Hmm..
no idea on this one..

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That sounds like a guess.  If it was, it was a really good guess.  By the time of planning for operation Overlord, the Japanese JN-25 code had been broken and was routinely read.  However the Japanese  never knew that, although some lower-level officers suspected JN-25 was cracked after the Yamamoto ambush, but the powers-to-be dismissed that notion.

Anyway, the Germans were sharing with the Japanese Embassy in Berlin it's disposition of defenses along the French/Belgium coast to defend against an allied invasion.  And the Japanese Embassy was dutifully  sending to Tokyo that information.  And the allies were eagerly reading those messages.

Ah dang! I knew the answer to this one! I remember reading about it in The Wotan Mission by Leo Kessler (Charles Whiting). They had a big submarine and helped with supplies and codes!



A.

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Just to point out that not all the codebreakers at Bletchley Park were British by any means. At its peak it was staffed by thousands of people from all walks of life & many different nationalities. There was a Japanese decoding section at Bletchley staffed by a large proportion of American codebreakers. Due to the large number of characters in the ordinary written language I've always thought that decoding Japanese messages must have been far more difficult than German or any other European language. Come to that I could never visualise a standard Japanese typewriter or keyboard.

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The D-Day Invasion of Normandy in 1944 was a combined Allied operation planned by American & British military experts with an American general in overall command. Every piece of intelligence would have been vital to its success wherever it came from.

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I don't know the 'purple machine' you refer to but this might be similar to the Enigma codes. The Allies already had a working example of the machine at Bletchley Park by 1940 but this wasn't much use without the codes. These were changed every day & had to be planned in advance so both the sender & receiver could use the correct daily code without which the coded message could not be deciphered by anyone except the person who sent it. This is where those codebooks come into it as the Enigma machine itself was automatic. I got most of my knowledge on this subject from a recent visit & guided tour of the once top secret 'Station X' codebreaking establishment at Bletchley Park where most of this work was done throughout WWII. http://www.bletchleypark.org.uk/

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Interesting. At first sight it has certain similarities to the Enigma machine but the principle it works on is not quite as easily understood, not by me anyway. The Enigma machine is beautiful in its simplicity & also very effective. I'm no cipher expert & have enough problems doing a crossword myself.

but yes, the basic principle of all the cryptographic gear used at the time was the same.