JN-25 is the name used by Western cryptography organizations for the main secure command and control communications scheme used by the Imperial Japanese Navy (JIN) during and before WWII (it was the 25th Japanese Navy system identified). It was an encyphered code, producing 5 numeral groups as the traffic which was actually broadcast. It was frequently revised during its lifetime, and each new version required a more or less fresh cryptanalytic start. New code books were introduced from time to time and new superencyphering books were also introduced, sometimes even at the same time. In particular, JN-25 was significantly changed immediately before the Pearl Harbor attack in December 1941. It was that edition of the JN-25 system which was sufficiently broken by late May 1942 to provide the forewarning which led to the US victory at the Battle of Midway.

The British and Americans (and the Dutch before the Japanese takeover of the Dutch East Indies) cooperated on attacks against JN-25 beginning well before the Pearl Harbor attack. Because the Japanese Navy was not engaged in actual battle until late 1941, there was little traffic available with which to work. JIN discussions and orders could generally travel by more secure routes than encrypted broadcast, such as courier or direct delivery by a JIN vessel. Publicly available accounts differ, but the credible ones agree that the JN-25 version in use before December 1941 was not more than perhaps 10% broken at the time of the attack. 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.

Note that the Purple cipher, used by the Japanese Foreign Office as its most secure system, had no cryptographic connection with any version of JN-25, or indeed with any of the encryption systems used by the Japanese military before or during the War. Purple traffic was diplomatic, not military, and in the period before the Pearl Harbor attack, the Japanese military, who controlled Japanese policy, did not trust the Foreign Office enough to tell it much. JN-25 traffic, on the other hand, was limited to military matters, mostly JIN operational ones, from which strategic or tactical information could be inferred.