If you need to send a message in a box to a person so that no one can intercept it and get the message, how do you do so if you have a padlock and key and they have a padlock and key
This solution has the box being locked anytime it's in transit, and does not require any exchange of keys:
A puts the message in the box, padlocks it, and sends it to B. B padlocks the box, and sends it back to A. A unlocks their padlock and sends the still-locked box back to B. B unlocks their padlock and opens the box.