Alan Turing’s codebreaking feats
The founding father of computing played a vital role in breaking German codes during the Second World War. Joel Greenberg deciphers the brilliant but troubled life of Alan Turing
“The number of possible daily keys for the Enigma system was almost too big to imagine: 158.9 million, million, million possibilities”
In September 1939, just as the Second World War was declared, a young man arrived to stay at the Crown Inn in the hamlet of Shenley Brook End, Buckinghamshire. He was fit enough – an exceptional long-distance runner, in fact – and his new landlady, Mrs Ramshaw, voiced concerns that such a clearly able-bodied young man wasn’t doing his bit for the war effort by joining up.
Mrs Ramshaw’s indignation couldn’t have been more misplaced. The man was Alan Turing, and his work at nearby Bletchley Park – the secret base of the Government Code and Cypher School (GC&CS), the Foreign Office’s codebreaking section – was to prove crucial in thwarting German military actions.
Turing had returned to England the previous summer after several years of research at Princeton University, which led to his PhD. The University of Cambridge then renewed his fellowship at King’s College, to which he had first been elected in March 1935 after earning a first-class honours degree there.
In 1938, with the threat of conflict in Europe looming, Turing was among a number of British academics approached by GC&CS to undertake secret work for them in anticipation of the outbreak of war. He worked part-time for GC&CS, attending several training courses, and collaborated with Dilly Knox, a veteran First World War codebreaker, on attempts to break the Enigma machine.
Enigma variations
On 4 September 1939, the day after Britain declared war on Germany, Turing reported for duty at Bletchley Park and stepped up his work on Enigma. He would go on to lead the team named Hut 8, after the wooden hut in which it was initially based.
Contrary to popular belief, there was no single ‘Enigma code’. The Enigma machine – actually a family of portable encryption devices that substituted each letter of a message for another letter of the alphabet – was first developed in the 1920s and enhanced over subsequent years. By the late 1930s different versions were used by the various branches of the German military. The Germans’ operating procedures exploited the reciprocal nature of the machine. When two Enigma machines were set up the same way, if on one you typed ‘A’ and it turned it into ‘B’, on the other machine if you typed ‘B’, it would turn it into ‘A’.
The setting that governed these substitutions was known at Bletchley Park as the daily key, because it was usually changed every 24 hours. If the Bletchley Park codebreakers could work out the daily key, they could decrypt and read all of the intercepted German messages sent that day. This was done using replica Enigma machines, manufactured in Britain. But the number of possible daily keys was almost too big to imagine. In the case of the German army and air force Enigma, there were 158.9 million, million, million possibilities. It was this daily key that Turing and his colleagues were trying to work out.
In the preceding months, Knox had met with members of the Polish Cipher Bureau who were collaborating with French intelligence. Having worked on Enigma for several years, the Poles had enjoyed some success in breaking the system used by the German army and air force in the 1930s, but their methods no longer worked because of changes made to Enigma by the Germans. They had also designed a semi-automatic machine – a bomba kryptologiczna (reputedly named after a Polish ice cream dessert called a bomba) – to determine the settings that were vital to deciphering the codes produced by Enigma, hugely speeding up the process. In July 1939, they shared their findings with Knox.
At Bletchley Park, Turing devised a new and more powerful kind of electro-mechanical machine for determining the crucial Enigma settings. Another Cambridge mathematician working at Bletchley Park, Gordon Welchman, made a crucial addition that increased the effectiveness of the machine – called the Bombe – providing Bletchley Park with a vital codebreaking tool. By the end of the war, some 211 machines had been produced.
The Bombe, though, wasn’t the complete solution to Enigma. Early in 1940, Turing was asked to take on the task of breaking the German navy’s Enigma system, which used more secure procedures than those of the air force and army. Many at Bletchley believed it could not be broken – yet doing so was vital.
These were desperate times for Britain. The country became increasingly dependent on convoys of ships carrying vital supplies across the North Atlantic, and German U-boat attacks were wreaking havoc on these convoys: average monthly shipping losses in 1940 exceeded 220,000 tonnes. To tackle this, Turing’s Bletchley Park team was expanded.
The challenge was this. Having set up their machines using the daily key, each Enigma operator applied one final setting before encrypting a message. The operators for the German army and air force were allowed to choose this setting themselves, but the German navy issued code books for this purpose. In a remarkable piece of work, Turing managed to deduce, quite quickly, how these code books were being used, but realised that his team would need to acquire copies before further progress could be made.
It wasn’t till a German naval code book was captured that Turing and his colleagues began to achieve success in working out the daily key and reading encrypted German naval messages. Intelligence reports about Germany’s U-boat and ship movements could then be produced and sent to the Admiralty for dissemination.
The interception and decryption of German naval messages played a crucial role in the great sea battles of the Second World War. German ships and U-boats could be located and attacked, and Allied convoys could be diverted to reduce shipping losses.
Public recognition
At its peak, Hut 8 had more than 150 staff. It was part of a large codebreaking operation at Bletchley Park that broke a number of other enemy code and cipher systems as well as Enigma, and employed as many as 10,500 people – the operation truly was a team effort. Yet Turing’s contribution was fundamental.
In late 1940 Turing wrote a report describing the methods he and his colleagues were using to solve the German Enigma system. It was known as ‘Prof’s Book’, and it became essential reading for new recruits.
Years later, Bletchley Park codebreaker Peter Hilton explained that what set Turing apart from his colleagues was his ability to
“Years after publication it became clear that Turing’s paper had laid the foundations for the evolution of computing”
come up with ideas that Hilton felt he would not have thought of “in a million years”. These ideas gave rise to a number of statistical methods with colourful names such as ‘Banburismus’ and ‘Turingery’.
In June 1946 it was announced that Turing had (in 1945) been awarded the Order of the British Empire (OBE) for war services. There were rumours that he had been considered for a higher award, but that the OBE was the highest that could be awarded to civil servants of Turing’s official wartime rank – his true role not being revealed for another three decades.
After the war, Turing worked at the National Physical Laboratory in London, where he designed an early digital computer. In 1945, he took up a position at the University of Manchester and contributed to its pioneering computer developments. Biological research was now occupying much of his time and in November 1951 he completed a paper on morphogenetic theory. However, it was work he’d undertaken much earlier that brought him academic renown in later years.
In 1935 Turing had attended a lecture by mathematician Max Newman, discussing the Entscheidungsproblem (‘decision problem’) which asks for a way of determining which mathematical problems are computable. This had intrigued Turing, and his research yielded the paper ‘On Computable Numbers with an Application to the Entscheidungsproblem’, published by the London Mathematical Society in 1937. By the early 1950s, his fame as the author of ‘On Computable Numbers…’ was growing, and in 1953 the University of Manchester appointed Turing to a specially created readership in the theory of computing.
But while Turing’s academic renown was growing, his private life was in turmoil. On 31 March 1952 at a court in Knutsford, Cheshire, Turing was charged with being “party to the commission of an act of gross indecency” – in effect, he was charged with being homosexual. He pleaded guilty. Instead of imprisonment he opted for hormone ‘treatment’ – oestrogen injections that made him put on weight and enlarged his breasts.
On the morning of 8 June 1954, Turing was found dead in bed by his housekeeper. The coroner’s verdict found that he had taken his own life; there were reports that a partly eaten apple by his bed contained traces of cyanide.
It was not till many years after the publication of Turing’s 1937 paper that it became clear it had probably laid the foundations for the evolution of computing. His story has now been told on stage and screen; perhaps not surprisingly, he remains the only Bletchley Park figure to be widely known. Yet it was only after his death that much of Turing’s life and work, obscured for so long, was revealed.