A cipher in which the plaintext is combined with a random key used only once; considered theoretically unbreakable.
Description based on Crypto Museum. The International Spy Museum describes a one-time pad as a string of random numbers used once as a key; properly used, it makes a message mathematically unbreakable.1,2
The key word is “properly”. According to the US Department of Energy, Soviet intelligence cables from the Second World War were enciphered with one-time pads, which should have made them unreadable, but because of a procedural error on the Soviet side some of the messages up to 1945 were read under the VENONA programme. The decrypted cables helped to unmask, between 1948 and 1951, Klaus Fuchs and Julius Rosenberg among others; the USSR learned of the programme in 1949 from Kim Philby.3
One-time pads were also used to communicate with agents. According to B. G. Royden's article in Studies in Intelligence, the Soviet engineer Adolf Tolkachev, a CIA source in Moscow, sent letters to accommodation addresses containing hidden messages written in secret ink and enciphered with a one-time pad.4
Sources
- Crypto Museum, Glossary of covert expressions
- International Spy Museum (Washington, DC), Language of Espionage (glossary)
- US Department of Energy, Office of History and Heritage Resources, The Manhattan Project: Venona
- Barry G. Royden, ‘Studies in Intelligence’ 47 (3), 2003 (CIA), Tolkachev, A Worthy Successor to Penkovsky: An Exceptional Espionage Operation
Checked: 4 November 2026