JN-25 is the name given by codebreakers to the chief, and most secure, command and control communications scheme used by the Imperial Japanese Navy (IJN) during and slightly before World War II (it was the 25th Japanese Navy system identified). Introduced in 1939 to replace Blue, it was an enciphered code, producing five numeral groups in 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 superenciphering books were also introduced, sometimes simultaneously. In particular, JN-25 was significantly changed on 1 December 1940, and again on 4 December 1941, just before the attack on Pearl Harbor. 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 U.S. victory at the Battle of Midway.
The British, Australians, Dutch and Americans cooperated on attacks against JN-25 beginning well before the Pearl Harbor attack. The Japanese Navy was not engaged in significant battle operations until late 1941, so there was little traffic available with which to work. Before then, IJN discussions and orders could generally travel by more secure routes than encrypted 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 superencipherment. 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.
The American effort was directed from Washington, D.C. by the U.S. Navy's signals intelligence command, called OP-20-G. It was centered at Pearl Harbor at the U.S. Navy's Combat Intelligence Unit (Station HYPO, also known as COM 14), commanded by Commander Joseph Rochefort. With the assistance of Station CAST (also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) in the Philippines, and the British Far East Combined Bureau in Singapore, and using an IBM punched card tabulating machine (when available), a successful attack was mounted against the JN-25 edition which came into effect on 1 December 1941. Together they made considerable progress by early 1942. Cribs were used by exploiting common formalities in Japanese messages, such as "I have the honor to inform your excellency" and the use of formal, stylized titles (see known plaintext attack).
The Purple cipher (also sometimes referred to as AN-1), 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, which 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 IJN operational ones, from which strategic or tactical information could sometimes be inferred. Nevertheless, decrypted Purple traffic was very valuable, especially later in the war,
Read more about this topic: Japanese Naval Codes