Code breaking played an important role in World War II. Codes are systems of words, letters, or symbols that represent others. A code substitutes groups of two or more numbers or letters for words, phrases, or sentences making up a message. A cipher substitutes a letter or number for each letter of the message. After Samuel Morse invented his code of dots and dashes for telegraph communication in the 1830s, hundreds of different ways were soon invented to encode and decode the messages, and the invention of cipher machines and computers in the 20th century made the process of encoding and decoding faster and more complex. The German Enigma machine and the Japanese “J” or Type 97 Alphabetical Typewriter, or Purple machine, are the most famous examples.
Codes and ciphers were used during the American Civil War, the Russo-Japanese War of 1904-05, and World War I, but greater attention was devoted to code breaking following Britain’s founding of the Code and Cypher School at Bletchley Park in 1919. Numbering 150 people in 1939, the Bletchley Park staff increased to 3,500 in 1942 and to
10,000 by 1945. Approximately 65 Americans worked at Bletchley Park, working almost exclusively on the German Enigma machine ciphers, which produced Ultra intelligence, the code name for decrypted German and Italian radio messages. The British first deciphered German Enigma codes in May 1940.
The American code word Magic represented intelligence derived from Japanese diplomatic communications using the Purple machine, while Ultra referred to intercepted and decoded military communications. Unlike Great Britain and the USSR but like the Axis nations, the United States did not have a central code and cipher school and operated separate offices within the military services. The U. S. Navy Operations Division (N-2) and the War Department General Staff (G-2) began code and cipher operations (known as cryptanalysis, which was distinct from traffic analysis) in 1924 and 1929, respectively. Americans working for the U. S. Army Signal Intelligence Service, later known as the Special Intelligence Branch, Military Intelligence Service, under Colonel William F. Friedman, first broke the Japanese Purple diplomatic code in September 1940 and provided a machine to the British in 1941. Although only occasionally deciphered through 1940 and 1941, the Japanese Purple code was read with increasingly regularity by 1942. By spring 1942, American code breakers working for the U. S. Navy Communications Security Unit formed by Commander Laurence F. Safford, were also reading messages enciphered in the Japanese naval code, known as JN-25. Breaking the JN-25 code was an extraordinary feat as the code comprised some 45,000 five-digit groups, each signifying a word or phrase, further embedded in additional five-digit groups taken from a continually changing list of
50,000 random numbers. Perhaps the greatest triumph of this navy unit during the war, under Commander Joseph P. Rochefort, Jr., was the accurate prediction of the Japanese attack at the Battle of Midway in time for Americans forces to take effective countermeasures and turn the tide of the war in the Pacific theater in June 1942.
The Germans and Japanese used many codes and ciphers during the war, some of which were never broken. The Japanese had more than 50 codes, while the Allies knew of 200 German codes by 1945. The Axis powers were convinced that their systems were unbreakable, as were the Allies, although both sides were successful in breaking each other’s code to some extent. The Allied emphasis on strict security, especially between the Americans and British, prevented the Axis, and the Soviet Union, from breaking their codes following 1943, while Allied intelligence services had increasingly regular and direct knowledge of Axis plans through decrypted enemy communications after 1942.
Further reading: Stephen Budransky, Battle of Wits: The Complete Story of Codebreaking in World War II (New York: Free Press, 2000); Ronald Lewin, American Magic: Codes, Ciphers, and the Defeat of Japan (New York: Farrar, Straus & Giroux, 1982); Thomas D. Parrish, The Ultra Americans: The U. S. Role in Breaking the Nazi Codes (New York: Stein & Day, 1986).
—Clayton D. Laurie
World History
