Colossus computer

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Colossus computer
A Colossus Mark 2 computer being operated by Wrens. [1] The slanted control panel on the left was used to set the "pin" (or "cam") patterns of the Lorenz. The "bedstead" paper tape transport is on the right.
Developer Tommy Flowers, assisted by Sidney Broadhurst, William Chandler and for the Mark 2 machines, Allen Coombs
Manufacturer Post Office Research Station
TypeSpecial-purpose electronic digital programmable computer
GenerationFirst-generation computer
Release date
  • Mk 1: December 1943 (1943-12)
  • Mk 2: 1 June 1944 (1944-06-01)
Units shipped12
CPU Custom circuits using thermionic valves and thyratrons. A total of 1,600 in Mk 1 and 2,400 in Mk 2. Also relays and stepping switches
MemoryNone (no RAM)
DisplayIndicator lamp panel
Input Paper tape of up to 20,000 × 5-bit characters in a continuous loop
Power8.5 kW [2]

Colossus was a set of computers developed by British codebreakers in the years 1943–1945 to help in the cryptanalysis of the Lorenz cipher. Colossus used thermionic valves (vacuum tubes) to perform Boolean and counting operations. Colossus is thus regarded [3] as the world's first programmable, electronic, digital computer, although it was programmed by switches and plugs and not by a stored program. [4]

A computer is a device that can be instructed to carry out sequences of arithmetic or logical operations automatically via computer programming. Modern computers have the ability to follow generalized sets of operations, called programs. These programs enable computers to perform an extremely wide range of tasks. A "complete" computer including the hardware, the operating system, and peripheral equipment required and used for "full" operation can be referred to as a computer system. This term may as well be used for a group of computers that are connected and work together, in particular a computer network or computer cluster.

Cryptanalysis science

Cryptanalysis is the study of analyzing information systems in order to study the hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages, even if the cryptographic key is unknown.

Cryptanalysis of the Lorenz cipher decoding process

Cryptanalysis of the Lorenz cipher was the process that enabled the British to read high-level German army messages during World War II. The British Government Code and Cypher School (GC&CS) at Bletchley Park decrypted many communications between the Oberkommando der Wehrmacht in Berlin and their army commands throughout occupied Europe, some of which were signed "Adolf Hitler, Führer". These were intercepted non-Morse radio transmissions that had been enciphered by the Lorenz SZ teleprinter rotor stream cipher attachments. Decrypts of this traffic became an important source of "Ultra" intelligence, which contributed significantly to Allied victory.


Colossus was designed by research telephone engineer Tommy Flowers to solve a problem posed by mathematician Max Newman at the Government Code and Cypher School (GC&CS) at Bletchley Park. Alan Turing's use of probability in cryptanalysis (see Banburismus) contributed to its design. It has sometimes been erroneously stated that Turing designed Colossus to aid the cryptanalysis of the Enigma. [5] Turing's machine that helped decode Enigma was the electromechanical Bombe, not Colossus. [6]

Tommy Flowers British engineer, helped to design Colossus during World War II

Thomas Harold Flowers, BSc, DSc, MBE was an English engineer with the British Post Office. During World War II, Flowers designed and built Colossus, the world's first programmable electronic computer, to help solve encrypted German messages.

Max Newman British mathematician

Maxwell Herman Alexander Newman, FRS,, generally known as Max Newman, was a British mathematician and codebreaker. His work in World War II led to the construction of Colossus, the world's first operational, programmable electronic computer, and he established the Royal Society Computing Machine Laboratory at the University of Manchester, which produced the world's first working, electronic stored-program electronic computer in 1948, the Manchester Baby.

Bletchley Park British country house

Bletchley Park is a nineteenth-century mansion and estate near Milton Keynes in Buckinghamshire, constructed during the years following 1883 for the English financier and politician Sir Herbert Samuel Leon in the Victorian Gothic, Tudor, and Dutch Baroque styles, on the site of older buildings of the same name. It has received latter-day fame as the central site for British codebreakers during World War II, although at the time of their operation this fact was a closely guarded secret. During the Second World War, the estate housed the British Government Code and Cypher School (GC&CS), which regularly penetrated the secret communications of the Axis Powers – most importantly the German Enigma and Lorenz ciphers; among its most notable early personnel the GC&CS team of codebreakers included Alan Turing, Gordon Welchman, Hugh Alexander and Stuart Milner-Barry.

