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rfc:rfc2759

Network Working Group G. Zorn Request for Comments: 2759 Microsoft Corporation Category: Informational January 2000

              Microsoft PPP CHAP Extensions, Version 2

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 The Point-to-Point Protocol (PPP) [1] provides a standard method for
 transporting multi-protocol datagrams over point-to-point links.  PPP
 defines an extensible Link Control Protocol and a family of Network
 Control Protocols (NCPs) for establishing and configuring different
 network-layer protocols.
 This document describes version two of Microsoft's PPP CHAP dialect
 (MS-CHAP-V2).  MS-CHAP-V2 is similar to, but incompatible with, MS-
 CHAP version one (MS-CHAP-V1, described in [9]).  In particular,
 certain protocol fields have been deleted or reused but with
 different semantics.  In addition, MS-CHAP-V2 features mutual
 authentication.
 The algorithms used in the generation of various MS-CHAP-V2 protocol
 fields are described in section 8.  Negotiation and hash generation
 examples are provided in section 9.

Specification of Requirements

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as
 described in [3].

Zorn Informational [Page 1] RFC 2759 Microsoft MS-CHAP-V2 January 2000

Table of Contents

 1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2. LCP Configuration . . . . . . . . . . . . . . . . . . . . . . .  3
 3. Challenge Packet  . . . . . . . . . . . . . . . . . . . . . . .  3
 4. Response Packet . . . . . . . . . . . . . . . . . . . . . . . .  4
 5. Success Packet  . . . . . . . . . . . . . . . . . . . . . . . .  4
 6. Failure Packet  . . . . . . . . . . . . . . . . . . . . . . . .  5
 7. Change-Password Packet  . . . . . . . . . . . . . . . . . . . .  6
 8. Pseudocode  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
 8.1. GenerateNTResponse()  . . . . . . . . . . . . . . . . . . . .  7
 8.2. ChallengeHash() . . . . . . . . . . . . . . . . . . . . . . .  8
 8.3. NtPasswordHash()  . . . . . . . . . . . . . . . . . . . . . .  9
 8.4. HashNtPasswordHash()  . . . . . . . . . . . . . . . . . . . .  9
 8.5. ChallengeResponse() . . . . . . . . . . . . . . . . . . . . .  9
 8.6. DesEncrypt()  . . . . . . . . . . . . . . . . . . . . . . . . 10
 8.7. GenerateAuthenticatorResponse() . . . . . . . . . . . . . . . 10
 8.8. CheckAuthenticatorResponse()  . . . . . . . . . . . . . . . . 12
 8.9. NewPasswordEncryptedWithOldNtPasswordHash() . . . . . . . . . 12
 8.10. EncryptPwBlockWithPasswordHash() . . . . . . . . . . . . . . 13
 8.11. Rc4Encrypt() . . . . . . . . . . . . . . . . . . . . . . . . 13
 8.12. OldNtPasswordHashEncryptedWithNewNtPasswordHash()  . . . . . 14
 8.13. NtPasswordHashEncryptedWithBlock() . . . . . . . . . . . . . 14
 9. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
 9.1. Negotiation Examples  . . . . . . . . . . . . . . . . . . . . 14
 9.1.1. Successful authentication . . . . . . . . . . . . . . . . . 15
 9.1.2. Authenticator authentication failure  . . . . . . . . . . . 15
 9.1.3. Failed authentication with no retry allowed . . . . . . . . 15
 9.1.4. Successful authentication after retry . . . . . . . . . . . 15
 9.1.5. Failed hack attack with 3 attempts allowed  . . . . . . . . 15
 9.1.6. Successful authentication with password change  . . . . . . 16
 9.1.7. Successful authentication with retry and password change. . 16
 9.2. Hash Example  . . . . . . . . . . . . . . . . . . . . . . . . 16
 9.3. Example of DES Key Generation . . . . . . . . . . . . . . . . 17
 10. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
 13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
 14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 20

Zorn Informational [Page 2] RFC 2759 Microsoft MS-CHAP-V2 January 2000

1. Introduction

 Where possible, MS-CHAP-V2 is consistent with both MS-CHAP-V1 and
 standard CHAP.  Briefly, the differences between MS-CHAP-V2 and MS-
 CHAP-V1 are:
  • MS-CHAP-V2 is enabled by negotiating CHAP Algorithm 0x81 in LCP

option 3, Authentication Protocol.

