





Network Working Group                                            S. Cobb
Informational Memo                                             Microsoft
Revision 1.22                                              December 1995


                     Microsoft PPP CHAP Extensions


Status of this Memo

    This document has no official Internet Engineering Task Force
    status.  It is submitted as an example of one vendor's working
    solution to several authentication issues not yet standardized by
    the Point-to-Point Working Group.

    The protocol described is implemented in Microsoft Windows NT 3.5
    and 3.51 and in Microsoft Windows95.  Differences between the
    platforms are noted in the text.  This information, plus that in
    the references, is believed sufficient to implement an
    interoperating peer.

    Distribution of this memo is unlimited.


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 Microsoft's PPP CHAP dialect (MS-CHAP),
    which extends the user authentication functionality provided on
    Windows networks to remote workstations.  MS-CHAP is closely
    derived from the PPP Challenge/Handshake Authentication Protocol
    described in RFC 1334 [2], which the reader should have at hand.


History

    Rev 1.21: (Sect 6) Fix error in implicit challenge description
    Rev 1.22: (Sect 7) Fix error in sub-field table ordering









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Table Of Contents

    1. Introduction.................................................3
    2. LCP Configuration............................................4
    3. Challenge Packet.............................................4
    4. Response Packet..............................................4
    5. Success Packet...............................................8
    6. Failure Packet...............................................8
    7. Change Password Packet (version 1)...........................9
    8. Change Password Packet (version 2)..........................12
    9. Negotiation Examples........................................16

    REFERENCES.....................................................18
    CHAIR'S ADDRESS................................................19
    AUTHOR'S ADDRESS...............................................19








































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1. Introduction

    Microsoft created MS-CHAP to authenticate remote Windows
    workstations, providing the functionality to which LAN-based users
    are accustomed.

    The closest fit available in standard PPP is CHAP which is
    primarily used for mutual secure authentication between WAN-aware
    routers.  Unfortunately, CHAP is not widely used in support of
    remote workstations where providers commonly require an insecure
    text login session in place of PPP authentication protocols.  To
    date, several remote workstation issues have not been adequately
    addressed in CHAP.  MS-CHAP addresses these issues and also
    integrates the encryption and hashing algorithms used on Windows
    networks.

    Where possible, MS-CHAP is consistent with standard CHAP, and the
    differences are easily modularized.  Microsoft implements MS-CHAP
    as extensions to it's standard CHAP code base.  Briefly,
    differences between MS-CHAP and standard CHAP are:

      * MS-CHAP is enabled by negotiating CHAP Algorithm 0x80 in LCP
        option 3, Authentication Protocol.

      * The MS-CHAP Response packet is in a format designed for
        compatibility with Microsoft Windows NT 3.5 and 3.51,
        Microsoft Windows95, and Microsoft LAN Manager 2.x networking
        products.  The MS-CHAP format does not require the
        authenticator to store a clear or reversibly encrypted
        password.

      * MS-CHAP provides an authenticator controlled authentication
        retry mechanism.

      * MS-CHAP provides an authenticator controlled change password
        mechanism.

      * MS-CHAP defines a set of reason-for-failure codes returned in
        the Failure packet Message field.
















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2. LCP Configuration

    The LCP configuration for MS-CHAP is identical to that for
    standard CHAP, except that the Algorithm field has value 0x80,
    rather than the MD5 value 0x05.  Non-MS-CHAP-aware implementations
    that correctly implement LCP Config-Rej have no problem dealing
    with this non-standard option.

    Microsoft currently negotiates authentication only on the
    server->workstation configuration.  Mutual authentication may be
    supported in the future.


3. Challenge Packet

    The MS-CHAP Challenge packet is identical in format to the
    standard CHAP Challenge packet.

    MS-CHAP authenticators send an 8-octet challenge Value field.  It
    is not necessary for peers to duplicate Microsoft's algorithm for
    selecting the 8-octet value, but the CHAP guidelines on randomness
    should be observed.

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


4. Response Packet

    The MS-CHAP Response packet is identical in format to the standard
    CHAP Response packet.  However, the Value field is sub-formatted
    differently as follows:

        24 octets: LAN Manager compatible challenge response
        24 octets: Windows NT compatible challenge response
         1 octet : "Use Windows NT compatible challenge response" flag

    The LAN Manager compatible challenge response is an encoded
    function of the password and the received challenge as output by
    the pseudo-code routine LmChallengeResponse below.  LAN Manager
    passwords are limited to 14 case-insensitive OEM characters.














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        LmChallengeResponse(
            IN  8-octet          Challenge,
            IN  0-to-14-oem-char Password,
            OUT 24-octet         Response )
        {
            LmPasswordHash(
                Password,
                giving PasswordHash )

            ChallengeResponse(
                Challenge,
                PasswordHash,
                giving Response )
        }

        LmPasswordHash(
            IN  0-to-14-oem-char Password,
            OUT 16-octet         PasswordHash )
        {
            Set UcasePassword to the uppercased Password
            Zero pad UcasePassword to 14 characters

            DesHash(
                1st 7-octets of UcasePassword,
                giving 1st 8-octets of PasswordHash )

            DesHash(
                2nd 7-octets of UcasePassword,
                giving 2nd 8-octets of PasswordHash )
        }

        DesHash(
            IN  7-octet Clear,
            OUT 8-octet Cypher )
        {
            Make Cypher an irreversibly encrypted form of Clear by
            encrypting known text [6] using Clear as the secret key,
            that is...

            DesEncrypt(
                StdText,
                Clear,
                giving Cypher )
        }











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        DesEncrypt(
            IN  8-octet Clear,
            IN  7-octet Key,
            OUT 8-octet Cypher )
        {
            Use the DES encryption algorithm [3] 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.
        }

        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 )
        }


    The Windows NT compatible challenge response is an encoded
    function of the password and the received challenge as output by
    the NtChallengeResponse routine below.  The Windows NT password is
    a string of 0 to (theoretically) 256 case-sensitive Unicode
    characters.  Current versions of Windows NT limit passwords to 14
    characters, mainly for compatibility reasons, though this may
    change in the future.









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        NtChallengeResponse(
            IN  8-octet               Challenge,
            IN  0-to-256-unicode-char Password,
            OUT 24-octet              Response )
        {
            NtPasswordHash(
                Password,
                giving PasswordHash )

            ChallengeResponse(
                Challenge,
                PasswordHash,
                giving Response )
        }

        NtPasswordHash(
            IN  0-to-256-unicode-char Password,
            OUT 16-octet              PasswordHash )
        {
            Use the MD4 algorithm [4] to irreversibly hash Password
            into PasswordHash.  Only the password is hashed without
            including any terminating 0.
        }

    The "use Windows NT compatible challenge response" flag, if 1,
    indicates that the Windows NT response is provided and should be
    used in preference to the LAN Manager response.  The LAN Manager
    response will still be used if the account does not have a Windows
    NT password hash, e.g. if the password has not been changed since
    the account was uploaded from a LAN Manager 2.x account database.
    The LAN Manager response need not be provided (set to 0's) if the
    implementation expects all user accounts to be stored only in
    fresh Windows NT account databases or ones where all uploaded
    passwords have been changed.  However, doing so may sacrifice
    downward compatibility with non-Windows-NT servers.

    If the flag is 0, the Windows NT response is ignored and the LAN
    Manager response is used.  If the password is LAN Manager
    compatible, interoperability may be achieved without providing the
    Windows NT challenge response (set to 0's), and providing only the
    LAN Manager response.  This is what Microsoft Windows95 does,
    though this may change in the future.  Doing so may sacrifice
    interoperability with OEM-specific versions of Windows NT designed
    for maximum security in Windows-NT-only networks.

    Implementors seeking the broadest possible interoperability are
    advised to supply both responses when the password is LAN Manager
    compatible.  This is what Microsoft Windows NT does.







