【作者】 张哲；

【导师】 禹思敏； 方晓乐；

【作者基本信息】 广东工业大学 ， 控制工程（专业学位）， 2021， 硕士

【摘要】 混沌密码学是一门融合了密码学和混沌理论的交叉学科,早在二十世纪80年代末就引起各方学者的关注,尤其是在当前大数据时代的背景下,数字图像成为信息交换的主要方式,所以各界研究学者在将混沌密码学用于数字图像加密的过程中,更加注重图像加密系统所具备的实用性和安全性。因此,当前各界学者开展对图像混沌加密算法的密码分析工作具有重要的现实意义。本文对两个图像混沌加密算法进行安全分析,发现算法中可能存在的安全缺陷,然后根据分析结果,采取选择明文攻击的方式成功破译了两个加密算法,对两个加密算法提出相关改进意见,其中针对第二种加密算法在第四章具体描述了算法的改进步骤,以抵抗包括选择明文攻击、选择密文攻击在内的多种攻击方式。本文重点工作为:(1)对2014年提出的一种像素位置与比特双重置乱的图像混沌加密算法进行密码分析。该算法是一种新型置乱—替代结构,具体的加密步骤有两大步:首先是对明文图像进行像素位置的全局置乱,获得置乱图像;其次是进行基于比特置乱的像素值替代加密操作,得到最终加密图像。为了测试算法的整体性能,原算法对提出的加密算法进行了实验仿真与统计分析,并从仿真结果和统计分析数据中发现,该算法不仅具有一般“置乱-替代”结构的所具有的性能而且在抵御选择明文攻击和选择密文攻击的方面比一般“置乱-替代”结构所具有的保密性能有明显增强,另外该加密算法在密钥空间、统计特性等方面的得到的仿真参数与原有的“置乱-替代”加密算法相比有较大提升。但是经过仿真发现,即使该算法置乱过程跟明文相关联,但是置乱前后图像统计特征没有发生根本性改变,说明算法无法抵御选择明文攻击。攻击者依然可以利用选择明文攻击进行破译,于是在第三章中通过构建多幅特殊明文图像并将其输入加密算法中来分步得到破译算法所需的等效加密密钥,最后利用等效加密密钥成功还原了明文图像。(2)对2020年提出的基于循环移位和混沌系统的图像加密算法进行安全分析。原算法包括循环移位置乱和异或扩散两个部分,主要特点是根据明文图像的统计特性来决定循环移位的位数。原作者通过加密算法得到加密图像,并对算法进行了安全分析,据此声称算法具有良好的安全性能,但分析原算法后发现算法存在较大的安全漏洞。根据分析结果采用选择明文攻击方法来对原算法进行密码分析,发现不管是加密过程中的循环移位置乱还是异或扩散操作都能破解出等效密钥,然后把获得的等效加密密钥和加密后的图像输入等效解密算法中恢复明文图像,最后提出对原算法的具体改进意见,对改进算法进行相关安全性能测试。在安全性能测试部分列举出了改进后的加密算法与原文加密算法的各项数据对比结果,根据数据对比结果发现改进后加密算法比原加密算法相比各项安全性能得到提高,密钥空间也更大,足以抵抗各类攻击。更多还原

【Abstract】 Chaos cryptography is a cross-discipline that combines cryptography and chaos theory.It has attracted the attention of scholars from all over the world as early as the late 1980 s.Especially in the context of the current era of big data,digital images have become the main method of information exchange.Therefore,researchers from all walks of life pay more attention to the practicability and security of image encryption systems in the process of using chaotic cryptography for digital image encryption.Sex.Therefore,it is of great practical significance for scholars from various parties to carry out the cryptanalysis of the image chaotic encryption algorithm.This paper analyzes the security of two image chaotic encryption algorithms and finds the possible security flaws in the algorithms.Then,based on the analysis results,the two encryption algorithms are successfully deciphered by selecting the plaintext attack method,and specific details are proposed for the second encryption algorithm.Improve the steps to increase the complexity of the algorithm to resist multiple attack methods including selected plaintext attacks and selected ciphertext attacks.The focus of this article is as follows:(1)The cryptanalysis of an image chaotic encryption algorithm proposed in 2014 with pixel position and bit double reset.The algorithm is a new type of scrambling-replacement structure.The specific encryption steps have two major steps: firstly,perform global scrambling of the pixel position of the plaintext image to obtain the scrambled image;secondly,perform pixel value replacement based on bit scrambling Encryption operation to get the final encrypted image.In order to test the overall performance of the algorithm,the original algorithm performed experimental simulation and statistical analysis on the proposed encryption algorithm,and found from the simulation results and statistical analysis data that the algorithm not only has the performance of a general encryption method with a "scrambling-replacement" structure In addition,the protection against selected plaintext attacks and selected ciphertext attacks is significantly stronger than the existing "scrambling-substitution" encryption method.In addition,the encryption algorithm also improves the performance index of the algorithm in other aspects,so it is compared with the original Some "scrambling-replacement" encryption algorithms have been greatly improved in terms of security and practicability.However,through simulation,it is found that although the scrambling process of the algorithm is related to the plaintext,the statistical characteristics of the image before and after the scrambling have not fundamentally changed,indicating that the algorithm cannot resist the plaintext selection attack.The attacker can still use the selective plaintext attack to decipher.By constructing multiple special plaintext images and inputting them into the encryption algorithm,they can obtain the equivalent encryption key required by the decryption algorithm step by step,and use the equivalent encryption key to successfully restore the plaintext image.(2)The security analysis of the image encryption algorithm based on cyclic shift and chaos system proposed in 2020.The original algorithm includes two parts of cyclic shift position chaos and XOR diffusion.The main feature is to determine the number of cyclic shift bits according to the statistical characteristics of the plaintext image.The original author obtained the encrypted image through the encryption algorithm,and carried out a security analysis on the algorithm.According to the analysis results,the original algorithm was cryptanalyzed by selecting the plaintext attack method,and it was found that the equivalent key can be cracked regardless of the cyclic shift in the encryption process or the XOR diffusion operation,and then successfully obtained the equivalent of the original algorithm Key and restore the plaintext image,and finally propose specific improvements to the original algorithm.In the security performance test part,the data comparison results of the improved encryption algorithm and the original encryption algorithm are also listed.According to the data comparison results,it is found that the security performance of the improved encryption algorithm is higher than that of the original encryption algorithm,and the key space It is also larger enough to resist all kinds of attacks.From the comparison of the data before and after and the theoretical analysis,it is not difficult to find that the original algorithm cannot resist the selected plaintext attack,but the security of the improved algorithm is greatly improved.更多还原