TIL about Leo Marks, WW2 cryptographer, who was given a 20 minute training exercise on his first day at work which took him the whole day to complete. He had accidentally not been given the cipher key and ended up breaking a code that was meant to be secure.

Leo Marks, who was codes and ciphers chief at the Special Operations Executive (SOE) during the second world war, and worked closely with agents dropped behind enemy lines, has died aged 80. He also pursued a long and sometimes controversial postwar career as a writer for stage and screen, culminating in the film Peeping Tom, which hurt the reputation of its leading British director, Michael Powell.

Obeying Churchill’s order to “set Europe ablaze”, SOE, a parallel secret service run by a mixture of brilliant brains and bungling amateurs, sent spies into occupied Europe to gather information and work with resistance groups. Casualties were high, as when the Germans penetrated its Dutch network and executed some 50 agents, despite warnings from Holland, and Marks’s own suspicions, disastrously ignored by the SOE command.

It fell to Marks to provide individual agents with the ciphers with which to send information to London by radio. Some of these were based on poems so familiar that an educated German was quite capable of recognising them and guessing the cipher, a fact that had not occurred to his superiors.

He therefore wrote new poems for people such as Violette Szabo, one of his most famous agents, who dropped into France armed with the usual concealed cyanide pill (in case of torture) and the following verse, inspired by the death of a woman Marks had loved: “The life that I have/ Is all that I have/ And the life that I have/ Is yours.”

Marks’s later means of protecting individual agents’ ciphers was to issue them with pads on easily concealed squares of silk, to be destroyed after use. Such ciphers were virtually unbreakable in the pre-computer age, and were not new; but Marks refined a stratagem still used long after the war.

He also managed to stop his superiors from insisting that agents repeat partly indecipherable messages for London’s administrative convenience, since this hugely increased their chances of capture by the Gestapo. Instead, he used his knowledge of individual agents’ foibles to work out what they meant, and his quicksilver intelligence to fill in gaps.

Marks’s deduction that the Germans had broken into the Dutch network was based on the fact that all messages from Holland in 1943 came across without a single error. He ended one dummy message to Holland with the letters HH, a common German sign-off standing for Heil Hitler. When the reply carried the automatic HH, he knew there was a German on the key.

Leopold Marks was educated at St Paul’s school, west London. His father, Benjamin, owned a London bookshop, later made famous by Helen Hanff’s memoir (also filmed) of her postal relationship with a dealer at the shop, Frank Doel: 84 Charing Cross Road. At the age of eight, Leopold, an only child, read Edgar Allen Poe’s story, The Gold Bug, about a treasure protected by a code; at the same age, he also cracked his father’s secret pricing code for second-hand books in a few minutes.

Conscripted in January 1942, Marks was trained as a cryptographer, completing a week’s decipherment exercise in hours. Unlike the rest of his intake, who were sent to the main British codebreaking centre at Bletchley Park, he was assigned to SOE. There, he apparently failed another test when he took all day to break a text he was expected to decipher (with the aid of a key) in 20 minutes. Not untypically, SOE had forgotten to supply the key.

Marks’s extraordinary ability brought rapid promotion to leadership of cryptography and cryptanalysis (making and breaking codes and ciphers) at SOE. Soon he was running a mini Bletchley at Grendon Underwood, in Buckinghamshire, staffed with 400 female volunteers specially chosen by him. But the young genius was not always comfortable at SOE, convinced that his Jewishness was held against him by some.

Awarded an inconspicuous MBE for his war work, Marks kept in touch with the intelligence world after demobilisation, and went into a successful career writing for the theatre and films.

Much of a not always brilliant output was derived from his SOE experience. In 1960, he wrote the script for Peeping Tom, about a serial killer who films young women as he stabs them to death. Filmed as if through the killer’s eye, it made the viewer a voyeur, if not an accomplice, a device which undermined Powell’s career: the film was first shown on television only in 1997. But the US film-maker Martin Scorsese acknowledged his debt to this picture, condemned at the time as pornographic and evil. There was clearly a dark side to Marks’s mind. He also worked for the Boulting brothers.

Marks did not take over the bookshop on his father’s death. Strongly built, yet notably soft-voiced, he was proud of his wartime exploits, though he kept the real secrets secret. But I remember his vivid description, at the Anglo-Dutch conference on Holland At War, in London in 1969, of his double struggle to frustrate the German “England spiel” in Holland, and to persuade SOE to change its ways.

He challenged the version of the story included by the historian MRD Foot in his book on SOE. Publication of Marks’s memoirs, Between Silk And Cyanide, in 1998, had been blocked by Whitehall for more than 10 years.

Illness, money problems and the breakdown of his childless, 34-year marriage to the painter Elena Gaussen last year brought a bitter end to the life of an octogenarian who may be seen to have peaked in the secret service world when he was in his early 20s.

• Leopold Samuel Marks, cryp- tographer and screenwriter, born September 24 1920; died January 15 2001

• This article was amended on 1 August 2017. An earlier version said that Helene Hanff’s 84 Charing Cross Road was based on her correspondence with Marks’ father. This has been corrected to say it was based on her correspondence with Frank Doel, a bookdealer who worked in the shop.