The codebreakers at Britain's Bletchley Park helped keep the Allies one step ahead of the Nazi war machine during World War II.
Their contribution was decisive in the outcome of the war, but Bletchley Park is also remembered for another reason. As home to the world's first programmable electronic computer, Colossus, and workplace of Alan Turing, seen by many as the father of computer science, Bletchley Park has an important place in the history of computing.
And yet, for decades the codebreakers were unable to talk about what they'd done – compelled by the Official Secrets Act to hide the extent of their contribution to the war, even from their loved ones.
Today the surviving codebreakers are in their 90s but still have vivid memories of the work they remember as being comparable to a high-stakes crossword puzzle.
Captain Jerry Roberts is the last survivor of the nine cryptographers who, from 1942, worked on cracking Tunny, the top level cipher used to protect communiqués from the German High Command to the top generals and field marshals – even messages from Hitler himself.
Tens of thousands of Tunny radio messages were intercepted by the British and broken at Bletchley Park by Roberts and his fellow codebreakers. Tunny provided information that changed the course of the war in Europe and saved lives at critical junctures like the D-Day landings. After the war, General Eisenhower said that the intelligence gleaned at Bletchley had shortened the fighting by at least two years.
Roberts recalls the pressure to decipher the messages as quickly as possible, but also the "fantastic spirit" in the small team and the excitement of the work.
"The pressure was internal, we knew from the signatures that this was important stuff, so we knew we had to get it out as quickly we could," he said, speaking at the launch of an exhibition to commemorate the life of Alan Turing.
"Doing the actual breaking was fascinating – a colleague of mine said that it was the most exciting work that he'd ever done and I would second that. It was like doing a very important crossword puzzle."
The codebreaking itself involved laborious and complex statistical analysis, and Roberts said that they had to break about 50 consecutive pieces of cipher text before messages were able to be decrypted.
"They had to be consecutive places, not 50 places anywhere, it was very challenging. Some times it would take half an hour, other days it would take one or two eight hour shifts. It was a mix of calculation and inspiration."
The need to develop a fast and reliable way to decrypt Tunny traffic led to the creation of the first programmable electronic computer, the Colossus. Built by post office engineer Tommy Flowers, the Mark II Colossus was room-sized machine comprised of 2,500 valves that rattled through some 25,000 characters a second. The Colossus is credited for being able to crunch through the statistical analysis needed to decrypt a message in a fraction of the time that it took the human codebreakers, but Roberts said that the importance of Colossus in cracking Tunny is sometimes overstated.
"It helped in a rather limited way. People make a great fuss about Colossus but in fact we were doing well at the job that Colossus did," he said.
"It did speed up the process, probably by half a shift, four hours, it was not huge but it was perhaps more reliable, you knew that it was going to come up [with something]."
Ten versions of the Colossus were built but by 1960, in order to keep the machine's existence secret, all had been dismantled and all drawings of the machine were burnt - so the machine didn't have a direct impact on the development of future computers.
However Turing was aware of the Colossus, and went on to draw up the plans for the Automatic Computing Engine or ACE, whose capabilities were a step beyond those of Colossus, in that it was a stored-program and general-purpose computer. The 1MHz ACE Pilot, which ran its first program at the National Physical Laboratory in Teddington in 1950, inspired a commercial model called DEUCE that became the foundation of the emerging British computer industry.
Cracking the Enigma
Bletchley is perhaps more famous for being home to the codebreakers that cracked Enigma, and in doing so ended the German U-boat threat, which at one point looked like it might cost Britain the war.
As Roberts tells it, the cracking of the German naval Enigma by the British mathematician and father of the computer Alan Turing came at a vital time for Britain.
"In the spring of 1941, Britain was losing the war. The German Wolf Packs were sinking the ships bringing in food and raw materials to Britain left, right and centre - and of course we didn't know where they were out there, waiting, lurking," he said.
"At that juncture, Turing made his fantastic achievement of breaking naval Enigma. At that juncture, there was no other salvation for Britain. Once naval Enigma was broken, the sinkings dropped by 75 per cent."
But despite Turing's critical achievements at Bletchley and his crucial role in the development of the computer, Roberts recalls him as a reserved figure.
"We never worked together but I used to see him walking the corridors with his gaze averted because he was a very shy man," he said.
"He was an amazing hero, but he didn't project himself in that way - the opposite in fact."
Lord Asa Briggs, aged 90, was working in hut six at Bletchley, decrypting Enigma messages used by the German Army and Navy. Each day the German military would change the settings used to encrypt messages and each day the Briggs and his colleagues were engaged in a race against time to crack that day's code.
"I was in the very heart of the Enigma codebreaking group," he said.
"We had to break a different code each day and no two days which were exactly the same. I enjoyed it. It was a remarkable activity that I would never have had experienced if it hadn't of been for the war."
Briggs and his colleagues worked to identify cribs – fragments of German text that provided a clue as to how the Enigma machine that encrypted the message had been set up. These cribs were used to set up electromechanical machines called Bombes which would calculate the Enigma settings that had produced the message, allowing it to be decrypted. It was a joint effort between man and machine, by identifying the cribs the Enigma team narrowed the number of checks needed to identify the encryption settings from 158 million, million, million to about one million. The Bombes would then check each setting until it found a possible valid key.
The Bombe was designed by Turing in 1939, and by the end of the war, more than 200 of the machines were being used in the UK to crack codes.
Working in eight hour shifts, Briggs said that he was acutely aware of how important it was to decipher the intercepts and the impact they could have on the course of the war.
"We worked very hard. It could be very tiring and extremely frustrating because we knew that the only purpose to our existence was to break those codes. If we couldn't break them it would play on your mind."
The codebreakers were a vital part of a much larger chain of people dedicated to intercepting, deciphering, translating and understanding the German intelligence – many of whom knew only as much as their demanded about the work they were doing. At the peak of Bletchley's wartime activities were some 10,000 people working there, and up to two thirds of the staff were women.
"The most important point is that is it was a team effort. Everybody treated each other on equal terms and women played a fundamental part," said Briggs.
Briggs released memoirs of his time at Bletchley last year, and says that his wartime experiences are still fresh in his mind, still recalling the excitement he felt as the war drew to a close and the soon to be defeated Germans gave up on encrypting their communications. However time has given him new perspectives on what he did at Bletchley.
"It was a very important role for a young person and a great privilege...I was 90 this year and I go on thinking about Bletchley a lot in the light of my own changing experiences."
Nick Heath is chief reporter for TechRepublic UK. He writes about the technology that IT-decision makers need to know about, and the latest happenings in the European tech scene.