In ancient times, there lived a prince and a princess who loved each other. They belonged to neighboring kingdoms. But their plans pf happiness together came to a sudden end when the two kingdoms went to war. Their parents locked them up in two towers and had them guarded day and night.
One day the prince bribed the guard so that he would deliver and collect written massages from himself and the princess. The prince however, did not trust the guard not to read the letters. Even if he were to seal the letters, the guard could open, read the reseal them without the princess knowing. The clever prince and princess then devised a way of making it impossible for the guard to read the letters before they were delivered.
How did they do it?