Riddle: You want to send a valuable object to a friend. You he a box which is more than large enough to contain the object. You he several locks with keys. The box has a locking ring which is more than large enough to he a lock attached. But your friend does not he the key to any lock that you he. How do you do it? Note that you cannot send a key in an unlocked box, since it might be copied.
Answer: 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
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