Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/drafts/draft_ietf_q_t/draft-ietf-tls-protocol-02.txt

Text File | 1997-03-24 | 163KB | 3,867 lines

Transport Layer Security Working Group                      Tim Dierks
INTERNET-DRAFT                                   Consensus Development
Expires September 22, 1997                           Christopher Allen
                                                 Consensus Development
                                                        March 24, 1997







                          The TLS Protocol
                             Version 1.0


                   <draft-ietf-tls-protocol-02.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                                               3
   2.       Goals                                                      4
   3.       Goals of this document                                     5
   4.       Presentation language                                      5
   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                                                   8
   4.7.     Cryptographic attributes                                   9
   4.8.     Constants                                                 10
   5.       The TLS Record Protocol                                   11
   5.1.     Connection states                                         11
   5.2.     HMAC and the pseudorandom function                        14
   5.3.     Record layer                                              15
   5.3.1.   Fragmentation                                             15
   5.3.2.   Record compression and decompression                      16
   5.3.3.   Record payload protection                                 17
   5.3.3.1. Null or standard stream cipher                            17
   5.3.3.2. CBC block cipher                                          18
   5.4.     Key calculation                                           19
   5.4.1.   Export key generation example                             21
   6.       The TLS Handshake Protocol                                21
   6.1.     Change cipher spec protocol                               22
   6.2.     Alert protocol                                            22
   6.2.1.   Closure alerts                                            23
   6.2.2.   Error alerts                                              24
   6.3.     Handshake Protocol overview                               26
   6.4.     Handshake protocol                                        29
   6.4.1.   Hello messages                                            30
   6.4.1.1. Hello request                                             30
   6.4.1.2. Client hello                                              31
   6.4.1.3. Server hello                                              33
   6.4.2.   Server certificate                                        34
   6.4.3.   Server key exchange message                               36
   6.4.4.   Certificate request                                       38
   6.4.5.   Server hello done                                         39
   6.4.6.   Client certificate                                        39
   6.4.7.   Client key exchange message                               40
   6.4.7.1. RSA encrypted premaster secret message                    40
   6.4.7.2. Client Diffie-Hellman public value                        41
   6.4.8.   Certificate verify                                        41
   6.4.9.   Finished                                                  42
   7.       Cryptographic computations                                43
   7.1.     Computing the master secret                               43

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   7.1.1.   RSA                                                       44
   7.1.2.   Diffie-Hellman                                            44
   8.       Application data protocol                                 44
   A.       Protocol constant values                                  44
   A.1.     Reserved port assignments                                 44
   A.2.     Record layer                                              45
   A.3.     Change cipher specs message                               46
   A.4.     Alert messages                                            46
   A.5.     Handshake protocol                                        46
   A.5.1.   Hello messages                                            47
   A.5.2.   Server authentication and key exchange messages           47
   A.5.3.   Client authentication and key exchange messages           49
   A.5.4.   Handshake finalization message                            49
   A.6.     The CipherSuite                                           49
   A.7.     The Security Parameters                                   51
   B.       Glossary                                                  51
   C.       CipherSuite definitions                                   55
   D.       Implementation Notes                                      57
   D.1.     Temporary RSA keys                                        57
   D.2.     Random Number Generation and Seeding                      57
   D.3.     Certificates and authentication                           58
   D.4.     CipherSuites                                              58
   E.       Backward Compatibility With SSL                           58
   E.1.     Version 2 client hello                                    59
   E.2.     Avoiding man-in-the-middle version rollback               61
   F.       Security analysis                                         61
   F.1.     Handshake protocol                                        61
   F.1.1.   Authentication and key exchange                           61
   F.1.1.1. Anonymous key exchange                                    62
   F.1.1.2. RSA key exchange and authentication                       62
   F.1.1.3. Diffie-Hellman key exchange with authentication           63
   F.1.2.   Version rollback attacks                                  63
   F.1.3.   Detecting attacks against the handshake protocol          64
   F.1.4.   Resuming sessions                                         64
   F.1.5.   MD5 and SHA                                               64
   F.2.     Protecting application data                               65
   F.3.     Final notes                                               65
   G.       Patent Statement                                          65
            References                                                66
            Credits                                                   68
            Comments                                                  69

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:


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     - The connection is private. Symmetric cryptography is used for
       data encryption (e.g., DES[DES], RC4[RC4], etc.) The keys for
       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 without 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 any
|      authenticated connection the secret cannot be obtained, even 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 parties
|      to the communication.

