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rfc:rfc6101

Internet Engineering Task Force (IETF) A. Freier Request for Comments: 6101 P. Karlton Category: Historic Netscape Communications ISSN: 2070-1721 P. Kocher

                                                Independent Consultant
                                                           August 2011
        The Secure Sockets Layer (SSL) Protocol Version 3.0

Abstract

 This document is published as a historical record of the SSL 3.0
 protocol.  The original Abstract follows.
 This document specifies version 3.0 of the Secure Sockets Layer (SSL
 3.0) protocol, a security protocol that provides communications
 privacy over the Internet.  The protocol allows client/server
 applications to communicate in a way that is designed to prevent
 eavesdropping, tampering, or message forgery.

Foreword

 Although the SSL 3.0 protocol is a widely implemented protocol, a
 pioneer in secure communications protocols, and the basis for
 Transport Layer Security (TLS), it was never formally published by
 the IETF, except in several expired Internet-Drafts.  This allowed no
 easy referencing to the protocol.  We believe a stable reference to
 the original document should exist and for that reason, this document
 describes what is known as the last published version of the SSL 3.0
 protocol, that is, the November 18, 1996, version of the protocol.
 There were no changes to the original document other than trivial
 editorial changes and the addition of a "Security Considerations"
 section.  However, portions of the original document that no longer
 apply were not included.  Such as the "Patent Statement" section, the
 "Reserved Ports Assignment" section, and the cipher-suite registrator
 note in the "The CipherSuite" section.  The "US export rules"
 discussed in the document do not apply today but are kept intact to
 provide context for decisions taken in protocol design.  The "Goals
 of This Document" section indicates the goals for adopters of SSL
 3.0, not goals of the IETF.
 The authors and editors were retained as in the original document.
 The editor of this document is Nikos Mavrogiannopoulos
 (nikos.mavrogiannopoulos@esat.kuleuven.be).  The editor would like to
 thank Dan Harkins, Linda Dunbar, Sean Turner, and Geoffrey Keating
 for reviewing this document and providing helpful comments.

Freier, et al. Historic [Page 1] RFC 6101 The SSL Protocol Version 3.0 August 2011

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for the historical record.
 This document defines a Historic Document for the Internet community.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6101.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Freier, et al. Historic [Page 2] RFC 6101 The SSL Protocol Version 3.0 August 2011

Table of Contents

 1. Introduction ....................................................5
 2. Goals ...........................................................5
 3. Goals of This Document ..........................................6
 4. Presentation Language ...........................................6
    4.1. Basic Block Size ...........................................7
    4.2. Miscellaneous ..............................................7
    4.3. Vectors ....................................................7
    4.4. Numbers ....................................................8
    4.5. Enumerateds ................................................8
    4.6. Constructed Types ..........................................9
         4.6.1. Variants ...........................................10
    4.7. Cryptographic Attributes ..................................11
    4.8. Constants .................................................12
 5. SSL Protocol ...................................................12
    5.1. Session and Connection States .............................12
    5.2. Record Layer ..............................................14
         5.2.1. Fragmentation ......................................14
         5.2.2. Record Compression and Decompression ...............15
         5.2.3. Record Payload Protection and the CipherSpec .......16
    5.3. Change Cipher Spec Protocol ...............................18
    5.4. Alert Protocol ............................................18
         5.4.1. Closure Alerts .....................................19
         5.4.2. Error Alerts .......................................20
    5.5. Handshake Protocol Overview ...............................21
    5.6. Handshake Protocol ........................................23
         5.6.1. Hello messages .....................................24
         5.6.2. Server Certificate .................................28
         5.6.3. Server Key Exchange Message ........................28
         5.6.4. Certificate Request ................................30
         5.6.5. Server Hello Done ..................................31
         5.6.6. Client Certificate .................................31
         5.6.7. Client Key Exchange Message ........................31
         5.6.8. Certificate Verify .................................34
         5.6.9. Finished ...........................................35
    5.7. Application Data Protocol .................................36
 6. Cryptographic Computations .....................................36
    6.1. Asymmetric Cryptographic Computations .....................36
         6.1.1. RSA ................................................36
         6.1.2. Diffie-Hellman .....................................37
         6.1.3. FORTEZZA ...........................................37
    6.2. Symmetric Cryptographic Calculations and the CipherSpec ...37
         6.2.1. The Master Secret ..................................37
         6.2.2. Converting the Master Secret into Keys and
                MAC Secrets ........................................37
 7. Security Considerations ........................................39
 8. Informative References .........................................40

Freier, et al. Historic [Page 3] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Appendix A. Protocol Constant Values ..............................42
   A.1. Record Layer ...............................................42
   A.2. Change Cipher Specs Message ................................43
   A.3. Alert Messages .............................................43
   A.4. Handshake Protocol .........................................44
     A.4.1. Hello Messages .........................................44
     A.4.2. Server Authentication and Key Exchange Messages ........45
   A.5. Client Authentication and Key Exchange Messages ............46
     A.5.1. Handshake Finalization Message .........................47
   A.6. The CipherSuite ............................................47
   A.7. The CipherSpec .............................................49
 Appendix B. Glossary ..............................................50
 Appendix C. CipherSuite Definitions ...............................53
 Appendix D. Implementation Notes ..................................56
   D.1. Temporary RSA Keys .........................................56
   D.2. Random Number Generation and Seeding .......................56
   D.3. Certificates and Authentication ............................57
   D.4. CipherSuites ...............................................57
   D.5. FORTEZZA ...................................................57
     D.5.1. Notes on Use of FORTEZZA Hardware ......................57
     D.5.2. FORTEZZA Cipher Suites .................................58
     D.5.3. FORTEZZA Session Resumption ............................58
 Appendix E. Version 2.0 Backward Compatibility ....................59
   E.1. Version 2 Client Hello .....................................59
   E.2. Avoiding Man-in-the-Middle Version Rollback ..............61
 Appendix F. Security Analysis .....................................61
   F.1. Handshake Protocol .........................................61
     F.1.1. Authentication and Key Exchange ........................61
     F.1.2. Version Rollback Attacks ...............................64
     F.1.3. Detecting Attacks against the Handshake Protocol .......64
     F.1.4. Resuming Sessions ......................................65
     F.1.5. MD5 and SHA ............................................65
   F.2. Protecting Application Data ................................65
   F.3. Final Notes ................................................66
 Appendix G. Acknowledgements ......................................66
   G.1. Other Contributors .........................................66
   G.2. Early Reviewers ............................................67

Freier, et al. Historic [Page 4] RFC 6101 The SSL Protocol Version 3.0 August 2011

1. Introduction

 The primary goal of the SSL protocol is to provide privacy and
 reliability between two communicating applications.  The protocol is
 composed of two layers.  At the lowest level, layered on top of some
 reliable transport protocol (e.g., TCP [RFC0793]), is the SSL record
 protocol.  The SSL record protocol is used for encapsulation of
 various higher level protocols.  One such encapsulated protocol, the
 SSL handshake protocol, allows the server and client to authenticate
 each other and to negotiate an encryption algorithm and cryptographic
 keys before the application protocol transmits or receives its first
 byte of data.  One advantage of SSL is that it is application
 protocol independent.  A higher level protocol can layer on top of
 the SSL protocol transparently.  The SSL protocol provides connection
 security that has three basic properties:
 o  The connection is private.  Encryption is used after an initial
    handshake to define a secret key.  Symmetric cryptography is used
    for data encryption (e.g., DES [DES], 3DES [3DES], RC4 [SCH]).
 o  The peer's identity can be authenticated using asymmetric, or
    public key, cryptography (e.g., RSA [RSA], DSS [DSS]).
 o  The connection is reliable.  Message transport includes a message
    integrity check using a keyed Message Authentication Code (MAC)
    [RFC2104].  Secure hash functions (e.g., SHA, MD5) are used for
    MAC computations.

2. Goals

 The goals of SSL protocol version 3.0, in order of their priority,
 are:
 1.  Cryptographic security
        SSL should be used to establish a secure connection between
        two parties.
 2.  Interoperability
        Independent programmers should be able to develop applications
        utilizing SSL 3.0 that will then be able to successfully
        exchange cryptographic parameters without knowledge of one
        another's code.

Freier, et al. Historic [Page 5] RFC 6101 The SSL Protocol Version 3.0 August 2011

        Note: It is not the case that all instances of SSL (even in
        the same application domain) will be able to successfully
        connect.  For instance, if the server supports a particular
        hardware token, and the client does not have access to such a
        token, then the connection will not succeed.
 3.  Extensibility
        SSL seeks to provide a framework into which new public key and
        bulk encryption methods can be incorporated as necessary.
        This will also accomplish two sub-goals: to prevent the need
        to create a new protocol (and risking the introduction of
        possible new weaknesses) and to avoid the need to implement an
        entire new security library.
 4.  Relative efficiency
        Cryptographic operations tend to be highly CPU intensive,
        particularly public key operations.  For this reason, the SSL
        protocol has incorporated an optional session caching scheme
        to reduce the number of connections that need to be
        established from scratch.  Additionally, care has been taken
        to reduce network activity.

3. Goals of This Document

 The SSL protocol version 3.0 specification is intended primarily for
 readers who will be implementing the protocol and those doing
 cryptographic analysis of it.  The spec has been written with this in
 mind, and it is intended to reflect the needs of those two groups.
 For that reason, many of the algorithm-dependent data structures and
 rules are included in the body of the text (as opposed to in an
 appendix), providing easier access to them.
 This document is not intended to supply any details of service
 definition or 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 External Data
 Representation (XDR) [RFC1832] in both its syntax and intent, it

Freier, et al. Historic [Page 6] RFC 6101 The SSL Protocol Version 3.0 August 2011

 would be risky to draw too many parallels.  The purpose of this
 presentation language is to document SSL only, not to have general
 application beyond that particular goal.

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 byte stream, 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 */

Freier, et al. Historic [Page 7] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Variable-length vectors are defined by specifying a subrange of legal
 lengths, inclusively, using the notation <floor..ceiling>.  When
 encoded, the actual length precedes the vector's contents in the byte
 stream.  The length will be in the form of a number consuming as many
 bytes as required to hold the vector's specified maximum (ceiling)
 length.  A variable-length vector with an actual length field of zero
 is referred to as an empty vector.
      T T'<floor..ceiling>;
 In the following example, mandatory is a vector that must contain
 between 300 and 400 bytes of type opaque.  It can never be empty.
 The actual length field consumes two bytes, a uint16, sufficient to
 represent the value 400 (see Section 4.4).  On the other hand, longer
 can represent up to 800 bytes of data, or 400 uint16 elements, and it
 may be empty.  Its encoding will include a two-byte actual length
 field prepended to the vector.
      opaque mandatory<300..400>;
            /* length field is 2 bytes, cannot be empty */
      uint16 longer<0..800>;
            /* zero to 400 16-bit unsigned integers */

4.4. Numbers

 The basic numeric data type is an unsigned byte (uint8).  All larger
 numeric data types are formed from fixed-length series of bytes
 concatenated as described in Section 4.1 and are also unsigned.  The
 following numeric types are predefined.
      uint8 uint16[2];
      uint8 uint24[3];
      uint8 uint32[4];
      uint8 uint64[8];

4.5. Enumerateds

 An additional sparse data type is available called enum.  A field of
 type enum can only assume the values declared in the definition.
 Each definition is a different type.  Only enumerateds of the same
 type may be assigned or compared.  Every element of an enumerated
 must be assigned a value, as demonstrated in the following example.
 Since the elements of the enumerated are not ordered, they can be
 assigned any unique value, in any order.
      enum { e1(v1), e2(v2), ... , en(vn), [[(n)]] } Te;

Freier, et al. Historic [Page 8] RFC 6101 The SSL Protocol Version 3.0 August 2011

 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;
 Optionally, one may specify a value without its associated tag to
 force the width definition without defining a superfluous element.
 In the following example, Taste will consume two bytes in the data
 stream but can only assume the values 1, 2, or 4.
      enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
 The names of the elements of an enumeration are scoped within the
 defined type.  In the first example, a fully qualified reference to
 the second element of the enumeration would be Color.blue.  Such
 qualification is not required if the target of the assignment is well
 specified.
      Color color = Color.blue;     /* overspecified, legal */
      Color color = blue;           /* correct, type implicit */
 For enumerateds that are never converted to external representation,
 the numerical information may be omitted.
      enum { low, medium, high } Amount;

4.6. Constructed Types

 Structure types may be constructed from primitive types for
 convenience.  Each specification declares a new, unique type.  The
 syntax for definition is much like that of C.
    struct {
        T1 f1;
        T2 f2;
        ...
        Tn fn;
    } [[T]];
 The fields within a structure may be qualified using the type's name
 using a syntax much like that available for enumerateds.  For
 example, T.f2 refers to the second field of the previous declaration.
 Structure definitions may be embedded.

