Bombe - The British Bombe

The British Bombe

The Polish cryptologic bomb (Polish: bomba kryptologiczna) had been useful only as long as three conditions were met. First, the form of the indicator had to include the repetition of the message key; second, the number of rotors available had to be limited to three, giving six different "wheel orders" (the three rotors and their order within the machine); and third, the number of plug-board leads had to remain relatively small so that the majority of letters were unsteckered. Six machines were built, one for each possible rotor order. They were delivered in November 1938, but barely a month later the Germans introduced two additional rotors for loading into the Enigma scrambler, increasing the number of wheel orders by a factor of ten. Building a further 54 bomby was beyond the Poles' resources. Also, on 1 January 1939, the number of plug-board leads was increased to ten. The Poles had therefore had to return to manual methods, the Zygalski sheets.

Alan Turing designed the British bombe on a more general principle, the assumption of the presence of text, called a crib, that cryptanalysts could predict was likely to be present at a defined point in the message. This technique is termed a known plaintext attack and had been used to a limited extent by the Poles, e.g., the Germans' use of "ANX" — German for "To," followed by "X" as a spacer.

The bombes were built by the British Tabulating Machine Company at Letchworth under the direction of Harold 'Doc' Keen. Each machine was about 7 feet (2.1 m) wide, 6 feet 6 inches (1.98 m) tall, 2 feet (0.61 m) deep and weighed about a ton. On the front of each bombe were 108 places where drums could be mounted. The drums were in three groups of 12 triplets. Each triplet, arranged vertically, corresponded to the three rotors of an Enigma scrambler. The bombe drums' input and output contacts went to cable connectors, allowing the bombe to be wired up according to the menu. The 'fast' drum rotated at a speed of 50.4 rpm in the first models and 120 rpm in later ones, when the time to set up and run through all 17,576 possible positions for one rotor order was about 20 minutes.

The first bombe, which was based on Turing's original design and so lacked a diagonal board, was installed at Bletchley Park on 18 March 1940 and was named "Victory". The second bombe, named "Agnus dei", later shortened to "Agnes", or "Aggie", was equipped with Welchman's diagonal board, and was installed on 8 August 1940; "Victory" was later returned to Letchworth to have a diagonal board fitted.

During 1940, 178 messages were broken on the two machines, nearly all successfully. Because of the danger of bombes at Bletchley Park being lost if there were to be a bombing raid, five bombe outstations were established, at Adstock, Gayhurst, Wavendon, Stanmore, and Eastcote. The bombe was referred to by Group Captain Winterbotham as a "Bronze Goddess" because of its colour. The devices were more prosaically described by operators as being "like great big metal bookcases".

Production of bombes by BTM at Letchworth in wartime conditions, was nowhere near as rapid as the Americans later achieved at NCR in Dayton, Ohio.

After World War II, some fifty bombes were retained at Eastcote, while the rest were destroyed. The surviving bombes were put to work, possibly on Eastern bloc ciphers. The official history of the bombe states that "some of these machines were to be stored away but others were required to run new jobs and sixteen machines were kept comparatively busy on menus. It is interesting to note that most of the jobs came up and the operating, checking and other times maintained were faster than the best times during the war periods."

A team led by John Harper conducted a 13-year project to reconstruct a working bombe. This was successful, and the reconstructed bombe can be seen at Bletchley Park Museum. In March 2009 it won an Engineering Heritage Award.