Japanese naval codes

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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.

World War II 1939–1945 global war

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

Battle of Midway World War II naval battle in the Pacific Ocean

The Battle of Midway was a decisive naval battle in the Pacific Theater of World War II that took place between 4 and 7 June 1942, only six months after Japan's attack on Pearl Harbor and one month after the Battle of the Coral Sea. The United States Navy under Admirals Chester Nimitz, Frank Jack Fletcher, and Raymond A. Spruance defeated an attacking fleet of the Imperial Japanese Navy under Admirals Isoroku Yamamoto, Chūichi Nagumo, and Nobutake Kondō near Midway Atoll, inflicting devastating damage on the Japanese fleet that proved irreparable. Military historian John Keegan called it "the most stunning and decisive blow in the history of naval warfare".

Isoroku Yamamoto Japanese Marshal Admiral

Isoroku Yamamoto was a Japanese Fleet Admiral of the Imperial Japanese Navy and the commander-in-chief of the Combined Fleet during World War II until his death.


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.

Imperial Japanese Navy Naval branch of the Empire of Japan

The Imperial Japanese Navy was the navy of the Empire of Japan from 1868 until 1945, when it was dissolved following Japan's surrender in World War II. The Japan Maritime Self-Defense Force (JMSDF) was formed after the dissolution of the IJN.

Red code

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. [1] [2] [3]

In cryptography, black-bag cryptanalysis is a euphemism for the acquisition of cryptographic secrets via burglary, or other covert means – rather than mathematical or technical cryptanalytic attack. The term refers to the black bag of equipment that a burglar would carry or a black bag operation.


A cipher machine developed for Japanese naval attaché ciphers. It was not used much. [4] [5]


A cipher machine used by the Imperial Japanese Navy from late 1942 to 1944 and similar to CORAL; see JADE (cypher machine).

JADE (cypher machine) cypher machine

JADE was the codename given by US codebreakers to a Japanese World War II cipher machine. The Imperial Japanese Navy used the machine for communications from late 1942 until 1944. JADE was similar to another cipher machine, CORAL, with the main difference that JADE was used to encipher messages in katakana using an alphabet of 50 symbols.

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. [6]


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


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 Feet, leading to a decisive U.S. victory at the Battle of Midway in 1942. [7] :p.155


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. 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. [8]

Introduced from 1 June 1939 to replace Blue (and the most recent descendant of the Red code), [9] 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. In particular, JN-25 was significantly changed on 1 December 1940, [10] and again on 4 December 1941, [11] just before the attack on Pearl Harbor. The 1941 edition (JN-25b) was sufficiently broken by late May 1942 to provide the critical forewarning of the Japanese attack on Midway.

British, Australian, Dutch and American workers were cooperating in attacks on 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 [12] , 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, OP-20-G; at Pearl Harbor it was centered at the Navy's Combat Intelligence Unit (Station HYPO, also known as COM 14), [13] led by Commander Joseph Rochefort. With the assistance of Station CAST (also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) [14] in the Philippines, and the British Far East Combined Bureau in Singapore, and using a punched card tabulating machine manufactured by International Business Machines, a successful attack was mounted against the December 1941 code edition. Together they made considerable progress by early 1942. "Cribs" exploited common formalities in Japanese messages, such as "I have the honor to inform your excellency" (see known plaintext attack).


This was a naval code used by merchant ships (commonly known as the "maru code"), [15] broken in May 1940. 28 May 1941, when the whale factory ship Nisshin Maru No. 2 (1937) visited San Francisco, U.S. Customs Service Agent George Muller and Commander R. P. McCullough of the U.S. Navy's 12th Naval District (responsible for the area) boarded her and seized her codebooks, without informing Office of Naval Intelligence (ONI). Copies were made, clumsily, and the originals returned. [16] The Japanese quickly realized JN-39 was compromised, and replaced it with JN-40. [17]


JN-39 was replaced by JN-40, which was originally believed to be a code super-enciphered with a numerical additive in the same way as JN-25. However, in September 1942, an error by the Japanese gave clues to John MacInnes and Brian Townend, codebreakers at the British FECB, Kilindini. It was a fractionating transposition cipher based on a substitution table of 100 groups of two figures each followed by a columnar transposition. By November 1942, they were able to read all previous traffic and break each message as they received it. Enemy shipping, including troop convoys, was thus trackable, exposing it to Allied attack. Over the next two weeks they broke two more systems, the "previously impenetrable" JN167 and JN152. [18] [17]


The "minor operations code" often contained useful information on minor troop movements. [19]


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. [20]


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

Chicago Tribune incident

In June 1942 the Chicago Tribune , run by isolationist Col. Robert R. McCormick, published an article implying that the United States had broken the Japanese codes, saying the U.S. Navy knew in advance about the Japanese attack on Midway Island, and published dispositions of the Japanese invasion fleet. The executive officer of Lexington, Commander Morton T. Seligman, had shown Nimitz's executive order to reporter Stanley Johnston.

