               case diffie_hellman: DiffieHellmanClientPublicValue;
           } exchange_keys;
       } ClientKeyExchange;

       struct {
           ProtocolVersion client_version;
           opaque random[46];
       } PreMasterSecret;

       struct {
           public-key-encrypted PreMasterSecret pre_master_secret;
       } EncryptedPreMasterSecret;

       enum { implicit, explicit } PublicValueEncoding;

       struct {
           select (PublicValueEncoding) {
               case implicit: struct {};
               case explicit: opaque DH_Yc<1..2^16-1>;
           } dh_public;

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       } ClientDiffieHellmanPublic;

       struct {
           Signature signature;
       } CertificateVerify;


A.5.1 Handshake finalization message

       struct {
           opaque md5_hash[16];
           opaque sha_hash[20];
       } Finished;


A.6 The CipherSuite

   The following values define the CipherSuite codes used in the client
   hello and server hello messages.

   A CipherSuite defines a cipher specifications supported in TLS
   Version 1.0.

    CipherSuite TLS_NULL_WITH_NULL_NULL                = { 0x00,0x00 };

   The following CipherSuite definitions require that the server
   provide an RSA certificate that can be used for key exchange. The
   server may request either an RSA or a DSS signature-capable
   certificate in the certificate request message.

    CipherSuite TLS_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
    CipherSuite TLS_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
    CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
    CipherSuite TLS_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
    CipherSuite TLS_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
    CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
    CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
    CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
    CipherSuite TLS_RSA_WITH_DES_CBC_SHA               = { 0x00,0x09 };
    CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA          = { 0x00,0x0A };

   The following CipherSuite definitions are used for
   server-authenticated (and optionally client-authenticated)
   Diffie-Hellman. DH denotes cipher suites in which the server's
   certificate contains the Diffie-Hellman parameters signed by the
   certificate authority (CA). DHE denotes ephemeral Diffie-Hellman,
   where the Diffie-Hellman parameters are signed by a DSS or RSA
   certificate, which has been signed by the CA. The signing algorithm
   used is specified after the DH or DHE parameter. In all cases, the
   client must have the same type of certificate, and must use the
   Diffie-Hellman parameters chosen by the server.


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    CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
    CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA            = { 0x00,0x0C };
    CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x0D };
    CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
    CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA            = { 0x00,0x0F };
    CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x10 };
    CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
    CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA           = { 0x00,0x12 };
    CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x13 };
    CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
    CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA           = { 0x00,0x15 };
    CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x16 };

   The following cipher suites are used for completely anonymous
   Diffie-Hellman communications in which neither party is
   authenticated. Note that this mode is vulnerable to
   man-in-the-middle attacks and is therefore strongly discouraged.

    CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
    CipherSuite TLS_DH_anon_WITH_RC4_128_MD5           = { 0x00,0x18 };
    CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };
    CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
    CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };

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

   Note:
         Additional cipher suites will be considered for implementation
         only with submission of notarized letters from two independent
         entities. Netscape Communications Corp. will act as an interim
         registration office, until a public standards body assumes
         control of TLS.


A.7 The Security Parameters

   These security parameters are determined by the TLS Handshake
   Protocol and provided as parameters to the TLS Record Layer in order
   to initialize a connection state. SecurityParameters includes:

       enum { null(0), (255) } CompressionMethod;

       enum { server, client } ConnectionEnd;

       enum { null, rc4, rc2, des, 3des, des40 }
           BulkCipherAlgorithm;

       enum { stream, block } CipherType;


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       enum { true, false } IsExportable;

       enum { null, md5, sha } MACAlgorithm;

   /* The algorithms specified in CompressionMethod,
      BulkCipherAlgorithm, and MACAlgorithm may be added to. */

       struct {
           ConnectionEnd entity;
           BulkCipherAlgorithm bulk_cipher_algorithm;
           CipherType cipher_type;
           uint8 key_size;
           uint8 key_material_length;
           IsExportable is_exportable;
           MACAlgorithm mac_algorithm;
           uint8 hash_size;
           uint8 whitener_length;
           CompressionMethod compression_algorithm;
           opaque master_secret[48];
           opaque client_random[32];
           opaque server_random[32];
       } SecurityParameters;

                       Appendix B


B. Glossary

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

   asymmetric cipher
         See public key cryptography.

   authentication
         Authentication is the ability of one entity to determine the
         identity of another entity.

