The job posting could have read: Clerical worker needed at country manor convenient to London transport. Reasonable wage, room, board and regular social activities provided.

A few vital details were left out. Key workers had to sign a draconian document ensuring secrecy for over 30 years. Hours were long and the work itself often tedious if mentally stimulating. Coworkers could be eccentric, intractable and unpleasant. The nature of the work had to be hidden from your family and friends. Lastly, the fate of millions could be affected by every result and decision. Such was the nature of the work of cryptanalysts at Bletchley Park - the heart of Great Britain's codebreaking efforts during World War 2. Joan Clarke was the rare woman accepted and highly respected within the senior cadre of male cryptanalysts that included university acquaintance computing pioneer Alan Turing.

In 1939, Joan Elizabeth Lowther Clarke was a Cambridge graduate with double firsts in Mathematics. Knowing of her intellectual powers, her school supervisor leading mathematician Gordon Welchman recruited Clarke to work at the Government Code and Cypher School (GCCS) at Bletchley. To reflect her true role and parity with the male codebreakers, Clarke was given the title of Linguist despite not speaking any foreign languages whatsoever. Clarke was one of four female codebreakers.

GCCS' reason for existence could be summed up in one word - Enigma. This was the cypher machine used by the German military to send and receive messages. Its encoded messages were considered by many to be unbreakable. Great Britain was highly dependent on supplies of food and fuel transported on merchant ships from the United States. Despite military escort ships, the convoys were vulnerable to crafty and ruthless German submarines. Breaking the Enigma code was crucial to Britain's survival.

Must Be Good At Games

Clarke worked in Hut 8 joining the team dedicated to cracking a more complex variant of the Enigma cypher used solely by the navy. Using what facts were known about the cypher, Turing created a codebreaking technique dubbed Banburismus which was intended to discover the settings used in creating the coded text. Using large sheets of paper on which the coded texts were described, analysts strived to find identical sequences of text that could indicate that the messages used the same machine settings.

The hunt for these sequences became a game of which Clarke became an enthusiastic and skilled player. Clarke would often work well into the night reluctant to stop and hand her sheets to the next shift to complete. Each of the eight Banburists in Hut 8 employed their own methods to using the sheets. Clarke was so dedicated that she invented a much faster method of sequential analysis. Despite the intellectual talent on hand, manual decoding took days not hours.

In 1941 with the aid of a German substitution table, the team broke the code at last. The Hut 8 team successfully decrypted 6 days worth of messages. Clarke solved 3 of the 6 days herself. The value of this intelligence work yielded results that year. Convoy losses fell in November to 62,000 tons from 120,000 tons in July.

It is known that Clarke assisted Turing in many ways. Turing treated her as a fellow mathematician and did not talk down to her. She was the first reader of "prof's book" which would later be named Turing's Treatise on the Enigma. Clarke reviewed and tested all the instructions, equations and methods described to make certain they made sense and were essentially correct. She assisted Turing and Welchman on the early work on the bombe machine which would later accelerate the decoding process.

Through the rest of the war, Clarke and the team continued to decipher the naval Enigma as well as helping decode another cipher called Shark. Clarke rose to the position of Deputy Head of Hut 8. At war's end, she transferred to the GCCS' successor organization the Government Communications Headquarters (GCHQ) responsible for gathering information and securing Britain's communication systems. Clarke was appointed a Member of the British Empire (MBE) in 1947.

An Enigma to the End

Clarke met and married her husband a retired army intelligence officer John Murray in 1952. She left the GCHQ and moved to Scotland. Through her husband, Clarke discovered an interest in the study of currency, coins and medals. She published several well-regarded papers on Scottish coinage. Clarke and Murray returned to work at GCHQ in 1962. Clarke's involvement in Enigma came to light in 1974 when restrictions of the National Secrets Act expired. She retired in 1977. She continued her study of numismatics receiving the British Numismatist Society's Sanford Saltus Gold Medal in 1986. Clarke died on September 1996 aged 80.

Today few details are known about Joan Clarke's work on intelligence and cryptanalysis. After the war, Winston Churchill ordered all documents and notes regarding Enigma be destroyed along with the computing machines developed there. The cryptanalyst fraternity has long maintained a veil of secrecy over its members' achievements because the methods and expertise developed in wartime continued to be employed through to the Cold War. Clarke's reserved nature did not invite scrutiny. She has been described by family and close friends as a warm, reticent individual who valued discretion, modesty and plain speaking.

Joan Clarke's intelligence, persistence and analytical powers are unquestioned. She proved her mettle among male peers at a time when women were not seen as a man's equal. Whether or not we will ever know more about her work during and after the war one thing is certain. Joan Clarke was a woman of extraordinary ability who lived an uncommon life of intrigue and achievement.