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Japanese naval codes
Japanese naval codes
Japanese naval codes
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The vulnerability of Japanese naval codes and ciphers was crucial to the conduct of the Pacific War 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 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

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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 attaché 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

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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

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Main article: JADE (cipher machine)

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

Dockyard codes

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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

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

JN-20

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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

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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, Government Communications Security Bureau) 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

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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

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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

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

JN-152

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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

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A merchant-shipping cipher (see JN-594).

Chicago Tribune incident

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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 (FRUMEL) heard Japanese messages, using a superseded lower-grade code. Changes were made to codebooks and the call-sign system, starting with the new JN-25c 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]

References

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Japanese naval codes

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Japanese naval codes encompassed the cryptographic systems developed and employed by the (IJN) for secure operational and administrative communications, primarily during the and . These systems evolved from earlier book ciphers like the (pre-1930) and (1931–1938) codes to more complex additive-based ciphers such as JN-25, introduced in 1939, which utilized a of approximately 27,500–30,000 five-digit groups superenciphered with a 300-page additive book of random numerals. The vulnerability of these codes to Allied , particularly by U.S. Navy teams using tabulating machines and , provided critical intelligence that shaped outcomes, including the in 1942. The IJN's cryptographic efforts began in the early , with the Red Book serving as a foundational operational code until its replacement by the Blue Book in , which U.S. cryptanalysts fully solved by 1933 to monitor Japanese fleet exercises. By 1938, the Black Code had superseded the Blue, but it too proved insecure, leading to the deployment of JN-25 in June 1939 as the primary fleet general-purpose system, handling thousands of messages monthly by late 1941. Variants like JN-25B (from December 1940) incorporated frequent additive book changes—such as in August and December 1941—to enhance security, yet these measures were undermined by stereotyped message formats (e.g., reports) and limited manpower in Japanese cryptographic sections. Beyond JN-25, the IJN relied on specialized systems including the Coral for naval traffic (1939–1945) and over a dozen minor codes for administrative, logistical, and merchant liaison purposes, generating around 40,000 messages annually by 1942. Allied decryption successes, bolstered by U.S.-British collaboration beginning in spring 1941, allowed partial reads of 10–15% of JN-25 traffic by early 1942, revealing operational plans despite challenges like codebook changes and resource constraints in units. These breakthroughs, however, came too late to prevent the attack in December 1941, as a major JN-25 revision on December 1, 1940, delayed real-time intelligence until mid-1942.

Early Codebooks

Red Code

The Red Code, also known as the Red Book, was the Imperial Japanese Navy's primary codebook system for secure communications, introduced circa 1918 during and employed as the main operational code through the 1920s until its replacement in late 1930. This manual system relied on a to translate phrases, words, and concepts into numerical groups, primarily using katakana syllables to represent Japanese terms, which were then obfuscated through an additive process to prevent straightforward decryption. It served as the foundational tool for naval operations, training, and supply coordination, reflecting Japan's early 20th-century emphasis on manual cryptosystems amid rising tensions in the Pacific. The structure of the Red Code centered on a comprehensive containing approximately 100,000 entries, each assigning a four- or five-digit numerical code to common naval terminology, locations, and commands expressed in . To enhance security, these code groups were superenciphered using a separate additive book, where numerical values—derived from sequential or random tables—were added to the code entries 10,000, creating variability in each message without altering the underlying meaning. Messages often included an indicator specifying the starting point in the additive sequence, allowing recipients to reverse the process, though this relied on shared codebooks and daily key changes to maintain secrecy. This additive method, while simple compared to later mechanical systems, provided sufficient obfuscation for interwar naval traffic, though it was vulnerable to systematic analysis of repeated patterns. In 1923, U.S. Navy conducted a covert " to photograph a copy of the Red Code from the luggage of a Japanese naval at a U.S. , providing American cryptanalysts with direct access to the system's core. This acquisition, kept highly classified, marked a pivotal coup during the , enabling targeted exploitation amid growing U.S. concerns over Japanese naval expansion. Cryptanalytic efforts began shortly after the theft, with U.S. Navy cryptologist Agnes Driscoll leading the breakthrough in the late 1920s by reconstructing the base from intercepted traffic and the stolen photographs, leveraging and known-plaintext attacks on routine naval messages. By 1928, Driscoll's team had fully recovered the additive tables through exhaustive reconstruction of daily variations, allowing routine decryption of Red Code messages for several years. These advances were achieved manually, without mechanical aids, and built on to identify message structures and common additives. The successful of the Red Code enabled the U.S. to monitor Japanese naval deployments, exercises, and strategic intentions throughout the interwar era, informing American policy and preparedness in the Pacific. This intelligence foundation proved instrumental in developing techniques for breaking successor systems, such as the Blue Code introduced in , which shared structural similarities but incorporated refinements to counter earlier vulnerabilities.

