In May 1976, collaborating with Stanford computer scientist Martin Hellman, Diffie cracked both problems and his scheme was called public-key cryptography. It was a brilliant breakthrough. Every user in the system has two keys — a public key and a private key. The public key can be widely distributed without compromising security; the private key, however, is held more closely than an ATM password, and you don't grant access to anybody. For relatively secret mathematical reasons, a message encoded with either key can be decoded with the other. For instance, if I want to send you a secure letter, I encrypt it with your public key (which I received from you), and send you the cyphertext and you decipher it using your private key. Likewise, if you send a message to me, you can encrypt it with my public key, and I'll switch it back to plaintext with my private key. This principle can also be used for authentication. Only one person can encrypt text with my private key, and that's me. If you can decode a message with my public key, you know beyond a doubt that it's straight from my machine to yours. The message bears my digital signature.

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