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Colossus computer

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The Colossus as it was — seen from behind.

Colossus was the world's first electronic, digital, programmable computer. British codebreakers used Colossus to read secret German messages during World War II. Colossus worked using vacuum tubes.

At Bletchley Park mathematician Max Newman had a problem. It was how to build a machine which would help decoding encrypted messages. Post Office engineers worked on the solution.[1][2] Colossus Mark 1 first worked in December 1943. In February 1944 it was working at Bletchley Park. Colossus Mark 2 was even better. It first worked on 1 June 1944, just in time for the D-Day landings in Normandy. Ten Colossus computers were in use by the end of the war.

The Colossus computers helped decipher teleprinter messages. British codebreakers called the messages "Fish". The messages had been encrypted by the German Lorenz machine. They called the machine and its encryption "Tunny". Colossus would read the encrypted message from a paper tape, and try many settings of the Lorenz machine. Colossus looked at the setting and the tape. It counted each time a letter was the same. If it counted a lot, then it would write on an electric typewriter. It kept trying other settings.

The Colossus was not meant to decrypt all of a message. It just found close settings for the Lorenz machines. The basic idea was that the frequencies of letters and numerals in German messages would give a start to translating the message itself. In English, for example, the letter 'E' is by far the most common letter. Another idea is that German military messages all started with some preliminary information, such as, where the message was from, and who it was addressed to. Of course, the machine disguised this by adding some meaningless symbols to the message, and sending the numbers in packets of five to disguise word length. Even so, the signs of a message can be picked up. The output from the Colossus was analysed by staff who were fluent in the German language.

Until the 1970s Colossus computers were very secret. After the war all the computers were broken into bits, so no one would find out about them. The designs were destroyed. No one knew the first people to make computers. Then a working copy of a Colossus computer was made in 2007.

A Colossus Mark 2 computer. The operator on the left is Dorothy Duboisson. The slanted control panel on the left was used to set the pin patterns on the Lorenz. The "bedstead" paper tape transport is on the right.

Purpose and origins

The Lorenz machine was used by the Germans to encrypt high-level teleprinter communications. It had 12 wheels, and a total of 501 pins.

The Colossus computers were used in the cryptanalysis (code breaking) of high-level German communications. It worked on messages which had been encrypted using the Lorenz SZ 40/42 machine. Part of what the fast electronic Colossus did was to work like the slow electromechanical Lorenz machine. To encrypt a message with the Lorenz machine, the plaintext was mixed with groups of five key bits. The keystream was made by twelve pinwheels. The British called five of them χ ("chi") wheels, another five ψ ("psi") wheels, and the remaining two the "motor wheels". The χ wheels moved regularly as each letter was encrypted. The ψ wheels did not move regularly, they were controlled by the motor wheels.

Bill Tutte was a code breaker at Bletchley Park. He found the keystream made by the Lorenz machine was not random. This meant it was weak and could be broken, so messages could be read. In order to read messages, there were two things that needed to be performed. The first thing was wheel breaking, which was finding the pin patterns for all the wheels. These patterns were set up once on the Lorenz machine and then used for a fixed period of time and for a number of different messages. The second task was wheel setting, which could be tried once the pin patterns were known. Each message encrypted using Lorenz was enciphered at a different start position for the wheels. The process of wheel setting found the start position for a message. Colossus was first used to help with wheel setting. Later Colossus was found useful for wheel breaking too.

The mathematician Max Newman was in charge of the part of Bletchley Park for making machines to break Lorenz messages. Before Colossus they made a machine called "Heath Robinson". It was an opto-mechanical machine — not electronic.

Heath Robinson was not a very good machine. The electro-mechanical relays were slow. The paper tapes did not work well. The tapes would stretch when going at 2000 letters each second. The machine had one tape for the enciphered message. The second was for patterns made by the wheels of the Lorenz machine. Sometimes they would not line up; then the counts were wrong.

Tommy Flowers worked for the Post Office Research Station at Dollis Hill. He was called in to look into the design of the Heath Robinson. He thought it was a bad machine. Without being asked he designed an electronic machine. It would store the letters from one of the tapes inside itself. In February 1943 he show Max Newman this design. The design had one to two thousand thermionic valves (vacuum tubes). Not many people thought that many could work without breaking all the time. Instead more Robinsons were ordered from Dollis Hill. Flowers kept with the idea and got support from the person in charge of the Research Station.

Building Colossus

Tommy Flowers spent eleven months from early February to December 1943 designing and building Colossus. He did it at the Post Office Research Station, Dollis Hill, in northwest London. They tested it in December 1943. Then they took Colossus apart and moved it to Bletchley Park. It got there on 18 January 1944. Harry Fensom and Don Horwood put it back together.[3] Colossus read its first message on 5 February.[4] After the first Colossus (Mark 1) were nine Mark 2 machines. The first Mark 2 was started in June 1944. The Mark 1 machine was changed into a Mark 2. At the end of the war one more Colossus was almost done. Mark 2 was designed while Mark 1 was being made. Allen Coombs became in charge of making Colossus Mark 2 when Tommy Flowers went on to do other things.

Colossus Mark 1 had 1,500 electronic valves (tubes). Colossus Mark 2 had 2,400 valves. Mark 2 was 5x faster than the Mark 1. It was much easier to start than the Mark 1. Bletchley Park could decode many more messages. Later stored-program computers like the Manchester Mark 1 of 1949 used about 4,200 valves; ENIAC (1946) used 17,468 valves.

Colossus did not have the second tape of the Heath Robinson design. Colossus made the wheel patterns electronically. Reading at 5,000 letters/second, the paper tape moved at 40 ft/s (12 m/s) or 27.3 mph (43.9 km/h). The work was synchronized by a clock signal made by the sprocket holes of the punched tape. The speed of working was limited by the tape reader. Tommy Flowers tested the tape reader up to 9,700 letters/second (53 mph (85 km/h)) before the tape was destroyed. He selected 5,000 letters/second as the best speed for regular work.

Sometimes, two or more Colossus computers tried different wheel patterns at once. This is now called parallel computing. Decoding went much faster. Colossus had the first ever use of shift registers and systolic arrays. These allowed five tests at once. Each test was doing up to 100 Boolean calculations, on each of the five channels on the punched tape. Normally only one or two channels were looked at in any run.

At first Colossus was only used to find the starting wheel places used for a message (called wheel setting). The Mark 2 has ways to help find pin patterns (wheel breaking). Both models were programmable using switches and plugs in a way the Robinsons had not been.

Design and use

In 2006, a man called Tony Sale (right) is in charge. They are breaking an enciphered message with the completed machine. Since 1994 his team has been building a new Colossus computer at Bletchley Park.

Colossus used electronics that were new back then. It used vacuum tubes (also called thermionic valves), thyratrons and photomultipliers. It read a paper tape with light. It then used a programmable logical function on every letter. It counted how often this function was "true". Machines with many valves were known to break a lot. They knew that valves break most when turning on. So the Colossus machines were never turned off[4] unless there was a break.

Colossus was the first of the electronic digital machines that could have a program. Today it is seems the program can only change a little:[5]

  • it had no program inside itself. A person used plugs, wires and switches to change the program. This is how it was set it up for a new task.
  • Colossus was not a general-purpose machine. It was designed for only one code breaking task. That task was counting and Boolean operations.

It was not a general Turing-complete computer, even though Alan Turing was at Bletchley Park. Back then they didn't know that it is important to be Turing-complete. Most of the other early modern computing machines were also not Turing-complete. For example: the Atanasoff–Berry Computer, the Harvard Mark I electro-mechanical relay machine, the Bell Labs relay machines (by George Stibitz and others), or the first designs of Konrad Zuse). The idea of a computer as a general purpose machine—that is, as more than a calculator only for the solution of one hard problem—did not become common for many years.

Some computers came before Colossus. Many of them were first in some way. Konrad Zuse made the Z3 computer. It was the first functional fully program-controlled computer. It was based on electromechanical relays, as were the Bell Labs machines. The Bell Labs machines made by George Stibitz and others in the late 1930s. They were not as good as the Z3. The Atanasoff–Berry Computer was electronic (no moving parts like electrical relays) and binary (only on and off, not many like an analog computer) but could not have a program. Some analog computers could have some of their program changed; some of these came well before the 1930s (for example Vannevar Bush). Charles Babbage's Analytical engine design in the mid-19th century. It was a decimal, programmable, made only from moving parts. Only parts of it were made and it did not go while Babbage was alive. Difference engine No. 2 was made in 1991 and does go. Colossus was the first to combine digital, programmable (in part), and electronic. The first all programmable digital electronic computer was the ENIAC which was made in 1946.

Defining characteristics of some early digital computers of the 1940s (In the history of computing hardware)
Name First operational Numeral system Computing mechanism Programming Turing complete
Zuse Z3 (Germany) May 1941 Binary Electro-mechanical Program-controlled by punched film stock Yes (1998)
Atanasoff–Berry Computer (US) mid-1941 Binary Electronic Not programmable—single purpose No
Colossus (UK) January 1944 Binary Electronic Program-controlled by patch cables and switches No
Harvard Mark I – IBM ASCC (US) 1944 Decimal Electro-mechanical Program-controlled by 24-channel punched paper tape (but no conditional branch) No
ENIAC (US) November 1945 Decimal Electronic Program-controlled by patch cables and switches Yes
Manchester Small-Scale Experimental Machine (UK) June 1948 Binary Electronic Stored-program in Williams cathode ray tube memory Yes
Modified ENIAC (US) September 1948 Decimal Electronic Program-controlled by patch cables and switches plus a primitive read-only stored programming mechanism using the Function Tables as program ROM Yes
EDSAC (UK) May 1949 Binary Electronic Stored-program in mercury delay line memory Yes
Manchester Mark 1 (UK) October 1949 Binary Electronic Stored-program in Williams cathode ray tube memory and magnetic drum memory Yes
CSIRAC (Australia) November 1949 Binary Electronic Stored-program in mercury delay line memory Yes

