Enthusiasts have succeeded in rebuiling a Nazi code cracking device, signaling the culmination of a 10-year project.

The replica Turing Bombe, a recreation of an electromagnetic machine used by British codebreakers to help decipher Nazi codes used during World War Two, was unveiled on Wednesday at Bletchley Park, the centre of British code-breaking efforts during the war.

Bombes automated the process of cracking the Nazi's Enigma code. Enigma devices had three rotors, each with 26 possible positions, creating 17,576 possible combinations for each letter. The devices tried every possible rotor position and applied test to weed out a much smaller number of possible solutions, which were then checked by hand. The whole process relied on using a small section of ciphertext, to which cryptographers had guessed corresponding plain text in order to extract the likely settings used to produce a much longer message.

The Bombe was the brainchild of mathematical geniuses Alan Turing and Gordon Welchman, combined with the engineering efforts of the British Tabulating Machine Company in Hertfordshire. Its design was based in part on earlier Polish code-breaking devices.

The machines enabled Bletchley Park's cryptographers to decode over 3,000 enemy messages a day, giving the allies a vital edge in military intelligence that helped turn the course of World War Two. Without this information the Battle of Britain and the Battle of the North Atlantic could have been lost. Turing's work helped pave the way for later development of mainstream computer technology. It was only in the 1970s that the veil of secrecy surrounding the devices was lifted.

The Bombes used 108 electromagnetic spinning drums to test combinations of letters and reveal the likely keys to the Enigma code used in a particular message. By the end of the war, boffins at Bletchley had built 200 Bombes. Churchill ordered the devices to be taken apart after the war. Simply acquiring blueprints for the devices took two years, according to retired computer engineer John Harper, 69, the leader of the restoration project.

"We were fortunate in having copies of most of the blueprints of the individual parts returned to Bletchley Park by the GCHQ [Britain's signal intelligence agency]," Harper told Reuters. "But there were no assembly drawings...and the blueprints covered more than one model, so it was a bit of a paperchase to work out which drawings applied to which model."

More than 60 volunteers worked on the restoration project, which was backed by the British Computer Society.

Wednesday's media event marked the first time in 60 years it has been possible to re-create the way the supposedly unbreakable Enigma code was cracked using functioning World War Two equipment.

The commissioning phase of the recreation Bombe will be opened to the public on the weekend of 23 and 24 September 2006, an event that will also mark the reunion of Bletchley Park's World War Two veterans. Special demonstrations by re-enactors in period dress will also be taking place. ®