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

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

   cipher block chaining
         Mode (CBC) CBC is a mode in which every plaintext block
         encrypted with the block cipher is first exclusive-ORed with
         the previous ciphertext block (or, in the case of the first

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         block, with the initialization vector).

   certificate
         As part of the X.509 protocol (a.k.a. ISO Authentication
         framework), certificates are assigned by a trusted Certificate
         Authority and provide verification of a party's identity and
         may also supply its public key.

   client
         The application entity that initiates a connection to a server

   client write key
         The key used to encrypt data written by the client.

   client write MAC secret
         The secret data used to authenticate data written by the
         client.

   connection
         A connection is a transport (in the OSI layering model
         definition) that provides a suitable type of service. For TLS,
         such connections are peer to peer relationships. The
         connections are transient. Every connection is associated with
         one session.

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

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

   digital signatures
         Digital signatures utilize public key cryptography and one-way
         hash functions to produce a signature of the data that can be
         authenticated, and is difficult to forge or repudiate.

   handshake
         An initial negotiation between client and server that
         establishes the parameters of their transactions.

   Initialization Vector
         (IV) When a block cipher is used in CBC mode, the
         initialization vector is exclusive-ORed with the first
         plaintext block prior to encryption.

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

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

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

   MD5
         MD5 [7] is a secure hashing function that converts an
         arbitrarily long data stream into a digest of fixed size.

   public key cryptography
         A class of cryptographic techniques employing two-key ciphers.
         Messages encrypted with the public key can only be decrypted
         with the associated private key. Conversely, messages signed
         with the private key can be verified with the public key.

   one-way hash function
         A one-way transformation that converts an arbitrary amount of
         data into a fixed-length hash. It is computation- ally hard to
         reverse the transformation or to find collisions. MD5 and SHA
         are examples of one-way hash functions.

   RC2, RC4
         Proprietary bulk ciphers from RSA Data Security, Inc. (There
         is no good reference to these as they are unpublished works;
         however, see [RSADSI]). RC2 is block cipher and RC4 is a
         stream cipher.

   RSA
         A very widely used public-key algorithm that can be used for
         either encryption or digital signing.

   salt
         Non-secret random data used to make export encryption keys
         resist precomputation attacks.

   server
         The server is the application entity that responds to requests
         for connections from clients. The server is passive, waiting
         for requests from clients.

   session
         A TLS session is an association between a client and a server.
         Sessions are created by the handshake protocol. Sessions
         define a set of cryptographic security parameters, which can
         be shared among multiple connections. Sessions are used to
         avoid the expensive negotiation of new security parameters for
         each connection.


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   session identifier
         A session identifier is a value generated by a server that
         identifies a particular session.

   server write key
         The key used to encrypt data written by the server.

   server write MAC secret
         The secret data used to authenticate data written by the
         server.

   SHA
         The Secure Hash Algorithm is defined in FIPS PUB 180-1. It
         produces a 20-byte output [SHA].

   SSL
         Netscape's Secure Socket Layer protocol [SSL3]. TLS is based
         on SSL Version 3.0

   stream cipher/
         An encryption algorithm that converts a key into a
         cryptographically-strong keystream, which is then
         exclusive-ORed with the plaintext.

   symmetric cipher
          See bulk cipher.


Appendix C

C. CipherSuite definitions

CipherSuite                 Is         Key            Cipher       Hash
                            Exportable Exchange

