From: norm@netcom.com (Norman Hardy)
Date: Tue, 26 Jul 94 09:37:56 PDT
To: cypherpunks@toad.com
Subject: Re: CYPHERPUNKS TO THE RESCUE
Message-ID: <199407261637.JAA21688@netcom.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 09:51 1994/07/26 -0400, Russell Nelson wrote:
>Why not generate a random number, checksum it, and sign it using a
>public key?  Or is that overkill?
...
Seems good. But to thwart replay of the signed message the garage unit must
never accept the same signed number twice. How about the car unit signing
successive numbers. The garage unit would remember the last number that it
accepted and only accept signed numbers larger than that. Garbled
transmissions would then cause no problems. They would be fixed by yet new
transmissions, just as with current units.

P.S. Better yet: There is no need of Public key technology. It suffices for
the car unit to send DES(k, n) on the nth transmission. k is a constant
secret key shared between car unit and garage unit. Garage unit decodes and
verifies that n is greater than it has seen before.