• Kyodo

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Britain was the first country to crack some of Japan’s most important naval codes in the 1930s and 1940s, and the long-held view that the United States played the main role as Japanese code-breaker is false, according to a new book.

“The Emperor’s Codes,” written by Daily Telegraph defense correspondent Michael Smith and published last week, says Britain was responsible for breaking most of Japan’s prewar naval codes. It says British code-breakers first cracked JN25, the Imperial navy’s main code during World War II, months before the Americans, who took all the glory for breaking it.

Cracking JN25 helped to contribute to the destruction of the Japanese fleet at Midway in 1942.

Smith says Britain did not reveal the fact that it had cracked the Japanese codes because it wanted to continue intercepting the messages of other countries without being discovered.

During the war, Japanese codes were cracked at Bletchley Park, about 80 km northwest of London, where Germany’s Enigma code and others were also broken.

The book reveals that Britain was deciphering Japanese codes as early as 1926 and had its first major success in 1934, when Hugh Foss broke a new machine cipher used by Japanese naval attaches in their embassies — 15 months ahead of the Americans.

Foss also later broke a Foreign Ministry machine cipher, two years before the U.S. did, according to Smith.

Smith’s revelations are based on his research of recently released documents at the Public Records Office in London.

The man given the credit for cracking JN25 and a number of other Japanese super-enciphered codes was British cryptographer John Tiltman, who worked at Bletchley Park.

“JN25 first appeared in June 1939, and within weeks Tiltman had broken it. The Americans later claimed to have broken JN25. They did, but not until many months later,” Smith wrote in Wednesday’s Daily Telegraph.

“News also leaked out of how the U.S. Navy had read a JN25 message that allowed its aircraft to shoot down the head of the Japanese navy, Adm. Isoruku Yamamoto. This publicity gave the lasting impression the Americans had broken the Japanese codes,” he said.

“By contrast, the British clamped down on any mention of the remarkable achievements of their own code-breakers so that they could continue to intercept the communications of other countries with impunity,” he added.

The book also argues that the U.S. Navy’s reluctance to share its code-breaking successes with Britain, and even with its own army, led to unnecessary casualties in the Pacific during the war, mainly American.

Smith also dismissed claims that the British knew of the Pearl Harbor attack through JN25 but kept quiet about the plans in order to draw the U.S. into the war.

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