<P> The Fleet Auxiliary System, derived from the JN - 40 merchant - shipping code . Important for the information on troop convoys and orders of battle . </P> <P> JN - 25 is the name given by codebreakers to the main, and most secure, command and control communications scheme used by the IJN during World War II . Named as the 25th Japanese Navy system identified, it was initially given the designation AN - 1 as a "research project" rather than a "current decryption" job . The project required reconstructing the meaning of thirty thousand code groups and piercing together thirty thousand random additives . </P> <P> Introduced from 1 June 1939 to replace Blue (and the most recent descendant of the Red code), it was an enciphered code, producing five - numeral groups for transmission . New code books and super-enciphering books were introduced from time to time, each new version requiring a more or less fresh cryptanalytic attack . In particular, JN - 25 was significantly changed on 1 December 1940, and again on 4 December 1941, just before the attack on Pearl Harbor . The 1941 edition (JN - 25b) was sufficiently broken by late May 1942 to provide the critical forewarning of the Japanese attack on Midway . </P> <P> British, Australian, Dutch and American workers were cooperating in attacks on JN - 25 well before the Pearl Harbor attack, but because the Japanese Navy was not engaged in significant battle operations before then, there was little traffic available to use as raw material . Before then, IJN discussions and orders could generally travel by routes more secure than broadcast, such as courier or direct delivery by an IJN vessel . Publicly available accounts differ, but the most credible agree that the JN - 25 version in use before December 1941 was not more than perhaps 10% broken at the time of the attack, and that primarily in stripping away its super-encipherment . JN - 25 traffic increased immensely with the outbreak of naval warfare at the end of 1941 and provided the cryptographic "depth" needed to succeed in substantially breaking the existing and subsequent versions of JN - 25 . </P>

When did the us break the japanese code