From: Hal <hfinney@shell.portal.com>
Date: Wed, 13 Sep 95 15:28:19 PDT
To: cypherpunks@toad.com
Subject: Re: Can GAK be made "not interoperable" with PGP?
Message-ID: <199509132227.PAA03114@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


From: Duncan Frissell <frissell@panix.com>
> At 10:26 AM 9/13/95 -0700, Timothy C. May wrote:
> 
> >But is this even possible, to make a GAK system "not interoperable" with,
> >say, PGP?
> >
> >Unless the GAK system has some sort of entropy analyzer, and can recognize
> >high-entropy sources which it presumes to be encrypted data (*), one can of
> >course PGP-encrypt a text file and then GAK the resulting file.
> 
> I took it to mean that they were saying that an approved program on one end
> of a communication exchange could not exchange encrypted messages or
> established an encrypted session of some kind with an un approved program on
> the other end.  Not trying to outlaw superencryption (PGP on both ends using
> a GAKed channel) but GAK on one end working with an unapproved system on the
> other end.  A ringer GAK-work-alike that would defeat the intent of GAK.

Yes, I think this was the idea of the original "software key escrow"
proposal, from TIS as I recall.  The sender would encode the session key
with a government public key but there was some trick by which the
receiver would verify that the session key was in fact encoded correctly
and refuse to operate if it was wrong.  So any attempt to corrupt or
remove the LEAF would be detected if you were talking to a compliant
receiver.

That is part of why Matt Blaze's Clipper attacks were so significant,
because they went to the heart of this requirement.  It was always clear
that you could superencrypt with Clipper, but Matt found a way in which
you could send a LEAF which would be accepted by a regular Clipper phone
but which had bogus data for law enforcement.  So this defeated the
requirement of not interoperating with rogues.

Hal