Return to puzzles

You want to send a valuable item to a friend. You have a box that is bigger than the item itself. You have a few locks with keys. The box has a ring (loop), which is more than it would be enough for the lock. But your friend's keys are from none of your lock. What to do?
Note: You can not send the key in an unlocked box, as it can be copied.

Answer: Put the item in the box and lock it. Send the box to a friend. A friend closes the box also to his lock and sends the box to you; You open your lock and send the box to a friend. He opens his castle.