Featured image: Mrs. Agnes Driscoll
The collaboration between the Army and the Navy on Japanese Diplomatic crypto‑systems did not extend to the Japanese Military (Army and Navy) crypto‑systems. A secret divulged to a third party is no longer a secret. The U.S. Navy withheld all details of its success with Japanese Naval crypto‑systems from the Army and in turn made no inquiries about the Army’s progress with Japanese Army crypto‑systems. When the Japanese Army invaded Manchuria in 1931 the U.S. Navy intercept station at Peking (manned by Marine Corps operators) went to watch‑and‑watch condition and obtained a wealth of tactical intercepts. These were all turned over to the War Department for exploitation ‑ and no embarrassing questions were ever asked.

From 1936 on, Navy intercept stations in the Far East copied considerable Japanese Army traffic which was turned over to the War Department. For some strange reason the U.S. Army posts at Tientsin (China) and Manila failed to profit from the wealth of Japanese Army messages available at slight effort. Not till the spring of 1941 did the War Department attempt to set up an intercept unit in the Philippines and sent a Signal Corps officer to take charge. The Navy collaborated with the three‑months’ loan of an experienced and qualified Chief Radioman to act as instructor, and the supply of all available technical literature on intercept operator training, Japanese radio procedure, Japanese radio organization, Japanese call‑and‑address system, etc., but left the Army “on their own” so far as Japanese military crypto‑systems were concerned.

On 1 December 1930 the old 1918 Japanese Naval Code was replaced by a 1930 Naval Code which remained in effect until 31 October 1938, giving the U.S. Navy COMINT organization a severe, although temporary, setback. The new code was never used without a cipher; the cipher had to be stripped off, before the code could be reconstructed. To make a long story short the Navy cryptanalyst, spearheaded by Mrs. Agnes Driscoll “accomplished the impossible”, solved the ciphers and then reconstructed the code. This was the most difficult cryptanalytic task ever performed up to that date and possibly the most brilliant as there were no “cribs” and “translations” to help out as in subsequent Army solution of the Purple machine. International Business Machines (IBM) tabulating machinery was introduced by the Navy incident to the solution of the 1930 Naval Operations Code. This machinery greatly speeded solution and increased the per capita output of the Decrypting Unit. In 1941 similar IBM equipment was sent to Pearl Harbor and to Corregidor.

The Japanese Navy held grand maneuvers every three years. With the 1930 Grand Maneuvers fully digested, comprehensive plans were made for the 1933 Grand Maneuvers. Subsequent events proved these maneuvers were a dress rehearsal for the conquest of China ‑ while warding off intervention from the U.S. Fleet. The U.S. Navy tested its theories of traffic analysis under simulated war conditions and found them practicable and reliable. The success of the Asiatic CI Unit convinced CINCAF (Admiral Upham) of the necessity of a permanent Navy COMINT installation on Corregidor. The project was begun in 1938 and completed in September 1941. On 7 December 1941 the Asiatic CI Unit consisted of nine officers and 61 men, located in a bomb proof tunnel on Corregidor, and functioning with 100% efficiency. This unit was subsequently evacuated to Australia by submarine and played an important part in the Battle of Coral Sea and in the Battle of Midway.

Extensive arrangements (including a mobile intercept unit aboard a destroyer) were made to cover the 1936 grand maneuvers of the Japanese Navy. But these maneuvers were delayed and finally turned into the real thing, the Invasion of China, as forecast by the 1933 grand maneuvers. The Navy COMINT organization gave the CNO and CINCAF advance information on all important moves and this information was later verified without exception. It proved what could be done by COMINT, even without radio direction finders, and HFDFs (we hoped) were “just around the corner”. The 1930 Naval Operations Code was thoroughly reconstructed by that time and the only limits to our detailed knowledge of what was going on inside the Japanese Navy was the acute shortage of translators and the fact that sometimes the Japanese did not entrust important secret matters to radio communications. The “China Incident” highlighted the need for a secure COMINT post in the Ultimate Defense Area of the Philippines. The Corregidor Project was revived; the CNO finally beat down the objections of the Army Chief of Staff which had delayed the project for two years. The two years additional delay before this project was really commenced were due to cussedness and cowardice on the part of certain high ranking officers 4  in the Navy Department itself.

The most important and certainly the most dramatic incident connected with the 1930 Naval Code was the message reporting the class of Japanese ships (NAGATO’S) post modernization trials in 1936. We were fortunate enough to intercept the message and got a solid translation. The NAGATO’s new speed was better than 26 knots ‑ the same as the four KONGO‑class battle cruisers. There was no doubt as to the correctness of this information. By inference, this was the prospective speed of the modernized Japanese ship (MUTSU) and minimum speed for the new Japanese battleships of the YAMATO class. This information created consternation in the higher echelons of the Navy Department because the MUTSU class was believed good for only 23‑1/2 knots, and our newbattleships (then in the blueprint stage) were going to have a speed of only 24 knots. The information was referred to the General Board; the maximum speed for battleships NORTH CAROLINA (BB-55) and WASHINGTON (BB-56) was raised to 27 knots, and for later battleships to 28 knots. The 12 battleships of the new building program were thus given a superiority in speed over the Japanese battleships. 5 It proved impossible to get any COMINT information on the tonnage, speed, or main‑battery caliber of the YAMATO class: The Japanese never sent this information by radio.

Source: SRH-149