ms-chap.txt

pptp第二层隧道模块

Network Working Group                                            S. Cobb
Informational Memo                                             Microsoft
Revision 1.3                                                  March 1997


                     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
    Rev 1.3:  (Sect 10) Added hash example section








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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
    10. Hash Example...............................................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,