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JN 25 code was not cracked before Dec 7 1941 , JN 25 was first introduced by the IJN Japanese Navy in 1939. The fantasy of revisionist Pearl Harbor historians can not even admit that the United States especially the Navy did not have a cohesive unit of codebreakers until well after Pearl Harbor.

U.S. intelligence efforts then 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.

Using complex mathematical analysis, IBM punch-card tabulating machines, and a cipher machine, which did not exist before Dec.7 1941' the United States 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 code breaking efforts and stamped at the top of all deciphered messages.