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war and social upheaval: World War II -- Japasnese codes
Japanese World War II Codes
Figure 1.--This is Ford Island at Pearl Harbor, Hawaii, as seen from an attacking Japanese plane during the assault on the U.S. Pacific Fleet thriusting America into World War II (December 7, 1941). Battleship Row is in the center of the photograph and was the foicus of the Japanese attack, but notice the oil strorange tanks which were left untouched. If tghe Japonese had hit them, the Pcifuic Fleet would have had to wiuthdraw to the West Coast. Pear Harbor is seen as the greatest American intelligence failure in history. At the time, American cryptologists had cracked the Jaoanese Diplomtic Coide (Purple), but not the all imprtant naval code (JN-25). Ameucan military commnders believed that the Japanese were preoaring to attck, but when and where were unknown. The consensus in Washington and at Pearl was that an attack on Peal was was unlikely if not impossible.
Both the Americans and the British broke Japanese codes after World War I. The Japanese had no idea that their codes had been broken. The secret was never uncovered by the Japanese counter-intelligence services. Rather the Americans announced what they had done. The unemployed American cytologist, H.O. Yardley, published a book describing his achievements. [Yardley] As a result, the Japanese changed their entire code and cipher system, closing off access to the Americans for several years. The Japanese developed a cipher machine similar to the German Enigma Machine which the Foreign Ministry used. The American code breakers referred to as Red. which the Foreign Ministry used. Subsequently they introduced a more advanced machine which the Americans called called Purple (1937). The Japanese Foreign Ministry considered it unbreakable and used it for their most sensitive communications. As World War II approached, American and British cryptologists began working together on Japanese codes. This was a very difficult task because so few Americans spoke Japanese. And the Japanese complicated task since they employed a jumble of kanji, hiragana, and romanized Japanese words. American cryptologists broke the Japanese diplomatic codes. They broke into Red before World War II. The cracked Purple after the War began in Europe, but before the Japanese launched the Pacific War. The resulting intelligence program became known as Magic. This did not prevent the devastating Pearl Harbor attack because the American naval cytologists had not yet broke into the naval code--JN-25. Subsequently American code breakers finally cracked the coveted Japanese JN-25. This played a key role element in the American naval victory at Midway (June 1942). The Imperial Navy was in the process of changing the codes, but their successful offensive in the South Pacific had created many new posts and isolated facilities that the effort needed to distribute the updated codes took considerable time. Given the Yardley revelations, one would have thought that the Japanese would have been very careful with their codes. Even after the American carriers unexpectedly turned up in the Coral Sea and at Midway, Japanese naval authorities concluded that their codes were still secure. Breaking the Japanese codes also allowed American pilots to shoot down Admiral Yamamoto--the architect of the attack on Pearl Harbor. American cryptologists continued to gain valuable information during the New Guinea campaign (1943-44) and the critical Marianas campaign (1944). Ironically, Magic proved to be the best source of information outside of Ultra on German operations. Japan and Germany were Axis allies. The Japanese Ambassador to Germany, Osima, was ardently pro-NAZI and wanted Japan to join in Barbarossa. He had very cordial relations with high NAZI officials. He thus obtained and relayed very accurate and detailed messages on to Tokyo about the conduct of the War in Europe. These messages as a result of Magic were thus available to American officials. Through Magic intercepts, the United States learned of Operation Barbarossa and were the source of the warning President Roosevelt gave Stalin (without divulging the source) just before the surprise NAZI attack.
Post-World War I
Both the Americans and the British broke Japanese codes after World War I. The effort was a reaction to Japanese behavior at the end of the War, especially policies toward China. The United States began to increasingly see Japan as a possible naval adversary. Negotiations with Japan at the Washington Naval Talks to limit naval forces proved very difficult. The Japanese had no idea that their codes had been broken. The secret was never uncovered by the Japanese counter-intelligence services. Rather the Americans announced what they had done. The now unemployed American cytologist, H.O. Yardley, published a book describing his achievements. [Yardley] As a result, the Japanese changed their entire code and cipher system, closing off access to the Americans for several years.
World War II
At the time of World War II all kinds of different codes were in use involving varying levels of difficulty. This has led to concision in the public mind and has fed into the Pearl Harbor conspiracy nonsense. The United States at the time of Pearl Harbor (December 1941) had been reading the Japanese diplomatic codes for about a year. American code breakers had not, however, cracked the naval code (JN-25). Thus there was no definitive advance warming of the Pearl Harbor attack. There was a general consensus in Washington that the Japanese were preparing to attack, but where and when were unknown. Pearl Harbor was seen as unlikely. And such details were not available in the diplomatic traffic. There were also entirely unrelated Army codes. The Imperial Army and Navy were barely on speaking terms. The Army codes after Pearl Harbor were less of a priority than the naval codes. Vital in the Pacific War were the maru codes. Given the dimensions of the trackless Pacific, braking the maru codes were vital if the U.S. Navy was to cut off Japanese war industries from the vital Southern Resource Zone they had seized. The Imperial Navy did not convoy marus, seeing this as essentially beneath the dignity of a warrior. And once the War began the Japanese were less and less able to provide escorts. Rather the Japanese marus relied on the vast distance of the Pacific to make the marus hard to find. .
Diplomatic Codes
The Japanese developed a cipher machine similar to the German Enigma Machine which the Foreign Ministry used. The American code breakers referred to as Red. which the Foreign Ministry used. Subsequently the Foreign Ministry introduced a more advanced machine which the Americans called called Purple (1937). The Japanese Foreign Ministry considered it unbreakable and used it for their most sensitive communications. As World War II approached, American and British cryptologists began working together on Japanese codes. This was a very difficult task because so few Americans spoke Japanese. And the Japanese complicated task since they employed a jumble of kanji, hiragana, and romanized Japanese words. American cryptologists broke the Japanese diplomatic codes. They broke into Red before World War II. The cracked Purple after the War began in Europe, but before the Japanese launched the Pacific War. The resulting intelligence program to read Purple became known as Magic, although some times the entire American cryptographic effort is sometimes called Magic.
Ironically, Magic proved to be the best source of information outside of Ultra on German operations. Japan and Germany were Axis allies. The Japanese Ambassador to Germany, Osima, was ardently pro-NAZI and wanted Japan to join in Barbarossa. He had very cordial relations with high NAZI officials. He even met occasionally with Hitler himself. He thus obtained and relayed very accurate and detailed messages on to Tokyo about the conduct of the War in Europe. These messages as a result of Magic were thus available to American officials. Through Magic intercepts, the United States learned of Operation Barbarossa and were the source of the warning President Roosevelt gave Stalin (without divulging the source) just before the surprise NAZI attack.
Naval codes
Cracking Purple did not prevent the devastating Pearl Harbor attack because the American naval cryptologists had not yet broke into the naval code--JN-25. Subsequently American code breakers finally cracked the coveted Japanese JN-25. This played a key role element in the American naval victory at Midway (June 1942). The Imperial Navy was in the process of changing the codes, but their successful offensive in the South pacific had created many new posts and isolated facilities that the effort needed to distribute the updated codes took considerable time. Given the Yardley revelations, one would have thought that the Japanese would have been very careful with their codes. Even after the American carriers unexpectedly turned up in the Coral Sea and at Midway. The Pacific is a vast ocean. The chances that American carriers (the Pacific Fleet had precious few) would turn up at just the right time and place to blunt important operations defies logic. The fact that the American carriers were mauled in the Coral sea perhaps explained why more questions were not asked, but it boggles the mind that Japanese naval authorities would not have realized their codes had been broken. A Chicago newspaper even broke the story at the time. Japanese naval authorities, however, concluded that their codes were still secure. It is difficult to see how they could have reached this conclusion. We can only conclude that the very strong conviction of superiority and under estimating American capabilities can explain the conclusion. The Imperial Navy changed the codes after Midway, but this was a preciously scheduled change. And they did not change the basic system. Thus American cryptologists were quickly back into the system. Breaking JN-25 allowed American pilots to shoot down Admiral Yamamoto--the architect of the attack on Pearl Harbor. American cryptologists continued to gain valuable information during the New Guinea campaign (1943-44). The key Hollandia operation was made possible by the information obtained. MacArthur was provided with detailed information on Japanese strength and troop disposition. Important information was obtained on the critical Marianas campaign (1944).
