Alice and Bob¹

Abstract

Alice and Bob live apart from each other and can only communicate by “snail” mail. But they know that the mailman reads all their letters. Alice has a message for Bob and doesn’t want the mailman to read it. What can she do? She has already thought of having the message delivered in a padlocked box. But how can she get the key to Bob? She can’t send it inside the box, because then Bob couldn’t open the box.

¹This article, together with the next two articles, was awarded the first prize in the 2003 Raising Public Awareness of Mathematics competition organized by the European Mathematical Society. They are reproduced here with some minor changes.