AS CHRISTMAS APPROACHED in 1942, a young cryptographer named Leo Marks sat in an office on Baker Street in London, trying to understand what was bothering him. Marks worked for the Special Operations Executive (SOE), the clandestine outfit Prime Minister Winston Churchill had ordered to “set Europe ablaze” with sabotage operations. His job was to improve the security of communications with agents behind enemy lines who transmitted and received encrypted messages in Morse code via portable radio sets.
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There were plenty of things to trouble the 22-year-old: he seemed to be on the point of being fired for insubordination, he couldn’t get a girlfriend, and his neighbors believed his lack of a uniform meant he was a coward. But it wasn’t any of those things that were on his mind. It was something about the messages SOE was getting from its agents in the Netherlands.
For a start, some weren’t using their security checks. Agents were given secret signals—often a deliberate spelling mistake—to include in a message to show that they weren’t “controlled”: that they hadn’t been captured and forced to transmit at gunpoint. Typically agents would be given two such checks: one they could confess to under torture and one they were supposed to keep secret. One Dutch agent had never used any of his checks; another had started out doing so and then stopped. That ought to have been a red flag, but when Marks asked the Dutch section’s controllers about it, he was told not to worry.
Part of the trouble was that all of SOE’s communications security was lax. Marks had been horrified, on joining earlier that year, at how easy the organization’s ciphers were to break. That made the main part of his job particularly important: ensuring that the agents spent as little time as possible on the air. Once an agent began transmitting, Nazis using direction-finding equipment would start hunting for the signal’s source. If caught, the agent faced torture and death. One cause of extra transmissions was when an agent made an encoding error, meaning that the message couldn’t be decoded at the other end. Until Marks arrived, SOE had tended to ask agents to send their message again. But Marks viewed that as an unnecessary risk. He set about teaching SOE’s signals staff how to work out where the agent had gone wrong and decode their messages so they didn’t have to retransmit.
Although, when Marks thought about it, this was one problem the Dutch agents didn’t have. Their messages were always perfectly encrypted. It would be several weeks before he realized the significance of that.