   One advantage of TLS is that it is application protocol independent.
|  Higher level protocols can layer on top of the TLS Protocol
]  transparently. The TLS standard, however, does not specify how
]  protocols add security with TLS; the decisions on how to initiate
]  TLS handshaking and how to interpret the authentication certificates
]  exchanged are left up to the judgement of the designers and
]  implementors of protocols which run on top of TLS.

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.


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    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.

 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. There is no required set of ciphers
|      for minimal compliance, so some implementations may be unable to
|      communicate.

    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 and the TLS protocol itself are based on the SSL 3.0
|  Protocol Specification as published by Netscape. The differences
|  between this protocol and SSL 3.0 are not dramatic, but they are
|  significant enough that TLS 1.0 and SSL 3.0 do not interoperate
|  (although TLS 1.0 does incorporate a mechanism by which a TLS
|  implementation can back down to SSL 3.0). 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.

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

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   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.

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

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


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

       T T'<floor..ceiling>;

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

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

4.4. Numbers

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

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

|  All values, here and elsewhere in the specification, are stored in
|  "network" or "big-endian" order; the uint32 represented by the hex
|  bytes 01 02 03 04 is equivalent to the decimal value 16909060.

4.5. Enumerateds

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

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



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

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

   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.

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

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   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.

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).


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   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.

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;

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       } 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.

|  Four record protocol clients are described in this document: the
|  handshake protocol, the alert protocol, the change cipher spec
|  protocol, and the application data protocol. In order to allow
|  extension of the TLS protocol, additional record types can be
|  supported by the record protocol. Any new record types should
|  allocate type values immediately beyond the ContentType values for
|  the four record types described here (see Appendix A.2). If a TLS
|  implementation receives a record type it does not understand, it
|  should just ignore it. Any protocol designed for use over TLS must
|  be carefully designed to deal with all possible attacks against it.

5.1. Connection states

   A 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.

   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.

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   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 which is returned by
|      the MAC algorithm.

   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 { server, client } ConnectionEnd;

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

       enum { stream, block } CipherType;

       enum { true, false } IsExportable;

       enum { null, md5, sha } MACAlgorithm;

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

       /* 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;

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           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 5.4.

   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 running in CBC mode, 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 block ciphers in other modes, whatever state
|      is necessary to sustain encryption or decryption must be
|      maintained. 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.

   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

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       under a particular connection state should use sequence number
       0.

5.2. HMAC and the pseudorandom function

|  A number of operations in the TLS record and handshake layer
|  required a keyed MAC; this is a secure digest of some data protected
|  by a secret. Forging the MAC is infeasible without knowledge of the
|  MAC secret. Finding two data messages which have the same MAC is
|  also cryptographically infeasible. The construction we use for this
|  operation is known as HMAC, described in [HMAC].

|  HMAC can be used with a variety of different hash algorithms. TLS
]  uses it in the handshake with two different algorithms: MD5 and
]  SHA-1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
]  data). Additional hash algorithms can be defined by cipher suites
]  and used to protect record data, but these hashes are hard coded
]  into the description of the handshaking for this version of the
]  protocol.

]  In addition, a construction is required to do expansion of secrets
]  into blocks of data for the purposes of key generation or
]  validation. This pseudo-random function (PRF) takes as input a
]  secret and a seed and produces an output of arbitrary length.

]  In order to make the PRF as secure as possible, it uses two hash
]  algorithms in a way which should guarantee its security if either
]  algorithm remains secure.

]  First, we define a data expansion function, P_hash(secret, data)
]  which uses a single hash function to expand a secret and seed into
]  an arbitrary quantity of output:

]      P_hash(secret, seed) = HMAC_hash(secret, A(1) + data) +
]                             HMAC_hash(secret, A(2) + data) +
]                             HMAC_hash(secret, A(3) + data) + ...

]  Where + indicates concatenation.

]  A() is defined as:
]      A(0) = seed
]      A(i) = HMAC_hash(secret, A(i-1))

]  P_hash can be iterated as many times as is necessary to produce the
]  required quantity of data. For example, if P_SHA-1 was being used to
]  create 64 bytes of data, it would have to be iterated 4 times
]  (through A(4)), creating 80 bytes of output data; the last 16 bytes
]  of the final iteration would then be discarded, leaving 64 bytes of
]  output data.




