Birth Date: 24 June, 1917
Date of Death: 4 September, 1996
Place of Birth: London, United Kingdom
Place of Death: Headington, Oxfordshire, United Kingdom
Nationality: English
Occupation/Field of Study English cryptanalyst and numismatic expert, mainly known for her work at Bletchley Park as a decoder of the Nazi Army’s Naval Enigma Machine and for her contributions to Scottish Numismatics.


KEYWORDS: Joan Clark, Decoder, Bletchley Park, The Enigma Machine, The Naval Enigma, Numismatics



Joan Elizabeth Lowther Clark was a British citizen whose great interest in mathematics led her to work at Bletchley Park in the decoding of the Nazis’ Enigma Machine, used as their means of communication during World War II.

Thanks to their expertise, Clarke and her team “were given the important task of breaking the codes related to German U-boats, which had been particularly destructive to the Allies”1. Despite being a woman, she quickly rose through the ranks at Bletchley Park and made very important contributions to the decoding of the Naval Enigma (also referred to as the Dolphin)2.

At the end of the war and after the decoding was successful, Clarke moved to Scotland with her husband and developed a great interest in Scottish Numismatics4. Apart from joining the British Numismatic Society, she dedicated herself to the collection and study of 15th-century coins and medals and published a significant number of papers related to the field2. Ultimately, she became the “only recipient of the Sanford Saltus Medal ever to have attained this honour entirely on account of contributions to Scottish numismatics”3.

However important her contributions were to the decoding of the Naval Enigma, there is still much secrecy surrounding the events at Bletchley Park: “the full extent of Clarke’s achievements remains unknown […]; [she] never sought the spotlight and rarely contributed to accounts of the Enigma Project”4.

Overall, Joan Clarke should go down in history for her remarkable work as a decoder at Bletchley Park, whose work is estimated by historians to “have shortened the war by two to three years, saving thousands of lives”1.



Joan Elisabeth Lowther Clarke was born on 24 June 1917 in London, the youngest daughter of William Kemp Lowther Clarke and Dorothy Elisabeth Clarke. Along with her three brothers and her sister, Clarke grew up in a clerical and scholarly environment, as many members of her father’s side of the family had been in Holy Orders and fellows of Cambridge Colleges3.

She attended Dulwich High School of Girls, where she was awarded a scholarship to enroll in Newnham College (Cambridge). In 1937, she was First in Part I of the Mathematical Tripos – “examinations in broad subjects […] divided into one or more Parts […] [which are needed] to qualify for the B.A. degree”5 which she completed in 1939. Although she graduated with double firsts, “this was merely the title of her degree, as Cambridge did not admit women to full membership of the body academic until after the end of the Second World War”2.

By the time she had finished her University education, there was a top secret Government Code and Cipher School (GCCS) project being carried out 50 miles away from London at a Victorian building under the name of Bletchley Park1. Gordon Welchman, mathematician and GCCS recruiter, had supervised Clarke’s Geometry works during her undergraduate years and, being aware of her abilities, invited her to join Bletchley Park2. After a period of clerical work when she first became part of the project, Clarke’s “abilities soon led her to become one of the few women codebreakers at Bletchley Park”6. Her work and expertise allowed her to become a member of the Hut 8 teams in charge of decoding the Dolphin code and the Shark Enigma until 1943 when the United States took over the task of decoding the Shark Enigma: “as a result, many of the Hut 8 staff transferred […]. However, Joan and […] her team continued to break the Naval Enigma until the end of the war”2. During her time at Bletchley Park, Joan Clarke developed a friendship with her colleague Alan Turning, one of the most notable codebreakers of the war period, and this would result in a marriage proposal4. Shortly thereafter, however, Turing called the engagement off by claiming that “it might not be an ideal marriage because he had what he termed ‘homosexual tendencies’”7 (p.184); even so, a close friendship between them remained until Turing’s death in 1954.

Once the war was over, Clarke moved to what came to be known as Bletchely’s Park successor, the Government Communications Headquarters, where she met a retired army officer, Lieutenant-Colonel J (Jock) K R Murray, whom she married in 1952. Due to her husband’s poor health, the couple had to move to Scotland3; there, and due to her husband’s influence, she developed an interest in numismatic history and ended up writing several relevant papers on the Scottish coinage of the 16th and 17th centuries2.

After living in Scotland for some years, the couple moved to Cheltenham (United Kingdom) and rejoined the Government Communications Headquarters (GCHQ), where Clarke remained until her retirement in 1977 at the age of 608.

As a consequence of her husband’s death in 1986, Joan Clarke moved to Headington (near Oxford, United Kingdom) and she collaborated with Sir Harry Hinsley on the development of the Appendix 20 to Volume 3, Issue 2 of the 1988 British Intelligence on the Second World War, a “substantially revised assessment of the Polish, French and British contributions to breaking the Enigma3. On 4 September 1996, Joan Clarke died at the  age of 79.



Joan Clarke’s main work took place at Bletchley Park, where she collaborated in the decoding of Enigma, an important upturn for the Allies’ victory in World War II. The Enigma Machine “invented in 1919 by Hugo Koch […] looked like a typewriter and was originally employed for business purposes. The German army adapted the machine for time use and consider[ed] its encoding system unbreakable”9. These portable machines “generated the countless millions of different letter combinations in which most coded German communications were sent”7 (p. 12). In order to defeat the Germans, it was imperative for the British Government to break the Enigma code; hence, Churchill searched for the best mathematicians, chess champions, and Egyptologists in the country in order to assist in this association7. (p. 13-14)

While Joan Clarke was still an undergraduate student at Cambridge, her supervisor of Part II Geometry – Gordon Welchman – who was very aware of her mathematical skills, recruited Clarke for the ‘Government Code and Cypher School’ at Bletchley Park2.

Upon her arrival at Bletchley Park in 1940, Joan Clark took up a humble position where she only earned £2 and joined a large group of women for the development of administrative and clerical work in Hut 6. “As typical for girls at Bletchley, (and they were universally referred to as girls, not women) Clarke was initially assigned clerical work […] [earning] significantly less than her male counterparts”4.

In June 1940, she was recruited as a mathematician in Hut 8. However, “the principle of equal pay and rank being stoutly resisted by the civil service, she had to be promoted to the humble rank of ‘linguist’ that the pre-war establishment reserved for women”10 (p. 370). Hence, Clarke was promoted to enable her to earn more money and recognise her contributions to the research.

Once in Hut 8, she she started working with other mathematicians, such as Alan Turing, Tony Kendrick and Peter Twin in order to complete the non-routine tasks of trying to break the complex Naval Enigma (also referred to as the Dolphin)2. The Naval Enigma was different from that of the Army and Air Forces and much more complex in its decoding due to the additional rotors it included11. The Naval Enigma used “different ciphers, each with its own daily key (rotor order, ring settings, plugboard connections and ground setting)”11, which left the decoders with 336 possible wheel orders for each message.

Following the occupation of France by the Germans in 1940, the need to decode Enigma became more and more pressing. Indeed, the German Army had now access to the Atlantic through the Bay of Biscay and, as Great Britain depended on imported products, merchant ships were forced to change routes because they were constantly being sunk by the Wehrmacht9.

By the end of 1940, rotors VI-VII-VIII had been recovered by Clarke’s team and Bletchley park had built up a library of cribs, which were “assembled by using anticipated text from German weather ships that were relaying messages in the German Meteorological cipher”2. Despite such developments, there were still too many possible meanings for each message. This is  why Clarke’s friend Alan Turning developed what came to be known as the Banburismusm Process, which “involved the use of paper sheets on which cipher-text messages were represented as punched holes […] [which] had to be moved against each other and coincident holes laboriously counted before the sophisticated statistical methods could be used”10 (p. 436). The development of this technique led to the reduction of possible meanings to only 20. Despite such reduction, other methods were needed in order to speed up the process; Clarke used the Yoxallimus method (“based on a statistical approach to find the Stecker connections […], it assumes that every letter of a message is the most frequent letter in the alphabet”12). Added to this, and due to her determination to break the code, Clarke developed her own method for speeding up the process. To her surprise, it turned out to be the same as the Dillysimus Process, invented by Dillwin (Dilly) Knox, one of the few cryptographic experts from World War I and also the person who headed the attack on the German Enigma in the first place2.

Clarke’s team’s breakthrough came about between February and June 1941, after the some trawlers were seized which held cipher equipment and codes. After some time applying the Banburismus method, the team managed to reduce the number of sunken boats to almost 80%: it went from 282,000 tons of shipping per month to 62,000 tons13 (p. 308).

After this turning point, Clarke and the Naval Enigma team were allotted responsibility for breaking the Shark Enigma (a 4-rotor key which was introduced by the Germans in late 1941 and which came to be “a special cipher for the Atlantic and Mediterranean U-boats”14 (p.3). Hut 8 managed to break the code in mid-December 1942; however, in 1943, the US codebreaking unit (OP-20-G) took over the tasks related to the Shark Enigma2. As a consequence of this, many of the staff at Bletchley Park were transferred to other departments or buildings, but Joan Clarke remained in Hut 8 and became Deputy Head in early 1944, directing the decoding of the Naval Enigma until the end of the war.



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 to shorten the war and thereby save thousands of lives.

Ann Lord, Lynsey (2008), “Joan Elisabeth Lowther Clarke Murray”, Honours Project, University of St Andrews.


Joan Clarke’s ingenious work as a codebreaker during WW2 saved countless lives, and her talents were formidable enough to command the respect of some of the greatest minds of the 20th Century, despite the sexism of the time.

Miller, Joe (10 Nov. 2014), “Joan Clarke, woman who cracked Enigma cyphers with Alan Turing”, BBC News, BBC.


Women like Margaret Rock, Joan Clarke Murray and Mavis Lever Batey are marvelous examples of women meeting their potential in a time of conflict, opening the door to long careers in codebreaking that they would never have been exposed to before the war.

Citing Howard, Kerrhy, “Joan Murray (formerly Clarke”, Bletchley Park Research, Bletchley Park Research.



1936: scholarship to study at Newnham College
1939: Philippa Fawcet Prize
1939-1940: Helen Gladstone Scholarship
1947: appointed Member of the Most Excellent Order of the British Empire (MBE)
1987: Stanford Status Medal



Influenced by her husband, who came from a Scottish family and who was enthused by the 16th and 17th centuries, Joan Clarke developed an interest in coins and medals both as a collector and as a student: “Joan concentrated on the fifteenth century where she soon mastered the existing literature and began to make important observations of her own”3.

She started publishing papers in the late 1960s and soon completed her own study of the heavy groats and unicorns of James III and IV. Even though some of her papers remain unpublished to this date, they were used for later relevant studies2.

Even after her retirement and her husband’s death, Clarke continued doing a considerable amount of technical numismatic work and kept on writing joint papers on the matter.



The Imitation Game (2014), dir. Morten Tyldum, perf. Benedict Cumberbartch, Keira Knightley, Matthew Goode, Black Bear Pictures and Bristol Automotive, Film.

Sage UK editors (2017), Joan Clarke, poster, Sage UK, London. [Her portrait was turned into a poster which was part of the series Traiblazers, unveiled in March 2017 to mark Women’s History Month].



Although there are no publications by Joan Clarke related to her outstanding work at Bletchley Park as a codebreaker, she did write many relevant papers for the world of Numismatics. The following is a personal selection of Clarke’s works listed by the British Numismatic Society2:


with Stewart, B.H.I.H (1967), “Unpublished Scottish Coins IV. Early James III”, NC, pp. 147-61

“Review of The Scottish Coinage, with supplement (with comments on the revaluation of silver coins of Charles II in 1681)” (1968), BNK XXXVII, pp. 201-02

“Hoards in Scotland under James IV” (1969), NCirc, June, p. 199

with Stewart, B.H.I.H (1970), “Unpublished Scottish Coins V. Light Groats and Base Groats of James III”, NC, pp. 163-86

“The Early Unicorns and the Heavy Groats of James III and IV” (1971), BNJ XL, pp. 62-96

“The First Gold Coinage of Mary Queen of Scotts” (1979), BNJ XLIX, pp. 82-86

with M. R. Cowell (1980), “Some Placks and Base Groats of James III of Scotland”, Metallurgy in Numismatics I, pp. 180-83

with Stuart, I. (1983), “St. Andrews Mint under David I”, BNJ, 53, pp. 178-80

“The Coinage of the Marians in Edinburgh Castle in 1572” (1987), BNJ, 57, pp.47-53

“The Location of the Edinburgh Mint, 1538 to 1463 – and Linlithgow Mint” (1991), BNJ, 61, pp- 126-29



Hodges, Andrew (1983), Alan Turing: The Enigma, New Jersey, Princeton University Press.

McKay, Sinclair (2012), The Secret Life of Bletchley Park: The WWII codebreaking centre and the men and women who worked there, London, Aurum Press Ltd.

Erskine, Ralph and Eric A. Weiss Good (March 2017), “100 Years Ago: Joan Clarke”, Notices of the AMS, vol. 64, nº 3, pp. 252-55,  (last accessed 19 Apr. 2017).

Smith, Michael (2011), The Secrets of Station X: How the Bletchley Park Decoders Helped Win The War, London, Biteback Publishing Ltd.

Haufler, Hervie (2003), Codebreakers Victory: How the Allied Cryptographers Won World War II, New York: Open Road Integrated Media Inc.



  1. Linik, Joyce Riha (4 Oct. 2016), “The Female Code Crackers of Bletchley Park”, IQ Magazine, Intel, (last accessed 19 Apr. 2017).
  2. Ann Lord, Lynsey (2008), “Joan Elisabeth Lowther Clarke Murray”, Honours Project, University of St Andrews, (last accessed 19 Apr. 2017).
  3. Stewartby, Lord (1996), “Obituary”, British Numismatic Journal, vol. 67, nº 13, Scotland, pp. 162-67, (last accessed 19 Apr. 2017).
  4. Miller, Joe (10 Nov. 2014), “Joan Clarke, woman who cracked Enigma cyphers with Alan Turing”, BBC News, BBC, (last accessed 19 Apr. 2017).
  5. “The Structure of undergraduate courses at Cambridge”, University of Cambridge, (last accessed 19 Apr. 2017).
  6. “Women Codebreakers”, Bletchley Park Research, Japan, (last accessed 19 Apr. 2017).
  7. “Women Codebreakers”, Bletchley Park Research, Japan, (last accessed 19 Apr. 2017).
  8. McKay, Sinclair (2012), The Secret Life of Bletchley Park: The WWII codebreaking centre and the men and women who worked there, London, Aurum Press Ltd.
  9. Simkin, John (2007), “Joan Clarke”, Spartacus Educational Project, Spartacus Educational Publishers Ltd, Great Britain, (last accessed 19 Apr. 2017).
  10. “1941: Enigma key broken”, History Channel, A&E Television Networks LLC, New York, (last accessed 19 Apr. 2017).
  11. Hodges, Andrew (1983), Alan Turing: The Enigma, New Jersey, Princeton University Press.
  12. “Technologies: Enigma”, net, Gudmundur Helgason, (last accessed 19 Apr. 2017).
  13. Leslie, Albert, “Yoxallismus Codebreaking Method”, Yoxall One Name Study, Darrin Lythgoe, (last accessed 19 Apr. 2017).
  14. Carlisle, Rodney P. (2015), Encyclopedia of intelligence and counterintelligence. Volume One-Two, New York, Routledge Taylor & Francis Group.
  15. Erskine, Ralph (2001), “Breaking Naval Enigma on Both Sides of the Atlantic”, in Action This Day: Bletchley Park from the breaking of the Enigma Code to the birth of the modern computer, United Kingdom, Bantam Press, Ch. 10.


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