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This is a reconstructed Bombe, one of the two main British codebreaking machines. It was used to break the codes produced by the German Enigma machines.
Over 200 Bombes were eventually built, based on the expertise of Alan Turing and Gordon Welchman. Turing and Welchman achieved their codebreaking success by recognising several weaknesses in the Germans' communications.
Firstly, the Enigma machines would never encrypt a letter as itself, which markedly reduced the number of permutations. Secondly, the Germans often sent encrypted weather forecasts. Because Bletchley Park could predict the weather, codebreakers could accurately guess the contents of the message and, therefore, the encryption used.
After the war, all the codebreaking machines were destroyed on Winston Churchill's orders so that, in the early years of the Cold War, Russia could not learn of Britain's codebreaking efforts.
However, thanks to a 12-year rebuild project, based on partial component diagrams recovered from GCHQ, this model is now working and is used today to decode mock enemy messages.
Despite the British starting to crack the Enigma code, Germany had another trick up its sleeve: the Lorenz machine. More complex than the Enigma, the 12-rotor Lorenz was used exclusively for the most important messages between central high command in Berlin and German army field marshals.
It was rather larger than the Enigma and, as a result, not portable. It used the so-called "International Teleprinter Code", with each letter represented by five electrical impulses. Messages were encoded by adding a series of apparently randomly generated letters to the original plain text.
Max Newman, another Cambridge graduate, was assigned the task of building machines to break the Lorenz code.
Newman's answer was the Colossus. His 2,500-valve machine was the first to break the Lorenz code, although Britain had to wait until December 1943 to get the first one installed at Bletchley Park. Nevertheless, that gave time enough to verify that Adolf Hitler had swallowed the D-Day deception campaigns.
A Colossus Mark II was finally reconstructed last autumn after 14 years of work by Tony Sale (pictured). Widely acknowledged as the world's first, practical, electronic, digital, information-processing machine, tape passes through the machine's wheels at a 30 miles per hour, meaning it can read 5,000 characters every second.
When the rebuild was completed, it was pitted in a codebreaking competition against radio enthusiasts who were allowed to employ whatever computing means they had at their disposal. Ironically, Colossus was beaten in the challenge by a German. The competition is due to be repeated in June.