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]  TLS's PRF is created by splitting the secret into two halves and
]  using one half to generate data with P_MD5 and the other half to
]  generate data with P_SHA1, then exclusive-or'ing the outputs of
]  these two expansion functions together.

]  S1 and S2 are the two halves of the secret and each is the same
]  length. S1 is taken from the first half of the secret, S2 from the
]  second half. Their length is created by rounding up the length of
]  the overall secret divided by two; thus, if the original secret is
]  an odd number of bytes long, the last byte of S1 will be the same as
]  the first byte of S2.

]      L_S = length in bytes of secret;
]      L_S1 = L_S2 = ceil(L_S / 2);

]  The secret is partitioned into two halves (with the possibility of
]  one shared byte) as described above, S1 taking the first L_S1 bytes
]  and S2 the last L_S2 bytes.

]  The PRF is then defined as the result of mixing the two pseudorandom
]  streams by exclusive-or'ing them together.

]      PRF(secret, seed) = P_MD5(S1, seed) XOR P_SHA-1(S2, seed);

]  Note that because MD5 produces 16 byte outputs and SHA-1 produces 20
]  byte outputs, the boundaries of their internal iterations will not
]  be aligned; to generate a 80 byte output will involve P_MD5 being
]  iterated through A(5), while P_SHA-1 will only iterate through A(4).

5.3. Record layer

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

5.3.1. 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;



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       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, 1 }. The
|      version value 3.1 is historical: TLS version 1.0 is a minor
|      modification to the SSL 3.0 protocol, which bears the version
|      value 3.0. (See Appendix A.1.1).

   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.

5.3.2. 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 a
   TLSPlaintext structure into a 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 a
   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;


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   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.

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

5.3.3. Record payload protection

   The encryption and MAC functions translate a TLSCompressed structure
   into a 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.

5.3.3.1. 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.




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

   The MAC is generated as:

|      HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.version +
|                    TLSCompressed.type + TLSCompressed.length +
|                    TLSCompressed.fragment));

   where "+" denotes concatenation.

   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.

5.3.3.2. 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 5.3.3.1.

   padding
       Padding that is added to force the length of the plaintext to be
|      an even multiple of the block cipher's block length. The padding
|      may be any length up to 255 bytes long, as long as it results in
|      the TLSCiphertext.length being an even multiple of the block
|      length. Lengths longer than necessary might be desirable to
|      frustrate attacks on a protocol based on analysis of the lengths

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|      of exchanged messages. The padding data must be filled with the
|      padding length repeated to fill the array.

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

Example: If the block length is 8 bytes, the content length
|        (TLSCompressed.length) is 61 bytes, and the MAC length is 20
|        bytes, the length before padding is 82 bytes. Thus, the
|        padding length modulo 8 must be equal to 6 in order to make
|        the total length an even multiple of 8 bytes (the block
|        length). The padding length can be 6, 14, 22, and so on,
|        through 254. If the padding length were the minimum necessary,
|        6, the padding would be 6 bytes, each containing the value 6.

|Note: With block ciphers in CBC mode (Cipher 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.

5.4. 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 = PRF(SecurityParameters.master_secret,
]                         SecurityParameters.server_random +
]                         SecurityParameters.client_random);


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   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]
       server_write_IV[SecurityParameters.IV_size]

   The client_write_IV and server_write_IV are only generated for
   non-export block ciphers. For exportable block ciphers, the
   initialization vectors are generated later, as described below. Any
   extra key_block material is discarded.

   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.

   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 =
|      PRF(SecurityParameters.client_write_key,
|                                 SecurityParameters.client_random +
|                                 SecurityParameters.server_random);
|      final_server_write_key =
|      PRF(SecurityParameters.server_write_key,
|                                 SecurityParameters.client_random +
|                                 SecurityParameters.server_random);

|  Exportable encryption algorithms derive their IVs solely from the
]  random values from the hello messages:

|      iv_block = PRF("", SecurityParameters.client_random +
|                         SecurityParameters.server_random);

|  The iv_block is partitioned into two initialization vectors as the
|  key_block was above:

|      client_write_IV[SecurityParameters.IV_size]
|      server_write_IV[SecurityParameters.IV_size]

|  Note that the PRF is used without a secret in this case: this just
|  means that the secret has a length of zero bytes and contributes
|  nothing to the hashing in the PRF.




