Colossus computer: the story on HearLore

Colossus computer

In the early hours of the 5th of February 1944, a massive machine named Colossus successfully decrypted its first message, marking the beginning of a secret war within a war that would ultimately shorten the conflict by many months. This machine, built from thousands of glowing vacuum tubes, was not merely a calculator but a weapon of intelligence that allowed the Allies to read the highest level of German military communications. The existence of these computers remained a state secret for thirty years after the war ended, and their very existence was erased from history until the mid-1970s. While the world celebrated the end of the war, the ten Colossus machines at Bletchley Park were dismantled, and their blueprints were burned in a furnace by order of the government. The engineers who built them were forbidden from speaking about their work, and the public was left to believe that the first electronic computer was the American ENIAC, which did not appear until 1946. The truth was that the British had already been using programmable electronic computers to break the Lorenz cipher, a system used by the German High Command to communicate with their army commands across occupied Europe. This intelligence, known as Ultra, provided the Allies with a vast amount of high-level military information that was impossible to obtain through any other means. The secrecy surrounding Colossus meant that its creators, including the brilliant engineer Tommy Flowers and the mathematician Max Newman, were denied the recognition they deserved for decades. The machine was so advanced for its time that it was not until the 1970s that historians began to piece together the true story of its development and impact. The destruction of the machines and the burning of the documentation ensured that the legacy of Colossus would remain hidden, buried under layers of official secrecy and the passage of time. The story of Colossus is not just about technology, but about the power of secrecy and the cost of keeping the truth hidden from the very people who built it.

The Blunder That Changed Everything

A fatal error by German operators in August 1941 provided the key to unlocking the Lorenz cipher, a mistake that would eventually lead to the creation of the world's first programmable electronic computer. Two versions of the same message were transmitted with identical machine settings, a blunder that allowed British cryptanalyst John Tiltman to derive a keystream of almost 4000 characters. This keystream was then passed to Bill Tutte, a newly arrived member of the Research Section at Bletchley Park, who used it to deduce the logical structure of the Lorenz machine. Tutte discovered that the machine had twelve wheels, which he named the chi and psi wheels, and the remaining two he called the 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. This discovery was the foundation upon which the entire cryptanalytic effort was built, and it was the first step in a process that would eventually lead to the creation of Colossus. The Lorenz machine used a Vernam ciphering technique on message characters in the standard 5-bit ITA2 telegraph code, combining the plaintext characters with a stream of key characters using the XOR Boolean function to produce the ciphertext. The Tunny machine did this well, but 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. The cryptanalysts then used this method to break the cipher, and the process was further refined by Alan Turing, who invented a method of wheel-breaking that became known as Turingery. The method was further developed into Rectangling, for which Colossus could produce tables for manual analysis. The success of this method was a testament to the ingenuity of the British cryptanalysts, who were able to turn a simple mistake into a powerful tool for breaking the German code. The story of the blunder is a reminder that even the most secure systems can be vulnerable to human error, and that the smallest mistake can have the biggest consequences.

When did the Colossus computer successfully decrypt its first message?

The Colossus computer successfully decrypted its first message on the 5th of February 1944. This event marked the beginning of a secret war within a war that ultimately shortened the conflict by many months.

Who designed the Colossus computer and when was the prototype operational?

Tommy Flowers designed the Colossus computer and the prototype Mark 1 became operational on the 8th of December 1943. The prototype was then dismantled and shipped to Bletchley Park where it was delivered on the 18th of January 1944.

How many Colossus machines were operational at Bletchley Park by V-E Day?

There were ten Colossus machines operational at Bletchley Park by V-E Day. An eleventh machine had started assembly but was not completed before the end of the war.

When was the existence of the Colossus computer finally revealed to the public?

The existence of the Colossus computer remained a state secret until the mid-1970s. The secrecy was broken when Group Captain Winterbotham published his book The Ultra Secret in 1974 and Brian Randell presented a paper on the wartime development in 1976.

Where is the reconstructed Colossus Mark 2 currently displayed?

The reconstructed Colossus Mark 2 is displayed at The National Museum of Computing in H Block Bletchley Park in Milton Keynes, Buckinghamshire. The reconstruction was built between 1993 and 2008 by a team led by Tony Sale.

The Engineer Who Defied Skepticism

Tommy Flowers, a senior electrical engineer and Head of the Switching Group at the Post Office Research Station at Dollis Hill, was the man who designed the Colossus computer, a machine that would change the course of history. Flowers 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. He was recommended to Max Newman by Alan Turing, who had been impressed by his work on the Bombes. Flowers 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 machine. He presented this design to Max Newman in February 1943, but the idea that the one to two thousand thermionic valves proposed could work together reliably was greeted with great scepticism. Flowers persisted with the idea and obtained support from the Director of the Research Station, W Gordon Radley. Flowers and his team of some fifty people in the switching group 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. The prototype, Mark 1 Colossus, contained 1,600 thermionic valves and performed satisfactorily at Dollis Hill on the 8th of December 1943. It was dismantled and shipped to Bletchley Park, where it was delivered on the 18th of January and re-assembled by Harry Fensom and Don Horwood. It was operational in January and it successfully attacked its first message on the 5th of February 1944. The machine was a large structure and was dubbed Colossus. A memo held in the National Archives written by Max Newman on the 18th of January 1944 records that Colossus arrives today. The story of Tommy Flowers is a testament to the power of persistence and the importance of trusting one's own instincts, even in the face of overwhelming skepticism. Flowers' ability to see beyond the limitations of the existing technology and to design a machine that could perform complex calculations at high speed was a remarkable achievement. His work on Colossus was a turning point in the history of computing, and his legacy continues to be felt today.

