CYPHERPUNKS TO THE RESCUE

[Eric Weaver]

   From: wb8foz at nrk.com (David Lesher)
   Date: Tue, 26 Jul 1994 18:51:19 +0000 (GMT)

   A challenge /response may make sense crypto-wise, but not $$-wise.
   The car would then need a receiver too, & the house a transmitter.
   More things to buy & break.

   A one-way solution is needed to make it fly here.

Okay, here's my "bright" "idea"...

A 32-bit counter, a 32-bit somewhat-random "salt", a 32-bit fixed
authenticator and a 32-bit checksum, two DES blocks.  The transmitter
just counts up each time the button is pressed, and the whole thing is
DES'd in CBC mode with the symmetric key or what have you.

The receiver decrypts, verifies the checksum and perhaps the
authenticator and just checks for the count to be greater than the
last time it received a signal.  This handles replays and doesn't
require exact sync between remote and base.

The receiver can have a reset button inside so the owner can push it
and click the remote if somehow the receiver gets skipped way ahead.

Counterexamples, anyone?