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.

History and use

Using Polish cryptological techniques, British cryptanalysts at Bletchley Park were, at the beginning of World War II, able to read Enigma messages by exploiting weaknesses in German operating procedures. The British cryptologists were concerned that the Germans might at any moment change their procedures, rendering those cryptological methods obsolete.

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To preempt this, British mathematician Alan Turing designed the bombe on a more general principle – the assumption of the presence of text that analysts could guess somewhere in the message, a cryptanalytical technique known as cribbing, also termed a "known-plaintext attack." (Actually, the Poles had likewise exploited "cribs," e.g. the Germans' use of "ANX" ? German for "To," followed by "X" as a spacer.)

Related Topics:
Alan Turing - Known-plaintext attack

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The first bombe, which was based on Turing's original design and so lacked a diagonal board, arrived at Bletchley Park in March 1940 and was named "Victory." The second bombe – "Agnus" – was equipped with Welchman's diagonal board, and was installed on 8 August 1940; bombes of this type were called "Spider" bombes.

Related Topics:
1940 - 8 August

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By the end of March 1941, a more advanced version of the Bombe had been developed, the "Jumbo" machine.

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During 1940, 178 messages were broken on the two machines, nearly all successfully. By the end of 1941, there were 16 bombes in use. By the end of 1942, this had increased to 49; at the end of 1943, that figure had more than doubled to 99 bombes in operation. By May 1945, there were 211 operational machines, requiring nearly 2,000 staff to run.

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The Germans generally changed settings each day at midnight; the British goal was to find the new settings before the day was out, preferably by noon. With a motor spinning at 120 RPM, all combinations could be tested in under 6 hours. On average, it took half that time to find the correct match.

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There were five bombe outstations off-site at Adstock, Gayhurst, Wavendon, Stanmore, and Eastcote.

Related Topics:
Adstock - Gayhurst - Wavendon - Stanmore - Eastcote

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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 (Smith, 1998). 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."

Related Topics:
Eastcote - Eastern bloc

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