Japanese naval codes

Last updated

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.

Contents

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.

Red code

The Red Book code was an IJN code book system used in World War I and after. It was called "Red Book" because the American photographs made of it were bound in red covers. [1] 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. [2] [3] [4]

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 JADE (cypher machine).

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, OP-20-G; at Pearl Harbor it was centered at the Navy's Combat Intelligence Unit (Station HYPO, also known as COM 14), [18] led by Commander Joseph Rochefort. [10] However, in 1942 not every cryptogram was decoded, as Japanese traffic was too heavy for the undermanned Combat Intelligence Unit. [19] With the assistance of Station CAST (also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) [20] 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 4 December 1941 edition (JN25b). 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).

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 "maru code"), [22] 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. [23] The Japanese quickly realized JN-39 was compromised, and replaced it with JN-40. [24]

JN-40

JN-40 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. [24] [25]

JN-147

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 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 (who was transferred to shore duties), 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 and because the Espionage Act did not cover enemy secrets, 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". Several in Britain believed that their worst fears about American security were realized. [29]

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). [30]

Related Research Articles

<span class="mw-page-title-main">Bletchley Park</span> WWII code-breaking site and British country house

Bletchley Park is an English country house and estate in Bletchley, Milton Keynes (Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following 1883 for the financier and politician Sir Herbert Leon in the Victorian Gothic, Tudor, and Dutch Baroque styles, on the site of older buildings of the same name.

<span class="mw-page-title-main">Cryptanalysis</span> Study of analyzing information systems in order to discover their hidden aspects

Cryptanalysis refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic security systems and gain access to the contents of encrypted messages, even if the cryptographic key is unknown.

<span class="mw-page-title-main">Enigma machine</span> German cipher machine

The Enigma machine is a cipher device developed and used in the early- to mid-20th century to protect commercial, diplomatic, and military communication. It was employed extensively by Nazi Germany during World War II, in all branches of the German military. The Enigma machine was considered so secure that it was used to encipher the most top-secret messages.

<span class="mw-page-title-main">Ultra</span> British designation for intelligence from decrypted enemy communications

Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School (GC&CS) at Bletchley Park. Ultra eventually became the standard designation among the western Allies for all such intelligence. The name arose because the intelligence obtained was considered more important than that designated by the highest British security classification then used and so was regarded as being Ultra Secret. Several other cryptonyms had been used for such intelligence.

<span class="mw-page-title-main">Fish (cryptography)</span>

Fish was the UK's GC&CS Bletchley Park codename for any of several German teleprinter stream ciphers used during World War II. Enciphered teleprinter traffic was used between German High Command and Army Group commanders in the field, so its intelligence value (Ultra) was of the highest strategic value to the Allies. This traffic normally passed over landlines, but as German forces extended their geographic reach beyond western Europe, they had to resort to wireless transmission.

Magic was an Allied cryptanalysis project during World War II. It involved the United States Army's Signals Intelligence Service (SIS) and the United States Navy's Communication Special Unit.

<span class="mw-page-title-main">Bombe</span> Codebreaking device created at Bletchley Park (United Kingdom)

The bombe was an electro-mechanical device used by British cryptologists to help decipher German Enigma-machine-encrypted secret messages during World War II. The US Navy and US Army later produced their own machines to the same functional specification, albeit engineered differently both from each other and from Polish and British bombes.

<span class="mw-page-title-main">Room 40</span> Cryptanalysis section of the British Admiralty during World War I

Room 40, also known as 40 O.B., was the cryptanalysis section of the British Admiralty during the First World War.

<span class="mw-page-title-main">Code (cryptography)</span> Method used to encrypt a message

In cryptology, a code is a method used to encrypt a message that operates at the level of meaning; that is, words or phrases are converted into something else. A code might transform "change" into "CVGDK" or "cocktail lounge". The U.S. National Security Agency defined a code as "A substitution cryptosystem in which the plaintext elements are primarily words, phrases, or sentences, and the code equivalents typically consist of letters or digits in otherwise meaningless combinations of identical length." A codebook is needed to encrypt, and decrypt the phrases or words.

Cryptanalysis of the Enigma ciphering system enabled the western Allies in World War II to read substantial amounts of Morse-coded radio communications of the Axis powers that had been enciphered using Enigma machines. This yielded military intelligence which, along with that from other decrypted Axis radio and teleprinter transmissions, was given the codename Ultra.

Cryptography was used extensively during World War II because of the importance of radio communication and the ease of radio interception. The nations involved fielded a plethora of code and cipher systems, many of the latter using rotor machines. As a result, the theoretical and practical aspects of cryptanalysis, or codebreaking, were much advanced.

Various conspiracy theories allege 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 why and how 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 46-page booklet entitled The Truth about Pearl Harbor, arguing that Roosevelt and his inner circle had been plotting to provoke the Japanese into an attack on the U.S. and thus provide a reason to enter the war since January 1941.

