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Much more important than breaking Purple (the Japanese diplomatic codes) was the Japanese Navy's code which American cryptologists called JN-25. (JN for Japanese Navy and 25 for the 25th code which they worked on.) The Purple diplomatic code was of great value. JN-25 was the mother lode. The Imperial Navy JN-25 proved much more difficult. Much of the final American work on JN-25B was conducted by the Navy at Pearl Harbor (Station HYPO) working with the British FECB . It was an enciphered code, producing five numeral groups which was what was actually broadcasted. It proved more difficult to break than Purple. Work on JN-25 was set back when the Japanese switched from JN-25A to JN-25B (December 1940). 【Boyd】 It was not until months after Pearl Harbor that the cryptologists in the Philippines (Station CAST) and the British FECB in Singapore began breaking into JN-25. They reported huge progress (January 1942). 【Boyd】 Both had to relocated. FCB to Colombo, Ceylon and Cast short after to the DEI and finally Melbourne, Australia. There was also an Australian team in Melbourne. This is when Station HYPO begins to play a vital role. While JN-25 it was only partially cracked. JN-25 decrypts played a major role in the Battle of the Coral Sea (May 1942). For the most part intercepts could be only partially read, but enough to make some sense of Japanese intentions. American and British and American code-breakers intercepted Japanese Navy signals which revealed a planned operation toward Australia (March 1942). Code-breakers at Station Hypo led Commander Joe Rochefort produced a decrypt that revealed a naval force from Rabaul would target Australian controlled areas of New Guinea (April 3). The precise target was not revealed, but Commander Rochefort assessed the targets as Port Moresby and the Solomon Islands. Another decrypt revealed the presence in Truck of a carrier strike force to be used for Operation MO along with the RZP invasion force at Rabaul (April 9). This confirmed the target. British and American code-breakers had previously learned that 'RZP' was the Japanese code for Port Moresby. British code-breakers in Ceylon intercepted and decrypted the final piece of the MO puzzle (Mid-April). The British informed Nimitz that two of Admiral Nagumo's first-line carriers had been temporarily detached from the First Air Fleet--Shokaku and Zuikaku. The Allies now knew that the Japanese naval offensive would be the Coral Sea to the east of Australia's Cape York. The American carriers in the Coral Sea sank a small carrier, but Lexington was lost. Even so Shokaku and Zuikaku as a result of the were not used for the Midway operation-MI. (Actually Zuikaku with Shokaku'sair squadron could have been used, but Yamamoto thought that it was not needed. Admiral Nagumo would have only four carriers. And Station Hypo produced decrypts which gave Admiral Nimitz the information he needed for the Battle of Midway (June 1942). Despite the inexplicable appearance of the American carriers, the Japanese remained supremely confident in the security of their codes. Even a German report failed to convince the Japanese. Important American newspapers virtually announced that the Japanese codes were broken. Even so, the United States continued to read Japanese naval messages throughout the War. The ability to read JN-25 also proved useful on other battlefields, including New Guinea and the Marianas. It was the ability to read JN-25 that led to Yamamoto shoot down (April 1943). After the Guadalcanal disaster, Yamamoto organized an inspection tour to meet with front-line units and improve their morale. The Yamamoto's headquarters sent a message to a unit on Bougainvillea doing battle with the Americans. The message contained the precise details of the Admiral's itinerary, including the times and locations, especially the arrival at Bougainvillea. And Bougainvillea was within range of American fighters on Guadalcanal. The coded message also included the number and types of planes that would transport the Admiral. This was important because it enabled the Americans to launch the necessary force as well as to identify Yamamoto's air squadron from other Japanese aircraft.
Much more important than breaking Purple (the Japanese diplomatic codes) was the Japanese Navy's code which American cryptologists called JN-25. (JN for Japanese Navy and 25 for the 25th code which they worked on.) The Purple diplomatic code was of great value. JN-25 was the mother lode. Avctual IJN traffic, including battlepolans.