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    The Name field identifies the authenticatee's user account name.
    The Windows NT domain name may prefix the user's account name in
    the typical Windows NT format, e.g. "redmond\stevec" where
    "redmond" is a Windows NT domain containing the user account
    "stevec".  If a domain is not provided, the backslash should also
    be omitted, e.g. "stevec".


5. Success Packet

    The Success packet is identical in format to the standard CHAP
    Success packet.


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 protocol.  The Message field format is:

        "E=eeeeeeeeee R=r C=cccccccccccccccc V=vvvvvvvvvv"

    where

        The "eeeeeeeeee" is the 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 a flag set to "1" if a retry is allowed, and "0" if
        not.  When authenticator sets this flag to "1" it disables
        short timeouts, expecting the authenticatee to prompt the user
        for new credentials and resubmit the response.

        The "cccccccccccccccc" is 16 hex digits representing an ASCII
        representation of a new challenge value.  This field is
        optional.  If it is not sent, authenticator expects the
        resubmitted response to be calculated based on the previous
        challenge value plus decimal 23 in the first octet, i.e. the
        one immediately following the Value Size field.  Windows95
        authenticators may send this field.  Windows NT authenticators
        do not, but may in the future.  Both systems implement
        authenticatee support of this field.




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        The "vvvvvvvvvv" is the decimal version code (need not be 10
        digits) indicating the MS-CHAP protocol version supported on
        the server.  Currently, this is interesting only in selecting
        a Change Password packet type.  If the field is not present
        the version should be assumed 1.

    Implementations should accept but ignore additional text they do
    not recognize.


7. Change Password Packet (version 1)

    The version 1 Change Password packet does not appear in standard
    CHAP.  It allows the authenticatee to change the password on the
    account specified in the previous Response packet.  The version 1
    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 Windows NT 3.5 and 3.51.  It is
    not supported by Windows95, though this may change in the future.
    See also Change Password Packet (version 2).

    The format of this packet is as follows:

       1 octet : Code (=5)
       1 octet : Identifier
       2 octets: Length (=72)
      16 octets: Encrypted LAN Manager Old password Hash
      16 octets: Encrypted LAN Manager New Password Hash
      16 octets: Encrypted Windows NT Old Password Hash
      16 octets: Encrypted Windows NT New Password Hash
       2 octets: Password Length
       2 octets: Flags


    Code

        5


    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

        72




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    Encrypted LAN Manager New Password Hash
    Encrypted LAN Manager Old Password Hash

        These fields contain the LAN Manager password hash of the new
        and old passwords encrypted with an 8-octet key value [6], as
        output by the pseudo-code routine LmEncryptedPasswordHash
        below.

        LmEncryptedPasswordHash(
            IN  0-to-14-oem-char Password,
            IN  8-octet          KeyValue,
            OUT 16-octet         Cypher )
        {
            LmPasswordHash(
                Password,
                giving PasswordHash )

            PasswordHashEncryptedWithBlock(
                PasswordHash,
                KeyValue,
                giving Cypher )
        }

        PasswordHashEncryptedWithBlock(
            IN  16-octet PasswordHash,
            IN  7-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,
                1st 7-octets Block,
                giving 2nd 8-octets Cypher )
        }


    Encrypted Windows NT New Password Hash
    Encrypted Windows NT Old Password Hash

        These fields contain the Windows NT password hash of the new
        and old passwords encrypted with an 8-octet key value [6], as
        output by the pseudo-code routine NtEncryptedPasswordHash
        below.








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        NtEncryptedPasswordHash(
            IN  0-to-14-oem-char Password
            IN  8-octet          Challenge
            OUT 16-octet         Cypher )
        {
            NtPasswordHash(
                Password,
                giving PasswordHash )

            PasswordHashEncryptedWithBlock(
                PasswordHash,
                Challenge,
                giving Cypher )
        }


    Password Length

        The length in octets of the LAN Manager compatible form of the
        new password.  If this value is less than or equal to 14 it is
        assumed that the encrypted LAN Manager password hash fields
        are valid.  Otherwise, it is assumed these fields are not
        valid, in which case the Windows NT compatible passwords must
        be provided.


