Communication Theory of Secrecy Systems is a paper published by Claude Shannon discussing cryptography from the viewpoint of information theory. It is one of the foundational treatments (primitives) of modern cryptography. It is also a proof that all theoretically unbreakable (un-crack-able) ciphers must have the same requirements as the one-time pad encryption algorithm.
Shannon published an earlier version of this research in the classified report A Mathematical Theory of Cryptography, Memorandum MM 45-110-02, Sept. 1, 1945, Bell Laboratories.
In 1948, Shannon published his great paper "A Mathematical Theory of Communication" which is considered one of the founding works of the field of information theory.
Related pages
Confusion and diffusion
Product cipher
References
Shannon, Claude. "Communication Theory of Secrecy Systems", Bell System Technical Journal, vol.28(4), page 656–715, 1949.