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[gnutls.git] / doc / protocol / draft-ietf-tls-rfc4346-bis-02.txt

                                                              Tim Dierks
                                                             Independent
                                                           Eric Rescorla
INTERNET-DRAFT                                   Network Resonance, Inc.
<draft-ietf-tls-rfc4346-bis-02.txt>    October 2006 (Expires April 2006)

                            The TLS Protocol
                              Version 1.2

Status of this Memo
   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies Version 1.2 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.

Table of Contents

   1.        Introduction                                                4
   1.1       Differences from TLS 1.1                                    5
   1.1       Requirements Terminology                                    5



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   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                                           14
   6.2.      Record layer                                                17
   6.2.1.    Fragmentation                                               17
   6.2.2.    Record compression and decompression                        18
   6.2.3.    Record payload protection                                   19
   6.2.3.1.  Null or standard stream cipher                              19
   6.2.3.2.  CBC block cipher                                            20
   6.2.3.3.  AEAD ciphers                                                23
   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                                          35
   7.4.1.    Hello messages                                              36
   7.4.1.1.  Hello request                                               36
   7.4.1.2.  Client hello                                                37
   7.4.1.3.  Server hello                                                40
   7.4.1.4   Hello Extensions                                            41
   7.4.1.4.1 Server Name Indication                                      43
   7.4.1.4.2 Maximum Fragment Length Negotiation                         44
   7.4.1.4.3 Client Certificate URLs                                     46
   7.4.1.4.4 Trusted CA Indication                                       46
   7.4.1.4.5 Truncated HMAC                                              48
   7.4.1.4.6 Certificate Status Request                                  49
   7.4.1.4.7 Cert Hash Types                                             50
   7.4.1.4.8 Procedure for Defining New Extensions                       51
   7.4.2.    Server certificate                                          52
   7.4.3.    Server key exchange message                                 53
   7.4.4.    CertificateStatus                                           56
   7.4.5.    Certificate request                                         56
   7.4.6.    Server hello done                                           58



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   7.4.7.    Client certificate                                          59
   7.4.8.    Client Certificate URLs                                     59
   7.4.9.    Client key exchange message                                 61
   7.4.9.1.  RSA encrypted premaster secret message                      62
   7.4.9.2.  Client Diffie-Hellman public value                          64
   7.4.10.   Certificate verify                                          65
   7.4.10.   Finished                                                    65
   8.        Cryptographic computations                                  66
   8.1.      Computing the master secret                                 67
   8.1.1.    RSA                                                         67
   8.1.2.    Diffie-Hellman                                              67
   9.        Mandatory Cipher Suites                                     67
   A.        Protocol constant values                                    71
   A.1.      Record layer                                                71
   A.2.      Change cipher specs message                                 72
   A.3.      Alert messages                                              72
   A.4.      Handshake protocol                                          74
   A.4.1.    Hello messages                                              74
   A.4.2.    Server authentication and key exchange messages             77
   A.4.3.    Client authentication and key exchange messages             78
   A.4.4.    Handshake finalization message                              79
   A.5.      The CipherSuite                                             80
   A.6.      The Security Parameters                                     83
   B.        Glossary                                                    84
   C.        CipherSuite definitions                                     88
   D.        Implementation Notes                                        90
   D.1       Random Number Generation and Seeding                        90
   D.2       Certificates and authentication                             90
   D.3       CipherSuites                                                90
   E.        Backward Compatibility                                      91
   E.1.      Version 2 client hello                                      92
   E.2.      Avoiding man-in-the-middle version rollback                 93
   F.        Security analysis                                           95
   F.1.      Handshake protocol                                          95
   F.1.1.    Authentication and key exchange                             95
   F.1.1.1.  Anonymous key exchange                                      95
   F.1.1.2.  RSA key exchange and authentication                         96
   F.1.1.3.  Diffie-Hellman key exchange with authentication             97
   F.1.2.    Version rollback attacks                                    97
   F.1.3.    Detecting attacks against the handshake protocol            98
   F.1.4.    Resuming sessions                                           98
   F.1.5     Extensions                                                  99
   F.1.5.1   Security of server_name                                     99
   F.1.5.2   Security of client_certificate_url                          100
   F.1.5.4.  Security of trusted_ca_keys                                 101
   F.1.5.5.  Security of truncated_hmac                                  101
   F.1.5.6.  Security of status_request                                  102
   F.2.      Protecting application data                                 102



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   F.3.      Explicit IVs                                                103
   F.4       Security of Composite Cipher Modes                          103
   F.5       Denial of Service                                           104
   F.6.      Final notes                                                 104


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.




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

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

1.1 Differences from TLS 1.1
   This document is a revision of the TLS 1.1 [TLS1.1] protocol which
   contains improved flexibility, particularly for negotiation of
   cryptographic algorithms. The major changes are:

     - Merged in TLS Extensions and AES Cipher Suites from external
     documents.

     - Replacement of MD5/SHA-1 combination in the PRF

     - Replacement of MD5/SHA-1 combination in the digitally-signed
     element.

     - Allow the client to indicate which hash functions it supports.

     - Allow the server to indicate which hash functions it supports

     - Addition of support for authenticated encryption with additional
     data modes.

1.1 Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
   "MAY" that appear in this document are to be interpreted as described
   in RFC 2119 [REQ].

2. Goals

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

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

    2. Interoperability: Independent programmers should be able to
       develop applications utilizing TLS that will then be able to
       successfully exchange cryptographic parameters without knowledge



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       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: to prevent
       the need to create a new protocol (and risking the introduction
       of possible new weaknesses) and to avoid the need to implement an
       entire new security library.

    4. Relative efficiency: Cryptographic operations tend to be highly
       CPU intensive, particularly public key operations. For this
       reason, the TLS protocol has incorporated an optional session
       caching scheme to reduce the number of connections that need to
       be established from scratch. Additionally, care has been taken to
       reduce network activity.

3. Goals of this document

   This document and the TLS protocol itself are based on the SSL 3.0
   Protocol Specification as published by Netscape. The differences
   between this protocol and SSL 3.0 are not dramatic, but they are
   significant enough that the various versions of TLS and SSL 3.0 do
   not interoperate (although each protocol incorporates a mechanism by
   which an implementation can back down to prior versions.) This
   document is intended primarily for readers who will be implementing
   the protocol and 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 nor interface definition, although it does cover select
   areas of policy as they are required for the maintenance of solid
   security.

4. Presentation language

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




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4.1. Basic block size

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

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

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

4.2. Miscellaneous

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

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

   Single byte entities containing uninterpreted data are of type
   opaque.

4.3. Vectors

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

       T T'[n];

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

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

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






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

       T T'<floor..ceiling>;

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




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   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]];






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

4.6.1. Variants

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

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

   For example:

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

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



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       orange VariantRecord

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

4.7. Cryptographic attributes

   The five cryptographic operations digital signing, stream cipher
   encryption, block cipher encryption, authenticated encryption with
   additional data (AEAD) encryption and public key encryption are
   designated digitally-signed, stream-ciphered, block-ciphered, aead-
   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, the output of the chosen hash function is encoded as
   a PKCS #1 DigestInfo and then signed using block type 01 as described
   in Section 8.1 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-1 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 which are block-ciphered
   will be an exact multiple of the cipher block length.



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   In AEAD encryption, the plaintext is simultaneously encrypted and
   integrity protected. The input may be of any length and the output is
   generally larger than the input in order to accomodate the integrity
   check value.

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

4.8. Constants

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

   For example,

       struct {
           uint8 f1;
           uint8 f2;
       } Example1;

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

5. HMAC and the pseudorandom function



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

   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.

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

       P_hash(secret, seed) = HMAC_hash(secret, A(1) + 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 was being used to
   create 64 bytes of data, it would have to be iterated 4 times
   (through A(4)), creating 80 bytes of output data; the last 16 bytes
   of the final iteration would then be discarded, leaving 64 bytes of
   output data.

   TLS's PRF is created by applying P_hash to the secret S as:

      PRF(secret, label, seed) = P_<hash>(secret, label + seed)

   Unless the cipher suite definition specifies otherwise, the hash
   function used in P MUST be the same hash function selected for the
   HMAC in the cipher suite. For existing cipher suites (which use MD5
   or SHA-1), the hash MUST be SHA-1. New ciphers which do not use HMAC
   MUST explicitly specify a PRF.

   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



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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, and reassembled, then delivered to
   higher level clients.

   Four record protocol clients are described in this document: the
   handshake protocol, the alert protocol, the change cipher spec
   protocol, and the application data protocol. In order to allow
   extension of the TLS protocol, additional record types can be
   supported by the record protocol. Any new record types SHOULD
   allocate type values immediately beyond the ContentType values for
   the four record types described here (see Appendix A.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, encryption algorithm,
   and 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 which 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:



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   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, stream, or AEAD cipher, 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 which is returned by
       the MAC algorithm.

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

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

   client random
       A 32 byte value provided by the client.

   server random
       A 32 byte value provided by the server.

   These parameters are defined in the presentation language as:

       enum { server, client } ConnectionEnd;

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

       enum { stream, block, aead } CipherType;

       enum { null, md5, sha, sha256, sha384, sha512} MACAlgorithm;

       /* The use of "sha" above is historical and denotes SHA-1 */

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

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

       struct {
           ConnectionEnd          entity;



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           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 the necessary state information
       is to allow the stream to continue to encrypt or decrypt data.

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

   sequence number
       Each connection state contains a sequence number, which is
       maintained 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



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       implementation would need to wrap a sequence number it must
       renegotiate instead. A sequence number is incremented after each
       record: specifically, the first record which is 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 1.2, which uses the version { 3, 3 }. The
       version value 3.3 is historical, deriving from the use of 3.1 for
       TLS 1.0. (See Appendix A.1).

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



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

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



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       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;
               case aead: GenericAEADCipher;
           } 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 +



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                     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
       TLS 1.2 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 which the cipher XORs with the
           plaintext prior to encryption of the first cipher block
           of the record.



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       In versions of TLS prior to 1.1, 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 equivalently cryptographically strong
       to 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 long, as long as it



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       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 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 TLSCompressed.length, CipherSpec.hash_size, and
   padding_length.

 Example: If the block length is 8 bytes, the content length
          (TLSCompressed.length) is 61 bytes, and the MAC length is 20
          bytes, the length before padding is 82 bytes (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 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



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       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.

6.2.3.3. AEAD ciphers

   For AEAD [AEAD] ciphers (such as [CCM] or [GCM]) the AEAD function
   converts TLSCompressed.fragment structures to and from AEAD
   TLSCiphertext.fragment structures.

       aead-ciphered struct {
           opaque IV[CipherSpec.iv_length];
           opaque aead_output[AEADEncrypted.length];
       } GenericAEADCipher;

   AEAD ciphers take as input a single key, optional IV (depending on
   the cipher), plaintext, and "additional data" to be included in the
   authentication check. I.e.,

      AEADEncrypted = AEAD-Encrypt(key, IV, plaintext,
                      additional_data)

   The key is either the client_write_key or the server_write_key.  When
   AEAD algorithms are used the MAC keys are of zero length and are not
   used. The length of the IV depends on the cipher suite.  If it is
   required it MUST be generated using a cryptographically strong random
   number generator. Note that the IV may be zero length.  The plaintext
   is the TLSCompressed.fragment. The additional_data is defined as
   follows:

      additional_data = seq_num + TLSCompressed.type +
                        TLSCompressed.version + TLSCompressed.length;

   Where "+" denotes concatenation.

   AEADEncrypted.length will generally be larger than
   TLSCompressed.length, but by an amount that varies with the cipher
   and the required padding (if any). AEAD algorithms MUST NOT produce
   an expansion of greater than 1024 bytes.

   In order to decrypt and verify, the cipher takes as input the key,
   IV, the "additional_data", and the AEADEncrypted value. The output is
   either the plaintext or an error indicating that the decryption
   failed. There is no separate integrity check.  I.e.,

   TLSCompressed.fragment = AEAD-Decrypt(write_key, IV, AEADEncrypted,
                   TLSCiphertext.type + TLSCiphertext.version +
                   TLSCiphertext.length);



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   If the decryption fails, a fatal bad_record_mac alert MUST be
   generated.

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, which are generated from the master secret in
   that order. Unused values are empty.

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

   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 which requires the most material is
       AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte
       keys and 2 x 20 byte MAC secrets, for a total 104 bytes of key
       material.

7. The TLS Handshaking Protocols

       TLS has three subprotocols which are used to allow peers to agree
       upon security parameters for the record layer, authenticate
       themselves, instantiate negotiated security parameters, and
       report error conditions to each other.




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       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 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.



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   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 7.4.11

 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),



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           decrypt_error(51),
           export_restriction_RESERVED(60),
           protocol_version(70),
           insufficient_security(71),
           internal_error(80),
           user_canceled(90),
           no_renegotiation(100),
           unsupported_extension(110),           /* new */
           certificate_unobtainable(111),        /* new */
           unrecognized_name(112),               /* new */
           bad_certificate_status_response(113), /* new */
           bad_certificate_hash_value(114),      /* new */
           (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



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   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 an 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 which had a length more than
       2^14+2048 bytes, or a record decrypted to a TLSCompressed record



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       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.




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   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 makes it impossible to continue (such as a memory
       allocation failure). 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 would be 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



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       point. This message is always a warning.

       The following error alerts apply only to the extensions described
       in Section XXX. To avoid "breaking" existing clients and servers,
       these alerts MUST NOT be sent unless the sending party has
       received an extended hello message from the party they are
       communicating with.

   unsupported_extension
       sent by clients that receive an extended server hello containing
       an extension that they did not put in the corresponding client
       hello (see Section 2.3).  This message is always fatal.

   unrecognized_name
       sent by servers that receive a server_name extension request, but
       do not recognize the server name.  This message MAY be fatal.

   certificate_unobtainable
       sent by servers who are unable to retrieve a certificate chain
       from the URL supplied by the client (see Section 3.3).  This
       message MAY be fatal - for example if client authentication is
       required by the server for the handshake to continue and the
       server is unable to retrieve the certificate chain, it may send a
       fatal alert.

   bad_certificate_status_response
       sent by clients that receive an invalid certificate status
       response (see Section 3.6).  This message is always fatal.

   bad_certificate_hash_value
       sent by servers when a certificate hash does not match a client
       provided certificate_hash.  This message is always fatal.

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

   New alerts 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



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   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 TLS always
   negotiating the strongest possible connection between two peers:
   there are a number of ways a man in the middle attacker can attempt
   to make two entities drop down to the least secure method they
   support. The protocol has been designed to minimize this risk, but
   there are still attacks available: for example, an attacker could
   block access to the port a secure service runs on, or attempt to get
   the peers to negotiate an unauthenticated connection. The fundamental
   rule is that higher levels must be cognizant of what their security
   requirements are and never transmit information over a channel less
   secure than what they require. The TLS protocol is secure, in that
   any cipher suite offers its promised level of security: if you
   negotiate 3DES with a 1024 bit RSA key exchange with a host whose
   certificate you have verified, you can expect to be that secure.













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   However, you SHOULD never send data over a link encrypted with 40 bit
   security unless you feel that data is worth no more than the effort
   required to break that encryption.

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

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

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





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   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*
                                               CertificateStatus*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                   <--------      ServerHelloDone
      Certificate*
      CertificateURL*
      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.



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      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), certificate_url(21), certificate_status(22),
        (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;
               case certificate_url:     CertificateURL;



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               case certificate_status:  CertificateStatus;
           } 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 described only in its first position. The one
   message which 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.




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   Structure of this message:
       struct { } HelloRequest;

 Note: This message MUST NOT be included in the message hashes which 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, this
   connection, or another currently active connection. The second option
   is useful if the client only wishes to update the random structures
   and derived values of a connection, while the third option makes it
   possible to establish several independent secure connections without
   repeating the full handshake protocol. These 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 removed due to aging or because



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   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;

   If the client wishes to use extensions (see Section XXX),
   it may send an ExtendedClientHello:

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



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           Extension client_hello_extension_list<0..2^16-1>;
       } ExtendedClientHello;

   These two messages can be distinguished by determining whether there
   are bytes following what would be the end of the 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 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.

   client_hello_extension_list
       Clients MAY request extended functionality from servers by
       sending data in the client_hello_extension_list.  Here the new
       "client_hello_extension_list" field contains a list of
       extensions.  The actual "Extension" format is defined in Section
       XXX.

       In the event that a client requests additional functionality
       using the extended client hello, and this functionality is not
       supplied by the server, the client MAY abort the handshake.



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       A server that supports the extensions mechanism MUST accept only
       client hello messages in either the original or extended
       ClientHello ormat, and (as for all other messages) MUST check
       that the amount of data in the message precisely matches one of
       these formats; if not then it MUST send a fatal "decode_error"
       alert.


   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.


7.4.1.3. Server hello

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

   Structure of this message:
   struct {
       ProtocolVersion server_version;
       Random random;
       SessionID session_id;
       CipherSuite cipher_suite;
       CompressionMethod compression_method;
   } ServerHello;

   If the server is sending an extension, it should use the
   ExtendedServerHello:

       struct {
           ProtocolVersion server_version;
           Random random;
           SessionID session_id;
           CipherSuite cipher_suite;
           CompressionMethod compression_method;
           Extension server_hello_extension_list<0..2^16-1>;
       } ExtendedServerHello;

   These two messages can be distinguished by determining whether there
   are bytes following what would be the end of the ServerHello.







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   server_version
   This field will contain the lower of that suggested by the client in
   the client hello and the highest supported by the server. For this
   version of the specification, the version is 3.2 (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.

   server_hello_extension_list
   A list of extensions. Note that only extensions offered by the client
   can appear in the server's list.

7.4.1.4 Hello Extensions

   The extension format for extended client hellos and extended server
   hellos is:

         struct {
             ExtensionType extension_type;
             opaque extension_data<0..2^16-1>;
         } Extension;




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   Here:

     - "extension_type" identifies the particular extension type.

     - "extension_data" contains information specific to the particular
   extension type.

   The extension types defined in this document are:

         enum {
             server_name(0), max_fragment_length(1),
             client_certificate_url(2), trusted_ca_keys(3),
             truncated_hmac(4), status_request(5),
             cert_hash_types(6), (65535)
         } ExtensionType;

   The list of defined extension types is maintained by the IANA. The
   current list can be found at (http://www.iana.org/assignments/tls-
   extensions). See sections 7.4.1.4.8 and 11.1 for more information on
   how new values are added.

   Note that for all extension types (including those defined in
   future), the extension type MUST NOT appear in the extended server
   hello unless the same extension type appeared in the corresponding
   client hello.  Thus clients MUST abort the handshake if they receive
   an extension type in the extended server hello that they did not
   request in the associated (extended) client hello.

   Nonetheless "server oriented" extensions may be provided in the
   future within this framework - such an extension, say of type x,
   would require the client to first send an extension of type x in the
   (extended) client hello with empty extension_data to indicate that it
   supports the extension type. In this case the client is offering the
   capability to understand the extension type, and the server is taking
   the client up on its offer.

   Also note that when multiple extensions of different types are
   present in the extended client hello or the extended server hello,
   the extensions may appear in any order.  There MUST NOT be more than
   one extension of the same type.

   An extended client hello may be sent both when starting a new session
   and when requesting session resumption.  Indeed a client that
   requests resumption of a session does not in general know whether the
   server will accept this request, and therefore it SHOULD send an
   extended client hello if it would normally do so for a new session.
   In general the specification of each extension type must include a
   discussion of the effect of the extension both during new sessions



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   and during resumed sessions.

   Note also that all the extensions defined in this document are
   relevant only when a session is initiated. When a client includes one
   or more of the defined extension types in an extended client hello
   while requesting session resumption:

     - If the resumption request is denied, the use of the extensions
       is negotiated as normal.

     - If, on the other hand, the older session is resumed, then the
       server MUST ignore the extensions and send a server hello
       containing none of the extension types; in this case the
       functionality of these extensions negotiated during the original
       session initiation is applied to the resumed session.

7.4.1.4.1 Server Name Indication

   [TLS1.1] does not provide a mechanism for a client to tell a server
   the name of the server it is contacting.  It may be desirable for
   clients to provide this information to facilitate secure connections
   to servers that host multiple 'virtual' servers at a single
   underlying network address.

   In order to provide the server name, clients MAY include an extension
   of type "server_name" in the (extended) client hello.  The
   "extension_data" field of this extension SHALL contain
   "ServerNameList" where:

         struct {
             NameType name_type;
             select (name_type) {
                 case host_name: HostName;
             } name;
         } ServerName;

         enum {
             host_name(0), (255)
         } NameType;

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

         struct {
             ServerName server_name_list<1..2^16-1>
         } ServerNameList;

   Currently the only server names supported are DNS hostnames, however
   this does not imply any dependency of TLS on DNS, and other name



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   types may be added in the future (by an RFC that Updates this
   document).  TLS MAY treat provided server names as opaque data and
   pass the names and types to the application.

   "HostName" contains the fully qualified DNS hostname of the server,
   as understood by the client. The hostname is represented as a byte
   string using UTF-8 encoding [UTF8], without a trailing dot.

   If the hostname labels contain only US-ASCII characters, then the
   client MUST ensure that labels are separated only by the byte 0x2E,
   representing the dot character U+002E (requirement 1 in section 3.1
   of [IDNA] notwithstanding). If the server needs to match the HostName
   against names that contain non-US-ASCII characters, it MUST perform
   the conversion operation described in section 4 of [IDNA], treating
   the HostName as a "query string" (i.e. the AllowUnassigned flag MUST
   be set). Note that IDNA allows labels to be separated by any of the
   Unicode characters U+002E, U+3002, U+FF0E, and U+FF61, therefore
   servers MUST accept any of these characters as a label separator.  If
   the server only needs to match the HostName against names containing
   exclusively ASCII characters, it MUST compare ASCII names case-
   insensitively.

   Literal IPv4 and IPv6 addresses are not permitted in "HostName".  It
   is RECOMMENDED that clients include an extension of type
   "server_name" in the client hello whenever they locate a server by a
   supported name type.

   A server that receives a client hello containing the "server_name"
   extension, MAY use the information contained in the extension to
   guide its selection of an appropriate certificate to return to the
   client, and/or other aspects of security policy.  In this event, the
   server SHALL include an extension of type "server_name" in the
   (extended) server hello.  The "extension_data" field of this
   extension SHALL be empty.

   If the server understood the client hello extension but does not
   recognize the server name, it SHOULD send an "unrecognized_name"
   alert (which MAY be fatal).

   If an application negotiates a server name using an application
   protocol, then upgrades to TLS, and a server_name extension is sent,
   then the extension SHOULD contain the same name that was negotiated
   in the application protocol.  If the server_name is established in
   the TLS session handshake, the client SHOULD NOT attempt to request a
   different server name at the application layer.

7.4.1.4.2 Maximum Fragment Length Negotiation




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   By default, TLS uses fixed maximum plaintext fragment length of 2^14
   bytes.  It may be desirable for constrained clients to negotiate a
   smaller maximum fragment length due to memory limitations or
   bandwidth limitations.

   In order to negotiate smaller maximum fragment lengths, clients MAY
   include an extension of type "max_fragment_length" in the (extended)
   client hello.  The "extension_data" field of this extension SHALL
   contain:

         enum{
             2^9(1), 2^10(2), 2^11(3), 2^12(4), (255)
         } MaxFragmentLength;

   whose value is the desired maximum fragment length.  The allowed
   values for this field are: 2^9, 2^10, 2^11, and 2^12.

   Servers that receive an extended client hello containing a
   "max_fragment_length" extension, MAY accept the requested maximum
   fragment length by including an extension of type
   "max_fragment_length" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL contain
   "MaxFragmentLength" whose value is the same as the requested maximum
   fragment length.

   If a server receives a maximum fragment length negotiation request
   for a value other than the allowed values, it MUST abort the
   handshake with an "illegal_parameter" alert.  Similarly, if a client
   receives a maximum fragment length negotiation response that differs
   from the length it requested, it MUST also abort the handshake with
   an "illegal_parameter" alert.

   Once a maximum fragment length other than 2^14 has been successfully
   negotiated, the client and server MUST immediately begin fragmenting
   messages (including handshake messages), to ensure that no fragment
   larger than the negotiated length is sent.  Note that TLS already
   requires clients and servers to support fragmentation of handshake
   messages.

   The negotiated length applies for the duration of the session
   including session resumptions.

   The negotiated length limits the input that the record layer may
   process without fragmentation (that is, the maximum value of
   TLSPlaintext.length; see [TLS] section 6.2.1).  Note that the output
   of the record layer may be larger.  For example, if the negotiated
   length is 2^9=512, then for currently defined cipher suites and when
   null compression is used, the record layer output can be at most 793



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   bytes: 5 bytes of headers, 512 bytes of application data, 256 bytes
   of padding, and 20 bytes of MAC.  That means that in this event a TLS
   record layer peer receiving a TLS record layer message larger than
   793 bytes may discard the message and send a "record_overflow" alert,
   without decrypting the message.

7.4.1.4.3 Client Certificate URLs

   Ordinarily, when client authentication is performed, client
   certificates are sent by clients to servers during the TLS handshake.
   It may be desirable for constrained clients to send certificate URLs
   in place of certificates, so that they do not need to store their
   certificates and can therefore save memory.

   In order to negotiate to send certificate URLs to a server, clients
   MAY include an extension of type "client_certificate_url" in the
   (extended) client hello.  The "extension_data" field of this
   extension SHALL be empty.

   (Note that it is necessary to negotiate use of client certificate
   URLs in order to avoid "breaking" existing TLS 1.0 servers.)

   Servers that receive an extended client hello containing a
   "client_certificate_url" extension, MAY indicate that they are
   willing to accept certificate URLs by including an extension of type
   "client_certificate_url" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL be empty.

   After negotiation of the use of client certificate URLs has been
   successfully completed (by exchanging hellos including
   "client_certificate_url" extensions), clients MAY send a
   "CertificateURL" message in place of a "Certificate" message.  See
   Section XXX.

7.4.1.4.4 Trusted CA Indication

   Constrained clients that, due to memory limitations, possess only a
   small number of CA root keys, may wish to indicate to servers which
   root keys they possess, in order to avoid repeated handshake
   failures.

   In order to indicate which CA root keys they possess, clients MAY
   include an extension of type "trusted_ca_keys" in the (extended)
   client hello.  The "extension_data" field of this extension SHALL
   contain "TrustedAuthorities" where:

         struct {
             TrustedAuthority trusted_authorities_list<0..2^16-1>;



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         } TrustedAuthorities;

         struct {
             IdentifierType identifier_type;
             select (identifier_type) {
                 case pre_agreed: struct {};
                 case key_sha1_hash: SHA1Hash;
                 case x509_name: DistinguishedName;
                 case cert_sha1_hash: SHA1Hash;
             } identifier;
         } TrustedAuthority;

         enum {
             pre_agreed(0), key_sha1_hash(1), x509_name(2),
             cert_sha1_hash(3), (255)
         } IdentifierType;

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

   Here "TrustedAuthorities" provides a list of CA root key identifiers
   that the client possesses.  Each CA root key is identified via
   either:

     -  "pre_agreed" - no CA root key identity supplied.

     -  "key_sha1_hash" - contains the SHA-1 hash of the CA root key.
   For
       DSA and ECDSA keys, this is the hash of the "subjectPublicKey"
       value.  For RSA keys, the hash is of the big-endian byte string
       representation of the modulus without any initial 0-valued bytes.
       (This copies the key hash formats deployed in other
       environments.)

     -  "x509_name" - contains the DER-encoded X.509 DistinguishedName
       of
       the CA.

     -  "cert_sha1_hash" - contains the SHA-1 hash of a DER-encoded
       Certificate containing the CA root key.

   Note that clients may include none, some, or all of the CA root keys
   they possess in this extension.

   Note also that it is possible that a key hash or a Distinguished Name
   alone may not uniquely identify a certificate issuer - for example if
   a particular CA has multiple key pairs - however here we assume this
   is the case following the use of Distinguished Names to identify
   certificate issuers in TLS.



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   The option to include no CA root keys is included to allow the client
   to indicate possession of some pre-defined set of CA root keys.

   Servers that receive a client hello containing the "trusted_ca_keys"
   extension, MAY use the information contained in the extension to
   guide their selection of an appropriate certificate chain to return
   to the client.  In this event, the server SHALL include an extension
   of type "trusted_ca_keys" in the (extended) server hello.  The
   "extension_data" field of this extension SHALL be empty.

7.4.1.4.5 Truncated HMAC

   Currently defined TLS cipher suites use the MAC construction HMAC
   with either MD5 or SHA-1 [HMAC] to authenticate record layer
   communications.  In TLS the entire output of the hash function is
   used as the MAC tag.  However it may be desirable in constrained
   environments to save bandwidth by truncating the output of the hash
   function to 80 bits when forming MAC tags.

   In order to negotiate the use of 80-bit truncated HMAC, clients MAY
   include an extension of type "truncated_hmac" in the extended client
   hello.  The "extension_data" field of this extension SHALL be empty.

   Servers that receive an extended hello containing a "truncated_hmac"
   extension, MAY agree to use a truncated HMAC by including an
   extension of type "truncated_hmac", with empty "extension_data", in
   the extended server hello.

   Note that if new cipher suites are added that do not use HMAC, and
   the session negotiates one of these cipher suites, this extension
   will have no effect.  It is strongly recommended that any new cipher
   suites using other MACs consider the MAC size as an integral part of
   the cipher suite definition, taking into account both security and
   bandwidth considerations.

   If HMAC truncation has been successfully negotiated during a TLS
   handshake, and the negotiated cipher suite uses HMAC, both the client
   and the server pass this fact to the TLS record layer along with the
   other negotiated security parameters.  Subsequently during the
   session, clients and servers MUST use truncated HMACs, calculated as
   specified in [HMAC].  That is, CipherSpec.hash_size is 10 bytes, and
   only the first 10 bytes of the HMAC output are transmitted and
   checked.  Note that this extension does not affect the calculation of
   the PRF as part of handshaking or key derivation.

   The negotiated HMAC truncation size applies for the duration of the
   session including session resumptions.




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7.4.1.4.6 Certificate Status Request

   Constrained clients may wish to use a certificate-status protocol
   such as OCSP [OCSP] to check the validity of server certificates, in
   order to avoid transmission of CRLs and therefore save bandwidth on
   constrained networks.  This extension allows for such information to
   be sent in the TLS handshake, saving roundtrips and resources.

   In order to indicate their desire to receive certificate status
   information, clients MAY include an extension of type
   "status_request" in the (extended) client hello.  The
   "extension_data" field of this extension SHALL contain
   "CertificateStatusRequest" where:

         struct {
             CertificateStatusType status_type;
             select (status_type) {
                 case ocsp: OCSPStatusRequest;
             } request;
         } CertificateStatusRequest;

         enum { ocsp(1), (255) } CertificateStatusType;

         struct {
             ResponderID responder_id_list<0..2^16-1>;
             Extensions  request_extensions;
         } OCSPStatusRequest;

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

   In the OCSPStatusRequest, the "ResponderIDs" provides a list of OCSP
   responders that the client trusts.  A zero-length "responder_id_list"
   sequence has the special meaning that the responders are implicitly
   known to the server - e.g., by prior arrangement.  "Extensions" is a
   DER encoding of OCSP request extensions.

   Both "ResponderID" and "Extensions" are DER-encoded ASN.1 types as
   defined in [OCSP].  "Extensions" is imported from [PKIX].  A zero-
   length "request_extensions" value means that there are no extensions
   (as opposed to a zero-length ASN.1 SEQUENCE, which is not valid for
   the "Extensions" type).

   In the case of the "id-pkix-ocsp-nonce" OCSP extension, [OCSP] is
   unclear about its encoding; for clarification, the nonce MUST be a
   DER-encoded OCTET STRING, which is encapsulated as another OCTET
   STRING (note that implementations based on an existing OCSP client
   will need to be checked for conformance to this requirement).




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   Servers that receive a client hello containing the "status_request"
   extension, MAY return a suitable certificate status response to the
   client along with their certificate.  If OCSP is requested, they
   SHOULD use the information contained in the extension when selecting
   an OCSP responder, and SHOULD include request_extensions in the OCSP
   request.

   Servers return a certificate response along with their certificate by
   sending a "CertificateStatus" message immediately after the
   "Certificate" message (and before any "ServerKeyExchange" or
   "CertificateRequest" messages). Section XXX describes the
   CertificateStatus message.

7.4.1.4.7 Cert Hash Types

   The client MAY use the "cert_hash_types" to indicate to the server
   which hash functions may be used in the signature on the server's
   certificate. The "extension_data" field of this extension contains:

         enum{
             md5(0), sha1(1), sha256(2), sha384(3), sha512(4), (255)
         } HashType;

         struct {
               HashType<255> types;
         } CertHashTypes;

   These values indicate support for MD5 [MD5], SHA-1, SHA-256, SHA-384,
   and SHA-512 [SHA] respectively. The server MUST NOT send this
   extension.

   Clients SHOULD send this extension if they support any algorithm
   other than SHA-1. If this extension is not used, servers SHOULD
   assume that the client supports only SHA-1. Note: this is a change
   from TLS 1.1 where there are no explicit rules but as a practical
   matter one can assume that the peer supports MD5 and SHA-1.

 HashType 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.




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


7.4.1.4.8 Procedure for Defining New Extensions

   The list of extension types, as defined in Section 2.3, is
   maintained by the Internet Assigned Numbers Authority (IANA). Thus
   an application needs to be made to the IANA in order to obtain a new
   extension type value. Since there are subtle (and not so subtle)
   interactions that may occur in this protocol between new features and
   existing features which may result in a significant reduction in
   overall security, new values SHALL be defined only through the IETF
   Consensus process specified in [IANA].

   (This means that new assignments can be made only via RFCs approved
   by the IESG.)

   The following considerations should be taken into account when
   designing new extensions:

     -  All of the extensions defined in this document follow the
       convention that for each extension that a client requests and that
       the server understands, the server replies with an extension of
       the same type.

     -  Some cases where a server does not agree to an extension are error
       conditions, and some simply a refusal to support a particular
       feature.  In general error alerts should be used for the former,
       and a field in the server extension response for the latter.

     -  Extensions should as far as possible be designed to prevent any
       attack that forces use (or non-use) of a particular feature by
       manipulation of handshake messages.  This principle should be
       followed regardless of whether the feature is believed to cause a
       security problem.

       Often the fact that the extension fields are included in the
       inputs to the Finished message hashes will be sufficient, but
       extreme care is needed when the extension changes the meaning of
       messages sent in the handshake phase. Designers and implementors
       should be aware of the fact that until the handshake has been
       authenticated, active attackers can modify messages and insert,
       remove, or replace extensions.

     -  It would be technically possible to use extensions to change major
       aspects of the design of TLS; for example the design of cipher



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       suite negotiation.  This is not recommended; it would be more
       appropriate to define a new version of TLS - particularly since
       the TLS handshake algorithms have specific protection against
       version rollback attacks based on the version number, and the
       possibility of version rollback should be a significant
       consideration in any major design change.


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

       Key Exchange Algorithm  Certificate Key Type

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

       DHE_DSS                 DSS public key.

       DHE_RSA                 RSA public key which 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, 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.




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

   Structure of this message:
       opaque ASN.1Cert<1..2^24-1>;

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

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



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       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 to encrypt the premaster secret with, or a Diffie-Hellman
       public key with which the client can complete a key exchange
       (with the result being the premaster secret.)

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

   If the SignatureAlgorithm being used to sign the ServerKeyExchange
   message is DSA, the hash function used MUST be SHA-1. If the
   SignatureAlgorithm it must be the same hash function used in the
   signature of the server's certificate (found in the Certificate)
   message. This algorithm is denoted Hash below. Hash.length is the
   length of the output of that algorithm.

   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.



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       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.

       hash
           Hash(ClientHello.random + ServerHello.random + ServerParams)

       sha_hash
           SHA1(ClientHello.random + ServerHello.random + ServerParams)

       enum { anonymous, rsa, dsa } SignatureAlgorithm;


       struct {
           select (SignatureAlgorithm) {
               case anonymous: struct { };
               case rsa:
                   digitally-signed struct {



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

7.4.4. CertificateStatus

   If a server returns a
   "CertificateStatus" message, then the server MUST have included an
   extension of type "status_request" with empty "extension_data" in the
   extended server hello.

         struct {
             CertificateStatusType status_type;
             select (status_type) {
                 case ocsp: OCSPResponse;
             } response;
         } CertificateStatus;

         opaque OCSPResponse<1..2^24-1>;

   An "ocsp_response" contains a complete, DER-encoded OCSP response
   (using the ASN.1 type OCSPResponse defined in [OCSP]).  Note that
   only one OCSP response may be sent.

   The "CertificateStatus" message is conveyed using the handshake
   message type "certificate_status".

   Note that a server MAY also choose not to send a "CertificateStatus"
   message, even if it receives a "status_request" extension in the
   client hello message.

   Note in addition that servers MUST NOT send the "CertificateStatus"
   message unless it received a "status_request" extension in the client
   hello message.

   Clients requesting an OCSP response, and receiving an OCSP response
   in a "CertificateStatus" message MUST check the OCSP response and
   abort the handshake if the response is not satisfactory.


7.4.5. Certificate request




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   When this message will be sent:
       A non-anonymous server can optionally request a certificate from
       the client, if appropriate for the selected cipher suite. This
       message, if sent, will immediately follow the Server Key Exchange
       message (if it is sent; otherwise, the Server Certificate
       message).

   Structure of this message:
       enum {
           rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
           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>;
           HashType certificate_hash<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_types
           A list of the types of certificate types which the client may
           offer.
              rsa_sign        a certificate containing an RSA key
              dss_sign        a certificate containing a DSS key
              rsa_fixed_dh    a certificate signed with RSA and containing
                              a static DH key.
              dss_fixed_dh    a certificate signed with DSS and containing
                              a static DH key

           Certificate types rsa_sign and dss_sign SHOULD contain
           certificates signed with the same algorithm. However, this is
           not required. This is a holdover from TLS 1.0 and 1.1.


       certificate_hash
           A list of acceptable hash algorithms to be used in
           certificate signatures.

       certificate_authorities



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

7.4.6. 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.




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       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;

7.4.7. 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.

7.4.8. Client Certificate URLs

   After negotiation of the use of client certificate URLs has been
   successfully completed (by exchanging hellos including
   "client_certificate_url" extensions), clients MAY send a
   "CertificateURL" message in place of a "Certificate" message.

         enum {
             individual_certs(0), pkipath(1), (255)
         } CertChainType;

         enum {
             false(0), true(1)
         } Boolean;

         struct {
             CertChainType type;
             URLAndOptionalHash url_and_hash_list<1..2^16-1>;
         } CertificateURL;



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         struct {
             opaque url<1..2^16-1>;
             Boolean hash_present;
             select (hash_present) {
                 case false: struct {};
                 case true: SHA1Hash;
             } hash;
         } URLAndOptionalHash;

         opaque SHA1Hash[20];

   Here "url_and_hash_list" contains a sequence of URLs and optional
   hashes.

   When X.509 certificates are used, there are two possibilities:

     -  if CertificateURL.type is "individual_certs", each URL refers to
       a single DER-encoded X.509v3 certificate, with the URL for the
       client's certificate first, or

     -  if CertificateURL.type is "pkipath", the list contains a single
       URL referring to a DER-encoded certificate chain, using the type
       PkiPath described in Section 8.

   When any other certificate format is used, the specification that
   describes use of that format in TLS should define the encoding format
   of certificates or certificate chains, and any constraint on their
   ordering.

   The hash corresponding to each URL at the client's discretion is
   either not present or is the SHA-1 hash of the certificate or
   certificate chain (in the case of X.509 certificates, the DER-encoded
   certificate or the DER-encoded PkiPath).

   Note that when a list of URLs for X.509 certificates is used, the
   ordering of URLs is the same as that used in the TLS Certificate
   message (see [TLS] Section 7.4.2), but opposite to the order in which
   certificates are encoded in PkiPath.  In either case, the self-signed
   root certificate MAY be omitted from the chain, under the assumption
   that the server must already possess it in order to validate it.

   Servers receiving "CertificateURL" SHALL attempt to retrieve the
   client's certificate chain from the URLs, and then process the
   certificate chain as usual.  A cached copy of the content of any URL
   in the chain MAY be used, provided that a SHA-1 hash is present for
   that URL and it matches the hash of the cached copy.

   Servers that support this extension MUST support the http: URL scheme



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   for certificate URLs, and MAY support other schemes. Use of other
   schemes than "http", "https", or "ftp" may create unexpected
   problems.

   If the protocol used is HTTP, then the HTTP server can be configured
   to use the Cache-Control and Expires directives described in [HTTP]
   to specify whether and for how long certificates or certificate
   chains should be cached.

   The TLS server is not required to follow HTTP redirects when
   retrieving the certificates or certificate chain.  The URLs used in
   this extension SHOULD therefore be chosen not to depend on such
   redirects.

   If the protocol used to retrieve certificates or certificate chains
   returns a MIME formatted response (as HTTP does), then the following
   MIME Content-Types SHALL be used: when a single X.509v3 certificate
   is returned, the Content-Type is "application/pkix-cert" [PKIOP], and
   when a chain of X.509v3 certificates is returned, the Content-Type is
   "application/pkix-pkipath" (see Section XXX).

   If a SHA-1 hash is present for an URL, then the server MUST check
   that the SHA-1 hash of the contents of the object retrieved from that
   URL (after decoding any MIME Content-Transfer-Encoding) matches the
   given hash.  If any retrieved object does not have the correct SHA-1
   hash, the server MUST abort the handshake with a
   "bad_certificate_hash_value" alert.

   Note that clients may choose to send either "Certificate" or
   "CertificateURL" after successfully negotiating the option to send
   certificate URLs. The option to send a certificate is included to
   provide flexibility to clients possessing multiple certificates.

   If a server encounters an unreasonable delay in obtaining
   certificates in a given CertificateURL, it SHOULD time out and signal
   a "certificate_unobtainable" error alert.

7.4.9. 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



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   Diffie-Hellman parameters which will allow each side to agree upon
   the same premaster secret. When the key exchange method is DH_RSA or
   DH_DSS, client certification has been requested, and the client was
   able to respond with a certificate which contained a Diffie-Hellman
   public key whose parameters (group and generator) matched those
   specified by the server in its certificate, this message 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;

7.4.9.1. RSA encrypted premaster secret message

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

   Structure of this message:
   struct {
       ProtocolVersion client_version;
       opaque random[46];
   } PreMasterSecret;

   client_version
           The latest (newest) version supported by the client. This is
           used to detect version roll-back attacks. Upon receiving the
           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;



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       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 which 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 it receives an
       incorrectly formatted RSA block, a server 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 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 which use static RSA



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       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.version, 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 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]

7.4.9.2. Client Diffie-Hellman public value

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

   Structure of this message:
       enum { implicit, explicit } PublicValueEncoding;

       implicit
           If the client certificate already contains a suitable Diffie-
           Hellman key, then Yc is implicit and does not need to be sent
           again. In this case, the client key exchange message will be
           sent, but 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).



