Леопольд Маркс: Криптограф, драматург и война в тени.

British cryptographer, playwright and screenwriter

Leopold Samuel Marks, (24 September 1920 – 15 January 2001) was an English writer, screenwriter, and cryptographer. During the Second World War he headed the codes office supporting resistance agents in occupied Europe for the secret Special Operations Executive organisation. After the war, Marks became a playwright and screenwriter, writing scripts that frequently utilised his war time cryptographic experiences. He wrote the script for Peeping Tom, the controversial film directed by Michael Powell that had a disastrous effect on Powell's career, but was later described by Martin Scorsese as a masterpiece. In 1998, towards the end of his life, Marks published a personal history of his experiences during the war, Between Silk and Cyanide, which was critical of the leadership of SOE.

Marks was born into a devout Jewish family. He was the son of Benjamin Marks, the joint owner of Marks & Co, an antiquarian bookseller in Charing Cross Road, London. He was introduced at an early age to cryptography when his father showed him Edgar Allan Poe's story, "The Gold Bug". From this early interest, he demonstrated his skill at codebreaking by deciphering the secret price codes that his father wrote inside the covers of books.

Marks was conscripted into the British Army in January 1942 and trained as a cryptographer; apparently he demonstrated the ability to complete one week's work in decipherment exercise in a few hours.

One of Marks's first challenges was to phase out double transposition ciphers using keys based on poems. These poem ciphers had the limited advantage of being easy to memorise, but significant disadvantages,

Gestapo signal tracers endangered clandestine radio operators, and their life expectancy in occupied France averaged about six weeks. Therefore, short and less frequent transmissions from the codemaster were of value. The pressure could cause agents to make mistakes encoding messages, and the practice was for the home station to tell them to recode it (usually a safe activity) and retransmit it (dangerous, and increasingly so the longer it took). In response to this problem, Marks established, staffed and trained a group based at Grendon Underwood, Buckinghamshire to cryptanalyse garbled messages ("indecipherables") so they could be dealt with in England without forcing the agent to risk retransmitting from the field. Other innovations of his simplified encoding in the field, which reduced errors and made shorter messages possible, both of which reduced transmission time.

The Germans generally did not execute captured radio operators out of hand. The goal was to turn and use them, or to extract enough information to imitate them. For the safety of entire underground "circuits", it was important to determine if an operator was genuine and still free, but means of independently checking were primitive. Marks claims that he became convinced (but was unable to prove) that their agents in the Netherlands had been compromised by the German counter intelligence Abwehr. Marks wrote the script for Michael Powell's film Peeping Tom (1960), the story of a serial killer who films his victims while stabbing them. The film provoked critical revulsion at the time, and was described as "evil and pornographic." In 1998, Marks published his account of his work in SOE – Between Silk and Cyanide: A Codemaker's Story 1941–1945. The book was written in the early 1980s, but didn't receive UK Government approval for publication until 1998.