Colossus Lives!

I love a good thriller, especially when old computers are involved. Colossus, the famous British code-breaking machine from World War II, is just such a computer at the heart of a thriller. The Allied forces used Colossus Mark II computers to decipher the Lorenz-coded (often mistaken for Enigma-coded) radio transmissions of the Nazi high command. Some brilliant mathematicians, cryptographers, engineers, and scientists created a very fast, hardwired, parallel machine that recovered the Lorenz crypto-machine’s wheel settings using only ciphertext.

A Colossus Mark II, based on just 2500 vacuum tubes and paper tape, was not a stored-program computer but it was certainly a computing machine. This crypto machine is so powerful that it can still outperform today’s mid-grade, multi-GHz PCs programmed for the same task even though Colossus runs at roughly 5000 Hz, at a speed determined solely by the paper-tape feed rate into the machine! The gear-sprocket holes in the paper tape served as the clock for the entire operation. Now that’s a truly impressive feat for digital engineering as it existed 65 years ago and it’s a testament to the amount of unrealized parallelism in today’s computing hardware.

Colossus was so powerful that the British government tried to erase it and all knowledge about it from existence. There were ten Colossus Mark IIs operating by the end of the war and Winston Churchill directed that the machines be reduced to scrap where each piece was no larger than a man’s hand. We have a pulley from a Colossus in the Computer History Museum in Mountain View. Colossus’ schematics and engineering drawings were all burned in 1960.

So it’s truly, mind-bogglingly amazing that a Colossus Mark II is now fully operational in Bletchley Park, which served as the wartime home for Britain’s secret codebreakers and the ten Colossus Mark IIs and now serves as Britain’s National Museum of Computing. Many people worked on the Colossus reconstruction, but the driver in this project was Tony Sale, who decided that there might be enough information still available to recreate a Colossus while he was working on computer restoration at the London Science Museum in the early 1990s. That was quite a controversial opinion because the only documentation that appeared to remain about the machines was eight old photos plus some illegally saved scraps with partial circuit diagrams.

Even so, Sale set out to recreate this truly historic computing machine. As a result of Sale’s efforts over a decade, plus the contributions of many people including some who worked on the development of the original machines, luck, secret and detailed NSA documents about Colossus that were placed into the public US National Archives in 1995 by an application of the Freedom of Information Act, and financial support from a few true believers, a Colossus Mark II lives again at Bletchley Park. You can read the whole fascinating story here.

Oh, and per the first comment below: yes, the resurrected Colossus does indeed work.