Question

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

Answer 1

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.

Comments

I agree.

Answer 2

You ask the friend to send you a lock of which he has the key.
You then seal the box using that lock and send it to your friend.
As the friend still has the key he can open it.