Enigma

A German-built Enigma machine.

The Enigma was a portable cipher machine, roughly the size of a typewriter, which could encrypt and decrypt secret messages. It used electro-mechanical rotors to convert a message to a secret code, and then another machine could decode that same message and print out the original message. This machine was commercially available beginning in the early 1920s. The Nazi German military version generated codes widely thought to be unbreakable.

The Enigma machine was an electromechanical device, equipped with a 26-letter keyboard, with one light per letter of the alphabet. Inside the machine was a set of wired drums for scrambling the input. To encipher or encrypt a letter, the operator pushed the relevant key and observed which of the lights lit. The communication between two operators required that each had an Enigma machine set up in the same way as each other. The large number of possibilities for setting the rotors (and a plugboard that could swap letters) made for an astronomical number of possible configurations, each producing a different cipher. The Germans changed the settings daily, such that the machine had to be rebroken each day in order to decrypt the messages.

Decryption required an understanding of how Enigma functioned, the wiring of the rotors and the daily settings (both rotors and plugboard).

Nazi Germany used the Enigma before and during World War II to send messages about its military maneuvers. But in 1932 a 27-year-old Polish mathematician (cryptologist) named Marian Rejewski had used advanced mathematics (permutation theory with some group theory) to secretly break ("crack") the Enigma codes. He then helped design working "doubles" of the Enigmas that could find the keys necessary for decryption. By 1938 the Poles were decrypting most of the German messages, and the Poles shared their marvelous discovery with Great Britain and France.

The breakthrough enabled the Allies to anticipate German military maneuvers, although sometimes the Allies would allow an attack to occur successfully in order to avoid signaling to the Germans that their code had been broken. The Allied code-breaking effort was known as "Ultra", as in "ultra-secret".

Y'all noobs! ~icewedge

At the end of the war, Allied General Dwight Eisenhower said that Ultra was "decisive" to the Allied victory over Germany. British Prime Minister Winston Churchill also declared, "It was thanks to Ultra that we won the war." The breaking of the German codes prevented it from completely blockading Britain with U-boats and enabled the Allied forces to make key strategic decisions throughout the war.