Blue Code

The Blue Code, also known as the Blue Book, was a Japanese naval system introduced in February 1931 to replace the earlier Red Book, which had been abandoned in late 1930. It served as the primary enciphered code for general naval operations, facilitating secure communications amid Japan's expanding military activities in during . The system remained in use until October 1938, when it was phased out in favor of more advanced codes like the Black Code. Structurally, the Blue Code built on the Red Book's framework but incorporated enhancements for greater security, including an expanded of approximately 85,000 groups, each represented by four symbols with the third serving as a control or . Messages were enciphered using additives combined with columnar transposition, featuring daily-changing grids, permutations, nulls, and blanks to obscure patterns, along with a separate reciphering step before transmission. This manual system relied on randomization improvements over its predecessor to resist , though it retained similarities in vocabulary that later aided cryptanalytic recovery. In the historical context of , the Blue Code supported Japan's naval maneuvers and territorial ambitions, particularly in resource-rich areas like the , with traffic volume increasing significantly due to expanded operations in the and broader region. It enabled coordination of fleet exercises and deployments, reflecting the Imperial Japanese Navy's growing assertiveness ahead of . Cryptanalysis of the Blue Code was achieved independently by multiple Allied efforts. The U.S. Navy's unit, led by figures such as Captain Laurance Safford, Lieutenant Thomas Dyer, and Agnes Driscoll, solved it in 1933 through painstaking and code recovery without the aid of or captured documents, employing tabulating machines to process the abundant traffic. Concurrently, Dutch cryptanalysts at the Bandoeng (Batavia) station, under Johannes Nuboer, broke the code starting February 20, 1935, using permutation analysis on repeated code groups, transposition reconstruction, and systematic codebook rebuilding, with full recovery by summer 1935. These breakthroughs leveraged prior knowledge of the Red Code's structure to accelerate recovery. The decryption of the Blue Code yielded significant pre-war intelligence, including details on Japanese fleet dispositions, battleship speeds exceeding 26 knots, and exercise patterns from 1934–1935, which directly influenced U.S. naval planning for faster warships. For the Dutch, it provided tactical insights into Japanese movements, enhancing defensive preparations in the . Overall, these successes established cryptanalytic techniques, such as additive stripping and group frequency matching, that informed approaches to subsequent Japanese codes like the JN series.

Cipher Machines

Coral

The Coral cipher machine was an electromechanical encryption device introduced by the Imperial Japanese Navy in the late 1930s as part of its 1937 series of cryptographic systems, designed specifically for secure communications by naval attachés stationed abroad. It saw limited deployment, primarily to protect the exchange of sensitive diplomatic and military intelligence with Axis partners, contrasting with the later Jade, an upgraded machine also used primarily by naval attachés. The machine operated as a stepping-switch system, employing telephone selector switches with 25 positions each to substitute characters, functioning analogously to the wired rotors in early Enigma variants but without removable wiring. It included a plugboard for additional settings and enciphered —typically Romanized Japanese or numerical data—into five-digit numerical groups for transmission, with the switches advancing automatically after each character. This design evolved from earlier Japanese reverse-engineering of European technology, such as Damm printers, to create a fully for faster, more secure encoding. Deployed to Japanese naval attaché stations in Europe and the United States, Coral facilitated high-level exchanges, including reports from figures like Vice Admiral Katsuo Abe on Axis military matters during council meetings in Berlin. Its use persisted from 1939 until at least 1945, though production was restricted to a small number of units due to its specialized role in diplomatic channels rather than frontline naval command. Allied cryptanalysts, including British experts from the Government Code and Cypher School collaborating with U.S. Navy teams, broke the Coral system in March 1944 using intercepted traffic and reconstructed machine settings led by codebreakers Hugh Foss and Hugh Alexander. This effort, building on earlier work at , exploited the machine's limited traffic volume and predictable patterns in attaché messages. Although its scope was narrow compared to major operational codes, the decryption of Coral messages provided the Allies with valuable insights into Axis coordination, including Japanese relays of German deployment details that informed European theater strategies. recovery of Coral machines and documentation further aided cryptographic studies, highlighting vulnerabilities in stepping-switch designs.

