Bombe - The Enigma Machine

The Enigma Machine

The Enigma was an electro-mechanical rotor machine used for the encryption and decryption of secret messages. It was developed in Germany in the 1920s. The repeated changes of the electrical pathway from the keyboard to the lampboard implemented a polyalphabetic substitution cipher, which turned plaintext into ciphertext and back again. Used properly, this provided a very high degree of security. The Enigma's scrambler contained rotors with 26 electrical contacts on each side, whose wiring diverted the current to a different position on the two sides. On depressing a key on the keyboard, an electrical current flowed through an entry drum at the right-hand end of the scrambler, then through the set of rotors to a reflecting drum (or reflector) which turned it back through the rotors and entry drum, and out to cause one lamp on the lampboard to be illuminated.

At each key depression, at least one of the rotors (the right-hand or "fast" rotor) advanced one position, which caused the encipherment to alter. At a certain point, the right-hand rotor caused the middle rotor to advance and in a similar way, the middle rotor caused the left-hand (or "slow") rotor to advance. Each rotor caused the "turnover" of the rotor to its left after a full rotation. The Enigma operator could rotate the wheels by hand to change the letter of the alphabet showing through a window, to set the start position of the rotors for enciphering a message. This three-letter sequence was "message key". There were 26 × 26 × 26 = 17,576 possible positions of the set of three rotors, and hence different message keys. By opening the lid of the machine and releasing a compression bar, the set of three rotors on their spindle could be removed from the machine and their sequence (called the "wheel order" at Bletchley Park) could be altered. Multiplying 17,576 by the six possible wheel orders gives 105,456 different ways that the scrambler could be set up.

In 1930 an additional security feature was introduced for the military Enigmas. This was a plugboard (Steckerbrett in German, shortened to "Stecker") that further scrambled the letters. Letters were swapped in pairs: if A was transformed into R then R was transformed into A. This regularity was exploited by Welchman's "diagonal board" enhancement to the bombe, which vastly increased its efficiency. With six plug leads in use (leaving 14 letters "unsteckered") this gives 100,391,791,500 possible ways of setting up the plugboard.

An important feature of the machine from a cryptanalyst's point of view, and indeed Enigma's Achilles' heel, was that the reflector in the scrambler meant that a letter was never enciphered as itself. Any putative solution that gave, for any location, the same letter in the proposed plaintext and the ciphertext, could therefore be eliminated.

In the lead up to World War II, the Germans made successive improvements to their military Enigma machines. By January 1939, additional rotors had been introduced so that there was a choice of three from five (i.e. 60 wheel orders) for the army and airforce Enigmas, and three out of eight (336 wheel orders) for the navy machines. In addition, ten leads were used on the plugboard leaving only six letters unsteckered. This meant that the airforce and army Enigmas could be set up 15x10^18 ways. In 1941 the German navy introduced a version of Enigma with rotatable reflector (the M4 or Four-rotor Enigma) for communicating with its U-boats. This could be set up in 18x10^19 different ways.