Type B Cipher Machine

Purple code was the code name used by the US military to identify the main cryptographic system used by the Japanese government during World War II. In operation, purple code, or J-machine, messages were encrypted using an analog computer (that is, a code machine). The practical result was virtually a new code for every single character. It was thought to be unbreakable. It was broken by William Friedman.

The purple code was first used by Japan in 1939, but US and British cryptographers had broken it before Pearl Harbor. It is said that US Navy cryptographers translated the 14-part Japanese diplomatic message of December 7, 1941 but were unable to alert the base at Pearl Harbor. Nonetheless, knowledge of the code being used by its enemy gave the US a great advantage in the Pacific war. The similar German encoding system, the Enigma, was also defeated early in the war by Allied cryptanalysis an effort led by Alan Turing.