Freier, et al. Historic [Page 9] RFC 6101 The SSL Protocol Version 3.0 August 2011

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;

Freier, et al. Historic [Page 10] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Variant structures may be qualified (narrowed) by specifying a value
 for the selector prior to the type.  For example, an
      orange VariantRecord
 is a narrowed type of a VariantRecord containing a variant_body of
 type V2.

4.7. Cryptographic Attributes

 The four cryptographic operations digital signing, stream cipher
 encryption, block cipher encryption, and public key encryption are
 designated digitally-signed, stream-ciphered, block-ciphered, and
 public-key-encrypted, respectively.  A field's cryptographic
 processing is specified by prepending an appropriate key word
 designation before the field's type specification.  Cryptographic
 keys are implied by the current session state (see Section 5.1).
 In digital signing, one-way hash functions are used as input for a
 signing algorithm.  In RSA signing, a 36-byte structure of two hashes
 (one SHA and one MD5) is signed (encrypted with the private key).  In
 DSS, the 20 bytes of the SHA hash are run directly through the
 Digital Signature Algorithm with no additional hashing.
 In stream cipher encryption, the plaintext is exclusive-ORed with an
 identical amount of output generated from a cryptographically secure
 keyed pseudorandom number generator.
 In block cipher encryption, every block of plaintext encrypts to a
 block of ciphertext.  Because it is unlikely that the plaintext
 (whatever data is to be sent) will break neatly into the necessary
 block size (usually 64 bits), it is necessary to pad out the end of
 short blocks with some regular pattern, usually all zeroes.
 In public key encryption, one-way functions with secret "trapdoors"
 are used to encrypt the outgoing data.  Data encrypted with the
 public key of a given key pair can only be decrypted with the private
 key, and vice versa.  In the following example:
      stream-ciphered struct {
          uint8 field1;
          uint8 field2;
          digitally-signed opaque hash[20];
      } UserType;
 The contents of hash are used as input for the signing algorithm,
 then the entire structure is encrypted with a stream cipher.

Freier, et al. Historic [Page 11] RFC 6101 The SSL Protocol Version 3.0 August 2011

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. SSL Protocol

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

5.1. Session and Connection States

 An SSL session is stateful.  It is the responsibility of the SSL
 handshake protocol to coordinate the states of the client and server,
 thereby allowing the protocol state machines of each to operate
 consistently, despite the fact that the state is not exactly
 parallel.  Logically, the state is represented twice, once as the
 current operating state and (during the handshake protocol) again as
 the pending state.  Additionally, separate read and write states are
 maintained.  When the client or server receives a change cipher spec
 message, it copies the pending read state into the current read
 state.  When the client or server sends a change cipher spec message,
 it copies the pending write state into the current write state.  When
 the handshake negotiation is complete, the client and server exchange
 change cipher spec messages (see Section 5.3), and they then
 communicate using the newly agreed-upon cipher spec.
 An SSL session may include multiple secure connections; in addition,
 parties may have multiple simultaneous sessions.

Freier, et al. Historic [Page 12] RFC 6101 The SSL Protocol Version 3.0 August 2011

 The session state includes the following elements:
 session identifier:  An arbitrary byte sequence chosen by the server
    to identify an active or resumable session state.
 peer certificate:  X509.v3 [X509] certificate of the peer.  This
    element of the state may be null.
 compression method:  The algorithm used to compress data prior to
    encryption.
 cipher spec:  Specifies the bulk data encryption algorithm (such as
    null, DES, etc.) and a MAC algorithm (such as MD5 or SHA).  It
    also defines cryptographic attributes such as the hash_size.  (See
    Appendix A.7 for formal definition.)
 master secret:  48-byte secret shared between the client and server.
 is resumable:  A flag indicating whether the session can be used to
    initiate new connections.
 The connection state includes the following elements:
 server and client random:  Byte sequences that are chosen by the
    server and client for each connection.
 server write MAC secret:  The secret used in MAC operations on data
    written by the server.
 client write MAC secret:  The secret used in MAC operations on data
    written by the client.
 server write key:  The bulk cipher key for data encrypted by the
    server and decrypted by the client.
 client write key:  The bulk cipher key for data encrypted by the
    client and decrypted by the server.
 initialization vectors:  When a block cipher in Cipher Block Chaining
    (CBC) mode is used, an initialization vector (IV) is maintained
    for each key.  This field is first initialized by the SSL
    handshake protocol.  Thereafter, the final ciphertext block from
    each record is preserved for use with the following record.

Freier, et al. Historic [Page 13] RFC 6101 The SSL Protocol Version 3.0 August 2011

 sequence numbers:  Each party maintains separate sequence numbers for
    transmitted and received messages for each connection.  When a
    party sends or receives a change cipher spec message, the
    appropriate sequence number is set to zero.  Sequence numbers are
    of type uint64 and may not exceed 2^64-1.

5.2. Record Layer

 The SSL record layer receives uninterpreted data from higher layers
 in non-empty blocks of arbitrary size.

5.2.1. Fragmentation

 The record layer fragments information blocks into SSLPlaintext
 records of 2^14 bytes or less.  Client message boundaries are not
 preserved in the record layer (i.e., multiple client messages of the
 same ContentType may be coalesced into a single SSLPlaintext record).
      struct {
          uint8 major, minor;
      } ProtocolVersion;
      enum {
          change_cipher_spec(20), alert(21), handshake(22),
          application_data(23), (255)
      } ContentType;
      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          opaque fragment[SSLPlaintext.length];
      } SSLPlaintext;
 type:  The higher level protocol used to process the enclosed
    fragment.
 version:  The version of protocol being employed.  This document
    describes SSL version 3.0 (see Appendix A.1).
 length:  The length (in bytes) of the following
    SSLPlaintext.fragment.  The length should not exceed 2^14.
 fragment:  The application data.  This data is transparent and
    treated as an independent block to be dealt with by the higher
    level protocol specified by the type field.

Freier, et al. Historic [Page 14] RFC 6101 The SSL Protocol Version 3.0 August 2011

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

5.2.2. Record Compression and Decompression

 All records are compressed using the compression algorithm defined in
 the current session state.  There is always an active compression
 algorithm; however, initially it is defined as
 CompressionMethod.null.  The compression algorithm translates an
 SSLPlaintext structure into an SSLCompressed structure.  Compression
 functions erase their state information whenever the CipherSpec is
 replaced.
 Note: The CipherSpec is part of the session state described in
 Section 5.1.  References to fields of the CipherSpec are made
 throughout this document using presentation syntax.  A more complete
 description of the CipherSpec is shown in Appendix A.7.
 Compression must be lossless and may not increase the content length
 by more than 1024 bytes.  If the decompression function encounters an
 SSLCompressed.fragment that would decompress to a length in excess of
 2^14 bytes, it should issue a fatal decompression_failure alert
 (Section 5.4.2).
      struct {
          ContentType type;       /* same as SSLPlaintext.type */
          ProtocolVersion version;/* same as SSLPlaintext.version */
          uint16 length;
          opaque fragment[SSLCompressed.length];
      } SSLCompressed;
 length:  The length (in bytes) of the following
    SSLCompressed.fragment.  The length should not exceed 2^14 + 1024.
 fragment:  The compressed form of SSLPlaintext.fragment.
 Note: A CompressionMethod.null operation is an identity operation; no
 fields are altered (see Appendix A.4.1.)
 Implementation note: Decompression functions are responsible for
 ensuring that messages cannot cause internal buffer overflows.

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5.2.3. Record Payload Protection and the CipherSpec

 All records are protected using the encryption and MAC algorithms
 defined in the current CipherSpec.  There is always an active
 CipherSpec; however, initially it is SSL_NULL_WITH_NULL_NULL, which
 does not provide any security.
 Once the handshake is complete, the two parties have shared secrets
 that are used to encrypt records and compute keyed Message
 Authentication Codes (MACs) on their contents.  The techniques used
 to perform the encryption and MAC operations are defined by the
 CipherSpec and constrained by CipherSpec.cipher_type.  The encryption
 and MAC functions translate an SSLCompressed structure into an
 SSLCiphertext.  The decryption functions reverse the process.
 Transmissions also include a sequence number so that missing,
 altered, or extra messages are detectable.
      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          select (CipherSpec.cipher_type) {
              case stream: GenericStreamCipher;
              case block: GenericBlockCipher;
          } fragment;
      } SSLCiphertext;
 type:  The type field is identical to SSLCompressed.type.
 version:  The version field is identical to SSLCompressed.version.
 length:  The length (in bytes) of the following
    SSLCiphertext.fragment.  The length may not exceed 2^14 + 2048.
 fragment:  The encrypted form of SSLCompressed.fragment, including
    the MAC.

5.2.3.1. Null or Standard Stream Cipher

 Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.7)
 convert SSLCompressed.fragment structures to and from stream
 SSLCiphertext.fragment structures.
      stream-ciphered struct {
          opaque content[SSLCompressed.length];
          opaque MAC[CipherSpec.hash_size];
      } GenericStreamCipher;

Freier, et al. Historic [Page 16] RFC 6101 The SSL Protocol Version 3.0 August 2011

 The MAC is generated as:
      hash(MAC_write_secret + pad_2 +
           hash(MAC_write_secret + pad_1 + seq_num +
                SSLCompressed.type + SSLCompressed.length +
                SSLCompressed.fragment));
 where "+" denotes concatenation.
 pad_1:  The character 0x36 repeated 48 times for MD5 or 40 times for
    SHA.
 pad_2:  The character 0x5c repeated 48 times for MD5 or 40 times for
    SHA.
 seq_num:  The sequence number for this message.
 hash:  Hashing algorithm derived from the cipher suite.
 Note that the MAC is computed before encryption.  The stream cipher
 encrypts the entire block, including the MAC.  For stream ciphers
 that do not use a synchronization vector (such as RC4), the stream
 cipher state from the end of one record is simply used on the
 subsequent packet.  If the CipherSuite is SSL_NULL_WITH_NULL_NULL,
 encryption consists of the identity operation (i.e., the data is not
 encrypted and the MAC size is zero implying that no MAC is used).
 SSLCiphertext.length is SSLCompressed.length plus
 CipherSpec.hash_size.

5.2.3.2. CBC Block Cipher

 For block ciphers (such as RC2 or DES), the encryption and MAC
 functions convert SSLCompressed.fragment structures to and from block
 SSLCiphertext.fragment structures.
      block-ciphered struct {
          opaque content[SSLCompressed.length];
          opaque MAC[CipherSpec.hash_size];
          uint8 padding[GenericBlockCipher.padding_length];
          uint8 padding_length;
      } GenericBlockCipher;
 The MAC is generated as described in Section 5.2.3.1.
 padding:  Padding that is added to force the length of the plaintext
    to be a multiple of the block cipher's block length.

Freier, et al. Historic [Page 17] RFC 6101 The SSL Protocol Version 3.0 August 2011

 padding_length:  The length of the padding must be less than the
    cipher's block length and may be zero.  The padding length should
    be such that the total size of the GenericBlockCipher structure is
    a multiple of the cipher's block length.
 The encrypted data length (SSLCiphertext.length) is one more than the
 sum of SSLCompressed.length, CipherSpec.hash_size, and
 padding_length.
 Note: With CBC, the initialization vector (IV) for the first record
 is provided by the handshake protocol.  The IV for subsequent records
 is the last ciphertext block from the previous record.

5.3. Change Cipher Spec Protocol

 The change cipher spec protocol exists to signal transitions in
 ciphering strategies.  The protocol consists of a single message,
 which is encrypted and compressed under the current (not the pending)
 CipherSpec.  The message consists of a single byte of value 1.
      struct {
          enum { change_cipher_spec(1), (255) } type;
      } ChangeCipherSpec;
 The change cipher spec message is sent by both the client and server
 to notify the receiving party that subsequent records will be
 protected under the just-negotiated CipherSpec and keys.  Reception
 of this message causes the receiver to copy the read pending state
 into the read current state.  The client sends a change cipher spec
 message following handshake key exchange and certificate verify
 messages (if any), and the server sends one after successfully
 processing the key exchange message it received from the client.  An
 unexpected change cipher spec message should generate an
 unexpected_message alert (Section 5.4.2).  When resuming a previous
 session, the change cipher spec message is sent after the hello
 messages.

5.4. Alert Protocol

 One of the content types supported by the SSL 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.

Freier, et al. Historic [Page 18] RFC 6101 The SSL Protocol Version 3.0 August 2011

      enum { warning(1), fatal(2), (255) } AlertLevel;
      enum {
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decompression_failure(30),
          handshake_failure(40),
          no_certificate(41),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter (47)
          (255)
      } AlertDescription;
      struct {
          AlertLevel level;
          AlertDescription description;
      } Alert;

5.4.1. Closure Alerts

 The client and the server must share knowledge that the connection is
 ending in order to avoid a truncation attack.  Either party may
 initiate the exchange of closing messages.
 close_notify:  This message notifies the recipient that the sender
    will not send any more messages on this connection.  The session
    becomes unresumable if any connection is terminated without proper
    close_notify messages with level equal to warning.
 Either party may initiate a close by sending a close_notify alert.
 Any data received after a closure alert is ignored.
 Each party is required to send a close_notify alert before closing
 the write side of the connection.  It is required that the other
 party respond with a close_notify alert of its own and close down the
 connection immediately, discarding any pending writes.  It is not
 required for the initiator of the close to wait for the responding
 close_notify alert before closing the read side of the connection.
 NB: It is assumed that closing a connection reliably delivers pending
 data before destroying the transport.

Freier, et al. Historic [Page 19] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.4.2. Error Alerts

 Error handling in the SSL handshake protocol is very simple.  When an
 error is detected, the detecting party sends a message to the other
 party.  Upon transmission or receipt of a fatal alert message, both
 parties immediately close the connection.  Servers and clients are
 required to forget any session identifiers, keys, and secrets
 associated with a failed connection.  The following error alerts are
 defined:
 unexpected_message:  An inappropriate message was received.  This
    alert is always fatal and should never be observed in
    communication between proper implementations.
 bad_record_mac:  This alert is returned if a record is received with
    an incorrect MAC.  This message is always fatal.
 decompression_failure:  The decompression function received improper
    input (e.g., data that would expand to excessive length).  This
    message is always fatal.
 handshake_failure:  Reception of a handshake_failure alert message
    indicates that the sender was unable to negotiate an acceptable
    set of security parameters given the options available.  This is a
    fatal error.
 no_certificate:  A no_certificate alert message may be sent in
    response to a certification request if no appropriate certificate
    is available.
 bad_certificate:  A certificate was corrupt, contained signatures
    that did not verify correctly, etc.
 unsupported_certificate:  A certificate was of an unsupported type.
 certificate_revoked:  A certificate was revoked by its signer.
 certificate_expired:  A certificate has expired or is not currently
    valid.
 certificate_unknown:  Some other (unspecified) issue arose in
    processing the certificate, rendering it unacceptable.
 illegal_parameter:  A field in the handshake was out of range or
    inconsistent with other fields.  This is always fatal.

Freier, et al. Historic [Page 20] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.5. Handshake Protocol Overview

 The cryptographic parameters of the session state are produced by the
 SSL handshake protocol, which operates on top of the SSL record
 layer.  When an SSL client and server first start communicating, they
 agree on a protocol version, select cryptographic algorithms,
 optionally authenticate each other, and use public key encryption
 techniques to generate shared secrets.  These processes are performed
 in the handshake protocol, which can be summarized as follows: the
 client sends a client hello message to which the server must respond
 with 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.
 Following the hello messages, the server will send its certificate,
 if it is to be authenticated.  Additionally, a server key exchange
 message may be sent, if it is required (e.g., if their server has no
 certificate, or if its certificate is for signing only).  If the
 server is authenticated, it may request a certificate from the
 client, if that is appropriate to the cipher suite selected.  Now the
 server will send the server hello done message, indicating that the
 hello-message phase of the handshake is complete.  The server will
 then wait for a client response.  If the server has sent a
 certificate request message, the client must send either the
 certificate message or a no_certificate alert.  The client key
 exchange message is now sent, and the content of that message will
 depend on the public key algorithm selected between the client hello
 and the server hello.  If the client has sent a certificate with
 signing ability, a digitally-signed certificate verify message is
 sent to explicitly verify the certificate.
 At this point, a change cipher spec message is sent by the client,
 and the client copies the pending CipherSpec into the current
 CipherSpec.  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 CipherSpec, and send its finished message under the new
 CipherSpec.  At this point, the handshake is complete and the client
 and server may begin to exchange application layer data.  (See flow
 chart below.)

Freier, et al. Historic [Page 21] RFC 6101 The SSL Protocol Version 3.0 August 2011

    Client                                                Server
    ClientHello                   -------->
                                                     ServerHello
                                                    Certificate*
                                              ServerKeyExchange*
                                             CertificateRequest*
                                  <--------      ServerHelloDone
    Certificate*
    ClientKeyExchange
    CertificateVerify*
    [ChangeCipherSpec]
    Finished                      -------->
                                              [ChangeCipherSpec]
                                  <--------             Finished
    Application Data              <------->     Application Data
  • Indicates optional or situation-dependent messages that are not

always sent.

 Note: To help avoid pipeline stalls, ChangeCipherSpec is an
 independent SSL protocol content type, and is not actually an SSL
 handshake message.
 When the client and server decide to resume a previous session or
 duplicate an existing session (instead of negotiating new security
 parameters) the message flow is as follows:
 The client sends a 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 SSL client and
 server perform a full handshake.

Freier, et al. Historic [Page 22] RFC 6101 The SSL Protocol Version 3.0 August 2011

    Client                                                Server
    ClientHello                   -------->
                                                     ServerHello
                                            [change cipher spec]
                                  <--------             Finished
    change cipher spec
    Finished                      -------->
    Application Data              <------->     Application Data
 The contents and significance of each message will be presented in
 detail in the following sections.

5.6. Handshake Protocol

 The SSL handshake protocol is one of the defined higher level clients
 of the SSL record protocol.  This protocol is used to negotiate the
 secure attributes of a session.  Handshake messages are supplied to
 the SSL record layer, where they are encapsulated within one or more
 SSLPlaintext structures, which are processed and transmitted as
 specified by the current active session state.
      enum {
          hello_request(0), client_hello(1), server_hello(2),
          certificate(11), server_key_exchange (12),
          certificate_request(13), server_hello_done(14),
          certificate_verify(15), client_key_exchange(16),
          finished(20), (255)
      } HandshakeType;
      struct {
          HandshakeType msg_type;    /* handshake type */
          uint24 length;             /* bytes in message */
          select (HandshakeType) {
              case hello_request: HelloRequest;
              case client_hello: ClientHello;
              case server_hello: ServerHello;
              case certificate: Certificate;
              case server_key_exchange: ServerKeyExchange;
              case certificate_request: CertificateRequest;
              case server_hello_done: ServerHelloDone;
              case certificate_verify: CertificateVerify;
              case client_key_exchange: ClientKeyExchange;
              case finished: Finished;
          } body;
      } Handshake;

Freier, et al. Historic [Page 23] RFC 6101 The SSL Protocol Version 3.0 August 2011

 The handshake protocol messages are presented in the order they must
 be sent; sending handshake messages in an unexpected order results in
 a fatal error.

5.6.1. Hello messages

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

5.6.1.1. Hello Request

 The hello request message may be sent by the server at any time, but
 will be ignored by the client if the handshake protocol is already
 underway.  It is a simple notification that the client should begin
 the negotiation process anew by sending a client hello message when
 convenient.
 Note: Since handshake messages are intended to have transmission
 precedence over application data, it is expected that the negotiation
 begin in no more than one or two times the transmission time of a
 maximum-length application data message.
 After sending a hello request, servers should not repeat the request
 until the subsequent handshake negotiation is complete.  A client
 that receives a hello request while in a handshake negotiation state
 should simply ignore the message.
 The structure of a hello request message is as follows:
      struct { } HelloRequest;

5.6.1.2. Client Hello

 When a client first connects to a server it is required to send the
 client hello as its first message.  The client can also send 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.  The client hello message includes a random structure,
 which is used later in the protocol.

Freier, et al. Historic [Page 24] RFC 6101 The SSL Protocol Version 3.0 August 2011

    struct {
        uint32 gmt_unix_time;
        opaque random_bytes[28];
    } Random;
 gmt_unix_time:  The current time and date in standard UNIX 32-bit
    format according to the sender's internal clock.  Clocks are not
    required to be set correctly by the basic SSL protocol; higher
    level or application protocols may define additional requirements.
 random_bytes:  28 bytes generated by a secure random number
    generator.
 The client hello message includes a variable-length session
 identifier.  If not empty, the value identifies a session between the
 same client and server whose security parameters the client wishes to
 reuse.  The session identifier may be from an earlier connection,
 this connection, or another currently active connection.  The second
 option is useful if the client only wishes to update the random
 structures and derived values of a connection, while the third option
 makes it possible to establish several simultaneous independent
 secure connections without repeating the full handshake protocol.
 The actual contents of the SessionID are defined by the server.
      opaque SessionID<0..32>;
 Warning: Servers must not place confidential information in session
 identifiers or let the contents of fake session identifiers cause any
 breach of security.
 The CipherSuite list, passed from the client to the server in the
 client hello message, contains the combinations of cryptographic
 algorithms supported by the client in order of the client's
 preference (first choice first).  Each CipherSuite defines both a key
 exchange algorithm and a CipherSpec.  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.  If the
 server supports none of those specified by the client, the session
 must fail.
      enum { null(0), (255) } CompressionMethod;
 Issue: Which compression methods to support is under investigation.

Freier, et al. Historic [Page 25] RFC 6101 The SSL Protocol Version 3.0 August 2011

 The structure of the client hello is as follows.
      struct {
          ProtocolVersion client_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suites<2..2^16-1>;
          CompressionMethod compression_methods<1..2^8-1>;
      } ClientHello;
 client_version:  The version of the SSL protocol by which the client
    wishes to communicate during this session.  This should be the
    most recent (highest valued) version supported by the client.  For
    this version of the specification, the version will be 3.0 (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, sorted with the client's first preference first.
    If the session_id field is not empty (implying a session
    resumption request), this vector must include at least the
    cipher_suite from that session.  Values are defined in
    Appendix A.6.
 compression_methods:  This is a list of the compression methods
    supported by the client, sorted by client preference.  If the
    session_id field is not empty (implying a session resumption
    request), this vector must include at least the compression_method
    from that session.  All implementations must support
    CompressionMethod.null.
 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.
 Implementation note: Application data may not be sent before a
 finished message has been sent.  Transmitted application data is
 known to be insecure until a valid finished message has been
 received.  This absolute restriction is relaxed if there is a
 current, non-null encryption on this connection.

