In 1948, Claude E. Shannon published the paper A Mathematical Theory of Communication, which is seen as the foundation of modern information theory.
In 1949, Shannon published Communication Theory of Secrecy Systems which relates cryptography to information theory, and should be seen as the foundation of modern cryptography.
Both papers derive from a technical report, A Mathematical Theory of Cryptography, written by Shannon in 1945. In this report, Shannon defined, and mathematically proved, perfect secrecy.