The government at first wanted to prosecute the Tribune under the Espionage Act of 1917. For various reasons, including the desire not to bring more attention to the article, the charges were dropped. A grand jury investigation did not result in prosecution but generated further publicity and, according to Walter Winchell, "tossed security out of the window". Britain's worst fears about American security were realized. [21]

In early August, a RAN intercept unit in Melbourne heard Japanese messages, using a superseded lower-grade code. Changes were made to codebooks and the call-sign system, starting with the new JN-25 codebook (issued two months before). However the changes indicated the Japanese believed the Allies had worked out the fleet details from traffic analysis or had obtained a codebook and additive tables, being reluctant to believe that anyone could have broken their codes (least of all a Westerner). [22]

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Fish (cryptography) Allied codename for any of several German teleprinter stream ciphers used during World War II

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Laurance Safford American cryptographer

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Joseph Rochefort American cryptographer

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Agnes Meyer Driscoll American cryptographer

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Pearl Harbor advance-knowledge conspiracy theory

The Pearl Harbor advance-knowledge conspiracy theory is the argument that U.S. Government officials had advance knowledge of Japan's December 7, 1941, attack on Pearl Harbor. Ever since the Japanese attack, there has been debate as to how and why the United States had been caught off guard, and how much and when American officials knew of Japanese plans for an attack. In September 1944, John T. Flynn, a co-founder of the non-interventionist America First Committee, launched a Pearl Harbor counter-narrative when he published a forty-six page booklet entitled The Truth about Pearl Harbor.

OP-20-G or "Office of Chief Of Naval Operations (OPNAV), 20th Division of the Office of Naval Communications, G Section / Communications Security", was the U.S. Navy's signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications from Japanese, German, and Italian navies. In addition OP-20-G also copied diplomatic messages of many foreign governments. The majority of the sections effort was directed towards Japan and included breaking the early Japanese "Blue" book fleet code. This was made possible by intercept and High Frequency Direction Finder (HFDF) sites in the Pacific, Atlantic, and continental U.S., as well as a Japanese telegraphic code school for radio operators in Washington, D.C.

Station HYPO, also known as Fleet Radio Unit Pacific was the United States Navy signals monitoring and cryptographic intelligence unit in Hawaii during World War II. It was one of two major Allied signals intelligence units, called Fleet Radio Units in the Pacific theaters, along with FRUMEL in Melbourne, Australia. The station took its initial name from the phonetic code at the time for "H" for Heʻeia, Hawaii radio tower. The precise importance and role of HYPO in penetrating the Japanese naval codes has been the subject of considerable controversy, reflecting internal tensions amongst US Navy cryptographic stations.

Station CAST was the United States Navy signals monitoring and cryptographic intelligence fleet radio unit at Cavite Navy Yard in the Philippines, until Cavite was captured by the Japanese forces in 1942, during World War II. It was an important part of the Allied intelligence effort, addressing Japanese communications as the War expanded from China into the rest of the Pacific theaters. As Japanese advances in the Philippines threatened CAST, its staff and services were progressively transferred to Corregidor in Manila Bay, and eventually to a newly formed US-Australian station, FRUMEL in Melbourne, Australia.

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Type A Cipher Machine cipher machine

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Far East Combined Bureau

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The Short Weather Cipher, also known as the weather short signal book, was a cipher, presented as a codebook, that was used by the radio telegraphists aboard U-boats of the German Navy (Kriegsmarine) during World War II. It was used to condense weather reports into a short 7-letter message, which was enciphered by using the naval Enigma and transmitted by radiomen to intercept stations on shore, where it was deciphered by Enigma and the 7-letter weather report was reconstructed.

Japanese army and diplomatic codes. This article is on Japanese army and diplomatic ciphers and codes used up to and during World War II, to supplement the article on Japanese naval codes. The diplomatic codes were significant militarily, particularly those from diplomats in Germany.


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  2. "Red Code" . Retrieved 2009-05-01.
  3. Budiansky 2000.
  4. http://www.mkheritage.co.uk/bpt/japcdsch2.html
  5. Smith, p.125
  6. Smith, p.93
  7. Mundy, Liza (2017). Code Girls. Hachette Books. ISBN   978-0-316-35253-6.
  8. Budiansky, Stephen (2000). Battle of Wits: The complete story of Codebreaking in World War II. New York: Free Press. pp. 7–12. ISBN   0-684-85932-7.
  9. Wilford, Timothy. "Decoding Pearl Harbor: USN Cryptanalysis and the Challenge of JN-25B in 1941", in The Northern Mariner XII, No.1 (January 2002), p.18.
  10. Wilford, p.18.
  11. Wilford, p.20: citing Kahn, The Codebreakers.
  12. Twomey, Steve (2016). Countdown to Pearl Harbor: The Twelve Days to the Attack. New York, NY 10020: Simon & Schuster. p. 152. ISBN   978-1-4767-7646-0.
  13. Wilford, p.19.
  14. Wilford, pp.19 and 29.
  15. Blair, Silent Victory, passim
  16. Farago, Ladislas. The Broken Seal (New York: Bantam, 1968), pp.393-395.
  17. 1 2 "Obituary: Brian Townend". London: The Times. March 2, 2005. Retrieved 1 May 2009.
  18. Smith 2000, p. 150.
  19. Smith 2000, pp. 191.
  20. Smith (2000) page 153 & (2001) pp140-143
  21. Gabriel Schoenfeld (March 2006). "Has the "New York Times" Violated the Espionage Act?". Commentary Magazine. Archived from the original on 2012-01-18. Retrieved 2017-10-04.
  22. Smith 2000, pp. 142,143.