TheCodeBreakers
The Navy complicated it by not using the full
complement of 30 strips every time. Instead it eliminated from zero to five
strips from one day to another. Thus one day's messages might use only
25 strips, the next day's, 27, the next, 30.
Japan had captured strip ciphers on Wake and Kiska, and with these
she attacked the intercepts. Her methods mixed sophistication and
naivete. To determine how many strips had been eliminated, the Tokumu
Han used I.B.M. tabulators of the First Life and the Maiji Life Insurance
companies of Tokyo. These took frequency counts at intervals of 30, 29,
28, . . ., 25 and compared them; the interval that showed the most
repetitions indicated the correct encipherment length. Many of the strip
messages were sent by American submarines; these were identifiable by
their indicators—BIMEC or FEMYH—and by their transmission from close to
the Japanese coast. The Tokumu Han could know that at that position a
annaabi.ee 
