From: Mike Ingle <MIKEINGLE@delphi.com>
Date: Wed, 27 Apr 94 02:38:04 PDT
To: cypherpunks@toad.com
Subject: Re: Clipper Key Exchange
Message-ID: <01HBNPYJTZDE9S87TE@delphi.com>
MIME-Version: 1.0
Content-Type: text/plain


matsb@sos.sll.se (Mats Bergstrom) writes:
Subj:   Clipper Session Key

>How do two Clipper/Capstone/Skipjack/Tessera units agree on a session key 
>without compromising it to the bad guys tapping the line? If not known 
>outside of the NSA, what possible solutions are there?

In the case of the MYK-78, the simple Clipper chip to be used in phones
and similar devices, that's your problem. You have to do a Diffie-Hellman
exchange or RSA or something externally. Or hire a courier. This will cause
compatibility problems between different manufacturers' Clipper devices,
unless they agree on a standard up front. Probably everyone will clone AT&T
Clipper devices, since they are going to be the first to market. If the
government buys a large batch, that will set the standard. Without at least
a de-facto protocol standard, the system would be almost useless. There is
also the problem of adding encryption to the V.x standards for fax, LAPM
for modems, etc. Each of these devices will need some kind of key exchange.

The Capstone and Tessera (same thing, different package) have the ability
to do a modexp operation. From this you can internally do DH exchange or
RSA. As I remember, the Capstone does up to 1024 bit modexp, comparable
to PGP in security.

Skipjack is just a symmetric cipher like DES, except it has an 80-bit key
and has (according to the review team) all of DES's problems fixed.

--- Mike