The prototype, Colossus Mark 1, was shown to be working in December 1943 and was in use at Bletchley Park by early 1944. An improved Colossus Mark 2 that used shift registers to quintuple the processing speed, first worked on 1 June 1944, just in time for the Normandy landings on D-Day. [7] Ten Colossi were in use by the end of the war and an eleventh was being commissioned. [7] Bletchley Park's use of these machines allowed the Allies to obtain a vast amount of high-level military intelligence from intercepted radiotelegraphy messages between the German High Command (OKW) and their army commands throughout occupied Europe.

In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the "data" input of the next flip-flop in the chain, resulting in a circuit that shifts by one position the "bit array" stored in it, "shifting in" the data present at its input and 'shifting out' the last bit in the array, at each transition of the clock input.

Normandy landings D-Day: First day of the invasion of Nazi-held Europe in World War II

The Normandy landings were the landing operations on Tuesday, 6 June 1944 of the Allied invasion of Normandy in Operation Overlord during World War II. Codenamed Operation Neptune and often referred to as D-Day, it was the largest seaborne invasion in history. The operation began the liberation of German-occupied France from Nazi control, and laid the foundations of the Allied victory on the Western Front.

Allies of World War II Grouping of the victorious countries of World War II

The Allies of World War II, called the United Nations from the 1 January 1942 declaration, were the countries that together opposed the Axis powers during the Second World War (1939–1945). The Allies promoted the alliance as a means to control German, Japanese and Italian aggression.

The existence of the Colossus machines was kept secret until the mid-1970s; the machines and the plans for building them had previously been destroyed in the 1960s as part of the effort to maintain the secrecy of the project. [8] [9] This deprived most of those involved with Colossus of the credit for pioneering electronic digital computing during their lifetimes. A functioning rebuild of a Mark 2 Colossus was completed in 2008 by Tony Sale and some volunteers; it is on display at The National Museum of Computing at Bletchley Park. [10] [11] [12]

Tony Sale Electronics engineer, computer hardware engineer, and curator

Anthony Edgar "Tony" Sale, FBCS was a British electronic engineer, computer programmer, computer hardware engineer, and historian of computing. He led the construction of fully functional Mark 2 Colossus computer between 1993 and 2008. The rebuild is exhibited at The National Museum of Computing at Bletchley Park in England.

The National Museum of Computing museum in the United Kingdom

The National Museum of Computing is a museum in the United Kingdom dedicated to collecting and restoring historic computer systems. The museum is based in rented premises at Bletchley Park in Milton Keynes, Buckinghamshire and opened in 2007. The building — Block H — was the first purpose-built computer centre in the world, hosting six Colossus computers by the end of World War II.

Purpose and origins

The Lorenz SZ machines had 12 wheels, each with a different number of cams (or "pins").
Wheel number
BP wheel name
Number of cams (pins)
23 SZ42-6-wheels-lightened.jpg
The Lorenz SZ machines had 12 wheels, each with a different number of cams (or "pins").
Wheel number123456789101112
BP wheel nameψ1ψ2ψ3ψ4ψ5μ37μ61χ1χ2χ3χ4χ5
Number of cams (pins)434751535937614131292623
Cams on wheels 9 and 10 showing their raised (active) and lowered (inactive) positions. An active cam reversed the value of a bit (0-1 and 1-0). Lorenz Cams.jpg
Cams on wheels 9 and 10 showing their raised (active) and lowered (inactive) positions. An active cam reversed the value of a bit (0→1 and 1→0).

The Colossus computers were used to help decipher intercepted radio teleprinter messages that had been encrypted using an unknown device. Intelligence information revealed that the Germans called the wireless teleprinter transmission systems "Sägefisch" (sawfish). This led the British to call encrypted German teleprinter traffic "Fish", [14] and the unknown machine and its intercepted messages "Tunny" (tunafish). [15]

Teleprinter device for transmitting messages in written form by electrical signals

A teleprinter is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initially they were used in telegraphy, which developed in the late 1830s and 1840s as the first use of electrical engineering. The machines were adapted to provide a user interface to early mainframe computers and minicomputers, sending typed data to the computer and printing the response. Some models could also be used to create punched tape for data storage and to read back such tape for local printing or transmission.

