Network Working Group                                          T. Dierks
Request for Comments: 4346                                   Independent
Obsoletes: 2246                                              E. Rescorla
Category: Standards Track                                     RTFM, Inc.
                                                              April 2006

The Transport Layer Security (TLS) Protocol
Version 1.1

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

   This document specifies Version 1.1 of the Transport Layer Security
   (TLS) protocol.  The TLS protocol provides communications security
   over the Internet.  The protocol allows client/server applications to
   communicate in a way that is designed to prevent eavesdropping,
   tampering, or message forgery.

   1. Introduction ....................................................4
      1.1. Differences from TLS 1.0 ...................................5
      1.2. Requirements Terminology ...................................5
   2. Goals ...........................................................5
   3. Goals of This Document ..........................................6
   4. Presentation Language ...........................................6
      4.1. Basic Block Size ...........................................7
      4.2. Miscellaneous ..............................................7
      4.3. Vectors ....................................................7
      4.4. Numbers ....................................................8
      4.5. Enumerateds ................................................8
      4.6. Constructed Types ..........................................9
           4.6.1. Variants ...........................................10
      4.7. Cryptographic Attributes ..................................11
      4.8. Constants .................................................12
   5. HMAC and the Pseudorandom Function .............................12
   6. The TLS Record Protocol ........................................14
      6.1. Connection States .........................................15
      6.2. Record layer ..............................................17
           6.2.1. Fragmentation ......................................17
           6.2.2. Record Compression and Decompression ...............19
           6.2.3. Record Payload Protection ..........................19
                  6.2.3.1. Null or Standard Stream Cipher ............20
                  6.2.3.2. CBC Block Cipher ..........................21
      6.3. Key Calculation ...........................................24
   7. The TLS Handshaking Protocols ..................................24
      7.1. Change Cipher Spec Protocol ...............................25
      7.2. Alert Protocol ............................................26
           7.2.1. Closure Alerts .....................................27
           7.2.2. Error Alerts .......................................28
      7.3. Handshake Protocol Overview ...............................31
      7.4. Handshake Protocol ........................................34
           7.4.1. Hello Messages .....................................35
                  7.4.1.1. Hello request .............................35
                  7.4.1.2. Client Hello ..............................36
                  7.4.1.3. Server Hello ..............................39
           7.4.2. Server Certificate .................................40
           7.4.3. Server Key Exchange Message ........................42
           7.4.4. Certificate request ................................44
           7.4.5. Server Hello Done ..................................46
           7.4.6. Client certificate .................................46
           7.4.7. Client Key Exchange Message ........................47
                  7.4.7.1. RSA Encrypted Premaster Secret Message ....47
                  7.4.7.2. Client Diffie-Hellman Public Value ........50
           7.4.8. Certificate verify .................................50
           7.4.9. Finished ...........................................51

   8. Cryptographic Computations .....................................52
      8.1. Computing the Master Secret ...............................52
           8.1.1. RSA ................................................53
           8.1.2. Diffie-Hellman .....................................53
   9. Mandatory Cipher Suites ........................................53
   10. Application Data Protocol .....................................53
   11. Security Considerations .......................................53
   12. IANA Considerations ...........................................54
   A. Appendix - Protocol constant values ............................55
           A.1. Record layer .........................................55
           A.2. Change cipher specs message ..........................56
           A.3. Alert messages .......................................56
           A.4. Handshake protocol ...................................57
           A.4.1. Hello messages .....................................57
           A.4.2. Server authentication and key exchange messages ....58
           A.4.3. Client authentication and key exchange messages ....59
           A.4.4.Handshake finalization message ......................60
           A.5. The CipherSuite ......................................60
           A.6. The Security Parameters ..............................63
   B. Appendix - Glossary ............................................64
   C. Appendix - CipherSuite definitions .............................68
   D. Appendix - Implementation Notes ................................69
           D.1 Random Number Generation and Seeding ..................70
           D.2 Certificates and authentication .......................70
           D.3 CipherSuites ..........................................70
   E. Appendix - Backward Compatibility With SSL .....................71
           E.1. Version 2 client hello ...............................72
           E.2. Avoiding man-in-the-middle version rollback ..........74
   F. Appendix - Security analysis ...................................74
           F.1. Handshake protocol ...................................74
           F.1.1. Authentication and key exchange ....................74
           F.1.1.1. Anonymous key exchange ...........................75
           F.1.1.2. RSA key exchange and authentication ..............75
           F.1.1.3. Diffie-Hellman key exchange with authentication ..76
           F.1.2. Version rollback attacks ...........................77
           F.1.3. Detecting attacks against the handshake protocol ...77
           F.1.4. Resuming sessions ..................................78
           F.1.5. MD5 and SHA ........................................78
           F.2. Protecting application data ..........................78
           F.3. Explicit IVs .........................................79
           F.4  Security of Composite Cipher Modes ...................79
           F.5  Denial of Service ....................................80
           F.6. Final notes ..........................................80
   Normative References ..............................................81
   Informative References ............................................82

1. Introduction

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

   -  The connection is private.  Symmetric cryptography is used for
      data encryption (e.g., DES [DES], RC4 [SCH] 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 to the judgment of the designers and implementors
   of protocols that run on top of TLS.

1.1. Differences from TLS 1.0

   This document is a revision of the TLS 1.0 [TLS1.0] protocol, and
   contains some small security improvements, clarifications, and
   editorial improvements.  The major changes are:

   -  The implicit Initialization Vector (IV) is replaced with an
      explicit IV to protect against CBC attacks [CBCATT].

   -  Handling of padding errors is changed to use the bad_record_mac
      alert rather than the decryption_failed alert to protect against
      CBC attacks.

   -  IANA registries are defined for protocol parameters.

   -  Premature closes no longer cause a session to be nonresumable.

   -  Additional informational notes were added for various new attacks
      on TLS.

   In addition, a number of minor clarifications and editorial
   improvements were made.

1.2. Requirements Terminology

   In this document, the keywords "MUST", "MUST NOT", "REQUIRED",
   "SHOULD", "SHOULD NOT" and "MAY" are to be interpreted as described
   in RFC 2119 [REQ].

2. Goals

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

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

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

   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: preventing
      the need to create a new protocol (and risking the introduction of
      possible new weaknesses) and avoiding 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 1.1, TLS 1.0, and SSL 3.0 do not
   interoperate (although each protocol incorporates a mechanism by
   which an implementation can back down prior versions).  This document
   is intended primarily for readers who will be implementing the
   protocol and for those doing cryptographic analysis of it.  The
   specification has been written with this in mind, and it is intended
   to reflect the needs of those two groups.  For that reason, many of
   the algorithm-dependent data structures and rules are included in the
   body of the text (as opposed to in an appendix), providing easier
   access to them.

   This document is not intended to supply any details of service
   definition or of interface definition, although it does cover select
   areas of policy as they are required for the maintenance of solid
   security.

4. Presentation Language

   This document deals with the formatting of data in an external
   representation.  The following very basic and somewhat casually
   defined presentation syntax will be used.  The syntax draws from
   several sources in its structure.  Although it resembles the
   programming language "C" in its syntax and 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; it has
   no 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 */

   Variable-length vectors are defined by specifying a subrange of legal
   lengths, inclusively, using the notation <floor..ceiling>.  When

   these are 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.  The length of an encoded vector must
   be an even multiple of the length of a single element (for example, a
   17-byte vector of uint16 would be illegal).

