Breaking Tunny

During the Second World War codebreakers of the GC&CS - Government Code and Cipher School - in Bletchley Park, England, were tasked to break German telex messages that were encrypted with the Lorenz SZ40/42 machine. This machine was used to protect the top secret high-level communications of the German armed forces. British codebreakers called all encrypted German telex traffic "Fish" and codenamed the Lorenz machine and its traffic "Tunny". The Lorenz was an on-line cipher machine that used 12 irregular moving pinwheels to create a pseudo-random 5 bit stream which was mixed with the plain 5-bit telex signal. When codebreakers at Bletchley discovered statistical biases in the random bits, generated by the Lorenz machine, they developed cryptanalytic attacks to break the Lorenz messages.

Building the Colossus

The codebreakers soon realized that they needed a machine to process the large number of Tunny messages. Early 1943 the mathematician Max Newman designed the "Heath Robinson", an electro-mechanical machine that was used to break the Lorenz traffic. The prototype became operational in June 1943 but had several technical problems. Meanwhile, a team lead by the brilliant engineer Tommy Flowers, assisted by Max Newman, was busy with a top secret project in the Post Office Research Station in Dollis Hill near London. His goal was to develop and build a digital and programmable advanced version of the Heath Robinson.

By the end of 1943 the Colossus Mark I was ready and all parts were moved to Bletchley Park and assembled during the Christmas holidays by Don Horwood and Harry Fensom. Mid January 1944 the Colossus was ready and began its task to break the top secret German communications. In June 1944 the Colossus Mark II, an improved version followed. The Mark II was 5 times faster and easier to program than the Mark I. The existing Colossus Mark I was revised, more Mark II's were built and by the end of the war 10 Colossus computers were operational.

Colossus at Work

The Colossus was the first ever to use shift registers and counters, and could perform Boolean calculations. It was programmable by switches and plug panels. The Mark I had two racks with 1,500 vacuum tubes, Mark II 2,500 tubes. They were used to store the virtual Lorenz settings and to do the calculations. Colossus could perform parallel calculations which gave the speed a big boost. A 5-bit paper tape with the ciphertext was read by a tape reading system and the Colossus was electronically synchronized with the tape, which limited its speed. The specially designed high speed tape reader could operate at speeds up to 9,700 characters per second. For safer use it was limited to 5,000 characters, which was still an incredible speed for a small paper ribbon! I had the opportunity to check out the running Colossus and its speed was very impressive!

The Colossus computers were very successful and broke enormous numbers of top secret German messages. The information obtained from the broken Tunny messages was codenamed ULTRA. The classification ULTRA was used for all critical and very sensitive information that was obtained from messages cracked by the British codebreakers. One other source of ULTRA was the breaking of the German radio traffic, encrypted with the Enigma cipher machine. ULTRA played a decisive role in the outcome of the Second World War.

After the War

After the war Bletchley Park was closed, eight Colossus computers were destroyed for security reasons, and the two remaining computers were moved together with the GC&CS to Cheltenham. They were dismantled in 1960. The Colossus computers and their task during the war were top secret and this remained so for decades after the war. No one knew about the Colossus, and for many years the American ENIAC received the undeserved credit of being the first programmable digital computer. Ironically, it was the National Security Agency, releasing old archives at the end of the 1970's under the Freedom of Information Act, that showed that Colossus had beaten them by two years. It is now recognized that the Colossus, and the people who developed it, had a significant influence on the development of early computers.

Computer expert Tony Sale and his team, which included some of the original engineers, have reconstructed the Colossus computer. The input of the original designers was invaluable since most of the secret plans of Colossus were destroyed. The operational and running Colossus is located in Bletchley Park and I can highly recommend a visit.

Additional Information

• The Colossus Rebuild Project. Tony Sale's pages on the reconstruction of the Colossus. On his site you can also find more information on the Lorenz cipher machine. Visit his well documented website for more information on cipher machines and codebreaking during the Second World War.
• The design of Colossus by Engineer Tommy Flowers.
• Bletchley Park, England Official site of the Bletchley Park Thrust. In the Second World War, this was the center of all British code breaking efforts.
• Colossus on Wikipedia A good description of the Colossus.

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