Freier, et al. Historic [Page 26] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Forward compatibility note: In the interests of forward
 compatibility, it is permitted for a client hello message to include
 extra data after the compression methods.  This data must be included
 in the handshake hashes, but must otherwise be ignored.

5.6.1.3. Server Hello

 The server processes the client hello message and responds with
 either a handshake_failure alert or server hello message.
      struct {
          ProtocolVersion server_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suite;
          CompressionMethod compression_method;
      } ServerHello;
 server_version:  This field will contain the lower of that suggested
    by the client in the client hello and the highest supported by the
    server.  For this version of the specification, the version will
    be 3.0 (see Appendix E for details about backward compatibility).
 random:  This structure is generated by the server and must be
    different from (and independent of) ClientHello.random.
 session_id:  This is the identity of the session corresponding to
    this connection.  If the ClientHello.session_id was non-empty, the
    server will look in its session cache for a match.  If a match is
    found and the server is willing to establish the new connection
    using the specified session state, the server will respond with
    the same value as was supplied by the client.  This indicates a
    resumed session and dictates that the parties must proceed
    directly to the finished messages.  Otherwise, this field will
    contain a different value identifying the new session.  The server
    may return an empty session_id to indicate that the session will
    not be cached and therefore cannot be resumed.
 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.

Freier, et al. Historic [Page 27] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.6.2. Server Certificate

 If the server is to be authenticated (which is generally the case),
 the server sends its certificate immediately following the server
 hello message.  The certificate type must be appropriate for the
 selected cipher suite's key exchange algorithm, and is generally an
 X.509.v3 certificate (or a modified X.509 certificate in the case of
 FORTEZZA(tm) [FOR]).  The same message type will be used for the
 client's response to a certificate request message.
      opaque ASN.1Cert<1..2^24-1>;
      struct {
          ASN.1Cert certificate_list<1..2^24-1>;
      } Certificate;
 certificate_list:  This is a sequence (chain) of X.509.v3
    certificates, ordered with the sender's certificate first followed
    by any certificate authority certificates proceeding sequentially
    upward.
 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.

5.6.3. Server Key Exchange Message

 The server key exchange message is sent by the server if it has no
 certificate, has a certificate only used for signing (e.g., DSS [DSS]
 certificates, signing-only RSA [RSA] certificates), or FORTEZZA KEA
 key exchange is used.  This message is not used if the server
 certificate contains Diffie-Hellman [DH1] parameters.
 Note: According to current US export law, RSA moduli larger than 512
 bits may not be used for key exchange in software exported from the
 US.  With this message, larger RSA keys may be used as signature-only
 certificates to sign temporary shorter RSA keys for key exchange.
      enum { rsa, diffie_hellman, fortezza_kea }
             KeyExchangeAlgorithm;
      struct {
          opaque rsa_modulus<1..2^16-1>;
          opaque rsa_exponent<1..2^16-1>;
      } ServerRSAParams;

Freier, et al. Historic [Page 28] RFC 6101 The SSL Protocol Version 3.0 August 2011

 rsa_modulus:  The modulus of the server's temporary RSA key.
 rsa_exponent:  The public exponent of the server's temporary RSA key.
      struct {
          opaque dh_p<1..2^16-1>;
          opaque dh_g<1..2^16-1>;
          opaque dh_Ys<1..2^16-1>;
      } ServerDHParams;     /* Ephemeral DH parameters */
 dh_p:  The prime modulus used for the Diffie-Hellman operation.
 dh_g:  The generator used for the Diffie-Hellman operation.
 dh_Ys:  The server's Diffie-Hellman public value (gX mod p).
      struct {
          opaque r_s [128];
      } ServerFortezzaParams;
 r_s:  Server random number for FORTEZZA KEA (Key Exchange Algorithm).
      struct {
          select (KeyExchangeAlgorithm) {
              case diffie_hellman:
                  ServerDHParams params;
                  Signature signed_params;
              case rsa:
                  ServerRSAParams params;
                  Signature signed_params;
              case fortezza_kea:
                  ServerFortezzaParams params;
          };
      } ServerKeyExchange;
 params:  The server's key exchange parameters.
 signed_params:  A hash of the corresponding params value, with the
    signature appropriate to that hash applied.
 md5_hash:  MD5(ClientHello.random + ServerHello.random +
    ServerParams);

Freier, et al. Historic [Page 29] RFC 6101 The SSL Protocol Version 3.0 August 2011

 sha_hash:  SHA(ClientHello.random + ServerHello.random +
    ServerParams);
      enum { anonymous, rsa, dsa } SignatureAlgorithm;
      digitally-signed struct {
          select(SignatureAlgorithm) {
              case anonymous: struct { };
              case rsa:
                  opaque md5_hash[16];
                  opaque sha_hash[20];
              case dsa:
                  opaque sha_hash[20];
          };
      } Signature;

5.6.4. Certificate Request

 A non-anonymous server can optionally request a certificate from the
 client, if appropriate for the selected cipher suite.
      enum {
          rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4),
          rsa_ephemeral_dh(5), dss_ephemeral_dh(6), fortezza_kea(20),
          (255)
      } ClientCertificateType;
      opaque DistinguishedName<1..2^16-1>;
      struct {
          ClientCertificateType certificate_types<1..2^8-1>;
          DistinguishedName certificate_authorities<3..2^16-1>;
      } CertificateRequest;
 certificate_types:  This field is a list of the types of certificates
    requested, sorted in order of the server's preference.
 certificate_authorities:  A list of the distinguished names of
    acceptable certificate authorities.
 Note: DistinguishedName is derived from [X509].
 Note: It is a fatal handshake_failure alert for an anonymous server
 to request client identification.

Freier, et al. Historic [Page 30] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.6.5. Server Hello Done

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

5.6.6. Client Certificate

 This is the first message the client can send after receiving a
 server hello done message.  This message is only sent if the server
 requests a certificate.  If no suitable certificate is available, the
 client should send a no_certificate alert instead.  This alert is
 only a warning; however, the server may respond with a fatal
 handshake failure alert if client authentication is required.  Client
 certificates are sent using the certificate defined in Section 5.6.2.
 Note: Client Diffie-Hellman certificates must match the server
 specified Diffie-Hellman parameters.

5.6.7. Client Key Exchange Message

 The choice of messages depends on which public key algorithm(s) has
 (have) been selected.  See Section 5.6.3 for the KeyExchangeAlgorithm
 definition.
      struct {
          select (KeyExchangeAlgorithm) {
              case rsa: EncryptedPreMasterSecret;
              case diffie_hellman: ClientDiffieHellmanPublic;
              case fortezza_kea: FortezzaKeys;
          } exchange_keys;
      } ClientKeyExchange;
 The information to select the appropriate record structure is in the
 pending session state (see Section 5.1).

Freier, et al. Historic [Page 31] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.6.7.1. RSA Encrypted Premaster Secret Message

 If RSA is being used for key agreement and authentication, the client
 generates a 48-byte premaster secret, encrypts it under the public
 key from the server's certificate or temporary RSA key from a server
 key exchange message, and sends the result in an encrypted premaster
 secret message.
      struct {
          ProtocolVersion client_version;
          opaque random[46];
      } PreMasterSecret;
 client_version:  The latest (newest) version supported by the client.
    This is used to detect version roll-back attacks.
 random:  46 securely-generated random bytes.
      struct {
          public-key-encrypted PreMasterSecret pre_master_secret;
      } EncryptedPreMasterSecret;
 pre_master_secret:  This random value is generated by the client and
    is used to generate the master secret, as specified in
    Section 6.1.

5.6.7.2. FORTEZZA Key Exchange Message

 Under FORTEZZA, the client derives a token encryption key (TEK) using
 the FORTEZZA Key Exchange Algorithm (KEA).  The client's KEA
 calculation uses the public key in the server's certificate along
 with private parameters in the client's token.  The client sends
 public parameters needed for the server to generate the TEK, using
 its own private parameters.  The client generates session keys, wraps
 them using the TEK, and sends the results to the server.  The client
 generates IVs for the session keys and TEK and sends them also.  The
 client generates a random 48-byte premaster secret, encrypts it using
 the TEK, and sends the result:

Freier, et al. Historic [Page 32] RFC 6101 The SSL Protocol Version 3.0 August 2011

      struct {
          opaque y_c<0..128>;
          opaque r_c[128];
          opaque y_signature[40];
          opaque wrapped_client_write_key[12];
          opaque wrapped_server_write_key[12];
          opaque client_write_iv[24];
          opaque server_write_iv[24];
          opaque master_secret_iv[24];
          block-ciphered opaque encrypted_pre_master_secret[48];
      } FortezzaKeys;
 y_signature:  y_signature is the signature of the KEA public key,
    signed with the client's DSS private key.
 y_c:  The client's Yc value (public key) for the KEA calculation.  If
    the client has sent a certificate, and its KEA public key is
    suitable, this value must be empty since the certificate already
    contains this value.  If the client sent a certificate without a
    suitable public key, y_c is used and y_signature is the KEA public
    key signed with the client's DSS private key.  For this value to
    be used, it must be between 64 and 128 bytes.
 r_c:  The client's Rc value for the KEA calculation.
 wrapped_client_write_key:  This is the client's write key, wrapped by
    the TEK.
 wrapped_server_write_key:  This is the server's write key, wrapped by
    the TEK.
 client_write_iv:  The IV for the client write key.
 server_write_iv:  The IV for the server write key.
 master_secret_iv:  This is the IV for the TEK used to encrypt the
    premaster secret.
 pre_master_secret:  A random value, generated by the client and used
    to generate the master secret, as specified in Section 6.1.  In
    the above structure, it is encrypted using the TEK.

Freier, et al. Historic [Page 33] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.6.7.3. Client Diffie-Hellman Public Value

 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.
      enum { implicit, explicit } PublicValueEncoding;
 implicit:  If the client certificate already contains the public
    value, then it is implicit and Yc does not need to be sent again.
 explicit:  Yc needs to be sent.
      struct {
          select (PublicValueEncoding) {
              case implicit: struct { };
              case explicit: opaque dh_Yc<1..2^16-1>;
          } dh_public;
      } ClientDiffieHellmanPublic;
 dh_Yc:  The client's Diffie-Hellman public value (Yc).

5.6.8. Certificate Verify

 This message is used to provide explicit verification of a client
 certificate.  This message is only sent following any client
 certificate that has signing capability (i.e., all certificates
 except those containing fixed Diffie-Hellman parameters).
        struct {
             Signature signature;
        } CertificateVerify;
      CertificateVerify.signature.md5_hash
                 MD5(master_secret + pad_2 +
                     MD5(handshake_messages + master_secret + pad_1));
      Certificate.signature.sha_hash
                 SHA(master_secret + pad_2 +
                     SHA(handshake_messages + master_secret + pad_1));
 pad_1:  This is identical to the pad_1 defined in Section 5.2.3.1.
 pad_2:  This is identical to the pad_2 defined in Section 5.2.3.1.
 Here, handshake_messages refers to all handshake messages starting at
 client hello up to but not including this message.

Freier, et al. Historic [Page 34] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.6.9. Finished

 A finished message is always sent immediately after a change cipher
 spec message to verify that the key exchange and authentication
 processes were successful.  The finished message is the first
 protected with the just-negotiated algorithms, keys, and secrets.  No
 acknowledgment of the finished message is required; parties may begin
 sending encrypted data immediately after sending the finished
 message.  Recipients of finished messages must verify that the
 contents are correct.
      enum { client(0x434C4E54), server(0x53525652) } Sender;
      struct {
          opaque md5_hash[16];
          opaque sha_hash[20];
      } Finished;
 md5_hash:  MD5(master_secret + pad2 + MD5(handshake_messages + Sender
    + master_secret + pad1));
 sha_hash:  SHA(master_secret + pad2 + SHA(handshake_messages + Sender
    + master_secret + pad1));
 handshake_messages:  All of the data from all handshake messages up
    to but not including this message.  This is only data visible at
    the handshake layer and does not include record layer headers.
 It is a fatal error if a finished message is not preceeded by a
 change cipher spec message at the appropriate point in the handshake.
 The hash contained in finished messages sent by the server
 incorporate Sender.server; those sent by the client incorporate
 Sender.client.  The value handshake_messages includes all handshake
 messages starting at client hello up to but not including this
 finished message.  This may be different from handshake_messages in
 Section 5.6.8 because it would include the certificate verify message
 (if sent).
 Note: Change cipher spec messages are not handshake messages and are
 not included in the hash computations.

