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A new exhibit opens today at Bletchley Park that illustrates the entire World War II codebreaking process from signal intercept to final decrypt by Tunny machines.

The original Tunny, a British project to re-engineer the then-unseen German Lorenz S42 cipher machine, was developed in 1942. After tens of thousands of man-hours and with only scattered information about the original, a team at The National Museum of Computing recently succeeded in completing a fully functioning rebuild of a Tunny machine.

The original machines were built by a team led by Bill Tutte that worked out the logical structure of the 12-rotor machine using samples of its encrypted output and the manual decrypts laboriously extracted by other teams at Bletchley Park. The 12 rotors of the Lorenz machine gave it 1.6 quadrillion (1.6 x 10¹⁵) possible start positions.

Tunny completed its first decrypts in 1942 but it really came into its own when the more celebrated Colossus was completed in 1944. Colossus was able to supply wheel settings much more quickly to Tunny than the earlier "Robinson" machines. Colussus reduced the difficult task of determining wheel settings in a matter of hours and reduced the total time spent deciphering a message from several weeks to up to four days.

Around 15 Tunny machines were built. All were dismantled after the war. The original Tunny project, though technically not as significant as the development of Colossus (the world's first modern computer), was still a remarkable achievement in its own right. The machines were an essential component in the decoding of Lorenz-encrypted communications.

The rebuild team had only a few photographs, partial circuit diagrams and the fading memories of a few original Tunny operators to go on. Nonetheless a team led by John Pether and John Whetter was able to complete this restoration work.

Pether explained that getting the electronics to work proved to be the most difficult part of the restoration process.

"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," he said. "Sourcing 200 suitable relays and dealing with the complex wiring schedules was difficult, but we really got in tune with the original team when we had to set up the electronic timing circuits. They were a continuous source of problems then as they are even now for the rebuild team – except the original team didn't even have the benefit of digital storage oscilloscopes."

The rebuild took place in four stages: the construction of a one-wheel Tunny to ensure that timing circuits and relays worked correctly, followed by progressively more complex five-, seven- and 12-wheel Tunny. At each stage, the rebuilds were tested. Key components for the Tunny rebuild were salvaged from decommissioned analogue telephone exchanges, donated by BT. The same components were used to complete the earlier Colussus rebuild project.

Andy Clark, a trustee and director of TNMOC, added:

"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." ®

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