From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 11-14:
In the first forty-eight hours after Pearl Harbor the CIC [Commander in Chief] had a flood of misinformation which left them ever afterwards with a healthy mistrust of ‘eyewitness’ accounts, not only from excited civilians but also from experienced Service personnel, both Allied and enemy. Japanese parachute troops were reported to have landed and to be engaged in a fierce pitched battle with US Marines. The uniforms worn by these mythical Japanese were described in the most minute sartorial detail. Strange vessels were reported arriving offshore, a large enemy fleet had been seen south of the islands and at least one Direction/Finding bearing (later judged to be ambiguous and inconclusive) was obtained. One officer sighted a dirigible over Honolulu, two degrees to the right of the moon and three degrees below it. To make matters more confusing, there were seemingly improbable reports of submarines in Pearl Harbor — but Japanese submarines did indeed take part in the attack.
In the earliest, defensive, stages of the war in the Central Pacific, radio intelligence was not just the most important source of intelligence; it was, for all practical purposes, the only source. There were no photographs of enemy-held positions. There were very few captured enemy documents and even fewer enemy prisoners-of-war. Apart from the Solomons and New Britain, spies and coast-watchers supplied no important intelligence.
Radio Intelligence embraced the interception and exploitation of all enemy radio transmissions which might yield intelligence, including the decryption of coded enemy messages; direction finding (D/F); navigational beacons and aids; enemy radar and infra-red transmissions; traffic analysis, which was the study of communications networks and the procedures, signals, callsigns and plain language messages passing over them; the monitoring of enemy radio broadcasts to the civilian population; and such refinements as the study of the types and peculiarities of particular transmitters and of the idiosyncratic morse characteristics of individual operators.
Fortunately for the Allies, distances in the Pacific were vast — by 1942 the perimeter of the area Japan had conquered was between 3000 and 4000 miles from Tokyo and overland or undersea communications, such as cable, telephones and telex, were scarce or non-existent. Thus the Imperial Japanese Navy routinely generated a huge amount of radio traffic. Again because of the distances involved, much of it was transmitted by High Frequency which was detectable at long ranges by a ring of listening stations down the west coast of the United States, in the Aleutians and Australia and, before the war, at Cavite, Guam, Shanghai and Peking.
The most valuable radio intelligence was obtained from the interception and decryption of encoded or encyphered enemy messages. The Japanese themselves regarded their language as a sacred mystery, not to be vouchsafed to outsiders. Japanese hearing for the first time a Westerner speak their language were known to shake their heads dis-believingly. Such a thing was not possible; they must be dreaming.
Learning to speak or read Japanese was in itself a formidable challenge to western minds. To unravel Japanese in code would seem a virtually impossible mental obstacle. In fact, many Allied cryptanalysts found that decyphering Japanese was a matter of persistence, of ‘quantity and time rather than difficulty’. It was, if anything, tedious rather than difficult.
That is not to say that the task was easy. Whereas the Germans used versions of the Enigma machine for encyphering virtually all Kriegsmarine, Wehrmacht, Luftwaffe, SS, police and diplomatic signal traffic, the Japanese used many different crypto systems. An operational history of Japanese naval communications from December, 1941–August, 1945, compiled under Allied direction by former Japanese officers who had served during the war, lists three naval code books for strategic and administrative use; six naval code books, a joint Army-Navy code book and a Combined Fleet special code book, for tactical use; for intelligence, an overseas secret telegraph code book, two more naval code books, and five variations of a code distributed to naval officers appointed pre-war as intelligence agents stationed in Europe, the Americas and all over the Far East, and a ‘New Code Book’ for naval officers stationed on the west coast of the USA; five code books for communications with service branches outside the Japanese Navy, such as merchant ships over 1,000 tons and fishing vessels, and a standard code book used by the Navy, Army and Foreign Ministries, distributed to diplomatic officials stationed in East Asia and principal Navy and Army headquarters. There were also other publications such as books of abbreviations, address codes and call signs, and books of visual signals.