Freier, et al. Historic [Page 35] RFC 6101 The SSL Protocol Version 3.0 August 2011

5.7. Application Data Protocol

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

6. Cryptographic Computations

 The key exchange, authentication, encryption, and MAC algorithms are
 determined by the cipher_suite selected by the server and revealed in
 the server hello message.

6.1. Asymmetric Cryptographic Computations

 The asymmetric algorithms are used in the handshake protocol to
 authenticate parties and to generate shared keys and secrets.
 For Diffie-Hellman, RSA, and FORTEZZA, the same algorithm is used to
 convert the pre_master_secret into the master_secret.  The
 pre_master_secret should be deleted from memory once the
 master_secret has been computed.
      master_secret =
        MD5(pre_master_secret + SHA('A' + pre_master_secret +
            ClientHello.random + ServerHello.random)) +
        MD5(pre_master_secret + SHA('BB' + pre_master_secret +
            ClientHello.random + ServerHello.random)) +
        MD5(pre_master_secret + SHA('CCC' + pre_master_secret +
            ClientHello.random + ServerHello.random));

6.1.1. 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.

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6.1.2. Diffie-Hellman

 A conventional Diffie-Hellman computation is performed.  The
 negotiated key (Z) is used as the pre_master_secret, and is converted
 into the master_secret, as specified above.
 Note: Diffie-Hellman parameters are specified by the server, and may
 be either ephemeral or contained within the server's certificate.

6.1.3. FORTEZZA

 A random 48-byte pre_master_secret is sent encrypted under the TEK
 and its IV.  The server decrypts the pre_master_secret and converts
 it into a master_secret, as specified above.  Bulk cipher keys and
 IVs for encryption are generated by the client's token and exchanged
 in the key exchange message; the master_secret is only used for MAC
 computations.

6.2. Symmetric Cryptographic Calculations and the CipherSpec

 The technique used to encrypt and verify the integrity of SSL records
 is specified by the currently active CipherSpec.  A typical example
 would be to encrypt data using DES and generate authentication codes
 using MD5.  The encryption and MAC algorithms are set to
 SSL_NULL_WITH_NULL_NULL at the beginning of the SSL handshake
 protocol, indicating that no message authentication or encryption is
 performed.  The handshake protocol is used to negotiate a more secure
 CipherSpec and to generate cryptographic keys.

6.2.1. The Master Secret

 Before secure encryption or integrity verification can be performed
 on records, the client and server need to generate shared secret
 information known only to themselves.  This value is a 48-byte
 quantity called the master secret.  The master secret is used to
 generate keys and secrets for encryption and MAC computations.  Some
 algorithms, such as FORTEZZA, may have their own procedure for
 generating encryption keys (the master secret is used only for MAC
 computations in FORTEZZA).

6.2.2. Converting the Master Secret into Keys and MAC Secrets

 The master secret is hashed into a sequence of secure bytes, which
 are assigned to the MAC secrets, keys, and non-export IVs required by
 the current CipherSpec (see Appendix A.7).  CipherSpecs require a
 client write MAC secret, a server write MAC secret, a client write
 key, a server write key, a client write IV, and a server write IV,
 which are generated from the master secret in that order.  Unused

Freier, et al. Historic [Page 37] RFC 6101 The SSL Protocol Version 3.0 August 2011

 values, such as FORTEZZA keys communicated in the KeyExchange
 message, are empty.  The following inputs are available to the key
 definition process:
        opaque MasterSecret[48]
        ClientHello.random
        ServerHello.random
 When generating keys and MAC secrets, the master secret is used as an
 entropy source, and the random values provide unencrypted salt
 material and IVs for exportable ciphers.
 To generate the key material, compute
      key_block =
        MD5(master_secret + SHA(`A' + master_secret +
                                ServerHello.random +
                                ClientHello.random)) +
        MD5(master_secret + SHA(`BB' + master_secret +
                                ServerHello.random +
                                ClientHello.random)) +
        MD5(master_secret + SHA(`CCC' + master_secret +
                                ServerHello.random +
                                ClientHello.random)) + [...];
 until enough output has been generated.  Then, the key_block is
 partitioned as follows.
      client_write_MAC_secret[CipherSpec.hash_size]
      server_write_MAC_secret[CipherSpec.hash_size]
      client_write_key[CipherSpec.key_material]
      server_write_key[CipherSpec.key_material]
      client_write_IV[CipherSpec.IV_size] /* non-export ciphers */
      server_write_IV[CipherSpec.IV_size] /* non-export ciphers */
 Any extra key_block material is discarded.
 Exportable encryption algorithms (for which CipherSpec.is_exportable
 is true) require additional processing as follows to derive their
 final write keys:
      final_client_write_key = MD5(client_write_key +
                                   ClientHello.random +
                                   ServerHello.random);
      final_server_write_key = MD5(server_write_key +
                                   ServerHello.random +
                                   ClientHello.random);

Freier, et al. Historic [Page 38] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Exportable encryption algorithms derive their IVs from the random
 messages:
      client_write_IV = MD5(ClientHello.random + ServerHello.random);
      server_write_IV = MD5(ServerHello.random + ClientHello.random);
 MD5 outputs are trimmed to the appropriate size by discarding the
 least-significant bytes.

6.2.2.1. Export Key Generation Example

 SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 requires five random bytes for
 each of the two encryption keys and 16 bytes for each of the MAC
 keys, for a total of 42 bytes of key material.  MD5 produces 16 bytes
 of output per call, so three calls to MD5 are required.  The MD5
 outputs are concatenated into a 48-byte key_block with the first MD5
 call providing bytes zero through 15, the second providing bytes 16
 through 31, etc.  The key_block is partitioned, and the write keys
 are salted because this is an exportable encryption algorithm.
      client_write_MAC_secret = key_block[0..15]
      server_write_MAC_secret = key_block[16..31]
      client_write_key      = key_block[32..36]
      server_write_key      = key_block[37..41]
      final_client_write_key = MD5(client_write_key +
                                   ClientHello.random +
                                   ServerHello.random)[0..15];
      final_server_write_key = MD5(server_write_key +
                                   ServerHello.random +
                                   ClientHello.random)[0..15];
      client_write_IV = MD5(ClientHello.random +
                            ServerHello.random)[0..7];
      server_write_IV = MD5(ServerHello.random +
                            ClientHello.random)[0..7];

7. Security Considerations

 See Appendix F.

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8. Informative References

 [DH1]      Diffie, W. and M. Hellman, "New Directions in
            Cryptography", IEEE Transactions on Information Theory V.
            IT-22, n. 6, pp. 74-84, June 1977.
 [SSL-2]    Hickman, K., "The SSL Protocol", February 1995.
 [3DES]     Tuchman, W., "Hellman Presents No Shortcut Solutions To
            DES", IEEE Spectrum, v. 16, n. 7, pp 40-41, July 1979.
 [DES]      ANSI X3.106, "American National Standard for Information
            Systems-Data Link Encryption", American National
            Standards Institute, 1983.
 [DSS]      NIST FIPS PUB 186, "Digital Signature Standard", National
            Institute of Standards and Technology U.S. Department of
            Commerce, May 1994.
 [FOR]      NSA X22, "FORTEZZA: Application Implementers Guide",
            Document # PD4002103-1.01, April 1995.
 [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
            STD 9, RFC 959, October 1985.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
            September 1981.
 [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
            Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
 [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992.
 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, September 1981.
 [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
            Specification", STD 8, RFC 854, May 1983.
 [RFC1832]  Srinivasan, R., "XDR: External Data Representation
            Standard", RFC 1832, August 1995.

Freier, et al. Historic [Page 40] RFC 6101 The SSL Protocol Version 3.0 August 2011

 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104,
            February 1997.
 [IDEA]     Lai, X., "On the Design and Security of Block Ciphers",
            ETH Series in Information Processing, v. 1, Konstanz:
            Hartung-Gorre Verlag, 1992.
 [PKCS1]    RSA Laboratories, "PKCS #1: RSA Encryption Standard
            version 1.5", November 1993.
 [PKCS6]    RSA Laboratories, "PKCS #6: RSA Extended Certificate
            Syntax Standard version 1.5", November 1993.
 [PKCS7]    RSA Laboratories, "PKCS #7: RSA Cryptographic Message
            Syntax Standard version 1.5", November 1993.
 [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
            Obtaining Digital Signatures and Public-Key
            Cryptosystems", Communications of the ACM v. 21, n. 2 pp.
            120-126., February 1978.
 [SCH]      Schneier, B., "Applied Cryptography: Protocols,
            Algorithms, and Source Code in C", John Wiley & Sons,
            1994.
 [SHA]      NIST FIPS PUB 180-1, "Secure Hash Standard", May 1994.
            National Institute of Standards and Technology, U.S.
            Department of Commerce, DRAFT
 [X509]     CCITT, "The Directory - Authentication Framework",
            Recommendation X.509 , 1988.
 [RSADSI]   RSA Data Security, Inc., "Unpublished works".

Freier, et al. Historic [Page 41] RFC 6101 The SSL Protocol Version 3.0 August 2011

Appendix A. Protocol Constant Values

 This section describes protocol types and constants.

A.1. Record Layer

      struct {
          uint8 major, minor;
      } ProtocolVersion;
      ProtocolVersion version = { 3,0 };
      enum {
          change_cipher_spec(20), alert(21), handshake(22),
          application_data(23), (255)
      } ContentType;
      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          opaque fragment[SSLPlaintext.length];
      } SSLPlaintext;
      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          opaque fragment[SSLCompressed.length];
      } SSLCompressed;
      struct {
          ContentType type;
          ProtocolVersion version;
          uint16 length;
          select (CipherSpec.cipher_type) {
              case stream: GenericStreamCipher;
              case block:  GenericBlockCipher;
          } fragment;
      } SSLCiphertext;
      stream-ciphered struct {
          opaque content[SSLCompressed.length];
          opaque MAC[CipherSpec.hash_size];
      } GenericStreamCipher;
      block-ciphered struct {
          opaque content[SSLCompressed.length];

Freier, et al. Historic [Page 42] RFC 6101 The SSL Protocol Version 3.0 August 2011

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

A.2. Change Cipher Specs Message

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

A.3. Alert Messages

      enum { warning(1), fatal(2), (255) } AlertLevel;
      enum {
          close_notify(0),
          unexpected_message(10),
          bad_record_mac(20),
          decompression_failure(30),
          handshake_failure(40),
          no_certificate(41),
          bad_certificate(42),
          unsupported_certificate(43),
          certificate_revoked(44),
          certificate_expired(45),
          certificate_unknown(46),
          illegal_parameter (47),
          (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_done(14),
        certificate_verify(15), client_key_exchange(16),
        finished(20), (255)
    } HandshakeType;
      struct {
          HandshakeType msg_type;
          uint24 length;
          select (HandshakeType) {
              case hello_request: HelloRequest;
              case client_hello: ClientHello;
              case server_hello: ServerHello;
              case certificate: Certificate;
              case server_key_exchange: ServerKeyExchange;
              case certificate_request: CertificateRequest;
              case server_done: ServerHelloDone;
              case certificate_verify: CertificateVerify;
              case client_key_exchange: ClientKeyExchange;
              case finished: Finished;
          } body;
      } Handshake;