Influence and fate

What the Colossus computers were used for was very secret. The Colossus itself was highly secret even for many years after the War. This is why Colossus could not be included in the history of computing hardware for a long time. Nobody knew how important that Flowers and the other people who helped make it were.

Not many people knew about this secret computer, so it had little direct effect on the new design of later computers; EDVAC was the early design which had the most effect on later computer design.

Once Colossus was made, some people now knew that high-speed electronic (no moving parts like electrical relays) digital computing devices could be made and they did not break too much. Just this knowledge was enough to have a big effect on the designs of early computers in Britain and probably in the US. People who knew about Colossus were important in the early computer field in Britain. In 1972, Herman Goldstine wrote that:

Britain had such vitality (energy or drive) that it could right after the war start so many well-planned and well-done projects in the computer field.[6]

In writing that, Goldstine did not know of Colossus. He did not know what it left to projects of people who knew about it. People such as Alan Turing (with the Pilot ACE and ACE), and Max Newman and I. J. Good (with the Manchester Mark 1 and other early Manchester computers). Brian Randell later wrote that:

the COLOSSUS project was an important source of this vitality (energy or drive), one that has not been well understood or known, as has the importance of its places in the time line of the invention of the digital computer.[7]

Colossus plans and machinary were secret from the when they were made. They stayed so after the War, when Winston Churchill ordered the destruction of most of the Colossus machines into "parts no bigger than a man's hand"; Tommy Flowers himself burned the designs in a fireplace at Dollis Hill. Some parts, changed to look innocent, were taken to Newman's Royal Society Computing Machine Laboratory at Manchester University.[8] The Colossus Mark 1 was taken apart and the parts were sent back to the Post Office. Two Colossus computers, along with two copied Tunny machines, were kept. They were moved to GCHQ's new head office at Eastcote in April 1946. They moved again with GCHQ to Cheltenham between 1952 and 1954.[9] One of the computers, known as Colossus Blue, was taken apart in 1959; the other in 1960.[9] In their later years, the computers were used for training. Before that, there had been attempts to change them (sometimes well) for other purposes.[10] Jack Good was the first to use it after the war, getting NSA to use Colossus to do something for which they were planning to build a special purpose machine.[9] Colossus was also used to perform letter counts on one-time pad tape to test for non-randomness.[9]

At this time the Colossus was still secret, long after any of its technical details were of any importance. This was due to the UK's intelligence agencies use of Enigma-like machines which they got other governments to purchase. The agencies then broke the codes using different ways. Had the knowledge of the codebreaking machines been widely known, no one would have accepted these machines; rather, they would have developed their own methods for encryption, methods that the UK services might not have been able to break. The need for such secrets slowly went away as communications moved to digital transmission and all-digital encryption systems became common in the 1960s.

Colonel Winterbotham's book The Ultra Secret came out in 1975.[11] This broke the secrecy around the Colossus. After that, details about the computer began to become public in the late 1970s.

A 500-page technical report on the Tunny cipher and its code breaking – entitled General Report on Tunny – was given by GCHQ to the national Public Record Office in October 2000; the complete technical report is online.[12][13]

Making it again

Building of a working copy[14] of a Colossus Mark 2 was done by a team led by Tony Sale. The plans and machines had been destroyed, but a surprising amount of other material was not destoyed. It was mostly in engineers' notebooks, much of it in the U.S. The optical tape reader might have been the biggest problem, but Dr. Arnold Lynch, its designer, was able to redesign it from his own first writings. The rebuilt Colossus is on show at The National Museum of Computing, in H Block Bletchley Park in Milton Keynes, Buckinghamshire. This is where the Colossus No. 9 was used in the war.

