JN-25

JN-25

JN-25 is the name used by Western crypto organizations for the main secure communications scheme used by the Japanese Imperial Navy during and before WWII. It was a superencyphered 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 fresh cryptanalytic start. New code books were introduced and new superencryption books were also introduced. In particular, JN-25 was significantly changed immediately before the Pearl Harbor attack in December 1941. It was the version of JN-25 introduced immediately before the Pearl Harbor Attack which was sufficiently broken to provide the forewarning which led to the US victory at the Battle of Midway.

The British and Americans (and the Dutch before the fall of the Dutch E Indies) cooperated on attacks against JN-25 in the period leading up to 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 via more secure 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 JN-25.

Note that the Purple cypher used by the Japanese Foreign Office as its most secure system had no connection with any version of JN-25, or indeed any of the encryption systems used by the Japanese military before or during the War.