A.4.1. Hello Messages

      struct { } HelloRequest;
      struct {
          uint32 gmt_unix_time;
          opaque random_bytes[28];
      } Random;
      opaque SessionID<0..32>;
      uint8 CipherSuite[2];
      enum { null(0), (255) } CompressionMethod;
      struct {
          ProtocolVersion client_version;
          Random random;
          SessionID session_id;
          CipherSuite cipher_suites<0..2^16-1>;
          CompressionMethod compression_methods<0..2^8-1>;

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

A.4.2. Server Authentication and Key Exchange Messages

      opaque ASN.1Cert<2^24-1>;
      struct {
          ASN.1Cert certificate_list<1..2^24-1>;
      } Certificate;
      enum { rsa, diffie_hellman, fortezza_kea } 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 {
          opaque r_s [128]
      } ServerFortezzaParams
      struct {
          select (KeyExchangeAlgorithm) {
              case diffie_hellman:
                  ServerDHParams params;
                  Signature signed_params;
              case rsa:
                  ServerRSAParams params;
                  Signature signed_params;
              case fortezza_kea:
                  ServerFortezzaParams params;
          };
      } ServerKeyExchange;

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      enum { anonymous, rsa, dsa } SignatureAlgorithm;
      digitally-signed struct {
          select(SignatureAlgorithm) {
              case anonymous: struct { };
              case rsa:
                  opaque md5_hash[16];
                  opaque sha_hash[20];
              case dsa:
                  opaque sha_hash[20];
          };
      } Signature;
      enum {
          RSA_sign(1), DSS_sign(2), RSA_fixed_DH(3),
          DSS_fixed_DH(4), RSA_ephemeral_DH(5), DSS_ephemeral_DH(6),
          FORTEZZA_MISSI(20), (255)
      } CertificateType;
      opaque DistinguishedName<1..2^16-1>;
      struct {
          CertificateType certificate_types<1..2^8-1>;
          DistinguishedName certificate_authorities<3..2^16-1>;
      } CertificateRequest;
      struct { } ServerHelloDone;

A.5. Client Authentication and Key Exchange Messages

      struct {
          select (KeyExchangeAlgorithm) {
              case rsa: EncryptedPreMasterSecret;
              case diffie_hellman: DiffieHellmanClientPublicValue;
              case fortezza_kea: FortezzaKeys;
          } exchange_keys;
      } ClientKeyExchange;
      struct {
          ProtocolVersion client_version;
          opaque random[46];
      } PreMasterSecret;
      struct {
          public-key-encrypted PreMasterSecret pre_master_secret;
      } EncryptedPreMasterSecret;

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      struct {
          opaque y_c<0..128>;
          opaque r_c[128];
          opaque y_signature[40];
          opaque wrapped_client_write_key[12];
          opaque wrapped_server_write_key[12];
          opaque client_write_iv[24];
          opaque server_write_iv[24];
          opaque master_secret_iv[24];
          opaque encrypted_preMasterSecret[48];
      } FortezzaKeys;
      enum { implicit, explicit } PublicValueEncoding;
      struct {
          select (PublicValueEncoding) {
              case implicit: struct {};
              case explicit: opaque DH_Yc<1..2^16-1>;
          } dh_public;
      } ClientDiffieHellmanPublic;
      struct {
          Signature signature;
      } CertificateVerify;

A.5.1. Handshake Finalization Message

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

A.6. The CipherSuite

 The following values define the CipherSuite codes used in the client
 hello and server hello messages.
 A CipherSuite defines a cipher specifications supported in SSL
 version 3.0.
   CipherSuite SSL_NULL_WITH_NULL_NULL                = { 0x00,0x00 };
 The following CipherSuite definitions require that the server provide
 an RSA certificate that can be used for key exchange.  The server may
 request either an RSA or a DSS signature-capable certificate in the
 certificate request message.

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   CipherSuite SSL_RSA_WITH_NULL_MD5                  = { 0x00,0x01 };
   CipherSuite SSL_RSA_WITH_NULL_SHA                  = { 0x00,0x02 };
   CipherSuite SSL_RSA_EXPORT_WITH_RC4_40_MD5         = { 0x00,0x03 };
   CipherSuite SSL_RSA_WITH_RC4_128_MD5               = { 0x00,0x04 };
   CipherSuite SSL_RSA_WITH_RC4_128_SHA               = { 0x00,0x05 };
   CipherSuite SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5     = { 0x00,0x06 };
   CipherSuite SSL_RSA_WITH_IDEA_CBC_SHA              = { 0x00,0x07 };
   CipherSuite SSL_RSA_EXPORT_WITH_DES40_CBC_SHA      = { 0x00,0x08 };
   CipherSuite SSL_RSA_WITH_DES_CBC_SHA               = { 0x00,0x09 };
   CipherSuite SSL_RSA_WITH_3DES_EDE_CBC_SHA          = { 0x00,0x0A };
 The following CipherSuite definitions are used for server-
 authenticated (and optionally client-authenticated) Diffie-Hellman.
 DH denotes cipher suites in which the server's certificate contains
 the Diffie-Hellman parameters signed by the certificate authority
 (CA).  DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman
 parameters are signed by a DSS or RSA certificate, which has been
 signed by the CA.  The signing algorithm used is specified after the
 DH or DHE parameter.  In all cases, the client must have the same
 type of certificate, and must use the Diffie-Hellman parameters
 chosen by the server.
   CipherSuite SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0B };
   CipherSuite SSL_DH_DSS_WITH_DES_CBC_SHA            = { 0x00,0x0C };
   CipherSuite SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x0D };
   CipherSuite SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA   = { 0x00,0x0E };
   CipherSuite SSL_DH_RSA_WITH_DES_CBC_SHA            = { 0x00,0x0F };
   CipherSuite SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x10 };
   CipherSuite SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x11 };
   CipherSuite SSL_DHE_DSS_WITH_DES_CBC_SHA           = { 0x00,0x12 };
   CipherSuite SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x13 };
   CipherSuite SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x14 };
   CipherSuite SSL_DHE_RSA_WITH_DES_CBC_SHA           = { 0x00,0x15 };
   CipherSuite SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x16 };
 The following cipher suites are used for completely anonymous Diffie-
 Hellman communications in which neither party is authenticated.  Note
 that this mode is vulnerable to man-in-the-middle attacks and is
 therefore strongly discouraged.
   CipherSuite SSL_DH_anon_EXPORT_WITH_RC4_40_MD5     = { 0x00,0x17 };
   CipherSuite SSL_DH_anon_WITH_RC4_128_MD5           = { 0x00,0x18 };
   CipherSuite SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA  = { 0x00,0x19 };
   CipherSuite SSL_DH_anon_WITH_DES_CBC_SHA           = { 0x00,0x1A };
   CipherSuite SSL_DH_anon_WITH_3DES_EDE_CBC_SHA      = { 0x00,0x1B };

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 The final cipher suites are for the FORTEZZA token.
   CipherSuite SSL_FORTEZZA_KEA_WITH_NULL_SHA         = { 0X00,0X1C };
   CipherSuite SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA = { 0x00,0x1D };
   CipherSuite SSL_FORTEZZA_KEA_WITH_RC4_128_SHA      = { 0x00,0x1E };
 Note: All cipher suites whose first byte is 0xFF are considered
 private and can be used for defining local/experimental algorithms.
 Interoperability of such types is a local matter.

A.7. The CipherSpec

 A cipher suite identifies a CipherSpec.  These structures are part of
 the SSL session state.  The CipherSpec includes:
      enum { stream, block } CipherType;
      enum { true, false } IsExportable;
      enum { null, rc4, rc2, des, 3des, des40, fortezza }
          BulkCipherAlgorithm;
      enum { null, md5, sha } MACAlgorithm;
      struct {
          BulkCipherAlgorithm bulk_cipher_algorithm;
          MACAlgorithm mac_algorithm;
          CipherType cipher_type;
          IsExportable is_exportable
          uint8 hash_size;
          uint8 key_material;
          uint8 IV_size;
      } CipherSpec;

Freier, et al. Historic [Page 49] RFC 6101 The SSL Protocol Version 3.0 August 2011

Appendix B. Glossary

 application protocol:  An application protocol is a protocol that
    normally layers directly on top of the transport layer (e.g.,
    TCP/IP [RFC0793]/[RFC0791]).  Examples include HTTP [RFC1945],
    TELNET [RFC0959], FTP [RFC0854], and SMTP.
 asymmetric cipher:  See public key cryptography.
 authentication:  Authentication is the ability of one entity to
    determine the identity of another entity.
 block cipher:  A block cipher is an algorithm that operates on
    plaintext in groups of bits, called blocks. 64 bits is a typical
    block size.
 bulk cipher:  A symmetric encryption algorithm used to encrypt large
    quantities of data.
 cipher block chaining (CBC) mode:  CBC is a mode in which every
    plaintext block encrypted with the block cipher is first
    exclusive-ORed with the previous ciphertext block (or, in the case
    of the first block, with the initialization vector).
 certificate:  As part of the X.509 protocol (a.k.a.  ISO
    Authentication framework), certificates are assigned by a trusted
    certificate authority and provide verification of a party's
    identity and may also supply its public key.
 client:  The application entity that initiates a connection to a
    server.
 client write key:  The key used to encrypt data written by the
    client.
 client write MAC secret:  The secret data used to authenticate data
    written by the client.
 connection:  A connection is a transport (in the OSI layering model
    definition) that provides a suitable type of service.  For SSL,
    such connections are peer-to-peer relationships.  The connections
    are transient.  Every connection is associated with one session.
 Data Encryption Standard (DES):  DES is a very widely used symmetric
    encryption algorithm.  DES is a block cipher [DES] [3DES].

Freier, et al. Historic [Page 50] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Digital Signature Standard:  (DSS) A standard for digital signing,
    including the Digital Signature Algorithm, approved by the
    National Institute of Standards and Technology, defined in NIST
    FIPS PUB 186, "Digital Signature Standard," published May, 1994 by
    the U.S. Dept. of Commerce.
 digital signatures:  Digital signatures utilize public key
    cryptography and one-way hash functions to produce a signature of
    the data that can be authenticated, and is difficult to forge or
    repudiate.
 FORTEZZA:  A PCMCIA card that provides both encryption and digital
    signing.
 handshake:  An initial negotiation between client and server that
    establishes the parameters of their transactions.
 Initialization Vector (IV):  When a block cipher is used in CBC mode,
    the initialization vector is exclusive-ORed with the first
    plaintext block prior to encryption.
 IDEA:  A 64-bit block cipher designed by Xuejia Lai and James Massey
    [IDEA].
 Message Authentication Code (MAC):  A Message Authentication Code is
    a one-way hash computed from a message and some secret data.  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 [RFC1321] is a secure hashing function that converts an
    arbitrarily long data stream into a digest of fixed size.
 public key cryptography:  A class of cryptographic techniques
    employing two-key ciphers.  Messages encrypted with the public key
    can only be decrypted with the associated private key.
    Conversely, messages signed with the private key can be verified
    with the public key.
 one-way hash function:  A one-way transformation that converts an
    arbitrary amount of data into a fixed-length hash.  It is
    computationally hard to reverse the transformation or to find
    collisions.  MD5 and SHA are examples of one-way hash functions.

Freier, et al. Historic [Page 51] RFC 6101 The SSL Protocol Version 3.0 August 2011

 RC2, RC4:  Proprietary bulk ciphers from RSA Data Security, Inc.
    (There is no good reference to these as they are unpublished
    works; however, see [RSADSI]).  RC2 is a block cipher and RC4 is a
    stream cipher.
 RSA:  A very widely used public key algorithm that can be used for
    either encryption or digital signing.
 salt:  Non-secret random data used to make export encryption keys
    resist precomputation attacks.
 server:  The server is the application entity that responds to
    requests for connections from clients.  The server is passive,
    waiting for requests from clients.
 session:  An SSL session is an association between a client and a
    server.  Sessions are created by the handshake protocol.  Sessions
    define a set of cryptographic security parameters, which can be
    shared among multiple connections.  Sessions are used to avoid the
    expensive negotiation of new security parameters for each
    connection.
 session identifier:  A session identifier is a value generated by a
    server that identifies a particular session.
 server write key:  The key used to encrypt data written by the
    server.
 server write MAC secret:  The secret data used to authenticate data
    written by the server.
 SHA:  The Secure Hash Algorithm is defined in FIPS PUB 180-1.  It
    produces a 20-byte output [SHA].
 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.