    Flags

        Bit field of option flags where 0 is the least significant bit
        of the 16-bit quantity:

            0    : Set 1 indicates that the encrypted Windows NT
                   hashed passwords are valid and should be used.  If
                   0, the Windows NT fields are not used and the LAN
                   Manager fields must be provided.

                   For the broadest possible interoperability,
                   implementations are encouraged to provide both the
                   Windows NT and LAN Manager fields when the password
                   is LAN Manager compatible.  This is what Windows NT
                   does.

            1-15 : Reserved, always set 0.












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8. Change Password Packet (version 2)

    The version 2 Change Password packet does not appear in standard
    CHAP.  It allows the authenticatee to change the password on the
    account specified in the previous Response packet.  The version 2
    Change Password packet should be sent only if the authenticator
    reports ERROR_PASSWD_EXPIRED (E=648) and a version of 2 or more in
    the Message field of the Failure packet.

    This packet type is supported by Windows NT 3.51.  It is not
    supported by Windows NT 3.5 or Windows95, though the latter may
    change in the future.  The version 2 change password packet type
    is preferable to the version 1 type and should be offered and
    accepted where possible.

    The format of this packet is as follows:

         1 octet  : Code (=6)
         1 octet  : Identifier
         2 octet  : Length (=1070)
       516 octets : Password Encrypted with Old NT Hash
        16 octets : Old NT Hash Encrypted with New NT Hash
       516 octets : Password Encrypted with Old LM Hash
        16 octets : Old LM Hash Encrypted With New NT Hash
        24 octets : LAN Manager compatible challenge response
        24 octets : Windows NT compatible challenge response
         2-octet  : Flags


    Code

        6


    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

        1118


    Password Encrypted with Old NT Hash

        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 below:



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        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 )
        }

        EncryptPwBlockWithPasswordHash(
            IN  0-to-256-unicode-char Password,
            IN  16-octet              PasswordHash,
            OUT datatype-PWBLOCK      PwBlock )
        {
            Fill ClearPwBlock with random octet values
            lstrcpyW( to ClearPwBlock.Password, from Password )
            ClearPwBlock.PasswordLength = lstrlenW( Password )

            Rc4Encrypt(
                ClearPwBlock,
                sizeof( ClearPwBlock ),
                PasswordHash,
                sizeof( PasswordHash ),
                giving PwBlock )
        }

        Rc4Encrypt(
            IN  x-octet Clear,
            IN  integer ClearLength,
            IN  y-octet Key,
            IN  integer KeyLength,
            OUT x-octet Cypher )
        {
            Use the RC4 encryption algorithm [5] 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.
        }





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    Old NT Hash Encrypted with New NT Hash

        This field contains the old Windows NT password hash encrypted
        with the new Windows NT password hash, as output by the
        OldNtPasswordHashEncryptedWithNewNtPasswordHash routine below:

        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 )

            PasswordHashEncryptedWithBlock(
                OldPasswordHash,
                NewPasswordHash,
                giving EncrytptedPasswordHash )
        }


    Password Encrypted with Old LM Hash

        This field contains the PWBLOCK form of the new Windows NT
        password encrypted with the old LAN Manager password hash, as
        output by the NewPasswordEncryptedWithOldLmPasswordHash
        routine below:

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

            EncryptPwBlockWithPasswordHash(
                NewPassword,
                PasswordHash,
                giving EncryptedPwBlock )
        }








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    Old LM Hash Encrypted with New NT Hash

        This field contains the old LAN Manager password hash encrypted
        with the new Windows NT password hash, as output by the
        OldLmPasswordHashEncryptedWithNewNtPasswordHash routine below:

