Colossus and its successors were used by British codebreakers to help read encrypted German messages during World War II. They used thermionic valves (vacuum tubes) to perform the calculations.
Colossus was designed by engineer Tommy Flowers with input from Sidney Broadhurst, William Chandler, Allen Coombs and Harry Fensom at the Post Office Research Station, Dollis Hill to solve a problem posed by mathematician Max Newman at Bletchley Park.
The prototype, Colossus Mark 1, was shown to be working in December 1943 and was operational at Bletchley Park by February 1944. An improved Colossus Mark 2 first worked on 1 June 1944, just in time for the Normandy Landings.
Ten Colossus computers were in use by the end of the war.
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 sent as output to an electric typewriter.
The Colossus was used to find possible key combinations for the Lorenz machines – rather than decrypting an intercepted message in its entirety.
In spite of the destruction of most of the Colossus hardware and blueprints as part of the effort to maintain a project secrecy that was kept up into the 1970s—a secrecy that deprived some of the Colossus creators of credit for their pioneering advancements in electronic digital computing during their lifetimes—a functional replica of a Colossus computer was completed in 2007.