关键词: 五维超混沌系统, 选择明文攻击, 图像加密, SHA-256哈希值

Abstract: A five dimensional hyperchaotic system is firstly constructed based on the existing four-dimensional hyperchaotic system. A digital image encryption algorithm based on hyper chaotic sequence and shift cipher is analyzed. By applying chosen plaintext attacks, it shows that all the equivalent keys can be revealed, which can be exploited to reveal the plain image. Two improvements are made to overcome the flaws of the image encryption scheme analyzed. As one improvement, the newly constructed five dimensional hyperchaotic system is employed in encryption algorithm to obtain larger key space and enhance the security. Another improvement is that the initial value of the chaotic system is related to the SHA-256 Hash value of the plain text image, so that the key stream is related to the plain text image, which can achieve the effect of “one -time pad”. Security analyses including key sensitivity analysis, key space analysis, statistical analysis, differential analysis, information entropy analysis and anti cutting, anti noise, anti compression experimental analysis are performed. Compared to the original algorithm, all the experimental results demonstrate that the proposed image encryption scheme shows better security and effectiveness.

Key words: five dimensional hyperchaotic system, chosen plain text attack, image encryption, SHA-256 Hash value