Claude Shannon Writes the Communication Theory of Secrecy Systems

Claude Shannon's report, originally issued as a classified document entitled A Mathematical Theory of Cryptography, Memorandum MM 45-110-02, September 1, 1945, was formally published in 1949 as "Communication Theory of Secrecy Systems" in Bell System Technical Journal, 28 (1949)  656–715.  This paper, discussing cryptography from the viewpoint of information theory, contained a proof that all theoretically unbreakable ciphers must have the same requirements as the one-time pad.