The Enigma machine graphic

Classic Projects: the Enigma machine

DATE: First patent application Feb 1918; Wehrmacht Enigma introduced early 1930sDESIGNER: Arthur Scherbius

Bletchley Park's role in the defeat of the Axis Powers in the Second World War has become legendary. This is where a team of crypt-analysts of the Government Code and Cypher School worked on the Enigma machine code as well the Lorenz ciphers (and some 150 other diplomatic cryptosystems). But it is the decoding of enemy Enigma communications generated by a primitive-looking machine, resembling a manual typewriter with rotating code wheels, and its link with Alan Turing, that keeps Bletchley in the public imagination to this day.

Expressed in its simplest form, Enigma was an electro-mechanical rotor cipher machine used for scrambling a text message into ciphered text (a polyalphabetic substitution cipher). An operator could type in a message and then scramble it using notched rotors displaying different letters of the alphabet. It was invented by German engineer Arthur Scherbius towards the end of the Great War, but it wasn't to make its lasting mark on history until the Second World War, when British success in cracking Enigma-encoded messages (particularly those of the German Navy) contributed greatly to the Allied victory in 1945. The exact nature of the codebreaking work at Bletchley stayed shrouded in secrecy until the 1970s when details of Ultra, the code-name given to decryptions of German radio messages encoded on Enigma machines, were released.

Encrypted military communications date back to the time of the Greek general Lysander of Sparta, in the 5th century BC. By the standards of the Ancient Greeks, Enigma was light years ahead, capable of putting a message into code in more than 150 million million million different ways. Despite increased complexity being added to the system with ever-changing daily key settings and auxiliary documents, Enigma was eventually broken, not so much because of any technical flaws in the design of the system, but more due to the capture of codebooks on 9 May 1941 from German U-boat U-110 as well as procedural malpractices by Enigma operators.

Enigma machines were made up of a family of evolving models between the wars including the so-called Wehrmacht Enigma I, introduced in 1932 for the German Army and public authorities. It was one of the most common machines, replacing the previous commercial variant. The main difference was the addition of a plugboard that allowed the swapping of letter pairs to increase the strength of the encoding. The German Navy adopted this model in 1934 and modified it to the point where they believed it was unbreakable. In February 1942, a four-rotor Enigma called the M4 (pictured) was introduced for U-boat communications, creating problems for the Allies, not only through its increased complexity, but also because procedures used by the German Naval Enigma were more elaborate and secure than other services. The accompanying codebooks were printed in red water-soluble ink on pink paper.

For Enigma to work, the transmitting and receiving machines needed to have identical settings for rotor selection, order, starting position and plugboard connections. In practice this meant that each member of the network was working to a set of assigned settings for a predefined period, which were distributed in advance in codebooks. Confidence in the system came because there were so many possible permutations, while the settings were reconfigurated ever more frequently. Enigma machines that survived the Second World War were mostly confiscated by the Allies and sold on to other countries with wiring modifications.

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