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.