Hara Hisashi's Pseudo-Random Number Code
Hara Hisashi became head of the code section of the Seventh Division sometime after 1932 and was later transferred to the Third Section of the Army General Staff. Sometime between then and 1940, Hara devised a system that used a pseudo-random number additive to superencipher the three number code the Army already had in service.
Neither Takagawa nor Hiyama provide details about when this three-number code system was adopted for Army communications. A three-number code has a maximum of 10³, or 1000 groups — which is still too small for a strategic code and a far cry from the 25,000 that Yardley claims some Japanese codes had in the 1920s. However, it was a two-part code — an important improvement.
Read more about this topic: Japanese Cryptology From The 1500s To Meiji
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