Japanese naval codes

During World War II, the Imperial Japanese Navy (IJN) used many codes and ciphers, some more effective than others. Perhaps the best known was JN-25, whose partial break by the Americans made possible the victorious ambush at Midway. All of these cryptosystems were known differently by different organizations; the names listed below are those given by Western cryptanalytic operations.

JN-11

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

JN-25

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). 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 immediately before the Pearl Harbor attack on 7 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 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), commanded by Commander Joseph 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 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 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, and was generally referred to as "Magic".

JN-40

JN-25 was replaced by JN-40, 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 and break each message as they received it. Enemy shipping was thus trackable and Allied submarines, especially, successfully attacked it.

JN-152

A simple transposition and substitution cipher used for broadcasting navigation warnings.

JN-167

A merchant-shipping cipher.

Magic: The Untold Story

In his book "Magic: The Untold Story" (2000, Athena Press) David Lowman argued that the primary reason for the Japanese-American relocations and internments was to keep Japan from discovering that US intelligence had partly broken these codes.

References

*cite web
last =
first =
authorlink =
coauthors =
title = Bletchley Park In Mombasa
work =
publisher = Coastweek Newspapers Ltd.
date =
url = http://www.coastweek.com/codes.htm
format =
doi =
accessdaymonth = 19 October | accessyear=2006

*cite web
last =
first =
authorlink =
coauthors = Michael J. O'Neal
title = World War II, United States Breaking of Japanese Naval Codes
work =
publisher =
date =
url = http://www.espionageinfo.com/Vo-Z/World-War-II-United-States-Breaking-of-Japanese-Naval-Codes.html
format =
doi =
accessdaymonth = 19 October | accessyear=2006

External links

* [http://www.mkheritage.co.uk/bpt/JapCDSCH1.html Bletchley Park - Japanese Codes]

* [http://www.shoestringprod.org/MacShowers.mp4 Video interview with one of Rochefort's JN-25 codebreaking team]


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Japanese American internment — refers to the forcible relocation and internment of approximately 110,000 Japanese nationals and Japanese Americans to housing facilities called War Relocation Camps , in the wake of Imperial Japan s attack on Pearl Harbor. [… …   Wikipedia

  • Japanese cryptology from the 1500s to Meiji — The cipher system that Uesugi used is basically a simple substitution usually known as a Polybius square or “checkerboard.” The i ro ha alphabet contains forty eight letters, so a seven by seven square is used, with one of the cells left blank.… …   Wikipedia

  • naval warfare — Military operations conducted on, under, or over the sea and waged against other seagoing vessels or targets on land or in the air. The earliest naval attacks were raids by the armed men of a tribe or town using fishing boats or merchant ships.… …   Universalium

  • Japanese battleship Musashi — Musashi leaving Brunei in October 1944 for the Battle of Leyte Gulf Career …   Wikipedia

  • Japanese nationalism — (国家主義, Kokka shugi?) encompasses a broad range of ideas and sentiments harbored by the Japanese people over the last two centuries regarding their native country, its cultural nature, political form and historical destiny. It is useful to… …   Wikipedia

  • Naval flag signalling — covers various forms of flag signalling, such as semaphore or flaghoist, used by various navies; distinguished from maritime flag signalling by merchant or other non naval vessels or flags used for identification. Contents 1 History 2 NATO flags… …   Wikipedia

  • Naval officer ranks — Navies have military rank systems that often are quite different from those of armies or air forces. Sometimes, services that are considered parts of the navy – marine or amphibious corps – use the army style ranks instead, while the ranks listed …   Wikipedia

  • Japanese submarine I-1 — The Japanese submarine I 1 was a J1 type submarine built by Kawasaki, Kobe, for the Imperial Japanese Navy. She was a large cruiser submarine displacing 2,135 tons and was the lead of four boats built in the class. She was commissioned on 10 Mar… …   Wikipedia

  • Japanese school uniform — Japan introduced school uniforms in the late 19th century. Today, school uniforms are almost universal in the public and private school systems. They are also used in some women s colleges. The Japanese word for uniform is… …   Wikipedia

  • Japanese submarine I-401 — The Sen Toku class I 401 was once the largest submarine in the world. It was commanded by Lieutenant Commander Nambu. Capable of carrying three two seat Aichi M6A1 Seiran (Mountain Haze) float torpedo bombers, the Sen Toku s were built to launch… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.