Subject: PPP AUTHENTICATION

From: T Manidhar <[email-masked]>
Date: Wed, 16 Jun 1999 10:50:15 +0530

Hi,

Any body knows how to configure the RAS service in NT client and server to use the CHAP as a authentication protocol. 

Thanks 
Mani.

-----Original Message-----
From:	Archie Cobbs [SMTP:[email-masked]]
Sent:	Wednesday, June 16, 1999 10:26 AM
To:	[email-masked]
Cc:	[email-masked]; [email-masked]
Subject:	Re: PPTP AUTHENTICATION

Naveen.P.N. writes:
>     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: David Woolley <[email-masked]>
Date: Fri, 18 Jun 1999 22:28:38 +0100 (BST)

[ Re-wrapped to internet standards ]
> Any body knows how to configure the RAS service in NT client and server
> to use the CHAP as a authentication protocol.

It's impossible, for reasons which should be clear from the PPTP 
AUTHENTICATION thread; basically NT does not know the user form of
the password, but MD5-CHAP requires that both parties do know it++.

The free pppd generates the form that NT stores on the fly, but doesn't
have to worry about conforming with Orange book requirements not to
store user form passwords.

++ Pedantically it is only a protocol, and therefore isn't concerned with
user interface issues, but I'm sure you are expecting to work in terms of
what the user enters, not an internal, scrambled, version.

---

From: James Carlson <[email-masked]>
Date: 21 Jun 1999 07:35:17 -0400

> the password, but MD5-CHAP requires that both parties do know it++.

This doesn't really matter, unless all we're talking about is the
current (and broken) mode of operation.

> store user form passwords.

"Orange Book" standards don't really apply here.  Computers with
network attachments can't pass TC/SEC muster at all.  (They can be
used in a secure environment if the whole configuration is blessed by
a DAA, but, then, the same is true of a live chicken.  ;-})

> what the user enters, not an internal, scrambled, version.

As also outlined in that thread, there's also no particular reason to
prohibit the user interface from doing the "right thing" here.
(Brashly assuming, of course, that MS-CHAP is also doing the "right
thing.")

The right thing, by way of NT and dial-up clients, would be to have
the "client" end of the conversation detect that the peer is using NT
(perhaps by a distinguished value in the peer name field in the CHAP
Challenge message), and then use MD4 to hash the user's password
before using it as part of the standard CHAP MD5 hash.  (Note that the
MD4 hash in MS-CHAP is done over only the password given by the user.
It is thus effectively a constant for a given user password.)

The shared secret is thus the hashed password, and the user interface
is intact.  No new protocols need be invented, and this procedure is
no more or less secure than MS-CHAP.

(You don't even necessarily need this automatic-detection logic based
on peer name.  This feature could be a check-box in your user
interface or, if you're lucky, a second try at a CHAP Response value.
Some implementations allow a limited number of CHAP retries on
failure, so the first attempt could use the standard method, and, if
that fails, the second could try the MD4 hash first.)

-- 
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