Cryptanalysis Of Some Digital Chaotic Image Encryption Schemes

Some fundamental features of the chaotic dynamical systems such as ergodicity,mixing property, sensitivity to initial conditions/system parameter can be consid-ered analogous to some ideal cryptographic properties such as confusion, difusionand avalanche properties. The low efciency of the tranditional text encryptionschemes in protecting image data and the increasing security demand of image dataattract many researchers’ attention to use digital chaotic systems to design new im-age ciphers in the past decades. According to the citation report provided by Webof Science, at least one thousand papers on designing chaos-based image encryp-tion scheme have been published in the past fifteen years, while no more than onehundred of them have been cryptanalyzed successfully.This thesis focuses on evaluating the capability of three recently-proposedchaos-based image encryption schemes against several familiar attack methods, suchas known/chosen-plaintext attack, diferential attack. All of them are supported byboth theoretical analysis and experimental results. Then the author tries to givesome advices for designing chaos-based image encryption scheme. The main achieve-ments of the thesis are as follows:1. Based on the strategy of Divide and Conquer, a new chaos-based colour imageencryption scheme proposed by Wang et al. has been analyzed in detail. Thisthesis evaluates its security and finds that a few pairs of chosen plain-image andthe corresponding cipher-image are sufcient to reveal the equivalent secret keyused for permutation and difusion. Both theoretical analysis and experimentalresults are given to show the feasibility of the proposed chosen-plaintext attack.2. This thesis analyzes the security of a chaotic image encryption algorithm basedon perceptron model and finds that the equivalent secret key can be recon-structed with only one pair of known-plaintext and the corresponding cipher-text, which is supported by both mathematical proof and experimental results.In addition, some other security defects, such as low sensitivity with respect tochange of plain-image and insufcient randomness of the used PRBS, are alsoreported. 3. A comprehensive analysis on the security of a chaos-based image encryptionalgorithm using alternate structure (IEAS) is presented. The following facts arefound:(a) some properties of IEAS can support a diferential attack to recoverthe equivalent secret key with a small number of chosen plain-images when theparameter used for difusion is even;(b) the size of the key space is not largeenough;(c) the cipher-image is insensitive to changes of plain-image;(d) IEASis much weaker than DES with respect to encryption complexity and robustnessagainst diferential attack. Both theoretical analysis and experimental resultsare provided to support vulnerability of IEAS against the proposed attack.4. Based on the above security analysis, some advices for designing chaos-basedimage encryption scheme are given.