Leo Marks
He replaced SOE's fatally weak poem codes with true one-time pads, and wrote a love poem that became the most famous cipher key of the war.
Leo Marks
Leo Marks was the precocious son of the owner of 84 Charing Cross Road. He taught himself codes from his father's rare book collection and was recruited to the Special Operations Executive at age 21. Given command of SOE's coding section, he found field agents dying because their poem codes — ciphers based on memorised poems — were trivially breakable under torture or traffic analysis. German codebreakers at the SD were identifying the poems from the traffic alone. Marks invented the Letter One-Time Pad (LOTP): a silk-printed single-use random key small enough to be sewn into clothing, swallowed, or destroyed without trace. He simultaneously introduced "security checks" — deliberate mistakes kept secret from London — so that agents under German control could signal their capture covertly. The system arguably saved hundreds of SOE agents' lives. He also wrote original poems as keys for agents he judged unlikely to remember memorised texts under stress — including "The Life That I Have," which he gave to Violette Szabo in 1944, months before her capture and execution.
Why This Person Matters
Leo Marks sits at a unique intersection of operational cryptography, literary sensibility, and moral seriousness about what it meant to send people to their deaths with bad cipher systems. SOE's poem codes were a disaster: the SD (German security service) broke most of them within days, and the infamous Englandspiel deception in the Netherlands ran for nearly two years because London chose to ignore agents' distress signals. Marks was the internal critic who identified the problem, proposed the solution, and fought the bureaucratic resistance. His 1998 memoir Between Silk and Cyanide is both a technical document of cipher design under wartime pressure and one of the most emotionally candid accounts of intelligence work ever published. The poem he wrote for Violette Szabo survived her, won a BAFTA when used in the 1958 film of her life, and is now on the wall of the SOE memorial in London.
SOE's standard cipher was the "poem code": an agent memorised a poem, chose five words from it as key, and used transposition of the letters to encipher messages. The fatal flaw was that the key — a poem — was memorable but finite and known. German analysts at the SD's Funkabwehr unit would intercept traffic, see recurring transposition patterns, and try published poems systematically until one matched. Because agents used the same poem repeatedly, a single break unlocked all past and future traffic. Marks calculated that the Germans could identify the poem from the structure of the ciphertext alone in under 48 hours of traffic volume, and that under any serious interrogation an agent would give the poem within hours. He described poem codes as "a lethal weapon pointed at our own agents."
Marks's solution was the Letter One-Time Pad (LOTP): a printed page of random letters, grouped in sets of five, each group usable once and once only. An agent would use one group per message, cross it off, and the pad would be destroyed section by section. The key material was printed on silk (to survive water and be sewable into clothing linings) or on rice paper (edible). Because each key was used exactly once, there was no pattern for a frequency analyst to exploit and no poem to torture out of the agent. The theoretical security of the OTP was absolute given random key material — Marks printed his using a physical random source. The practical problem was key distribution to agents already in the field; Marks solved this by preparing individual pads for each agent before departure and by establishing an air-drop resupply system for active networks.
The poem Marks wrote for Violette Szabo was composed after the death of his girlfriend Ruth in a Canadian plane crash in 1943. He had been using it privately; when Szabo proved unable to memorise existing poems reliably before her first mission, he gave her the new one instead. She was parachuted into France in April 1944, returned, and went again in June — where she was captured, tortured, sent to Ravensbrück, and shot in January or February 1945. After the war the poem was read on BBC radio and caused such correspondence that the BBC produced a documentary about Szabo. It was used in the 1958 film Carve Her Name with Pride, won a BAFTA for best score/lyrics, and Marks donated all royalties to the SOE Memorial Fund. The poem's text — "The life that I have / Is all that I have / And the life that I have / Is yours" — is now carved at the SOE memorial at Lambeth Palace Road, London.
| Born | September 7, 1920, London |
| Died | January 15, 2001, London |
| Role | Head of Codes, SOE 1942–45 |
| Key innovation | Letter One-Time Pad (LOTP) for field agents |
| Famous poem | "The Life That I Have" |
| Post-war | Screenwriter; memoir Between Silk and Cyanide, 1998 |
| Legacy | SOE Memorial inscription; BAFTA |