  • MS-CHAP-V2 provides mutual authentication between peers by

piggybacking a peer challenge on the Response packet and an

    authenticator response on the Success packet.
  • The calculation of the "Windows NT compatible challenge response"

sub-field in the Response packet has been changed to include the

    peer challenge and the user name.
  • In MS-CHAP-V1, the "LAN Manager compatible challenge response"

sub-field was always sent in the Response packet. This field has

    been replaced in MS-CHAP-V2 by the Peer-Challenge field.
  • The format of the Message field in the Failure packet has been

changed.

  • The Change Password (version 1) and Change Password (version 2)

packets are no longer supported. They have been replaced with a

    single Change-Password packet.

2. LCP Configuration

 The LCP configuration for MS-CHAP-V2 is identical to that for
 standard CHAP, except that the Algorithm field has value 0x81, rather
 than the MD5 value 0x05.  PPP implementations which do not support
 MS-CHAP-V2, but correctly implement LCP Config-Rej, should have no
 problem dealing with this non-standard option.

3. Challenge Packet

 The MS-CHAP-V2 Challenge packet is identical in format to the
 standard CHAP Challenge packet.
 MS-CHAP-V2 authenticators send an 16-octet challenge Value field.
 Peers need not duplicate Microsoft's algorithm for selecting the 16-
 octet value, but the standard guidelines on randomness [1,2,7] SHOULD
 be observed.
 Microsoft authenticators do not currently provide information in the
 Name field.  This may change in the future.

Zorn Informational [Page 3] RFC 2759 Microsoft MS-CHAP-V2 January 2000

4. Response Packet

 The MS-CHAP-V2 Response packet is identical in format to the standard
 CHAP Response packet.  However, the Value field is sub-formatted
 differently as follows:
 16 octets: Peer-Challenge
  8 octets: Reserved, must be zero
 24 octets: NT-Response
  1 octet : Flags
 The Peer-Challenge field is a 16-octet random number.  As the name
 implies, it is generated by the peer and is used in the calculation
 of the NT-Response field, below.  Peers need not duplicate
 Microsoft's algorithm for selecting the 16-octet value, but the
 standard guidelines on randomness [1,2,7] SHOULD be observed.
 The NT-Response field is an encoded function of the password, the
 user name, the contents of the Peer-Challenge field and the received
 challenge as output by the routine GenerateNTResponse() (see section
 8.1, below).  The Windows NT password is a string of 0 to
 (theoretically) 256 case-sensitive Unicode [8] characters.  Current
 versions of Windows NT limit passwords to 14 characters, mainly for
 compatibility reasons; this may change in the future.  When computing
 the NT-Response field contents, only the user name is used, without
 any associated Windows NT domain name.  This is true regardless of
 whether a Windows NT domain name is present in the Name field (see
 below).
 The Flag field is reserved for future use and MUST be zero.
 The Name field is a string of 0 to (theoretically) 256 case-sensitive
 ASCII characters which identifies the peer's user account name.  The
 Windows NT domain name may prefix the user's account name (e.g.
 "BIGCO\johndoe" where "BIGCO" is a Windows NT domain containing the
 user account "johndoe").  If a domain is not provided, the backslash
 should also be omitted, (e.g. "johndoe").

5. Success Packet

 The Success packet is identical in format to the standard CHAP
 Success packet.  However, the Message field contains a 42-octet
 authenticator response string and a printable message.  The format of
 the message field is illustrated below.
 "S=<auth_string> M=<message>"

Zorn Informational [Page 4] RFC 2759 Microsoft MS-CHAP-V2 January 2000

 The <auth_string> quantity is a 20 octet number encoded in ASCII as
 40 hexadecimal digits.  The hexadecimal digits A-F (if present) MUST
 be uppercase.  This number is derived from the challenge from the
 Challenge packet, the Peer-Challenge and NT-Response fields from the
 Response packet, and the peer password as output by the routine
 GenerateAuthenticatorResponse() (see section 8.7, below).  The
 authenticating peer MUST verify the authenticator response when a
 Success packet is received.  The method for verifying the
 authenticator is described in section 8.8, below.  If the
 authenticator response is either missing or incorrect, the peer MUST
 end the session.
 The <message> quantity is human-readable text in the appropriate
 charset and language [12].

