The Soviet System of Encription

During the late 1930s, the Soviet Union had designed a sophisticated method of securing its diplomatic communications. Messages were sent by diplomatic pouch, shortwave radio, and international cable. While longer messages and scientific samples could be sent by pouch, it often took three months for them to reach Moscow from the United States. Shortwave radio was seen as having limited viability; messages could be intercepted and the discovery of a station alerted the host country’s counterintelligence service that the Soviet services were active. Learning from the mistakes of the British and French, the NKVD used a complex system to protect intelligence and diplomatic messages sent by international cable.
For example, the NKVD rezidentura in New York had to send a message to Moscow that Harry Gold (Gus) was traveling to New Mexico to meet an agent. The initial message would be: Gus traveling to Camp 2 (Los Alamos) to meet probationer (agent). The code clerk would then take the code book and find the five-digit code group for the words. In the case of proper names or locations, the word was spelled out; the code book had a five-digit code group for each letter.
Gus traveling Camp 2 meet probationer
45211 14402 34500 14521 22305
To ensure the security of the message, the clerk would then take a onetime pad, a list of random number groups, and would add a five-digit group to each of the code groups from the book. (Numbers were not carried as in “espionage arithmetic.”) The clerk ended the message with five-digit code groups from the one-time pad, which would show the recipient what numbers had been used in enciphering the message. Thus:
45217 14402 34500 14521 22305 Code book
12345 32503 13542 33454 61234 Added numbers
57552 46905 47042 47975 83549
As a last step to ensure the security of the message, the numbers would be translated into letters, using an established code (0 _ O, 1 _ I, 2 _ U, 3 _ Z, 4 _ T, 5 _ R, 6 _ E, 7 _ W, 8 _ A, 9 _ P.) The final message to Moscow would read:
RWRRU TEPOR TWOTU TWPWR AZRTP.
Even if an adversary had access to the Soviet code book, the system was secure if the code clerks used the one-time pads only once. During the first two years of the war, however, the NKVD and GRU code clerks went through existing one-time pads and began to use the numbered groups repeatedly. This allowed American and British cryptographers to see a pattern in the messages; over years of hard work, thousands of messages were decrypted.
Two excellent sources for the layperson on the Venona process are John Earl Haynes and Harvey Klehr, Venona: Decoding Soviet Espionage in America (New Haven, Conn.: Yale University Press, 1999), and Robert Louis Benson and Michael Werner, Venona: Soviet Espionage and the American Response (Washington, D.C.: National Security Agency, Central Intelligence Agency, 1996). Benson, now deceased, worked on Venona as a young cryptographer. Good places to begin a study of cryptology are David Kahn, The Codebreakers (New York: Scribner, 1996), and Rudolf Kippenhahn, Code Breaking: A History and an Exploration (New York: Overlook Press, 1999).

Historical dictionary of Russian and Soviet Intelligence. . 2014.

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