In cryptography, encryption is the process of encoding a message or information in such a way that only authorized parties can access it and those who are not authorized cannot. Encryption does not itself prevent interference, but denies the intelligible content to a would-be interceptor. In an encryption scheme, the intended information or message, referred to as plaintext, is encrypted using an encryption algorithm – a cipher – generating ciphertext that can be read only if decrypted. For technical reasons, an encryption scheme usually uses a pseudo-random encryption key generated by an algorithm. It is in principle possible to decrypt the message without possessing the key, but, for a well-designed encryption scheme, considerable computational resources and skills are required. An authorized recipient can easily decrypt the message with the key provided by the originator to recipients but not to unauthorized users.

Fish (cryptography) Allied codename for any of several German teleprinter stream ciphers used during World War II

Fish was the UK's GC&CS Bletchley Park codename for any of several German teleprinter stream ciphers used during World War II. Enciphered teleprinter traffic was used between German High Command and Army Group commanders in the field, so its intelligence value (Ultra) was of the highest strategic value to the Allies. This traffic normally passed over landlines, but as German forces extended their geographic reach beyond western Europe, they had to resort to wireless transmission.

Before the Germans increased the security of their operating procedures, British cryptanalysts diagnosed how the unseen machine functioned and built an imitation of it called "British Tunny".

It was deduced that the machine had twelve wheels and used a Vernam ciphering technique on message characters in the standard 5-bit ITA2 telegraph code. It did this by combining the plaintext characters with a stream of key characters using the XOR Boolean function to produce the ciphertext.

In August 1941, a blunder by German operators led to the transmission of two versions of the same message with identical machine settings. These were intercepted and worked on at Bletchley Park. First, John Tiltman, a very talented GC&CS cryptanalyst, derived a key stream of almost 4000 characters. [16] Then Bill Tutte, a newly arrived member of the Research Section, used this key stream to work out the logical structure of the Lorenz machine. He deduced that the twelve wheels consisted of two groups of five, which he named the χ ( chi ) and ψ ( psi ) wheels, the remaining two he called μ ( mu ) or "motor" wheels. The chi wheels stepped regularly with each letter that was encrypted, while the psi wheels stepped irregularly, under the control of the motor wheels. [17]

With a sufficiently random key stream, a Vernam cipher removes the natural language property of a plaintext message of having an uneven frequency distribution of the different characters, to produce a uniform distribution in the ciphertext. The Tunny machine did this well. However, the cryptanalysts worked out that by examining the frequency distribution of the character-to-character changes in the ciphertext, instead of the plain characters, there was a departure from uniformity which provided a way into the system. This was achieved by "differencing" in which each bit or character was XOR-ed with its successor. [18] After Germany surrendered, allied forces captured a Tunny machine and discovered that it was the electromechanical Lorenz SZ (Schlüsselzusatzgerät, cipher attachment) in-line cipher machine. [14]

A Lorenz SZ40 machine on display at the National Cryptologic Museum, Fort Meade, Maryland, USA. Lorenz SZ40 cipher machine (TUNNY) - National Cryptologic Museum - DSC07883.JPG
A Lorenz SZ40 machine on display at the National Cryptologic Museum, Fort Meade, Maryland, USA.

In order to decrypt the transmitted messages, two tasks had to be performed. The first was "wheel breaking", which was the discovery of the cam patterns for all the wheels. These patterns were set up on the Lorenz machine and then used for a fixed period of time for a succession of different messages. Each transmission, which often contained more than one message, was enciphered with a different start position of the wheels. Alan Turing invented a method of wheel-breaking that became known as Turingery. [19] Turing's technique was further developed into "Rectangling", for which Colossus could produce tables for manual analysis. Colossi 2, 4, 6, 7 and 9 had a "gadget" to aid this process. [20]

The second task was "wheel setting", which worked out the start positions of the wheels for a particular message, and could only be attempted once the cam patterns were known. [21] It was this task for which Colossus was initially designed. To discover the start position of the chi wheels for a message, Colossus compared two character streams, counting statistics from the evaluation of programmable Boolean functions. The two streams were the ciphertext, which was read at high speed from a paper tape, and the key stream, which was generated internally, in a simulation of the unknown German machine. After a succession of different Colossus runs to discover the likely chi-wheel settings, they were checked by examining the frequency distribution of the characters in processed ciphertext. [22] Colossus produced these frequency counts.

