As in other areas of Shannon’s life, his most important work in cryptography yielded a rigorous, theoretical underpinning for many of a field’s key concepts. Shannon’s exposure to day-to-day cryptographic work during the war, it seems, was important—but its primary purpose was as grist for a paper that would only be published in classified form on September 1, 1945—one day before the Japanese surrender was signed. This paper, “A Mathematical Theory of Cryptography—Case 20878,” contained important antecedents of Shannon’s later work—but it also provided the first-ever proof of a critical
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