TLS_NULL_WITH_NULL_NULL               * NULL           NULL        NULL
TLS_RSA_WITH_NULL_MD5                 * RSA            NULL         MD5
TLS_RSA_WITH_NULL_SHA                 * RSA            NULL         SHA
TLS_RSA_EXPORT_WITH_RC4_40_MD5        * RSA_EXPORT     RC4_40       MD5
TLS_RSA_WITH_RC4_128_MD5                RSA            RC4_128      MD5
TLS_RSA_WITH_RC4_128_SHA                RSA            RC4_128      SHA
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5    * RSA_EXPORT     RC2_CBC_40   MD5
TLS_RSA_WITH_IDEA_CBC_SHA               RSA            IDEA_CBC     SHA
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA     * RSA_EXPORT     DES40_CBC    SHA
TLS_RSA_WITH_DES_CBC_SHA                RSA            DES_CBC      SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA           RSA            3DES_EDE_CBC SHA
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA  * DH_DSS_EXPORT  DES40_CBC    SHA
TLS_DH_DSS_WITH_DES_CBC_SHA             DH_DSS         DES_CBC      SHA
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA        DH_DSS         3DES_EDE_CBC SHA
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA  * DH_RSA_EXPORT  DES40_CBC    SHA
TLS_DH_RSA_WITH_DES_CBC_SHA             DH_RSA         DES_CBC      SHA
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA        DH_RSA         3DES_EDE_CBC SHA

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TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC    SHA
TLS_DHE_DSS_WITH_DES_CBC_SHA            DHE_DSS        DES_CBC      SHA
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA       DHE_DSS        3DES_EDE_CBC SHA
TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC    SHA
TLS_DHE_RSA_WITH_DES_CBC_SHA            DHE_RSA        DES_CBC      SHA
TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA       DHE_RSA        3DES_EDE_CBC SHA
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5    * DH_anon_EXPORT RC4_40       MD5
TLS_DH_anon_WITH_RC4_128_MD5            DH_anon        RC4_128      MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA   DH_anon        DES40_CBC    SHA
TLS_DH_anon_WITH_DES_CBC_SHA            DH_anon        DES_CBC      SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA       DH_anon        3DES_EDE_CBC SHA

   * Indicates IsExportable is True

      Key             Description                        Key size limit
      Exchange
      Algorithm
      DHE_DSS         Ephemeral DH with DSS signatures   None
      DHE_DSS_EXPORT  Ephemeral DH with DSS signatures   DH = 512 bits
      DHE_RSA         Ephemeral DH with RSA signatures   None
      DHE_RSA_EXPORT  Ephemeral DH with RSA signatures   DH = 512 bits,
                                                         RSA = none
      DH_anon         Anonymous DH, no signatures        None
      DH_anon_EXPORT  Anonymous DH, no signatures        DH = 512 bits
      DH_DSS          DH with DSS-based certificates     None
      DH_DSS_EXPORT   DH with DSS-based certificates     DH = 512 bits
      DH_RSA          DH with RSA-based certificates     None
      DH_RSA_EXPORT   DH with RSA-based certificates     DH = 512 bits,
                                                         RSA = none
      NULL            No key exchange                    N/A
      RSA             RSA key exchange                   None
      RSA_EXPORT      RSA key exchange                   RSA = 512 bits

   Key size limit
         The key size limit gives the size of the largest public key
         that can be legally used for encryption in cipher suites that
         are exportable.


      Cipher     Cipher IsExpo  Key      Exp.    Effect  IV      Block
                 Type   rtable  Material Key Mat ive Key Size    Size
                                         erial   Bits

      NULL          Stream *      0       0       0       0       N/A
      IDEA_CBC      Block         16      16      128     8       8
      RC2_CBC_40    Block  *      5       16      40      8       8
      RC4_40        Stream *      5       16      40      0       N/A
      RC4_128       Stream        16      16      128     0       N/A
      DES40_CBC     Block  *      5       8       40      8       8
      DES_CBC       Block         8       8       56      8       8
      3DES_EDE_CBC  Block         24      24      168     8       8


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   * Indicates IsExportable is true.

   Key Material
         The number of bytes from the key_block that are used for
         generating the write keys.

   Expanded Key Material
         The number of bytes actually fed into the encryption algorithm

   Effective Key Bits
         How much entropy material is in the key material being fed
         into the encryption routines.

      Hash       Hash Size  Padding
      function              Size
      NULL       0          0
      MD5        16         48
      SHA        20         40


Appendix D

D. Implementation Notes

   The TLS protocol cannot prevent many common security mistakes. This
   section provides several recommendations to assist implementers.


