The codebreakers of the Japanese Foreign Ministry and the compromise of US codes prior to Pearl Harbor

Imperial Japan entered WWII with three separate codebreaking agencies under the control of the Army, Navy and Foreign Ministry. The Army and Navy signal intelligence agencies intercepted foreign radio traffic and decrypted several military and civilian US, UK, Chinese and Soviet cryptosystems. However relations between these two organizations were strained and in many cases they withheld valuable information from each other. Compared to them the decryption department of the Foreign Ministry was much smaller and had access to limited resources, both in terms of manpower and radio equipment.

Information on the decryption department of the Japanese Foreign Ministry is limited since their archives were destroyed twice during the war. First in a bombing on 25 May 1945 and then in August 1945, when they were ordered by their superiors to burn all secret documents.

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.

 

Overview of exploited foreign codes

British codes

In the case of Britain the Government Telegraph Code, R Code, Interdepartmental Cypher and Cypher M were read.

According to one of the Japanese analysts a 4-figure diplomatic codebook and its substitution tables were received from either the Army or the Navy in January 1940, thus a great deal of the traffic could be read. Even though the substitution tables changed every 4-6 months the Japanese were able to get a copy roughly one month after their introduction.

 

Chinese codes

The Chinese government used several codebooks but only a few were enciphered properly. This allowed the Japanese to solve most of the traffic. One of the codebooks they solved was the ’27 DEMPON’.

 

French codes

Some French codes and their substitution tables were received from the Army and thus it was possible to solve this traffic. These were called ‘PC 149’, ‘PC 150’, ‘PC 151’ and ‘CGX’ by the Japanese and they were used by the French embassies in Tokyo, Peking, Hanoi, Nanking and Chungking.

 

It seems that the numbered codes were used mostly for reports on administrative matters while ‘CGX’ carried important reports on the political and military developments.

German codes

Even though Japan and Germany were allies in WWII it seems that the Japanese authorities did not neglect to solve German diplomatic codes. According to DF-169, p2 a German diplomatic unenciphered code of 100.000 values was solved in part and from 1942 it was possible to read some messages even when they were enciphered with additive sequences, thanks to the reuse of the additive pads.

This must have been the German Foreign Ministry’s basic codebook used unenciphered for low level messages, enciphered with reusable additive pads for important messages and also with one time pads for the most important traffic.

 Swiss codes

The code of the Swiss legation in Tokyo was received from the military in summer 1945 and messages were read till the end of the war.

USA codes

The main target of the Foreign Ministry’s codebreakers were the diplomatic systems of the United States. The State Department used the Gray and Brown codes, the enciphered codebooks A1, B1, C1, D1 and the M-138-A strip cipher. By 1940 the Japanese had managed to get copies of Gray, Brown, A1 and several sets of strips of the M-138-A.

 

With these codes and with the M-138-A strips and keylists the Japanese could read all US diplomatic traffic in the period 1940-41. The importance of this compromise for Japanese foreign policy is something that needs to be investigated by historians.

During the war they received more strips and keylists from their Finnish and German allies.