You want to send a valuable object to a friend securely. You have a box which can be fitted with multiple locks, and you have several locks and their corresponding keys. However, your friend does not have any keys to your locks, and if you send a key in an unlocked box, the key could be copied en route.

How can you send the object securely?

If your friend has a lock, you can send the locked box, and he can add a lock of her own and send it back.
Then, you remove your lock, send the box back one more time, and he can remove his own lock and open the box.

That way, you never send the box unlocked, and never have to send a key at all.

Posted by DJ on 2003-07-18 18:34:01