6. Failure Packet

 The Failure packet is identical in format to the standard CHAP
 Failure packet.  There is, however, formatted text stored in the
 Message field which, contrary to the standard CHAP rules, does affect
 the operation of the protocol.  The Message field format is:
    "E=eeeeeeeeee R=r C=cccccccccccccccccccccccccccccccc V=vvvvvvvvvv

M=<msg>"

    where
    The "eeeeeeeeee" is the ASCII representation of a decimal error
    code (need not be 10 digits) corresponding to one of those listed
    below, though implementations should deal with codes not on this
    list gracefully.
       646 ERROR_RESTRICTED_LOGON_HOURS
       647 ERROR_ACCT_DISABLED
       648 ERROR_PASSWD_EXPIRED
       649 ERROR_NO_DIALIN_PERMISSION
       691 ERROR_AUTHENTICATION_FAILURE
       709 ERROR_CHANGING_PASSWORD
    The "r" is an ASCII flag set to '1' if a retry is allowed, and '0'
    if not.  When the authenticator sets this flag to '1' it disables
    short timeouts, expecting the peer to prompt the user for new
    credentials and resubmit the response.
    The "cccccccccccccccccccccccccccccccc" is the ASCII representation
    of a hexadecimal challenge value.  This field MUST be exactly 32
    octets long and MUST be present.

Zorn Informational [Page 5] RFC 2759 Microsoft MS-CHAP-V2 January 2000

    The "vvvvvvvvvv" is the ASCII representation of a decimal version
    code (need not be 10 digits) indicating the password changing
    protocol version supported on the server.  For MS-CHAP-V2, this
    value SHOULD always be 3.
    <msg> is human-readable text in the appropriate charset and
    language [12].

7. Change-Password Packet

 The Change-Password packet does not appear in either standard CHAP or
 MS-CHAP-V1.  It allows the peer to change the password on the account
 specified in the preceding Response packet.  The Change-Password
 packet should be sent only if the authenticator reports
 ERROR_PASSWD_EXPIRED (E=648) in the Message field of the Failure
 packet.
 This packet type is supported by recent versions of Windows NT 4.0,
 Windows 95 and Windows 98.  It is not supported by Windows NT 3.5,
 Windows NT 3.51, or early versions of Windows NT 4.0, Windows 95 and
 Windows 98.
 The format of this packet is as follows:
      1 octet  : Code
      1 octet  : Identifier
      2 octets : Length
    516 octets : Encrypted-Password
     16 octets : Encrypted-Hash
     16 octets : Peer-Challenge
      8 octets : Reserved
     24 octets : NT-Response
      2-octet  : Flags
 Code
    7
 Identifier
    The Identifier field is one octet and aids in matching requests
    and replies.  The value is the Identifier of the received Failure
    packet to which this packet responds plus 1.
 Length
    586

Zorn Informational [Page 6] RFC 2759 Microsoft MS-CHAP-V2 January 2000

 Encrypted-Password
    This field contains the PWBLOCK form of the new Windows NT
    password encrypted with the old Windows NT password hash, as
    output by the NewPasswordEncryptedWithOldNtPasswordHash() routine
    (see section 8.9, below).
 Encrypted-Hash
    This field contains the old Windows NT password hash encrypted
    with the new Windows NT password hash, as output by the
    OldNtPasswordHashEncryptedWithNewNtPasswordHash() routine (see
    section 8.12, below).
 Peer-Challenge
    A 16-octet random quantity, as described in the Response packet
    description.
 Reserved
    8 octets, must be zero.
 NT-Response
    The NT-Response field (as described in the Response packet
    description), but calculated on the new password and the challenge
    received in the Failure packet.
 Flags
    This field is two octets in length.  It is a bit field of option
    flags where 0 is the least significant bit of the 16-bit quantity.
    The format of this field is illustrated in the following diagram:
                  1
        5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Bits 0-15
          Reserved, always clear (0).

8. Pseudocode

 The routines mentioned in the text above are described in pseudocode
 in the following sections.

