[Pelham]

We had the ability to intercept and decrypt part of Japan’s secret PURPLE transmissions in December 1941.

But PURPLE was their diplomatic cable traffic and the Japanese military didn’t trust it or use it. Japan’s military used a different system, JN-25, and they kept all references to to their planned attack there. We didn’t have the ability to spy on JN-25 traffic until some time in 1942.

[rktman]

They should have put it in the "cloud". 👹

[jamaksin]

Citations from Congressional Hearings on Pearl Harbor, perhaps helpful for you:

37PHA1062-1063 - What means CI, RI, and "tranlation this day" ... and

Exhibit No. 151 - exchange technical information and translations is what?

Thanks.

P.S., Also suggest quick scan:

Provocation and Angst: FDR, Japan, Pearl Harbor, and the Entry into War in the Pacific, Burtness and Ober, The Hawaiian Journal of History, Volume 51 (2017), pages 91-114