       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;

   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,
   with 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
   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, an

       orange VariantRecord

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

4.7. Cryptographic Attributes

   The four cryptographic operations digital signing, stream cipher
   encryption, block cipher encryption, and public key encryption are
   designated digitally-signed, stream-ciphered, block-ciphered, and
   public-key-encrypted, respectively.  A field's cryptographic
   processing is specified by prepending an appropriate key word
   designation before the field's type specification.  Cryptographic
   keys are implied by the current session state (see Section 6.1).

   In digital signing, one-way hash functions are used as input for a
   signing algorithm.  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 RSA signing, a 36-byte structure of two hashes (one SHA and one
   MD5) is signed (encrypted with the private key).  It is encoded with
   PKCS #1 block type 1, as described in [PKCS1A].

   Note: The standard reference for PKCS#1 is now RFC 3447 [PKCS1B].
         However, to minimize differences with TLS 1.0 text, we are
         using the terminology of RFC 2313 [PKCS1A].

   In DSS, the 20 bytes of the SHA hash are run directly through the
   Digital Signing Algorithm with no additional hashing.  This produces
   two values, r and s.  The DSS signature is an opaque vector, as
   above, the contents of which are the DER encoding of:

       Dss-Sig-Value  ::=  SEQUENCE  {
            r       INTEGER,
            s       INTEGER
       }

   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 that 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.

   An RSA-encrypted value is encoded with PKCS #1 block type 2, as
   described in [PKCS1A].

   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, and
   then the entire structure is encrypted with a stream cipher.  The
   length of this structure, in bytes, would be equal to two 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 because 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;
       } Example1;

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

5. HMAC and the Pseudorandom Function

   A number of operations in the TLS record and handshake layer require
   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.  The construction we use for this operation is known as HMAC,
   and is 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 MD5 and SHA-1 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, a seed,
   and an identifying label 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 that should guarantee its security if either
   algorithm remains secure.

   First, we define a data expansion function, P_hash(secret, data) that
   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) + seed) +
                              HMAC_hash(secret, A(2) + seed) +
                              HMAC_hash(secret, A(3) + seed) + ...

   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 is being used to
   create 64 bytes of data, it will have to be iterated 4 times (through
   A(4)), creating 80 bytes of output data; the last 16 bytes of the
   final iteration will then be discarded, leaving 64 bytes of output
   data.

   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_SHA-1, then exclusive-ORing 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-ORing them together.

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

   The label is an ASCII string.  It should be included in the exact
   form it is given without a length byte or trailing null character.
   For example, the label "slithy toves" would be processed by hashing
   the following bytes:

73 6C 69 74 68 79 20 74 6F 76 65 73

   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.  Generating an 80-byte output will require that P_MD5
   iterate through A(5), while P_SHA-1 will only iterate through A(4).

6. 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, reassembled, and 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.1).  All such
   values must be defined by RFC 2434 Standards Action.  See Section 11
   for IANA Considerations for ContentType values.

   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.  Note that because the type and length of a record are
   not protected by encryption, care SHOULD be taken to minimize the
   value of traffic analysis of these values.

6.1. Connection States

   A TLS connection state is the operating environment of the TLS Record
   Protocol.  It specifies a compression algorithm, and encryption
   algorithm, and a MAC algorithm.  In addition, the parameters for
   these algorithms are known: the MAC secret and the bulk encryption
   keys 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 Change Cipher Spec 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 that has not been initialized with security
   parameters a current state.  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.

   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, and the block size
      of the cipher (if appropriate).

   MAC algorithm
      An algorithm to be used for message authentication.  This
      specification includes the size of the hash returned by the MAC
      algorithm.

   compression algorithm
      An algorithm to be used for data compression.  This specification
      must include all information the algorithm requires 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, idea, aes }
       BulkCipherAlgorithm;

       enum { stream, block } CipherType;

       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;
           MACAlgorithm           mac_algorithm;
           uint8                  hash_size;
           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 four items:

       client write MAC secret
       server write MAC secret
       client write key
       server write key

   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 6.3.

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

   compression state
      The current state of the compression algorithm.

   cipher state
      The current state of the encryption algorithm.  This will consist
      of the scheduled key for that connection.  For stream ciphers,
      this will also contain whatever state information is necessary 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 separately 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.  Sequence numbers do not wrap.  If a TLS
      implementation would need to wrap a sequence number, it must
      renegotiate instead.  A sequence number is incremented after each
      record: specifically, the first record transmitted under a
      particular connection state MUST use sequence number 0.

6.2. Record layer

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

6.2.1. Fragmentation

   The record layer fragments information blocks into TLSPlaintext
   records carrying data in chunks 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 a single message MAY be
   fragmented across several records).

       struct {
           uint8 major, minor;
       } ProtocolVersion;

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

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

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

   version
      The version of the protocol being employed.  This document
      describes TLS Version 1.1, which uses the version { 3, 2 }.  The
      version value 3.2 is historical: TLS version 1.1 is a minor
      modification to the TLS 1.0 protocol, which was itself a minor
      modification to the SSL 3.0 protocol, which bears the version
      value 3.0.  (See Appendix A.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 is 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.  However, records MUST be
   delivered to the network in the same order as they are protected by
   the record layer.  Recipients MUST receive and process interleaved
   application layer traffic during handshakes subsequent to the first
   one on a connection.

6.2.2. Record Compression and Decompression

   All records are compressed using the compression algorithm defined in
   the current session state.  There is always an active compression
   algorithm; however, initially it is defined as
   CompressionMethod.null.  The compression algorithm translates 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;

   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.

6.2.3. Record Payload Protection

   The encryption and MAC functions translate a TLSCompressed structure
   into a TLSCiphertext.  The decryption functions reverse the process.
   The MAC of the record also includes a sequence number so that
   missing, extra, or repeated 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.

6.2.3.1. Null or Standard Stream Cipher

   Stream ciphers (including BulkCipherAlgorithm.null, see Appendix A.6)
   convert TLSCompressed.fragment structures to and from stream
   TLSCiphertext.fragment structures.

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

   The MAC is generated as:

       HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type +
                     TLSCompressed.version + 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.

6.2.3.2. CBC Block Cipher

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

       block-ciphered struct {
           opaque IV[CipherSpec.block_length];
           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 6.2.3.1.

   IV
      Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
      IV in order to prevent the attacks described by [CBCATT].  We
      recommend the following equivalently strong procedures.  For
      clarity we use the following notation.

      IV
         The transmitted value of the IV field in the GenericBlockCipher
         structure.

      CBC residue
         The last ciphertext block of the previous record.

      mask
         The actual value that the cipher XORs with the plaintext prior
         to encryption of the first cipher block of the record.

      In prior versions of TLS, there was no IV field and the CBC
      residue and mask were one and the same.  See Sections 6.1,
      6.2.3.2, and 6.3, of [TLS1.0] for details of TLS 1.0 IV handling.

      One of the following two algorithms SHOULD be used to generate the
      per-record IV:

      (1) Generate a cryptographically strong random string R of length
          CipherSpec.block_length.  Place R in the IV field.  Set the
          mask to R.  Thus, the first cipher block will be encrypted as
          E(R XOR Data).

      (2) Generate a cryptographically strong random number R of length
          CipherSpec.block_length and prepend it to the plaintext prior
          to encryption.  In this case either:

          (a) The cipher may use a fixed mask such as zero.
          (b) The CBC residue from the previous record may be used as
              the mask.  This preserves maximum code compatibility with
              TLS 1.0 and SSL 3.  It also has the advantage that it does
              not require the ability to quickly reset the IV, which is
              known to be a problem on some systems.

          In either (2)(a) or (2)(b) the data (R || data) is fed into
          the encryption process.  The first cipher block (containing
          E(mask XOR R) is placed in the IV field.  The first block of
          content contains E(IV XOR data).

