By Captain Duane L. Whitlock, U.S. Navy, Retired
Notwithstanding, a firm decision was made to explore that potential on the occasion of the next Japanese navy Grand Maneuver, which was expected to occur in 1933. This decision was given considerable impetus when in December 1930 when the Japanese navy superseded its 1918 code book, thereby eliminating the American ability to decrypt its messages. The Red Book was now useless for all but historical purposes, and ONI found itself unable to obtain the new code in the way it had the old one.
Accordingly, the new book had to be reduced by cryptanalysts, a process that consumed most of the next five years.  Admiral Upham, the Commander in Chief of the Asiatic Fleet, therefore assigned his staff radio intelligence officer, Lieutenant Joseph Wenger, to derive all possible intelligence on the expected exercise by means other than cryptanalysts. Wenger arranged to receive the intercepts made by stations ABLE and BAKER and also some temporary sites. The IJN maneuver occurred as predicted, and Lieutenant Wenger spent the next six months performing after-the-fact traffic analysis. That effort produced a 115-page report laying out in considerable detail the composition and disposition of the forces involved, including the identity of the individual ships and commands. His report was sent back to Washington, where, three years later, enough of the new code (the “Blue Book”) had been recovered to affirm that Lieutenant Wenger had been essentially accurate. 
Admiral Upham did not wait for that assessment. He was so impressed with Wenger’s work that he immediately made a strong representation for a radio intelligence center in his command. Specifically, he requested that an intercept unit complete with a decryption center be located in what war plans referred to as the Ultimate Defense Area of Manila Bay, with the mission of preventing surprise attack.  This recommendation is historically significant for two reasons. It was undoubtedly the first time that a U.S. Navy fleet commander ever requested that a decryption center be placed under his command; second, it marked the beginning of a willingness in the U.S. Navy to amend the longstanding precept that the military commander should always be guided by estimates of enemy capabilities, never of intent.
Implementation soon began. Three graduates of Chief Kidder’s first class and two from his second had been sent to the Philippines, where late in 1929 they attempted to set up an intercept station at Olongapo. Unfortunately, they were delayed by having to assume primary responsibility for all regular Navy communications in and out of the base there.  As a result, they did not really get on with intercept duties until August 1932, as Station CAST. Station CAST was augmented as recommended by Admiral Upham; it was transferred from Olongapo to Mariveles and then to the Navy Yard in Cavite. In mid-October 1940 it would finally establish itself in a special tunnel built for the Navy at Monkey Point on Corregidor, two months later absorbing the mission and the personnel of Station ABLE in Shanghai, which was closed.  In the meantime, in 1935, a new intercept site, Station HYPO, was opened on the windward side of Oahu, and a decryption unit was established in the basement of the administration building at Pearl Harbor. In June 1939, however, the Japan navy once again changed its operating code, and the nine years of cryptanalytic effort that had gone into recovering and exploiting the Blue Book went down the drain.
The new code, designated JN-25 by the U.S. Navy, was of a drastically different type than its predecessors. Made up of five-digit groups, it allowed for forty-five thousand entries in each of two dictionaries-one set up alphabetically for encoding, the other arranged numerically for decoding. For the cryptanalyst, who had to gain entry to the code through the numerical dictionary, this provision had the effect of scrambling the alphabetical order of the code, thereby complicating the process of assigning meanings to individual code groups. But the Japanese had erected an even greater barrier to that entry by enciphering each already encoded message by adding arithmetically a different and arbitrary five-digit number to each of the five-digit code groups contained in the message. Generated randomly, these arbitrary enciphering groups were drawn systematically from a separate book and were added, one for one, to each of the basic code groups awaiting encipherment. “False addition” was used to combine these arbitrary groups, or “additives,” to the basic code groups. In false addition, no numbers are carried over; if the additive 49238 were added to the code group 77739, for example, the result would be 16967, which would be the five-digit group that was actually transmitted. Inasmuch as there were eighteen thousand of these additives in the enciphering book, there existed a potential to encipher any one of the forty-five thousand basic code groups in eighteen thousand ways. The system seemed unbreakable: one needed the additives in order to recover the basic code groups but had to have the basic code groups to find the additives. The cryptanalyst’s only hope was to sift all intercepted messages very carefully, hoping that Japanese code clerks would eventually make enough careless mistakes to permit entry into their system.
Responsibility for breaking JN-25 was shared between the Navy Department (now Station NEGAT) and the decryption unit at Station CAST, on Corregidor. The unit in Pearl Harbor, Station HYPO, was assigned to work on the Imperial Japanese Navy’s high level cipher used between admirals. That code was never broken, mainly because the infrequency of its use denied cryptanalysts a sufficient depth of message traffic.  Depth was not a problem with respect to JN-25, but, even with the opportunities provided by one or two errant Japanese code clerks, the very nature of that code made progress extremely slow and tedious. In a division of effort, Corregidor was to attack the code on a current operational basis; Washington was to follow up several weeks or months later, with the primary aim of building up the dictionaries to support operational exploitation. Between June 1939 and December 1941 Washington did decrypt a few JN-25 messages, but they provided little insight into the current intelligence picture.
The Japanese also made several changes in the code during that period, which put Corregidor and Washington out of sync.
11. Safford SRH-149 p. 12, and Safford, SRH-305, pp. 09-12. Back
12. Safford SRH-305 pp. 14-6. Back
13. Safford, SRH 149, p. 12; and Safford, SRH-305, p. 19. Back
14. “A Brief History of U.S. Naval Pre-World War II Radio Intelligence Activities in the Philippine Islands,” SRH-180, p. 3. Back
15. SRH-179, p. 24. Back
16. Safford, SRH-149, p. 15. Back
Source: Naval War College Review, Vol. 48, No. 4 (AUTUMN 1995)