browsing around and found this bit of info.
the seed and key, ie one module asks for a seed, the other gives him one, then requesting module then sends a matching key, if its legit the auth was complete. In the case of door locks I believe I've seen 3 byte seeds and keys, that I believe rarely repeat. Thats 16,777,215 possible combonations for you just to once unlock the door.
read the entire thread here canhack.org