From: rarachel@prism.poly.edu (Arsen Ray Arachelian)
Date: Tue, 26 Jul 94 06:06:04 PDT
To: sandfort@crl.com (Sandy Sandfort)
Subject: Re: CYPHERPUNKS TO THE RESCUE
In-Reply-To: <Pine.3.87.9407251452.A16249-0100000@crl2.crl.com>
Message-ID: <9407261252.AA19317@prism.poly.edu>
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8086's are very cheap these days.  They have enough computing power to run
something like IDEA, albeit very slowly.  I'd use a challenge/response method
with something like this, because you have to keep in mind that encrypted
signals can just as easily be captured.

You'd need a clock on the garage controller.  CMOS clock chips
anyone?  It doesn't have to be acurate to the second, but certainly to the
minute, and have date, month and year available to it.   The garage opener
would receive a signal from the remote, issue a challenge code based on a
hash of the time/date + some random numbers.  The remote would encrypt this
hash with the owner's IDEA key and send back the response.

Both units would need some sort of keypad to program the codes into them.  A
backup batery for both sides is also important along with a warning that
the main battery has failed.  You wouldn't want to lose access to your garage.

I suppose some backup entry system would also help... a two key system (using
physical keys with high security mushroom pin locks, etc.)  Remember that should
the remote opener fail, the driver would be damned pissed at crypto and we
want him very happy.