Microsoft PPP CHAP Extensions
RFC 2433

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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.

Zorn & Cobb                  Informational                      [Page 1]
RFC 2433             Microsoft PPP CHAP Extensions         Ocotober 1998

   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.

Zorn & Cobb                  Informational                      [Page 2]
RFC 2433             Microsoft PPP CHAP Extensions         Ocotober 1998

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