Enigma and Bletchley Park: the rotor machine and those who beat it

It looks like a typewriter enclosed in a wooden box, with a keyboard, a panel of lamps and a few toothed wheels on top. Yet Enigma was, during the Second World War, the main tool with which the German armed forces protected their communications. Its story, and that of those who managed to read its messages, is one of the best-known chapters of modern cryptography.

A machine born for business

The patent underlying Enigma was filed in 1918 by the German engineer Arthur Scherbius, who had in mind a product for banks and companies. The first commercial versions appeared in the 1920s. The German navy adopted its own variant in 1926 and the army a few years later, adding over time features that enormously increased the number of possible combinations.

The principle was that of rotors: wheels with twenty-six irregularly wired contacts which turned with each key press, changing the correspondence between letters. In practice the machine carried out Alberti's disk idea automatically, but with an extremely long series of different alphabets. A reflector sent the signal back through the rotors, and a plugboard further swapped pairs of letters.

The Polish contribution

The first to break Enigma were not the British but the mathematicians of the Polish Cipher Bureau. At the end of 1932 Marian Rejewski, using permutation theory and some documents obtained by French intelligence, reconstructed the wiring of the military rotors. With his colleagues Jerzy Różycki and Henryk Zygalski he developed methods and devices, including a machine called the bomba, to find the daily settings.

In July 1939, a few weeks before the German invasion of Poland, the Poles revealed their results to the British and French services at a meeting in the Pyry forest near Warsaw, also handing over replicas of the machine. Without that transfer of knowledge the Allies' later work would have been much slower and less certain.

Bletchley Park

At a Victorian mansion at Bletchley Park, in Buckinghamshire, the British government brought together mathematicians, linguists, chess champions and puzzle enthusiasts. Alan Turing and Gordon Welchman designed a new electromechanical machine, also called the bombe, which rapidly tested hypotheses about the settings starting from probable words in the messages, such as those in weather reports. The reflector helped: since no letter was ever enciphered as itself, many hypotheses could be discarded at once.

The intelligence obtained, known by the code name Ultra, played an important role in many phases of the war, particularly in the Battle of the Atlantic against the U-boats. Historians debate exactly how much weight it carried. Thousands of people worked at Bletchley, most of them women, and Colossus, one of the first electronic computers, was built at the same site, though it was intended for another German cipher, the Lorenz.

For decades all this remained secret. Only in the 1970s did the story of Ultra begin to be told publicly, and Turing in time received the recognition he deserves. Enigma remains a symbol of the shift from handcrafted cryptography to the cryptography of machines, and of the decisive role that computation was coming to play.