From: Bill Stewart <stewarts@ix.netcom.com>
Date: Fri, 29 Sep 95 00:00:30 PDT
To: jsw@neon.netscape.com (Jeff Weinstein)
Subject: Re: Netscpae & Fortezza (Or, say it Ain't so, Jeff?)
Message-ID: <199509290659.XAA09185@ix7.ix.netcom.com>
MIME-Version: 1.0
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>I for one am against any kind of GAK on moral grounds.  I also think
>that trying to implement mandatory GAK in a software only system
>would be a nightmare.

Unfortunately, it's quite simple, if your only intent is to get the keys,
and not to use it as a way to increase NSA leverage...
Carl Ellison's web page points out the simple version of this,
and it's easy to extend to make it more reliable.
1) Have the NSA/NIST/DEA/etc. generate public keys for their GAK agents.
2) Have each session-key-transfer encrypt a copy of the session key with
the public key of the GAK agent, and send it at the beginning of the connection.
3) To make it more robust, have the recipient of the session key also 
encrypt the session key with the GAK key and send it back,
so that a conformist receiver can rat the key even if the sender didn't.
(takes a little protocol support to make sure the sender doesn't mind
getting it echoed back to her.)

Unlike Steven Walker's fancy complex method (which Dorothy liked a lot),
this is simple and straightforward, and requires no validation by
the recipient.  (Both parties could send fakes, but they could
do that anyway.)  What it doesn't do is allow third parties to detect
whether the GAK field has the real session key in it or a fake,
but c'est la guerre.  You can even get fancier and support M-of-N splitting,
requiring M of N GAK agents to give out their keyparts; just do the
split and encrypt each piece with the corresponding GAK agent's public key.
This also works in a wide variety of environments (e.g. Diffie-Hellman).
You could also scrounge a few bits by using the GAK field as an IV if you 
need one.
#---
# Bill Stewart, Freelance Information Architect, stewarts@ix.netcom.com
# Phone +1-510-247-0664 Pager/Voicemail 1-408-787-1281
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