Maru codes
Although not as dramatic, cracking the Maru campaign was with the exception of the critical Midway operation, was probably the most important achievement of Allied cryptologists, but it was not done by the Americans. Resource poor Japan primarily went to war to obtain the critical resources of the Southern Resource Zone, especially the vital oil they needed. Those resources were, however, useless unless they could be transported back to the factories on the Home Islands. Japan began the War with a merchant or maru fleet barely adequate for its needs, especially after the conquest of vast new areas. The United States had an important submarine fleet based at Pearl Harbor. Early efforts were impaired by a range of factors, especially faulty torpedoes. Even after the torpedoes were corrected, the submarine force faced a daunting task. Only a small number of submarines were available for the vast Pacific. Patrolling with just luck to encounter marus would have yielded meager results. The Japanese solved this problem for the American submarines and the subs operating from Australia. Messages were broadcast providing detailed information on routes and position. American code breakers had cracked the maru code (JN-39) before the War (May 1940). This achievement, however, was compromised when a year later U.S. Customs Service Agent George Muller and Commander R. P. McCullough of the U.S. Navy's 12th Naval District (responsible for San Francisco) boarded the whale factory ship Nisshin Maru No. 2 which docked in San Francisco and seized her code books) (May 28, 1941). This was an action taken without informing the Office of Naval Intelligence (ONI). Copies were made somewhat clumsily, and the originals returned to the ship. [Farago, pp.393–95.] The Japanese quickly understood that JN-39 was compromised, and replaced it with JN-40. Fortunately for the Americans, the Japanese would be less aware about JN-25. American code breakers at first believed JN-40 to be a code super-enciphered with a numerical additive, similar to JN-25 which they were hard at work on. And they made less progress on JN-40. The priority of course was on JN-25. But after Pearl Harbor, enormous resources began to be given to code breaking. As was often the case at Bletchly Park, an error by the code operators led the Allied code breakers into JN-40 (September 1942). John MacInnes and Brian Townend, code breakers at the British Far East Combined Bureau (FECB) Kilindini in Mombasa made the break through. They determined that JN-40 was a fractionating transposition cipher based on a substitution table of 100 groups of two figures each followed by a columnar transposition. This was a far kless sophisticted systen=m than JN-25. As a result, the British code breakers were not only able to read all previous traffic, but more importantly break each message as they received it (November 1942). This meant that Japanese cargo shipping, including troop convoys, could be located and tracked. This exposed these vessels to Allied attack, including waters deep into Japanese controlled areas. They also cracked JN-167 and JN-152 which had been bedeviling American and British code breakers. [Smith, p. 150.] The result was the destruction of Japan's all important maru fleet primarily by the small American submarine force. A measure of the effectiveness of the submarine campaign was that the Japanese had to use much of their steel to build marus (especially tankers) rather than naval ships and most importantly oil deliveries slowed to a trickle. The Japanese had to base important elements of the Imperial fleet at Singapore close to the oil fields rather than the Home Islands. .
Army codes
We know less about Army Codes. The primary American concern during the Pacific War was with the naval and maru codes.
After the Japanese invasion of India at Impal failed. many of the Japanese units were destroyed as many failed to retreat or were so emaciated that they were unable to do so. British soldiers found a key army code book. This apparently enabled British and American cryptologists to read Japanese Army messages.
Historians both during and after the War did not know just why American carriers showed up at just the right time and place in the Coral Sea or at Midway. Neither did the Japanese. The answer of course was Magic. This was the code name for the top-secret code breaking operation which alerted the Americans to the Japanese plans. The same American code breaking operation that had learned of the Port Moresby operation, also warned Admiral Nimitz that the next target was Midway. American code breakers broke into the Japanese Foreign Office's top secret system for sending messages (September 1940). The American cryptologists named it Purple. The information gained from Purple decryption came to be called Magic within the U.S. government because the Foreign Office used it for only their most important messages. The location of the Magic operation in Washington meant that information from the decrypts were not sent to Pearl Harbor unless the War Department decided to send some of the intelligence obtained. The Purple machine was a successor to earlier machines used to read Japanese diplomatic messages. The Navy at Pearl Harbor had its own code breaking operation working on the Japanese naval code--JN-25. Station HYPO at Pearl finally cracked JN-25, leading to the Battle of the Coral Sea and Midway. Midway was the most decisive naval battle of the War. After Pearl Harbor, American code breakers in cooperation with the British began to focus on German communications.
Sources
Farago, Ladislas. The Broken Seal (New York: Bantam, 1968).
Smith, Michael. The Emperor's Codes: Bletchley Park and the Breaking of Japan's Secret Ciphers. London: Bantam Press, 2000).
