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

At 09:51 1994/07/26 -0400, Russell Nelson wrote: 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.