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7.4.10. 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. If the SignatureAlgorithm
       is DSA, then the sha_hash value must be used. If it is RSA,
       the same function (denoted Hash) must be used as was used to
       create the signature for the client's certificate.

       CertificateVerify.signature.hash
           Hash(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.

7.4.10. 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.




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       struct {
           opaque verify_data[12];
       } Finished;

       verify_data
           PRF(master_secret, finished_label, Hash(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".

           Hash denotes the negotiated hash used for the PRF. If a new
           PRF is defined, then this hash MUST be specified.

       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 7.4.10 because it
   would include the certificate verify message (if sent). Also, the
   handshake_messages for the finished message sent by the client will
   be different from that for the finished message sent by the server,
   because the one which is sent second will include the prior one.

 Note: Change cipher spec messages, 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



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   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



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

11. IANA Considerations

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

   Section 7.4.5 describes a TLS HashType Registry to be maintained by
   the IANA, as defining a number of such code point identifiers.
   HashType 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 [RFC 2434]
   Specification Required.  Identifier values from 224-255 (decimal)
   inclusive are reserved for RFC 2434 Private Use. The registry will be
   initially populated with the values in this document, Section 7.4.5.

   Section 7.4.5 describes a TLS ClientCertificateType Registry to be
   maintained by the IANA, as 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 [RFC 2434] Specification Required.  Identifier values from
   224-255 (decimal) inclusive are reserved for RFC 2434 Private Use.
   The registry will be initially populated with the values in this
   document, Section 7.4.5.

   Section A.5 describes a TLS Cipher Suite Registry to be maintained by
   the IANA, as well as defining 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 be initially populated with the values from Section A.5
   of this document, [TLSAES], and Section 3 of [TLSKRB].

   Section 6 requires that all ContentType values be defined by RFC 2434
   Standards Action. IANA SHOULD create 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
   [RFC 2434].

   Section 7.2.2 requires that all Alert values be defined by RFC 2434
   Standards Action. IANA SHOULD create a TLS Alert registry, initially
   populated with values from Section 7.2 of this document and Section 4



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   of [TLSEXT]. Future values MUST be allocated via Standards Action as
   described in [RFC 2434].

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


11.1 Extensions

   Sections XXX and XXX describes a registry of ExtensionType values to
   be maintained by the IANA. ExtensionType values are to be assigned
   via IETF Consensus as defined in RFC 2434 [IANA]. The initial
   registry corresponds to the definition of "ExtensionType" in Section
   2.3.

   The MIME type "application/pkix-pkipath" has been registered by the
   IANA with the following template:

      To: ietf-types@iana.org Subject: Registration of MIME media type
      application/pkix-pkipath

      MIME media type name: application
      MIME subtype name: pkix-pkipath

      Optional parameters: version (default value is "1")

      Encoding considerations:
         This MIME type is a DER encoding of the ASN.1 type PkiPath,
         defined as follows:
           PkiPath ::= SEQUENCE OF Certificate
           PkiPath is used to represent a certification path.  Within the
           sequence, the order of certificates is such that the subject of
           the first certificate is the issuer of the second certificate,
           etc.

         This is identical to the definition published in [X509-4th-TC1];
         note that it is different from that in [X509-4th].

         All Certificates MUST conform to [PKIX].  (This should be
         interpreted as a requirement to encode only PKIX-conformant
         certificates using this type.  It does not necessarily require
         that all certificates that are not strictly PKIX-conformant must
         be rejected by relying parties, although the security consequences
         of accepting any such certificates should be considered
         carefully.)



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         DER (as opposed to BER) encoding MUST be used.  If this type is
         sent over a 7-bit transport, base64 encoding SHOULD be used.

      Security considerations:
         The security considerations of [X509-4th] and [PKIX] (or any
         updates to them) apply, as well as those of any protocol that uses
         this type (e.g., TLS).

         Note that this type only specifies a certificate chain that can be
         assessed for validity according to the relying party's existing
         configuration of trusted CAs; it is not intended to be used to
         specify any change to that configuration.

      Interoperability considerations:
         No specific interoperability problems are known with this type,
         but for recommendations relating to X.509 certificates in general,
         see [PKIX].

      Published specification: this memo, and [PKIX].

      Applications which use this media type: TLS.  It may also be used by
         other protocols, or for general interchange of PKIX certificate

      Additional information:
         Magic number(s): DER-encoded ASN.1 can be easily recognized.
           Further parsing is required to distinguish from other ASN.1
           types.
         File extension(s): .pkipath
         Macintosh File Type Code(s): not specified

      Person & email address to contact for further information:
         Magnus Nystrom <magnus@rsasecurity.com>

      Intended usage: COMMON

      Change controller:
         IESG <iesg@ietf.org>














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A. Protocol constant values

   This section describes protocol types and constants.

A.1. Record layer

    struct {
        uint8 major, minor;
    } ProtocolVersion;

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

    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];



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

    aead-ciphered struct {
        opaque IV[CipherSpec.iv_length];
        opaque aead_output[AEADEncrypted.length];
    } GenericAEADCipher;

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),
            unsupported_extension(110),           /* new */
            certificate_unobtainable(111),        /* new */



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            unrecognized_name(112),               /* new */
            bad_certificate_status_response(113), /* new */
            bad_certificate_hash_value(114),      /* new */
            (255)
        } AlertDescription;

    struct {
        AlertLevel level;
        AlertDescription description;
    } Alert;









































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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), certificate_url(21), certificate_status(22),
     (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;
            case certificate_url:     CertificateURL;
            case certificate_status:  CertificateStatus;
        } 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;



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        SessionID session_id;
        CipherSuite cipher_suites<2..2^16-1>;
        CompressionMethod compression_methods<1..2^8-1>;
        Extension client_hello_extension_list<0..2^16-1>;
    } ClientHello;

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

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

    struct {
        ProtocolVersion server_version;
        Random random;
        SessionID session_id;
        CipherSuite cipher_suite;
        CompressionMethod compression_method;
     Extension server_hello_extension_list<0..2^16-1>;
    } ExtendedServerHello;

    struct {
        ExtensionType extension_type;
        opaque extension_data<0..2^16-1>;
    } Extension;

    enum {
        server_name(0), max_fragment_length(1),
        client_certificate_url(2), trusted_ca_keys(3),
        truncated_hmac(4), status_request(5),
        cert_hash_types(6), (65535)
    } ExtensionType;

    struct {
        NameType name_type;
        select (name_type) {
            case host_name: HostName;



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        } name;
    } ServerName;

    enum {
        host_name(0), (255)
    } NameType;

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

    struct {
        ServerName server_name_list<1..2^16-1>
    } ServerNameList;

    enum{
        2^9(1), 2^10(2), 2^11(3), 2^12(4), (255)
    } MaxFragmentLength;

    struct {
        TrustedAuthority trusted_authorities_list<0..2^16-1>;
    } TrustedAuthorities;

    struct {
        IdentifierType identifier_type;
        select (identifier_type) {
            case pre_agreed: struct {};
            case key_sha1_hash: SHA1Hash;
            case x509_name: DistinguishedName;
            case cert_sha1_hash: SHA1Hash;
        } identifier;
    } TrustedAuthority;

    enum {
        pre_agreed(0), key_sha1_hash(1), x509_name(2),
        cert_sha1_hash(3), (255)
    } IdentifierType;

    struct {
        CertificateStatusType status_type;
        select (status_type) {
            case ocsp: OCSPStatusRequest;
        } request;
    } CertificateStatusRequest;

    enum { ocsp(1), (255) } CertificateStatusType;

    struct {
        ResponderID responder_id_list<0..2^16-1>;
        Extensions  request_extensions;



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    } OCSPStatusRequest;

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

    struct {
        CertificateStatusType status_type;
        select (status_type) {
            case ocsp: OCSPResponse;
        } response;
    } CertificateStatus;

    opaque OCSPResponse<1..2^24-1>;

    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) {



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            case diffie_hellman:
                ServerDHParams params;
            case rsa:
                ServerRSAParams params;
        };
    } ServerParams;

    struct {
        select (SignatureAlgorithm) {
            case anonymous: struct { };
            case rsa:
                digitally-signed struct {
                    opaque hash[Hash.length];
                };
            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 {



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        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;

    enum {
        individual_certs(0), pkipath(1), (255)
    } CertChainType;

    enum {
        false(0), true(1)
    } Boolean;

    struct {
        CertChainType type;
        URLAndOptionalHash url_and_hash_list<1..2^16-1>;
    } CertificateURL;

    struct {
        opaque url<1..2^16-1>;
        Boolean hash_present;
        select (hash_present) {
            case false: struct {};
            case true: SHA1Hash;
        } hash;
    } URLAndOptionalHash;

    opaque SHA1Hash[20];

    struct {
        Signature signature;
    } CertificateVerify;

A.4.4. Handshake finalization message

    struct {



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        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 };
    CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA           = { 0x00, 0x2F };
    CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA           = { 0x00, 0x35 };
   The following CipherSuite definitions are used for server-
   authenticated (and optionally client-authenticated) Diffie-Hellman.
   DH denotes cipher suites in which the server's certificate contains
   the Diffie-Hellman parameters signed by the certificate authority
   (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
   parameters are signed by a DSS or RSA certificate, which has been
   signed by the CA. The signing algorithm used is specified after the
   DH or DHE parameter. The server can request an RSA or DSS signature-
   capable certificate from the client for client authentication or it
   may request a Diffie-Hellman certificate. Any Diffie-Hellman
   certificate provided by the client must use the parameters (group and
   generator) described by the server.