8.1. GenerateNTResponse()

 GenerateNTResponse(
 IN  16-octet              AuthenticatorChallenge,
 IN  16-octet              PeerChallenge,

Zorn Informational [Page 7] RFC 2759 Microsoft MS-CHAP-V2 January 2000

 IN  0-to-256-char         UserName,
 IN  0-to-256-unicode-char Password,
 OUT 24-octet              Response )
 {
    8-octet  Challenge
    16-octet PasswordHash
    ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName,
                   giving Challenge)
    NtPasswordHash( Password, giving PasswordHash )
    ChallengeResponse( Challenge, PasswordHash, giving Response )
 }

8.2. ChallengeHash()

 ChallengeHash(
 IN 16-octet               PeerChallenge,
 IN 16-octet               AuthenticatorChallenge,
 IN  0-to-256-char         UserName,
 OUT 8-octet               Challenge
 {
    /*
     * SHAInit(), SHAUpdate() and SHAFinal() functions are an
     * implementation of Secure Hash Algorithm (SHA-1) [11]. These are
     * available in public domain or can be licensed from
     * RSA Data Security, Inc.
     */
    SHAInit(Context)
    SHAUpdate(Context, PeerChallenge, 16)
    SHAUpdate(Context, AuthenticatorChallenge, 16)
    /*
     * Only the user name (as presented by the peer and
     * excluding any prepended domain name)
     * is used as input to SHAUpdate().
     */
    SHAUpdate(Context, UserName, strlen(Username))
    SHAFinal(Context, Digest)
    memcpy(Challenge, Digest, 8)
 }

Zorn Informational [Page 8] RFC 2759 Microsoft MS-CHAP-V2 January 2000

8.3. NtPasswordHash()

 NtPasswordHash(
 IN  0-to-256-unicode-char Password,
 OUT 16-octet              PasswordHash )
 {
    /*
     * Use the MD4 algorithm [5] to irreversibly hash Password
     * into PasswordHash.  Only the password is hashed without
     * including any terminating 0.
     */
 }

8.4. HashNtPasswordHash()

 HashNtPasswordHash(
 IN  16-octet PasswordHash,
 OUT 16-octet PasswordHashHash )
 {
    /*
     * Use the MD4 algorithm [5] to irreversibly hash
     * PasswordHash into PasswordHashHash.
     */
 }

8.5. ChallengeResponse()

 ChallengeResponse(
 IN  8-octet  Challenge,
 IN  16-octet PasswordHash,
 OUT 24-octet Response )
 {
    Set ZPasswordHash to PasswordHash zero-padded to 21 octets
    DesEncrypt( Challenge,
                1st 7-octets of ZPasswordHash,
                giving 1st 8-octets of Response )
    DesEncrypt( Challenge,
                2nd 7-octets of ZPasswordHash,
                giving 2nd 8-octets of Response )
    DesEncrypt( Challenge,
                3rd 7-octets of ZPasswordHash,
                giving 3rd 8-octets of Response )
 }

Zorn Informational [Page 9] RFC 2759 Microsoft MS-CHAP-V2 January 2000

8.6. DesEncrypt()

 DesEncrypt(
 IN  8-octet Clear,
 IN  7-octet Key,
 OUT 8-octet Cypher )
 {
    /*
     * Use the DES encryption algorithm [4] in ECB mode [10]
     * to encrypt Clear into Cypher such that Cypher can
     * only be decrypted back to Clear by providing Key.
     * Note that the DES algorithm takes as input a 64-bit
     * stream where the 8th, 16th, 24th, etc.  bits are
     * parity bits ignored by the encrypting algorithm.
     * Unless you write your own DES to accept 56-bit input
     * without parity, you will need to insert the parity bits
     * yourself.
     */
 }

8.7. GenerateAuthenticatorResponse()

 GenerateAuthenticatorResponse(
 IN  0-to-256-unicode-char Password,
 IN  24-octet              NT-Response,
 IN  16-octet              PeerChallenge,
 IN  16-octet              AuthenticatorChallenge,
 IN  0-to-256-char         UserName,
 OUT 42-octet              AuthenticatorResponse )
 {
    16-octet              PasswordHash
    16-octet              PasswordHashHash
    8-octet               Challenge
    /*
     * "Magic" constants used in response generation
     */
    Magic1[39] =
       {0x4D, 0x61, 0x67, 0x69, 0x63, 0x20, 0x73, 0x65, 0x72, 0x76,
        0x65, 0x72, 0x20, 0x74, 0x6F, 0x20, 0x63, 0x6C, 0x69, 0x65,
        0x6E, 0x74, 0x20, 0x73, 0x69, 0x67, 0x6E, 0x69, 0x6E, 0x67,
        0x20, 0x63, 0x6F, 0x6E, 0x73, 0x74, 0x61, 0x6E, 0x74};