<span class="mw-page-title-main">Hut 8</span>

Hut 8 was a section in the Government Code and Cypher School (GC&CS) at Bletchley Park tasked with solving German naval (Kriegsmarine) Enigma messages. The section was led initially by Alan Turing. He was succeeded in November 1942 by his deputy, Hugh Alexander. Patrick Mahon succeeded Alexander in September 1944.

Before the development of radar and other electronics techniques, signals intelligence (SIGINT) and communications intelligence (COMINT) were essentially synonymous. Sir Francis Walsingham ran a postal interception bureau with some cryptanalytic capability during the reign of Elizabeth I, but the technology was only slightly less advanced than men with shotguns, during World War I, who jammed pigeon post communications and intercepted the messages carried.

Hut 7 was a wartime section of the Government Code and Cypher School (GC&CS) at Bletchley Park tasked with the solution of Japanese naval codes such as JN4, JN11, JN40, and JN-25. The hut was headed by Hugh Foss who reported to Frank Birch, the head of Bletchley's Naval section.

The cipher system that the Uesugi are said to have used is 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. The rows and columns are labeled with a number or a letter. In the table below, the numbers start in the top left, as does the i-ro-ha alphabet. In practice these could start in any corner.

<span class="mw-page-title-main">Type A Cipher Machine</span> Japanese cipher machine

In the history of cryptography, 91-shiki ōbun injiki or Angōki Taipu-A, codenamed Red by the United States, was a diplomatic cryptographic machine used by the Japanese Foreign Office before and during World War II. A relatively simple device, it was quickly broken by western cryptographers. The Red cipher was succeeded by the Type B "Purple" machine which used some of the same principles. Parallel usage of the two systems assisted in the breaking of the Purple system.

<span class="mw-page-title-main">Far East Combined Bureau</span>

The Far East Combined Bureau, an outstation of the British Government Code and Cypher School, was set up in Hong Kong in March 1935, to monitor Japanese, and also Chinese and Russian (Soviet) intelligence and radio traffic. Later it moved to Singapore, Colombo (Ceylon), Kilindini (Kenya), then returned to Colombo.

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.

References

  1. 1 2 Greg Goebel. "US Codebreakers In The Shadow Of War". 2018.
  2. ↑ Sterling, Christopher H. (2007). Military Communications: From Ancient Times to the 21st Century. ABC-CLIO. pp. 126–127. ISBN   978-1-85109-732-6 . Retrieved 1 May 2009.
  3. ↑ "Red Code". Archived from the original on 8 May 2009. Retrieved 1 May 2009.
  4. ↑ Budiansky 2000, p. 5.
  5. ↑ "Bletchley Park Jewels Japanese Cryptographic Machines". mkheritage.co.uk.
  6. ↑ Smith 2000, p. 125.
  7. ↑ "Early Japanese Systems". NSA Center for Cryptologic History.
  8. ↑ Smith 2000, p. 93.
  9. ↑ Mundy, Liza (2017). Code Girls. Hachette Books. ISBN   978-0-316-35253-6.
  10. 1 2 Hanson, Victor Davis (18 December 2007). Carnage and Culture: Landmark Battles in the Rise to Western Power. Knopf Doubleday Publishing Group. ISBN   978-0-307-42518-8.
  11. ↑ Budiansky 2000, p.  7–12.
  12. 1 2 Wilford 2002, p. 18.
  13. ↑ Dufty 2017, p. 122,123.
  14. ↑ Dufty 2017, p. 14,64.
  15. ↑ Dufty 2017, p. 96.
  16. ↑ Wilford 2002, p. 20: citing Kahn, The Codebreakers.
  17. ↑ Twomey, Steve (2016). Countdown to Pearl Harbor: The Twelve Days to the Attack. New York: Simon & Schuster. p. 152. ISBN   978-1-4767-7646-0.
  18. ↑ Wilford 2002, p. 19.
  19. ↑ Kahn 1996, p. 566.
  20. ↑ Wilford 2002, pp. 19 & 29.
  21. ↑ Kahn 1996, pp. 99, 564, 567.
  22. ↑ Blair, Silent Victory, passim
  23. ↑ Farago, Ladislas. The Broken Seal (New York: Bantam, 1968), pp.393–395.
  24. 1 2 "Obituary: Brian Townend". The Times . London. 2 March 2005. Retrieved 1 May 2009.
  25. ↑ Smith 2000, p. 150.
  26. ↑ Smith 2000, pp. 191.
  27. ↑ Smith 2000, p. 153.
  28. ↑ Smith 2001, pp. 140–143.
  29. ↑ Gabriel Schoenfeld (March 2006). "Has the 'New York Times' Violated the Espionage Act?". Commentary Magazine. Archived from the original on 18 January 2012. Retrieved 4 October 2017.
  30. ↑ Smith 2000, pp. 142, 143.

Sources

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.