READING · 4 MIN
THE CODE THAT
GOT AGENTS KILLED.
In occupied Europe, a memorised poem was the only thing standing between an SOE agent and the Gestapo. A 22-year-old cryptographer named Leo Marks decided that wasn’t good enough - and rebuilt the system from scratch.
Who was Leo Marks? ◢
Leo Marks (1920-2001) was a British cryptographer conscripted into the codes section of the Special Operations Executive(SOE) in January 1942 - the wartime agency Winston Churchill tasked with sabotage and resistance support across Nazi-occupied Europe. Within about a year he was effectively running SOE’s codes office, still in his early twenties. He later wrote Between Silk and Cyanide: A Codemaker’s War 1941-1945, a memoir of that work that the UK government held back from publication for decades before clearing it in 1998.
The poem code’s fatal flaw ◢
Before Marks arrived, agents dropped into France or the Low Countries relied on the poem code: memorise a poem, pick a handful of words from it as an encryption key, and use the letters of those words to scramble a message by transposition. No paper key to be caught carrying - which sounded clever until you saw how it broke. Agents leaned on poems they already knew, and German cryptanalysts simply searched well-known verse until the key fell out. Worse, a memorised poem is not something you can burn: under interrogation, it could be dragged out of a captured agent, unlocking every message they’d ever sent under it. It’s a world away from something like a basic Caesar shift, where the “key” is just a number - the poem code’s danger was that the key lived in a person’s head, and people can be broken.
Marks’s fix: keys you could burn ◢
Marks’s answer was the Worked Out Key, or WOK: instead of a poem in an agent’s memory, each agent carried a set of pre-arranged transposition keys printed on a small sheet of silk. Silk held up to sweat and travel and could be sewn into a lining or a seam. Crucially, each key was used once - the agent cut or tore off that section of silk after sending the message and destroyed it, so nothing survived capture that could compromise past or future traffic. Later in the war SOE pushed this further still, moving some traffic to genuine one-time pads, also printed on silk for the same reasons: nothing memorised, nothing reused, nothing left to torture out of anyone.
Why it mattered ◢
This wasn’t an academic cipher exercise. SOE agents were operating alone, in disguise, inside Gestapo-patrolled cities, and a broken code could roll up an entire resistance network in one sweep. Marks spent the war treating every key as a life-or-death design problem, not a puzzle - which is precisely why the field moved from “memorable” toward “disposable.” It’s the same instinct behind every serious cipher since: the strongest ones are those where the secret can be destroyed the moment it’s used.
Play the ciphers in this story