The Imperial Navy JN-25 proved much more difficult. Much of the final American work on JN-25B was conducted by the Navy at Pearl Harbor (Station HYPO) working with the British FECB . It was an enciphered code, producing five numeral groups which was what was actually broadcasted. It proved more difficult to break than Purple. Work on JN-25 was set back when the Japanese switched from JN-25A to JN-25B (December 1940). 【Boyd】
The Japanese like the Germans with Enigma believed tht JN-25 could not be cracked. The orincipal trason was the complexirty of thesystem and the incomprehendibe number permutations-exasctly why the Germans nekieved that Enigma could not be cracked. But the Japanese had a nother reason for their confidence in JN-25. In fact even after theWar when told that the Americans hd crasked JN-35, many naval officrs would not blieve it. The reason for this was the complexity of the Japaenese language.presented a huge barrier in itself. The Hapan reason that there were not a 100 American fluent bin he Japnese language. (For ethnic reasons they discounted Japanese Americans.) And not one of those 100 Americans had the mathemtical skills to even attempt to crack JN-25. Most of the Jaoabnese officers invlved in security doubted that the Americans would even bother to try.
Cracking JN-25 was an internayional effort. But the British. Australins and Dutch were also involved. The British were the major foreign country, but for good reason Bletchly Park's main focus was on the Germans. The Americans could afford to concentrate primrily on the Japanese. The BRitish Far East Combinned Bureau (FECB) was working on JN-25. It was established in HO=ong JKong (1935). It chieved the firs brkthrough on JN-25 (1939). As a resukt John Tiltman with help from Alan Turing worked out the basic structure of JN-25a. They deduced that it was a five-digit code with a codebook to translate words into five digits and a second 'additive' book to ncomplicte the message. As well as the divisibility by three of all valid code groups. The Japanese introduced JN-25b (December 1, 1940). Bletchely immiditly discovered a dertaithat might have delayedcodebrakjers fior onths. The Japanese had for some rason changed the code book, but not the additiv tables. THis mean that messages sen in December could be used to trip way rghe enccryotion layer. FECB hd anafed to recover some 3000 code groups (May 1940). The Duch were also interested in JN-25. And unlike their homeland, the Dutch East Indies was still in the led Allied camp. Kamer 14 at Bandong Technicl College in Java was overseen by Lt. Cmdr Leo Brouwer. They began intercepting Japanese naval commuicatins (1936). Annd had develope a valuable knowledge of Japanese radio procedures. Brower anf his team were evacuated befor Jav fell andthey reacvhed Australia with boxes uith punch card continung partially decided messages. Hewould eventually winf up at Blechly (Hut 7).
OP-20-G was short hand for Office of Chief Of Naval Operations (OPNAV), 20th Division ( the Office of Naval Communications) G Section (Communications Security). This was the U.S. Navy's signals intelligence and cryptanalysis group. Its assugned mission during World War II was to intercept, decrypt, and analyze naval communications from adversary countries (Japanese, German, and Italian navies. OP-20-G also worked on diplomatic codes. The principal effort was directed at Japanese codes. They broke the Japanese "Blue" book fleet code. This was done through 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. OP-20 G also copperated with the U.S. Army in cracking "pyole" diplmtic codes. with the U.S. Army. efore the war, the Navy cipher bureau operated out of three main bases:
1) Station NEGAT at headquarters in Washington, D.C. 2) Station HYPO (or FRUPAC), at Pearl Harbor in Hawaii. 3) Station CAST, at Cavite Navy Yard in the Philippines, but evcuated to the fortified tunnels of Corregidor. They were supported by a network of listening and radio direction finding stations. Navy CAST signals intelligence personnel in Correigidor were evacuated by subnmasrine. Evacuated American Army Signals Corps intelligence personnel joined Central Bureau. OP-20-G was comanded by Cpt. John R. Redman. His brother Joseph Redman was the Director of Naval Intelligence. The Redman brothers who had the support of Adm. King wanted HYPO and CAST to report to OP-20-G and they would provide intlligence directives to field commanders. This caused a provlem when Cmdr. Joseph Rochefort developed prsonal relstionship with Adm Ninmitz. The Redman brothers used pure mathamatical analysis. Rceefrtif cou course usedmatha mathamatical analysis, but added his understanding of the Japanerse language and cultutr.
