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.
Codebreaker Biography
Leo Marks
Leo Marks joined SOE during the war and worked to improve agents’ communications security. His work included replacing vulnerable poem-derived keying with prepared material and letter one-time pads. He did not invent the general one-time-pad principle. Security checks were agreed with the receiving organisation, so their presence or absence could help signal operator authenticity or coercion; they were not kept secret from London.
Why This Person Matters
Marks’s memoir Between Silk and Cyanide describes the operational difficulties of training agents, distributing keys and interpreting security checks. It is a participant’s account, to be read alongside other SOE records. The Netherlands deception known as the Englandspiel also illustrates that warning signs are only useful if headquarters acts on them.
A poem code derives transposition keys from memorised text. Published poems provide a restricted set of possibilities, and repeated use, captured material or interrogation can expose keys. The amount of traffic and difficulty of a particular break vary; no universal 48-hour recovery time is established here.
A letter one-time pad provides secret, independently random key letters for the message. It requires at least as much fresh key material as the protected text, used only once: one five-letter group cannot secure an arbitrary message. Printing and distributing pads does not by itself guarantee random generation, non-reuse or protection from capture.
Marks wrote “The Life That I Have” in memory of Ruth, then assigned it to Violette Szabo as a poem key. It was not originally composed for Szabo. Her wartime service and the later film Carve Her Name with Pride made the poem widely known.
| Born | September 24, 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 |