D.1 Temporary RSA keys

   US Export restrictions limit RSA keys used for encryption to 512
   bits, but do not place any limit on lengths of RSA keys used for
   signing operations. Certificates often need to be larger than 512
   bits, since 512-bit RSA keys are not secure enough for high-value
   transactions or for applications requiring long-term security. Some
   certificates are also designated signing-only, in which case they
   cannot be used for key exchange.

   When the public key in the certificate cannot be used for
   encryption, the server signs a temporary RSA key, which is then
   exchanged. In exportable applications, the temporary RSA key should
   be the maximum allowable length (i.e., 512 bits). Because 512-bit
   RSA keys are relatively insecure, they should be changed often. For
   typical electronic commerce applications, it is suggested that keys
   be changed daily or every 500 transactions, and more often if
   possible. Note that while it is acceptable to use the same temporary
   key for multiple transactions, it must be signed each time it is
   used.

   RSA key generation is a time-consuming process. In many cases, a
   low-priority process can be assigned the task of key generation.


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   Whenever a new key is completed, the existing temporary key can be
   replaced with the new one.


D.2 Random Number Generation and Seeding

   TLS requires a cryptographically-secure pseudorandom number
   generator (PRNG). Care must be taken in designing and seeding PRNGs.
   PRNGs based on secure hash operations, most notably MD5 and/or SHA,
   are acceptable, but cannot provide more security than the size of
   the random number generator state. (For example, MD5-based PRNGs
   usually provide 128 bits of state.)

   To estimate the amount of seed material being produced, add the
   number of bits of unpredictable information in each seed byte. For
   example, keystroke timing values taken from a PC- compatible's 18.2
   Hz timer provide 1 or 2 secure bits each, even though the total size
   of the counter value is 16 bits or more. To seed a 128-bit PRNG, one
   would thus require approximately 100 such timer values.

   Note:
         The seeding functions in RSAREF and versions of BSAFE prior to
         3.0 are order-independent. For example, if 1000 seed bits are
         supplied, one at a time, in 1000 separate calls to the seed
         function, the PRNG will end up in a state which depends only
         on the number of 0 or 1 seed bits in the seed data (i.e.,
         there are 1001 possible final states). Applications using
         BSAFE or RSAREF must take extra care to ensure proper seeding.


D.3 Certificates and authentication

   Implementations are responsible for verifying the integrity of
   certificates and should generally support certificate revocation
   messages. Certificates should always be verified to ensure proper
   signing by a trusted Certificate Authority (CA). The selection and
   addition of trusted CAs should be done very carefully. Users should
   be able to view information about the certificate and root CA.


D.4 CipherSuites

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

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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 moving to newer versions.

   The following cipher specifications are carryovers from SSL Version
   2.0. These are assumed to use RSA for key exchange and
   authentication.

       V2CipherSpec SSL_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
       V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
       V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
       V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
                                                  = { 0x04,0x00,0x80 };
       V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
       V2CipherSpec SSL_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
       V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };

   Cipher specifications introduced in Version 3.0 can be included in
   Version 2.0 client hello messages using the syntax below. Any
   V2CipherSpec element with its first byte equal to zero will be
   ignored by Version 2.0 servers. Clients sending any of the above
   V2CipherSpecs should also include the Version 3.0 equivalent (see
   Appendix A.6):

       V2CipherSpec (see Version 3.0 name) = { 0x00, CipherSuite };


E.1 Version 2 client hello

   The Version 2.0 client hello message is presented below using this
   document's presentation model. The true definition is still assumed
   to be the SSL Version 2.0 specification.

       uint8 V2CipherSpec[3];

       struct {
           unit8 msg_type;
           Version version;
           uint16 cipher_spec_length;

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           uint16 session_id_length;
           uint16 challenge_length;
           V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
           opaque session_id[V2ClientHello.session_id_length];
           Random challenge;
       } V2ClientHello;

   msg_type
         This field, in conjunction with the version field, identifies
         a version 2 client hello message. The value should be one (1).

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

   cipher_spec_length
         This field is the total length of the field cipher_specs. It
         cannot be zero and must be a multiple of the V2CipherSpec
         length (3).

   session_id_length
         This field must have a value of either zero or 16. If zero,
         the client is creating a new session. If 16, the session_id
         field will contain the 16 bytes of session identification.

   challenge_length
         The length in bytes of the client's challenge to the server to
         authenticate itself. This value must be 32.

