Britain’s most brilliant cryptographer was a young mathematician named Alan Turing. He had read mathematics at King’s College, Cambridge, and then studied for his PhD at Princeton in the United States. At the age of 24 he published a brilliant paper on ‘computable numbers’ that was far ahead of its time, and he is widely regarded as one of the founding-fathers of modern computing science.
When the Polish consignment arrived in Britain, it was Turing who had the task of deciding what to do next. He constructed a design for an upgraded version of the Bombe with 108 different places where the decoding drums could be fitted. Each rotor could be set to one of 17,576 theoretical positions and the machines that Turing designed could try them all in 20 minutes. Manufacture of these Bombes was entrusted to the British Tabulating Machine Company at Letchworth, near London, where Harold Keen had led a research project to introduce punched-card technology, introduced from the United States, to calculating machines. The British Bombe machines were, as one commentator said, ‘the size of a large book-case’ and measured 7ft (2m) wide, 6ft 6in (1.9m) tall and 2ft (0.6m) deep. Each weighed about 1 ton.
In March 1940 the first was completed and installed at Bletchley Park under the code name Victory. A second one followed in August. This had a more advanced diagonal plug-board and was named Agnus Dei (the Lamb of God) but became known as Agnes, or Aggie, for short. Victory was soon upgraded to match Aggie’s specifications. Work at the British Tabulating Machine Company was redoubled, and five separate decoding stations were set up around North London, in case the facility at Bletchley Park was destroyed by a German raid. From that time on, the British were capable of overhearing the German military communications. Everything was carried out under conditions of top security and total secrecy, and the staff at Bletchley Park remained famously reticent about their work for decades following the war. London did not always appreciate the importance of their delicate, private approach; once the building was operational, a group of distinguished officials from London came down to inspect the progress. They arrived in a column of official cars with motorcycle outriders, flags waving, and everyone in full uniform; ‘So much for the secrecy,’ said one of the scientists.
In July 1942 the drawings and wiring diagrams were passed to the United States Navy, and in September 1942 funding of $2 million was requested for the construction of Bombe machines in America; the project was approved within 24 hours. This new version was built by the National Cash Register Corporation (NCR) in Dayton, Ohio, with full collaboration between the Americans and the Bletchley Park team and had a greatly increased capacity. An order for more than 300 of the American machines was placed.
Alan Turing was seconded to Washington DC to advise in December 1942 and went straight to NCR. He quickly calculated how the machines could be linked together, and determined that a smaller number would be sufficient — so the order was reduced to just 96 machines. The American machines were larger than the British Bombes, weighing 2.5 tons, and could run more than 30 times as rapidly. The first came on stream in May 1943 and in June two American Bombes named Adam and Eve broke a particularly difficult message which many believed it was impossible to decode.
A different design was chosen by the US Army and their contract was given to Bell Laboratories in September 1942. It was fitted with telephone relays instead of combinations of mechanical drums, so rotor changes could be done by pressing buttons rather than physically exchanging the rotors. It was intended only for three-rotor decryption, not the four-rotor traffic that the Navy Bombes could handle, but its refined design greatly reduced the time it took to decode messages.
Ultra was the code name for the British decryption project and it was at Bletchley Park that the Ultra project was based. The teams monitored, and successfully decoded, the messages sent from Germany to the U-boats during the battle of the Atlantic. The Bletchley teams also decoded all the orders sent out in the North Africa campaign in 1941, and General Auchinleck later confessed that ‘Rommel would have certainly got through to Cairo’ without the Allies having advance knowledge of all the German plans. During the planning of the Allied invasion of Normandy, Bletchley Park gave the Allied commanders full details of all but two of the 58 Nazi divisions situated on the Western Front. The United States were kept fully involved, and Americans had been invited to become part of the Bletchley Park teams from the start; but Churchill did not trust the Soviet authorities and they never knew of Bletchley Park for the duration of the war. Eventually, the term ‘Ultra’ was adopted by all the Western Allies to signify the decoding of German messages.
When we arrived at Bletchley station we were met by a leading Wren and marched up to a perimeter fence with sentries standing guard. We were then taken to an office in a grand Victorian mansion where we were told that the work we were going to be doing was of the utmost secrecy and vital to the war effort, and we were required to sign the official secrets act. One was left with the distinct impression that contravening it would mean a spell in the Tower at the very least. Next we were escorted across the large grounds to a concrete hut, and had to press a bell and wait to be admitted. When we went inside I was immediately aware of the large back machines making a terrible din and smelling of hot oil. These were the bombes, which were essential to the breaking of the German Enigma codes…
Work in the bombe hut went on 24 hours a day and we worked 8 hour watches with one meal break. The first week was from 8 am to 4 pm, the second 4 pm to midnight and the third midnight to 8 am, with one day off each week. On the fourth week we worked everybody else’s days off and then had 4 days leave. There were two Wrens to each machine and our job was to change the wheel orders, which has a different colour for each number, in accordance with the list given by the code-breakers in Huts 6 and 8. They also dictated how the alphabetised sockets on the back of the machine should be connected to each other. The piece of paper given to us for this purpose was called, interestingly, a menu — the same word we have for starting points on our computers today. Once all this had been done the machine was started up and it clanked and whirred and eventually it would stop. We noticed the wheel order running at that time and the position at which it had stopped and phoned it through to Hut 6 and 8. This could go for hours or even days, with several machines working on the same job, but eventually the cry would be heard: ‘Job up’, which meant they had a match. Occasionally the job didn’t come up and was abandoned because the next day’s settings would now be in operation.
It was quite physical work, plugging and unplugging the menu, working as fast as we could to change the wheel orders and, most importantly, checking the drums when we took them off the machine. The underside of each drum had four circular rows of small wire brushes which had to be kept in a state of perfection. A stray wire could cause a short and distort all the information gathered by the machine, so we had to straighten them with wire tweezers. I don’t think all that close work did our eyesight much good and the noise in Hut 11 properly affected our hearing, too.
Anne Chetwynd-Stapylton, courtesy of Bletchley Park.
There was just one major scare, when three bombs were dropped by a German aircraft in November 1940, just as the base was coming fully on stream. It turned out to be an error, for the bombs had actually been intended for the nearby rail station at Bletchley. The only effect was that one of the bombs blasted Hut 4, housing the Royal Navy Intelligence Unit, off its brick-built base. Workers simply jacked it up, back into position, and work went on as though nothing had happened. Those who worked at Bletchley Park took their task seriously, and never spoke of it afterwards. Even by the 1990s little had ever been disclosed. Churchill noted the unimpeachable level of personal security when, at the end of the war, he described Bletchley Park as ‘The Golden Goose that never cackled’.