Freier, et al. Historic [Page 52] RFC 6101 The SSL Protocol Version 3.0 August 2011

Appendix C. CipherSuite Definitions

CipherSuite Is Key Cipher Hash

                           Exportable Exchange

SSL_NULL_WITH_NULL_NULL * NULL NULL NULL SSL_RSA_WITH_NULL_MD5 * RSA NULL MD5 SSL_RSA_WITH_NULL_SHA * RSA NULL SHA SSL_RSA_EXPORT_WITH_RC4_40_MD5 * RSA_EXPORT RC4_40 MD5 SSL_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 SSL_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5 * RSA_EXPORT RC2_CBC_40 MD5 SSL_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA SSL_RSA_EXPORT_WITH_DES40_CBC_SHA * RSA_EXPORT DES40_CBC SHA SSL_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA SSL_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA SSL_DH_DSS_EXPORT_WITH_DES40_CBC_SHA * DH_DSS_EXPORT DES40_CBC SHA SSL_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA SSL_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA SSL_DH_RSA_EXPORT_WITH_DES40_CBC_SHA * DH_RSA_EXPORT DES40_CBC SHA SSL_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA SSL_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA * DHE_DSS_EXPORT DES40_CBC SHA SSL_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA * DHE_RSA_EXPORT DES40_CBC SHA SSL_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA SSL_DH_anon_EXPORT_WITH_RC4_40_MD5 * DH_anon_EXPORT RC4_40 MD5 SSL_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 SSL_DH_anon_EXPORT_WITH_DES40_CBC_SHA DH_anon DES40_CBC SHA SSL_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA SSL_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA SSL_FORTEZZA_KEA_WITH_NULL_SHA FORTEZZA_KEA NULL SHA SSL_FORTEZZA_KEA_WITH_FORTEZZA_CBC_SHA FORTEZZA_KEA FORTEZZA_CBC SHA SSL_FORTEZZA_KEA_WITH_RC4_128_SHA FORTEZZA_KEA RC4_128 SHA

Freier, et al. Historic [Page 53] RFC 6101 The SSL Protocol Version 3.0 August 2011

 +----------------+------------------------------+-------------------+
 |  Key Exchange  |          Description         |   Key Size Limit  |
 |    Algorithm   |                              |                   |
 +----------------+------------------------------+-------------------+
 |     DHE_DSS    |     Ephemeral DH with DSS    |        None       |
 |                |          signatures          |                   |
 | DHE_DSS_EXPORT |     Ephemeral DH with DSS    |   DH = 512 bits   |
 |                |          signatures          |                   |
 |     DHE_RSA    |     Ephemeral DH with RSA    |        None       |
 |                |          signatures          |                   |
 | DHE_RSA_EXPORT |     Ephemeral DH with RSA    |   DH = 512 bits,  |
 |                |          signatures          |     RSA = none    |
 |     DH_anon    |  Anonymous DH, no signatures |        None       |
 | DH_anon_EXPORT |  Anonymous DH, no signatures |   DH = 512 bits   |
 |     DH_DSS     |       DH with DSS-based      |        None       |
 |                |         certificates         |                   |
 |  DH_DSS_EXPORT |       DH with DSS-based      |   DH = 512 bits   |
 |                |         certificates         |                   |
 |     DH_RSA     |       DH with RSA-based      |        None       |
 |                |         certificates         |                   |
 |  DH_RSA_EXPORT |       DH with RSA-based      |   DH = 512 bits,  |
 |                |         certificates         |     RSA = none    |
 |  FORTEZZA_KEA  |     FORTEZZA KEA. Details    |        N/A        |
 |                |          unpublished         |                   |
 |      NULL      |        No key exchange       |        N/A        |
 |       RSA      |       RSA key exchange       |        None       |
 |   RSA_EXPORT   |       RSA key exchange       |   RSA = 512 bits  |
 +----------------+------------------------------+-------------------+
                                Table 1
 Key size limit:  The key size limit gives the size of the largest
    public key that can be legally used for encryption in cipher
    suites that are exportable.

Freier, et al. Historic [Page 54] RFC 6101 The SSL Protocol Version 3.0 August 2011

 +--------------+--------+-----+-------+-------+-------+------+------+
 | Cipher       | Cipher | IsE |  Key  |  Exp. | Effec |  IV  | Bloc |
 |              |  Type  | xpo | Mater |  Key  |  tive | Size |   k  |
 |              |        | rta |  ial  | Mater |  Key  |      | Size |
 |              |        | ble |       |  ial  |  Bits |      |      |
 +--------------+--------+-----+-------+-------+-------+------+------+
 | NULL         | Stream |  *  |   0   |   0   |   0   |   0  |  N/A |
 | FORTEZZA_CBC |  Block |     |   NA  |   12  |   96  |  20  |   8  |
 |              |        |     |  (**) |  (**) |  (**) | (**) |      |
 | IDEA_CBC     |  Block |     |   16  |   16  |  128  |   8  |   8  |
 | RC2_CBC_40   |  Block |  *  |   5   |   16  |   40  |   8  |   8  |
 | RC4_40       | Stream |  *  |   5   |   16  |   40  |   0  |  N/A |
 | RC4_128      | Stream |     |   16  |   16  |  128  |   0  |  N/A |
 | DES40_CBC    |  Block |  *  |   5   |   8   |   40  |   8  |   8  |
 | DES_CBC      |  Block |     |   8   |   8   |   56  |   8  |   8  |
 | 3DES_EDE_CBC |  Block |     |   24  |   24  |  168  |   8  |   8  |
 +--------------+--------+-----+-------+-------+-------+------+------+
  • Indicates IsExportable is true.
  • * FORTEZZA uses its own key and IV generation algorithms.
                                Table 2
 Key Material:  The number of bytes from the key_block that are used
    for generating the write keys.
 Expanded Key Material:  The number of bytes actually fed into the
    encryption algorithm.
 Effective Key Bits:  How much entropy material is in the key material
    being fed into the encryption routines.
             +---------------+-----------+--------------+
             | Hash Function | Hash Size | Padding Size |
             +---------------+-----------+--------------+
             |      NULL     |     0     |       0      |
             |      MD5      |     16    |      48      |
             |      SHA      |     20    |      40      |
             +---------------+-----------+--------------+
                                Table 3

Freier, et al. Historic [Page 55] RFC 6101 The SSL Protocol Version 3.0 August 2011

Appendix D. Implementation Notes

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

D.1. Temporary RSA Keys

 US export restrictions limit RSA keys used for encryption to 512
 bits, but do not place any limit on lengths of RSA keys used for
 signing operations.  Certificates often need to be larger than 512
 bits, since 512-bit RSA keys are not secure enough for high-value
 transactions or for applications requiring long-term security.  Some
 certificates are also designated signing-only, in which case they
 cannot be used for key exchange.
 When the public key in the certificate cannot be used for encryption,
 the server signs a temporary RSA key, which is then exchanged.  In
 exportable applications, the temporary RSA key should be the maximum
 allowable length (i.e., 512 bits).  Because 512-bit RSA keys are
 relatively insecure, they should be changed often.  For typical
 electronic commerce applications, it is suggested that keys be
 changed daily or every 500 transactions, and more often if possible.
 Note that while it is acceptable to use the same temporary key for
 multiple transactions, it must be signed each time it is used.
 RSA key generation is a time-consuming process.  In many cases, a
 low-priority process can be assigned the task of key generation.
 Whenever a new key is completed, the existing temporary key can be
 replaced with the new one.

D.2. Random Number Generation and Seeding

 SSL 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.

Freier, et al. Historic [Page 56] RFC 6101 The SSL Protocol Version 3.0 August 2011

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

D.3. Certificates and Authentication

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

D.4. CipherSuites

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

D.5. FORTEZZA

 This section describes implementation details for cipher suites that
 make use of the FORTEZZA hardware encryption system.

D.5.1. Notes on Use of FORTEZZA Hardware

 A complete explanation of all issues regarding the use of FORTEZZA
 hardware is outside the scope of this document.  However, there are a
 few special requirements of SSL that deserve mention.
 Because SSL is a full duplex protocol, two crypto states must be
 maintained, one for reading and one for writing.  There are also a
 number of circumstances that can result in the crypto state in the
 FORTEZZA card being lost.  For these reasons, it's recommended that
 the current crypto state be saved after processing a record, and
 loaded before processing the next.

Freier, et al. Historic [Page 57] RFC 6101 The SSL Protocol Version 3.0 August 2011

 After the client generates the TEK, it also generates two message
 encryption keys (MEKs), one for reading and one for writing.  After
 generating each of these keys, the client must generate a
 corresponding IV and then save the crypto state.  The client also
 uses the TEK to generate an IV and encrypt the premaster secret.  All
 three IVs are sent to the server, along with the wrapped keys and the
 encrypted premaster secret in the client key exchange message.  At
 this point, the TEK is no longer needed, and may be discarded.
 On the server side, the server uses the master IV and the TEK to
 decrypt the premaster secret.  It also loads the wrapped MEKs into
 the card.  The server loads both IVs to verify that the IVs match the
 keys.  However, since the card is unable to encrypt after loading an
 IV, the server must generate a new IV for the server write key.  This
 IV is discarded.
 When encrypting the first encrypted record (and only that record),
 the server adds 8 bytes of random data to the beginning of the
 fragment.  These 8 bytes are discarded by the client after
 decryption.  The purpose of this is to synchronize the state on the
 client and server resulting from the different IVs.

D.5.2. FORTEZZA Cipher Suites

 5) FORTEZZA_NULL_WITH_NULL_SHA: Uses the full FORTEZZA key exchange,
 including sending server and client write keys and IVs.

D.5.3. FORTEZZA Session Resumption

 There are two possibilities for FORTEZZA session restart: 1) Never
 restart a FORTEZZA session. 2) Restart a session with the previously
 negotiated keys and IVs.
 Never restarting a FORTEZZA session:
 Clients who never restart FORTEZZA sessions should never send session
 IDs that were previously used in a FORTEZZA session as part of the
 ClientHello.  Servers who never restart FORTEZZA sessions should
 never send a previous session id on the ServerHello if the negotiated
 session is FORTEZZA.
 Restart a session:
 You cannot restart FORTEZZA on a session that has never done a
 complete FORTEZZA key exchange (that is, you cannot restart FORTEZZA
 if the session was an RSA/RC4 session renegotiated for FORTEZZA).  If
 you wish to restart a FORTEZZA session, you must save the MEKs and

Freier, et al. Historic [Page 58] RFC 6101 The SSL Protocol Version 3.0 August 2011

 IVs from the initial key exchange for this session and reuse them for
 any new connections on that session.  This is not recommended, but it
 is possible.

