RFC 2759 (rfc2759) - Page 3 of 20

Microsoft PPP CHAP Extensions, Version 2

RFC 2759                  Microsoft MS-CHAP-V2              January 2000


1.  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 response 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.

2.  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.

3.  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 observed.

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



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