Bombe

In the history of cryptography, the bombe was an electromechanical device used by British and American cryptologists to help break German Enigma machine signals during World War II. The bombe was designed by Alan Turing, with an important refinement subsequently contributed by Gordon Welchman.

United States Navy bombes

By late 1941 the change in German Navy fortunes, combined with intelligence reports, convinced Admiral Karl Dönitz that the Allies could read German Navy communications, and a thin fourth rotor with unknown wiring was added to German Navy Enigmas to produce the Triton system. The Triton had a lock-out that allowed it to remain compatible with three-rotor machines when necessary. As before, the unknown wiring would prevent unauthorized reading of messages. Fortunately for the Allies, in December 1941, before the machine went into official service, a submarine accidentally sent a message using four rotors, then the same message again using only three, thus disclosing the wiring of the extra rotor. In February 1942 the change in number of rotors used became official, and British ability to read German submarines' messages largely ceased until new equipment became available that could use the information about the fourth-rotor wiring.

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That spring was the "Happy Time" for the submarines, with renewed German success in attacking Allied shipping due to the security of their own communications and their ability to read convoy messages sent in Allied Naval Cipher No. 3. Between January and March 1942, German submarines sank 216 ships off the US East Coast. In May 1942 the US began using the convoy system and requiring blackouts of coastal cities so that ships would not be silhouetted against their lights, but this yielded only slightly improved security for Allied shipping.

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A crash program was begun at Bletchley Park to design bombes that could decrypt the four-rotor system, with delivery scheduled for August or September 1942. The urgent need, doubts about the British design, and slow progress with it prompted the US to start investigating designs for a parallel effort, based in part on wiring diagrams provided to US Navy officers during a visit to Bletchley Park in July 1942. Funding for a full US development effort was requested on 3 September 1942 and approved the following day.

Related Topics:
Bletchley Park - 3 September

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The U.S. bombes became available starting in late May 1943. They were 10 feet wide, 7 feet high, 2 feet deep and weighed 2 1/2 tons. About 120 were made before production was stopped in September 1944 due to rapid progress in the war. The last-manufactured United States bombe is on display at the National Cryptologic Museum. Jack Ingram, Curator of the museum, describes being told of the existence of a second bombe and searching for it but not finding it whole. Whether it remains in storage in pieces, waiting to be discovered, or no longer exists, is unknown.

Related Topics:
1944 - National Cryptologic Museum

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