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Born the son of an antiquarian bookseller in London, he was first introduced to cryptography when his father showed him a copy of Edgar Allan Poe's story, The Gold Bug. From this early interest, he demonstrated his skill at codebreaking at an early age by deciphering his father's secret price codes.
Work in cryptography
Marks joined the Armed Services in January 1942, and went to Bedford to train as a cryptographer. His original and unorthodox mode of thought led to him being the only one of his class judged not good enough to be sent to Bletchley Park; instead, he was sent to a rival organisation of the intelligence services, the recently formed Special Operations Executive (SOE). When his abilities subsequently became evident, he was referred to by Bletchley Park as "one that got away".
Marks personally briefed many of the Allied agents being sent into occupied Europe, including Noor Inayat Khan, and his own close friend, the legendary White Rabbit, 'Tommy' Yeo-Thomas. A highly empathetic and imaginative personality (as well as a self-professed "coward"), Marks continually acted on the rarely expressed premise that agents in occupied territories deserved every conceivable bit of support that those enjoying safety and freedom could provide. An attitude which is only just, considering their contribution to freedom and a level of personal sacrifice that would make most veteran soldiers feel humble, yet an attitude which is not always common among individuals, still less among bureaucracies.
Although personally in charge only of agent codes, by making a habit of walking into bureaucratic lion's dens, the young and "cowardly" Mr. Marks saved lives in the field. One of his first challenges (stubbornly resisted by the establishment) was to phase out the use of double transposition ciphers using keys based on preselected poems. These poem cyphers had the limited advantage of being easy to memorise, but a number of significant disadvantages, including limited cryptographic security, substantial minimum message sizes (short ones were hopelessly easy to crack.), and the fact that the complexity of using them caused a significant number of encoding errors to be made resulting in garbled messages.
Cryptographic security was greatly enhanced by Marks' innovations, the "letter one-time pad" and one-time keys. While attempting to relegate poem codes to emergency use only, he enhanced their security by promoting the use of original poems in preference to widely known ones, thus forcing a would-be cracker to work it out the hard way for each message instead of being able to guess an agent's entire set of keys after breaking the key to a single message (or possibly even just part of the key.) Something of a poet himself, Marks wrote and gave to the agent Violette Szabo, the poignant The Life That I Have, which was also used in the 1958 film about her, Carve Her Name With Pride .
Gestapo signal tracers made clandestine radio operators an especially endangered species (life expectancy averaged about three weeks), so shorter and less frequent transmissions were the greatest gift a codemaster had to bestow. Being human and under pressure agents frequently made mistakes encoding messages, and the old practice was for the home station to tell them to recode it (usually a reasonably safe activity) and retransmit it (very dangerous, increasingly so the longer it takes.) In response to this problem Marks established, staffed, and trained a large group (based at Grendon Underwood, Buckinghamshire) to cryptanalyse garbled messages ("indecypherables") so that they could be dealt with in England without forcing the agent to run the risk of retransmission from the field. Other innovations of his simplified encoding in the field which reduced errors, and also made shorter messages possible, both of which reduced transmission time.
The Germans generally didn't just murder captured radio operators out of hand. The goal was to "turn" and use them, or at least to extract enough information to be able to convincingly imitate them. For the safety of entire underground "circuits" it was important to be able to determine if a given operator was genuine and still free, and existing means of independently checking were primitive. When, due to the unusual lack of transcription errors in messages from agents in Holland, Marks became increasingly convinced (but not able to "prove" it) that the situation in the Netherlands was completely out of SOE's control and that they were being toyed with by the Germans (who among themselves actually did call it a game), he was repeatedly told (for basically political reasons) to keep his mouth shut while as many as fifty agents were delivered directly to the Gestapo. Subsequently he invented a new form of "security check" (a duress code) which is still classified even as of 2003.
He left SOE in 1946 and declined an offer of employment from the Secret Intelligence Service (SIS). He went on to write a number of marginally successful plays and films, including The Girl Who Couldn't Quite (1947), Cloudburst (1951), The Best Damn Lie (1957), Sebastian (1967) and Twisted Nerve (1968). Marks also wrote the script for Michael Powell's intelligent and highly controversial Peeping Tom (1960), the story of a serial killer who films his victims while stabbing them to death. Marks also provided the voice of the Devil in Martin Scorsese's The Last Temptation of Christ (1988).
In 1998, Marks published a book about his work in SOE — Between Silk and Cyanide: A Codemaker's Story 1941-1945. The book was reportedly written in the early 1980s, but didn't receive the UK Government's approval for publication until 1998.
- Leo Marks, Between Silk and Cyanide: A Codemaker's Story 1941-1945. (HarperCollins, 1998). ISBN 068486780X.
- Philippe Ganier-Raymond, The Tangled Web, (Arthur Barker 1968, Warner Paperback ISBN 0446659347, originally published in French as Le Réseau Éntranglé) one of the central stories in Marks' book, the betrayal of the SOE Dutch network, told from the Dutch and German points of view.
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