Jade

The Jade cipher machine, designated as such by Allied codebreakers and known to the Japanese as the Type 97 cipher machine, was introduced by the in late 1942 as a successor to the Coral machine for enhanced secure communications. It was primarily employed by Japanese naval attachés stationed in neutral and Axis countries to transmit reports on Axis naval activities, strategic assessments, and updates from the European theater back to , operating until 1944 amid intensifying Pacific campaigns. A unit was captured by U.S. forces in Saipan in June 1944, aiding further analysis. Structurally, Jade was an upgraded rotor-like cipher machine utilizing telephone stepping switches as wired rotors, each with 25 contacts, to encipher messages in the 50-symbol Katakana syllabary, outputting them as 5-digit numeral groups for transmission. This design represented an improvement over Coral by adapting the system specifically for Katakana encoding, incorporating additional switch complexity and a reflector mechanism to increase cryptographic depth and resistance to frequency analysis. Allied cryptanalysts, leveraging prior breaks of the Coral predecessor, exploited Jade through known-plaintext attacks on intercepted attaché traffic, achieving initial recoveries at U.S. Navy stations like in , with contributions from British efforts at . Key figures such as Mary H. "Polly" Budenbach, Arthur Levenson, and Francis Raven computerized analytic techniques to decrypt messages, focusing on captured depth variants and repetitive phrases in diplomatic reports. The intelligence derived from Jade provided the Allies with marginal yet valuable insights into Japanese assessments of Allied naval movements and broader war perceptions, supplementing higher-priority breaks like JN-25, though its volume was limited by the specialized attaché role. Usage declined and ceased by late 1944 as Japanese overseas networks collapsed amid mounting defeats in the Pacific.

Specialized Codes

Dockyard Codes

The Dockyard Codes constituted a series of low-security communication systems utilized by the for administrative and logistical exchanges between shipyards, naval bases, and installations. Introduced in 1939, these codes facilitated routine operations such as coordinating ship repairs, managing supply requisitions, and signaling base activities, remaining in service throughout . Unlike more complex operational ciphers, they prioritized simplicity to support rapid, non-sensitive transmissions in dockyard environments. The codes saw heightened traffic volumes in 1941, with messages indicating the recall of warships to ports for refitting—a clear harbinger of impending hostilities. This surge in activity provided early indicators of Japan's mobilization efforts. British codebreakers at the Far East Combined Bureau (FECB) in achieved penetration of the Dockyard Codes by 1939, employing fundamental frequency analysis to identify and recover recurring code groups from intercepted traffic. These efforts were aided by the codes' repetitive phrasing in administrative contexts. The decryption of the Dockyard Codes yielded early on Japanese naval preparations, alerting Allied forces to aggressive intentions and highlighting vulnerabilities in logistics, such as repair schedules and supply chains. However, their limited strategic impact stemmed from the non-operational focus of the messages, offering tactical insights only in conjunction with higher-grade intercepts like the related JN-39 merchant shipping code.

