Abstract: With the development of computing power and cryptanalysis technology, unconditional security of cryptosystem is more and more important. This study presents a review of the literature on the unconditional security of cryptosystem. One-time pad and most quantum key distribution protocols were proved to be unconditionally secure. The development and extension of unconditional security is represented. As unconditional security is strict and hard to reach, some moderate security ideas are proposed and an outlook is given.