Colossus I

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Colossus I


Colossus I was the world's first programmable computer. Colossus I was created during World War II by the British to speed up the decryption of German messages encoded by the Lorenz Schlüsselzusatz (SZ) 40 and 42 machines.

In 1940, the British began to intercept German non-Morse teleprinter text that used the Baudot Code, an international standard where each letter is represented by five binary elements. In modern binary notation, A is 11000, B equals 10011, and G is 01011. The Lorenz machine used a code devised by Gilbert Vernam (18901960) in 1918. Obscuring letters were added in modulo 2 addition, where adding like to like gives a 0, while like and unlike equals 1.

The Lorenz machine added two obscuring letters generated by two sets of five-toothed wheels, and two motor wheels arranged in any order and starting position. The British did manage to break this system when multiple messages were sent using the same initial settings, but decoding was time-consuming and only partially successful. Eventually, the internal workings of the SZ machines were deduced, and allowed decoding, providing the starting position of the wheels could be found. Decoding by hand, however, took several weeks. Max Newman (18971984) used the ideas of Alan Turing (19121954) to design a machine to speed up the process. Called "Robinson" after Heath Robinson, the British cartoonist and designer of fantastic machines, it compared the coded text with another piece of tape that represented the Lorenz wheel settings. However, keeping the two paper tapes in sync at high speed was difficult, and they frequently tore.

Tommy Flowers (19051998), an engineer, had the idea of representing the Lorenz wheel settings electronically, doing away with the need for synchronised tapes. Despite many doubters, Flowers spent ten months building the Colossus Mark I, completed on December 8, 1943. Colossus contained 1500 valves, more than had been previously used in a single device, and used photocells to read punched paper tape at 5000 characters per second. It had a limited memory, of five-bits, and used pluggable logic gates. The wheel settings of the Lorenz ciphers were simulated in collections of thyratons, gas-filled triodes that acted as one-bit stores. The results were then printed via a typewriter.

The Colossus Mark I was quickly outdated by the Colossus II, the first of which was finished by June, 1944. Nine Mark IIs were built, and the original machine was upgraded to become the tenth machine, each one occupying a large room. The Colossus II used around 2500 values and read the tape five times as fast as its predecessor.

The Colossus machines counted through the length of the encoded text many times, effectively trying out billions of combinations to determine which initial wheel settings of the Lorenz encoder were statistically significant. The Colossus output did not give a decoded message, but rather the settings of the first set of five wheels. Humans, using a combination of statistics, language skills, and intuition did the remaining decoding. Finally, the complete wheel settings were fed into a device that produced the deciphered message. Later, the versatile Colossi were reprogrammed to do more of the code-breaking work, but there was always considerable input from their human operators.

Breaking the Lorenz cipher gained the Allies crucial information that aided in major operations, such as the Battle of Kursk, and the D-Day landings. Colossus showed that Turing's ideas of a universal computer could be made into practical machines. However, the existence of Colossus was kept secret for many years, and so the American Electronic Numerical Integrator and Computer (ENIAC), completed by the U.S. Army in 1946, was considered the world's first computer until information on Colossus was finally declassified in the 1970s. In 1996, a Colossus was reconstructed, and it can be seen at the Bletchley Park Museum.



Hinsley, F. H., et al. British Intelligences in the Second World War: Its Influence on Strategy and Operations, Volume Three, Part I. London: Her Majesty's Stationary Office, 1984.

Sale, Anthony E. "The Colossus of Bletchley ParkThe German Cipher System," in Raùl Rojas and Ulf Hashagen The First Computers: History and Architectures. Cambridge, MA: MIT Press, 2000.

Smith, Michael. Station X: The Codebreakers of Bletchley Park. London: Channel 4 Books, 2000.


WWII Codes and Ciphers. <> (December 19, 2002).


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