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Last Updated: Thursday, 15 November 2007, 12:16 GMT
How Colossus decoded messages from the enemy
Annotated image - Tony Sale and Colossus replica
Tony Sale demonstrates the working replica of Colossus

Colossus was used during World War II to decrypt German codes. The computer is back in operation at Bletchley Park in the UK, and to celebrate its restoration it is cracking codes once more. But how does it work?

During the war Colossus was used to decrypt teleprinter orders and messages enciphered by a Lorenz SZ40/42 machine that were sent via radio to Hitler's generals.

This enciphered traffic was susceptible to cryptanalysis because the stream of scrambled characters produced by the Lorenz machine was not completely random. Colossus was built to analyse these deviations at high speed.

Cracking the messages was a many-stage process. First, captured radio signals of the enciphered teleprinter messages were punched on to paper tape. This was fed at a rate of 5,000 characters per second into Colossus where it was held in the machine's memory.

Colossus then subjected this to statistical analysis to reveal how the wheels of a Lorenz machine might have been set up to encipher the text.

The patch panel, plugs and program switches were used to set up Colossus to carry out the statistical analysis. The message was analysed many times and the end result, with luck, would be a printed tape that revealed the Lorenz wheel settings so cryptographers could decipher the message.

Colossus typically took about six hours to break a message and reveal the wheel settings.

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