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

   session_id
         If this field's length is not zero, it will contain the
         identification for a session that the client wishes to resume.

   challenge
         The client challenge to the server for the server to identify
         itself is a (nearly) arbitrary length random. The Version 3.0
         server will right justify the challenge data to become the
         ClientHello.random data (padded with leading zeroes, if
         necessary), as specified in this Version 3.0 protocol. If the
         length of the challenge is greater than 32 bytes, only the
         last 32 bytes are used. It is legitimate (but not necessary)
         for a V3 server to reject a V2 ClientHello that has fewer than
         16 bytes of challenge data.

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

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E.2 Avoiding man-in-the-middle version rollback

   When SSL Version 3.0 clients fall back to Version 2.0 compatibility
   mode, they use special PKCS #1 block formatting. This is done so
   that Version 3.0 servers will reject Version 2.0 sessions with
   Version 3.0-capable clients.

   When Version 3.0 clients are in Version 2.0 compatibility mode, they
   set the right-hand (least-significant) 8 random bytes of the PKCS
   padding (not including the terminal null of the padding) for the RSA
   encryption of the ENCRYPTED-KEY- DATA field of the CLIENT-MASTER-KEY
   to 0x03 (the other padding bytes are random). After decrypting the
   ENCRYPTED- KEY-DATA field, servers that support TLS should issue
   an error if these eight padding bytes are 0x03. Version 2.0 servers
   receiving blocks padded in this manner will proceed normally.


Appendix F

F. Security analysis

   The TLS protocol is designed to establish a secure connection
   between a client and a server communicating over an insecure
   channel. This document makes several traditional assumptions,
   including that attackers have substantial computational resources
   and cannot obtain secret information from sources outside the
   protocol. Attackers are assumed to have the ability to capture,
   modify, delete, replay, and otherwise tamper with messages sent over
   the communication channel. This appendix outlines how TLS has been
   designed to resist a variety of attacks.


F.1 Handshake protocol

   The handshake protocol is responsible for selecting a CipherSpec and
   generating a MasterSecret, which together comprise the primary
   cryptographic parameters associated with a secure session. The
   handshake protocol can also optionally authenticate parties who have
   certificates signed by a trusted certificate authority.


F.1.1 Authentication and key exchange

   TLS supports three authentication modes: authentication of both
   parties, server authentication with an unauthenticated client, and
   total anonymity. Whenever the server is authenticated, the channel
   should be secure against man-in- the-middle attacks, but completely
   anonymous sessions are inherently vulnerable to such attacks.
   Anonymous servers cannot authenticate clients, since the client
   signature in the certificate verify message may require a server
   certificate to bind the signature to a particular server. If the
   server is authenticated, its certificate message must provide a

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   valid certificate chain leading to an acceptable certificate
   authority. Similarly, authenticated clients must supply an
   acceptable certificate to the server. Each party is responsible for
   verifying that the other's certificate is valid and has not expired
   or been revoked.

   The general goal of the key exchange process is to create a
   pre_master_secret known to the communicating parties and not to
   attackers. The pre_master_secret will be used to generate the
   master_secret (see Section 6.1). The master_secret is required to
   generate the finished messages, encryption keys, and MAC secrets
   (see Sections 5.6.9 and 6.2.2). By sending a correct finished
   message, parties thus prove that they know the correct
   pre_master_secret.


F.1.1.1 Anonymous key exchange

   Completely anonymous sessions can be established using RSA or
   Diffie-Hellman 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. (Note that no anonymous RSA Cipher
   Suites are defined in this document).

   With Diffie-Hellman, the server's public parameters are contained in
   the server key exchange message and the client's are sent in the
   client key exchange message. Eavesdroppers who do not know the
   private values should not be able to find the Diffie-Hellman result
   (i.e. the pre_master_secret).

