World War II, United States Breaking of Japanese Naval Codes

World War II, United States Breaking of Japanese Naval Codes


On December 7, 1941, Japanese military forces attacked the United States naval fleet anchored at Pearl Harbor on the Hawaiian island of Oahu. The surprise attack was devastating to the U.S. Navy. Nearly every American plane on Oahu was destroyed; three cruisers, three destroyers, and eight battleships were severely damaged, and two battleships, the Oklahoma and the Arizona were destroyed; over 2,300 U.S. servicemen lost their lives. In the weeks and months that followed, fears ran deep among shocked Americans that Japan had the ability to launch an invasion on the West Coast of the United States. At the very least, it was feared that the Japanese Navy, facing only the remnants of a tattered American fleet, could effectively control the Pacific Ocean, cutting the United States off from vital resources and shipping lanes.

Over the next three and a half years, in a series of fierce sea and island battles, American forces managed to push the Japanese empire back to its own shores. They were able to do so not only through courage and resolve, but also through the efforts of hundreds of men and women who labored in secrecy, many of them twelve hours a day, seven days a week, cracking the codes that Japanese forces used to transmit messages. Without the information revealed by breaking these codes, the U.S. military could never have countered Japanese offensives throughout the vast expanse of the Pacific, for they would never have known where the Japanese intended to strike next.

Early code-breaking efforts. Even before World War I, the United States had been regularly deciphering coded messages sent by foreign diplomats. On the basis of decoded diplomatic messages, for example, the United States and Great Britain knew what arms limitations the Japanese would accept in the peace talks following the war, and negotiators bargained accordingly. The effort to break Japanese diplomatic codes continued into the 1920s and 1930s under the direction of William Friedman, a Russian immigrant who was appointed chief cryptanalyst of the Army Signal Intelligence Service (SIS) in 1922. In the late 1930s, SIS cryptanalysts succeeding in breaking the Purple code, also designated AN-1, which was the principal cipher Japan used to send diplomatic messages. (While the terms code and cipher are often used interchangeably, a code is a substitution of one character or string of characters for another; reading a cipher, however, usually requires application of some kind of mathematical operation specified by a cipher key; a simple cipher, for example, might require the decoder to subtract a designated value from a string of numbers to arrive at the true string of numbers that encodes a letter, word, or phrase.)

JN–25. On June 1, 1939, the Japanese introduced what American cryptanalysts called JN–25. JN means simply Japanese Navy, and JN–25, consisting eventually of about 33,000 words, phrases, and letters, was the primary code the Japanese used to send military, as opposed to diplomatic, messages. After Pearl Harbor, U.S. intelligence efforts focused on cracking JN–25. Leading the effort, code-named Magic, was the U.S. Navy's Combat Intelligence Unit, called OP–20–G and consisting of 738 naval personnel. The unit, housed in the basement of the 14th Naval District Administration at Pearl Harbor, was under the command of Commodore John Rochefort, who combined fluency in Japanese with single-minded dedication to the task. Using complex mathematical analysis, IBM punch-card tabulating machines, and a cipher machine, Friedman had developed the ECM Mark III, the unit was able to crack most of the code by January 1942. The blanket name given to any information gained by deciphering JN–25 was Ultra, a word borrowed from British codebreaking efforts and stamped at the top of all deciphered messages.

The Combat Intelligence Unit worked tirelessly, but the unit had some help from the Japanese themselves. For example, messages, primarily radio transmissions, often began with such stylized phrases as "I have the honor to inform your excellency" and with the names of ships, locations, commanders, the time and date, and similar repeated information that could be easily verified; many referred to military and other officials by formal, stylized titles. These weaknesses, combined with the fact that the Japanese introduced changes to the code only every three to six months, gave American cryptanalysts a toehold into the code. Soon they were able to read the code, which consisted of strings of five digits. Thus, for example, the string 97850 meant submarine, although because JN–25 was really a cipher, the cryptanalyst had to subtract a value from the string of digits to arrive at the correct meaning. Making this task somewhat easier for Americans, the Japanese changed their cipher key infrequently.

Putting the code to use. Armed with the ability to read Japanese operational messages, the U.S. Navy was able turn back the Japanese advance in the Pacific in mid-1942. In April of that year, decrypted messages revealed that Japanese forces were preparing for an assault on Port Moresby, an Australian base in New Guinea, on May 7. In response, U.S. Admiral Chester Nimitz moved his fleet into the Coral Sea between New Guinea and Australia. While the ensuing two-day Battle of Coral Sea was considered a draw, U.S. forces inflicted enough damage on the Japanese navy to force it to withdraw, giving the United States and Australia time to reinforce Allied defenses in New Guinea.

Perhaps the most dramatic success that resulted from breaking the Japanese naval code was the Battle of Midway in June 1942. The plan of Japanese commander Admiral Isoroku Yamamoto was to assemble an aircraft carrier task force, launch a diversionary raid off the Aleutian Islands, and lure the U.S. Navy to Midway Island and into a decisive battle that would destroy what remained of the American fleet after Pearl Harbor. From decrypted messages, U.S. naval commanders knew the general outlines of the plan, even the timetable. The messages, however, did not say where the Japanese intended to strike; the target was simply designated "AF." It was Rochefort who proposed a ruse to determine what AF stood for. Suspecting that it was Midway Island, he arranged for American forces on the island to send out a radio message saying that they were running short of fresh water. Rochefort and his group waited anxiously to see if Japan would take the bait. Finally, codebreakers intercepted a Japanese message: AF was running short of fresh water. Knowing that the assault was to come at Midway, the U.S. Navy was ready. On June 4, 1942, after a fierce three-day battle, U.S. pilots sank all four Japanese aircraft carriers in Yamamoto's task force, effectively turning the tide in the Pacific. Later, in an unintended breach of wartime security, the Chicago Tribune published a story revealing that the navy had known about Japanese intentions in advance, in effect revealing that JN–25 had been broken. The Japanese never found out about the article.

In a postscript to the Battle of Midway, Admiral Yamamoto lost his life as a result of a decrypted message. Codebreakers learned that the admiral was scheduled to inspect a naval base on Bougainville in the Solomon Islands on April 18, 1943. Some U.S. policy makers were hesitant to use this information for fear that doing so would tip off the Japanese that their codes had been broken. Nevertheless, the decision was made to assassinate Yamamoto. That morning, eighteen P–38 fighters left their base at Guadalcanal at the other end of the Solomon chain and arrived at Bougainville just as Yamamoto's plane was making its approach. The admiral was killed in the attack, depriving Japan of its most experienced and accomplished admiral and sapping Japanese morale. To maintain the fiction that the fighters had arrived by chance, the air force flew other patrols in the area, both before and after the attack. The Japanese did not change JN–25, and for the remainder of the war, U.S. intelligence intercepted and read thousands of Japanese messages.



Benson, Robert Louis. A History of U.S. Communications Intelligence During World War II: Policy and Administration. Washington, D.C.: Center for Cryptologic History, National Security Agency, 1997.

Farago, Ladislas. The Broken Seal. New York: Random House, 1967.

Persico, Joseph E. Roosevelt's Secret War: FDR and World War II Espionage. New York: Random House, 2001.

Winton, John. Ultra in the Pacific: How Breaking Japanese Codes & Cyphers Affected Naval Operations Against Japan: 1941–1945. London: Leo Cooper, 1993.


"Cryptography in the Modern Age." < > (January 9,2003).

Singh, Simon. "US Codebreakers in World War II." < > (January 9, 2003).


Cipher Machines
Codes and Ciphers
Cryptology, History
Pearl Harbor, Japanese Attack on
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