Subject: PPTP AUTHENTICATION

From: "Naveen.P.N." <[email-masked]>
Date: Tue, 15 Jun 1999 19:55:36 -0700

Hi,
    Is MS-CHAP a must to logon to Windows-NT (Ver 4.0) PPTP Server ?
    Is it possible to use CHAP (MD5) or PAP for the above purpose ? If
Yes,
    Please suggest me the means.

    Thanks in advance.
Regards,
Naveen

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From: Archie Cobbs <[email-masked]>
Date: Tue, 15 Jun 1999 21:55:49 -0700 (PDT)

>     Is MS-CHAP a must to logon to Windows-NT (Ver 4.0) PPTP Server ?

Yes, because MPPE encryption uses the challenge sequence from the
MS-CHAP login. Also, NT servers store the password as hashes instead
of in cleartext (I'm pretty sure about that but don't quote me) so
they can't even do MD5 CHAP if they wanted to.

-Archie

___________________________________________________________________________
Archie Cobbs   *   Whistle Communications, Inc.  *   http://www.whistle.com

---

From: James Carlson <[email-masked]>
Date: 16 Jun 1999 13:11:08 -0400

[email-masked] (Archie Cobbs) writes:
> MS-CHAP login.

They are doing this, but the rationale is weak.  The same procedure
described in draft-ietf-pppext-mschapv2-keys-02.txt could be applied
to standard CHAP, if they desired to be compatible with others.

> Also, NT servers store the password as hashes instead

That's not at all an impediment.  The NT server stores what is
effectively a cleartext (if not plaintext) password.  The password
just happens to be an MD4 hash of the what the *user* thinks of as his
password.  That the hash itself is the MS-CHAP shared secret doesn't
affect the problem.

So, the behavior expected here by the dial-up user's implementation
(aka the "client") would be to prompt the user for a password with a
fancy dialog box, hash that password using MD4, and then proceed to
use the result of the hash as the shared secret for industry-standard
CHAP authentication.  Et voila.  Standard CHAP authentication.

In fact, since NT dial-in server boxes never bother to authenticate
themselves to anyone (nobody wants to be certain he is always reaching
his actual ISP and isn't being spoofed, does he?), you could have the
CHAP peer name issued by the NT system include a distinguished string
(like "NT-RAS" or some such) so that the "client" machine can
correctly determine whether to do the bogus MD4 hash before using the
password value or not.  That would make it as invisible as any
marketeer would like.

There's nothing in the standard protocols that would prevent them from
doing that and eliminating MS-CHAP-v-anything.

-- 
James Carlson, Software Architect                   <[email-masked]>
IronBridge Networks / 55 Hayden Avenue   71.246W   Vox:  +1 781 372 8132
Lexington MA  02421-7996 / USA           42.423N   Fax:  +1 781 372 8090
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp