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As was normal practise, various codes were used by the Japanese for their messages. Regular communications at consular level used J-19 which both the British and Americans could read, but as the keys changed daily, it was laborious and time-consuming work. The more sophisticated diplomatic code, Purple, was readable by use of the machine known as MAGIC and the Japanese naval code, JN-25, was thought to be unreadable at the relevant time - late 1941. The espionage undertaken in Honolulu by the Japanese consulate was reported to Tokyo using J-19 and these were, in themselves, sufficient to warn the USA of the planned attack on Pearl Harbor.

See also: CIU; espionage; JN-25; MAGIC; Pearl Harbor