An improved chaotic cryptosystem with external key

In recent years, external key was introduced to chaotic cryptography by Pareek et al. and find its application in several discrete chaotic cryptosystems. The first part of this paper is devoted to the analysis of their essential weaknesses as well as some redundancies that contribute little to the security of those cryptosystems. Then, an improved scheme with all existing deficiencies and redundancies eliminated, is proposed. Theoretic analysis and numerical simulation both verify its superiority and security.