In the history of cryptography, the Turing bombe was an electromechanical device used by British cryptologists to help break Enigma-machine generated codes during World War II. Bombes were built by the British Tabulating Machine Company at Letchworth, UK.

These machines used several stacks of rotors spinning together to test multiple hypotheses about possible setups of the Enigma machine, such as the order of the rotors in the stack. While Turing's bombe worked in theory, it required impracticably long cribs to rule out sufficiently large numbers of settings. Gordon Welchman came up with a way of using the symmetry of the Enigma stecker to increase the power of the bombe. His suggestion was an attachment called the diagonal board that further improved the bombe's effectiveness

Bombes were built under the direction of Harold 'Doc' Keen and the project was codenamed CANTAB. Each British bombe was about 7 feet (2.1 m) wide, 6 feet 6 inches (2.0 m) tall and 2 feet (0.61 m) deep and weighed about a ton. On the front of each bombe were 108 places where rotors could be mounted. The rotors were in three groups of 12 triplets. Each triplet, arranged vertically, corresponded to the three Enigma rotors. The bombe rotors had a double set of contacts and wiring to emulate the Enigma reflection. The input and output of each triplet of rotors went to cable connectors, allowing the bombe to be rewired according to the Turing and Welchman methodologies as applied to individual ciphertexts.

The original devices were destroyed after the war for security reasons, but in 1970, a set of blueprints turned up at Bletchley and the idea to reconstruct a Bombe was born. The rebuild team, led by volunteer John Harper, has finally succeeded in putting the machine together.

Tuesday March 24, 2009 marked the day for honouring a rebuilt replica of the Turing Bombe, the machine invented by mathematician Alan Turing. It was a forerunner of the modern day computer. Presented at Bletchley Park, the Engineering Heritage Award recognized the 13 years of hard work into recreating the legendary Enigma-busting machine.

Internal view of the Turing Bombe. It was the brainchild of Alan Turing with an important refinement suggested by Gordon Welchman. A total of 210 machines were manufactured by the British Tabulator Machine Company during WWII. (Photo via The Register Newspaper - UK)