Decryption processes

Notation [23]
Kkey – the sequence of characters used in binary XOR with
the plaintext to give the ciphertext
chi component of key
psi component of key
extended psi – the actual sequence of characters added by
the psi wheels, including those when they do not advance [24]
Dde-chi—the ciphertext with the chi component of the key removed [23]
Δany of the above XOR'ed with its successor character or bit [18]
the XOR operation [25] [26]
Bletchley Park shorthand for telegraphy code space (zero)
xBletchley Park shorthand for telegraphy code mark (one)

By using differencing and knowing that the psi wheels did not advance with each character, Tutte worked out that trying just two differenced bits (impulses) of the chi-stream against the differenced ciphertext would produce a statistic that was non-random. This became known as Tutte's "1+2 break in". [27] It involved calculating the following Boolean function:

∆Z1 ⊕ ∆Z2 ⊕ ∆1 ⊕ ∆2 =

and counting the number of times it yielded "false" (zero). If this number exceeded a pre-defined threshold value known as the "set total", it was printed out. The cryptanalyst would examine the printout to determine which of the putative start positions was most likely to be the correct one for the chi-1 and chi-2 wheels. [28]

This technique would then be applied to other pairs of, or single, impulses to determine the likely start position of all five chi wheels. From this, the de-chi (D) of a ciphertext could be obtained, from which the psi component could be removed by manual methods. [29] If the frequency distribution of characters in the de-chi version of the ciphertext was within certain bounds, "wheel setting" of the chi wheels was considered to have been achieved, [22] and the message settings and de-chi were passed to the "Testery". This was the section at Bletchley Park led by Major Ralph Tester where the bulk of the decrypting work was done by manual and linguistic methods. [30]

Colossus could also derive the start position of the psi and motor wheels, but this was not much done until the last few months of the war, when there were plenty of Colossi available and the number of Tunny messages had declined.

Design and construction

Colossus was developed for the "Newmanry", [31] the section headed by the mathematician Max Newman that was responsible for machine methods against the twelve-rotor Lorenz SZ40/42 on-line teleprinter cipher machine (code named Tunny, for tunafish). The Colossus design arose out of a prior project that produced a counting machine dubbed "Heath Robinson". Although it proved the concept of machine analysis for this part of the process, it was initially unreliable. The electro-mechanical parts were relatively slow and it was difficult to synchronise two looped paper tapes, one containing the enciphered message, and the other representing part of the key stream of the Lorenz machine, [32] also the tapes tended to stretch when being read at up to 2000 characters per second.

Stepping switch from an original Colossus presented by the Director of GCHQ to the Director of the NSA to mark the 40th anniversary of the UKUSA Agreement in 1986 COLOSSUS, part of the machine, presented by Director GCHQ to Director NSA in 1986 - National Cryptologic Museum - DSC07890.JPG
Stepping switch from an original Colossus presented by the Director of GCHQ to the Director of the NSA to mark the 40th anniversary of the UKUSA Agreement in 1986

Tommy Flowers MBE [34] was a senior electrical engineer and Head of the Switching Group at the Post Office Research Station at Dollis Hill. Prior to his work on Colossus, he had been involved with GC&CS at Bletchley Park from February 1941 in an attempt to improve the Bombes that were used in the cryptanalysis of the German Enigma cipher machine. [35] He was recommended to Max Newman by Alan Turing, who had been impressed by his work on the Bombes. [36] The main components of the Heath Robinson machine were as follows.

