A cipher system based on the single use of a cipher, thereby avoiding the repetition that allows the cryptographer an advantage. The system is usually based on disposable sheets of randomly generated five-figure numbers which are used as additives to alter an enciphered text. Although theoretically ciphers based on OTPs should be unsolvable except by the parties with access to the pads, the British Government Code & Cypher School found in 1942 that it was possible to mimic the supposedly random numbers generated by the Lorenz machine used as the basis of the OTPs used by the German Foreign Ministry. This traffic, code-named FLORADORA, was read consistently until the end of hostilities and proved excellent intelligence.
Duplicate keys spotted in Soviet cable traffic in 1943 led to the discovery that between 1940 and 1948 the Soviets had depended on identical sheets inserted separately into the OTPs used for diplomatic, consular, trade, NKVD, GRU, and Naval GRU communications. This source later became known by the generic code name VENONA, although precisely how the random numbers were generated, before they were printed on their pads’ disposable sheets, remains a mystery.