>>Based as it was on cryptanalysis, the core evidence is so sensitive that it remains secret even today.
Actually the story of Japanese intelligence rings in the U.S. is reasonably well-documented. The technical details of the breaking of the Purple code may not be, but the results of those intercepts are, and the thinking in Washington that led to mass internment as a way to avoid revealing we’d broken the code.
Magic: The Untold Story of U.S. Intelligence and the Evacuation of Japanese Residents from the West Coast During Ww II
https://www.amazon.com/Magic-Intelligence-Evacuation-Japanese-Residents/dp/0960273611
I think that I have that book in my stacks of what I intend to read, eventually. Despite increasing public access to the primary source material and path-breaking historical accounts, the point remains that even though US cryptanalysis established the existence of Japanese spy and sabotage rings, radio detection and other CI methods had not led to their shut down, even in the more intense security environment after Pearl Harbor. With US war industries and military installations regarded as vulnerable and a Japanese strike at the US West Coast seen as imminent, internment made a lot of sense. I do not see though how internment helped to protect the secret of US cryptanalytic success.