*Originally posted by rgoudie*

**I must admit this as a plausible solution, but not the particular solution that I am looking for, How about if I add these two extra constraints:
**

No lock may be sent unless it is properly locked shut.

No keys may be sent at all at any time.

-Ray.

These constraints together with your original ones make this

problem impossible.

Bbarr's solution is what I had in mind. That is the essence

of public key encryption, which is more or less the state of

the art in electronic security. If your newly stated problem

had a solution, it would be a major breakthrough in that field

and you could earn untold fortunes on it. But it doesn't,

unless you have stated it wrong, or unless I am misunderstanding it.

Your first new constraint is the killer. Any new security methods based

on keys always must address the "key distribution problem," and

your problem specifically disallows this aspect to be addressed, since

an open lock is essentially analogous to a "public key".

Dr. Cribs