基于快速置换和可选择像素扩散的医疗图像加密算法的安全性分析

Cryptanalysis of Medical Image Encryption Algorithm Using High-speed Scrambling and Pixel Adaptive Diffusion

图像加密算法的安全性是最基本和最重要的。医疗图像加密是保护患者隐私的一种手段,分析医疗图像加密算法的安全性,对设计医疗图像加密算法、增强算法的安全性和促进医疗图像加密算法的应用非具有常重要的意义。最近,Hua等提出了一种基于快速置换和可选择像素扩散的医疗图像加密方案。加密方案的一个关键操作是在图像的四周插入随机值,然后通过置乱使得随机值分散到整幅图像,最后通过扩散混乱等操作加密整幅图像。每次加密都会产生不同的随机值,即使加密相同的图像,每次加密得到的密文也不一样,这就保证了“一次一密”的加密效果。文中采用差分分析和选择密文攻击,从理论上详细地分析了Hua等提出的算法。首先分析解密过程,通过差分分析构造明文-密文的线性关系,并根据构造的线性关系建立密码本;然后使用密码本攻击便可破解该算法。密码本的大小与图像尺寸相关,若密文图像的尺寸为M×N,则构造的密码本包含(M×N+1)个明文-密文对。仿真实验验证了理论分析的正确性。为了提高该算法的安全性,抵抗文中提出的密码本攻击,进一步提出了一种基于差分分析的改进方案。该方案引入了与明文相关的置换矩阵。仿真实验结果和统计分析结果表明,改进方案不仅继承了原算法的优点,而且具有很好的抗差分攻击能力。

Security is essential and important for every image encryption algorithm.Medical image encryption is a means to protect patients’privacy.Analyzing the security of medical image encryption algorithm is very meaningful for the design of medical image encryption algorithm,enhancing the security of algorithm and promoting the application of medical image encryption algorithm.Recently,Hua et al.proposed a medical image encryption algorithm using high-speed scrambling and pixel adaptive diffusion.The key operation of the scheme is insertion of a random sequence around an image,then the random values are dispersed to the whole image by scrambling,finally,the whole image is scrambled by diffusion.Because different random values are generated in each encryption,even for one unchanged image,the cipher-image is different in every encryption such that Hua et al’s scheme is similar to one time one pad system.In this paper,the security of the algorithm was analyzed by differential cryptanalysis and chosen ciphertext attack in detail.The decryption process is analyzed theoretically by differential cryptanalysis and linear relationship is constructed between plain-images and cipher-images.Based on the linear relationship,a codebook is established,and the codebook attack breaks Hua et al’s algorithm.The size of the codebook is determined by the size of the cipher-image.If the size of the cipher-image is,the constructed codebook contains pairs of plain-image/cipher-image.The experimental results verify the theoretical analysis.To improve the security of Hua et al’s algorithm and to resist the differential cryptanalysis,an improved scheme was proposed.In the improved scheme,plaintext-related permutation matrices are introduced.The simulation and statistical results show that the improved scheme not only inherits the advantages of the original algorithm,but also resist the differential cryptanalysis and the codebook attack.