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5.4.1. 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. The PRF output is
]  stored in the key_block. The key_block is partitioned, and the write
]  keys are salted because this is an exportable encryption algorithm.

|      key_block               = PRF(master_secret,
|                                    master_secret +
|                                    server_random +
|                                    client_random)[0..41]
       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  = PRF(client_write_key,
|                                    client_random +
|                                    server_random)[0..15]
|      final_server_write_key  = PRF(server_write_key,
|                                    client_random +
|                                    server_random)[0..15]
|      iv_block                = PRF("", client_random +
|                                    server_random)[0..15]
|      client_write_IV = iv_block[0..7]
|      server_write_IV = iv_block[8..15]

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
       X509v3[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

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       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.

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.



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

       enum {
           close_notify(0),
           unexpected_message(10),
           bad_record_mac(20),
|          decryption_failed(21),
|          record_overflow(22),
           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),
|          unknown_ca(48),
|          access_denied(49),
|          decode_error(50),
|          decrypt_error(51),
|          export_restriction(60),
|          protocol_version(70),
|          insufficient_security(71),
|          internal_error(80),
|          user_canceled(90),
|          no_renegotiation(100),
           (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.




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   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.

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

|  decryption_failed
|      A TLSCiphertext decrypted in an invalid way: either it wasn`t an
|      even multiple of the block length or its padding values, when
|      checked, weren`t correct. This message is always fatal.

|  record_overflow
|      A TLSCiphertext record was received which had a length more than
|      2^14+2048 bytes, or a record decrypted to a TLSCompressed record
|      with more than 2^14+1024 bytes. 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

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       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.

|  unknown_ca
|      A valid certificate chain or partial chain was received, but the
|      certificate was not accepted because the CA certificate could
|      not be located or couldn`t be matched with a known, trusted CA.
|      This message is always fatal.

|  access_denied
|      A valid certificate was received, but when access control was
|      applied, the sender decided not to proceed with negotiation.
|      This message is always fatal.

|  decode_error
|      A message could not be decoded because some field was out of the
|      specified range or the length of the message was incorrect. This
|      message is always fatal.

|  export_restriction
|      A negotiation not in compliance with export restrictions was
|      detected; for example, attemption to transfer a 1024 bit
|      ephemeral RSA key for the RSA_EXPORT handshake method. This
|      message is always fatal.

|  protocol_version
|      The protocol version the client has attempted to negotiate is
|      recognized, but not supported. (For example, old protocol
|      versions might be avoided for security reasons). This message is
|      always fatal.



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|  insufficient_security
|      Returned instead of handshake_failure when a negotiation has
|      failed specifically because the server requires ciphers more
|      secure than those supported by the client. This message is
|      always fatal.

|  internal_error
|      An internal error unrelated to the peer or the correctness of
|      the protocol makes it impossible to continue (such as a memory
|      allocation failure). This message is always fatal.

|  user_cancelled
|      This handshake is being cancelled for some reason unrelated to a
|      protocol failure. If the user cancels an operation after the
|      handshake is complete, just closing the connection by sending a
|      close_notify is more appropriate. This alert should be followed
|      by a close_notify. This message is generally a warning.

|  no_renegotiation
|      Sent by the client in response to a hello request or by the
|      server in response to a client hello after initial handshaking.
|      Either of these would normally lead to renegotiation; when that
|      is not appropriate, the reciepient should respond with this
|      alert; at that point, the original reqester can decide whether
|      to proceed with the connection. One case where this would be
|      appropriate would be where a server has spawned a process to
|      satisfy a request; the process might receive secuirty parameters
|      (key length, authentication, etc.) at startup and it might be
|      difficult to communicate changes to these parameters after that
|      point. This message is always a warning.

   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
   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 the following goals:

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


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     - 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.

]  Note that higher layers should not be overly reliant on TLS always
]  negotiating the strongest possible connection between two peers:
]  there are a number of ways a man in the middle attacker can attempt
]  to make two entities drop down to the least secure method they
]  support. The protocol has been designed to minimize this risk, but
]  there are still attacks available: for example, an attacker could
]  block access to the port a secure service runs on, or attempt to get
]  the peers to negotiate an unauthenticated connection. The
]  fundamental rule is that higher levels must be cognizant of what
]  their security requirements are and never transmit information over
]  a channel less secure than what they require. The TLS protocol is
]  secure, in that any cipher suite offers its promised level of
]  security: if you negotiate 3DES with a 1024 bit RSA key exchange
]  with a host whose certificate you have verified, you can expect to
]  be that secure. However, you should never send data over a link
]  encrypted with 40 bit security unless you feel that data is worth no
]  more than the effort required to break that encryption.