Flowers had been brought in to design the Heath Robinson's combining unit. [37] He was not impressed by the system of a key tape that had to be kept synchronised with the message tape and, on his own initiative, he designed an electronic machine which eliminated the need for the key tape by having an electronic analogue of the Lorenz (Tunny) machine. [38] He presented this design to Max Newman in February 1943, but the idea that the one to two thousand thermionic valves (vacuum tubes and thyratrons) proposed, could work together reliably, was greeted with great scepticism, [39] so more Robinsons were ordered from Dollis Hill. Flowers, however, knew from his pre-war work that most thermionic valve failures occurred as a result of the thermal stresses at power up, so not powering a machine down reduced failure rates to very low levels. [40] Additionally, the heaters were started at a low voltage then slowly brought up to full voltage to reduce the thermal stress. The valves themselves were soldered in to avoid problems with plug-in bases, which could be unreliable.[ citation needed ] Flowers persisted with the idea and obtained support from the Director of the Research Station, W Gordon Radley. [41] Flowers and his team of some fifty people in the switching group [42] [43] spent eleven months from early February 1943 designing and building a machine that dispensed with the second tape of the Heath Robinson, by generating the wheel patterns electronically. Flowers used some of his own money for the project. [44] [45]

This prototype, Mark 1 Colossus, contained 1600 thermionic valves (tubes). [42] It performed satisfactorily at Dollis Hill on 8 December 1943 [46] and was dismantled and shipped to Bletchley Park, where it was delivered on 18 January and re-assembled by Harry Fensom and Don Horwood. [47] [48] It was operational in January [49] [50] and it successfully attacked its first message on 5 February 1944. [51] It was a large structure and was dubbed 'Colossus', supposedly by the WRNS operators. However, a memo held in the National Archives written by Max Newman on 18 January 1944 records that 'Colossus arrives today". [52]

During the development of the prototype, an improved design had been developed – the Mark 2 Colossus. Four of these were ordered in March 1944 and by the end of April the number on order had been increased to twelve. Dollis Hill was put under pressure to have the first of these working by 1 June. [53] Allen Coombs took over leadership of the production Mark 2 Colossi, the first of which – containing 2400 valves – became operational at 08:00 on 1 June 1944, just in time for the Allied Invasion of Normandy on D-Day. [54] Subsequently, Colossi were delivered at the rate of about one a month. By the time of V-E Day there were ten Colossi working at Bletchley Park and a start had been made on assembling an eleventh. [53]

Colossus 10 with its extended bedstead in Block H at Bletchley Park in the space now containing the Tunny galley of The National Museum of Computing Wartime photo of Colossus 10.png
Colossus 10 with its extended bedstead in Block H at Bletchley Park in the space now containing the Tunny galley of The National Museum of Computing

The main units of the Mark 2 design were as follows. [38] [55]

Most of the design of the electronics was the work of Tommy Flowers, assisted by William Chandler, Sidney Broadhurst and Allen Coombs; with Erie Speight and Arnold Lynch developing the photoelectric reading mechanism. [56] Coombs remembered Flowers, having produced a rough draft of his design, tearing it into pieces that he handed out to his colleagues for them to do the detailed design and get their team to manufacture it. [57] The Mark 2 Colossi were both five times faster and were simpler to operate than the prototype. [58]

Data input to Colossus was by photoelectric reading of a paper tape transcription of the enciphered intercepted message. This was arranged in a continuous loop so that it could be read and re-read multiple times – there being no internal store for the data. The design overcame the problem of synchronizing the electronics with the speed of the message tape, by generating a clock signal from reading its sprocket holes. The speed of operation was thus limited by the mechanics of reading the tape. During development the tape reader was tested up to 9700 characters per second (53 mph) before the tape disintegrated. So 5000 characters/second (40 ft/s (12.2 m/s; 27.3 mph)) was settled on as the speed for regular use. Flowers designed a 6-character shift register, which was used both for computing the delta function (ΔZ) and for testing five different possible starting points of Tunny's wheels in the five processors. [59] [60] This five-way parallelism [61] enabled five simultaneous tests and counts to be performed giving an effective processing speed of 25,000 characters per second. [60] The computation used algorithms devised by W. T. Tutte and colleagues to decrypt a Tunny message. [62] [63]


Colossus selection panel showing selections amongst others, of the far tape on the bedstead, and for input to the algorithm: DZ, D
{\displaystyle \chi }
and D
{\displaystyle \psi }
. Colossus computer selection panel.png
Colossus selection panel showing selections amongst others, of the far tape on the bedstead, and for input to the algorithm: ΔZ, Δ and Δ.

