The Colossus Computer

The British Colossus was the first ever programmable digital computer. This top secret computer came into service at the end of 1943 and was developed to break German telex traffic, encrypted with the Lorenz SZ40/42 machines. Although Colossus operated two years earlier than the publicly well-known American ENIAC, it never received the proper credit due to its top secret status during and after the war. It was only in the late 1970's that information about this wonderful machine became available. Unfortunately, this magnificent machine is hardly mentioned in history books.

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 secured the top secret high-level communications of the German armed forces. British codebreakers gave the codename "Fish" to all encrypted German telex traffic and the Lorenz machine and its traffic was codenamed "Tunny".

The Lorenz was an on-line cipher machine and 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 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 an advanced digital and programmable version of the Heath Robinson. Its name was Colossus.

The Colossus Mark I was ready by the end of 1943 and all parts were moved to Bletchley Park and assembled during the Christmas holidays by Don Horwood and Harry Fensom. By mid-January 1944, Colossus was finished and began its task to break the top secret German communications. Colossus Mark II, an improved version followed entered service in June 1944 . 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 a total of 10 Colossus computers were operational by the end of the war .

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 had 2,500 tubes. They were used to store the virtual Lorenz settings and to do the statistical calculations. Colossus could perform parallel calculations, enabling very high speed. the 5-bit paper tape, containing the message ciphertext, was read by a tape reading system and the Colossus was electronically synchronized with the tape.

The tape speed was the limiting factor of the Colossus. Therefore, they designed a unique high-speed tape reader that 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 an enormous number of top secret German messages. The information, obtained from the broken Tunny messages, was codenamed ULTRA. This classification was used for all critical and very sensitive information, obtained by cryptanalysis of enemy messages. One other source of ULTRA was the broken 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

Bletchley Park closed after the war and eight Colossus computers were destroyed for security reasons. The two remaining computers were moved together with the GC&CS to Cheltenham. They were dismantled in 1960. After the war, the Colossus computers and their success remained top secret for decades. 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 who, by releasing old archives under the Freedom of Information Act at the end of the 1970's, showed that Colossus had beaten them by two years. It is now recognized that the Colossus, and the people who designed 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.

Colossus in action during the war

Tony Sale and his reconstructed Colossus

Full front view of the Colossus

Tony Sale and I in front of high-speed tape reader

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Dirk Rijmenants 2004. Last changes: 31 March 2022