In November 2007, to mark both the end of the work and the start of fundraising (asking for money) they ran a competition. The money would help The National Museum of Computing with a Cipher Competition[15] where the rebuilt Colossus competed with radio amateurs all around the world. The first to hear and decode three messages enciphered would win. They would be enciphered using the Lorenz SZ42 and transmitted from the radio station in the Heinz Nixdorf MuseumsForum computer museum in Germany. The competition was easily won by radio amateur Joachim Schüth. Schüth had gotten ready[16] for the event. He made his own signal processing and code-breaking program using Ada.[17] The Colossus team lost because the wanted to use World War II radios,.[18] They were a day late because of bad radio conditions. The winner's 1.4 GHz laptop, running his own program, took less than a minute to find the settings for all 12 wheels. The German codebreaker said: "My laptop worked on ciphertext at a speed of 1.2 million letters per second—240x faster than Colossus. If you compare the two computers, you could say Colossus had a speed of 5.8 MHz. That is very fast for a computer built in 1944."[19]

Other pages


  2. Bletchley's code-cracking Colossus
  3. The Colossus Rebuild
  4. 4.0 4.1 Jack Copeland, "Machine against Machine", p. 75 (entire article pp. 64-77) in B. Jack Copeland, ed., Colossus: The Secrets of Bletchley Park's Codebreaking Computers, Oxford University Press, 2006
  5. A Brief History of Computing. Jack Copeland, June 2000
  6. The Computer from Pascal to von Neuman (pp. 321)
  7. B. Randell, "The Colossus", in A History of Computing in the Twentieth Century (N. Metropolis, J. Howlett and G. C. Rota, Eds.), pp.47-92, Academic Press, New York, 1980., p. 87
  8. "A Brief History of Computing". Retrieved 2010-01-26.
  9. 9.0 9.1 9.2 9.3 Copeland, 2006, p. 173-175
  10. Horwood, 1973
  11. Winterbotham, Frederick William (15 November 1975). The Ultra Secret. New York. ISBN 978-0440190615 .
  12. Jack Good; Donald Michie, and Geoffrey Timms (1945). "General Report on Tunny".
  13. "TR17-003".
  14. Sale, Tony. "The Colossus Rebuild Project". Retrieved 30 October 2011.
  15. "Cipher Challenge". Archived from the original on August 1, 2008. Retrieved February 1, 2012.
  16. "SZ42 Codebreaking Software".
  17. "Cracking the Lorenz Code (interview with Schüth)".
  18. Ward, Mark (16 November 2007). "BBC News Article". Retrieved 2 January 2010.
  19. "German Codebreaker receives Bletchley Park Honours".
  • I. J. Good, Early Work on Computers at Bletchley (IEEE Annals of the History of Computing, Vol. 1 (No. 1), 1979, pp. 38–48)
  • I. J. Good, Pioneering Work on Computers at Bletchley (in Nicholas Metropolis, J. Howlett, Gian-Carlo Rota, (editors), A History of Computing in the Twentieth Century, Academic Press, New York, 1980)
  • T. H. Flowers, The Design of Colossus (Annals of the History of Computing, Vol. 5 (No. 3), 1983, pp. 239–252)
  • D C Horwood, A technical description of COLOSSUS I, August 1973, PRO HW 25/24.
  • Brian Randell, Colossus: Godfather of the Computer, 1977 (reprinted in The Origins of Digital Computers: Selected Papers, Springer-Verlag, New York, 1982)
  • Brian Randell, The COLOSSUS (in A History of Computing in the Twentieth Century)
  • Sale, Tony (2000). "The Colossus of Bletchley Park - The German Cipher System". In Rojas, Raúl; Hashagen, Ulf. The First Computers: History and Architecture. Cambridge, Massachusetts: The MIT Press. pp. 351–364. ISBN 0-262-18197-5
  • Albert W. Small, The Special Fish Report (December, 1944) describe the operation of Colossus to break Tunny messages

Further reading

  • Harvey G. Cragon, From Fish to Colossus: How the German Lorenz Cipher was Broken at Bletchley Park (Cragon Books, Dallas, 2003; ISBN 0-9743045-0-6) – A detailed description of the cryptanalysis of Tunny, and some details of Colossus (contains some minor errors)
  • Ted Enever, Britain's Best Kept Secret: Ultra's Base at Bletchley Park (Sutton Publishing, Gloucestershire, 1999; ISBN 0-7509-2355-5) – A guided tour of the history and geography of the Park, written by one of the founder members of the Bletchley Park Trust
  • Tony Sale, The Colossus Computer 1943–1996: How It Helped to Break the German Lorenz Cipher in WWII (M.&M. Baldwin, Kidderminster, 2004; ISBN 0-947712-36-4) – A slender (20 page) booklet, containing the same material as Tony Sale's website (see below)
  • Michael Smith, Station X, 1998. ISBN 0-330-41929-3.
  • Gannon, Paul (2007), Colossus: Bletchley Park's Greatest Secret, Atlantic Books, ISBN 978 1843543312
  • Jack Copeland: Colossus. The Secrets of Bletchley Park's Codebreaking Computers. Oxford University Press 2006. ISBN 0-19-284055-X
  • R. Rojas, U. Hashagen (eds.): The First Computers: History and Architectures. MIT Press 2000. ISBN 0-262-18197-5. – Comparison of the first computers, with a chapter about Colossus and its reconstruction by Tony Sale.

Other websites