The Newmanry was staffed by cryptanalysts, operators from the Women's Royal Naval Service (WRNS) – known as "Wrens" – and engineers who were permanently on hand for maintenance and repair. By the end of the war the staffing was 272 Wrens and 27 men. [53]

The first job in operating Colossus for a new message, was to prepare the paper tape loop. This was performed by the Wrens who stuck the two ends together using Bostik glue, ensuring that there was a 150-character length of blank tape between the end and the start of the message. [64] Using a special hand punch they inserted a start hole between the third and fourth channels 2 12 sprocket holes from the end of the blank section, and a stop hole between the fourth and fifth channels 1 12 sprocket holes from the end of the characters of the message. [65] [66] These were read by specially positioned photocells and indicated when the message was about to start and when it ended. The operator would then thread the paper tape through the gate and around the pulleys of the bedstead and adjust the tension. The two-tape bedstead design had been carried on from Heath Robinson so that one tape could be loaded whilst the previous one was being run. A switch on the Selection Panel specified the "near" or the "far" tape. [67]

After performing various resetting and zeroizing tasks, the Wren operators would, under instruction from the cryptanalyst, operate the "set total" decade switches and the K2 panel switches to set the desired algorithm. They would then start the bedstead tape motor and lamp and, when the tape was up to speed, operate the master start switch. [67]


Colossus K2 switch panel showing switches for specifying the algorithm (on the left) and the counters to be selected (on the right). Colossus computer Q panel.png
Colossus K2 switch panel showing switches for specifying the algorithm (on the left) and the counters to be selected (on the right).
Colossus 'set total' switch panel Picture111Shopped.jpg
Colossus 'set total' switch panel

Howard Campaigne, a mathematician and cryptanalyst from the US Navy's OP-20-G, wrote the following in a foreword to Flowers' 1983 paper "The Design of Colossus".

My view of Colossus was that of cryptanalyst-programmer. I told the machine to make certain calculations and counts, and after studying the results, told it to do another job. It did not remember the previous result, nor could it have acted upon it if it did. Colossus and I alternated in an interaction that sometimes achieved an analysis of an unusual German cipher system, called "Geheimschreiber" by the Germans, and "Fish" by the cryptanalysts. [68]

Colossus was not a stored-program computer. The input data for the five parallel processors was read from the looped message paper tape and the electronic pattern generators for the chi, psi and motor wheels. [69] The programs for the processors were set and held on the switches and jack panel connections. Each processor could evaluate a Boolean function and count and display the number of times it yielded the specified value of "false" (0) or "true" (1) for each pass of the message tape.

Input to the processors came from two sources, the shift registers from tape reading and the thyratron rings that emulated the wheels of the Tunny machine. [70] The characters on the paper tape were called Z and the characters from the Tunny emulator were referred to by the Greek letters that Bill Tutte had given them when working out the logical structure of the machine. On the selection panel, switches specified either Z or ΔZ, either or Δ and either or Δ for the data to be passed to the jack field and 'K2 switch panel'. These signals from the wheel simulators could be specified as stepping on with each new pass of the message tape or not.

The K2 switch panel had a group of switches on the left hand side to specify the algorithm. The switches on the right hand side selected the counter to which the result was fed. The plugboard allowed less specialized conditions to be imposed. Overall the K2 switch panel switches and the plugboard allowed about five billion different combinations of the selected variables. [64]

As an example: a set of runs for a message tape might initially involve two chi wheels, as in Tutte's 1+2 algorithm. Such a two-wheel run was called a long run, taking on average eight minutes unless the parallelism was utilised to cut the time by a factor of five. The subsequent runs might only involve setting one chi wheel, giving a short run taking about two minutes. Initially, after the initial long run, the choice of next algorithm to be tried was specified by the cryptanalyst. Experience showed, however, that decision trees for this iterative process could be produced for use by the Wren operators in a proportion of cases. [71]

Influence and fate

Although the Colossus was the first of the electronic digital machines with programmability, albeit limited by modern standards, [72] it was not a general-purpose machine, being designed for a range of cryptanalytic tasks, most involving counting the results of evaluating Boolean algorithms.

