You want to send a valuable object to a friend& You have a box to send the object.
The box has a locking ring which is more than large enough to have a lock attached
You and your friend have several locks with keys.
Butthe problem is that your friend does not have the key to any lock that you have, and vice versa. How do you do it?
Note: you cannot send a key in an unlocked box
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Attach a lock to the ring. Send it to her. She attaches her own lock and sends it back. You remove your lock and send it back to her. She removes her lock.