Joan Clarke

Key Facts & Summary

  • Joan Elisabeth Lowther Murray was an English cryptanalyst and numismatist best known for her work as a code-breaker at Benchley Park during World War II.
  • Her role in the Enigma project that decrypted Nazi Germany’s secret communications, earned her awards and citations, such as the appointment as a Member of the Order of the British Empire or MBE in 1946.
  • Joan Clarke was awarded the distinguished Philippa Fawcett Prize and in 1939-1940 the Helen Gladstone Scholarship.
  • Her controversial relationship with Alan Turing sometimes overshadows her contributions to the war effort.
  • Her husband Murray, who had published works on the Scottish coinage of the 16th and 17th century, raised Clarke’s interest in numismatic history.
  • She would go on and establish the sequence of the complex series of gold unicorn and heavy groat coins that were in circulation in Scotland during the reign of James III and James IV. Her contributions were recognized and she was awarded the Sanford Saltus Gold Medal.


Joan Clarke was born to William Kemp Lowther Clarke, a Clergyman and Dorothy Elisabeth Clarke in 1917. She was the youngest child and was educated at Dulwich High School and in 1936 matriculated at Newnham College, Cambridge, to study Mathematics. In 1937 and 1939 respectively, she achieved a First in Part I and Part II of the Mathematical Tripos – a three-year course leading to a BA degree and became a Wrangler.

In 1939 she graduated, achieving a double First in Mathematics. However this was merely a title, as Cambridge did not admit women to “full membership of the body academic” until after the end of World War II. Due to her prowess in the Mathematical field, she attracted the attention of Gordon Welchman, one of the four top mathematicians recruited in 1939 to set up decoding operations at Bletchley Park. Gordon was responsible for recruiting Clarke to join the “Government Code and Cypher School” or GCCS, at Bletchley Park.

Code breaker

In order to defeat Germany, it was imperative to break the Enigma code. Churchill’s government searched the country for the best mathematicians, chess champions, Egyptologists and others of suitable ability who would know anything about the possible permutations of formal systems, to assist in the operation of cracking the enigma code. In August 1939, the GCCS was set up in privacy at Bletchley Park, its sole mission: To break the enigma code. Bletchley Park was far safer than London for the code breakers, it was at the junction of a major road.

This was a very difficult task, Alastair Denniston, who was to become the first Head of Bletchley Park, thought it would be impossible. He is recorded as telling his fellow code breakers: “…all German codes were unbreakable.” However, Joan Clarke and her colleagues were destined to prove him wrong, eventually. Clarke initially didn’t even know what the job would entail, but she nonetheless accepted the job and thus the challenge. She started to work at Bletchley Park in June 1940 after she completed Part III of the Mathematical Tripos.

Arriving at her destination, she was placed in Hut 8, which only women stayed in and were mostly engaged in clerical work. The ration of women to men at the Bletchley Park was 8:1. Clarke’s first promotion at work was to Linguist Grade, even though she did not know another language. This action was engineered to enable her to earn extra money, thereby acknowledging her workload and contributions to the team. She believed she struggled to get further promotion purely because she was a woman. Commander Edward Travis later told her that she might have to enroll in the Women’s Royal Naval Service, in order to earn significantly more money, but Clarke did not want to pursue this route.

While staying in Hut 8, she was quickly promoted to her own small room, joining a team which included Alan Turing, Tony Kendrick and Peter Twinn. Collectively they were applying themselves to non-routine tasks of trying to break the complex Naval Enigma – codenamed Dolphin. William F Friedman, the founder of modern US cryptology wrote that a code breaker required unusual powers of inductive and deductive reasoning, much concentration, perseverance and a vivid imagination. The fact that Joan Clarke was able to move so quickly into the male cryptology area at Bletchley Park indicates she possessed these attributes.

The Naval Enigma was different to the Army and Luftwaffe Enigma and more complex to break. Firstly, two extra wheels had been added so there was now a choice of three from five giving a total number of 336 possible wheel orders. Secondly, to give added security, a different indicator system was applied; instead of transmitting the indicators directly, they were super enciphered using bigram tables.

Naval Enigma Code

The need for breaking the Naval Enigma code was growing greater by the day. By mid-1940, following the German occupation of France, German U-Boats now had access to the Atlantic from the Bay of Biscay. Britain had become extremely dependent on imports and was importing half of its food and all of its oil. The provisions now had to come across the Atlantic from North America, and the convoys rapidly become targets for the U-boats. At one stage, Britain was only three days from running out of food and therefore it was crucial that the Naval Enigma code was broken.

In May 1940, matched plaintext and Enigma cypher text became available from a German patrol boat that was captured off the Norwegian Coast. Joan Clarke’s first task on arriving at Bletchley Park was to use a new key-finding aid called the Bombe, against the recovered data. This successfully resulted in Clarke and her colleagues breaking approximately six days of April traffic over a period of three months. By the end of 1940 rotors VI, VII and VIII had been recovered and a library of cribs built up – the cribs were assembled by using anticipated text from German weather ships that were relaying messages in the German Meteorological cipher. This provided Clarke and the team, with the knowledge of what information to expect in a message and how the naval indicator system worked.

Alan Turing invented a new code breaking technique, called the Banburismus. It involved the use of long sheets of paper printed in Banbury and thus the name of the technique. The method exploited the German cryptographic mistake and wheels VI, VII, and VIII all had the different positions of turnover for each wheel. There were eight male Banburists and Joan Clarke was the only female Banburist. However, she was one of the best Banburists and was enthusiastic and fascinated with the technique.

She would sometimes be unwilling to hand over her workings at the end of her shift and would continue to see if a few more tests would produce a result. A method known as Yoxallismus was devised to speed up this work and was named after its inventor Leslie Yoxall. Shortly afterwards, Clarke devised a method of her own to also speed up the technique, and she was told, to her surprise, that she had used pure Dillysimus.  This was a method which had been invented by Dillwin (Dilly) Knox, one of the few cryptographic experts of World War One, who had originally headed the attack on the German enigma. Banburismus was impossible without the Bigram substitution tables and therefore without them, very little progress against the Naval Enigma was accomplished. The breakthrough came in February and June 1941, when trawlers were captured along with cipher equipment and codes.

Clarke and her co-workers successfully performed Banburismus for two years, only stopping in August 1943 when ultra-fast Bombes became available. The successful results of their efforts were evident immediately. Between March and June 1941, the Wolf Packs (a term used to describe the mass attack tactics used against convoys by U-boats), had sunk 282,000 tons of shipping a month. From July, the figure dropped to 120,000 tons a month and by November, to 62,000 tons.

Alan Turing

During her stay in Hut 8, Joan Clarke developed a close friendship with her colleague Alan Turing. They became inseparable and thus Turing even arranged their shifts so they could work together. They spent many of their off-duty days together. Soon after, Turing proposed to Clarke and she accepted, however a few days after the proposal Turing confessed to Clark that he had homosexual tendencies.

Although Turing expected this affair to end, Clarke was undeterred by his decision and the engagement continued. In this period in history marriage for women was considered a social duty, and it was not necessary that marriage should correspond with sexual desires. They both met each other’s families but they hid their relationship from their colleagues. In late summer 1941 their engagement however ended, as Turin considered that his tendencies would lead to a failed marriage. They remained close friends but soon after Turin’s sexual preferences were discovered, he was imprisoned and was given estrogen injections for a year. He then committed suicide.

Clarke and the Naval Enigma team continued to work and try to break the Dolphin Enigma as well as the Shark Enigma code. They succeeded in breaking the Dolphin Enigma, however the Shark Enigma was taken over by the US code breaking unit. This resulted in the relocation of the staff of Hut 8 in other parts of the Bletchley Park. Yet, Joan Clarke remained in Hut 8, becoming Deputy Head in 1944. She and her team continued to break the Naval Enigma until the end of the war. When the war ended in 1845, all the workers had vacated Bletchley Park and every scrap of evidence of their secret code breaking exploits was disposed of.

After the War

Due to her code breaking expertise and contributions during the war, Joan Clarke was appointed a Member of the British Empire or MBE, in 1947. But her work was to remain confidential due to the restraints of the Official Secrets Act.

She went on to marry Colonel J.K. Murray in 1952, though they did not have any children. Due to her husband’s poor health, they moved to Scotland where they both developed a keen interest in Scottish history. This introduction into history from her husband also pushed Clarke’s interest to study numismatic history. In 1962, they returned to England and Clarke rejoined the GCHQ, where she remained until retiring at the age of 60 in 1977.

The lifting of the Official Secrets Act in 1974 set loose the details of the code breakers brilliant work, they became widely known throughout the world. In 1986 Clarke’s husband had died, she moved near Oxford and continued her numismatic research.

In 1987 she was awarded the Sandford Saltus Medal, the Society’s premier distinction, voted for by its members for scholarly contributions to British Numismatics.


Joan Clarke died in 1996 at her home in Oxford. The full extent of Clarke’s mathematical contributions and acomplishments, sadly still remain unknown due to the continuing secrecy amongst cryptanalysts. Clarke’s work is still somewhat overshadowed by her relationship with Alan Turing. Nonetheless it is clear that Joan Clarke played a notable role in Britain’s crucial achievements during World War II.

And it is clear that her mathematical expertise on the Naval Enigma helped shorten the war and thus saved thousands of lives.


[1.] Women Codebreakers”. Bletchley Park Research. 3 October 2013. Retrieved 3 November 2013.
[2.] “Sanford Saltus Gold Medal”. British Numismatic Society. British Numismatic Society. 2013. Retrieved 14 November 2014.
[3.] Burman, Annie (2013). Gendering Decryption—Decrypting Gender: The Gender Discourse of Labour at Bletchley Park, 1939–1945 [65 pp.]
[4.] Welchman, Gordon (2005) [1997], The Hut Six story: Breaking the Enigma codes, Cleobury Mortimer, England: M&M Baldwin, pp. 138–145
[5.] Anderson, L. V. (3 December 2014). “The Imitation Game: Fact vs Fiction”
[6.] F H Hinsley and Alan Stripp, Codebreakers: The inside story of Bletchley Park (Oxford University Press, Oxford, 1993).
[7.] Lord, Lynsey Ann (2008). “Joan Elisabeth Lowther Clarke Murray”
[8.] M Smith, Station X: The Code Breakers of Bletchley Park (Pan Books, 2004).

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