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Codebreaking Colossus returns to Bletchley Park

World's first programmable computer on show

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Colossus Mk2, the first ever programmable computer and a crucial piece of WWII history, has been rebuilt. Old code breakers who worked with the machine during the war were given an preview of the machine in action, as part of the D-Day celebrations in at the Science Museum in London.

Colossus will go on display alongside a piece of the original Colossus machine, on show to the public for the first time.

Colossus Mk2 was an upgrade to the original Colossus machine. It was first introduced at Bletchley in December 1943, and was operational by 1944. Within months, ten Colossi were up and running. Between them, they were able to decipher a message sent using the Lorenz system in a matter of hours, and the information they yielded was vital to the allies in the run up to D-Day.

Murlyn Hakon, an expert on the machine at Bletchley Park, gave El Reg a potted history of the great machines.

"Enigma was going well by this stage, but intelligence realised there must be another system when a stray teleprinter message was picked up by a listening post in Kent.

"The messages were sent by Hitler to his field generals and senior staff. They contained detailed information about large scale troop movements. It was essential that the allies decrypt them before D-Day."

The allies knew how this cipher worked, (you can read more about it here) and they could build a machine that could replicate it. However, it was reset every time a message was sent, and each time this was done the key to the cipher needed to be broken.

"The allies built a replica Lorenz system which they called Tunny. Tunny looked nothing like the original Lorenz machine, but the function of the two was identical," Hakon said.

The sole purpose of the Colossus machines was to decipher the key setting of messages encrypted with the Lorenz machine. Colossus used a statistical attack to break the key - the first time this had been done with a machine.

Once they had the key, the code-breakers at Bletchley Park ran the message through a replica of the Lorenz system they had built, quickly deciphering the whole message.

Colossus was built by Dr Tommy Flowers at the Post Office research labs in London. He originally conceived of a programmable machine to automate telephone call switches. However, the theory was equally applicable to the decryption problem.

The machine had 1,500 valves in it, which generated a healthy scepticism: people thought the valves would blow, like they did in family's radio sets. However, Flowers knew the valves were reliable if they were not switched off and on, so once Colossus was switched on, it was not switched off until the end of the war.

It used an optical reader system capable of handling data input at over 5000 characters per second. It was fed the intercepted teleprinter characters on a looped tape. At the start and finish of each text were punched holes so that the machine could recognise the beginning and end of each input.

It was a groundbreaking machine in many respects: it used an early form of double buffering during its analysis and optical valves for input. Because it employed parallel processing to attack its problem, the Colossus machine was enormously powerful. Even today, a desktop PC would take a similar amount of time to crack a Lorenz key.

It was the first ever programmable electronic computer, predating the American Eniac (Electronic Numerical Integrator and Computer) by two years. The Colossus Rebuild Project started in 1993. You can read the full story of the mammoth effort here. ®

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