Leo Marks, the son of a Jewish bookseller, was born in London on 24th September, 1920. Marks joined the British Army in January 1942. Trained as a cryptographer he was assigned to the Special Operations Executive (SOE).
Marks became an expert in cryptanalysis (making and breaking codes and ciphers) and eventually became head of SOE's codes and ciphers with a staff of 400 people. It was Marks's responsibility to provide agents with the ciphers with which to send information to London by radio.
These ciphers were often based on famous poems or brief passages of memorable prose such as the Lord's Prayer. Marks argued that the enemy might know the poem or the prose passage and would then be able to break the cipher. To overcome this problem Marks provided unknown poems for his agents. This included the poem The Life That I Have, that had originally been written for his girlfriend, Ruth Hambro who had been killed in an air crash in Canada. He later gave the poem as a cipher to the SOE agent Violette Szabo when she was sent to France during the war.
When agents based in Holland began sending messages without any errors, Marks suspected they had been arrested by the Gestapo. To test his theory he sent indecipherable messages to the agents. When they did not complain he knew that the short-wave morse transceivers were under the control of the Germans. His warnings were ignored by Maurice Buckmaster and agents continued to be sent to Holland where they were arrested and in most cases executed.
On 23rd June, 1943, three key members of the Prosper Network, Andrée Borrel, Francis Suttill and Gilbert Norman, were arrested by the Gestapo. Noor Inayat Khan reported back to the Special Operations Executive that she had lost contact with the rest of the group and feared they were in the hands of the Germans. Jack Agazarian, who was on leave at the time, told the SOE that if this was the case, he suspected that they had been betrayed by Henri Déricourt, a former pilot in the French Air Force, whose job it was to find suitable landing grounds and organize receptions for agents brought by air.
Gilbert Norman continued to send messages to London. Marks, was convinced that Norman was under the control of the Gestapo. Major Nicholas Bodington disagreed and persuaded Maurice Buckmaster to let him go to France to find out what had happened. Jack Agazarian was recalled from leave and the two men were taken to France.
Messages from the wireless owned by Gilbert Norman were still being sent to the Special Operations Executive in London. Instructions were passed on to Bodington by the SOE to arrange a meeting with Norman at the address he had sent them. Bodington later claimed that he and Jack Agazarian tossed to decide who should visit the address. Agazarian, who was convinced it was a trap, lost, and when he arrived at the address he was immediately arrested. Agazarian was tortured by the Gestapo for six months at Fresnes Prison before being sent to Flossenburg where he was kept in solitary confinement.
After the war Marks became a writer for stage and screen. This included writing the script for Peeping Tom. Directed by Michael Powell in 1960 it tells the story of a serial killer who films young women as he stabs them to death. Condemned as pornographic and evil, it was not shown on television until 1997.
Marks also had trouble with his autobiography Between Silk and Cyanide, that challenged the official history of the Special Operations Executive written by M.R.D. Foot. Although written in the early 1980s it was blocked by Whitehall and only appeared in 1998. He also published The Life That I Have in 1999. Leo Marks died on 15th January 2001.