From: Brad Huntting <huntting@glarp.com>
Date: Tue, 26 Jul 94 09:26:45 PDT
To: norm@netcom.com (Norman Hardy)
Subject: Re: CYPHERPUNKS TO THE RESCUE
In-Reply-To: <199407260711.AAA10426@netcom.netcom.com>
Message-ID: <199407261607.KAA02397@misc.glarp.com>
MIME-Version: 1.0
Content-Type: text/plain



> 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.

I think a simple key seeded MD5 work work fine for garage doors:

The remote can transmit: (n, M(n^k))

Where n is random (and so doesn't repeat often), k is a shared key
known only to the remote and the door opener, and M is a reasonably
strong hash function.  k could be set by a bank dip switches, but
to get a large enough key space would require alot of switches.


brad