      The following alternative procedure MAY be used; however, it has
      not been demonstrated to be as cryptographically strong as the
      above procedures.  The sender prepends a fixed block F to the
      plaintext (or, alternatively, a block generated with a weak PRNG).
      He then encrypts as in (2), above, using the CBC residue from the
      previous block as the mask for the prepended block.  Note that in
      this case the mask for the first record transmitted by the
      application (the Finished) MUST be generated using a
      cryptographically strong PRNG.

      The decryption operation for all three alternatives is the same.
      The receiver decrypts the entire GenericBlockCipher structure and
      then discards the first cipher block, corresponding to the IV
      component.

   padding
      Padding that is added to force the length of the plaintext to be
      an integral multiple of the block cipher's block length.  The
      padding MAY be any length up to 255 bytes, as long as it results
      in the TLSCiphertext.length being an integral multiple of the
      block length.  Lengths longer than necessary might be desirable to
      frustrate attacks on a protocol that are based on analysis of the
      lengths of exchanged messages.  Each uint8 in the padding data
      vector MUST be filled with the padding length value.  The receiver

      MUST check this padding and SHOULD use the bad_record_mac alert to
      indicate padding errors.

   padding_length
      The padding length MUST be such that the total size of the
      GenericBlockCipher structure is a multiple of the cipher's block
      length.  Legal values range from zero to 255, inclusive.  This
      length specifies the length of the padding field exclusive of the
      padding_length field itself.

   The encrypted data length (TLSCiphertext.length) is one more than the
   sum of CipherSpec.block_length, 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, then the length before padding is 82 bytes (this does
            not include the IV, which may or may not be encrypted, as
            discussed above).  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.  Thus, the last 8 octets
            of the GenericBlockCipher before block encryption would be
            xx 06 06 06 06 06 06 06, where xx is the last octet of the
            MAC.

   Note: With block ciphers in CBC mode (Cipher Block Chaining), it is
         critical that the entire plaintext of the record be known
         before any ciphertext is transmitted.  Otherwise, it is
         possible for the attacker to mount the attack described in
         [CBCATT].

   Implementation Note: Canvel et al. [CBCTIME] have demonstrated a
                        timing attack on CBC padding based on the time
                        required to compute the MAC.  In order to defend
                        against this attack, implementations MUST ensure
                        that record processing time is essentially the
                        same whether or not the padding is correct.  In
                        general, the best way to do this is to compute
                        the MAC even if the padding is incorrect, and
                        only then reject the packet.  For instance, if
                        the pad appears to be incorrect, the
                        implementation might assume a zero-length pad
                        and then compute the MAC.  This leaves a small
                        timing channel, since MAC performance depends to
                        some extent on the size of the data fragment,

                        but it is not believed to be large enough to be
                        exploitable, due to the large block size of
                        existing MACs and the small size of the timing
                        signal.

6.3. Key Calculation

   The Record Protocol requires an algorithm to generate keys, 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 and keys required by the current
   connection state (see Appendix A.6).  CipherSpecs require a client
   write MAC secret, a server write MAC secret, a client write key, and
   a server write key, each of which is generated from the master secret
   in that order.  Unused values are empty.

   When keys and MAC secrets are generated, the master secret is used as
   an entropy source.

   To generate the key material, compute

       key_block = PRF(SecurityParameters.master_secret,
                          "key expansion",
                          SecurityParameters.server_random +
             SecurityParameters.client_random);

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

       client_write_MAC_secret[SecurityParameters.hash_size]
       server_write_MAC_secret[SecurityParameters.hash_size]
       client_write_key[SecurityParameters.key_material_length]
       server_write_key[SecurityParameters.key_material_length]

   Implementation note: The currently defined cipher suite that requires
   the most material is AES_256_CBC_SHA, defined in [TLSAES].  It
   requires 2 x 32 byte keys, 2 x 20 byte MAC secrets, and 2 x 16 byte
   Initialization Vectors, for a total of 136 bytes of key material.

7. The TLS Handshaking Protocols

   TLS has three subprotocols that are used to allow peers to agree upon
   security parameters for the record layer, to authenticate themselves,
   to instantiate negotiated security parameters, and to 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
      cryptographic attributes such as the hash_size.  (See Appendix A.6
      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.

7.1. 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 the
   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 MUST instruct the
   record layer to make the write pending state the write active state.
   (See Section 6.1.)  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 7.4.9).

   Note: If a rehandshake occurs while data is flowing on a connection,
         the communicating parties may continue to send data using the
         old CipherSpec.  However, once the ChangeCipherSpec has been
         sent, the new CipherSpec MUST be used.  The first side to send
         the ChangeCipherSpec does not know that the other side has
         finished computing the new keying material (e.g., if it has to
         perform a time consuming public key operation).  Thus, a small
         window of time, during which the recipient must buffer the
         data, MAY exist.  In practice, with modern machines this
         interval is likely to be fairly short.

7.2. Alert Protocol

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

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

             enum {
                 close_notify(0),
                 unexpected_message(10),
                 bad_record_mac(20),
                 decryption_failed(21),
                 record_overflow(22),
                 decompression_failure(30),
                 handshake_failure(40),
                 no_certificate_RESERVED (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_RESERVED(60),
                 protocol_version(70),
                 insufficient_security(71),
                 internal_error(80),
                 user_canceled(90),
                 no_renegotiation(100),
                 (255)
             } AlertDescription;

             struct {
                 AlertLevel level;
                 AlertDescription description;
             } Alert;

7.2.1. 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.  Note that as of TLS 1.1,
      failure to properly close a connection no longer requires that a
      session not be resumed.  This is a change from TLS 1.0 to conform
      with widespread implementation practice.

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

   Unless some other fatal alert has been transmitted, each party is
   required to send a close_notify alert before closing the write side
   of the connection.  The other party MUST 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.

   If the application protocol using TLS provides that any data may be
   carried over the underlying transport after the TLS connection is
   closed, the TLS implementation must receive the responding
   close_notify alert before indicating to the application layer that
   the TLS connection has ended.  If the application protocol will not
   transfer any additional data, but will only close the underlying
   transport connection, then the implementation MAY choose to close the

   transport without waiting for the responding close_notify.  No part
   of this standard should be taken to dictate the manner in which a
   usage profile for TLS manages its data transport, including when
   connections are opened or closed.

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

7.2.2. 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 a fatal alert message, both
   parties immediately close the connection.  Servers and clients MUST
   forget any session-identifiers, keys, and secrets associated with a
   failed connection.  Thus, any connection terminated with a fatal
   alert MUST NOT be resumed.  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 alert also MUST be returned if an alert is sent because
      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.

   decryption_failed
      This alert MAY be returned if 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.

   Note: Differentiating between bad_record_mac and decryption_failed
         alerts may permit certain attacks against CBC mode as used in
         TLS [CBCATT].  It is preferable to uniformly use the
         bad_record_mac alert to hide the specific type of the error.

   record_overflow
         A TLSCiphertext record was received that 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_RESERVED
         This alert was used in SSLv3 but not in TLS.  It should not be
         sent by compliant implementations.

   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.

   decrypt_error
         A handshake cryptographic operation failed, including being
         unable to correctly verify a signature, decrypt a key exchange,
         or validate a finished message.

   export_restriction_RESERVED
         This alert was used in TLS 1.0 but not TLS 1.1.

   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.

   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 (such as a memory allocation failure) makes it
         impossible to continue.  This message is always fatal.

   user_canceled
         This handshake is being canceled 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 recipient should respond with this
         alert.  At that point, the original requester can decide
         whether to proceed with the connection.  One case where this
         would be appropriate is where a server has spawned a process to
         satisfy a request; the process might receive security
         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 that are transmitted
   with a level of fatal MUST be treated as fatal messages.