The Machine That Outpaced The War

The Mark 2 Colossus, an improved version of the prototype, was designed to be five times faster and simpler to operate than the original. 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 the 1st of June. Allen Coombs took over leadership of the production Mark 2 Colossi, the first of which, containing 2,400 valves, became operational at 08:00 on the 1st of June 1944, just in time for the Allied Invasion of Normandy on D-Day. 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. Seven of the Colossi were used for wheel setting and three for wheel breaking. The Mark 2 design included a tape transport with an 8-photocell reading mechanism, a six character FIFO shift register, twelve thyratron ring stores that simulated the Lorenz machine generating a bit-stream for each wheel, and panels of switches for specifying the program and the set total. 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 before the tape disintegrated. So 5000 characters per second 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 and for testing five different possible starting points of Tunny's wheels in the five processors. This five-way parallelism enabled five simultaneous tests and counts to be performed, giving an effective processing speed of 25,000 characters per second. The computation used algorithms devised by W. T. Tutte and colleagues to decrypt a Tunny message. The Mark 2 Colossus was a marvel of engineering, and its speed and efficiency were crucial to the success of the cryptanalytic effort. The machine was able to process messages at a rate that was far beyond the capabilities of any other machine of the time, and its success was a testament to the skill and dedication of the engineers who built it. The Mark 2 Colossus was a key factor in the Allied victory, and its legacy continues to be felt today.

The Women Who Kept The Lights On

The Newmanry was staffed by cryptanalysts, operators from the Women's Royal Naval Service, 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. 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. Using a special hand punch they inserted a start hole between the third and fourth channels sprocket holes from the end of the blank section, and a stop hole between the fourth and fifth channels sprocket holes from the end of the characters of the message. 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. 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. The Wrens were the unsung heroes of the Colossus project, and their work was crucial to the success of the machine. They were the ones who kept the lights on, and their dedication and skill were essential to the operation of the machine. The story of the Wrens is a reminder that the success of the Colossus project was not just the result of the work of the engineers and cryptanalysts, but also the result of the hard work and dedication of the women who operated the machine.

The Erasure Of A Legacy

The existence of the Colossus machines was kept secret until the mid-1970s, and all but two of the Colossi 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. Two Colossi, along with two Tunny machines, were retained and moved to GCHQ's new headquarters at Eastcote in April 1946, and then to Cheltenham between 1952 and 1954. One of the Colossi, known as Colossus Blue, was dismantled in 1959; the other in the 1960s. Tommy Flowers was ordered to destroy all documentation. He duly burnt them in a furnace and later said of that order: The Colossi were adapted for other purposes, with varying degrees of success; in their later years they were used for training. 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. Colossus was also used to perform character counts on one-time pad tape to test for non-randomness. 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, Max Newman and Harry Huskey, wrote that Professor Brian Randell, who unearthed information about Colossus in the 1970s, commented on this, saying that Randell's efforts started to bear fruit in the mid-1970s. The secrecy about Bletchley Park had been broken when Group Captain Winterbotham published his book The Ultra Secret in 1974. Randell was researching the history of computer science in Britain for a conference on the history of computing held at the Los Alamos Scientific Laboratory, New Mexico on 10 to the 15th of June 1976, and got permission to present a paper on wartime development of the COLOSSI at the Post Office Research Station, Dollis Hill. In October 1975 the British Government had released a series of captioned photographs from the Public Record Office. The interest in the revelations in his paper resulted in a special evening meeting when Randell and Coombs answered further questions. Coombs later wrote that no member of our team could ever forget the fellowship, the sense of purpose and, above all, the breathless excitement of those days. In 1977 Randell published an article The First Electronic Computer in several journals. The story of the erasure of the Colossus legacy is a reminder of the power of secrecy and the cost of keeping the truth hidden from the very people who built it. The destruction of the machines and the burning of the documentation ensured that the legacy of Colossus would remain hidden, buried under layers of official secrecy and the passage of time. The story of Colossus is not just about technology, but about the power of secrecy and the cost of keeping the truth hidden from the very people who built it.

The Machine That Refused To Die

A team led by Tony Sale built a fully functional reconstruction of a Colossus Mark 2 between 1993 and 2008. In spite of the blueprints and hardware being destroyed, a surprising amount of material had 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 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 for the event and developed his own signal processing and code-breaking code using Ada. The Colossus team were hampered by their wish to use World War II radio equipment, 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. The Cipher Challenge verified the successful completion of the rebuilding 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. The story of the reconstruction of the Colossus is a testament to the enduring legacy of the machine and the dedication of the people who worked to bring it back to life. The reconstruction is a reminder that the legacy of Colossus is not just about technology, but about the power of human ingenuity and the importance of preserving the past for future generations.