CYPHERPUNKS TO THE RESCUE

[hughes@ah.com (Eric Hughes)]

   Why not generate a random number, checksum it, and sign it using a
   public key?  Or is that overkill?

That's overkill.  For an affordable microprocessor for the price point
of an electronic lock, you can't do a modular exponentiation in a
reasonable amount of time.  A two-second delay is likely too long for
_mass_ market, even if certain markets would bear it.  Sandy also
suggest public key.

A shared secret key for a symmetric cipher is sufficient, since the
binding between a single garage and a single opener is usually not
broken.  If your opener had to work with multiple doors, and if the
usual case pertained where two people share the permission to open
some doors but not others, then public key woudl be needed.

So you can do challenge/response, but there's no need to use public
key.  DES would be sufficient.

Eric