How volunteers rebuilt World War II computers

A single photograph, scraps of circuit diagrams drawn from memory and a pile of disused components – it isn’t much to go on, but from such meagre beginnings, engineers rebuilt one of the precursors to the modern computer.

The Tunny decryption machine – on display at The Museum of National Computing at Bletchley Park, Buckinghamshire – was a feat of engineering both during World War II when it was created, and over the past five years when it was rebuilt. This is the story of how a team of volunteers turned scraps of information into a fully functioning replica of a machine that helped to win the war.

What was the Tunny?

Stories of WWII code-cracking are now the stuff of Hollywood legend, but the attention is normally thrust upon machines such as Enigma and Colossus. However, the Tunny played no lesser role in the code-cracking effort.

The Tunny is an emulator of the German 12-rotor Lorenz cipher machine, which encrypted messages from the German high command using a new machine-generated code each time – not entirely unlike modern disposable passcodes.

During the war, workers at the code-breaking centre at Bletchley Park would intercept German radio signals, typing the code onto punched tape. That tape was fed into the Tunny, whose settings were controlled by the programmable Colossus machine, which was also rebuilt with the help of volunteers. The message was decrypted and printed out in German text, giving British commanders an invaluable insight into enemy plans.

The restoration team’s achievement is undoubtedly impressive, but consider the engineers and mathematicians at Bletchley Park who initially developed the emulator: while the rebuilders had only a single photograph to work from, the original Tunny was built blind: it was developed in 1942, three years before British forces laid eyes on a Lorenz.

“The people trying to break into it here at Bletchley Park had never seen it,” said John Pether, a member of the restoration team. “They had no idea what it looked like or how it worked.”

The people trying to break into it here at Bletchley Park had never seen it

It may never have been possible without a lucky break. “There was a rather fortunate incident in August 1941, when some German operators on one of these radio teleprinter links became lazy in sending the message,” explained Pether. “They actually had to send it twice; the first time they sent it, it became corrupted. They just set the cipher machines back to the same settings as the first time they sent the message to resend it. That gave the people working to break the machine the keyhole into breaking it – basically, because a couple of Germans got lazy on procedures.”

That mistake gave the British two versions of the same 4,000-character message, but with enough minor differences between them – such as shortening “number” to “nm” – to let Bletchley Park crack the code and allow, over the course of four months, mathematician Bill Tutte to deduce how the Lorenz cipher machine actually worked. Tutte gave the General Post Office’s (GPO) research team enough information to build a replica at Dollis Hill in North London.

Starting from scratch

Between 12 and 15 Tunny machines were eventually built, but all were dismantled for parts – and for security reasons – after the war ended. “As far as we know, they were all broken up at the end of the war. That’s the official story,” said Pether.