JN-39

The JN-39, also known as the "maru code," was a system employed by the for merchant marine communications, facilitating the coordination of commercial shipping traffic including tracking and cargo reporting. Introduced in alongside the JN-25 operational code, it supported the Navy's oversight of maritime during Japan's pre-war expansion in the Pacific. The structure of JN-39 centered on a augmented by daily additives to obscure messages, with content tailored to naval-merchant interactions such as vessel positions, routes, and supply details. This system emphasized terminology for shipping operations, distinguishing it from more general fleet codes. In the historical context of Japan's early Pacific ambitions, JN-39 traffic was monitored by U.S. stations on the West Coast, providing insights into merchant activities around ports like amid rising tensions. The code's leveraged to identify patterns and recover elements. Complete recovery followed in 1941 after U.S. Customs agents seized a set of JN-39 codebooks from the Japanese whale factory ship Nisshin Maru No. 2 during its stop in on May 28, 1941. The compromise of JN-39 enabled U.S. intelligence to track Japanese merchant movements and assess economic logistics in the lead-up to , yielding critical data on convoy dispositions and resource flows. It was subsequently replaced by the JN-40, a direct successor retaining a comparable and additive framework.

JN-40

JN-40 was introduced in early as the successor to the compromised JN-39, serving as the primary cryptographic system for Japanese merchant shipping during the campaigns. This code facilitated communications for convoy formations, cargo routing, and logistical support essential to sustaining operations across the vast theater. As the predecessor JN-39 had been broken by U.S. cryptanalysts, its replacement with JN-40 aimed to secure vital maritime traffic amid escalating Allied pressure. The structure of JN-40 consisted of a superenciphered using subtractive additives, followed by transposition for additional security. The system incorporated specific indicators for routing, such as designations for formation types and navigational waypoints, enabling efficient coordination of vessels often referred to as the "Maru fleet." This design reflected wartime enhancements over JN-39, prioritizing volume and speed for high-traffic supply missions. In historical context, JN-40 played a pivotal role in Japan's efforts to maintain supply lines to isolated island garrisons, particularly after the in June 1942, when message volume surged to support reinforcements and resupply amid mounting losses. The code's heavy usage underscored its importance to the Japanese war economy, as merchant shipping carried critical resources like oil, rice, and munitions across contested waters. Cryptanalysis of JN-40 was achieved by the British Far East Combined Bureau (FECB) at Kilindini in 1942, building on prior knowledge of JN-39's structure and exploiting repeated messages with extraneous data that revealed patterns. U.S. Navy stations contributed to recovering the subtractive additive tables through traffic analysis and depth recovery techniques. The decryption of JN-40 had profound operational impact, enabling Allied forces to intercept and target Japanese supply convoys with precision. This intelligence was instrumental in U.S. submarine warfare, which sank over 5 million tons of Japanese merchant shipping by war's end, severely disrupting logistics and isolating forward bases. By providing real-time details on convoy schedules and routes, JN-40 breaks shifted the balance in the Pacific, contributing to Japan's strategic attrition.

JN-152

JN-152 was a specialized employed by the for issuing navigation warnings and meteorological reports to its fleets, enabling safe operations amid potential hazards like mines or adverse weather. Introduced before the entered in late 1941, it saw extensive use through 1943, particularly in the Pacific theater where timely alerts were critical for maintaining operational secrecy. The system's structure was designed for efficiency in high-volume radio traffic, featuring a compact augmented by fixed additives to obscure patterns. Messages were typically enciphered into concise groups, allowing for rapid transmission and reducing the risk of interception errors over long distances. This simplicity prioritized speed over complexity, making it suitable for routine alerts rather than high-level commands. In historical context, JN-152 supported Japanese efforts to mask patrol activities from Allied forces, with messages often coordinating submarine wolf packs and reconnaissance aircraft in contested waters. By broadcasting warnings about enemy sightings or environmental threats, it helped preserve the element of surprise in early war operations, such as those preceding major engagements in 1942. Cryptanalytic success against JN-152 came from the British Far East Combined Bureau (FECB) at Kilindini in 1942, who exploited the code's repetitive phrasing in standard weather reports—such as consistent descriptors for storm fronts or visibility—to deduce key mappings. Through methodical traffic analysis and partial recoveries, the team achieved full decryption by mid-1942, transforming what was intended as a low-profile system into a vulnerability. The breakthrough had significant operational impact, furnishing Allied commanders with detailed maps of Japanese patrol zones and anticipated routes, which informed strategies and protections. Moreover, insights from JN-152's structure and usage patterns indirectly accelerated the of the primary operational JN-25 by revealing additive reuse tendencies. This was broken alongside the related JN-167 merchant shipping .

