Solution: Boris and Natasha
 

0. Boris puts the ring in a box, locks the box with a padlock, and sends it to Natasha.

1. Natasha receives the box and adds a padlock of her own to the box. (At this time, one would need to unlock both padlocks in order to open the box.) She then sends the box back to Boris.

2. Boris unlocks his padlock, returning the box to Natasha.

3. Natasha opens the remaining padlock on the box, opens the box, and receives the ring.

You can use this technique to send encrypted messages between two people without any shared key—each padlock is a random bit pattern and the operation of locking or unlocking is XOR. Why is it we don’t use this simple scheme for all our encryption needs?