The Colossus Mark 1 computer is delivered to Bletchley Park

December 1943
The Colossus Mark 1 computer is delivered to Bletchley Park

The Colossus Mark 1 machine, used to help break the German Lorenz cipher in WW2, was delivered to the Government Code and Cipher School at Bletchley Park, Buckinghamshire in December 1943.  It was functional by February 1944.  Colossus is considered to be the world's first semi programmable, electronic, digital computer.

The ciphers most commonly associated with the Bletchley Park 'codebreakers' are those produced by the German Enigma machines.  Enigma was used to disguise short battlefield and immediate military messages. The Lorenz cipher machine however was used for strategic communications entailing the encryption of much longer messages, many thousands of characters in length.  Adolf Hitler called Lorenz his 'secret writer' and the breaking of this cipher was of extreme importance to Bletchley Park as it would provide the Allies with an insight into Hitler's longer term plans and strategic intentions.   

A catastrophic error in the use of Lorenz on August 30th 1941 would lead to Bletchley Park codebreakers untangling one message from its cipher and known as the Tiltman Break.  After a great effort by codebreaker Bill Tutte, the logic behind the Lorenz cipher machine was discovered and the equivalent of Lorenz was built at Bletchley Park and known as the Tunny machine.

The breaking of subsequent German Lorenz encrypted ciphers now required the deployment of complex statistical calculations on each message and this would take five or six weeks of work by hand. A faster approach to the calculations was needed.

In January 1943 a small team at the Post Office Research Station, led by Tommy Flowers and working to functional requirements drawn up at Bletchley Park by the mathematician Max Newman, designed an all-electronic  'computer', the Colossus Mk1. This was assembled at the Post Office Research Station at Dollis Hill, London, and began useful work at Bletchley Park in February 1944.

This special-purpose computer contained 1,500 thermionic valves and proved that large numbers of electronic circuits could be made to do reliable calculations at speed. Tommy Flowers and his team would overcome many of the reliability issues of working with valves by leaving the Colossus permanently switched on for the duration of the war.  

A system of wheels on Colossus (on the right in the photograph),  guided the punched paper tape, containing the German encrypted message, through an optical reader as a repetitive loop of punched paper tape in 5-bit teleprinter code.  Characters on the tape were read into Colossus over and over again at the amazing rate of 5,000 characters per second.

Once the paper tape was set up and the machine configured, typically 4-6 hours would pass before Colossus would output the results of the statistical analysis of the message.  These results together with further work by the codebreakers, would result in the breaking of the German Lorenz cipher to reveal the strategic message it disguised.

Colossus Mark 1 contained 1,500 thermionic valves - far more than any single electronic device built up to that time.  Mark 2 systems, which began work on June 1, 1944, contained 2,500 valves and used shift registers to greatly improve processing speed.  Colossus contained electronics for counting, comparison, simple binary arithmetic and logical operations. Output was via an electric typewriter and the 'program' was controlled from plug-boards and switches. 

Ten Colossus units were constructed during the war, providing vital intelligence to Allied forces.  All but two were dismantled after the war, and these last two units were decommissioned in 1959 and 1960.  The existence of the system was only revealed in the 1970s. 

Tony Sale and his project team constructed a rebuild of a Colossus Mark 2 and it is on display at The National Museum of Computing at Bletchley Park.





The Colossus Mark 1 computer is delivered to Bletchley Park

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