After completing his Ph.D. in mathematical logic at Princeton University in 1938, Alan Turing returned to his native Britain and a fellowship at King’s College, University of Cambridge. With the outbreak of war with Germany the following year, he joined the wartime headquarters of the Government Code and Cypher School, housed at Bletchley Park, Buckinghamshire, some 55 miles northwest of London. Over the next year, Turing and his fellow cryptologists worked to design a machine known as the Bombe, which successfully decoded messages encrypted by Nazi Germany’s Enigma machine. By early 1942, the team at Bletchley Park was decoding about 39,000 intercepted messages each month; the monthly total later rose to more than 84,000.
Turing also developed a system known as Banburismus, which Turing based on a mistake the Germans made in the Enigma’s wheel design. Using this system, Allied code-breakers punched holes that represented encrypted messages into two long sheets of paper, then slide the two sheets on paper over each other until they lined up. This gave Turing’s cryptologists insight into the Enigma’s rotor settings, which the Germans changed daily. The sheets of paper they used became known as Banbury sheets, as the stationery was printed in Banbury, Oxfordshire.
During a recent large-scale renovation of Bletchley Park, workers found a number of these Banbury sheets stuffed into the walls of one of the huts where Turing and his colleagues did their code-breaking work. The documents, which bear notes handwritten by Turing’s colleagues (although not Turing himself) in pencil and crayon, represent the only known examples of Banbury sheets to survive, given the wartime security mandate to destroy all documentary evidence of the code-breaking process. Instead, these notes appear to have been used to block holes in the walls and ceiling of the hut, which like the others at Bletchley had no insulation or heating. As Iain Stander, chief executive of the Bletchley Park Trust, told MKWeb: “The fact that these papers were used to block draughty holes in the primitive hut walls reminds us of the rudimentary conditions under which these extraordinary people were working.”
When Turing and his colleagues began cracking the Enigma code, World War II was decidedly headed in Nazi Germany’s favor. Their work is thought to have done a good deal to turn the tide of the conflict toward the Allies, and may have shortened the war by as much as two years’ time. At war’s end, Turing was made an officer of the Order of the British Empire for his code-breaking work. In the years after the war, he turned his efforts to developing some of the earliest computers. He also pioneered the emerging field of artificial intelligence, developing the so-called Turing test to serve as a criterion for whether a machine can “think.”
In 1952, Turing was prosecuted and convicted of committing homosexual acts, which were then a criminal offense in England. Due to his conviction, he lost his security clearance and access to ongoing government work with codes and computers. In lieu of going to prison, he accepted chemical castration (with estrogen treatments). Turing died two years later of cyanide poisoning; his death was officially ruled a suicide.
Due to its top-secret nature, Turing’s WWII-era code-cracking was not revealed until the 1970s. By the end of the 20th century, he was being honored for that important work, as well as for his groundbreaking achievements in computer science. In 2009, British Prime Minister Gordon Brown publicly apologized for Turing’s treatment by the justice system, and Queen Elizabeth II issued Turing a posthumous pardon in 2013. The current Academy Award-nominated film “The Imitation Game,” starring Benedict Cumberbatch and Keira Knightley, dramatizes the wartime work of Turing and his fellow code-breakers at Bletchley Park, including the daily race to decipher German messages using the Banburismus system.
The surviving Banbury sheets, which the workers discovered in the walls of Bletchley Park’s Hut No. 6 in 2013, were immediately frozen to prevent further deterioration. They have now been restored, and will go on exhibition at Bletchley Park later this month.