Securing Unsecure Courier

You are on an island with a box, a chain, a padlock with key, and a diamond. You have a friend on another island with a box, a chain, and a padlock with key. Only your key opens your lock; only your friend's key opens your friend's lock. You need to get your diamond to your friend, and you need to use the Pirate Courier Service (tm). The only problem is the pirates are thieves, and they will steal anything without a locked lock on it. How can you get your diamond to your friend?

Solution

1. You put diamond in box, tighten box with chain, lock the chain, send to friend. (Keep your key.)
2. Friend locks the chain in addition to your lock, sends back to you. (Friend keeps key.)
3. You now unlock your lock, send to friend.

   The chained box is still locked by friend's lock, so it is still safe to send; and that is the only locking left, so friend can reach the diamond.