The Bombe helped break Enigma by shrinking the number of settings to search. Once wartime additions were counted, an Enigma machine could be set up in 158 million million million ways. Simon Greenish, then executive director of the Bletchley Park Trust, told the BBC that the Bombe and its mathematics brought that down to about a million. Greenish added that a million was still an unmanageable number.
more
Why speed mattered
German operators reset the wheels across the whole network every day, usually around midnight. Each reset produced a new key, and Bletchley Park had to find it again. A message read three months late was of no use, as Greenish put it.
Once a day’s wheel and plugboard settings were known, every other message sent on that key that day could be read. The intelligence it yielded went under the codename Ultra.
Who built it
The machine began with Polish work. Polish mathematicians had a method for Enigma before the war, but the Germans made the machine more complex just as the war began. The Poles handed their system to the British, and Bletchley developed it from there.
Alan Turing and Gordon Welchman designed the British Bombe, drawing partly on the earlier Polish design. Each standard machine carried 36 Enigma equivalents, and each of those held three drums wired to copy the enciphering. Paul Kellar, an engineer on the project to rebuild a working Bombe, credited Welchman’s diagonal board with making it work. He said it let the machine run on shorter cribs and throw up fewer false positives.
How big it got
By the end there were about 213 Bombes, by the count of Ruth Bourne, a Wren who operated them. They could not all fit at Bletchley Park, so many ran at satellite sites such as Eastcote and Stanmore. Bourne worked there from May 1944 until VJ Day, on eight-hour shifts that ran around the clock.
The operators were told almost nothing. Bourne said she was told only that the work meant breaking German codes, and never heard the word Enigma until long after the war. Her job was to put wheels on, take them off, and set the plugs, with accuracy drilled in above all.
The output was large. Late in the war Bletchley was breaking more than 3,000 messages a day, according to Greenish. After the atomic bomb fell on Hiroshima in August 1945, the Bombes stopped and the dismantling began.
A rebuilt Bombe now stands at the National Museum of Computing. Bourne, then 92, said she still knew how to operate it. With a second person, she said, she could plug it up as well.
