C.e. Shannon's communication theory of security system is a famous paper on information theory by C.E. Shannon, which was published in Bell Laboratory Technology Journal, Volume 28, No.4 (page 656 ~ 7 15), 1949. In this paper, Shannon further developed the concept and method of information theory established in his classic paper "Mathematical Theory of Communication" published in 1948, and brilliantly expounded the scientific thoughts on the analysis, evaluation and design of cryptographic systems. Some important concepts such as mathematical model, random cipher, pure cipher, perfect secrecy, ideal secrecy system, unique solution distance, theoretical secrecy and actual secrecy are put forward, and five criteria for evaluating secrecy system are put forward, namely, secrecy, number of keys, complexity of encryption operation, error propagation and message expansion. This paper has created a new way to study cryptography with information theory, which has always been valued by cryptography researchers. It is not only an important tool to analyze classical cryptography (such as single table substitution and multi-table substitution), but also a powerful weapon to explore modern cryptography. The combination of the computational complexity theory proposed in this paper and the computational complexity theory in computer theory has become an important standard to evaluate the security of passwords.