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?
You send key#1 in the box locked with lock#2.
You send key#2 without a box.
The friend unlocks the box, takes key#1 and sends the box back.
You send the object in the box locked with lock#1.
The friend unlocks the box with key#1, takes out the object and congratulates you on your cleverness... ;)
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