JN-167

JN-167 was introduced prior to late 1942 by the as a secure replacement for earlier merchant shipping systems, specifically designed to protect high-priority supply runs amid escalating Allied in the Pacific. This code facilitated communications for convoys carrying essential resources from the Japanese home islands to distant outposts, such as those in the and , where supply lines were increasingly vulnerable to interception. The structure of JN-167 featured an advanced augmented by variable additives and indicators to enhance security against cryptanalytic attacks. Messages were encoded in a 5-digit format, providing greater complexity than prior systems and allowing for concise transmission of routing, , and scheduling details. In historical context, JN-167's deployment coincided with intensified U.S. operations, which by 1943 had begun systematically targeting Japanese vessels, sinking over two-thirds of the fleet between 1942 and 1944 and severely straining Japan's logistics. Traffic under this code primarily supported the defense of isolated garrisons, but poor discipline and delayed adoption of protective measures like limited its effectiveness. Cryptanalysis of JN-167 was achieved through Anglo-American efforts led by the Far East Combined Bureau at Kilindini in 1942, after on JN-40, using cross-traffic analysis to identify patterns in communications; full recovery of additives followed, enabling routine decryption alongside the JN-152 auxiliary code. The decryption of JN-167 proved pivotal, providing Allies with actionable on routes and timings that facilitated the targeting of Japan's remaining fleet, ultimately contributing to the logistical collapse and starvation of isolated garrisons by mid-1944.

Main Operational Codes

JN-25

JN-25 was the primary operational code of the , introduced on June 1, 1939, as the main fleet command system for transmitting orders, dispositions, and strategic communications, replacing the earlier and codes. It served as the core cipher for the Combined Fleet's high-level traffic throughout , enabling coordination of major naval operations across the Pacific. The code's structure consisted of a containing approximately 30,000 five-digit groups, each representing words, phrases, or concepts in kana-based Japanese expressed in Romaji for transmission. These groups were superenciphered using daily numerical additives drawn from a 300-page of random five-digit numbers (30,000 total), with an indicator specifying the page and line to select the additive for subtraction from the code group. Variants evolved over time, including JN-25A (introduced in 1939 with the initial codebook), JN-25B (deployed December 1, 1940, featuring larger books and new key generation, which was partially broken before the ), and subsequent JN-25C and JN-25D (introduced in 1942 and 1943, respectively, with further book changes to enhance security). JN-25 played a pivotal role in key Japanese operations, including planning for the attack in 1941, the in 1942, and —the 1943 mission targeting Admiral . Its traffic, primarily from headquarters, provided insights into fleet movements and intentions, though initial Allied unreadability limited its exploitation at . U.S. of JN-25 began with partial successes by the Navy's unit in 1940, recovering additives and code groups from the JN-25A variant using tabulating machines and depth analysis of repeated messages. By May 1942, Station HYPO in , under Commander , achieved full additive recovery for JN-25B through high-volume , cross-referencing with decrypted diplomatic messages, and exploiting Japanese retention of old keys; this yielded 10-15% readability of messages pre-Midway, sufficient for identifying "AF" as the target outpost (). Readability improved dramatically to around 80% by 1943 as cryptanalysts recovered more code groups and adapted to variants. The breaks in JN-25 had profound impacts, enabling U.S. forces to the Japanese carrier fleet at Midway in June 1942, marking a turning point in the , and providing the precise itinerary that led to Yamamoto's downing on April 18, 1943. Overall, JN-25 decrypts contributed to Allied intelligence dominance, allowing preemptive strikes and strategic superiority throughout the conflict; the break of the auxiliary JN-20 code further aided recovery efforts by providing contextual traffic insights.

