From: George A. Gleason <gg@well.sf.ca.us>
Date: Tue, 20 Oct 92 01:56:22 PDT
To: hkhenson@cup.portal.com
Subject: Re:  one time pads.
Message-ID: <199210200855.AA27037@well.sf.ca.us>
MIME-Version: 1.0
Content-Type: text/plain


Re use fo CD ROMs for OTPs.  That would seem to be a variation on the old
theme of the book cipher, which is *not* random and therefore *not* secure.
Also, agreeing on key procedures over the *phone* in *clear voice* is kind
of like sex without a condom.  Not secure.  Not safe.  Potentially deadly.

While we're on the subject of link encryption, the same response pertains to
the suggestion made by someone from SJG here (whose name I ought to know but
my memory for names has more holes than Bush's economic plan) about key
distribution and transmission of cleartext over phone lines.  It *doesn't
matter* if it's encrypted at the BBS server, if it goes in clear over the
phone.  Keyword in context recognition is no problem when dealing with
ASCII.  Various agencies (you know who) have raised this to a fine art.  The
telephone line is precisely the link in the chain which is weakest and needs
most to be protected.  If the nasty-wasties break down your door and go for
your hard drive, it's already too late.  Like worrying about a condom after
the fact.  

Now as a matter of record, 
it's been demonstrated that a processor emits electromagnetic radiation back
over the phone lines as well as electrical power lines, which can be picked
up at a distance.  So consider for a moment that there is a BBS serving
dissidents, who Big Brother wants to monitor.  Seeing all the encrypted
traffic, they install a device on the phone line and the AC to pick up what
the mircoprocessor is doing.  And they see it doing crypto, and they see the
cleartext which is recovered, and get the keys, and the whole damn thing may
as well be transparent.  So for BBSs and such, I would argue that it's
necessary to have electromechanical relays to isolate the computer from the
phone lines when encrypting or decrypting; ideally isolate it from the AC as
well (using an uninterruptable power supply, which will run the computer on
batteries for some period of time, say 45 minutes).  So you have a great big
red toggle switch next to the computer, and for some period during the day
when doing all the crypto processing, throw the switch to the Safe position
to get it off the phones and AC lines.  This might make BBS use a little
less convenient (in that email becomes available the day after it was
posted), but at least it's not literally leaking everyone's secrets into the
airwaves.  

This general area is part of a larger topic called TEMPEST.  Anyone else
interested in pursuing this angle...?

-gg