The Vigenère cipher is an early form of polyalphabetic cipher invented in the 16th century, but the most famous polyalphabetic cipher is the Enigma machine. Invented by Arthur Scherbius, this mechanical version of the Vigenère cipher was used by Germany prior to and throughout the Second World War. Looking rather like a typewriter, each letter on the keyboard was connected to a letter on the lampboard by 26 wires. However, the machine was not hardwired. The wiring passed through rotors, which turned after each key was pressed, so the circuits were continually changing.

A crucial feature of the Enigma cipher (and most crypto algorithms) is that the machine has billions of possible settings, such as the starting orientations of the rotors. Each complete setting is called a key. The Germans knew that a machine would eventually fall into that hands of the Allies, but such a machine could not be used to decipher a message unless the key used to encrypt the message was known. The significance of the key is an enduring principle of cryptography, and it was definitively stated in 1883 by the Dutch linguist Auguste Kerckhoffs von Nieuwenhof: “The security of a cryptosystem must not depend on keeping secret the crypto algorithm. The security depends only on keeping secret the key.”