Zorn Informational [Page 10] RFC 2759 Microsoft MS-CHAP-V2 January 2000

    Magic2[41] =
       {0x50, 0x61, 0x64, 0x20, 0x74, 0x6F, 0x20, 0x6D, 0x61, 0x6B,
        0x65, 0x20, 0x69, 0x74, 0x20, 0x64, 0x6F, 0x20, 0x6D, 0x6F,
        0x72, 0x65, 0x20, 0x74, 0x68, 0x61, 0x6E, 0x20, 0x6F, 0x6E,
        0x65, 0x20, 0x69, 0x74, 0x65, 0x72, 0x61, 0x74, 0x69, 0x6F,
        0x6E};
    /*
     * Hash the password with MD4
     */
    NtPasswordHash( Password, giving PasswordHash )
    /*
     * Now hash the hash
     */
    HashNtPasswordHash( PasswordHash, giving PasswordHashHash)
    SHAInit(Context)
    SHAUpdate(Context, PasswordHashHash, 16)
    SHAUpdate(Context, NTResponse, 24)
    SHAUpdate(Context, Magic1, 39)
    SHAFinal(Context, Digest)
    ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName,
                   giving Challenge)
    SHAInit(Context)
    SHAUpdate(Context, Digest, 20)
    SHAUpdate(Context, Challenge, 8)
    SHAUpdate(Context, Magic2, 41)
    SHAFinal(Context, Digest)
    /*
     * Encode the value of 'Digest' as "S=" followed by
     * 40 ASCII hexadecimal digits and return it in
     * AuthenticatorResponse.
     * For example,
     *   "S=0123456789ABCDEF0123456789ABCDEF01234567"
     */
 }

Zorn Informational [Page 11] RFC 2759 Microsoft MS-CHAP-V2 January 2000

8.8. CheckAuthenticatorResponse()

 CheckAuthenticatorResponse(
 IN  0-to-256-unicode-char Password,
 IN  24-octet              NtResponse,
 IN  16-octet              PeerChallenge,
 IN  16-octet              AuthenticatorChallenge,
 IN  0-to-256-char         UserName,
 IN  42-octet              ReceivedResponse,
 OUT Boolean               ResponseOK )
 {
    20-octet MyResponse
    set ResponseOK = FALSE
    GenerateAuthenticatorResponse( Password, NtResponse, PeerChallenge,
                                   AuthenticatorChallenge, UserName,
                                   giving MyResponse)
    if (MyResponse = ReceivedResponse) then set ResponseOK = TRUE
    return ResponseOK
 }

8.9. NewPasswordEncryptedWithOldNtPasswordHash()

 datatype-PWBLOCK
 {
    256-unicode-char Password
    4-octets         PasswordLength
 }
 NewPasswordEncryptedWithOldNtPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT datatype-PWBLOCK      EncryptedPwBlock )
 {
    NtPasswordHash( OldPassword, giving PasswordHash )
    EncryptPwBlockWithPasswordHash( NewPassword,
                                    PasswordHash,
                                    giving EncryptedPwBlock )
 }

Zorn Informational [Page 12] RFC 2759 Microsoft MS-CHAP-V2 January 2000

8.10. EncryptPwBlockWithPasswordHash()

 EncryptPwBlockWithPasswordHash(
 IN  0-to-256-unicode-char Password,
 IN  16-octet              PasswordHash,
 OUT datatype-PWBLOCK      PwBlock )
 {
    Fill ClearPwBlock with random octet values
       PwSize = lstrlenW( Password ) * sizeof( unicode-char )
       PwOffset = sizeof( ClearPwBlock.Password ) - PwSize
       Move PwSize octets to (ClearPwBlock.Password + PwOffset ) from
 Password
       ClearPwBlock.PasswordLength = PwSize
       Rc4Encrypt( ClearPwBlock,
                   sizeof( ClearPwBlock ),
                   PasswordHash,
                   sizeof( PasswordHash ),
                   giving PwBlock )
    }

