Joan Clarke
Cryptanalyst, Banburist, code-breaker of the Enigma cipher machine during World War II.
The most common means of attack on Naval Enigma at the beginning of the war was Banburismus, a statistical attack which reduced the use of bombes. The second Naval key ever to be broken through Banburismus was broken by Clarke. According to Hugh Alexander, head of Hut 8 in Bletchley Park (1943-1944) – Clarke was one of the best Banburists in the section.
Joan Elisabeth Lowther Clarke was born on June 24, 1917 in West Norwood neighbourhood of London. She graduated from the Dulwich High School for girls in Southern London and in 1936 she was awarded a scholarship which allowed her to attend the Newnham College in Cambridge. She was an extraordinary student. Thanks to her outstanding mathematics skills she got top grades at every stage of her education. Despite that, she did not receive a full degree which until 1948, Cambridge University only granted to male students. Joan received a “double first” title and, at her last year, she also got a Wrangler’s title, awarded for highest finals results on math exams.
During her studies at Cambridge, Joan’s talent had been noticed by Gordon Welchman – one of the four best mathematicians hired in 1939 by the secret British estate Bletchley Park to oversee codebreaking efforts against the Enigma machine. Joan was scouted to work at Bletchley where she was hired by the Government Code and Cypher School (GC&CS); later known as the Government Communications Headquarters (GCHQ).
Around 7500 women worked as cryptanalysts at Bletchley Park during World War II. Until 1945, 75% of Bletchley’s personnel were women. Young British women got hired to operate cryptographic machines, translate Axis Powers’ documents, work as traffic analyst, office workers, secretaries. The Bletchley institute, though quantitatively dominated by women, was an environment created and governed by men and the responsibilities that women could take on were limited.
Joan Clarke arrived at Bletchley on June 17, 1940. At first, she was assigned to a group of women working office jobs, “The Girls”. After some time, due to her mathematical talent, she was moved to the building Hut 8, to Alan Turing’s team working on breaking the Enigma code. Clarke and Turing became fast friends and they remained close until Turing’s death in 1954. They shared many interests and character traits. Turing coordinated their shifts so that they could work together, they also spent most of their free time with each other. After she started working on the Enigma, Joan Clarke got promoted to a linguist position. It was the only way to ensure that she would be paid more for her job. The absurdity of the situation was that Joan did not know any foreign language. Even though she could not hope for a real career advancement, Joan worked just as hard as other cryptanalysts from Hut 8 on solving complex cryptologic problems. When Alan Turing created the cryptanalytical method known as Banburismus, Joan was the only woman alongside eight men – top cryptanalysts – who could use that method.
When in May of 1940 the Allies recovered a German decrypted text along with its coded equivalent from a German patrol boat, it was Joan Clarke who uncovered the cipher key used in these messages. It is estimated that she was able to reduce the amount of sunk shipments for the fighting Allies from 282 000 to 62 000 tones.
Clarke was also working on the issue of the time-consuming nature of decryption. Her teammate Leslie Yoxall dealt with the same problem. He is the author of the initial stage of a strategy that allows for a shorter decryption time. Clarke improved upon Yoxall’s procedure further, proposing her own solution. She found out, however, that she did not create anything original and has in fact rediscovered a method proposed by a famous cryptanalyst from the time of World War I, Dillwin Knox – the so called Dillismus. Yoxall got all the credit for streamlining the process of decryption and the method was called Yoxallismus.
After the main brunt of the Enigma decryption efforts had been delegated to the United States in 1943, some of the Hut 8 members were assigned to other tasks. Clarke however remained at her post. Hugh Alexander, a genius cryptanalyst and the leading Banburist, considered Joan to be the best of them all.
In 1944 Joan Clarke became deputy head of Hut 8. For her service in the Enigma project, she was appointed a Member of the British Empire. After the war, she continued working for the GCHQ until 1977 when she retired at the age of 60. She died in her home in Oxford on September 4, 1996.