   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.

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   Following the hello messages, the server will send its certificate,
   if it is to be authenticated. Additionally, a server key exchange
   message may be sent, if it is required (e.g. if their server has no
   certificate, or if its certificate is for signing only). If the
   server is authenticated, it may request a certificate from the
   client, if that is appropriate to the cipher suite selected. Now the
   server will send the server hello done message, indicating that the
   hello-message phase of the handshake is complete. The server will
   then wait for a client response. If the server has sent a
   certificate request message, the client must send either the
   certificate message or a no_certificate alert. The client key
   exchange message is now sent, and the content of that message will
   depend on the public key algorithm selected between the client hello
   and the server hello. If the client has sent a certificate with
   signing ability, a digitally-signed certificate verify message is
   sent to explicitly verify the certificate.

   At this point, a change cipher spec message is sent by the client,
   and the client copies the pending 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 a TLS
       handshake message.



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   When the client and server decide to resume a previous session or
   duplicate an existing session (instead of negotiating new security
   parameters) the message flow is as follows:

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

      Client                                                Server

      ClientHello                   -------->
                                                       ServerHello
                                                [ChangeCipherSpec]
                                    <--------             Finished
      [ChangeCipherSpec]
      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;

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               case server_hello:        ServerHello;
               case certificate:         Certificate;
               case server_key_exchange: ServerKeyExchange;
               case certificate_request: CertificateRequest;
               case server_hello_done:   ServerHelloDone;
               case certificate_verify:  CertificateVerify;
               case client_key_exchange: ClientKeyExchange;
               case finished:            Finished;
           } body;
       } Handshake;

   The handshake protocol messages are presented in the order they must
   be sent; sending handshake messages in an unexpected order results
|  in a fatal error. Unneeded handshake messages can be omitted,
|  however. The one exception is the Hello request message, which may
|  be sent by the server at any time.

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 independent secure
|  connections without repeating the full handshake protocol. These
|  independant connections may occur sequentially or simultaneously; a
|  SessionID becomes valid when the handshake negotiating it completes
|  with the exchange of Finished messages and persists until removed
|  due to aging or because a fatal error was encountered on a
|  connection associated with the session. The actual contents of the
|  SessionID are defined by the server.

       opaque SessionID<0..32>;

   Warning:
|      Because the SessionID is transmitted without encryption or
|      immediate MAC protection, servers must not place confidential
|      information in session identifiers or let the contents of fake
|      session identifiers cause any breach of security. (Note that the
|      contents of the handshake as a whole, including the SessionID,

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|      is protected by the Finished messages exchanged at the end of
|      the handshake.)

   The CipherSuite list, passed from the client to the server in the
   client hello message, contains the combinations of cryptographic
   algorithms supported by the client in order of the client's
   preference (first choice first). Each CipherSuite defines 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.

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

]  It also contains a vendor identification string, intended to
]  identify the manufacturer, platform, and/or version of the TLS
]  implementation running on the Client. While these are the intended
]  uses, this field is not specified and may contain any data thought
]  useful by the implementor, or no data at all. This string consists
]  of between 0 and 64 ISO Latin 1 characters.

]      opaque VendorID<0..64>;  /* Vendor-specified ID string */

       struct {
           ProtocolVersion client_version;
           Random random;
           SessionID session_id;
           CipherSuite cipher_suites<2..2^16-1>;
           CompressionMethod compression_methods<1..2^8-1>;
]          VendorID client_vendor;
       } 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 3.1 (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.

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   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. 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.

]  client_vendor
]      This freeform string contains ISO Latin 1 characters specifying
]      the implementation of TLS being used by the client. This is
]      intended solely for compatibility and debugging work and should
]      not be used by the server as a part of the protocol.

   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;
]          VendorID server_vendor;
       } ServerHello;

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   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
|      3.1 (See Appendix E for details about backward compatibility).

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

   session_id
       This is the identity of the session corresponding to this
       connection. If the ClientHello.session_id was non-empty, the
       server will look in its session cache for a match. If a match is
       found and the server is willing to establish the new connection
       using the specified session state, the server will respond with
       the same value as was supplied by the client. This indicates a
       resumed session and dictates that the parties must proceed
       directly to the finished messages. Otherwise this field will
       contain a different value identifying the new session. The
       server may return an empty session_id to indicate that the
]      session will not be cached and therefore cannot be resumed. If a
]      session is resumed, it must be resumed using the same cipher
]      suite it was originally negotiated with.

