rfc2433.txt

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Network Working Group                                            G. Zorn
Request for Comments: 2433                                       S. Cobb
Category: Informational                            Microsoft Corporation
                                                            October 1998


                     Microsoft PPP CHAP Extensions

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 (1998).  All Rights Reserved.

IESG Note

   The protocol described here has significant vulnerabilities.  People
   planning on implementing or using this protocol should read section
   12, "Security Considerations".

1.  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 1994
   [2], which the reader should have at hand.

   The algorithms used in the generation of various MS-CHAP protocol
   fields are described in an appendix.

2.  Introduction

   Microsoft created MS-CHAP to authenticate remote Windows
   workstations, providing the functionality to which LAN-based users
   are accustomed while integrating the encryption and hashing
   algorithms used on Windows networks.




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   Where possible, MS-CHAP is consistent with standard CHAP.  Briefly,
   the 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's Windows NT 3.5, 3.51 and 4.0, and
        Windows95 networking products.  The MS-CHAP format does not
        require the authenticator to store a clear-text or reversibly
        encrypted password.

      * MS-CHAP provides authenticator-controlled authentication retry
        and password changing mechanisms.

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

3.  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].

4.  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.  PPP implementations which do not support MS-CHAP,
   but correctly implement LCP Config-Rej, should have no problem
   dealing with this non-standard option.

5.  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.  Peers
   need not duplicate Microsoft's algorithm for selecting the 8-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.







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6.  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 routine
   LmChallengeResponse() (see section A.1, below).  LAN Manager
   passwords are limited to 14 case-insensitive OEM characters.  Note
   that use of the LAN Manager compatible challenge response has been
   deprecated; peers SHOULD NOT generate it, and the sub-field SHOULD be
   zero-filled.  The algorithm used in the generation of the LAN Manager
   compatible challenge response is described here for informational
   purposes only.

   The Windows NT compatible challenge response is an encoded function
   of the password and the received challenge as output by the routine
   NTChallengeResponse() (see section A.5, 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.

   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.  If the flag is
   0, the Windows NT response is ignored and the LAN Manager response is
   used.  Since the use of LAN Manager authentication has been
   deprecated, this flag SHOULD always be set (1) and the LAN Manager
   compatible challenge response field SHOULD be zero-filled.

   The Name field 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 "john-doe").  If a domain is not provided, the backslash
   should also be omitted, (e.g. "johndoe").







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7.  Success Packet

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

8.  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, affects 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 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 "cccccccccccccccc" is 16 hexadecimal digits representing an
         ASCII representation of a new challenge value.  This field is
         optional.  If it is not sent, the 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.  Windows 95
         authenticators may send this field.  Windows NT authenticators
         do not, but may in the future.  Both systems implement peer
         support of this field.

         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 to be 1; since use of the version 1



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         Change Password packet has been deprecated, this field SHOULD
         always contain a value greater than or equal to 2.

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

9.  Change Password Packet (version 1)

   The version 1 Change Password packet does not appear in standard
   CHAP.  It allows the peer 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) and V is either missing or equal to one
   in the Message field of the Failure packet.

   The use of the Change Password Packet (version 1) has been
   deprecated; the format of the packet is described here for
   informational purposes, but peers SHOULD NOT transmit it.

   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

      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 the last received challenge
         value, as output by the routine LmEncryptedPasswordHash() (see
         section A.8, below).



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      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 the last received challenge
         value, as output by the pseudo-code routine
         NtEncryptedPasswordHash() (see section A.10, below).

      Password Length
         The length in octets of the LAN Manager compatible form of the
         new password.  If this value is greater than or equal to zero
         and 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
         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:

            Bit 0
               If this bit is set (1), it indicates that the encrypted
               Windows NT hashed passwords are valid and should be used.
               If this bit is cleared (0), the Windows NT fields are not
               used and the LAN Manager fields must be provided.

            Bits 1-15
               Reserved, always clear (0).

10.  Change Password Packet (version 2)

   The version 2 Change Password packet does not appear in standard
   CHAP.  It allows the peer to change the password on the account
   specified in the preceding 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 greater in the
   Message field of the Failure packet.

   This packet type is supported by Windows NT 3.51, 4.0 and recent
   versions of Windows 95.  It is not supported by Windows NT 3.5 or
   early versions of Windows 95.

      The format of this packet is as follows:

           1 octet  : Code
           1 octet  : Identifier
           2 octets : Length
         516 octets : Password Encrypted with Old NT Hash



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          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 (see section A.11, below).

      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 (see
         section A.14, below).

      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