The Japanese devised a host of minor codes and at least four major four-digit systems: one for ground forces, another for air forces, another for high-level administrators, and another for the “water transport organization,” which was a vital lifeline of marus, or commercial merchant ships, commandeered by the military to carry resources, including oil, food, and equipment. Each system was identified by a discriminant—an unenciphered four-digit code group at the beginning. It was a huge tangle, and as of January 1943 there were just fifteen American civilians, twenty-three officers, and
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