    CipherSuite TLS_DH_DSS_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 };



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    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 };
    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_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 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 };



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   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 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.



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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;


















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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



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       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.




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   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.






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

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

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

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

   SHA
       The Secure Hash Algorithm is defined in FIPS PUB 180-2. 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.












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C. CipherSuite definitions

CipherSuite                             Key          Cipher      Hash
                                        Exchange

TLS_NULL_WITH_NULL_NULL                 NULL           NULL        NULL
TLS_RSA_WITH_NULL_MD5                   RSA            NULL         MD5
TLS_RSA_WITH_NULL_SHA                   RSA            NULL         SHA
TLS_RSA_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_RSA_WITH_AES_128_CBC_SHA            RSA            AES_128_CBC  SHA
TLS_RSA_WITH_AES_256_SHA                RSA            AES_256_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
TLS_DH_DSS_WITH_AES_128_CBC_SHA         DH_DSS         AES_128_CBC  SHA
TLS_DH_RSA_WITH_AES_128_CBC_SHA         DH_RSA         AES_128_CBC  SHA
TLS_DHE_DSS_WITH_AES_128_CBC_SHA        DHE_DSS        AES_128_CBC  SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA        DHE_RSA        AES_128_CBC  SHA
TLS_DH_anon_WITH_AES_128_CBC_SHA        DH_anon        AES_128_CBC  SHA
TLS_DH_DSS_WITH_AES_256_CBC_SHA         DH_DSS         AES_256_CBC  SHA
TLS_DH_RSA_WITH_AES_256_CBC_SHA         DH_RSA         AES_256_CBC  SHA
TLS_DHE_DSS_WITH_AES_256_CBC_SHA        DHE_DSS        AES_256_CBC  SHA
TLS_DHE_RSA_WITH_AES_256_CBC_SHA        DHE_RSA        AES_256_CBC  SHA
TLS_DH_anon_WITH_AES_256_CBC_SHA        DH_anon        AES_256_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



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      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
       How much data needs 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











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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. To seed a 128-bit PRNG, one
   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
   which provide no or minimal security. A proper implementation will
   probably not support many cipher suites. For example, 40-bit
   encryption is easily broken, so implementations requiring strong
   security should not allow 40-bit keys. Similarly, anonymous Diffie-
   Hellman is strongly discouraged because it cannot prevent man-in-the-
   middle attacks. Applications should also enforce minimum and maximum
   key sizes. For example, certificate chains containing 512-bit RSA
   keys or signatures are not appropriate for high-security
   applications.







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E. Backward Compatibility

   For historical reasons and in order to avoid a profligate consumption
   of reserved port numbers, application protocols which are secured by
   TLS, 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 1.2, 1.1, 1.0, and SSL 3.0 are very similar; thus,
   supporting them all at the same time is relatively 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, 3} for the client version field to note that
   they support TLS 1.2 and {3, 0} for the record version field (because
   the SSLv3 record format is being used--although the cleartext record
   format is the same for all versions).  If the server supports only a
   downrev version it will respond with a downrev 3.0 server hello; if
   it supports TLS 1.2 it will respond with a TLS 1.2 server hello. The
   negotiation then proceeds as appropriate for the negotiated protocol.

   Similarly, a TLS 1.2  server which wishes to interoperate with
   downrev clients SHOULD accept downrev client hello messages and
   respond with appropriate version fields. Note that the version in the
   server hello message and in the record header are the same.

   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 };



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       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 1.2 clients may generate the SSLv2 EXPORT cipher suites in
   handshakes for backward compatibility but MUST NOT negotiate them in
   TLS 1.2 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).



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   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



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   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.
















































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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 7.4.10, 7.4.11 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



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   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 may be either contained in the server's certificate or may
   be a temporary RSA key sent in a server key exchange message.  When
   temporary RSA keys are used, they are signed by the server's RSA
   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



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   the certificate verify message (see Section 7.4.10). The client signs
   a value derived from the master_secret and all preceding handshake
   messages. These handshake messages include the server certificate,
   which binds the signature to the server, and ServerHello.random,
   which binds the signature to the current handshake process.

F.1.1.3. Diffie-Hellman key exchange with authentication

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

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

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

   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 so 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




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   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 be using 40-bit
   encryption keys anyway. Altering the padding of the least-significant
   8 bytes of the PKCS padding does not impact security for the size of
   the signed hashes and RSA key lengths used in the protocol, since
   this is essentially equivalent to increasing the input block size by
   8 bytes.

F.1.3. Detecting attacks against the handshake protocol

   An attacker might try to influence the handshake exchange to make the
   parties select different encryption algorithms than they would
   normally 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



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   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 Extensions

   Security considerations for the extension mechanism in general, and
   the design of new extensions, are described in the previous section.
   A security analysis of each of the extensions defined in this
   document is given below.

   In general, implementers should continue to monitor the state of the
   art, and address any weaknesses identified.


F.1.5.1 Security of server_name

   If a single server hosts several domains, then clearly it is
   necessary for the owners of each domain to ensure that this satisfies
   their security needs.  Apart from this, server_name does not appear
   to introduce significant security issues.

   Implementations MUST ensure that a buffer overflow does not occur
   whatever the values of the length fields in server_name.

   Although this document specifies an encoding for internationalized
   hostnames in the server_name extension, it does not address any
   security issues associated with the use of internationalized
   hostnames in TLS - in particular, the consequences of "spoofed" names
   that are indistinguishable from another name when displayed or
   printed.  It is recommended that server certificates not be issued
   for internationalized hostnames unless procedures are in place to
   mitigate the risk of spoofed hostnames.

   6.2. Security of max_fragment_length

   The maximum fragment length takes effect immediately, including for
   handshake messages.  However, that does not introduce any security
   complications that are not already present in TLS, since [TLS]
   requires implementations to be able to handle fragmented handshake
   messages.

   Note that as described in section XXX, once a non-null cipher suite
   has been activated, the effective maximum fragment length depends on
   the cipher suite and compression method, as well as on the negotiated
   max_fragment_length.  This must be taken into account when sizing
   buffers, and checking for buffer overflow.



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F.1.5.2 Security of client_certificate_url

   There are two major issues with this extension.

   The first major issue is whether or not clients should include
   certificate hashes when they send certificate URLs.

   When client authentication is used *without* the
   client_certificate_url extension, the client certificate chain is
   covered by the Finished message hashes.  The purpose of including
   hashes and checking them against the retrieved certificate chain, is
   to ensure that the same property holds when this extension is used -
   i.e., that all of the information in the certificate chain retrieved
   by the server is as the client intended.

   On the other hand, omitting certificate hashes enables functionality
   that is desirable in some circumstances - for example clients can be
   issued daily certificates that are stored at a fixed URL and need not
   be provided to the client.  Clients that choose to omit certificate
   hashes should be aware of the possibility of an attack in which the
   attacker obtains a valid certificate on the client's key that is
   different from the certificate the client intended to provide.
   Although TLS uses both MD5 and SHA-1 hashes in several other places,
   this was not believed to be necessary here.  The property required of
   SHA-1 is second pre-image resistance.

   The second major issue is that support for client_certificate_url
   involves the server acting as a client in another URL protocol.  The
   server therefore becomes subject to many of the same security
   concerns that clients of the URL scheme are subject to, with the
   added concern that the client can attempt to prompt the server to
   connect to some, possibly weird-looking URL.

   In general this issue means that an attacker might use the server to
   indirectly attack another host that is vulnerable to some security
   flaw.  It also introduces the possibility of denial of service
   attacks in which an attacker makes many connections to the server,
   each of which results in the server attempting a connection to the
   target of the attack.

   Note that the server may be behind a firewall or otherwise able to
   access hosts that would not be directly accessible from the public
   Internet; this could exacerbate the potential security and denial of
   service problems described above, as well as allowing the existence
   of internal hosts to be confirmed when they would otherwise be
   hidden.

