Rebuilt Tunny machine adds to knowledge of early computing

Bletchley Park codebreaking veterans were present at the opening of a museum gallery dedicated to recognising the technological achievements of the men and women who contributed to the process in the 1940s.

Jerry Roberts, Helen Curry, John Croft and Gil Hayward were guests of honour at the unveiling of the new Tunny exhibit at The National Museum of Computing, which shows the entire wartime code-breaking operation from interception to decryption. The centrepiece of the gallery is a fully-functioning rebuild of a Tunny machine that produced the final decrypts of enciphered German High Command communications from 1942 to 1945.

The original Tunny, a British re-engineering of the then unseen German Lorenz S42 cipher machine, was completed in 1942 by team led by Bill Tutte who worked in the Testery at Bletchley Park. With only fragmentary information about the original, the rebuilt and functioning Tunny has recently been completed by a volunteer team at The National Museum of Computing.

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

Tunny was a re-engineering of the Lorenz SZ42 cipher machine used in German radio teleprinter communications. No-one at the Testery saw an actual Lorenz until after the war, so Tutte had to work out the logical structure of the highly sophisticated 12-rotor machine using samples of its encrypted output and the manual decrypts laboriously and ingeniously achieved by the Testery. The 12 rotors of the Lorenz machine gave it 1.6 million billion possible start positions, so the work of the Testery was a nigh impossible achievement.

Built by the Post Office Research Station at Dollis Hill, London, Tunny started its first decrypts in 1942. When the Colossus ‘computer’ was completed in 1944, it supplied the wheel settings much more quickly than the earlier ‘Robinson’ computing machines, and the number of Tunny machines was increased to between 12 and 15. By the end of the war, they were working 24/7 deciphering about 300 messages a week to gain vital intelligence for the allied war effort.

The original Tunny, though technically not as significant as the development of Colossus (now generally recognised as the world’s first modern computer), was a remarkable feat in its own right, Whetter adds. By the end of the war, the Tunny machines are thought to have numbered between 12 and 15; they were dismantled and recycled after the war.

The National Museum of Computing at Bletchley Park, an independent charity, houses the largest collection of functional historic computers in Europe, including a rebuilt Colossus, the world’s first electronic programmable computer. The Museum complements the Bletchley Park Trust’s story of code breaking up to the Colossus and allows visitors to follow the development of computing from the ultra-secret pioneering efforts of the 1940s through the mainframes of the 1960s and 1970s, and the rise of personal computing in the 1980s.
See E&T Video’s Tunny Machine video report from the National Museum of Computing -

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