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 In the history of cryptography, the Colossus was the first programmable (to a limited extent) digital electronic computer. In World War II, Colossus was used for breaking the German Lorenz SZ 40/42 machine, codenamed "Tunny" by the British. Colossus was designed by Tommy Flowers at the British Post Office Research Station, Dollis Hill. Colossus was preceded by several computers, many first in some category. Zuse's Z3 was the world's first functional fully program-controlled computer, and was based on electromechanical relays, as were the (less advanced) Bell Labs machines of the late 1930s (George Stibitz, et al). Assorted analog computers were semiprogrammable, some of these much predated the 1930s (eg, Vannevar Bush). Babbage's Analytical engine antedated all these (in the mid-1800s), and was both digital and programmable, but was only partially constructed and never functioned at the time (a replica of his Difference engine No. 2, built in 1991 does work, however). Colossus was the first combining all of digital, (partially) programmable, and electronic.
PurposeThe Colossus was designed for cryptanalysis to assist the reading of messages encrypted using the Lorenz SZ 40/42 machine, used for securing high-level German communications. These machines were in the spirit of those first proposed by Colonel Parker Hitt of the US Army around WWI. The Lorenz machine generated a pseudo-random stream of bits, grouped in fives — a stream cipher. However, the keystream and ciphertext produced by the machine exhibited a very subtle statistical bias deviating from that would be expected from a random bitstream; Colossus exploited these statistical weaknesses. OriginsThe idea for Colossus developed out of a prior project which produced a special purpose opto-mechanical comparator machine called the Heath Robinson, and its successors the Old Robinson and Super Robinson. The main problem with the Robinsons was synchronising two paper tapes, one punched with the enciphered message, the other representing the patterns produced by the wheels of the Lorenz machine, that tended to stretch when being read at over 1000 characters per second. Colossus dispensed with the second tape by generating the wheel patterns electronically, and could process 5,000 characters (40 feet / 12m of tape) per second. The Colossus Mark II was simpler to operate as well as being more advanced, and so greatly speeded the deciphering process, which was largely still carried out by hand. It included the first ever use of shift registers and systolic arrays, enabling five simultaneous tests, each involving up to 100 Boolean calculations, on each of the five channels on the punched tape (although in normal operation only one or two channels were examined in any run). Initially Colossus was only used to determine the initial wheel positions used for a particular message (termed wheel setting); the Mark II included mechanisms intended to help determine pin patterns (wheel breaking). Both models were programmable using switches and plug panels, in a way the Robinsons had not been. Work on the design of the Mark I started early in 1943, and the prototype was finished and installed by February 1944. It was followed into service by the Mark II Colossus in June 1944. Ten Mark II Colossus machines were in use at Bletchley Park by the end of the war. Most were destroyed after the war as part of 'protecting secrets' although two survived for many years and were used during the cold war. Design and operationMissing image Colossus-rebuild.jpg In 1994, a team led by Tony Sale began a reconstruction of a Colossus. The machine is nearly complete, and has required over 6,000 man-days of volunteer work. Colossus used state-of-the-art vacuum tubes (valves), thyratrons and photomultipliers to optically read a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true". Although valves were generally considered to be liable to high failure rates it was recognised that failure occurred at power on and off so the Colossus machines, once turned on, were never powered down until the end of the war. The Colossus was efficient for its purpose. Even in 2004, Tony Sale notes that "Colossus is so fast and parallel that a modern PC programmed to do the same code-breaking task takes as long as Colossus to achieve a result!". While Colossus featured limited programmability and was the first of the electronic digital machines to do so, it was not a true general purpose computer, not being Turing-complete. It was not then realized that Turing-completeness was significant; most of the other pioneering modern computing machines were not either (e.g. the ABC machine, the Harvard Mark I electro-mechanical relay machine, the Bell Labs relay machines (by George Stibitz et al), Konrad Zuse's first two designs, and so on). The notion of a computer as a general purpose machine, and not simply a massive calculator devoted to solving difficult but single-minded problems, did not become prominent until many years later. InfluenceThe use to which the Colossi were put was of the highest secrecy, and the Colossus itself was highly secret. It therefore had little influence on the development of later computers; being unknown. EDVAC was the early design which had the most influence on subsequent computer architecture. Colossus documentation and hardware were classified from the moment of their creation and remained so after the War. After the war Winston Churchill specifically ordered the destruction of the Colossus machines into 'pieces no bigger than a man's hand'; Tommy Flowers personally burned blueprints in a furnace at Dollis Hill. However, two machines continued in use after the war at GCHQ in Cheltenham, until their destruction in the 1960s. Information about Colossus emerged publicly in the late 1970s after the secrecy imposed by the Official Secrets Act ended in 1976. Thus, Colossus could not be included in the history of computing hardware for many years. Newman and his associates also were deprived of the recognition they were due. A 500-page technical report on the Tunny cipher and its cryptanalysis — entitled General Report on Tunny — was released by GCHQ to the national Public Record Office in October 2000; the complete report is available online [1] (http://www.alanturing.net/turing_archive/archive/index/tunnyreportindex.html), and it contains a fascinating paean to Colossus by the cryptographers who worked with it:
ReconstructionIn May 2004, the construction of a replica of a Colossus Mark II was completed by a team led by Tony Sale. It currently is on display in the Bletchley Park Museum in Milton Keynes, Buckinghamshire. See alsoFurther reading
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Other meanings: Colossus was the name of a fictional computer that takes over the world in the 1969 science fiction film Colossus: the Forbin Project, loosely based on the novel Colossus by Dennis Feltham Jones. It has been speculated that Jones named his rogue computer after the "real" Colossus, because of the secrecy that surrounded the project.
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