An unbreakable cipher when used properly. Used improperly by the Soviet NKVD during World War II, it opened up much of the story of Soviet espionage efforts against the United States.
The one-time pad contains thousands of groups, usually of five digits. Each group represents a single word or a phrase. Then, to ensure security, the person using the pad next transposes or converts the numbers of his enciphered message to another set of digits by using a specific, but randomly chosen page of the pad (the double, or superencipher, process). The key to the pages and lines used on the pad would precede the encrypted message.
The one-time pad takes various forms, from a thick booklet the size of a postage stamp to a scroll about the size of a cigarette butt. The important feature is that the pad must be small, so that it can be easily concealed. The booklets can be very thick, several hundred pages, and sometimes in two colors, to distinguish enciphering and deciphering sections. The "printing" is often a form of reduced photography.
Further, the "paper" of the one-time pad can be made of cellulose nitrate or some other highly flammable material that enables its rapid destruction. During World War II the U.S. Office of Strategic Services and the British Special Air Service employed "pads" printed on silk handkerchiefs, printed with 600 cipher groups.
In theory there are but two copies of a specific one - time pad: one for the agent or diplomat, and one for the official with whom the agent or diplomat communicates.
Cryptography historian David Kahn cites 1918 as the year one-time pad ciphers were invented. In 1930 the Soviet Union began to employ the one-time pad for messages from its overseas diplomatic and intelligence officials, and subsequently agents, making their communications invulnerable to deciphering. The one-time pads were to be used only once.
The U.S. Army's Signal Intelligence Service and successor agencies were able to intercept, but not decrypt, messages sent by Soviet officials in New York to Moscow beginning in 1939. Not until Feb. 1, 1943, did the Army begin a major effort to decipher these messages under the Venona program. During World War II the cryptographic material production office of the NKVD apparently reused some of the pages from one-time pads, and these were used by Soviet legals and illegals in the United States in their communications with Moscow.
Because of the duplication of one-time pad pages, the Army obtained some minor decrypts of messages in 1943. The situation was more favorable for the codebreakers in 1944, even more so in 1945. Finally, in the summer of 1946 major breakthroughs were made, opening up the wartime spying efforts of the Soviet Union against the United States.
One-time pads continued in use into the Cold War era. Cuban revolutionary Che Guevara was carrying sheets of a one-time pad cipher when he was killed in Bolivia in 1967.
