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

AbstractDuring 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.Keywords: Leo Marksmental one-time pad (MOP)transposition-substitution ciphersWilliam FriedmanWorld War II ciphers Disclosure statementNo potential conflict of interest was reported by the authors.Additional informationNotes on contributorsThomas LarsenThomas Larsen is an Australian data scientist in the commercial sector as well as in critical care research at the University of Melbourne and Monash University. He specializes in accurate and interpretable (A+I) machine learning techniques including symbolic regression.