Show me the Tunny

Opinion by Nick Barron

Bletchley Park's new gallery provides an important working record of history-making technology.

Bletchley Park's new gallery provides an important working record of history-making technology.

Recently I was fortunate to receive an invite to the opening of The Tunny Gallery at The National Museum of Computing at Bletchley Park. Here I discovered that the Nazis' Enigma cipher machine was not, contrary to popular belief, cracked by Colossus. The latter was in fact created to accelerate the key recovery for the Germans' Lorenz SZ42 encryption device. A different machine, the Bombe, was the primary device used against Enigma.

The cryptological attack on the SZ42 was nothing short of spectacular. Unlike the Enigma, which had been seen in the flesh to some degree by the boffins at Bletchley, the SZ42 was broken with intercepted traffic alone. After an unfortunate (for the Germans) incident involving a 4,000-character message being sent twice using the same key, it was possible to reconstruct the internal mathematics of the device and work out how to determine the key settings.

What is often forgotten is that key recovery is only half the battle. Once you have the keys you then need to exploit them to decrypt the intercepted traffic, and it is only after this process that you gain the valuable intelligence. For example, although messages before the attack on Pearl Harbor were intercepted and the cipher was cracked, they were not decrypted in time.

The Tunny Gallery reproduces the decryption process, from radio interception to transcription to revelation. It gets its name from the Tunny machine, a rather noisy device that simulates the SZ42 using telephone switch hardware. With the intercepted message and relevant key settings, the Tunny clatters out the decrypted German text.

It was a real thrill for a geek like me to see the SZ42, but impressive as the hardware was, it paled beside the handful of Bletchley veterans who attended the gallery's launch and spoke about their experiences. Captain Jerry Roberts gave a fascinating overview of the process and explained how Tunny decrypts provided critical intelligence used by the Allies for the Battle of Kursk and the D-Day landings. Joking about the high-level strategic intercepts, he noted: “I didn't know much German, but I knew enough to command the German army.”

The charming Helen Currie, one of the operators at Bletchley during World War Two, remarked that the recruitment and vetting process was much simpler in the 1940s: “They asked me two things: would I like to work in the country, and could I keep a secret?”

It was also revealed by the veterans that some of the design notes for the Tunny were being used as toilet paper in Bletchley towards the end of the war, which gives a whole new meaning to the phrase ‘data sanitisation'.

Equally impressive was the team that rebuilt the equipment on display. As you might expect, design documentation for sensitive machinery of this kind was not easy to find, so most of the work involved reverse engineering with the assistance of the remaining Bletchley veterans. One glance at the back of the Tunny with its incredibly complicated wiring left me in complete awe of their skill and patience.

The National Museum of Computing's work to preserve this and other milestones of computing technology is wonderful, and I can only hope it will continue to expand and secure further funding. Sadly, the strict requirements of military security remove a lot of the historical record for such achievements, and I pray that someone is maintaining a suitably secure history of contemporary developments for publication when national security permits.

The museum at Bletchley is well worth a visit for anyone with an interest in WW2 or the history of computing in general. It is always looking for sponsorship, volunteers and old computer equipment. Bletchley Park also provides conference facilities, and would make a great location for a security-related marketing event. Visit the museum or park's websites for details.


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