Cryptanalyses of Some Multimedia Encryption Schemes

Since early 1990s, chaos has been widely investigated to construct multimedia encryption scheme for its good cryptography-like characteristics, such as the ergodicity, mixing and exactness property and the sensitivity to initial conditions. This thesis is concerned with the cryptanalyses of some recently-proposed chaos related multimedia encryption schemes. The security of the schemes against some familiar attack methods, such as brute-force attack, known/chosen-plaintext attack, is investigated in detail with theoretical analyses and experimental results. The main achievements are as follows: 1. Based on a normalized encryption/decryption model, from a general perspective this thesis analyzes the security of permutation-only multimedia ciphers. It is pointed out that all permutation-only image ciphers are insecure against known/chosen-plaintext attacks in the sense that only O (logL(MN)) known/chosen plain-images are enough to break the ciphers, where MN is the size of the image and L is the number of all possible different pixel values. Also, it is found that the attack complexity is only O(n ·(MN)), where n is the number of known/chosen plain-images used. A recently proposed permutation-only image cipher called hierarchical chaotic image encryption (HCIE) is served as a concretized example to show how the attack work. Experiments are shown to verify the feasibility of the known/chosen-plaintext attacks. 2. The security of a recently proposed chaos-based image encryption scheme called RCES (also called RSES) was analyzed and we found that it can be broken with only one or two known/chosen-plaintexts. In addition, the security of RCES against the brute-force attack was overestimated. Both theoretical and experimental analyses are given to show the performance of the suggested known/chosen-plaintext attacks. 3. This thesis analyzes the security of a new multistage encryption system (MES) recently proposed in ISCAS’2004. It is found that MES is insecure against a differential chosen-plaintext/ciphertext attack. Experiments are given to support the proposed attack. It is also pointed out that the security of MES against brute-force attacks is not sufficiently high. 4. This thesis analyzes the security of a new domino signal encryption algorithm (DSEA), and points out the following weaknesses: 1) its security against the brute-force attack was overestimated; 2) it is not sufficiently secure against ciphertext-only attacks, and only one ciphertext is enough to get some information about the plaintext and to break the value of a sub-key; 3) it is