   New alert values MUST be defined by RFC 2434 Standards Action.  See
   Section 11 for IANA Considerations for alert values.

7.3. 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 involves the following steps:

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

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

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

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

   -  Provide security parameters to the record layer.

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

   Note that higher layers should not be overly reliant on whether TLS
   always negotiates the strongest possible connection between two
   peers.  There are a number of ways in which 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, one SHOULD never send data over a link encrypted with 40-bit
   security unless one feels 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
   specifying a format for these messages and by defining the use of the
   messages to allow the client and server to agree upon a shared
   secret.  This secret MUST be quite long; currently defined key
   exchange methods exchange secrets that range from 48 to 128 bytes in
   length.

   Following the hello messages, the server will send its certificate,
   if it is to be authenticated.  Additionally, a server key exchange
   message may be sent, if it is required (e.g., if the 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.  Next,
   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 the certificate
   message.  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.)  Application data MUST NOT be sent prior to the
   completion of the first handshake (before a cipher suite other
   TLS_NULL_WITH_NULL_NULL is established).

      Client                                               Server

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

             Fig. 1. Message flow for a full handshake

      * 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.

   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

          Fig. 2. Message flow for an abbreviated handshake

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

7.4. Handshake Protocol

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

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

      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case hello_request:       HelloRequest;
              case client_hello:        ClientHello;

              case server_hello:        ServerHello;
              case certificate:         Certificate;
              case server_key_exchange: ServerKeyExchange;
              case certificate_request: CertificateRequest;
              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 below 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.  Note one exception to the ordering: the
   Certificate message is used twice in the handshake (from server to
   client, then from client to server), but is described only in its
   first position.  The one message that is not bound by these ordering
   rules is the Hello Request message, which can be sent at any time,
   but which should be ignored by the client if it arrives in the middle
   of a handshake.

   New Handshake message type values MUST be defined via RFC 2434
   Standards Action.  See Section 11 for IANA Considerations for these
   values.

7.4.1. 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.

7.4.1.1. 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, or the client may, if it wishes, respond

      with a no_renegotiation alert.  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 receive 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 MUST NOT be included in the message hashes that
         are maintained throughout the handshake and used in the
         finished messages and the certificate verify message.

7.4.1.2. 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 (seconds since the midnight starting Jan 1, 1970, GMT,
      ignoring leap seconds) 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,
   from this connection, or from 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, and the third
   option makes it possible to establish several independent secure
   connections without repeating the full handshake protocol.  These
   independent 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 it is
   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 content of the handshake as a
            whole, including the SessionID, 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 (favorite 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;

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

   client_version
      The version of the TLS protocol by which the client wishes to
      communicate during this session.  This SHOULD be the latest
      (highest valued) version supported by the client.  For this
      version of the specification, the version will be 3.2.  (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 if the
      client wishes to generate new security parameters.

   cipher_suites
      This is a list of the cryptographic options supported by the
      client, with the client's first preference first.  If the
      session_id field is not empty (implying a session resumption
      request) this vector MUST include at least the cipher_suite from
      that session.  Values are defined in Appendix A.5.

   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.

   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 that a client hello message 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.

      Note: For the intended use of trailing data in the ClientHello,
         see RFC 3546 [TLSEXT].

7.4.1.3. Server Hello

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

   Structure of this message:

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

   server_version
      This field will contain the lower of that suggested by the client
      in the client hello and the highest supported by the server.  For
      this version of the specification, the version is 3.2.  (See
      Appendix E for details about backward compatibility.)

   random
      This structure is generated by the server and MUST be
      independently generated from the 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.

7.4.2. 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 that matches the key exchange
      method, as follows.  Unless otherwise specified, the signing
      algorithm for the certificate MUST be the same as the algorithm
      for the certificate key.  Unless otherwise specified, the public
      key MAY be of any length.

      Key Exchange Algorithm  Certificate Key Type

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

      DHE_DSS                 DSS public key.

      DHE_RSA                 RSA public key that can be used for
                              signing.

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

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

   All certificate profiles and key and cryptographic formats are
   defined by the IETF PKIX working group [PKIX].  When a key usage
   extension is present, the digitalSignature bit MUST be set for the
   key to be eligible for signing, as described above, and the
   keyEncipherment bit MUST be present to allow encryption, as described
   above.  The keyAgreement bit must be set on Diffie-Hellman
   certificates.

   As CipherSuites that 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
      independently, the self-signed certificate that specifies the root
      certificate authority may optionally be omitted from the chain,
      under the assumption that the remote end must already 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.

7.4.3. 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:

           DHE_DSS
           DHE_RSA
           DH_anon

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

           RSA
           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 with
      which to encrypt the premaster secret, 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 that include new key
   exchange algorithms, the server key exchange message will be sent if
   and only if the certificate type associated with the key exchange
   algorithm does not provide enough information for the client to
   exchange a premaster secret.

   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;

      struct {
          select (KeyExchangeAlgorithm) {
              case diffie_hellman:
                  ServerDHParams params;
              case rsa:
                  ServerRSAParams params;
          };
       } ServerParams;

      params
          The server's key exchange parameters.

      signed_params
          For non-anonymous key exchanges, a hash of the corresponding
          params value, with the signature appropriate to that hash
          applied.

      md5_hash
          MD5(ClientHello.random + ServerHello.random + ServerParams);

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

      enum { anonymous, rsa, dsa } SignatureAlgorithm;

      struct {
          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;

7.4.4. Certificate request

   When this message will be sent:

      A non-anonymous server can optionally request a certificate from
      the client, if it is 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),
       rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
       fortezza_dms_RESERVED(20),
          (255)

      } ClientCertificateType;

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

      struct {
          ClientCertificateType certificate_types<1..2^8-1>;
          DistinguishedName certificate_authorities<0..2^16-1>;
      } CertificateRequest;

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

      certificate_authorities
         A list of the distinguished names of acceptable certificate
         authorities.  These distinguished names may specify a desired
         distinguished name for a root CA or for a subordinate CA; thus,
         this message can be used to describe both known roots and a
         desired authorization space.  If the certificate_authorities
         list is empty then the client MAY send any certificate of the
         appropriate ClientCertificateType, unless there is some
         external arrangement to the contrary.

   ClientCertificateType values are divided into three groups:

      1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
         reserved for IETF Standards Track protocols.

      2. Values from 64 decimal (0x40) through 223 decimal (0xDF)
         inclusive are reserved for assignment for non-Standards Track
         methods.

      3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
         inclusive are reserved for private use.

   Additional information describing the role of IANA in the allocation
   of ClientCertificateType code points is described in Section 11.

   Note: Values listed as RESERVED may not be used.  They were used in
         SSLv3.

   Note: DistinguishedName is derived from [X501].  DistinguishedNames
         are represented in DER-encoded format.

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

7.4.5. 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;

7.4.6. 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.  That is, the certificate_list structure has a
      length of zero.  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 7.4.2.

   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.

7.4.7. Client Key Exchange Message

   When this message will be sent:

      This message is always sent by the client.  It MUST immediately
      follow the client certificate message, if it is sent.  Otherwise
      it MUST 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 transmission of the RSA-encrypted secret or by the
      transmission of Diffie-Hellman parameters that 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
      that contained a Diffie-Hellman public key whose parameters (group
      and generator) matched those specified by the server in its
      certificate, this message MUST not contain any data.

   Structure of this message:

      The choice of messages depends on which key exchange method has
      been selected.  See Section 7.4.3 for the KeyExchangeAlgorithm
      definition.

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

7.4.7.1. 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 and is 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 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 8.1.

   Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be
         used to attack a TLS server that is using PKCS#1 v 1.5 encoded
         RSA.  The attack takes advantage of the fact that, by failing
         in different ways, a TLS server can be coerced into revealing
         whether a particular message, when decrypted, is properly
         PKCS#1 v1.5 formatted or not.

         The best way to avoid vulnerability to this attack is to treat
         incorrectly formatted messages in a manner indistinguishable
         from correctly formatted RSA blocks.  Thus, when a server
         receives an incorrectly formatted RSA block, it should generate
         a random 48-byte value and proceed using it as the premaster
         secret.  Thus, the server will act identically whether the
         received RSA block is correctly encoded or not.

         [PKCS1B] defines a newer version of PKCS#1 encoding that is
         more secure against the Bleichenbacher attack.  However, for
         maximal compatibility with TLS 1.0, TLS 1.1 retains the
         original encoding.  No variants of the Bleichenbacher attack

         are known to exist provided that the above recommendations are
         followed.

   Implementation Note: Public-key-encrypted data is represented as an
                        opaque vector <0..2^16-1> (see Section 4.7).
                        Thus, the RSA-encrypted PreMasterSecret in a
                        ClientKeyExchange is preceded by two length
                        bytes.  These bytes are redundant in the case of
                        RSA because the EncryptedPreMasterSecret is the
                        only data in the ClientKeyExchange and its
                        length can therefore be unambiguously
                        determined.  The SSLv3 specification was not
                        clear about the encoding of public-key-encrypted
                        data, and therefore many SSLv3 implementations
                        do not include the length bytes, encoding the
                        RSA encrypted data directly in the
                        ClientKeyExchange message.

                        This specification requires correct encoding of
                        the EncryptedPreMasterSecret complete with
                        length bytes.  The resulting PDU is incompatible
                        with many SSLv3 implementations.  Implementors
                        upgrading from SSLv3 must modify their
                        implementations to generate and accept the
                        correct encoding.  Implementors who wish to be
                        compatible with both SSLv3 and TLS should make
                        their implementation's behavior dependent on the
                        protocol version.

   Implementation Note: It is now known that remote timing-based attacks
                        on SSL are possible, at least when the client
                        and server are on the same LAN.  Accordingly,
                        implementations that use static RSA keys SHOULD
                        use RSA blinding or some other anti-timing
                        technique, as described in [TIMING].

   Note: The version number in the PreMasterSecret MUST be the version
         offered by the client in the ClientHello, not the version
         negotiated for the connection.  This feature is designed to
         prevent rollback attacks.  Unfortunately, many implementations
         use the negotiated version instead, and therefore checking the
         version number may lead to failure to interoperate with such
         incorrect client implementations.  Client implementations, MUST
         and Server implementations MAY, check the version number.  In
         practice, since the TLS handshake MACs prevent downgrade and no
         good attacks are known on those MACs, ambiguity is not
         considered a serious security risk.  Note that if servers
         choose to check the version number, they should randomize the

         PreMasterSecret in case of error, rather than generate an
         alert, in order to avoid variants on the Bleichenbacher attack.
         [KPR03]

7.4.7.2. 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 and 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 it MUST 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).

7.4.8. 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
      except those containing fixed Diffie-Hellman parameters).  When
      sent, it MUST immediately follow the client key exchange message.

   Structure of this message:

      struct {
           Signature signature;
      } CertificateVerify;

      The Signature type is defined in 7.4.3.

      CertificateVerify.signature.md5_hash
          MD5(handshake_messages);

      CertificateVerify.signature.sha_hash
          SHA(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 7.4, exchanged thus far.

7.4.9. 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.

      struct {
          opaque verify_data[12];
      } Finished;

      verify_data
          PRF(master_secret, finished_label, MD5(handshake_messages) +
          SHA-1(handshake_messages)) [0..11];

      finished_label
          For Finished messages sent by the client, the string "client
          finished".  For Finished messages sent by the server, the
          string "server finished".

      handshake_messages
          All of the data from all messages in this handshake (not
          including any HelloRequest 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
          7.4, exchanged thus far.

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

   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 7.4.8
   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 that is sent second will include the
   prior one.

   Note: Change cipher spec messages, alerts, and any other record types
         are not handshake messages and are not included in the hash
         computations.  Also, Hello Request messages are omitted from
         handshake hashes.

8. 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.

8.1. 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, "master secret",
                           ClientHello.random + ServerHello.random)
       [0..47];

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

8.1.1. 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.

8.1.2. Diffie-Hellman

   A conventional Diffie-Hellman computation is performed.  The
   negotiated key (Z) is used as the pre_master_secret, and is converted
   into the master_secret, as specified above.  Leading bytes of Z that
   contain all zero bits are stripped before it is used as the
   pre_master_secret.

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

9. Mandatory Cipher Suites

   In the absence of an application profile standard specifying
   otherwise, a TLS compliant application MUST implement the cipher
   suite TLS_RSA_WITH_3DES_EDE_CBC_SHA.

10. 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.

11. Security Considerations

Security issues are discussed throughout this memo, especially in
Appendices D, E, and F.

12. IANA Considerations

   This document describes a number of new registries that have been
   created by IANA.  We recommended that they be placed as individual
   registries items under a common TLS category.

   Section 7.4.3 describes a TLS ClientCertificateType Registry to be
   maintained by the IANA, defining a number of such code point
   identifiers.  ClientCertificateType identifiers with values in the
   range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards
   Action.  Values from the range 64-223 (decimal) inclusive are
   assigned via [RFC2434] Specification Required.  Identifier values
   from 224-255 (decimal) inclusive are reserved for RFC 2434 Private
   Use.  The registry will initially be populated with the values in
   this document, Section 7.4.4.

   Section A.5 describes a TLS Cipher Suite Registry to be maintained by
   the IANA, and it defines a number of such cipher suite identifiers.
   Cipher suite values with the first byte in the range 0-191 (decimal)
   inclusive are assigned via RFC 2434 Standards Action.  Values with
   the first byte in the range 192-254 (decimal) are assigned via RFC
   2434 Specification Required.  Values with the first byte 255
   (decimal) are reserved for RFC 2434 Private Use.  The registry will
   initially be populated with the values from Section A.5 of this
   document, [TLSAES], and from Section 3 of [TLSKRB].

   Section 6 requires that all ContentType values be defined by RFC 2434
   Standards Action.  IANA has created a TLS ContentType registry,
   initially populated with values from Section 6.2.1 of this document.
   Future values MUST be allocated via Standards Action as described in
   [RFC2434].

   Section 7.2.2 requires that all Alert values be defined by RFC 2434
   Standards Action.  IANA has created a TLS Alert registry, initially
   populated with values from Section 7.2 of this document and from
   Section 4 of [TLSEXT].  Future values MUST be allocated via Standards
   Action as described in [RFC2434].

   Section 7.4 requires that all HandshakeType values be defined by RFC
   2434 Standards Action.  IANA has created a TLS HandshakeType
   registry, initially populated with values from Section 7.4 of this
   document and from Section 2.4 of [TLSEXT].  Future values MUST be
   allocated via Standards Action as described in [RFC2434].

Appendix A. Protocol Constant Values

This section describes protocol types and constants.