JN-20

The JN-20 was an auxiliary system employed by the from 1941 to 1943, primarily for short-range communications between islands, regional fleet messaging, coordination of supplies, and reports from outposts in the Central Pacific. It served as a tactical tool for localized operations, complementing the primary operational code JN-25 by handling lower-volume traffic that required rapid transmission. The structure of JN-20 featured a simplified overlaid with changing additives to obscure meanings; this design facilitated quick encoding and decoding for low-traffic scenarios, prioritizing speed over the complexity of main fleet systems. U.S. cryptanalysts at Station HYPO in achieved a break in early through recovery of code groups and additives. In historical context, JN-20 traffic from the Central Pacific proved vital during early 1942, providing valuable intelligence that contributed to preparations for the in June 1942, shifting the Pacific War's momentum. The rapid exploitation of JN-20 underscored the fragility of auxiliary codes, which, despite their limited scope, exposed operational vulnerabilities when patterns in routine communications were exploited.

JN-11

The JN-11 code, also known as the Fleet Auxiliary System, was introduced by the Imperial Japanese Navy in mid-1942 as a derivative of the JN-40 merchant shipping code framework. It served primarily for secure communications related to auxiliary vessels and convoy escorts, facilitating the coordination of logistical support in naval operations. This system addressed the need for encrypted messaging in non-combatant fleet elements, distinct from higher-level command codes. Structurally, JN-11 utilized an adapted with approximately 6,000 entries, incorporating shared additive tables from JN-40 to encipher messages, while emphasizing code groups for personnel rosters, equipment inventories, and supply manifests. These additives, typically numerical, were superimposed on four-numeral code groups to obscure , allowing for concise transmission of details and vessel statuses. The design prioritized efficiency in repetitive logistical reporting, with built-in checks to detect transmission errors. JN-11 saw extensive use during the from August 1942 onward and throughout the Allied island-hopping offensives in the and beyond, where it supported the monitoring and direction of troop reinforcements to contested positions. Japanese forces relied on it to schedule destroyer runs and merchant convoys delivering soldiers and under threat of , contributing to sustained efforts amid escalating attrition. Cryptanalytic efforts by U.S. Navy stations at (HYPO) and Australian facilities in achieved a breakthrough against JN-11 in late , capitalizing on prior recoveries of JN-40 materials to reconstruct additives and values. Traffic analysis proved essential, as recurring patterns in message volume and routing indicators revealed schedules before full decryption. This combined approach allowed rapid exploitation of intercepted signals. The intelligence derived from JN-11 decrypts enabled Allied commanders to anticipate Japanese reinforcement attempts, such as troop-laden convoys to , permitting preemptive ambushes and air strikes that inflicted heavy losses on auxiliary shipping. This foresight bolstered U.S. amphibious operations by enabling targeted disruptions of enemy , shifting the balance in the theater.

JN-147

JN-147 was a minor operational code introduced by the in 1943 for use by subordinate units, patrols, and in minor engagements, characterized by low-volume traffic. It supplemented higher-level systems like JN-25 for in localized scenarios. The code's structure featured a compact with approximately 3,000 entries, employing simple additives to encode messages, making it suitable for urgent, short transmissions rather than complex strategic directives. In historical context, JN-147 saw deployment during the defensive phases of the , including operations around the and the Marianas, where it facilitated tracking of small-scale unit movements and patrol activities amid dwindling resources. Allied cryptanalysts, through combined efforts involving U.S. and British under agreements like BRUSA, broke JN-147 in early using volume analysis of intercepted traffic, with recovery accelerated by its inherent simplicity. The code's decryption had a marginal strategic impact but proved valuable for Allied mopping-up operations against isolated Japanese forces, highlighting the desperation and fragmentation in late-war Japanese naval command structures.

