You want to send a secret message to your friend Alex in the mail. But Sam, who you don't trust, has access to all your mail. So you put your message in a box with a lock. But you are not allowed to send a key! How can you get your message through securely?
- Send the box with the lock to Alex.
- Alex can't open it, but can put another lock on the box.
- Alex sends this box with the 2 locks back to you, you unlock your lock and send it back to Alex again.
- So there is just Alex's lock on the box and Alex can now open it.