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Since the dawn of civilization, people have tried to keep secrets from others. For example, allies, both in war and in business, have tried to keep strategic messages from their adversaries. Once languages developed, it was natural to think of ways to create a code that scrambled a message to make it difficult for enemies to decode, yet easy for a trusted friend who possessed the right key to unscramble. In 1977, a remarkable discovery showed how prime numbers could be used to devise public-key systems: if Alice wants to enable Bob to send secret messages to her, she can tell Bob--over a completely public channel--what to do. Knowing how Bob encodes his message does not help the eavesdropper decode the message!