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 in 1942 (by breaking code JN-25b) and the shooting down of Japanese admiral Isoroku Yamamoto a year later in Operation Vengeance.

The

Red code

The Red Book code was an IJN

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

Knowledge of the Red Book code helped crack the similarly constructed Blue Book code.[1]

Coral

A cipher machine developed for Japanese naval attaché ciphers, similar to JADE. It was not used extensively,^[5][6] but Vice Admiral Katsuo Abe, a Japanese representative to the Axis Tripartite Military Commission, passed considerable information about German deployments in CORAL, intelligence "essential for Allied military decision making in the European Theater."^[7]

Jade

A cipher machine used by the Imperial Japanese Navy from late 1942 to 1944 and similar to CORAL; see

Dockyard codes

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.^[8]

JN-11

The Fleet Auxiliary System, derived from the JN-40 merchant-shipping code. Important for information on troop convoys and orders of battle.

JN-20

An inter-island cipher that provided valuable intelligence, especially when periodic changes to JN-25 temporarily blacked out U.S. decryption. JN-20 exploitation produced the "AF is short of water" message that established the main target of the Japanese Fleet, leading to a decisive U.S. victory at the Battle of Midway in 1942.^[9]: p.155

JN-25

JN-25 is the name given by codebreakers to the main, and most secure, command and control communications scheme used by the IJN during World War II.^[10] Named as the 25th Japanese Navy system identified, it was initially given the designation AN-1 as a "research project" rather than a "current decryption" job. The project required reconstructing the meaning of thirty thousand code groups and piecing together thirty thousand random additives.^[11]

Introduced from 1 June 1939 to replace Blue (and the most recent descendant of the Red code),^[12] it was an enciphered code, producing five-numeral groups for transmission. New code books and super-enciphering books were introduced from time to time, each new version requiring a more or less fresh cryptanalytic attack. John Tiltman with some help from Alan Turing (at GCSB) had "solved" JN25 by 1941, i.e. they knew that it was a five-digit code with a codebook to translate words into five digits and there was a second "additive" book that the sender used to add to the original numbers "But knowing all this didn’t help them read a single message".

By April 1942 JN25 was about 20 percent readable, so codebreakers could read "about one in five words" and traffic analysis was far more useful. ^[13] Tiltman had devised a (slow; neither easy nor quick) method of breaking it and had noted that all the numbers in the codebook were divisible by three.^[14] "Breaking" rather than "solving" a code involves learning enough code words and indicators so that any given message can be read. ^[15]

In particular, JN-25 was significantly changed on 1 December 1940 (JN25a);^[12] and again on 4 December 1941 (JN25b),^[16] just before the attack on Pearl Harbor.

British, Australian, Dutch and American cryptanalysts co-operated on breaking JN-25 well before the Pearl Harbor attack, but because the Japanese Navy was not engaged in significant battle operations before then, there was little traffic available to use as raw material. Before then, IJN discussions and orders could generally travel by routes more secure than 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,^[17] and that primarily in stripping away its super-encipherment. 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,

Later versions of JN-25 were introduced: JN-25c from 28 May 1942, deferred from 1 April then 1 May; providing details of the attacks on Midway and Port Moresby. JN-25d was introduced from 1 April 1943, and while the additive had been changed, large portions had been recovered two weeks later, which provided details of Yamamoto's plans that were used in Operation Vengeance, the shooting-down of his plane.^[21]

JN-39

This was a naval code used by merchant ships (commonly known as the "

The "minor operations code" often contained useful information on minor troop movements.[26]

JN-152

A simple transposition and substitution cipher used for broadcasting navigation warnings. In 1942 after breaking JN-40 the FECB at Kilindini broke JN-152 and the previously impenetrable JN-167, another merchant shipping cypher.^[27][28]

JN-167

A merchant-shipping cipher (see JN-152).

Chicago Tribune incident

In June 1942 the