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


F.1.1.2 RSA key exchange and authentication

   With RSA, key exchange and server authentication are combined. The
   public key may be either contained in the server's certificate or
   may be a temporary RSA key sent in a server key exchange message.
   When temporary RSA keys are used, they are signed by the server's
   RSA or DSS certificate. The signature includes the current
   ClientHello.random, so old signatures and temporary keys cannot be
   replayed. Servers may use a single temporary RSA key for multiple
   negotiation sessions.

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   Note:
         The temporary RSA key option is useful if servers need large
         certificates but must comply with government-imposed size
         limits on keys used for key exchange.

   After verifying the server's certificate, the client encrypts a
   pre_master_secret with the server's public key. By successfully
   decoding the pre_master_secret and producing a correct finished
   message, the server demonstrates that it knows the private key
   corresponding to the server certificate.

   When RSA is used for key exchange, clients are authenticated using
   the certificate verify message (see Section 5.6.8). The client signs
   a value derived from the master_secret and all preceding handshake
   messages. These handshake messages include the server certificate,
   which binds the signature to the server, and ServerHello.random,
   which binds the signature to the current handshake process.


F.1.1.3 Diffie-Hellman key exchange with authentication

   When Diffie-Hellman key exchange is used, the server can either
   supply a certificate containing fixed Diffie-Hellman parameters or
   can use the server key exchange message to send a set of temporary
   Diffie-Hellman parameters signed with a DSS or RSA certificate.
   Temporary parameters are hashed with the hello.random values before
   signing to ensure that attackers do not replay old parameters. In
   either case, the client can verify the certificate or signature to
   ensure that the parameters belong to the server.

   If the client has a certificate containing fixed Diffie- Hellman
   parameters, its certificate contains the information required to
   complete the key exchange. Note that in this case the client and
   server will generate the same Diffie- Hellman result (i.e.,
   pre_master_secret) every time they communicate. To prevent the
   pre_master_secret from staying in memory any longer than necessary,
   it should be converted into the master_secret as soon as possible.
   Client Diffie- Hellman parameters must be compatible with those
   supplied by the server for the key exchange to work.

   If the client has a standard DSS or RSA certificate or is
   unauthenticated, it sends a set of temporary parameters to the
   server in the client key exchange message, then optionally uses a
   certificate verify message to authenticate itself.


F.1.2 Version rollback attacks

   Because TLS includes substantial improvements over SSL Version 2.0,
   attackers may try to make TLS-capable clients and servers fall back
   to Version 2.0. This attack can occur if (and only if) two
   TLS-capable parties use an SSL 2.0 handshake.

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   Although the solution using non-random PKCS #1 block type 2 message
   padding is inelegant, it provides a reasonably secure way for
   Version 3.0 servers to detect the attack. This solution is not
   secure against attackers who can brute force the key and substitute
   a new ENCRYPTED-KEY-DATA message containing the same key (but with
   normal padding) before the application specified wait threshold has
   expired. Parties concerned about attacks of this scale should not be
   using 40-bit encryption keys anyway. Altering the padding of the
   least-significant 8 bytes of the PKCS padding does not impact
   security, since this is essentially equivalent to increasing the
   input block size by 8 bytes.


F.1.3 Detecting attacks against the handshake protocol

   An attacker might try to influence the handshake exchange to make
   the parties select different encryption algorithms than they would
   normally choose. Because many implementations will support 40-bit
   exportable encryption and some may even support null encryption or
   MAC algorithms, this attack is of particular concern.

   For this attack, an attacker must actively change one or more
   handshake messages. If this occurs, the client and server will
   compute different values for the handshake message hashes. As a
   result, the parties will not accept each others' finished messages.
   Without the master_secret, the attacker cannot repair the finished
   messages, so the attack will be discovered.


F.1.4 Resuming sessions

   When a connection is established by resuming a session, new
   ClientHello.random and ServerHello.random values are hashed with the
   session's master_secret. Provided that the master_secret has not
   been compromised and that the secure hash operations used to produce
   the encryption keys and MAC secrets are secure, the connection
   should be secure and effectively independent from previous
   connections. Attackers cannot use known encryption keys or MAC
   secrets to compromise the master_secret without breaking the secure
   hash operations (which use both SHA and MD5).

