There is some evidence out there that the Japanese had broken some of our diplomatic codes as well. Much of what existed was destroyed however and it is impossible to figure out to what degree they had access to our own communications. If the Japanese were reading our diplomatic codes at this time, then our code breaking progress between the two nations at this moment would be a wash since we are only reading PURPLE at this point. The JN-25 code is only is being worked on, but we are only having limited success right now and the code will be changed on December 1st erasing some of that progress made. It wont be until early next year that we make the real breakthrough on the Japanese naval codes.
It is known that older U.S. diplomatic codes (Gray from the 1920s, and Brown from the 30s) were broken by the Japanese. To what degree, if any, that we used these codes in the early 40s I couldn’t say without doing some research which unfortunately, I just don’t have time to do at the moment.
also impressive is the degree to which the Japanese were able to disguise that traffic, and the level of detail and planning that went into the Japanese concealment effort.
one of the most intersting things i read: the FBI was tapping all the phones of suspected Japanese spies and monitoring the calls back to Japan. One in particular was reporting on “flowers” and other stuff. this agent correctly figured out that this was a report on naval activity and in fact pretty much called the Dec 7 attack based on his work,
but no one really would listen. combined with rocheforts report that he had lost the location of the jap carriers, and other intel from decoded messages in DC, it all sounds like 9-11 failure to connect the dots. in the retrospectoscope there was plenty of evidence when and where the japs would attack.
its still unbelievable that they managed to arrive off hawaii totally undetected.