Since the release of the movie “The Imitation Game,” the name “Turing” has become synonymous with “Enigma.” Alan Turing was a brilliant mathematician. Born in London in 1912, he studied at both Cambridge and Princeton Universities. He was already working part-time for the British Government’s Code and Cypher School before the Second World War broke out. In 1939, Turing took up a full-time role at Bletchley Park in Buckinghamshire–where top secret work was carried out to decipher the military codes used by Germany and its allies. Along with breaking the Japanese diplomatic cryptosystem, usually referred to as “Purple,” probably the greatest example of Allied cryptanalytic success in World War II was the breaking of the German Enigma machine.
The main focus of Turing’s work at Bletchley was in cracking the ‘Enigma’ code. The Enigma was a type of enciphering machine used by the German armed forces to send messages securely. Although Polish mathematicians had worked out how to read Enigma messages and had shared this information with the British, the Germans increased its security at the outbreak of war by changing the cipher system daily. This made the task of understanding the code even more difficult.
It was a simple yet complex machine and even more complex to describe how it works: The standard military Enigma used three 26-point wired metal and black plastic rotors selected from a set of five to eight. Each rotor was a cylinder with a large, moveable notched wheel on one end with an alphabet, or numbers, around its circumference. One face of the cylinder had twenty-six spring-loaded copper pins protruding from it, and the other face had twenty-six flush copper contacts. Inside each cylinder was a wired “maze” connecting the contacts to the pins.
With this rotor set up, the letters “Roosevelt” would come out as “smmgzczdt.” Not too difficult to read…just monoalphabetic substitution. But Enigma used three rotors, each one wired differently, and a reflector. When a letter was input, the rightmost wheel would turn one space forward and an electrical pulse would route through each wheel in turn, then through the reflector, then back through the three wheels by a different route, and a glow lamp would light to show the encrypted character. When the notch on that wheel progressed around to the reading point, the middle wheel would advance one space; and when the notch on the middle wheel progressed to its reading point,the leftmost wheel would advance one space. It would take 16,900 characters to return the three wheels to their initial position. By limiting allowable message length, this “cycling” would not happen, so there were few predictable patterns.
Turing played a key role in solving this decryption by inventing, along with fellow code-breaker Gordon Welchman, a machine known as the Bombe. This device helped to significantly reduce the work of the code-breakers. From mid-1940, German Air Force signals were being read at Bletchley and the intelligence gained from them was helping the war effort.
Turing also worked to decrypt the more complex German naval communications that had defeated many others at Bletchley. German U-boats were inflicting heavy losses on Allied shipping and the need to understand their signals was crucial. With the help of captured Enigma material, and Turing’s work in developing a technique he called “Banburismus,” the naval Enigma messages were able to be read from 1941.
He headed the ‘Hut 8’ team at Bletchley, which carried out cryptanalysis of all German naval signals. This meant that, for the most part, Allied convoys could be directed away from the German U-boat “wolf-packs.” Turing’s role was pivotal in helping the Allies during the Battle of the Atlantic and his other works contributed greatly to the Allied war effort and, ultimately, their victory.
David P. Mowry. German Cipher Machines of World War II. Center for Cryptologic History, National Security Agency. 2014.