Details of their successes against enemy codes have been hard to find because after Japan’s surrender, in September 1945, they had time to destroy their records and disperse their personnel. Still the few remaining documents in Japan combined with decoded Japanese messages found in the British archives can provide a basis for assessing their operations during the war.
In order to keep an eye on US fleet movements several monitoring stations were operated prior and during the war. An interesting case was the undercover L agency. In 1938 a small unit called the ‘L Agency’ (L-Kikan) was established in Mexico to monitor US Fleet traffic in the Atlantic and also the commercial RCA radio from New York City.
During the Pacific war most US military codes proved secure. There was only limited success with the US Navy’s CSP-642 strip cipher. However the codes used by merchant ships had been received from the Germans and their enciphering tables were solved in Japan.
Foreign Ministry’s decryption
According to the recently declassified TICOM report DF-169 ‘Cryptanalytic section Japanese Foreign Office’ this department was established in 1923 and by the end of WWII had approximately 14 officials and 16 clerks. The radio intercept unit supplying it with messages had a station in Tokyo equipped with 10 receivers and 19 operators. They usually intercepted 40-60 messages per day with 100 being the maximum.
The emphasis was on the solution of the codes of the United States, Britain, China and France but some German, Turkish, Spanish, Italian, Swiss, Thailand and Portuguese codes were also read. Despite their limited resources it seems that the Foreign Ministry’s codebreakers were able to achieve their goals mainly thanks to compromised material that they received from their Army and Navy counterparts.
Japanese radio security services
Cooperation with foreign powers
A Japanese mission headed by Colonel Tahei Hayashi, former head of the Army’s cryptologic agency visited Germany in 1941 and exchanged US and British codes with systems solved by the Germans. This promising start did not lead to closer cooperation as communications between Japan and Germany were problematic. Moreover the Germans did not trust the Japanese with their most recent codebreaking successes. Things changed in summer ’44, when under Hitler’s orders several high level systems (including the latest strips for the M-138-A cipher) were given to the Japanese.
According to Wilhelm Fenner, head of the cryptanalysis department of OKW/Chi, about 200 decoded messages were passed on to the Japanese in 1944-45. For example:
In September 1944 Finland signed an armistice with the Soviet Union. Paassonen and Hallamaa anticipated this move and fearing a Soviet take-over of the country had taken measures to relocate the radio service to Sweden. This operation was called Stella Polaris (Polar Star). Roughly 700 people, comprising members of the intelligence services and their families were transported by ship to Sweden. The Finns had come to an agreement with the Swedish intelligence service that their people would be allowed to stay and in return the Swedes would get the Finnish crypto archives and their radio equipment. Their archives were also sold to the Japanese military attaché Makoto Onodera.
military and NKVD border guard figure codes were read (OKK, OK40, PK1), thanks in
part to Finnish and German help. These were used to monitor the movements and readiness
of Soviet units in the Far East.
The Soviet diplomatic code used in the East by consulates/embassies in Seoul, Dairen, and Hakodate for their communications with Moscow and Vladivostok was read by the Japanese from 1943 onwards. This was not the standard Soviet diplomatic system of codebook plus one time pads but a simpler system
Intelligence reports from Australia were copied by a Japanese spy working in the Soviet embassy in Harbin, China. These messages came from Soviet agents in the Australian government and contained information on Allied political and military plans.
State Department codes Gray, Brown, A1 and the M-138-A strip cipher were read by the Japanese with varying degrees of success. All these systems had been physically compromised. Through these systems the messages of the US ambassador in Japan Joseph Grew, as well as other embassies abroad, could be solved.
Success with the high level M-138-A in 1943-44 depended on help from the Finns and the Germans.
British diplomatic messages can be found in the archives of the Japanese Foreign Ministry in the file ‘U.S.-Japan Relations, Miscellaneous Diplomatic Correspondence-Special Information File’:
Acknowledgments: I have to thank mr Ken Kotani for answering my questions on WWII Japanese cryptologic history and the staff of the Japan Center for Asian Historical Records for the links to the files in ‘U.S.-Japan Relations, Miscellaneous Diplomatic Correspondence-Special Information File’