Leo Marks’ “mental one-time pad”: an experimental cipher for British special operations in World War II

Abstract

During World War II, Leo Marks designed ciphers for the British Special Operations Executive (SOE). Tasked with ensuring that SOE undercover agents could encrypt radio transmissions from Nazi-occupied Europe to British headquarters, Marks introduced one-time pad ciphers with random keys printed on materials like paper and silk. However, carrying such materials was risky for agents. In an attempt to mitigate this risk, Marks experimented with a system he called the “Mental One-Time Pad” (MOP) cipher. Contrary to its name, the MOP cipher was not a genuine one-time pad system, but rather used double columnar transposition with memorized texts to create unique substitution keys. In this article, we’ll explore the design, vulnerabilities, and historical significance of the MOP cipher, and discuss potential improvements from US Army cryptographer William Friedman.