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Versions: 00 01 02 03 04 rfc2759                                        
Network Working Group                                            G. Zorn
Internet-Draft                                     Microsoft Corporation
Category: Informational                                    November 1998

                Microsoft PPP CHAP Extensions, Version 2

1.  Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as ``work in progress''.

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

This memo provides information for the Internet community.  This memo
does not specify an Internet standard of any kind.  The distribution of
this memo is unlimited.  It is filed as <draft-ietf-pppext-mschap-
v2-02.txt> and expires May 20, 1999.  Please send comments to the PPP
Extensions Working Group mailing list (ietf-ppp@merit.edu) or to the
author (gwz@acm.org).

2.  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

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fields are described in an appendix.

3.  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 reponse 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.

4.  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 [2].

5.  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.

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6.  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

Microsoft authenticators do not currently provide information in the
Name field.  This may change in the future.

7.  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
A.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

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account "johndoe").  If a domain is not provided, the backslash should
also be omitted, (e.g. "johndoe").

8.  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 of the form


where <auth_string> 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 A.6, 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 A.7, below.  If the authenticator response is
either missing or incorrect, the peer MUST end the session.

9.  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"


      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.


      The "r" is an ASCII flag set to '1' if a retry is allowed, and '0'

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      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.

      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.

Implementations should accept but ignore additional text they do not

10.  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
       24 octets : Peer-Challenge
       24 octets : NT-Response
        2-octet  : Flags


      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.

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      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 A.8, below).

      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 A.11, below).

      A 16-octet random quantity, as described in the Response packet

      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.

      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:

          5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
         |                               |

         Bits 0-15
            Reserved, always clear (0).

11.  Pseudocode

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

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11.1.  GenerateNTResponse()

   IN  16-octet              AuthenticatorChallenge,
   IN  16-octet              PeerChallenge,
   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 )

11.2.  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.

      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)

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      memcpy(Challenge, Digest, 8)

11.3.  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.

11.4.  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 )

11.5.  DesEncrypt()

   IN  8-octet Clear,
   IN  7-octet Key,
   OUT 8-octet Cypher )

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       * 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.

11.6.  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};

      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,

       * Hash the password with MD4

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      NtPasswordHash( Password, giving PasswordHash )

       * Now hash the hash

      HashNtPasswordHash( PasswordHash, giving PasswordHashHash)

      SHAUpdate(Context, PasswordHashHash, 16)
      SHAUpdate(Context, NTResponse, 24)
      SHAUpdate(Context, Magic1, 39)
      SHAFinal(Context, Digest)

      ChallengeHash( PeerChallenge, AuthenticatorChallenge, UserName,
                     giving Challenge)

      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"


11.7.  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

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      set ResponseOK = FALSE
      GenerateAuthenticatorResponse( Password, NtResponse, PeerChallenge,
                                     AuthenticatorChallenge, UserName,
                                     giving MyResponse)

      if (MyResponse = ReceivedResponse) then set ResponseOK = TRUE
      return ResponseOK

11.8.  NewPasswordEncryptedWithOldNtPasswordHash()

      256-unicode-char Password
      4-octets         PasswordLength

   IN  0-to-256-unicode-char NewPassword,
   IN  0-to-256-unicode-char OldPassword,
   OUT datatype-PWBLOCK      EncryptedPwBlock )
      NtPasswordHash( OldPassword, giving PasswordHash )

      EncryptPwBlockWithPasswordHash( NewPassword,
                                      giving EncryptedPwBlock )

11.9.  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 ),
                  sizeof( PasswordHash ),

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                  giving PwBlock )

11.10.  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.

11.11.  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,
                                        giving EncryptedPasswordHash )

11.12.  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 )

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      DesEncrypt( 2nd 8-octets PasswordHash,
                  2nd 7-octets Block,
                  giving 2nd 8-octets Cypher )

12.  Examples

12.1.  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.

12.1.1.  Successful authentication

            <- Challenge
        Response ->
            <- Success

12.1.2.  Failed authentication with no retry allowed

            <- Challenge
        Response ->
            <- Failure (E=691 R=0)

12.1.3.  Successful authentication after retry

            <- Challenge
        Response ->
            <- Failure (E=691 R=1), disable short timeout
        Response (++ID) to challenge in failure message ->
            <- Success

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12.1.4.  Failed hack attack with 3 attempts allowed

            <- Challenge
        Response ->
            <- 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)

12.1.5.  Successful authentication with password change

            <- Challenge
        Response ->
            <- Failure (E=648 R=0 V=3), disable short timeout
        ChangePassword (++ID) to challenge in Failure message ->
            <- Success

12.1.6.  Successful authentication with retry and password change

            <- Challenge
        Response ->
            <- 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

12.2.  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

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   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

   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:

12.3.  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 Appendix A.12).

   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

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13.  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.

14.  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., et. al., "Randomness Recomnendations 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", draft-ietf-
     pppext-mschap-00.txt (work in progress), March 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

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15.  Acknowledgements

Thanks (in no particular order) to Bruce Johnson
(bjohnson@microsoft.com), Tony Bell (tonybe@microsoft.com), Paul Leach
(paulle@microsoft.com, Terence Spies (terences@microsoft.com), Dan Simon
(dansimon@microsoft.com), Narendra Gidwani (nareng@microsoft.com),
Gurdeep Singh Pall (gurdeep@microsoft.com), Jody Terrill
(jodyt@extendsys.com), Brad Robel-Forrest (brad@watchguard.com) and Joe
Davies (josephd@microsoft.com) for useful suggestions and feedback.

16.  Chair's Address

The PPP Extensions Working Group can be contacted via the current chair:

   Karl Fox
   Ascend Communications
   3518 Riverside Drive
   Suite 101
   Columbus, OH 43221

   Phone: +1 614 326 6841
   Email: karl@ascend.com

17.  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

18.  Expiration Date

This memo is filed as <draft-ietf-pppext-mschap-v2-02.txt> and expires
on May 20, 1999.

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