Station Cast atvthe Cavite Naval Yard moved to Coregidor when the Japanese invaed the Phiippines (December 1941). They made an mprtant contribution. CAST detrmined that AF was the Jaoanese designation for Midway. The relayed this to Station HYPO (March 1942). CAST personnel were so valuable that they were some of the few Americans evauated by submarine (February-April 1942). Not only were they invaluable, but if the Japanese got their hands on a single member they would have learned about the work on JN-25. CAST personnel were reformed as RUMEL in Melbourne
The Japanese submarine I-124 was based on a minelayer design. The Japanese used a GeranU-boat received as Worl dWar I reparations. It supported the invasion of the Philippines (Decembner 1941), but during anti-shipping operations off Darwin, Australia was sunk (January 1942). It was the first Japanese sunmrine sunk by the Royal Australian Navy. The sub settled in relatively shallow waters. U.S. Navy duvers arrempted to recover code books, but it is unclear if they had any success. We have noted differing reports.
It was not until months after Pearl Harbor that the cryptologists in the Philippines (Station CAST) and the British FECB reloocated to Singapore began breaking into JN-25. They reported huge progress (January 1942). 【Boyd】 Both had to relocated becsause of the ightening speed of Japanese sadvances. FECB was relcated to Colombo, Ceylon. CAST shortly was moved to the DEI and finally Melbourne, Australia. There was also an Australian team in Melbourne. This is when Station HYPO begins to play a vital role. While JN-25 it was only partially cracked. JN-25 decrypts played a major role in the Battle of the Coral Sea (May 1942). For the most part intercepts could be only partially read, but enough to make some sense of Japanese intentions. Estimates vary, but theAllies by May 1942 were intercepting sone 60 percent of Jpanese radio naval commuivations and decrupting some waht abut 40 percent of tose messages. The Japanese had no idea tht tghis was happning.
American and British and American code-breakers intercepted Japanese Navy signals which revealed a planned operation toward Australia (March 1942). Code-breakers at Station Hypo led Commander Joe Rochefort produced a decrypt that revealed a naval force from Rabaul would target Australian controlled areas of New Guinea (April 3). The precise target was not revealed, but Commander Rochefort assessed the targets as Port Moresby and the Solomon Islands. Another decrypt revealed the presence in Truck of a carrier strike force to be used for Operation MO along with the RZP invasion force at Rabaul (April 9). This confirmed the target. British and American code-breakers had previously learned that 'RZP' was the Japanese code for Port Moresby. British code-breakers in Ceylon intercepted and decrypted the final piece of the MO puzzle (Mid-April). The British informed Nimitz that two of Admiral Nagumo's first-line carriers had been temporarily detached from the First Air Fleet--Shokaku and Zuikaku. The Allies now knew that the Japanese naval offensive would be the Coral Sea to the east of Australia's Cape York. The American carriers in the Coral Sea sank a small carrier, but Lexington was lost.
After Coral Sea, it soon became evident that the Jaoanese ewere preparing a major offensive attacking identified oly ass AF. The question was where and when. OP-20-G issued an assessment that AF could be the Aleutin Islands, Perl,or the Pacific West Coast (May 14). Such an assessment of widely sepsatre targets seems to be more a beaurcratic CYA asessment than a legitimate decrypotion of Japanese intercepts. HYPO told Adm. Nimitz at Pearl that AF was Midway. This led to a rare confronation between King and Nimitz. The Redman brihers concince king that Roichefort was a rebnegade officer acting largeky on emoton and seeing what he wanted in ambiguous intercepts. . Onlywhen a ruse over the Midway water supply suggest the HYPO was correct did King relent still concerned about HYPO violaing the chain of commnd. This allowed Ninitz to commit his three carriers to the defense of Midway rather than the Pacific coast. Nimitz was putting his caeer on the line. Nimitz is today lionized, and for good reason, but in May 1942 he was yet to win his laurels.
