By Captain Duane L. Whitlock, U.S. Navy, Retired

Three days after the Pearl Harbor attack, the decryption unit of Station HYPO joined in working on JN-25, but it was not until 16 March 1942 that the combined recovery effort finally allowed Corregidor to issue its first complete decrypt of a current Japanese message. That message revealed that the Japanese had assigned the geographical designator “AF” to the island of Midway.

Until that date and since June 1939, the only current radio intelligence available on the Japanese navy had been produced by methods short of decryption-that is, by traffic analysis. [17] Most of that intelligence was being produced by a few self-taught enlisted analysts who had moved up from intercept duties. Much of it was of a technical nature, relating to the identification of encrypted call signs, which the Japanese changed periodically, and to the recovery of the enemy’s communication plan. In pursuing these tasks, the analysts learned a considerable amount about how the IJN was organized, and they periodically produced and updated tables of organization (i.e., orders of battle) for its entire fleet.

For years, information of this nature had been included in the monthly report that each intercept station was required to file. At Station CAST that practice began to change in the summer of 1941, when Lieutenant Jefferson Dennis, detached from duty as the Asiatic Fleet radio intelligence officer, arrived on Corregidor to await transportation back to the Navy Department. A very capable analyst, while at Station CAST he began to produce a brief daily summary on the location and activity of various Japanese ships and aircraft as deduced from communications patterns; he sent it by radio to the Commander in Chief, Asiatic Fleet. Studying his methods, the unit’s three enlisted analysts began to produce a few items of current intelligence, which Dennis included in his summaries. Initially, these items related only to events of the current date, correlated wherever possible with previous estimates, but soon a new dimension was added-interpreting enemy intent.

The change came about when one of the enlisted analysts noted an intriguing combination of addresses on a short encrypted message; it led him to suspect that a division of destroyers was departing from Taiwan to the island group of Palau, east of the Philippine island of Mindanao. Lieutenant Dennis, after careful thought, tended to disagree, stating that Japanese ships destined for Palau nominally proceeded by way of Saipan (due east of Manila) or Truk (over a thousand miles east of Palau). Two days later, however, a PBY reconnaissance aircraft flying out of Cavite reported three Japanese destroyers two hundred miles east of Manila on a southeasterly course-toward Palau. Thus it became apparent that traffic analysis had some potential to predict enemy intent, but because JN-25 was not yet yielding current intelligence, there existed no way to assess the limits of that potential.

When Lieutenant Dennis departed, the enlisted analysts of Station CAST continued to originate periodic intelligence estimates, dispatching them through the Commandant of the Sixteenth Naval District to the commanders in chief of the Asiatic and Pacific fleets, Station HYPO at Pearl Harbor, and CNO. Following its lead, HYPO began filing similar estimates, through the Commandant of the Fourteenth Naval District. In the fall of 1941, both stations detected and reported several organizational changes within the Japanese navy, one of which saw the consolidation of all land-based air units into a new air fleet consisting of several newly formed flotillas. In October, the analysts on Corregidor incorporated all of these changes into a new order of battle, which was submitted to the Asiatic Fleet for transmittal to commander in chief of the Pacific Fleet and CNO. In his endorsement, the Asiatic Fleet intelligence officer stated that the report could be construed in no way except that the Japanese navy had assumed a wartime disposition.

In early November 1941, those same analysts reported that Japanese merchant ships were being inducted into the navy in alarming numbers and that some 250 of those ships had been sent to ports bordering the Taiwan Strait, just north of the Philippines. Then, in late November, traffic analysts both at Pearl Harbor and on Corregidor reported that two powerful Japanese task forces were approaching the Philippines, from the north and the east; on the basis of communications patterns, the report laid out the composition of both of those forces in considerable detail. Corregidor, unfortunately, made the mistake of estimating that the large Japanese carriers were still in home waters, although none of them had been noted communicating for several days. [18] While this error was to contribute virtually nothing to the debacle at Pearl Harbor, it did teach both the producers and users of radio intelligence a valuable lesson about the limits of traffic analysis: it has little if any effectiveness against a target that elects to observe radio silence.

Cryptanalysis, of course, is also hampered by that stratagem, but only to a lesser degree, for if cryptanalysts have broken the enemy’s code, they can read messages being broadcast to the target “in the blind” (i.e., without established two-way contact). They can do so provided, of course, that traffic analysts have broken the enemy’s call-sign system and can identify the intended recipients. In this regard, most decrypted messages are never fully intelligible until their encrypted call signs and addresses have been deciphered. The complementary nature of these two basic forms of analysis became more and more apparent as the Pacific war progressed. The first indication of Japanese intent with respect to Midway, for instance, appeared in a traffic analysis report filed by Corregidor in early March 1942; that estimate was backed up three days later by the JN-25 decrypt assigning the “AF” designator to the island. By that time, traffic analysts on Corregidor and in Hawaii were reporting almost daily on the buildup of ships and aircraft at Truk and Rabaul. These reports collectively revealed quite clearly a Japanese intention to launch a major thrust into the Coral Sea and probably toward Midway. They also provided the basis for the estimate given to Admiral Chester W. Nimitz by his intelligence officer, Edwin T. Layton, that the Japanese had amassed in the Truk area some three hundred ships for impending operations. (After the war, the Japanese indicated that the figure had been 280.) [19]


17. Frederick D. Parker, “The Unsolved Messages of Pearl Harbor,” Cryptologia, October 1991, pp. 297-9. Back

18. Ibid., p. 299. Back

19. Informal discussion with Roger Pineau, October 1992. Back

Source: Naval War College Review, Vol. 48, No. 4 (AUTUMN 1995)

Featured Image: Lieutenant Jefferson Dennis