Like the Japanese Navy, the Japanese Army cryptographers were fond of sum checks as a guard against garble, but they had devised a method different from the “divisible by three” sum check of the Japanese Navy. In this system, the code breakers discovered, the four-digit code groups were actually three digits, plus a fourth digit that served as a sum check. If a code group was 0987, for example, 098 was the actual code group, and 7 (using false math) was the sum check: 0 + 9 + 8 = 7. Reconstructing an enemy codebook is called “book-breaking,” and Ann Caracristi, working with the linguists,
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