A.1. Record Layer

   struct {
       uint8 major, minor;
   } ProtocolVersion;

   ProtocolVersion version = { 3, 2 };     /* TLS v1.1 */

   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 {
       opaque IV[CipherSpec.block_length];

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

A.2. Change Cipher Specs Message

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

A.3. Alert Messages

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

       enum {
           close_notify(0),
           unexpected_message(10),
           bad_record_mac(20),
           decryption_failed(21),
           record_overflow(22),
           decompression_failure(30),
           handshake_failure(40),
           no_certificate_RESERVED (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_RESERVED(60),
           protocol_version(70),
           insufficient_security(71),
           internal_error(80),
           user_canceled(90),
           no_renegotiation(100),
           (255)
       } AlertDescription;

   struct {
       AlertLevel level;
       AlertDescription description;
   } Alert;

A.4. Handshake Protocol

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

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

A.4.1. Hello messages

   struct { } HelloRequest;

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

   opaque SessionID<0..32>;

   uint8 CipherSuite[2];

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

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

   } ClientHello;

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

A.4.2. Server Authentication and Key Exchange Messages

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

   struct {
       ASN.1Cert certificate_list<0..2^24-1>;
   } Certificate;

   enum { rsa, diffie_hellman } KeyExchangeAlgorithm;

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

   struct {
       opaque dh_p<1..2^16-1>;
       opaque dh_g<1..2^16-1>;
       opaque dh_Ys<1..2^16-1>;
   } ServerDHParams;

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

   enum { anonymous, rsa, dsa } SignatureAlgorithm;

   struct {
       select (KeyExchangeAlgorithm) {
           case diffie_hellman:
               ServerDHParams params;

           case rsa:
               ServerRSAParams params;
       };
   } ServerParams;

   struct {
       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),
    rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6),
    fortezza_dms_RESERVED(20),
    (255)
   } ClientCertificateType;

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

   struct {
       ClientCertificateType certificate_types<1..2^8-1>;
       DistinguishedName certificate_authorities<0..2^16-1>;
   } CertificateRequest;

   struct { } ServerHelloDone;

A.4.3. Client Authentication and Key Exchange Messages

   struct {
       select (KeyExchangeAlgorithm) {
           case rsa: EncryptedPreMasterSecret;
           case diffie_hellman: ClientDiffieHellmanPublic;
       } 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.4.4. Handshake Finalization Message

   struct {
       opaque verify_data[12];
   } Finished;

A.5. 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
   1.1.

   TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a
   TLS connection during the first handshake on that channel, but must
   not be negotiated, as it provides no more protection than an
   unsecured connection.

    CipherSuite TLS_NULL_WITH_NULL_NULL                = { 0x00,0x00 };

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

    CipherSuite TLS_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
    CipherSuite TLS_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
    CipherSuite TLS_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
    CipherSuite TLS_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
    CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
    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 that 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_WITH_DES_CBC_SHA            = { 0x00,0x0C };
    CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x0D };
    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_WITH_DES_CBC_SHA           = { 0x00,0x12 };
    CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x13 };
    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_WITH_RC4_128_MD5           = { 0x00,0x18 };
    CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
    CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };

   When SSLv3 and TLS 1.0 were designed, the United States restricted
   the export of cryptographic software containing certain strong
   encryption algorithms.  A series of cipher suites were designed to
   operate at reduced key lengths in order to comply with those
   regulations.  Due to advances in computer performance, these
   algorithms are now unacceptably weak, and export restrictions have
   since been loosened.  TLS 1.1 implementations MUST NOT negotiate
   these cipher suites in TLS 1.1 mode.  However, for backward
   compatibility they may be offered in the ClientHello for use with TLS

   1.0 or SSLv3-only servers.  TLS 1.1 clients MUST check that the
   server did not choose one of these cipher suites during the
   handshake.  These ciphersuites are listed below for informational
   purposes and to reserve the numbers.

    CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
    CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
    CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
    CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
    CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
    CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
    CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
    CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
    CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };

   The following cipher suites were defined in [TLSKRB] and are included
   here for completeness.  See [TLSKRB] for details:

    CipherSuite    TLS_KRB5_WITH_DES_CBC_SHA           = { 0x00,0x1E }:
    CipherSuite    TLS_KRB5_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1F };
    CipherSuite    TLS_KRB5_WITH_RC4_128_SHA           = { 0x00,0x20 };
    CipherSuite    TLS_KRB5_WITH_IDEA_CBC_SHA          = { 0x00,0x21 };
    CipherSuite    TLS_KRB5_WITH_DES_CBC_MD5           = { 0x00,0x22 };
    CipherSuite    TLS_KRB5_WITH_3DES_EDE_CBC_MD5      = { 0x00,0x23 };
    CipherSuite    TLS_KRB5_WITH_RC4_128_MD5           = { 0x00,0x24 };
    CipherSuite    TLS_KRB5_WITH_IDEA_CBC_MD5          = { 0x00,0x25 };

   The following exportable cipher suites were defined in [TLSKRB] and
   are included here for completeness.  TLS 1.1 implementations MUST NOT
   negotiate these cipher suites.

    CipherSuite  TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA    = { 0x00,0x26};
    CipherSuite  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA    = { 0x00,0x27};
    CipherSuite  TLS_KRB5_EXPORT_WITH_RC4_40_SHA        = { 0x00,0x28};
    CipherSuite  TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5    = { 0x00,0x29};
    CipherSuite  TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5    = { 0x00,0x2A};
    CipherSuite  TLS_KRB5_EXPORT_WITH_RC4_40_MD5        = { 0x00,0x2B};

   The following cipher suites were defined in [TLSAES] and are included
   here for completeness.  See [TLSAES] for details:

         CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA      = { 0x00, 0x2F };
         CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA   = { 0x00, 0x30 };
         CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA   = { 0x00, 0x31 };
         CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA  = { 0x00, 0x32 };
         CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA  = { 0x00, 0x33 };
         CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA  = { 0x00, 0x34 };

         CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA      = { 0x00, 0x35 };
         CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA   = { 0x00, 0x36 };
         CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA   = { 0x00, 0x37 };
         CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA  = { 0x00, 0x38 };
         CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA  = { 0x00, 0x39 };
         CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA  = { 0x00, 0x3A };

    The cipher suite space is divided into three regions:

      1. Cipher suite values with first byte 0x00 (zero) through decimal
         191 (0xBF) inclusive are reserved for the IETF Standards Track
         protocols.

      2. Cipher suite values with first byte decimal 192 (0xC0) through
         decimal 254 (0xFE) inclusive are reserved for assignment for
         non-Standards Track methods.

      3. Cipher suite values with first byte 0xFF are reserved for
         private use.

   Additional information describing the role of IANA in the allocation
   of cipher suite code points is described in Section 11.

   Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
         reserved to avoid collision with Fortezza-based cipher suites
         in SSL 3.

A.6. 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, aes, idea }
            BulkCipherAlgorithm;

            enum { stream, block } CipherType;

            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;
                MACAlgorithm mac_algorithm;
                uint8 hash_size;
                CompressionMethod compression_algorithm;
                opaque master_secret[48];
                opaque client_random[32];
                opaque server_random[32];
            } SecurityParameters;

Appendix B. Glossary

   Advanced Encryption Standard (AES)
      AES is a widely used symmetric encryption algorithm.  AES is a
      block cipher with a 128, 192, or 256 bit keys and a 16 byte block
      size. [AES] TLS currently only supports the 128 and 256 bit key
      sizes.

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

   asymmetric cipher
      See public key cryptography.

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

   block cipher
      A block cipher is an algorithm that operates on plaintext in
      groups of bits, called blocks. 64 bits is a common 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 a strong binding between a party's identity
      or some other attributes and its public key.

   client
      The application entity that initiates a TLS connection to a
      server.  This may or may not imply that the client initiated the
      underlying transport connection.  The primary operational
      difference between the server and client is that the server is
      generally authenticated, while the client is only optionally
      authenticated.

