Crypto Education
Joan Clarke — the woman that cracked Enigma
A short biography on one of early cryptography’s key female figures.
Introduction
Joan Clarke was born June 24 1917 in London and was the youngest child of Dorothy and the Revd William Kemp Lowther Clarke, a clergyman. She attended Dulwich High School for Girls in London and won a scholarship to Newnham College, Cambridge in 1936. There she studied mathematics.
In 1937 and 1939 she achieved a First in Part I and Part II of the Mathematical Tripos (a three-year course leading to a Bachelor’s Degree) and became a Wrangler. However, she was denied a full degree because Cambridge didn’t allow women to advance in their academic careers until after the Second World War. Cambridge only awarded this to men until 1948. In 1939 Clarke was awarded the Philippa Fawcett Prize and in 1939–1940 the Helen Gladstone Scholarship.
Clarke’s geometry tutor at Cambridge, Gordon Welchman, knew of her mathematical abilities and recruited her to work at Bletchley Park as a part of the ‘Government Code and Cypher School’ (GCCS). On June 17, 1940, one day before France capitulated, Joan Clarke arrived at Bletchley Park.
Early Career
Her initial position was quite modest. She joined a large group of women who undertook clerical work. By 1941 the number of people working there had reached 7,000, two-thirds of the employees being women. Fairly quickly she progressed through the ranks and became part of a group working in a section known as Hut 8 that included Alan Turing, Tony Kendrick and Peter Twinn. Clarke modestly said that this quick promotion “was obviously because of my degree, and before I had any chance of proving myself”. Colleagues described Clarke as shy, kind, truthful, agreeable and an intelligent worker.
Together they carried out difficult tasks. For example they worked on cracking naval enigma code types such as “Dolphin”. The principal cipher was Heimisch (Heimische Gewässer — known to Bletchley Park as Dolphin) for U-boats and surface ships in ‘Home Waters’.
What is Enigma ?
Enigma was a portable encryption machine used to encrypt and decrypt secret messages. The significance of cracking this machine cannot be overstated. The cipher-machine was being worked upon by the best minds available, top mathematicians, Egyptologists, chess champions, anyone who could assist in cracking the Enigma code.
The encryption process begins with setting the key for the machine. An unencrypted message is then entered into the machine’s keyboard. As it is passing through the machine, various mechanisms and electrical impulses transform it into encrypted text which is output on another section of the machine with indicators which light. Many alterations can be made to the structure of this process adding more layers of difficulty to the decryption process of an intercepted message.
Another Enigma machine, which receives the message and holds the correct key (settings used for the original encryption) can be used to pass the message through in the opposite direction. When this occurs the text is transformed from encrypted to original form.
The naval (Kriegsmarine) Enigma machine was different from the type used by the field and air branches (Wehrmacht/Luftwaffe) of the military. Initially the naval machine used additional rotor types, one of several methods of making the codes generated even more complex. Later the machine housed a fourth rotor of one of two types, Beta or Gamma which could be manually set to any of 26 positions. One of the 26 positions made the machine perform identically to the three-rotor machine.
Clarke was tasked with breaking encryption in real time. The messages decoded resulting in immediate military action made it one of the operations most high-pressure tasks. Thanks to the success of Clarke and other cryptanalysts who cracked German ciphers, the countries of the anti-Reich coalition gained vital information about key battles and actions of the war. However, this machine was not considered indestructible for nothing. Some Enigma cipher versions (Aegir and TGD), were never broken during the war.
Bridging the gap
While Clarke worked at Bletchley Park she only knew one other female cryptanalyst, the field was male dominated as cryptology and many other fields were not considered ’a job fitting for a woman’ at that time in England. Initially she was paid £1.50 a week. Men, on the other hand, were paid considerably more for the same work. In order to get a raise, she was promoted to linguist, even though she only spoke English and knew no other languages. On a questionnaire for the raise, she answered with a joke: “Grade — Linguist. Languages — none”.
She became the only female practitioner of Banburismus, a cryptanalytic process developed by Alan Turing that reduced the need for bombes, the electromechanical devices used by British cryptologists Welchman and Turing to decrypt messages.
The new technique involved the use of long pieces of paper, printed in Banbury. This method helped reduce possible wheel orders needed to find the correct key from 336 to as little as 20. Another method, Yoxallismus, was also developed to speed things up. Clarke herself also developed a method, but it turned out it was similar to Dillysimus — a method invented by Dillwin (Dilly) Knox, one of the first cryptography experts of the First World War. The Banburismus method was used successfully until the advent of ultra-fast Bombes.
As well as cracking ‘Dolphin’, Clarke and the Enigma team were responsible for cracking ‘Shark’, a code name for the 4-rotor key system introduced in late 1941. Her success as a cryptanalyst resulted in Joan becoming Deputy Head of Hut 8 in 1944. An achievement in and of itself made during a time when intelligence in women was not appreciated. She and her team continued to solve the mysteries of intercepted codes until the end of the war. The war officially ended on 9th of May 1945. By March 1946 all the workers had left Bletchley Park and all evidence of their secret code-breaking exploits had been destroyed.
Joan Clarke was made a Member of the Order of the British Empire in 1947 for her work during the war.
Later life
In the post-war period she moved to Scotland where she published many works on numismatics, the study of currency. One such work was a record of a complex series of gold unicorn and heavy groat coins that were in circulation in Scotland during the reigns of James III and James IV. Coins from this period are particularly difficult to date and sequence, Clarke possessed a real talent for analysing data. Supplemented by extensive historical research, Clarke presented her findings in several numismatic publications, most frequently for the British Numismatic Journal albeit under her married name Joan.E.L.Murray.
In 1962 she returned to England, where she re-joined the GCHQ (Government Communications Headquarters). She continued to work there until her retirement in 1977. In the 1980s she assisted Sir Harry Hinsley with an appendix to Volume 3, Part 2 of British Intelligence in the Second World War. In 1986 her other research works were recognised by the British Numismatic Society when she was awarded the Sanford Saltus Gold Medal.
Joan Clarke died in 1996 in her 79th year. Joan Clarke’s contribution to Enigma cryptanalysis went unappreciated for many years. The practices developed at Bletchley Park were shared with American and other counterparts. It is now clear that her work and knowledge of cryptography was a massive contribution to the war effort and the field of cryptography.
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