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

摘要： 在已有的四维超混沌系统的基础上构造了一个五维超混沌系统，对一种结合超混沌序列和移位密码的数字图像加密算法进行了分析，通过选择明文攻击的方法，成功破解了该算法中用于像素位置置乱以及像素值扩散和混淆的等效密钥，从而能利用破解的等效密钥解密出目标明文。为此对该算法进行了两个方面的改进。一个改进是加密系统中使用了新构造的五维超混沌系统，使得改进算法的密钥空间更大，进一步提高了安全性。另一个改进是设计混沌系统的初始值与明文图像的SHA-256哈希值有关，从而使得密钥流与明文图像相关，达到“一次一密”的效果。密钥空间分析、密钥敏感性分析、统计分析、信息熵分析、差分攻击分析、抗剪切、抗噪声、抗压缩分析等实验结果表明，改进后的图像加密算法比原加密算法更加安全有效。

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