Locks and keys

You want to send a valuable object to a friend. You pack it inside a large box, which has a locking ring around it, where you can put several locks and can send it. Unfortunately, your friend does not have any keys for these locks and also you should not send the keys separately for fear of its being copied out. Nevertheless, you must send it safely. How?


Answer:

Just attach locks to the ring and send it to your friend. Your friend attaches his/her own lock and sends it back. You remove your locks and send it again to your friend. He/She removes his/her lock and opens the parcel.