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Old November 23rd, 2012, 01:56 AM   #1
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Does anybody know why the Japanese didn't change secret codes that were broken by US?


During the pacific war, Japanese used naval codes that were easily broken by US intelligence, and this was a public secret among US media, which reported it. Despite that, Japanese didn't change their codes but remained confident in the security, which led to the defeat in the midway and other naval clashes.

do you know why? was Japan so incompetent with counter intelligence and scouting of us public information??
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Old November 23rd, 2012, 02:58 AM   #2
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No I don't know Why did they do that?
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Old November 23rd, 2012, 03:21 AM   #3

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The long and the short of it is that Japan believed that the JN-25 naval code was both unbroken and unbreakable. And to characterize breaking it as easy is a bit questionable. Had it been that easy Pearl Harbor would never have happened. The lead codebreaker for Hypo went on record as saying that they could decipher 15-20 percent of the actual text-the rest was guesswork.
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Old November 23rd, 2012, 03:41 AM   #4

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Didn't a great deal of intelligence about Japanese naval movements come from intercepting the communications of the Japanese ambassador in Berlin?
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Old November 23rd, 2012, 03:45 AM   #5
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Quote:
Originally Posted by demarche View Post
During the pacific war, Japanese used naval codes that were easily broken by US intelligence, and this was a public secret among US media, which reported it. Despite that, Japanese didn't change their codes but remained confident in the security, which led to the defeat in the midway and other naval clashes.

do you know why? was Japan so incompetent with counter intelligence and scouting of us public information??
They did change codebooks and enciphering tables throughout WWII.

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Originally Posted by Naomasa298 View Post
Didn't a great deal of intelligence about Japanese naval movements come from intercepting the communications of the Japanese ambassador in Berlin?
Why would details of military operations be available to the ambassador? Doesn't make much sense.
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Old November 23rd, 2012, 04:00 AM   #6

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Quote:
Originally Posted by Naomasa298 View Post
Didn't a great deal of intelligence about Japanese naval movements come from intercepting the communications of the Japanese ambassador in Berlin?
It did and the British and Americans shared a great deal.
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Old November 23rd, 2012, 04:06 AM   #7

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My readings leave me with the understanding that the Japanese did change their codes, though they had a tendency to leave each version in place too long. Yamamoto had decided the code needed changing in his planning of his last operation in the Solomons. It was decided that the operation would be delayed to a disadvantage if time was taken to distribute the books to all units. Keeping the existing code in place cost the Admiral his life.

The code changed after the death of Yamamoto was the version that was partially cracked that discovered the IJN's intentions towards Midway. But that version was changed, leaving the US with but an outline of the operation, and little else as to exact timing and accurate knowledge of actual force composition, leaving the US tactical response largely up to best guess.
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Old November 23rd, 2012, 07:19 AM   #8

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Originally Posted by Spartacuss View Post
My readings leave me with the understanding that the Japanese did change their codes, though they had a tendency to leave each version in place too long. Yamamoto had decided the code needed changing in his planning of his last operation in the Solomons. It was decided that the operation would be delayed to a disadvantage if time was taken to distribute the books to all units. Keeping the existing code in place cost the Admiral his life.

The code changed after the death of Yamamoto was the version that was partially cracked that discovered the IJN's intentions towards Midway. But that version was changed, leaving the US with but an outline of the operation, and little else as to exact timing and accurate knowledge of actual force composition, leaving the US tactical response largely up to best guess.
All true, but it bears noting that it was still an evolutionary change of the fundamental JN-25 code-not an entirely new code.
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Old November 23rd, 2012, 07:55 AM   #9

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I have posted about this before on another thread, Japanese military and diplomatic codes, including the JN-25 code, were broken in part or in whole by GC&CS ( Bletchley Park) on a regular basis through sections based in Hong Kong, Ceylon and other places. The British codebreakers started breaking Japanese codes in the 1920s and maintained their codebreaking sucesses right up to an through WW2.
What is VERY puzzling to me is that while Bletchley activities, files and reports regarding the breaking of German codes in WW2 have been transferred en mass to the publically available British Archive--no such transfer of documentation has taken place regarding work on Japanese codes with the exception of half a dozen files. In fact NO files were made available at all before 1990!
The Japanese JN-25, which is always the top-of-mind Japanese code was modified on a regular basis by the Japanese after its introduction and each change required a fresh cryptanalytic approach. However being a "war" code, very little traffic was available over airwaves for codebreakers to work on before December 1941. It was changed twice shortly before the Pearl Harbour attack, but there are clues that CC&CS were reading some traffic before then.
Conspiracy theorists are welcome to speculate.
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Old November 23rd, 2012, 08:29 AM   #10

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Yes right, the diplomatic code that delivered up Yamamoto were also changed somewhat but the fundamentals were known and some meanings were inferred from before the Pacific war when the breakers from the US, England, Australia and Holland expecting war began attacking the codes. Germany knew about these allied successes and actually sent a modified version of their Enigma machine to Japan which required allied codebreakers to start all over again. Little did they know that Enigma was also broken.

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After war broke out in Europe, the Japanese received encryption and security help from Nazi Germany. The Germans had discovered that U.S. intelligence was monitoring and decoding Japanese communications as early as 1935, but they did not immediately inform the Japanese. Later, Germany sent a copy of their infamous Enigma encryption machine, with a few modifications, to help secure Japanese communications. As a result, U.S. intelligence could no longer read Japanese intercepts. The painstaking work of U.S. cryptologists began anew.

With the aid of the British, United States intelligence made significant progress against Purple in a short time. A replica of the Japanese Purple machine, built in 1939 by American cryptologist William Friedman, was used to adapt a German Enigma bombe to decode Japanese Purple. Although the settings for each message had to be determined by hand, United States intelligence gained the ability to read Japanese code with greater ease, in a more timely manner, by 1942, six months after the Japanese bombing of Pearl Harbor and the entry of the United States into World War II.

Read more: Operation Magic -

Last edited by rehabnonono; November 23rd, 2012 at 08:37 AM.
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