Tunny Machine rebuild completes the story of code-breaking and decryption at Bletchley Park

By Richard Moss | 25 May 2011
a photo of part of a large deciphering machine with lots of dials and knobs
© Robert Dowell and The National Museum of Computing
The National Museum of Computing at Bletchley Park has completed its mission to display and explain the entire World War Two code breaking process - from signal intercept to final decrypt - with the unveiling of a new gallery.  

Four World War Two codebreakers who worked at Bletchley will be on hand for the unveiling of the new Tunny Gallery at Bletchley today (May 26 2011).

Its centrepiece is a fully functioning rebuild of a Tunny machine, which the British cleverly copied and duplicated from the Lorenz SZ42 cipher machine used by German High Command to produce the final decrypts of their enciphered communications.

The rebuilt Tunny was recently completed by a team led by John Pether and John Whetter at The National Museum of Computing, using only fragmentary information consisting of a few photographs, a handful of circuit diagrams and the fading memories of the men who originally used it.

As a result, the ancient deciphering machine took tens of thousands of painstaking man-hours to rebuild.  

"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,” says Pether.

a photo of a deciphering machine with knobs, dials and guages all over it
© Robert Dowell and The National Museum of Computing
Bill Tutte worked in the Testery at Bletchley Park during World War Two and figured out the logical structure of the highly sophisticated 12-rotor machine using only samples of its encrypted output and the manual decrypts laboriously and ingeniously achieved by the codebreakers at Bletchley.

The 12 rotors of the Lorenz machine gave it 1.6 million billion possible start positions, so the work of Tutte and his team in the Testery was an astonishing achievement.

"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,” adds Pether.  

“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."  

Built by the Post Office Research Station at Dollis Hill in London, the original Tunny started its first decrypts in 1942.

When Bletchley’s main decryption computer, Colossus, was completed in 1944, it started to decipher encrypted messages much more quickly than before. The number of Tunny machines feeding it information was increased to between 12 and 15.

By the end of the war they were working around the clock, helping to decipher about 300 messages a week to gain vital intelligence.

The completion of the new Tunny complements the rebuild of the Colossus computer by a team led by Tony Sale in 2007.

Now the whole process of Bletchley’s critical code-breaking work is being explained as it happened during World War II in the historic Block H on Bletchley Park.
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