Japanese army and diplomatic codes

Japanese army and diplomatic codes. This article is on Japanese army and diplomatic ciphers and codes used up to and during World War II, to supplement the article on Japanese naval codes. The diplomatic codes were significant military, particularly those from diplomats in Germany.

Japanese army (IJA) and diplomatic codes were studied at Arlington Hall (US), Bletchley Park (UK), Central Bureau or CBB (Australian, US; in Melbourne, then Brisbane), the FECB (British Far East Combined Bureau) at Hong Kong, Singapore, Kilindi then Colombo and the British Wireless Experimental Centre in Delhi.

Initially Arlington Hall had delayed study of the Army codes until 1942 because of the "high payoff" from diplomatic codes, but were not successful until 1943. Then with success on Army codes in April the increasing workload was put under Solomon Kullback in branch B-II in September. Other mainly diplomatic work was put under Frank Rowlett in B-III. Branch B-I translated Japanese.

Initially “brute-force” IBM runs on Army codes from April 1942 to the end of the year did not work. But US Army Sgt Joe Richard noticed that the system for 2468 changed every three weeks, so the messages could be arranged by IBM tabulators by group and time period. Richard was assisted at Central Bureau by Major Harry Clark and by the head Abe Sinkov, and broke 2468 on 6 April 1943; for which he was awarded the Legion of Merit. Wilfrid Noyce at the Wireless Experimental Centre had realised that the first letter of the third group of each message was not random, and that other groups were paired in "doublets".

Initially Arlington Hall could not find the non-randomness until Richard told them it changed about every four weeks. So with the tip Arlington Hall broke the code, as did the Wireless Experimental Centre. It used a 10 x 10 conversion square with the plain text digits 0-9 across the top key digits down the side and the table contained the cipher text digits. As well as "kana" the Chinese Telegraph Code was used to explain places or words, and the code groups 1951 or 5734 indicated that the CTC follows; an "absurd security flaw" as it was like "Stop" as a key. The CTC code group was often preceded by the "kana" groups for the same character.

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