Tuesday, October 21, 2008

Lorenz Cipher & Collosus

I went to a superb lecture at my institute - The IET - all about the German Lorenz cipher from the war and the Colossus machine built at Bletchley to crack that code. Tony Sale is a very engaging speaker and clearly a very competent engineer. He is the proprietor of the Computing Museum at Bletchley and has (for the last ten years!) been re-building a working Mk2 Colossus.
The Colossus computers were used to help decipher teleprinter messages which had been encrypted using the Lorenz SZ40/42 machine — British codebreakers referred to encrypted German teleprinter traffic as "Fish" and called the SZ40/42 machine and its traffic "Tunny". Colossus compared two data streams, counting each match based on a programmable Boolean function. The encrypted message was read at high speed from a paper tape. The other stream was generated internally, and was an electronic simulation of the Lorenz machine at various trial settings. If the match count for a setting was above a certain threshold, it would be output on an electric typewriter.

I did take copious notes, but Tony's website is excellent and Wikipedia has good articles on both the Lorenz Cipher, which (like the Enigma) is a symmetric stream cipher, and the Colossus computer.

Tony set a challenge a year ago to receive and break a Lorenz transmission and the German engineer who won was using a 1.4 GHz laptop which, running his own code, took less than a minute to find the settings for all 12 wheels. The German codebreaker said: “My laptop digested ciphertext at a speed of 1.2 million characters per second – 240 times faster than Colossus. If you scale the CPU frequency by Moore's Law, you get an equivalent clock of 5.8 MHz for Colossus. That is a remarkable speed for a computer built in 1944."

Whichever way you think about it the codebreakers at Bletchley shortened the war by months or even years and so can be considered the real heroes of WW2.

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