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WWII Code-breaking Machine Rebuilt

May 29, 2011

Four code-breaking machines used during World War II — of which one is a fully functioning rebuild — was presented at the opening of the new Tunny Gallery at the National Museum of Computing last week.

The gallery shows the entire wartime code-breaking process from intercept to decrypt and recognizes the remarkable achievements of the men and women who were part of the whole process back in the 1940s.

The rebuild of a Tunny machine that produced the final decrypts of enciphered communications of the German High Command is the centerpiece of the presentation. The original Tunny, a British re-engineering of the then unseen German Lorenz S42 cipher machine, was completed in 1942.

It took tens of thousands of man-hours for experts to rebuild a functioning Tunny as they only had fragmentary information about the original.

The design of the original Tunny machines was the result of the ingenuity of a team led by Bill Tutte, who worked in the Testery at Bletchley Park. Nobody at the Testery saw the Lorenz until one was captured after the war, so Tutte had to use samples of the machine’s encrypted output and manual decrypts to form a functioning rebuild. The feat was an astonishing achievement as the 12 rotors of the Lorenz machine gave it 1.6 million billion possible starting positions.

The restoration work on the Tunny was re-started in 2005 by a team led by computer conservationists John Pether and John Whetter. Pether said the lack of original material made the rebuild even more challenging.

“We’ve succeeded in rebuilding Tunny with scraps of evidence, and although we are very proud of our work it is rather different from the truly astonishing achievement of Bill Tutte’s re-engineering of the Lorenz machine,” said Pether in a statement.

“As far as I know there were no original circuit diagrams left,” said Pether. “All we had was a few circuit elements drawn up from memory by engineers who worked on the original.”

The hardest part of the rebuild, according to Pether, was getting the six timing circuits of the machine working in unison. The Tunny machines were dismantled and recycled for spare parts after World War II.

By the end of the war there were 12 to 15 Tunny machines in use and the information they revealed about Nazi battle plans aided the Russians during the battle of Kursk and helped to ensure the success of D-Day.

“We have a great deal of admiration for Bill Tuttle and those original engineers,” Whetter told BBC News.

“There were no standard drawings they could put together,” he said. “It was all original thought and it was incredible what they achieved.”

One reason the restoration project has succeeded, noted Whetter, was that the machines were built by the Post Office’s research lab at Dollis Hill. All the parts were typically used to build telephone exchanges, he added.

“The work of the team led by John Pether and John Whetter is fantastic and, we hope, a fitting tribute to the achievements of the wartime code breakers,” said Andy Clark, a trustee and director of TNMOC.

“We can now present the whole process of code-breaking as it happened during World War II in the historic Block H on Bletchley Park. The completion of the Tunny rebuild superbly complements the rebuild of Colossus by Tony Sale’s team in 2007 and will undoubtedly attract even more visitors to the array of fascinating working vintage computers at TNMOC,” Clark said.

The next restoration project being contemplated is that of the Heath Robinson machines, which were used to find SZ42 settings before the creation of Colossus. Those may be even more of a challenge, said Whetter.

“We have even less information about that than we had on Tunny,” he said.

Image Caption: The Battle of Britain ended the German advance in Western Europe. Credit: Wikipedia   

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