Secret Mail Puzzle - Solution

The Puzzle:

You want to send a secret message to your friend Alex in the mail.

But you suspect a spy has access to your mail!

So you put your message in a box with a lock.

But if you send a key then the spy might find it too, and make a copy!

How can you get your message through securely?

Send the box (with your message in), locked with your lock, to Alex.

Alex can't open it, but can put another lock on the box and send it back to you (it now has 2 locks).

You remove your lock and send the box back to Alex again (it now has only Alex's lock).

Alex can open his lock and get the message.