Communication Theory of Secrecy Systems

The paper serves as the foundation of secret-key cryptography, including the work of Horst Feistel, the Data Encryption Standard (DES), Advanced Encryption Standard (AES), and more. In the paper, Shannon defined unicity distance, and the principles of confusion and diffusion, which are key to a secure cipher.

Shannon published an earlier version of this research in the formerly classified report A Mathematical Theory of Cryptography, Memorandum MM 45-110-02, Sept. 1, 1945, Bell Laboratories. This report also precedes the publication of his "A Mathematical Theory of Communication", which appeared in 1948.