Japanese code library found in New Guinea

Photo By Amy Stork | American members of the Central Bureau in Brisbane, summer 1943. (Courtesy Oz at War, https://www.ozatwar.com/sigint/cbi.htm)  

FORT HUACHUCA, ARIZONA, UNITED STATES

01.13.2023

Courtesy Story

U.S. Army Intelligence Center of Excellence

by Lori S. Stewart, USAICoE Command Historian

On January 19, 1944, the Australian Ninth Infantry Division uncovered the Japanese 20th Division’s entire cryptographic library in Sio, New Guinea. This find allowed the Central Bureau, the Southwest Pacific Area’s multinational signals intelligence organization, to master the Imperial Army's codes and ciphers and provide timely intelligence to Allied forces.

In 1943, the Allies had occupied several islands in the New Guinea chain, but the Japanese maintained a large base at Rabaul on the island of New Britain from which they could threaten Allied operations. Instead of attacking the base outright, the Allies decided to bypass and isolate Rabaul and continue their advance through New Guinea and the Solomon Islands and eventually on to the Philippines. As part of this strategy, the Australian Ninth Infantry Division battled the enemy along the coast of the Huon Peninsula of Papua New Guinea.

By January 1944, as the Australians closed in on the town of Sio, the Japanese 20th Division withdrew north to Madang. Facing a trek over the rugged mountainous terrain and through swollen rivers, the 20th Division’s radio platoon chose to abandon a heavy steel trunk containing its entire code library. The platoon could not burn the contents because of heavy rains and the risk Allied aircraft would see the smoke. Instead, they tore the covers off the booklets as proof of destruction and buried the trunk in the mucky ground alongside a stream.

As the Ninth Infantry Division moved into Sio, engineers looking for Japanese booby traps and mines detected the trunk and dug it up. Allied Translator and Interpreter Service personnel serving with the division realized the value of the materials and contacted the Central Bureau in Brisbane, Australia. Upon seeing the contents, an excited but dismayed U.S. Army Maj. Charles Girhard, chief of the bureau’s cryptologic section, recalled, “What a mess! ...There was so much mildew on the material that each page had to be dried in the large commercial cooking ovens in our kitchen.” Other pages were hung on clotheslines and dried with electric heaters and fans. Upon completion of this process, the bureau had at hand all the materials needed to decrypt the Japanese Imperial Army’s mainline code used to communicate with its divisions.

The cryptanalysts at the Central Bureau were elated. While they had solved the Japanese Army’s air-to-ground radio code and a similar code for the Japanese Navy, they had difficulty with the mainline Japanese army code. Suddenly, however, they were decrypting about 2,000 messages every day. Overwhelmed, they copied the entire code library and sent it to the U.S. Army’s Signals Intelligence Service headquarters at Arlington Hall Station, Virginia, where an additional 36,000 messages were decrypted in the month of March alone.

This windfall came at a critical time in General Douglas MacArthur’s campaign in New Guinea. Message traffic revealed Japanese intentions in the Southwest Pacific, enemy strength and incoming reinforcements, the enemy’s defenses and weaknesses, the locations of Japanese fighter aircraft, and the results of Allied air and naval efforts. The intelligence gained from the enemy traffic, along with other sources, allowed MacArthur to refine and accelerate his planned assaults in the Admiralties and on the major Japanese air and resupply base at Hollandia in April 1944.

In retrospect, Col. Abraham Sinkov, the American assistant director of the Central Bureau, reflected that eventually the organization would have solved the code, but finding the trunk of materials “certainly spared us a great deal of effort.”

• Date Taken: 01.13.2023
• Date Posted: 01.14.2023 09:09
• Story ID: 436784
• Location: FORT HUACHUCA, ARIZONA, US