A Colossus computer was thus not a fully Turing complete machine. However, University of San Francisco professor Benjamin Wells has shown that if all ten Colossus machines made were rearranged in a specific cluster, then the entire set of computers could have simulated a universal Turing machine, and thus be Turing complete. [73] The notion of a computer as a general purpose machine — that is, as more than a calculator devoted to solving difficult but specific problems — did not become prominent until after World War II.[ citation needed ]

Colossus and the reasons for its construction were highly secret, and remained so for 30 years after the War. Consequently, it was not included in the history of computing hardware for many years, and Flowers and his associates were deprived of the recognition they were due. Colossi 1 to 10 were dismantled after the war and parts returned to the Post Office. Some parts, sanitised as to their original purpose, were taken to Max Newman's Royal Society Computing Machine Laboratory at Manchester University. [74] Tommy Flowers was ordered to destroy all documentation and burnt them in a furnace at Dollis Hill. He later said of that order:

That was a terrible mistake. I was instructed to destroy all the records, which I did. I took all the drawings and the plans and all the information about Colossus on paper and put it in the boiler fire. And saw it burn. [75]

Colossi 11 and 12, along with two replica Tunny machines, were retained, being moved to GCHQ's new headquarters at Eastcote in April 1946, and again with GCHQ to Cheltenham between 1952 and 1954. [76] One of the Colossi, known as Colossus Blue, was dismantled in 1959; the other in 1960. [76] There had been attempts to adapt them to other purposes, with varying success; in their later years they had been used for training. [77] Jack Good related how he was the first to use Colossus after the war, persuading the US National Security Agency that it could be used to perform a function for which they were planning to build a special-purpose machine. [76] Colossus was also used to perform character counts on one-time pad tape to test for non-randomness. [76]

A small number of people who were associated with Colossus—and knew that large-scale, reliable, high-speed electronic digital computing devices were feasible—played significant roles in early computer work in the UK and probably in the US. However, being so secret, it had little direct influence on the development of later computers; it was EDVAC that was the seminal computer architecture of the time. In 1972 Herman Goldstine, who was unaware of Colossus and its legacy to the projects of people such as Alan Turing (ACE), Max Newman (Manchester computers) and Harry Huskey (Bendix G-15), 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. [78]

Professor Brian Randell, who unearthed information about Colossus in the 1970s, commented on this, saying that:

It is my opinion 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. [79]

Randell's efforts started to bear fruit in the mid-1970s, after the secrecy about Bletchley Park was broken when Group Captain Winterbotham published his book The Ultra Secret in 1974. [80] In October 2000, a 500-page technical report on the Tunny cipher and its cryptanalysis—entitled General Report on Tunny [81] —was released by GCHQ to the national Public Record Office, and it contains a fascinating paean to Colossus by the cryptographers who worked with it:

It is regretted that 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. [82]


A team led by Tony Sale (right) reconstructed a Colossus Mark II at Bletchley Park. Here, in 2006, Sale supervises the breaking of an enciphered message with the completed machine. ColossusRebuild 11.jpg
A team led by Tony Sale (right) reconstructed a Colossus Mark II at Bletchley Park. Here, in 2006, Sale supervises the breaking of an enciphered message with the completed machine.

Construction of a fully functional rebuild [83] [84] of a Colossus Mark 2 was undertaken between 1993 and 2008 by a team led by Tony Sale. [12] [11] In spite of the blueprints and hardware being destroyed, a surprising amount of material survived, mainly in engineers' notebooks, but a considerable amount of it in the U.S. The optical tape reader might have posed the biggest problem, but Dr. Arnold Lynch, its original designer, was able to redesign it to his own original specification. The reconstruction is on display, in the historically correct place for Colossus No. 9, at The National Museum of Computing, in H Block Bletchley Park in Milton Keynes, Buckinghamshire.

