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?
(In reply to
re: solution by DJ)
Yes, the one key is sent insecurely, but the point is that the box with the corresponding lock has already arrived at the friend before the key for it is sent. Therefore copying this key doesn't help someone sitting between the sender and his friend. (To open the box he needs the copied key BEFORE the locked box goes through his hands.)
In fact, i just realised that my solution was overly complicated, the second key&lock isn't neccessairy.
Just send the locked box with the valuable object first and the key only after the box has arrived.
re(2): solution