8.11. Rc4Encrypt()

 Rc4Encrypt(
 IN  x-octet Clear,
 IN  integer ClearLength,
 IN  y-octet Key,
 IN  integer KeyLength,
 OUT x-octet Cypher )
 {
    /*
     * Use the RC4 encryption algorithm [6] to encrypt Clear of
     * length ClearLength octets into a Cypher of the same length
     * such that the Cypher can only be decrypted back to Clear
     * by providing a Key of length KeyLength octets.
     */
 }

Zorn Informational [Page 13] RFC 2759 Microsoft MS-CHAP-V2 January 2000

8.12. OldNtPasswordHashEncryptedWithNewNtPasswordHash()

 OldNtPasswordHashEncryptedWithNewNtPasswordHash(
 IN  0-to-256-unicode-char NewPassword,
 IN  0-to-256-unicode-char OldPassword,
 OUT 16-octet              EncryptedPasswordHash )
 {
    NtPasswordHash( OldPassword, giving OldPasswordHash )
    NtPasswordHash( NewPassword, giving NewPasswordHash )
    NtPasswordHashEncryptedWithBlock( OldPasswordHash,
                                      NewPasswordHash,
                                      giving EncryptedPasswordHash )
 }

8.13. NtPasswordHashEncryptedWithBlock()

 NtPasswordHashEncryptedWithBlock(
 IN  16-octet PasswordHash,
 IN  16-octet Block,
 OUT 16-octet Cypher )
 {
    DesEncrypt( 1st 8-octets PasswordHash,
                1st 7-octets Block,
                giving 1st 8-octets Cypher )
    DesEncrypt( 2nd 8-octets PasswordHash,
                2nd 7-octets Block,
                giving 2nd 8-octets Cypher )
 }

9. Examples

 The following sections include protocol negotiation and hash
 generation examples.

9.1. Negotiation Examples

 Here are some examples of typical negotiations.  The peer is on the
 left and the authenticator is on the right.
 The packet sequence ID is incremented on each authentication retry
 response and on the change password response.  All cases where the
 packet sequence ID is updated are noted below.
 Response retry is never allowed after Change Password.  Change
 Password may occur after response retry.

Zorn Informational [Page 14] RFC 2759 Microsoft MS-CHAP-V2 January 2000

9.1.1. Successful authentication

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Success/Authenticator Response
 (Authenticator Response verification succeeds, call continues)

9.1.2. Authenticator authentication failure

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Success/Authenticator Response
 (Authenticator Response verification fails, peer disconnects)

9.1.3. Failed authentication with no retry allowed

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Failure (E=691 R=0)
 (Authenticator disconnects)

9.1.4. Successful authentication after retry

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Failure (E=691 R=1), disable short timeout
     Response (++ID) to challenge in failure message ->
                       <- Success/Authenticator Response
 (Authenticator Response verification succeeds, call continues)

9.1.5. Failed hack attack with 3 attempts allowed

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Failure (E=691 R=1), disable short timeout
     Response (++ID) to challenge in Failure message ->
                       <- Failure (E=691 R=1), disable short timeout
     Response (++ID) to challenge in Failure message ->
                       <- Failure (E=691 R=0)

Zorn Informational [Page 15] RFC 2759 Microsoft MS-CHAP-V2 January 2000

9.1.6. Successful authentication with password change

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Failure (E=648 R=0 V=3), disable short
 timeout
     ChangePassword (++ID) to challenge in Failure message ->
                       <- Success/Authenticator Response
 (Authenticator Response verification succeeds, call continues)

9.1.7. Successful authentication with retry and password change

                       <- Authenticator Challenge
     Peer Response/Challenge ->
                       <- Failure (E=691 R=1), disable short timeout
     Response (++ID) to first challenge+23 ->
                       <- Failure (E=648 R=0 V=2), disable short
 timeout
     ChangePassword (++ID) to first challenge+23 ->
                       <- Success/Authenticator Response
 (Authenticator Response verification succeeds, call continues)

9.2. Hash Example

 Intermediate values for user name "User" and password "clientPass".
 All numeric values are hexadecimal.