   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.

]  server_vendor
]      This freeform string contains ISO Latin 1 characters specifying
]      the implementation of TLS being used by the server. This is
]      intended solely for compatibility and debugging work and should
]      not be used by the client as a part of the protocol.

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.509v3
       certificate. It must contain a key which matches the key

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       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.

       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.

]  All certificate profiles, key and cryptographic formats are defined
]  by the IETF PKIX working group [PKIX].

   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<0..2^24-1>;
       } Certificate;

   certificate_list
|      This is a sequence (chain) of X.509v3 certificates. The sender's
|      certificate must come first in the list. Each following
|      certificate must directly certify the one preceding it. Because
|      certificate validation requires that root keys be distributed
|      independantly, the self-signed certificate which specifies the
|      root certificate authority may optionally be omitted from the
|      chain, under the assumption that the remote end must already

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|      possess it in order to validate it in any case.

   The same message type and structure will be used for the client's
|  response to a certificate request message. Note that a client may
|  send no certificates if it does not have an appropriate certificate
|  to send in response to the server's authentication request.

 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 immediately after the server
|      certificate message (or the server hello message, if this is an
|      anonymous negotiation).

       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 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

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   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.

       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.



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       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];
               };
           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, if sent, will immediately follow the Server Key
|      Exchange message (if it is sent; otherwise, the Server
|      Certificate message).

]  Structure of this message:
]      enum {
]          rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
           (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.


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       certificate_authorities
           A list of the distinguished names of acceptable certificate
|          authorities. These distinguished names may specify a desired
|          distinguished name for a root CA or for a subordinate CA;
|          thus, this message can be used both to describe known roots
|          and a desired authorization space.

 Note: DistinguishedName is derived from [X509].

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

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 certificate message
|      containing no certificates. If client authentication is required
|      by the server for the handshake to continue, it may respond with
|      a fatal handshake failure alert. Client certificates are sent
|      using the Certificate structure defined in Section 6.4.2.

 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.




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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. 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. Upon receiving the

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|          premaster secret, the server should check that this value
|          matches the value transmitted by the client in the client
|          hello message.

       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. In this case, the Client Key Exchange message
|          will be sent, but will be empty.

       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
|          HMAC_MD5(master_secret, handshake_messages);

       Certificate.signature.sha_hash
|          HMAC_SHA(master_secret, handshake_messages);

   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. Recipients of
]      finished messages must verify that the contents are correct.
]      Once a side has sent its Finished message and received and
]      validated the Finished message from its peer, it may begin to
]      send and receive application data over the connection.

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

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

       md5_hash
|          HMAC_MD5(master_secret, handshake_messages + Sender);




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       sha_hash
|          HMAC_SHA(master_secret, handshake_messages + Sender);

       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 6.4.8 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. Also, Hello Request
|      messages are omitted from handshake hashes.

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.

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 = PRF(pre_master_secret,
]                          ClientHello.random + ServerHello.random);




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   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.

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, although the protocol should be useful over
|  any transport which can provide a reliable stream connection. 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).



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   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.2. Record layer

    struct {
        uint8 major, minor;
    } ProtocolVersion;

|   ProtocolVersion version = { 3, 1 };     /* TLS v1.0 */

    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;
        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 {

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        opaque content[TLSCompressed.length];
        opaque MAC[CipherSpec.hash_size];
        uint8 padding[GenericBlockCipher.padding_length];
        uint8 padding_length;
    } GenericBlockCipher;

A.3. Change cipher specs message

    struct {
        enum { change_cipher_spec(1), (255) } type;
    } ChangeCipherSpec;

A.4. Alert messages

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

    enum {
        close_notify(0),
        unexpected_message(10),
        bad_record_mac(20),
        decompression_failure(30),
        handshake_failure(40),
        no_certificate(41),
        bad_certificate(42),
        unsupported_certificate(43),
        certificate_revoked(44),
        certificate_expired(45),
        certificate_unknown(46),
        illegal_parameter (47),
        (255)
    } AlertDescription;

    struct {
        AlertLevel level;
        AlertDescription description;
    } Alert;

A.5. 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;

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            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.5.1. Hello messages

    struct { } HelloRequest;

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

    opaque SessionID<0..32>;

    uint8 CipherSuite[2];

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

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

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

A.5.2. Server authentication and key exchange messages

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

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

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    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;

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

]   enum {
]       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
]       (255)
]   } ClientCertificateType;

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

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

    struct { } ServerHelloDone;

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A.5.3. 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;
    } ClientDiffieHellmanPublic;

    struct {
        Signature signature;
    } CertificateVerify;

A.5.4. 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 specification 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.

