[4] It is also a proof that all theoretically unbreakable ciphers must have the same requirements as the one-time pad.
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.
[5] In the paper, Shannon defined unicity distance, and the principles of confusion and diffusion, which are key to a secure cipher.
[6] 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.
[7][8] This report also precedes the publication of his "A Mathematical Theory of Communication", which appeared in 1948.