0-to-256-char UserName: 55 73 65 72

0-to-256-unicode-char Password: 63 00 6C 00 69 00 65 00 6E 00 74 00 50 00 61 00 73 00 73 00

16-octet AuthenticatorChallenge: 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C 60 21 32 26 26 28

16-octet PeerChallenge: 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E

8-octet Challenge: D0 2E 43 86 BC E9 12 26

16-octet PasswordHash: 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE

Zorn Informational [Page 16] RFC 2759 Microsoft MS-CHAP-V2 January 2000

24 octet NT-Response: 82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33 11 4A 3D 85 D6 DF

16-octet PasswordHashHash: 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F

42-octet AuthenticatorResponse: "S=407A5589115FD0D6209F510FE9C04566932CDA56"

9.3. Example of DES Key Generation

 DES uses 56-bit keys, expanded to 64 bits by the insertion of parity
 bits.  After the parity of the key has been fixed, every eighth bit
 is a parity bit and the number of bits that are set (1) in each octet
 is odd; i.e., odd parity.  Note that many DES engines do not check
 parity, however, simply stripping the parity bits.  The following
 example illustrates the values resulting from the use of the password
 "MyPw" to generate a pair of DES keys (e.g., for use in the
 NtPasswordHashEncryptedWithBlock() described in section 8.13).
 0-to-256-unicode-char Password:
 4D 79 50 77
 16-octet PasswordHash:
 FC 15 6A F7 ED CD 6C 0E DD E3 33 7D 42 7F 4E AC
 First "raw" DES key (initial 7 octets of password hash):
 FC 15 6A F7 ED CD 6C
 First parity-corrected DES key (eight octets):
 FD 0B 5B 5E 7F 6E 34 D9
 Second "raw" DES key (second 7 octets of password hash)
 0E DD E3 33 7D 42 7F
 Second parity-corrected DES key (eight octets):
 0E 6E 79 67 37 EA 08 FE

10. Security Considerations

 As an implementation detail, the authenticator SHOULD limit the
 number of password retries allowed to make brute-force password
 guessing attacks more difficult.

Zorn Informational [Page 17] RFC 2759 Microsoft MS-CHAP-V2 January 2000

11. References

 [1]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
      1661, July 1994.
 [2]  Simpson, W., "PPP Challenge Handshake Authentication Protocol
      (CHAP)", RFC 1994, August 1996.
 [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [4]  "Data Encryption Standard (DES)", Federal Information Processing
      Standard Publication 46-2, National Institute of Standards and
      Technology, December 1993.
 [5]  Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, April
      1992.
 [6]  RC4 is a proprietary encryption algorithm available under
      license from RSA Data Security Inc.  For licensing information,
      contact:
           RSA Data Security, Inc.
           100 Marine Parkway
           Redwood City, CA 94065-1031
 [7]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
      Recommendations for Security", RFC 1750, December 1994.
 [8]  "The Unicode Standard, Version 2.0", The Unicode Consortium,
      Addison-Wesley, 1996. ISBN 0-201-48345-9.
 [9]  Zorn, G. and Cobb, S., "Microsoft PPP CHAP Extensions", RFC
      2433, October 1998.
 [10] "DES Modes of Operation", Federal Information Processing
      Standards Publication 81, National Institute of Standards and
      Technology, December 1980.
 [11] "Secure Hash Standard", Federal Information Processing Standards
      Publication 180-1, National Institute of Standards and
      Technology, April 1995.
 [12] Zorn, G., "PPP LCP Internationalization Configuration Option",
      RFC 2484, January 1999.

Zorn Informational [Page 18] RFC 2759 Microsoft MS-CHAP-V2 January 2000

12. Acknowledgements

 Thanks (in no particular order) to Bruce Johnson, Tony Bell, Paul
 Leach, Terence Spies, Dan Simon, Narendra Gidwani, Gurdeep Singh
 Pall, Jody Terrill, Brad Robel-Forrest, and Joe Davies for useful
 suggestions and feedback.

13. Author's Address

 Questions about this memo can also be directed to:
 Glen Zorn
 Microsoft Corporation
 One Microsoft Way
 Redmond, Washington 98052
 Phone: +1 425 703 1559
 Fax:   +1 425 936 7329
 EMail: gwz@acm.org

Zorn Informational [Page 19] RFC 2759 Microsoft MS-CHAP-V2 January 2000

14. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Zorn Informational [Page 20]

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