From: norm@netcom.com (Norman Hardy)
Date: Tue, 26 Jul 94 00:11:38 PDT
To: Sandy Sandfort <cypherpunks@toad.com>
Subject: Re: CYPHERPUNKS TO THE RESCUE
Message-ID: <199407260711.AAA10426@netcom.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 14:43 1994/07/25 -0700, Sandy Sandfort wrote:
>The questions are:  Could standard auto and garage door openers
>easily be retrofitted?  Could a "crypto remote" with its own CPU
>be made small enough to fit into a hand-held unit?  Could such a
>system be made for a reasonable cost?
...
Sounds like an application for a "challenge-response" system. But that
would require transmission from garage unit to car unit.

If there were syncnronized clocks then the signal could be a function of
time so that the above replay would fail. That requires only a PRNG.

Both units could compute the next password from the same PRNG but this
would require a "backspace" button on the car unit for those occasions
where the garage unit failed to hear a broadcast signal. A "reset to new
known state" for both units would be required for when the state became
hoplessly confused.