   Sessions cannot be resumed unless both the client and server agree.
   If either party suspects that the session may have been compromised,
   or that certificates may have expired or been revoked, it should
   force a full handshake. An upper 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

   TLS uses hash functions very conservatively. Where possible, both
   MD5 and SHA are used in tandem to ensure that non- catastrophic
   flaws in one algorithm will not break the overall protocol.


F.2 Protecting application data

   The master_secret is hashed with the ClientHello.random and
   ServerHello.random to produce unique data encryption keys and MAC
   secrets for each connection.

   Outgoing data is protected with a MAC before transmission. To
   prevent message replay or modification attacks, the MAC is computed
   from the MAC secret, the sequence number, the message length, the
   message contents, and two fixed character strings. The message type
   field is necessary to ensure that messages intended for one TLS
   Record Layer client are not redirected to another. The sequence
   number ensures that attempts to delete or reorder messages will be
   detected. Since sequence numbers are 64-bits long, they should never
   overflow. Messages from one party cannot be inserted into the
   other's output, since they use independent MAC secrets. Similarly,
   the server-write and client-write keys are independent so stream
   cipher keys are used only once.

   If an attacker does break an encryption key, all messages encrypted
   with it can be read. Similarly, compromise of a MAC key can make
   message modification attacks possible. Because MACs are also
   encrypted, message-alteration attacks generally require breaking the
   encryption algorithm as well as the MAC.

   Note:
       MAC secrets may be larger than encryption keys, so messages
       can remain tamper resistant even if encryption keys are broken.


F.3 Final notes

   For TLS to be able to provide a secure connection, both the client
   and server systems, keys, and applications must be secure. In
   addition, the implementation must be free of security errors.

   The system is only as strong as the weakest key exchange and
   authentication algorithm supported, and only trustworthy
   cryptographic functions should be used. Short public keys, 40-bit
   bulk encryption keys, and anonymous servers should be used with
   great caution. Implementations and users must be careful when
   deciding which certificates and certificate authorities are
   acceptable; a dishonest certificate authority can do tremendous
   damage.


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Appendix G

G. Patent Statement

   This version of the TLS protocol relies on the use of patented
   public key encryption technology for authentication and encryption.
   The Internet Standards Process as defined in RFC 1310 requires a
   written statement from the Patent holder that a license will be made
   available to applicants under reasonable terms and conditions prior
   to approving a specification as a Proposed, Draft or Internet
   Standard. The Massachusetts Institute of Technology has granted RSA
   Data Security, Inc., exclusive sub-licensing rights to the following
   patent issued in the United States:

          Cryptographic Communications System and Method ("RSA"),
          No. 4,405,829

   The Board of Trustees of the Leland Stanford Junior University have
   granted Caro-Kann Corporation, a wholly owned subsidiary
   corporation, exclusive sub-licensing rights to the following patents
   issued in the United States, and all of their corresponding foreign
   patents:

          Cryptographic Apparatus and Method ("Diffie-Hellman"), No.
          4,200,770

          Public Key Cryptographic Apparatus and Method
          ("Hellman-Merkle"), No. 4,218,582

   The Internet Society, Internet Architecture Board, Internet
   Engineering Steering Group and the Corporation for National Research
   Initiatives take no position on the validity or scope of the patents
   and patent applications, nor on the appropriateness of the terms of
   the assurance. The Internet Society and other groups mentioned above
   have not made any determination as to any other intellectual
   property rights which may apply to the practice of this standard.
   Any further consideration of these matters is the user's own
   responsibility.


References

   [SSL3] Frier, Karton and Kocher,
   internet-draft-tls-ssl-version3-00.txt: "The SSL 3.0 Protocol",
   Nov 18 1996.

   [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.

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   [DES] ANSI X3.106, "American National Standard for Information
   Systems-Data Link Encryption," American National Standards
   Institute, 1983.

   [DSS] NIST FIPS PUB 186, "Digital Signature Standard," National
   Institute of Standards and Technology, U.S. Department of Commerce,
   18 May 1994.

   [FTP] J. Postel and J. Reynolds, RFC 959: File Transfer Protocol,
   October 1985.

   [HTTP] T. Berners-Lee, R. Fielding, H. Frystyk, Hypertext Transfer
   Protocol -- HTTP/1.0, October, 1995.

   [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH
   Series in Information Processing, v. 1, Konstanz: Hartung-Gorre
   Verlag, 1992.

   [KRAW] H. Krawczyk, IETF Draft: Keyed-MD5 for Message
   Authentication, November 1995.

   [MD2] R. Rivest. RFC 1319: The MD2 Message Digest Algorithm. April
   1992.

   [MD5] R. Rivest. RFC 1321: The MD5 Message Digest Algorithm. April
   1992.

   [PKCS1] RSA Laboratories, "PKCS #1: RSA Encryption Standard,"
   version 1.5, November 1993.

   [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate Syntax
   Standard," version 1.5, November 1993.

   [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message Syntax
   Standard," version 1.5, November 1993.

   [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for
   Obtaining Digital Signatures and Public-Key Cryptosystems,"
   Communications of the ACM, v. 21, n. 2, Feb 1978, pp. 120- 126.

   [RSADSI] Contact RSA Data Security, Inc., Tel: 415-595-8782 [SCH] B.
   Schneier. Applied Cryptography: Protocols, Algorithms, and Source
   Code in C, Published by John Wiley & Sons, Inc. 1994.

   [SHA] NIST FIPS PUB 180-1, "Secure Hash Standard," National
   Institute of Standards and Technology, U.S. Department of Commerce,
   DRAFT, 31 May 1994. [TCP] ISI for DARPA, RFC 793: Transport Control
   Protocol, September 1981.

   [TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993. [X509] CCITT.
   Recommendation X.509: "The Directory - Authentication Framework".
   1988.

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   [XDR] R. Srinivansan, Sun Microsystems, RFC-1832: XDR: External Data
   Representation Standard, August 1995.


Working Group Chair

      Win Treese
      Open Market
      treeseopenmarket.com

Editors

      Tim Dierks                    Christopher Allen
      Consensus Development         Consensus Development
      timd@consensus.com            christophera@consensus.com

Authors

      Alan O. Freier                Paul C. Kocher
      Netscape Communications       Independent Consultant
      freier@netscape.com           pck@netcom.com

      Philip L. Karlton             Tim Dierks
      Netscape Communications       Consensus Development
      karlton@netscape.com          timd@consensus.com


Other contributors

      Martin Abadi                  Robert Relyea
      Digital Equipment Corporation Netscape Communications
      ma@pa.dec.com                 relyea@netscape.com

      Taher Elgamal                 Jim Roskind
      Netscape Communications       Netscape Communications
      elgamal@netscape.com          jar@netscape.com

      Anil Gangolli                 Micheal J. Sabin, Ph. D.
      Netscape Communications       Consulting Engineer
      gangolli@netscape.com         msabin@netcom.com

      Kipp E.B. Hickman             Tom Weinstein
      Netscape Communications       Netscape Communications
      kipp@netscape.com             tomw@netscape.com

Early reviewers

      Robert Baldwin                Clyde Monma
      RSA Data Security, Inc.       Bellcore
      baldwin@rsa.com               clyde@bellcore.com



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      George Cox                    Eric Murray
      Intel Corporation             ericm@lne.com
      cox@ibeam.jf.intel.com

      Cheri Dowell                  Avi Rubin
      Sun Microsystems              Bellcore
      cheri@eng.sun.com             rubin@bellcore.com

      Stuart Haber                  Don Stephenson
      Bellcore                      Sun Microsystems
      stuart@bellcore.com           don.stephenson@eng.sun.com

      Burt Kaliski                  Joe Tardo
      RSA Data Security, Inc.       General Magic
      burt@rsa.com                  tardo@genmagic.com

Comments

   Comments on this draft should be sent to the editors, Tim
   Dierks <timd@consensus.com> and Christopher Allen
   <christophera@consensus.com>, or to the IETF Transport Layer
   Security (TLS) Working Group.

   The discussion list for IETF-TLS is at IETF-TLS@W3.ORG. You
   subscribe and unsubscribe by sending to IETF-TLS-REQUEST@W3.ORG
   with subscribe or unsubscribe in the SUBJECT of the message.

   Archives of the list are at
        <http://lists.w3.org/Archives/Public/ietf-tls>
























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