Colossus computer

From Academic Kids

A Colossus Mark II computer. The slanted control panel on the left was used to set the pin patterns on the Lorenz; the paper tape transport is on the right.
A Colossus Mark II computer. The slanted control panel on the left was used to set the pin patterns on the Lorenz; the paper tape transport is on the right.

In the history of cryptography, the Colossus machines were the first programmable (to a limited extent) digital electronic computers. In World War II, Colossus machines were used to assist in 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 computer 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.


Purpose and origins

The 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. Bill Tutte, a cryptanalyst at Bletchley Park, discovered that 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.

The 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.

The advent of Colossus

Tommy Flowers, an engineer related to the codebreaking efforts at Bletchley, took the blueprints to the Post Office's research center at Dollis Hill, North London and spent ten months building Colossus. He delivered the working machine to Bletchley on December 8, 1943 two years prior to the completion of another early computer, ENIAC. Colossus contained 1,500 electronic valves and was programmable. For comparison, ENIAC contained 1,800 vacuum tubes (electronic valves), and could perform 5,000 operations 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.

Colossus 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 December 1943. 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 operation

Missing image
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!".

Colossus featured limited programmability and was the first of the electronic digital machines to do so. However, it was not a true general purpose computer, not being Turing-complete, even though Alan Turing on whose research this definition was based, worked at Bletchley Park where Colossus was put into operation. 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 a few years later.

Influence and fate

The use to which the Colossi were put was of the highest secrecy, and the Colossus itself was highly secret, and remained so for many years after the War. Thus, Colossus could not be included in the history of computing hardware for many years, and Flowers and his associates also were deprived of the recognition they were due.

Being not widely known, it therefore had little direct influence on the development of later computers; EDVAC was the early design which had the most influence on subsequent computer architecture.

However, the technology of Colossus, and the knowledge that reliable high-speed electronic digital computing devices were feasible, had a significant influence on the development of early computers in Britain. A number of people who were associated with the project and knew all about Colossus played significant roles in early computer work in Britain. In 1972, Herman Goldstine wrote that:

"Britain had such vitality that it could immediately after the war embark on so many well-conceived and well-executed projects in the computer field".

In writing that, Goldstine was unaware of Colossus, and its legacy to those projects of people such as Alan Turing (with the Pilot ACE and ACE), and Max Newman and I. J. Good (with the Manchester Mark I and other early Manchester computers). Brian Randell later wrote that:

"the COLOSSUS project was an important source of this vitality, one that has been largely unappreciated, as has the significance of its places in the chronology of the invention of the digital computer."

Colossus documentation and hardware were classified from the moment of their creation and remained so after the War, when 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 began to emerge publicly in the late 1970s, after the secrecy imposed by the Official Secrets Act ended in 1976. More recently, 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] (, and it contains a fascinating paean to Colossus by the cryptographers who worked with it:

It is regretted than it is not possible to give an adequate idea of the fascination of a Colossus at work; its sheer bulk and apparent complexity; the fantastic speed of thin paper tape round the glittering pulleys; the childish pleasure of not-not, span, print main header and other gadgets; the wizardry of purely mechanical decoding letter by letter (one novice thought she was being hoaxed); the uncanny action of the typewriter in printing the correct scores without and beyond human aid; the stepping of the display; periods of eager expectation culminating in the sudden appearance of the longed-for score; and the strange rhythms characterizing every type of run: the stately break-in, the erratic short run, the regularity of wheel-breaking, the stolid rectangle interrupted by the wild leaps of the carriage-return, the frantic chatter of a motor run, even the ludicrous frenzy of hosts of bogus scores. [2] (


In 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 also

Further reading

  • Harvey G. Cragon, From Fish to Colossus: How the German Lorenz Cipher was Broken at Bletchley Park (Cragon Books, Dallas, 2003; ISBN 0974304506) - A detailed description of the cryptanalysis of Tunny, and some details of Colossus (contains some minor errors)
  • Tony Sale, The Colossus Computer 1943-1996: How It Helped to Break the German Lorenz Cipher in WWII (M.&M. Baldwin, Kidderminster, 2004; ISBN 0947712364) - A slender (20 page) booklet, containing the same material as Tony Sale's web-site (see below)
  • Simon Singh, The Code Book (Anchor Books (Random House, Inc.), 1999l; ISBN 0-385-49532-3). Pages 243–244 contain a brief description of Colossus.


  • Brian Randall, Colossus: Godfather of the Computer, 1977 (reprinted in The Origins of Digital Computers: Selected Papers, Springer-Verlag, New York, 1982)
  • Brian Randall, The COLOSSUS ( (in Nicholas Metropolis, J. Howlett, Gian-Carlo Rota, (editors), A History of Computing in the Twentieth Century, Academic Press, New York, 1980)
  • I. J. Good, Early Work on Computers at Bletchley (Annals of the History of Computing, Vol. 1 (No. 1), 1979, pp. 38-48)
  • I. J. Good, Pioneering Work on Computers at Bletchley (in A History of Computing in the Twentieth Century)
  • T. H. Flowers, The Design of Colossus (Annals of the History of Computing, Vol. 5 (No. 3), 1983, pp. 239-252)
  • Allen W. M. Coombs, The Making of Colossus (Annals of the History of Computing, Vol. 5 (No. 3), 1983, pp.253-259)
  • W. W. Chandler, The Installation and Maintenance of Colossus (Annals of the History of Computing, Vol. 5 (No. 3), 1983, pp. 260-262)
  • Jack Copeland, Colossus: Its Origins and Originators (IEEE Annals of the History of Computing, 26(4), October–December 2004, pp. 38–45).

External links

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 es:Colossus id:Komputer Colossus nl:Colossus pl:Colossus sv:Colossus


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