Security Incidents

Theft of Red Code Materials

In the early , amid escalating naval rivalry between the and following the of 1922, the U.S. (ONI) conducted covert operations to gain insights into Japanese communications. These "black bag" jobs targeted Japanese diplomatic and naval assets in the to counterbalance Japan's growing fleet capabilities under the treaty's tonnage limits. A pivotal success occurred in when agents executed a against the luggage of a Japanese naval in Washington, D.C., successfully stealing and photographing pages of the Imperial Japanese Navy's Red Code book, also known as the Secret Operating Code from . The operation involved surreptitious access to the materials, allowing agents to document key sections without immediate detection by Japanese officials. This acquisition provided the U.S. with critical artifacts, including entries, which were smuggled out and developed into photographic copies for analysis. The photographed materials were promptly transferred to the Navy Department in Washington for cryptanalytic examination, though the original book was not retained by U.S. forces. These copies proved sufficient to initiate systematic decryption efforts, revealing patterns in the Code's structure and additives. Although the full volume was not captured, the partial recovery enabled breakthroughs in understanding Japanese naval and procedures. The theft's long-term impact was profound, directly contributing to the full solution of the Red Code by cryptanalyst Agnes Driscoll and her team in 1926 after three years of intensive work. This decryption yielded valuable on Japanese naval operations, training, and logistics, informing U.S. strategic planning in the . Moreover, the operation established a precedent for ONI's pre-World War II against Japanese codes, enhancing American communications capabilities amid rising tensions in the Pacific.

Chicago Tribune Incident

The Chicago Tribune incident refers to the publication of a article on June 7, 1942, that inadvertently revealed the ' success in breaking Japanese naval s during . Following the decisive American victory at the on June 4–7, 1942, reporter Stanley Johnston penned an article titled "Navy Had Word of Jap Plan to Strike at Sea," which detailed U.S. foreknowledge of Japanese fleet movements with such specificity that it strongly implied the decryption of the Imperial Japanese Navy's primary operational , JN-25. The piece, also syndicated to outlets like the , described how American forces had anticipated the Japanese carrier strike force's position and composition, attributing this to naval sources without explicitly naming codebreaking. Johnston obtained the sensitive information earlier in the war while aboard the USS Barnett, a transport ship returning from the in May 1942. Sharing a cabin with Morton T. Seligman, a staff officer to Admiral , Johnston gained access to a top-secret dispatch (Nimitz's message number 311221) that outlined decrypted Japanese plans. Seligman, possibly careless in handling classified material, discussed operational details with Johnston, who took notes on the JN-25-derived intelligence without realizing its full sensitivity at the time. This leak occurred amid strict wartime , but Johnston's article bypassed review due to its composition en route to publication. The article sparked immediate alarm within the U.S. government, as it risked alerting Japan to the compromise of JN-25 and prompting a code change that could blind Allied intelligence efforts. President ordered an FBI investigation, leading to a federal in on July 9, 1942, to consider Espionage Act charges against Johnston and Tribune managing editor J. Loy Maloney for allegedly aiding the enemy through disclosure of military secrets. , , intervened decisively by barring Navy cryptanalysts from testifying, citing concerns that a public trial would generate further publicity and force Japanese code alterations. Without expert evidence to substantiate the charges, the declined to indict on July 29, 1942, effectively ending the case. Despite the exposure, the Japanese military failed to recognize the code's vulnerability from the article, allowing continued U.S. decryption of JN-25 messages throughout the Pacific campaign. Seligman faced repercussions, including reassignment to shore duty and early retirement in 1944 without promotion, while the incident underscored the precarious balance between press freedom and wartime secrecy. Declassified documents from the grand jury proceedings, unsealed in 2017 after legal challenges, confirmed the government's fears but also revealed no malicious intent by the Tribune staff.

References

  1. https://www.nsa.gov/History/Cryptologic-History/Historical-Events/Article-View/Article/2740680/jn-25/
  2. https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/p/pearl-harbor-revisited-usn-communications-intelligence/part-2.html
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  7. https://www.britannica.com/biography/Agnes-Meyer-Driscoll
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  34. https://militaryhistorynow.com/2021/02/03/u-s-navy-codebreakers-americas-cryptanalysis-coup-at-the-battle-of-midway-was-20-years-in-the-making/
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  38. https://nsarchive.gwu.edu/briefing-book/intelligence/2017-10-25/secrecy-leaks-when-us-government-prosecuted-chicago-tribune
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