Shokaku and Zuikaku as a result of the were not used for the Midway operation. (Actually Zuikaku with Shokaku'sair squadron could have been used, but Yamamoto was so confident at this stage of the War, he thought that it would not be needed. Admiral Nagumo would have only four carriers. Station Hypo produced decrypts which gave Admiral Nimitz the information he needed for the Battle of Midway (June 1942). Never before in naval historty was a commander so fully briefed on the disposition and intent of his adversary before an important sea battle. .
Despite the inexplicable appearance of the American carriers, the Japanese remained supremely confident in the security of their codes. Even a German report failed to convince the Japanese. Important American newspapers virtually announced that the Japanese codes were broken. The Japanese confidence in JN-25 is bdifficult to understyand. The Pacific is a huge area. The Japanese even convinced themselves tha an American carrier battle group showed up by accident in the Coral Sea to oppose their invasion of Port Moresby. The chances that this would have occured by accident are astronomical. But that was just what the Japanese convinced themselves had occued (May 1942). Now they might have been forgiven once. But what is hard to understand, just a month later, the Japanese convinced themelves that lighting struck again at Midway (June 1942). The chances that the Americans carriers would show up at just the right time and place once werevastronomica, but that it woud occur twice were virtunatly infinite. No one looking at this today can understand how the Japanese did not immediately understand that JN-25 had been cracked. So powerful was their confidece in JN-25 that even comunications staff could get into trouble for queationing it. An American decrypt alerted Allied forces of a convoy carrying reinforcements to Rabul (January 1943). American destroyers intercepted the convoy and sank the transports. Commander Kato Tessuo survived. He was a communications officer and reported, "The Americans were waiting at the exact poin where we changed course. They must have broken our codes." The response of his commanders was to dismiss his report and conclude he suffered from battke fatigue. He was not only reassigned to shore duties, but ordered not to discuss his 'delusions' further. Intrcepting Yamamoto's flight was another dead give away (April 1943), but only a perfunctory assessment followed. Interestingly, Vice Admiral Matome Ugaki, Chief-of-Staff of the Combined Fleet under Admiral Yamamoto, was traveling with Yammoto, but in a separate plane when intrcepted by American P-38 fighters. He was also shot down, but survived. He wrote in his famous diary, "The enemy seems to have a sixth sense about our operations. Every movementis countered, every plan anticipated." Ugaki of course was correct, but he never knew how right he was. Both Yamaoto and Ugaki remained supremely confident in JN-25. The Japanese continued to believe in the immunity of JN-25 throught the War. The Americans continued to use JN-25 dycrypts to monitor Japanee compliance after the surrender.
The United States continued to read Japanese naval messages throughout the War. The ability to read JN-25 also proved useful on other battlefields, including New Guinea and the Marianas.
It was the ability to read JN-25 that led to Yamamoto shoot down (April 1943). After the Guadalcanal disaster, Yamamoto organized an inspection tour to meet with front-line units and improve their morale. The Yamamoto's headquarters sent a message to a unit on Bougainvillea doing battle with the Americans. The message contained the precise details of the Admiral's itinerary, including the times and locations, especially the arrival at Bougainvillea. And Bougainvillea was within range of American fighters on Guadalcanal. The coded message also included the number and types of planes that would transport the Admiral. This was important because it enabled the Americans to launch the necessary force as well as to identify Yamamoto's air squadron from other Japanese aircraft.
Loureiro, Pedro. "The imperial Japanese Navy and espionage: The Itaru Tachibana case," International Journal of Intelligence and CounterIntelligence. Vol. 3 Mo. 1 (1989). pp. 105-21.
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