Appendix E. Version 2.0 Backward Compatibility

 Version 3.0 clients that support version 2.0 servers must send
 version 2.0 client hello messages [SSL-2].  Version 3.0 servers
 should accept either client hello format.  The only deviations from
 the version 2.0 specification are the ability to specify a version
 with a value of three and the support for more ciphering types in the
 CipherSpec.
 Warning: The ability to send version 2.0 client hello messages will
 be phased out with all due haste.  Implementers should make every
 effort to move forward as quickly as possible.  Version 3.0 provides
 better mechanisms for transitioning to newer versions.
 The following cipher specifications are carryovers from SSL version
 2.0.  These are assumed to use RSA for key exchange and
 authentication.
      V2CipherSpec SSL_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
      V2CipherSpec SSL_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
      V2CipherSpec SSL_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
      V2CipherSpec SSL_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
                                                 = { 0x04,0x00,0x80 };
      V2CipherSpec SSL_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
      V2CipherSpec SSL_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
      V2CipherSpec SSL_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
 Cipher specifications introduced in version 3.0 can be included in
 version 2.0 client hello messages using the syntax below.  Any
 V2CipherSpec element with its first byte equal to zero will be
 ignored by version 2.0 servers.  Clients sending any of the above
 V2CipherSpecs should also include the version 3.0 equivalent (see
 Appendix A.6):
      V2CipherSpec (see Version 3.0 name) = { 0x00, CipherSuite };

E.1. Version 2 Client Hello

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

Freier, et al. Historic [Page 59] RFC 6101 The SSL Protocol Version 3.0 August 2011

      uint8 V2CipherSpec[3];
      struct {
          unit8 msg_type;
          Version version;
          uint16 cipher_spec_length;
          uint16 session_id_length;
          uint16 challenge_length;
          V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length];
          opaque session_id[V2ClientHello.session_id_length];
          Random challenge;
      } V2ClientHello;
 session msg_type:  This field, in conjunction with the version field,
    identifies a version 2 client hello message.  The value should
    equal one (1).
 version:  The highest version of the protocol supported by the client
    (equals ProtocolVersion.version; see Appendix A.1).
 cipher_spec_length:  This field is the total length of the field
    cipher_specs.  It cannot be zero and must be a multiple of the
    V2CipherSpec length (3).
 session_id_length:  This field must have a value of either zero or
    16.  If zero, the client is creating a new session.  If 16, the
    session_id field will contain the 16 bytes of session
    identification.
 challenge_length:  The length in bytes of the client's challenge to
    the server to authenticate itself.  This value must be 32.
 cipher_specs:  This is a list of all CipherSpecs the client is
    willing and able to use.  There must be at least one CipherSpec
    acceptable to the server.
 session_id:  If this field's length is not zero, it will contain the
    identification for a session that the client wishes to resume.
 challenge:  The client's challenge to the server for the server to
    identify itself is a (nearly) arbitrary length random.  The
    version 3.0 server will right justify the challenge data to become
    the ClientHello.random data (padded with leading zeroes, if
    necessary), as specified in this version 3.0 protocol.  If the
    length of the challenge is greater than 32 bytes, then 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.

Freier, et al. Historic [Page 60] RFC 6101 The SSL Protocol Version 3.0 August 2011

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

E.2. Avoiding Man-in-the-Middle Version Rollback

 When SSL version 3.0 clients fall back to version 2.0 compatibility
 mode, they use special PKCS #1 block formatting.  This is done so
 that version 3.0 servers will reject version 2.0 sessions with
 version 3.0-capable clients.
 When version 3.0 clients are in version 2.0 compatibility mode, they
 set the right-hand (least-significant) 8 random bytes of the PKCS
 padding (not including the terminal null of the padding) for the RSA
 encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
 to 0x03 (the other padding bytes are random).  After decrypting the
 ENCRYPTED-KEY-DATA field, servers that support SSL 3.0 should issue
 an error if these eight padding bytes are 0x03.  Version 2.0 servers
 receiving blocks padded in this manner will proceed normally.

Appendix F. Security Analysis

 The SSL 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 SSL has been designed to resist a variety
 of attacks.

F.1. Handshake Protocol

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

F.1.1. Authentication and Key Exchange

 SSL supports three authentication modes: authentication of both
 parties, server authentication with an unauthenticated client, and
 total anonymity.  Whenever the server is authenticated, the channel
 should be secure against man-in-the-middle attacks, but completely
 anonymous sessions are inherently vulnerable to such attacks.

Freier, et al. Historic [Page 61] RFC 6101 The SSL Protocol Version 3.0 August 2011

 Anonymous servers cannot authenticate clients, since the client
 signature in the certificate verify message may require a server
 certificate to bind the signature to a particular server.  If the
 server is authenticated, its certificate message must provide a valid
 certificate chain leading to an acceptable certificate authority.
 Similarly, authenticated clients must supply an acceptable
 certificate to the server.  Each party is responsible for verifying
 that the other's certificate is valid and has not expired or been
 revoked.
 The general goal of the key exchange process is to create a
 pre_master_secret known to the communicating parties and not to
 attackers.  The pre_master_secret will be used to generate the
 master_secret (see Section 6.1).  The master_secret is required to
 generate the finished messages, encryption keys, and MAC secrets (see
 Sections 5.6.9 and 6.2.2).  By sending a correct finished message,
 parties thus prove that they know the correct pre_master_secret.

F.1.1.1. Anonymous Key Exchange

 Completely anonymous sessions can be established using RSA, Diffie-
 Hellman, or FORTEZZA for key exchange.  With anonymous RSA, the
 client encrypts a pre_master_secret with the server's uncertified
 public key extracted from the server key exchange message.  The
 result is sent in a client key exchange message.  Since eavesdroppers
 do not know the server's private key, it will be infeasible for them
 to decode the pre_master_secret.
 With Diffie-Hellman or FORTEZZA, 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) or the FORTEZZA token encryption
 key (TEK).
 Warning: Completely anonymous connections only provide protection
 against passive eavesdropping.  Unless an independent tamper-proof
 channel is used to verify that the finished messages were not
 replaced by an attacker, server authentication is required in
 environments where active man-in-the-middle attacks are a concern.

F.1.1.2. RSA Key Exchange and Authentication

 With RSA, key exchange and server authentication are combined.  The
 public key either may be contained in the server's certificate or may
 be a temporary RSA key sent in a server key exchange message.  When
 temporary RSA keys are used, they are signed by the server's RSA or
 DSS certificate.  The signature includes the current

Freier, et al. Historic [Page 62] RFC 6101 The SSL Protocol Version 3.0 August 2011

 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.
 After verifying the server's certificate, the client encrypts a
 pre_master_secret with the server's public key.  By successfully
 decoding the pre_master_secret and producing a correct finished
 message, the server demonstrates that it knows the private key
 corresponding to the server certificate.
 When RSA is used for key exchange, clients are authenticated using
 the certificate verify message (see Section 5.6.8).  The client signs
 a value derived from the master_secret and all preceding handshake
 messages.  These handshake messages include the server certificate,
 which binds the signature to the server, and ServerHello.random,
 which binds the signature to the current handshake process.

F.1.1.3. Diffie-Hellman Key Exchange with Authentication

 When Diffie-Hellman key exchange is used, the server either can
 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.

Freier, et al. Historic [Page 63] RFC 6101 The SSL Protocol Version 3.0 August 2011

F.1.1.4. FORTEZZA

 FORTEZZA's design is classified, but at the protocol level it is
 similar to Diffie-Hellman with fixed public values contained in
 certificates.  The result of the key exchange process is the token
 encryption key (TEK), which is used to wrap data encryption keys,
 client write key, server write key, and master secret encryption key.
 The data encryption keys are not derived from the pre_master_secret
 because unwrapped keys are not accessible outside the token.  The
 encrypted pre_master_secret is sent to the server in a client key
 exchange message.

F.1.2. Version Rollback Attacks

 Because SSL version 3.0 includes substantial improvements over SSL
 version 2.0, attackers may try to make version 3.0-capable clients
 and servers fall back to version 2.0.  This attack is occurring if
 (and only if) two version 3.0-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,
 since this is essentially equivalent to increasing the input block
 size by 8 bytes.

F.1.3. Detecting Attacks against the Handshake Protocol

 An attacker might try to influence the handshake exchange to make the
 parties select different encryption algorithms than they would
 normally choose.  Because many implementations will support 40-bit
 exportable encryption and some may even support null encryption or
 MAC algorithms, this attack is of particular concern.
 For this attack, an attacker must actively change one or more
 handshake messages.  If this occurs, the client and server will
 compute different values for the handshake message hashes.  As a
 result, the parties will not accept each other's finished messages.
 Without the master_secret, the attacker cannot repair the finished
 messages, so the attack will be discovered.

Freier, et al. Historic [Page 64] RFC 6101 The SSL Protocol Version 3.0 August 2011

F.1.4. Resuming Sessions

 When a connection is established by resuming a session, new
 ClientHello.random and ServerHello.random values are hashed with the
 session's master_secret.  Provided that the master_secret has not
 been compromised and that the secure hash operations used to produce
 the encryption keys and MAC secrets are secure, the connection should
 be secure and effectively independent from previous connections.
 Attackers cannot use known encryption keys or MAC secrets to
 compromise the master_secret without breaking the secure hash
 operations (which use both SHA and MD5).
 Sessions cannot be resumed unless both the client and server agree.
 If either party suspects that the session may have been compromised,
 or that certificates may have expired or been revoked, it should
 force a full handshake.  An upper limit of 24 hours is suggested for
 session ID lifetimes, since an attacker who obtains a master_secret
 may be able to impersonate the compromised party until the
 corresponding session ID is retired.  Applications that may be run in
 relatively insecure environments should not write session IDs to
 stable storage.

F.1.5. MD5 and SHA

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

F.2. Protecting Application Data

 The master_secret is hashed with the ClientHello.random and
 ServerHello.random to produce unique data encryption keys and MAC
 secrets for each connection.  FORTEZZA encryption keys are generated
 by the token, and are not derived from the master_secret.
 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 SSL
 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.

Freier, et al. Historic [Page 65] RFC 6101 The SSL Protocol Version 3.0 August 2011

 If an attacker does break an encryption key, all messages encrypted
 with it can be read.  Similarly, compromise of a MAC key can make
 message modification attacks possible.  Because MACs are also
 encrypted, message-alteration attacks generally require breaking the
 encryption algorithm as well as the MAC.
 Note: MAC secrets may be larger than encryption keys, so messages can
 remain tamper resistant even if encryption keys are broken.

F.3. Final Notes

 For SSL to be able to provide a secure connection, both the client
 and server systems, keys, and applications must be secure.  In
 addition, the implementation must be free of security errors.
 The system is only as strong as the weakest key exchange and
 authentication algorithm supported, and only trustworthy
 cryptographic functions should be used.  Short public keys, 40-bit
 bulk encryption keys, and anonymous servers should be used with great
 caution.  Implementations and users must be careful when deciding
 which certificates and certificate authorities are acceptable; a
 dishonest certificate authority can do tremendous damage.

Appendix G. Acknowledgements

G.1. Other Contributors

 Martin Abadi                  Robert Relyea
 Digital Equipment Corporation Netscape Communications
 ma@pa.dec.com                 relyea@netscape.com
 Taher Elgamal                 Jim Roskind
 Netscape Communications       Netscape Communications
 elgamal@netscape.com          jar@netscape.com
 Anil Gangolli                 Micheal J. Sabin, Ph.D.
 Netscape Communications       Consulting Engineer
 gangolli@netscape.com         msabin@netcom.com
 Kipp E.B. Hickman             Tom Weinstein
 Netscape Communications       Netscape Communications
 kipp@netscape.com             tomw@netscape.com

Freier, et al. Historic [Page 66] RFC 6101 The SSL Protocol Version 3.0 August 2011

G.2. Early Reviewers

 Robert Baldwin                Clyde Monma
 RSA Data Security, Inc.       Bellcore
 baldwin@rsa.com               clyde@bellcore.com
 George Cox                    Eric Murray
 Intel Corporation             ericm@lne.com
 cox@ibeam.jf.intel.com
 Cheri Dowell                  Avi Rubin
 Sun Microsystems              Bellcore
 cheri@eng.sun.com             rubin@bellcore.com
 Stuart Haber                  Don Stephenson
 Bellcore                      Sun Microsystems
 stuart@bellcore.com           don.stephenson@eng.sun.com
 Burt Kaliski                  Joe Tardo
 RSA Data Security, Inc.       General Magic
 burt@rsa.com                  tardo@genmagic.com

Authors' Addresses

 Alan O. Freier
 Netscape Communications
 Philip Karlton
 Netscape Communications
 Paul C. Kocher
 Independent Consultant

Freier, et al. Historic [Page 67]

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