Joan Clarke: Breaking Enigma

On this 70th anniversary of the end of World War II, DiscovHER celebrates cryptanalyst Joan Clarke (Murray) one of wartime Britain’s few women code-breakers. Clarke’s decryption of German naval communications contributed to the Allied victory by averting U-boat attacks. Her role was recently brought to the public eye thanks to The Imitation Game, a film about her close friend, onetime fiancé and colleague Alan Turing.

When we first meet Joan Clarke in The Imitation Game, she beats a roomful of male-only candidates to solve a mathematical conundrum in under six minutes, winning a top secret job with mathematical genius Alan Turing. Though that scene is fictional – Clarke was recruited to the Government Code & Cypher School’s (GC&CS) wartime headquarters at Bletchley Park, not by Turing, but by a former Cambridge tutor– the film’s representation of her brilliance is not. In real life, she was a rare woman at the highest level of Bletchley’s naval code-cracking team.

Mathematical excellence when mathematics was not for women

From Dulwich High School for girls, Clarke studied mathematics at Newnham College, one of two Cambridge colleges that accepted women. Studying for its world-renowned mathematics examination Mathematical Tripos, where the brightest scholars problem-solved against the clock, would serve her later career. Exceptionally-gifted, she was awarded the Philippa Fawcett Prize, named for the woman mathematician who out-performed her male classmates in 1890, and the Helen Gladstone Scholarship. In 1939 Clarke graduated with a double-first, in title only, since Cambridge only awarded women full degrees from 1948.

A race to break enigma

Arriving at Bletchley in June 1940, Clarke joined Hut 8, responsible for solving naval enigma. Each day in this wooden hut on the mansion’s grounds was a race against time, since the Germans changed their naval enigma settings at midnight. As U-boats attacks threatened Britain’s Atlantic lifeline with the U.S., descrambling radio communications was critical. Yet the chance of breaking enigma, a scrambling typewriter using rotating wheels and rotors to turn ordinary text into code, was believed to be 159 million million million to one.

The only woman to crack the naval code

Together with electromechanical machines, or “bombes” as they were called, Clarke, and the team, used a cryptanalytic method called Banburismus, developed by Turing. Seen as the first example of sequential analysis, the process involved finding patterns by cross-referencing scrambled texts punched into card that could reveal Enigma’s likely wheel orders. Referred to as “one of the best Banburists,” Clarke was the only woman to begin to crack the naval code using the technique. She found Banburismus “so enthralling” she would even be unwilling to hand over her workings at the end of her shift.

A Clarke invention with no name

Clarke also developed a method to accelerate decryption of the double-enciphered messages sent to U-boat officers, known in German as “Offizier ciphers.” While some Bletchley decryption techniques were named after their inventor, Clarke’s wasn’t. As she recalls: “my name was not attached to it.” She was told she had used Dillyismus, an invention by renowned World War I cryptanalyst Dilly Knox. “Clarke had been accepted into the team, but evidently her name (even her gender-neutral surname) attached to a method of cryptanalysis was one step too far.”

“Grade: Linguist. Languages: None!”

Clarke challenged the traditional place of Bletchley’s “girls” as women were referred to. Primarily assigned support-staff clerical roles, she was one of very few women, alongside Mavis Batey, Margaret Rock and Rosalind Hudson, known to work in cryptanalysis. To enable a pay rise, Clarke was promoted to linguist grade, despite speaking no languages. She delighted in answering a questionnaire with “Grade linguist, languages, none.” 

“She Also Served”

Clarke would remain in Hut 8 throughout the war, becoming its deputy head in 1944, a rare senior role for a woman at the GC&CS. Her wartime contribution was recognized when she was awarded Member of the British Empire (MBE) in 1947.

A move to Numismatics  

Clarke married retired army officer Jock Murray in 1952, and continued to work at the Government Communication Headquarters into her Sixties. Turning the same brilliance for mathematics to Numismatics, her research into 14th to 16th century Scottish coinage gained her the last award of her life, the British Numismatics Society’s John Sanford Saltus Medal in 1986. She died in 1996.

70 years on, the Joan Clarke Maths Residential 

Known to the greater public largely for her relationship with Turing, the next generation may yet learn about Clarke’s brilliance. Sixth-form girls considering Maths, Physics and Engineering at University can participate in the four-day Joan Clarke Maths Residential at her former Newnham College, Cambridge starting this September.

Photo Credit : © 2014 THE WEINSTEIN COMPAGNY

For Women in Science

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