Operation Magic

Operation Magic


Operation Magic was the cryptonym given to United States efforts to break Japanese military and diplomatic codes during World War II. The United States Army Signals Intelligence Section (SIS) and the Navy Communication Special Unit worked in tandem to monitor, intercept, decode, and translate Japanese messages. Intelligence information gathered from the messages was sent to military command at the Office of Strategic Services (OSS). The ability to decipher and read Japanese communications was one of the key components of the Allied victory in the Pacific.

Even before the outbreak of World War II in Europe in 1939, the United States began its efforts to decode Japanese diplomatic and military communications. In 1923, a United States Navy intelligence officer obtained contraband copy of the World War I era Japanese Imperial Navy Secret Operating Code. Photographs of the codebook were passed on to the cryptologists at the Research Desk, where code was placed in red folders after the additive code keys were fully discovered. The simple additive code became known as "Red," after the folders in which it was stored.

For high-level communications, the Japanese replaced Red with Blue, a more sophisticated code in 1930. However, the new code too closely resembled its predecessor, allowing United States cryptologists to fully break the new cipher in less than two years. At the outbreak of World War II, the Japanese were still using both Red and Blue for various communications. U.S. military intelligence established listening stations throughout the Pacific to monitor ship-to-ship, command-to-fleet, and land-based communications.

After war broke out in Europe, the Japanese received encryption and security help from Nazi Germany. The Germans had discovered that U.S. intelligence was monitoring and decoding Japanese communications as early as 1935, but they did not immediately inform the Japanese. Later, Germany sent a copy of their infamous Enigma encryption machine, with a few modifications, to help secure Japanese communications. As a result, U.S. intelligence could no longer read Japanese intercepts. The painstaking work of U.S. cryptologists began anew.

U.S. cryptanalysts named the new code Purple. Applied to several variations of the initial Enigma code, Purple provided the most significant challenge to both United States and British intelligence during the war.

With the aid of information from Polish and Swedish cryptologists, the British military intelligence cryptanalysis unit at Bletchley Park first broke the German Enigma code. They then developed sophisticated decoding bombes and the first programmable computer to facilitate the deciphering of the complex Enigma code. By 1943, British intelligence was able to utilize almost real-time intelligence information received from translated Enigma intercepts.

In the United States, cryptologists struggled to break the Purple by hand. However, the structure of Japanese messages, always beginning with the same introductory phrase, aided code breakers in determining the sequencing of the multi-rotored Japanese cipher machine. United States code breakers had made significant progress on the Purple code by 1941, gaining the ability to read several lines of intercepts. The process remained slow, and the information gained from Purple was usually outdated by the time it was translated.

Aware of British successes against the German Enigma machine, United States military intelligence asked their ally to share code-breaking information. The British sent top Bletchley Park cryptographers and engineers to the United States to help train code breakers and build decoding bombes. However, they closely guarded, and did not share, the secret of Enigma code breaking efforts (code named Operation Ultra) that involved Colossus, the Bletchley Park decoding computer.

With the aid of the British, United States intelligence made significant progress against Purple in a short time. A replica of the Japanese Purple machine, built in 1939 by American cryptologist William Friedman, was used to adapt a German Enigma bombe to decode Japanese Purple. Although the settings for each message had to be determined by hand, United States intelligence gained the ability to read Japanese code with greater ease, in a more timely manner, by 1942, six months after the Japanese bombing of Pearl Harbor and the entry of the United States into World War II.

Utilizing their extensive network of listening stations in the Pacific, United States intelligence intercepted and decoded several other types of messages. Diplomatic Purple messages, paired with JN-25 intercepts, another broken Japanese Navy code, gave U.S. military command vital information about Japanese defenses at Midway. Operation Magic intercepts provided useful information during the ensuing Battle of Midway, turning the tide of the war in the Pacific in favor of the allied forces. A year later, Purple intercepts gave the U.S. information about a diplomatic flight on which Japanese General Yamamoto, the mastermind behind the Pearl Harbor attack, was traveling. U.S. planes shot down the Japanese aircraft.

Operation Magic provided critical intelligence information in both the Pacific and European theaters of war. Diplomatic messages between Berlin and Tokyo, encoded with Enigma and Purple, yielded British and United States intelligence information regarding German defenses in France. The information helped commanders plan the DDay invasion of Normandy in June 1944.

The Japanese government remained unaware that the United States broke the Purple code. Japanese Imperial forces continued to use codes broken by Operation Magic throughout the war and in the weeks following the Japanese surrender in 1945.



Boyd, Carl. Hitler's Japanese Confidant: General Oshima Hiroshi and MAGIC Intelligence, 1941–1945. Lawrence, KS: University Press of Kansas, 1993.

Budiansky, Stephen. Battle of Wits: The Complete Story of Codebreaking in World War II. New York: Touchstone Books, 2002.

Clark, Ronald William. The Man Who Broke Purple: The Life of Colonel William F. Friedman, Who Deciphered the Japanese Code in World War II. Boston: Little Brown, 1977.

Matthews, Tony. Shadows Dancing: Japanese Espionage Against the West. New York: St. Martin's Press, 1993.


Bletchley Park
Purple machine
Red code
Ultra, Operation
World War II
World War II, United States Breaking of Japanese Naval Codes

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