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    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. The server can
|  request an RSA or DSS signature-capable certificate from the client
|  for client authentication or it may request a Diffie-Hellman
|  certificate. Any Diffie-Hellman certificate provided by the client
|  must use the parameters (group and generator) described by the
|  server.

    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 deprecated.

    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.

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

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;

       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;

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.




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   asymmetric cipher
       See public key cryptography.

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

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

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

|  cipher block chaining (CBC)
|      CBC is a mode in which every plaintext block encrypted with a
|      block cipher is first exclusive-ORed with the previous
|      ciphertext block (or, in the case of the first block, with the
|      initialization vector). For decryption, every block is first
|      decrypted, then exclusive-ORed with the previous ciphertext
|      block (or IV).

   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 with a 56 bit key and an 8 byte block size. Note
|      that in TLS, for key generation purposes, DES is treated as
|      having an 8 byte key length (64 bits), but it still only
|      provides 56 bits of protection. DES can also be operated in a
|      mode where three independant keys and three encryptions are used
|      for each block of data; this uses 168 bits of key (24 bytes in

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|      the TLS key generation method) and provides the equivalent of
|      112 bits of security. [DES], [3DES]

   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.
       [DSS]

   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.
       [IDEA]

   Message Authentication Code (MAC)
       A Message Authentication Code is a one-way hash computed from a
|      message and some secret data. It is difficult to forge without
|      knowing the secret data and it is difficult to find messages
|      which hash to the same MAC. 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 is a secure hashing function that converts an arbitrarily
|      long data stream into a digest of fixed size (16 bytes). [MD5]

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

   one-way hash function
       A one-way transformation that converts an arbitrary amount of
       data into a fixed-length hash. It is computationally hard to

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       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. [RSA]

   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.

   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. Note that all references to SHA
|      actually use the modified SHA-1 algorithm. [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

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       with the plaintext.

   symmetric cipher
       See bulk cipher.

|  Transport Layer Security (TLS)
|      This protocol; also, the Transport Layer Security working group
|      of the Internet Engineering Task Force (IETF). See "Comments" at
|      the end of this document.

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
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
      Exchange
      Algorithm       Description                        Key size limit

      DHE_DSS         Ephemeral DH with DSS signatures   None
      DHE_DSS_EXPORT  Ephemeral DH with DSS signatures   DH = 512 bits

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      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.

                         Key      Expanded   Effective   IV    Block
    Cipher       Type  Material Key Material  Key Bits  Size   Size

    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

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




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   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.

   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.




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Warning: 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.
|        This may be accomplished by accumulating seed bits into a
|        buffer and processing them all at once or by processing an
|        incrementing counter with every seed bit; either method will
|        reintroduce order dependance into the seeding process.

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.

E. Backward Compatibility With SSL

|  For historical reasons and in order to avoid a profligate
|  consumption of reserved port numbers, application protocols which
|  are secured by TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share
|  the same connection port: for example, the https protocol (HTTP
|  secured by SSL or TLS) uses port 443 regardless of which security
|  protocol it is using. Thus, some mechanism must be determined to
|  distinguish and negotiate among the various protocols.

|  TLS version 1.0 and SSL 3.0 are very similar; thus, supporting both
|  is easy. TLS clients who wish to negotiate with SSL 3.0 servers
|  should send client hello messages using the SSL 3.0 record format
|  and client hello structure, sending {3, 1} for the version field to
|  note that they support TLS 1.0. If the server supports only SSL 3.0,
|  it will respond with an SSL 3.0 server hello; if it supports TLS,
|  with a TLS server hello. The negotiation then proceeds as
|  appropriate for the negotiated protocol.