In November 2007, to celebrate the project completion and to mark the start of a fundraising initiative for The National Museum of Computing, a Cipher Challenge [85] pitted the rebuilt Colossus against radio amateurs worldwide in being first to receive and decode three messages enciphered using the Lorenz SZ42 and transmitted from radio station DL0HNF in the Heinz Nixdorf MuseumsForum computer museum. The challenge was easily won by radio amateur Joachim Schüth, who had carefully prepared [86] for the event and developed his own signal processing and code-breaking code using Ada. [87] The Colossus team were hampered by their wish to use World War II radio equipment, [88] delaying them by a day because of poor reception conditions. Nevertheless, the victor's 1.4 GHz laptop, running his own code, took less than a minute to find the settings for all 12 wheels. The German codebreaker said: "My laptop digested ciphertext at a speed of 1.2 million characters per second—240 times faster than Colossus. If you scale the CPU frequency by that factor, you get an equivalent clock of 5.8 MHz for Colossus. That is a remarkable speed for a computer built in 1944." [89]

The Cipher Challenge verified the successful completion of the rebuild project. "On the strength of today's performance Colossus is as good as it was six decades ago", commented Tony Sale. "We are delighted to have produced a fitting tribute to the people who worked at Bletchley Park and whose brainpower devised these fantastic machines which broke these ciphers and shortened the war by many months." [90]

Front view of the Colossus rebuild showing, from right to left (1) The "bedstead" containing the message tape in its continuous loop and with a second one loaded. (2) The J-rack containing the Selection Panel and Plug Panel. (3) The K-rack with the large "Q" switch panel and sloping patch panel. (4) The double S-rack containing the control panel and, above the image of a postage stamp, five two-line counter displays. (5) The electric typewriter in front of the five sets of four "set total" decade switches in the C-rack. Frontal view of the reconstructed Colossus at The National Museum of Computing, Bletchley Park.jpg
Front view of the Colossus rebuild showing, from right to left (1) The "bedstead" containing the message tape in its continuous loop and with a second one loaded. (2) The J-rack containing the Selection Panel and Plug Panel. (3) The K-rack with the large "Q" switch panel and sloping patch panel. (4) The double S-rack containing the control panel and, above the image of a postage stamp, five two-line counter displays. (5) The electric typewriter in front of the five sets of four "set total" decade switches in the C-rack.

Other meanings

There was a fictional computer named Colossus in the 1970 movie Colossus: The Forbin Project which was based on the 1966 novel Colossus by D. F. Jones. This was sheer coincidence as it pre-dates the public release of information about Colossus, or even its name.

Neal Stephenson's novel Cryptonomicon (1999) also contains a fictional treatment of the historical role played by Turing and Bletchley Park.

See also


  1. The two operators have been variously identified as Dorothy Du Boisson (left) and Elsie Booker, Vivian Vorster (left) and Catherine Kennedy, and (unknown) and Patricia (Pat) Davis (right).[ citation needed ]
  2. Based on what the National Museum of Computing state is the power consumption of the Colossus rebuild. In the absence of information to the contrary, the original is presumed to be similar.
  3. Copeland 2006, Copeland, Jack, Introduction p. 2.
  4. Sale 2000.
  5. Golden, Frederic (29 March 1999), "Who Built The First Computer?", Time Magazine, vol. 153 no. 12
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  8. Barber, Nicola (21 December 2015). "Who Broke the Wartime Codes?". Capstone. Retrieved 26 October 2017 via Google Books.
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  18. 1 2 Good, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11C Wheel Patterns, (b) Differenced and Undifferenced Wheels, p. 11.
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  25. The Boolean or "truth" function XOR, also known as Exclusive disjunction and Exclusive or, is the same as binary modulo 2 addition and subtraction
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  58. For comparison, later stored-program computers such as the Manchester Mark 1 of 1949 used 4050 valves,Lavington, S. H. (July 1977), "The Manchester Mark 1 and Atlas: a Historical Perspective" (PDF), Communications of the ACM – Special issue on computer architecture, 21 (1): 4–12, doi:10.1145/359327.359331 , retrieved 8 February 2009 while ENIAC (1946) used 17,468 valves.
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  61. This would now be called a systolic array
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Further reading