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Making Crypto History: The Women of Bletchley Park

Remember the movie The Imitation Game? The film depicts Alan Turing and his team at Bletchley Park trying to crack the Enigma machine and decrypt German intelligence codes for the British government during World War II. While modern cryptography relies on complex algorithms and asymmetric key encryption to keep data secure, prior to the 1970s cryptanalysis was mostly a manual process used for military purposes.

At Bletchley Park, much of the cryptanalysis work was performed by women. Nearly 8,000 women were employed there at its peak. In honor of upcoming International Women’s Day and International Women in Engineering Day, let’s look at these trailblazing women who changed the course of the war and advanced the field of cryptography.

During World War II, Bletchley Park (about 50 miles north of London) was the central site for British cryptanalysis and the home of the Government Code and Cipher School. It employed mathematicians, linguists, and even chess masters to analyze secret communications (most notably the Enigma and Lorenz ciphers) to predict when and where the next invasion would occur. By 1945, more than 10,000 people worked at Bletchley―75% of them were women.

Initially the heads of Bletchley expanded their staff with debutantes to perform clerical duties. But as an entire generation of men was sent off to war, more and more women were gaining entrance into STEM programs. Bletchley tapped into this growing demographic of women who held degrees in math, physics, and engineering. They also recruited women who were fluent in multiple languages and those who exhibited creative problem solving and unconventional thinking, such as crossword puzzle experts.

Several women at Bletchley Park were part of the elite code-cracking team:

Mavis Lever
Lever’s work was pivotal to the success of D-Day. She began her cryptanalysis career at The Times (a daily British newspaper), gleaning the personal columns for coded spy messages. In 1940―at just 19 years old―she was recruited to work at Bletchley Park, where she became an expert in “rodding.”

Rodding was a new technique invented by Alfred Dilly Knox. Its linguistic (rather than mathematical) approach to breaking codes involved turning rods marked with wooden letters to work out the rules used to scramble messages. After Knox’s team had gone months without success, it was Lever who experimented with the rotor starting position and made the rodding breakthrough.

Lever broke into the Italian Naval Enigma machine’s framework and deciphered a message that read “Today’s the day minus three.” She and her colleagues discovered that the Italians intended to assault a Royal Navy convoy transporting supplies.

In 1941 she broke a message between Belgrade and Berlin that enabled Knox’s team to decrypt the output of the Abwehr (German Secret Service) Enigma machine. Thanks to Lever, British intelligence could then monitor Abwehr activities and even plant false information. Later she broke another Abwehr machine, the GGG. This work confirmed that the Germans believed the double-cross intelligence they were receiving from British spies.

Jane Hughes
Hughes helped decode a message that led to the sinking of the German battleship Bismarck. Fluent in German, she joined the code-breaking project at the age of 18. Hughes was assigned to a women-only “Decoding Room” where they would receive the daily Enigma keys and type them into their Typex machines. They would then determine if the messages were recognizable German. In 1941, Hughes decoded a message detailing the Bismarck’s position and destination in France. The battleship was then attacked by the Royal Navy and sunk.

Jean Valentine
Valentine was an operator of the bombe (electromechanical) decryption device designed by Alan Turing. Toward the end of the war, she began studying Japanese codes and ciphers. Valentine is still alive. In recent years, she has been involved with the reconstruction of the bombe at the Bletchley Park Museum where she gives tours and demonstrates how the machine was used.

Joan Clarke
Featured prominently in The Imitation Game, Clarke was a cryptanalyst and a numismatist (an expert in currency, coins, tokens, and paper money). She earned a double first degree in mathematics at Cambridge. Recruited by former classmates into the Government Code and Cipher School in 1939, Clarke was first placed in a women-only group performing routine clerical work and earning just £2 a week. She quickly became the only woman at Bletchley Park to practice (and master) Banburismus, a cryptanalytic process developed by Alan Turing that reduced the need for bombes.

Her abilities stood out and Clarke was soon moved into the small room in Hut 8 occupied by Turing and his team. The navy ciphers decoded by Clarke and her colleagues were much harder to break than other secret messages and aimed to prevent U-boats from sinking Allied ships carrying troops and supplies from the U.S. to Europe.

Clarke’s task was to break these ciphers in real time―one of the most high-pressure jobs at Bletchley. The messages Clarke decoded resulted in military action being taken immediately. U-boats would be sunk or circumnavigated, saving thousands of Allied lives and salvaging thousands of tons of precious supplies.

Clarke became deputy head of Hut 8 in 1944. Her role in the Enigma project led to her appointment as a Member of the Order of the British Empire in 1946.

Thousands of other women at Bletchley performed important auxiliary work such as operating communications machinery, translating Axis documents, making complex calculations by hand, and translating Morse Code. Of the 8,000 women who worked at Bletchley Park, only about 600 continued with cryptography or careers in secret service. Much of their work did not come to light until decades later as it had been classified under Britain’s Official Secrets Act.

We now have devices that fit into the palms of our hands that crunch numbers faster than all the people at Bletchley combined. But the women of Bletchley Park―who challenged the status quo―forever changed cryptographic history and inspired generations of women in tech.