A crucial element of the secrecy surrounding the Hawaii Operation was the radio silence preserved by the First Air Fleet. It was essential to conceal their position and therefore all messages within the fleet were passed either by signal lamp (blinker) or by flags. Messages from Japan were encoded using JN-25 but even then the content was enigmatic, virtually an additional code. The final order to proceed was, for example, "Climb Mount Niitaka" and was picked up by the battleship Hiei and passed on by visual signal. It is said that the code had been broken and the messages passed on by the British organisation FECB to London where they were suppressed. The evidence is, at best, circumstantial and the messages themselves were obscure, so this view of the espionage is doubtful. The preservation of radio silence by pilots from the cruiser Northampton may have contributed to the unopposed withdrawal of the First Air Fleetafter the attack.
You can find out more about the Hawaii Operation in the following Osprey book: