Francais | English | Espanõl

Japanese naval codes

From Wikipedia, the free encyclopedia

(Redirected from JN-25)

Jump to: navigation, search

The names given to the various codes were given by Western cryptography organizations.

1 JN-11
2 JN-25
3 JN-40
4 JN-152
5 JN-167
6 External links
7 References

[edit] JN-11

The Fleet Auxiliary System, derived from the JN-40 merchant-shipping code.

[edit] JN-25

JN-25 is the name given to 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 enciphered 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 superenciphering 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, Australians, Dutch and Americans cooperated on attacks against JN-25 beginning well before and after the Pearl Harbor attack. The Japanese Navy was not engaged in actual battle until late 1941, so 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.

The American effort was directed from Washington by the US Navy's signals intelligence command, called OP–20–G. in Pearl Harbor, it was centered at Pearl Harbor at the U.S. Navy's Combat Intelligence Unit, commanded by Commodore John Rochefort. With the assistance of Station CAST in the Philippines, and the British in Hong Kong and later Singapore, and IBM punch-card tabulating machines (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 by exploiting common formalities in Japanese messages, such as "I have the honor to inform your excellency" and the use of formal, stylized titles.

Note that the Purple cipher (also 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, 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. Nevertheless, decrypted Purple traffic was very valuable, especially later in the War, and was generally referred to as Magic.

[edit] JN-40

JN-25 was replaced by JN-4O, which was believed to be a code super-enciphered with a numerical additive in the same way as JN-25. In September 1942, an error by the Japanese gave clues to the code-breakers at Kilindini and, by November 1942, they were able to read all previous traffic break each message as they received it. Enemy shipping was thus able to be tracked and attacked by Allied submarines.