   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.  (The low bit of each key byte is presumed to be
      set to produce odd parity in that key byte.)  DES can also be
      operated in a mode where three independent keys and three
      encryptions are used for each block of data; this uses 168 bits of
      key (24 bytes in 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.  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 reverse
      the transformation or to find collisions.  MD5 and SHA are
      examples of one-way hash functions.

   RC2
      A block cipher developed by Ron Rivest at RSA Data Security, Inc.
      [RSADSI] described in [RC2].

   RC4
      A stream cipher invented by Ron Rivest.  A compatible cipher is
      described in [SCH].

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

   server
      The server is the application entity that responds to requests for
      connections from clients.  See also under client.

   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 that 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 180-2.  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
      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.

Appendix C. CipherSuite Definitions

CipherSuite                           Key Exchange   Cipher      Hash

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_WITH_RC4_128_MD5              RSA            RC4_128      MD5
TLS_RSA_WITH_RC4_128_SHA              RSA            RC4_128      SHA
TLS_RSA_WITH_IDEA_CBC_SHA             RSA            IDEA_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_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_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_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_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_WITH_RC4_128_MD5          DH_anon        RC4_128      MD5
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

      Key
      Exchange
      Algorithm     Description                        Key size limit

      DHE_DSS       Ephemeral DH with DSS signatures   None
      DHE_RSA       Ephemeral DH with RSA signatures   None
      DH_anon       Anonymous DH, no signatures        None
      DH_DSS        DH with DSS-based certificates     None
      DH_RSA        DH with RSA-based certificates     None
                                                       RSA = none
      NULL          No key exchange                    N/A
      RSA           RSA key exchange                   None

                         Key      Expanded     IV    Block
    Cipher       Type  Material Key Material   Size   Size

    NULL         Stream   0          0         0     N/A
    IDEA_CBC     Block   16         16         8      8
    RC2_CBC_40   Block    5         16         8      8
    RC4_40       Stream   5         16         0     N/A
    RC4_128      Stream  16         16         0     N/A
    DES40_CBC    Block    5          8         8      8
    DES_CBC      Block    8          8         8      8
    3DES_EDE_CBC Block   24         24         8      8

   Type
      Indicates whether this is a stream cipher or a block cipher
      running in CBC mode.

   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.

   IV Size
      The amount of data needed to be generated for the initialization
      vector.  Zero for stream ciphers; equal to the block size for
      block ciphers.

   Block Size
      The amount of data a block cipher enciphers in one chunk; a block
      cipher running in CBC mode can only encrypt an even multiple of
      its block size.

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

Appendix D. Implementation Notes

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

D.1. 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.  Seeding a 128-bit PRNG would
   thus require approximately 100 such timer values.

   [RANDOM] provides guidance on the generation of random values.

D.2 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.3 CipherSuites

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

Appendix E. Backward Compatibility with SSL

   For historical reasons and in order to avoid a profligate consumption
   of reserved port numbers, application protocols that are secured by
   TLS 1.1, 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 versions 1.1 and 1.0, and SSL 3.0 are very similar; thus,
   supporting both is easy.  TLS clients who wish to negotiate with such
   older servers SHOULD send client hello messages using the SSL 3.0
   record format and client hello structure, sending {3, 2} for the
   version field to note that they support TLS 1.1. If the server
   supports only TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0
   server hello; if it supports TLS 1.1 it will respond with a TLS 1.1
   server hello.  The negotiation then proceeds as appropriate for the
   negotiated protocol.

   Similarly, a TLS 1.1  server that wishes to interoperate with TLS 1.0
   or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages and
   respond with a SSL 3.0 server hello if an SSL 3.0 client hello with a
   version field of {3, 0} is received, denoting that this client does
   not support TLS.  Similarly, if a SSL 3.0 or TLS 1.0 hello with a
   version field of {3, 1} is received, the server SHOULD respond with a
   TLS 1.0 hello with a version field of {3, 1}.

   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 1.1 clients that support SSL Version 2.0 servers MUST send SSL
   Version 2.0 client hello messages [SSL2].  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.  Implementors 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.5):

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

   Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
       handshakes for backward compatibility but MUST NOT negotiate them
       in TLS 1.1 mode.

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.  Note that this message MUST
   be sent directly on the wire, not wrapped as an SSLv3 record

     uint8 V2CipherSpec[3];

     struct {
         uint16 msg_length;
         uint8 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];
         opaque challenge[V2ClientHello.challenge_length;
     } V2ClientHello;

   msg_length
      This field is the length of the following data in bytes.  The high
      bit MUST be 1 and is not part of the length.

   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).

   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 zero.

   challenge_length
      The length in bytes of the client's challenge to the server to
      authenticate itself.  When using the SSLv2 backward compatible
      handshake the client MUST use a 32-byte challenge.

   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
      This field MUST be empty.

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

      Note: Requests to resume a TLS session MUST 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. 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
   is secure against man-in-the-middle attacks, but completely anonymous
   sessions are inherently vulnerable to such attacks.  Anonymous
   servers cannot authenticate clients.  If the server is authenticated,
   its certificate message must provide a valid certificate chain
   leading to an acceptable certificate authority.  Similarly,
   authenticated clients must supply an acceptable certificate to the

   server.  Each party is responsible for verifying that the other's
   certificate is valid and has not expired or been revoked.

   The general goal of the key exchange process is to create a
   pre_master_secret known to the communicating parties and not to
   attackers.  The pre_master_secret will be used to generate the
   master_secret (see Section 8.1).  The master_secret is required to
   generate the finished messages, encryption keys, and MAC secrets (see
   Sections 7.4.8, 7.4.9, and 6.3).  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: 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 either may be contained in the server's certificate or may
   be a temporary RSA key sent in a server key exchange message.  When
   temporary RSA keys are used, they are signed by the server's RSA
   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.

   Note that if ephemeral RSA is not used, compromise of the server's
   static RSA key results in a loss of confidentiality for all sessions
   protected under that static key.  TLS users desiring Perfect Forward
   Secrecy should use DHE cipher suites.  The damage done by exposure of
   a private key can be limited by changing one's private key (and
   certificate) frequently.

   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 7.4.8).  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
   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.

   If the same DH keypair is to be used for multiple handshakes, either
   because the client or server has a certificate containing a fixed DH
   keypair or because the server is reusing DH keys, care must be taken
   to prevent small subgroup attacks.  Implementations SHOULD follow the
   guidelines found in [SUBGROUP].

   Small subgroup attacks are most easily avoided by using one of the
   DHE ciphersuites and generating a fresh DH private key (X) for each
   handshake.  If a suitable base (such as 2) is chosen, g^X mod p can
   be computed very quickly, therefore the performance cost is
   minimized.  Additionally, using a fresh key for each handshake
   provides Perfect Forward Secrecy.  Implementations SHOULD generate a
   new X for each handshake when using DHE ciphersuites.

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 expired.
   Parties concerned about attacks of this scale should not use 40-bit
   encryption keys.  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 chooses.

   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.

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. Explicit IVs

   [CBCATT] describes a chosen plaintext attack on TLS that depends on
   knowing the IV for a record.  Previous versions of TLS [TLS1.0] used
   the CBC residue of the previous record as the IV and therefore
   enabled this attack.  This version uses an explicit IV in order to
   protect against this attack.

F.4. Security of Composite Cipher Modes

   TLS secures transmitted application data via the use of symmetric
   encryption and authentication functions defined in the negotiated
   ciphersuite.  The objective is to protect both the integrity and
   confidentiality of the transmitted data from malicious actions by
   active attackers in the network.  It turns out that the order in
   which encryption and authentication functions are applied to the data
   plays an important role for achieving this goal [ENCAUTH].

   The most robust method, called encrypt-then-authenticate, first
   applies encryption to the data and then applies a MAC to the
   ciphertext.  This method ensures that the integrity and
   confidentiality goals are obtained with ANY pair of encryption and
   MAC functions, provided that the former is secure against chosen
   plaintext attacks and that the MAC is secure against chosen-message
   attacks.  TLS uses another method, called authenticate-then-encrypt,
   in which first a MAC is computed on the plaintext and then the
   concatenation of plaintext and MAC is encrypted.  This method has
   been proven secure for CERTAIN combinations of encryption functions
   and MAC functions, but it is not guaranteed to be secure in general.
   In particular, it has been shown that there exist perfectly secure
   encryption functions (secure even in the information-theoretic sense)
   that combined with any secure MAC function, fail to provide the
   confidentiality goal against an active attack.  Therefore, new
   ciphersuites and operation modes adopted into TLS need to be analyzed
   under the authenticate-then-encrypt method to verify that they
   achieve the stated integrity and confidentiality goals.

   Currently, the security of the authenticate-then-encrypt method has
   been proven for some important cases.  One is the case of stream
   ciphers in which a computationally unpredictable pad of the length of
   the message, plus the length of the MAC tag, is produced using a
   pseudo-random generator and this pad is xor-ed with the concatenation
   of plaintext and MAC tag.  The other is the case of CBC mode using a
   secure block cipher.  In this case, security can be shown if one
   applies one CBC encryption pass to the concatenation of plaintext and
   MAC and uses a new, independent, and unpredictable IV for each new
   pair of plaintext and MAC.  In previous versions of SSL, CBC mode was
   used properly EXCEPT that it used a predictable IV in the form of the
   last block of the previous ciphertext.  This made TLS open to chosen
   plaintext attacks.  This version of the protocol is immune to those
   attacks.  For exact details in the encryption modes proven secure,
   see [ENCAUTH].

F.5. Denial of Service

   TLS is susceptible to a number of denial of service (DoS) attacks.
   In particular, an attacker who initiates a large number of TCP
   connections can cause a server to consume large amounts of CPU doing
   RSA decryption.  However, because TLS is generally used over TCP, it
   is difficult for the attacker to hide his point of origin if proper
   TCP SYN randomization is used [SEQNUM] by the TCP stack.

   Because TLS runs over TCP, it is also susceptible to a number of
   denial of service attacks on individual connections.  In particular,
   attackers can forge RSTs, thereby terminating connections, or forge
   partial TLS records, thereby causing the connection to stall.  These
   attacks cannot in general be defended against by a TCP-using
   protocol.  Implementors or users who are concerned with this class of
   attack should use IPsec AH [AH-ESP] or ESP [AH-ESP].

F.6. 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.

Normative References

   [AES]      National Institute of Standards and Technology,
              "Specification for the Advanced Encryption Standard (AES)"
              FIPS 197.  November 26, 2001.

   [3DES]     W. Tuchman, "Hellman Presents No Shortcut Solutions To
              DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp. 40-41.

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

   [DSS]      NIST FIPS PUB 186-2, "Digital Signature Standard,"
              National Institute of Standards and Technology, U.S.
              Department of Commerce, 2000.

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:  Keyed-
              Hashing for Message Authentication", RFC 2104, 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.

   [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321,
              April 1992.

   [PKCS1A]   B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1:
              RSA Cryptography Specifications Version 1.5", RFC 2313,
              March 1998.

   [PKCS1B]   J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards
              (PKCS) #1: RSA Cryptography Specifications Version 2.1",
              RFC 3447, February 2003.

   [PKIX]     Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RC2]      Rivest, R., "A Description of the RC2(r) Encryption
              Algorithm", RFC 2268, March 1998.

   [SCH]      B. Schneier. "Applied Cryptography: Protocols, Algorithms,
              and Source Code in C, 2ed", Published by John Wiley &
              Sons, Inc. 1996.

   [SHA]      NIST FIPS PUB 180-2, "Secure Hash Standard," National
              Institute of Standards and Technology, U.S. Department of
              Commerce., August 2001.

   [REQ]      Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [TLSAES]   Chown, P., "Advanced Encryption Standard (AES)
              Ciphersuites for Transport Layer Security (TLS)", RFC
              3268, June 2002.

   [TLSEXT]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, June 2003.

   [TLSKRB]   Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

Informative References

   [AH-ESP]   Kent, S., "IP Authentication Header", RFC 4302, December
              2005.

              Eastlake 3rd, D., "Cryptographic Algorithm Implementation
              Requirements for Encapsulating Security Payload (ESP) and
              Authentication Header (AH)", RFC 4305, December 2005.

   [BLEI]     Bleichenbacher D., "Chosen Ciphertext Attacks against
              Protocols Based on RSA Encryption Standard PKCS #1" in
              Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462,
              pages:  1-12, 1998.

   [CBCATT]   Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
              Problems and Countermeasures",
              http://www.openssl.org/~bodo/tls-cbc.txt.

   [CBCTIME]  Canvel, B., "Password Interception in a SSL/TLS Channel",
              http://lasecwww.epfl.ch/memo_ssl.shtml, 2003.

   [ENCAUTH]  Krawczyk, H., "The Order of Encryption and Authentication
              for Protecting Communications (Or: How Secure is SSL?)",
              Crypto 2001.

   [KPR03]    Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
              Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
              March 2003.

   [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.

   [RANDOM]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

   [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.

   [SEQNUM]   Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [SSL2]     Hickman, Kipp, "The SSL Protocol", Netscape Communications
              Corp., Feb 9, 1995.

   [SSL3]     A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0
              Protocol", Netscape Communications Corp., Nov 18, 1996.

   [SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
              Attacks on the Diffie-Hellman Key Agreement Method for
              S/MIME", RFC 2785, March 2000.

   [TCP]      Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

   [TIMING]   Boneh, D., Brumley, D., "Remote timing attacks are
              practical", USENIX Security Symposium 2003.

   [TLS1.0]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [X501]     ITU-T Recommendation X.501: Information Technology - Open
              Systems Interconnection - The Directory: Models, 1993.

   [X509]     ITU-T Recommendation X.509 (1997 E): Information
              Technology - Open Systems Interconnection - "The Directory
              - Authentication Framework". 1988.

[XDR] Srinivasan, R., "XDR: External Data Representation
Standard", RFC 1832, August 1995.

Authors' Addresses

   Working Group Chairs

   Win Treese

   EMail: treese@acm.org

   Eric Rescorla

   EMail: ekr@rtfm.com

Editors

   Tim Dierks
   Independent

   EMail: tim@dierks.org

   Eric Rescorla
   RTFM, Inc.

   EMail: ekr@rtfm.com

Other Contributors

   Christopher Allen (co-editor of TLS 1.0)
   Alacrity Ventures
   EMail: ChristopherA@AlacrityManagement.com

   Martin Abadi
   University of California, Santa Cruz
   EMail: abadi@cs.ucsc.edu

   Ran Canetti
   IBM
   EMail: canetti@watson.ibm.com

   Taher Elgamal
   Securify
   EMail: taher@securify.com

   Anil Gangolli
   EMail: anil@busybuddha.org

   Kipp Hickman

   Phil Karlton (co-author of SSLv3)

   Paul Kocher (co-author of SSLv3)
   Cryptography Research
   EMail: paul@cryptography.com

   Hugo Krawczyk
   Technion Israel Institute of Technology
   EMail: hugo@ee.technion.ac.il

   Robert Relyea
   Netscape Communications
   EMail: relyea@netscape.com

   Jim Roskind
   Netscape Communications
   EMail: jar@netscape.com

   Michael Sabin

   Dan Simon
   Microsoft, Inc.
   EMail: dansimon@microsoft.com

   Tom Weinstein

Comments

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