In February 1942, however, the hypercautious German Navy added a fourth rotor to the naval U-boat Enigma machines, increasing the possible combinations by a factor of twenty-six. The Allies called this new four-rotor cipher “Shark,” and initially it proved impenetrable. The Allies lost the ability to read U-boats. The whole system went dark. This crushing turn of events occurred just months after the United States entered the war, and it began an eight-month period of death and destruction and helplessness, a time when ship after ship went down and it felt very much as though the war could
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