The original classified memo for Bell Labs that was republished in 1949 as ["Communication Theory of Secrecy Systems"](communication-theory-of-secrecy-systems.pdf).
Shannon said that his wartime insights into communication theory and cryptography developed simultaneously and that "they were so close together you couldn’t separate them". ["Communication Theory of Secrecy Systems"](communication-theory-of-secrecy-systems.pdf) incorporates many concepts and formulations that also appeared in his 1948 paper.