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|  Similarly, a TLS server which wishes to interoperate with SSL 3.0
|  clients should accept SSL 3.0 client hello messages and respond with
|  an SSL 3.0 server hello if an SSL 3.0 client hello is received which
|  has a version field of {3, 0}, denoting that this client does not
|  support TLS.

|  Whenever a client already knows the highest protocol known to a
|  server (for example, when resuming a session), it should initiate
|  the connection in that native protocol.

|  TLS 1.0 clients that support SSL Version 2.0 servers must send SSL
|  Version 2.0 client hello messages [SSL-2]. TLS servers should accept
|  either client hello format if they wish to support SSL 2.0 clients
|  on the same connection port. 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 TLS_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
|      V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
|      V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
|      V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
|                                                 = { 0x04,0x00,0x80 };
|      V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
|      V2CipherSpec TLS_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
|      V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };

|  Cipher specifications native to TLS 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 TLS equivalent (see Appendix A.6):

|      V2CipherSpec (see TLS 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];



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       struct {
           unit8 msg_type;
           Version version;
           uint16 cipher_spec_length;
           uint16 session_id_length;
           uint16 challenge_length;
           V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
           opaque session_id[V2ClientHello.session_id_length];
           Random challenge;
       } V2ClientHello;

   msg_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.

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|Note: Requests to resume a TLS session should use a TLS client hello.

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

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

|  When TLS 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 Master Secret, 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
   valid certificate chain leading to an acceptable certificate

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   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 7.1). The master_secret is required to
|  generate the certificate verify and finished messages, encryption
   keys, and MAC secrets (see Sections 6.4.8, 6.4.9 and 5.4). 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.

 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.

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   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 6.4.8). 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.

   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

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   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 for the size of the signed hashes and RSA key lengths used
|  in the protocol, 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.

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.




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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.

   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

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   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

   [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
   IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.

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

   [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.

   [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.




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

]  [HMAC] H. Krawczyk, RFC 2104, HMAC: Keyed-Hashing for Message
]  Authentication, February, 1997.

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

   [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.

]  [PKIX] R. Housley, W. Ford, W. Polk, D. Solo, Internet Public Key
]  Infrastructure: Part I: X.509 Certificate and CRL Profile,
]  <draft-ietf-pkix-ipki-part1-03.txt>, December 1996.

   [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: [phone] [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.

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

   [TCP] ISI for DARPA, RFC 793: Transport Control Protocol, September
   1981.

   [TEL] J. Postel and J. Reynolds, RFC 854/5, May, 1993.




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   [X509] CCITT. Recommendation X.509: "The Directory - Authentication
   Framework". 1988.

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

Credits

Working Group Chair

      Win Treese
      Open Market
      treeseopenmarket.com

Editors

      Tim Dierks                    Christopher Allen
      Consensus Development         Consensus Development
      [e-mail]            [e-mail]

Authors

      Alan O. Freier                Paul C. Kocher
      Netscape Communications       Independent Consultant
      [e-mail]           [e-mail]

      Philip L. Karlton             Tim Dierks
      Netscape Communications       Consensus Development
      [e-mail]          [e-mail]


Other contributors

      Martin Abadi                  Robert Relyea
      Digital Equipment Corporation Netscape Communications
      [e-mail]                 [e-mail]

      Taher Elgamal                 Jim Roskind
      Netscape Communications       Netscape Communications
      [e-mail]          [e-mail]

      Anil Gangolli                 Micheal J. Sabin, Ph. D.
      Netscape Communications       Consulting Engineer
      [e-mail]         [e-mail]

      Kipp E.B. Hickman             Tom Weinstein
      Netscape Communications       Netscape Communications
      [e-mail]             [e-mail]

Early reviewers



Dierks, T.                  Expires September, 1997            [Page 68]
INTERNET-DRAFT                     TLS 1.0                    March 1997

      Robert Baldwin                Clyde Monma
      RSA Data Security, Inc.       Bellcore
      [e-mail]               [e-mail]

      George Cox                    Eric Murray
      Intel Corporation             [e-mail]
      [e-mail]

      Cheri Dowell                  Avi Rubin
      Sun Microsystems              Bellcore
      [e-mail]             [e-mail]

      Stuart Haber                  Don Stephenson
      Bellcore                      Sun Microsystems
      [e-mail]           [e-mail]

      Burt Kaliski                  Joe Tardo
      RSA Data Security, Inc.       General Magic
      [e-mail]                  [e-mail]

Comments

]  Comments on this draft should be sent to the editors, Tim Dierks and
]  Christopher Allen at the address <[e-mail]>,
]  or to the IETF Transport Layer Security (TLS) Working Group.

]  The discussion list for the IETF TLS working group is located at the
]  e-mail address <[e-mail]>. Information on the group
]  and information on how to subscribe to the list is at
]  <http://www.consensus.com/ietf-tls/>.

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   Archives of the list are at:
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Dierks, T.                  Expires September, 1997            [Page 69]