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.
His father, Benjamin Marks, was joint owner of the Marks & Co bookshop at 84 Charing Cross Road, which achieved international fame with the 1970 book of that title by New York writer Helene Hanff and the later plays and movie.
As a teenager, he earned pocket money by setting the notoriously difficult Times cryptic crossword.
So begins his book, Between Silk and Cyanide, about his work in cryptography. Marks joined the Armed Services 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 "the 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 ciphers 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's innovations, especially "worked-out keys" (he was widely credited with inventing the "letter one-time pad," but while he did independently discover the method he was later to find that it was already in use at Bletchley). 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 gained popularity when it was used in the 1958 film about her, Carve Her Name With Pride. It is believed that Marks wrote the poem about a girlfriend, Ruth Hambro, who was killed in an air crash in Canada.
The life that I have is all that I have
And the life that I have is yours
The love that I have of the life that I have
Is yours and yours and yours.
A sleep I shall have, a rest I shall have
And death will be but a pause
For the years I shall have in the long green grass
Are yours and yours and yours.
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, and increasingly so the longer it took). In response to this problem Marks established, staffed and trained a large group (based at Grendon Underwood, Buckinghamshire) to cryptanalyse garbled messages ("indecipherables") 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 the Netherlands, 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—Das Englandspiel), 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. The other side of this story was published in 1953 by Marks's German "opposite number" in the Netherlands, H J Giskes in London Calling North Pole.
At one point in his book (pp. 222-3), he describes the memorandum he wrote detailing his conviction that the messages from the Netherlands were being sent by Germans or by agents who had been "turned." His main argument is that despite incredibly harrowing circumstances "not a single Dutch agent has been so overwrought that he's made a mistake in his coding...." Marks has to face Brigadier (later Sir) Colin Gubbins:
Gubbins grills Marks. In particular he wants to know who has seen this report, who typed it (Marks did):
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 (1968) 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.
He married the portrait painter Elena Gaussen in 1966, a marriage that lasted until shortly before his death in January 2001.
