Japanese naval codes

The vulnerability of Japanese naval codes and ciphers was crucial to the conduct of World War II, and had an important influence on foreign relations between Japan and the west in the years leading up to the war as well. Every Japanese code was eventually broken, and the intelligence gathered made possible such operations as the victorious American ambush of the Japanese Navy at Midway (JN-25b) and the shooting down of Isoroku Yamamoto in Operation Vengeance.

The Imperial Japanese Navy (IJN) used many codes and ciphers. All of these cryptosystems were known differently by different organizations; the names listed below are those given by Western cryptanalytic operations.

This was a code book system used in World War I and after. It was so called because the American copies made of it were bound in red covers. It should not be confused with the RED cipher used by the diplomatic corps.

This code consisted of two books. The first contained the code itself; the second contained an additive cipher which was applied to the codes before transmission, with the starting point for the latter being embedded in the transmitted message. A copy of the code book was obtained in a "black bag" operation on the luggage of a Japanese naval attache in 1923; after three years of work Agnes Driscoll was able to break the additive portion of the code.

In 1937 the alphabetic typewriter was used to encipher naval attache traffic.The U.S. called this the Purple machine. Although the Japanese purchased the Enigma machine they chose to base their cipher machine on a different technology. Rather than use rotors the Japanese used a stepping switch.

A cipher machine developed for Japanese naval attache ciphers. It was not much used.

A succession of codes used to communicate between Japanese naval installations. These were comparatively easily broken by British codebreakers in Singapore and are believed to have been the source of early indications of imminent naval war preparations.

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