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Machine for deciphering Japanese code
Machine for deciphering Japanese code
Photograph courtesy U.S. Naval Historical Center


The Japanese had begun to use machines to encipher their communications in the late 1930s. Led by U.S. Army cryptologist William F. Friedman, U. S. code breakers in 1940 were able to build an analog machine that could read the Japanese diplomatic cipher, which the code breakers named Purple.

The Purple breakthrough helped U.S. officials in their negotiations with the special Japanese envoys sent to Washington, D.C., in late November 1941. Of course, the diplomatic messages said nothing about the planned attack on Pearl Harbor.

Meanwhile, U.S. Navy code breakers were trying to penetrate Japanese naval codes. Some success was achieved before Pearl Harbor, but useful decryptions were not achieved against some naval codes until the spring of 1942. Some Japanese naval codes were never broken during the war.


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