It was built by British Post Office at its Dollis Hill Research facility by Thomas Flowers and crew to a design by Max Newman and associates of Bletchley Park. It was designed in an attempt to break one or more of the Fish ciphers (a Bletchley Park term) used by the German military for its most secure strategic communications. These were essentially attempts at an electro mechanical implementations of the one-time pad cypher invented by Gilbert Venam and Joseph Mauborgne in the US at the end of WWI.

The most important was a teletype based machine built by Lorenz Electric, the Schlusselzerasatz. Another, different machine was designed and built by Siemens, the T-52 Geheimfernschreiber. An early version of the Siemens machine was used to send signals between Germany and Norway over a cable running through Sweden. The Swedes tapped the cable, copied the traffic, and Arne Buerling, a Swedish mathematician, broke the cypher. Later production versions of the T-52 were harder, even for Bletchley Park.

The one-time pad requires a random sequence. It is combined with the plaintext (character by character) resulting in the ciphertext which is transmitted. On receipt, the same random sequence is combined with the ciphertext (again character by character), and because the combining operation is reversible in a particular way (see XOR) the output is the original plaintext.

In the Lorenz machine, the 'random' sequence was produced by various electromechnaical rotors, and wasn't actually random. Because there were patterns, they could be predicted if the cryptanalysts were sufficiently clever, and plaintexts recovered. In the case of the Lorenz machine, Col. John Tiltman and Bill Tutte of Bletchley Park were sufficiently clever.

The idea for Colossus developed out of a prior project which produced a special purpose comparator machine called the Heath Robinson. The Colossus was intended to be more flexible and faster; it was decided to make it programmable in a way the Heath Robinson had not been. The project was headed by the mathematician Max Newman who drew heavily on the concept of a universal machine conceived by Alan Turing. It started early in 1943 and the first version of the machine (Mark 1 Colossus) was finished and installed by about January 1944, to be followed by the improved Mark 2 Colossus in June 1944. Ten Mark 2 Colossus machines were in use at Bletchley Park by the end of the war.

Since solid state electronics had not yet been invented, the machine used vacuum tubes and optical devices to read a cyphertext from a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true".

It was a highly secret device, and had therefore not much influence on the development of later computers. Nearly all documentation and machinery was classified immediately after the war, and destroyed in 1960s. It is said that Winston Churchill specifically ordered the destruction of the Colossus machines into 'pieces no bigger than a man's hand' and that Tommy Flowers personally burned the blueprints in a furnace at Dollis Hill. Information about Colossus reemerged in the 1970s. Due to this secrecy, Colossus was not able to be included in the history of computing for many years.

A copy of one of the Colossus versions has been partly completed by Tony Sale and is on display in the Bletchley Park Museum in Milton Keynes, Buckinghamshire.

External links:
Lorenz Cipher and the Colossus
http://www.codesandciphers.org.uk/lorenz/index.htm

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Colossus".