This allowed them to identify depths, which were the key to recovering the numerals used in messages. Navy cryptanalysts used IBM machines to correlate and compare values in JN-25-based communications. Their first goal was to recover the indicator in each message which showed where in the additive book numbers were taken then recover and strip away the additives themselves to get down to the codebook values and, finally, recover the meanings in plaintext Japanese of the underlying codebook values, which would allow messages to be read, at least in part. cryptanalysts had to collate large numbers of Japanese messages over time. It should be noted that this additive book for JN-25 was not a one-time pad: the five-digit groups were re-used, as needed. The additive book consisted of 300 pages, each page containing 100 random five-digit groups. JN-25 consisted of a codebook with approximately 27,500 entries and an additive book for superenciphering the codebook values. JN-25, however, became one of the most widely used Japanese Navy systems - and, eventually, a critical source of intelligence for the Allies. The Flag Officers Code was never solved by the Americans. Navy got little intercept in the Flag Officers Code and the effort against it was abandoned in December 1941. Both systems were worked by Navy cryptanalysts, although the U.S. The Japanese Navy introduced two new enciphered codes for general purposes in 1939 American cryptanalysts dubbed one of the systems the "Flag Officers Code" and the other, JN-25. The BLACK CODE introduced the use of an additive book to superencipher messages. Navy terminology for the superseded system had been the BLUE CODE, based on the color binding used for the American recoveries to the system the new code was designed BLACK. In 1938, the Japanese Navy changed its operational code, replacing a system that had been in use since 1931.
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