   The detailed security concerns involved will depend on the URL



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   schemes supported by the server.  In the case of HTTP, the concerns
   are similar to those that apply to a publicly accessible HTTP proxy
   server.  In the case of HTTPS, the possibility for loops and
   deadlocks to be created exists and should be addressed.  In the case
   of FTP, attacks similar to FTP bounce attacks arise.

   As a result of this issue, it is RECOMMENDED that the
   client_certificate_url extension should have to be specifically
   enabled by a server administrator, rather than being enabled by
   default.  It is also RECOMMENDED that URI protocols be enabled by the
   administrator individually, and only a minimal set of protocols be
   enabled, with unusual protocols offering limited security or whose
   security is not well-understood being avoided.

   As discussed in [URI], URLs that specify ports other than the default
   may cause problems, as may very long URLs (which are more likely to
   be useful in exploiting buffer overflow bugs).

   Also note that HTTP caching proxies are common on the Internet, and
   some proxies do not check for the latest version of an object
   correctly.  If a request using HTTP (or another caching protocol)
   goes through a misconfigured or otherwise broken proxy, the proxy may
   return an out-of-date response.

F.1.5.4. Security of trusted_ca_keys

   It is possible that which CA root keys a client possesses could be
   regarded as confidential information.  As a result, the CA root key
   indication extension should be used with care.

   The use of the SHA-1 certificate hash alternative ensures that each
   certificate is specified unambiguously.  As for the previous
   extension, it was not believed necessary to use both MD5 and SHA-1
   hashes.

F.1.5.5. Security of truncated_hmac

   It is possible that truncated MACs are weaker than "un-truncated"
   MACs.  However, no significant weaknesses are currently known or
   expected to exist for HMAC with MD5 or SHA-1, truncated to 80 bits.

   Note that the output length of a MAC need not be as long as the
   length of a symmetric cipher key, since forging of MAC values cannot
   be done off-line: in TLS, a single failed MAC guess will cause the
   immediate termination of the TLS session.

   Since the MAC algorithm only takes effect after the handshake
   messages have been authenticated by the hashes in the Finished



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   messages, it is not possible for an active attacker to force
   negotiation of the truncated HMAC extension where it would not
   otherwise be used (to the extent that the handshake authentication is
   secure).  Therefore, in the event that any security problem were
   found with truncated HMAC in future, if either the client or the
   server for a given session were updated to take into account the
   problem, they would be able to veto use of this extension.

F.1.5.6. Security of status_request

   If a client requests an OCSP response, it must take into account that
   an attacker's server using a compromised key could (and probably
   would) pretend not to support the extension.  A client that requires
   OCSP validation of certificates SHOULD either contact the OCSP server
   directly in this case, or abort the handshake.

   Use of the OCSP nonce request extension (id-pkix-ocsp-nonce) may
   improve security against attacks that attempt to replay OCSP
   responses; see section 4.4.1 of [OCSP] for further details.


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.



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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 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 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



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       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 verson 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, terminating connections, or
       forge partial TLS records, 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] or ESP [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.













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Security Considerations

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

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, pp40-41.

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

   [DSS]    NIST FIPS PUB 186-2, "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.

   [HTTP]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
            L., Leach, P. and T. Berners-Lee, "Hypertext Transfer
            Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

   [IDNA]    Faltstrom, P., Hoffman, P. and A. Costello,
            "Internationalizing Domain Names in Applications (IDNA)",
            RFC 3490, March 2003.

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

   [OCSP]   Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
            Adams, "Internet X.509 Public Key Infrastructure: Online
            Certificate Status Protocol - OCSP", RFC 2560, June 1999.

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



Dierks & Rescorla            Standards Track                   [Page 105]\fdraft-ietf-tls-rfc4346-bis-02.txt  TLS                      October 2006


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

   [PKIOP]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
            Infrastructure - Operation Protocols: FTP and HTTP", RFC
            2585, May 1999.


   [PKIX]   Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
            Public Key Infrastructure: Part I: X.509 Certificate and CRL
            Profile", RFC 3280, April 2002.

   [RC2]    Rivest, R., "A Description of the RC2(r) Encryption
            Algorithm", RFC 2268, January 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] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
            Considerations Section in RFCs", RFC 3434, 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.,
            Wright, T., "Transport Layer Security (TLS) Extensions", RFC
            3546, June 2003.
   [TLSKRB] A. Medvinsky, M. Hur, "Addition of Kerberos Cipher Suites to
            Transport Layer Security (TLS)", RFC 2712, October 1999.


   [URI]    Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax", RFC 2396,
            August 1998.

   [UTF8]   Yergeau, F., "UTF-8, a transformation format of ISO 10646",
            RFC 3629, November 2003.

   [X509-4th] ITU-T Recommendation X.509 (2000) | ISO/IEC 9594- 8:2001,



Dierks & Rescorla            Standards Track                   [Page 106]\fdraft-ietf-tls-rfc4346-bis-02.txt  TLS                      October 2006


            "Information Systems - Open Systems Interconnection - The
            Directory:  Public key and Attribute certificate
            frameworks."

   [X509-4th-TC1] ITU-T Recommendation X.509(2000) Corrigendum 1(2001) |
            ISO/IEC 9594-8:2001/Cor.1:2002, Technical Corrigendum 1 to
            ISO/IEC 9594:8:2001.

Informative References

   [AEAD]   Mcgrew, D., "Authenticated Encryption", July 2006, draft-
            mcgrew-auth-enc-00.txt.

   [AH]     Kent, S., and Atkinson, R., "IP Authentication Header", RFC
            2402, November 1998.

   [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.

   [CCM]     "NIST Special Publication 800-38C: The CCM Mode for
            Authentication and Confidentiality",
            http://csrc.nist.gov/publications/nistpubs/SP800-38C.pdf.

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

   [ESP]     Kent, S., and Atkinson, R., "IP Encapsulating Security
            Payload (ESP)", RFC 2406, November 1998.

   [GCM]    "NIST Special Publication 800-38C: The CCM Mode for
            Authentication and Confidentiality",
            http://csrc.nist.gov/publications/nistpubs/SP800-38C.pdf.

   [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



Dierks & Rescorla            Standards Track                   [Page 107]\fdraft-ietf-tls-rfc4346-bis-02.txt  TLS                      October 2006


            Standard," version 1.5, November 1993.

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

   [RANDOM] D. Eastlake 3rd, S. Crocker, J. Schiller. "Randomness
            Recommendations for Security", RFC 1750, December 1994.

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

   [TCP]    Postel, J., "Transmission Control Protocol," STD 7, RFC 793,
            September 1981.

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

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

   [TLS1.1] Dierks, T., and Rescorla, E., "The TLS Protocol, Version
            1.1", RFC 4346, April, 2006.

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




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Credits

   Working Group Chairs
   Eric Rescorla
   EMail: ekr@networkresonance.com

   Pasi Eronen
   pasi.eronen@nokia.com


   Editors

   Tim Dierks                    Eric Rescorla
   Independent                   Network Resonance, Inc.

   EMail: tim@dierks.org         EMail: ekr@networkresonance.com



   Other contributors

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

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

   Steven M. Bellovin
   Columbia University
   smb@cs.columbia.edu

   Simon Blake-Wilson
   BCI
   EMail: sblakewilson@bcisse.com

   Ran Canetti
   IBM
   canetti@watson.ibm.com

   Pete Chown
   Skygate Technology Ltd
   pc@skygate.co.uk

   Taher Elgamal
   taher@securify.com
   Securify



Dierks & Rescorla            Standards Track                   [Page 109]\fdraft-ietf-tls-rfc4346-bis-02.txt  TLS                      October 2006


   Anil Gangolli
   anil@busybuddha.org

   Kipp Hickman

   David Hopwood
   Independent Consultant
   EMail: david.hopwood@blueyonder.co.uk

   Phil Karlton (co-author of SSLv3)

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

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

   Jan Mikkelsen
   Transactionware
   EMail: janm@transactionware.com

   Magnus Nystrom
   RSA Security
   EMail: magnus@rsasecurity.com

   Robert Relyea
   Netscape Communications
   relyea@netscape.com

   Jim Roskind
   Netscape Communications
   jar@netscape.com

   Michael Sabin

   Dan Simon
   Microsoft, Inc.
   dansimon@microsoft.com

   Tom Weinstein

   Tim Wright
   Vodafone
   EMail: timothy.wright@vodafone.com

Comments



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   The discussion list for the IETF TLS working group is located at the
   e-mail address <tls@ietf.org>. Information on the group and
   information on how to subscribe to the list is at
   <https://www1.ietf.org/mailman/listinfo/tls>

   Archives of the list can be found at:
       <http://www.ietf.org/mail-archive/web/tls/current/index.html>












































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