        OldLmPasswordHashEncryptedWithNewNtPasswordHash(
            IN  0-to-256-unicode-char NewPassword,
            IN  0-to-256-unicode-char OldPassword,
            OUT 16-octet              EncryptedPasswordHash )
        {
            LmPasswordHash(
                OldPassword,
                giving OldPasswordHash )

            NtPasswordHash(
                NewPassword,
                giving NewPasswordHash )

            PasswordHashEncryptedWithBlock(
                OldPasswordHash,
                NewPasswordHash,
                giving EncrytptedPasswordHash )
        }


    LAN Manager compatible challenge response
    Windows NT compatible challenge response

        The challenge response fields as described in the Response
        packet description, but calculated on the new password and the
        same challenge used in the last response.


    Flags

        Bit field of option flags:

            0    : The "use Windows NT compatible challenge response"
                   flag as described in the Response packet.

            1    : Set 1 indicates that the "Password Encrypted with
                   Old LM Hash" and "Old LM Hash Encrypted With New NT
                   Hash" fields are valid and should be used.  Set 0
                   indicates these fields are not valid.

                   For the broadest possible interoperability,
                   implementations are encouraged to provide both the
                   Windows NT and LAN Manager fields when the password
                   is LAN Manager compatible.  This is what Windows NT
                   does.

            2-15 : Reserved, always set 0.


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9. Negotiation Examples

    Here are some examples of typical negotiations.  The authenticatee
    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 either Change Password.
    Change Password may occur after Response retry.  The implied
    challenge form is shown in the examples, though all cases of
    "first challenge+23" should be replaced by the
    "C=cccccccccccccccc" challenge if authenticator supplies it in the
    Failure packet.


    Successful authentication

            <- Challenge
        Response ->
            <- Success


    Failed authentication with no retry allowed

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


    Successful authentication after retry

            <- Challenge
        Response ->
            <- Failure (E=691 R=1), disable short timeout
        Response (++ID) to first challenge+23 ->
            <- Success


    Failed hack attack with 3 attempts allowed

            <- Challenge
        Response ->
            <- Failure (E=691 R=1), disable short timeout
        Response (++ID) to first challenge+23 ->
            <- Failure (E=691 R=1), disable short timeout
        Response (++ID) to first challenge+23+23 ->
            <- Failure (E=691 R=0)






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    Successful authentication with password change

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

    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), disable short timeout
        ChangePassword (++ID) to first challenge+23 ->
            <- Success






































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REFERENCES

    [1] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC 1331,
        Daydreamer, May 1992

    [2] LLoyd, B and Simpson, W., "PPP Authentication Protocols",
        RFC 1334, L&A and Daydreamer respectively, Octobet 1992

    [3] "Data Encryption Standard (DES)" is Federal Information
        Processing Standard publication 46, National Institute of
        Standard and Techology.

    [4] Rivest, R., "MD4 Message Digest Algorithm", RFC 1320, MIT
        Laboratory for Computer Science and RSA Data Security, Inc.,
        April 1992.

    [5] RC4 is an encryption standard available from RSA Data Security
        Inc.

    [6] The 8-octet StdText string used in the LAN Manager compatible
        password hashing and the 8-octet KeyValue used in the Change
        Password (version 1) packet are not available for public
        distribution at this time.  Contact the Microsoft Developer
        Relations group (at time of writing dbeaver@microsoft.com) for
        details on obtaining these values.  On this particular point
        the author can't help you.





























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CHAIR'S ADDRESS

    The working group can be contacted via the current chair:

        Fred Baker
        Email: fred@cisco.com



AUTHOR'S ADDRESS

    The author is a developer in Microsoft's Windows NT
    Internetworking group, which monitors the ietf-ppp@merit.edu
    discussions.  Questions can also be directed as below, where email
    is preferred.

        Steve Cobb
        Microsoft Corporation
        One Microsoft Way
        Redmond, WA  98052-6399

        Email: stevec@microsoft.com

    The author maintains an informal mailing list of persons
    interested in MS-CHAP and other news regarding Windows NT support
    for PPP authentication protocols.  Send email if interested.





























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