Analysis On Some Image Encryption Algorithms Based On Chaos

Chaos is the inherent characteristics and common phenomenon of dynamical systems. Chaotic system is characterized by sensitive dependence on initial conditions and control parameters, pseudo-randomness and unpredictability, which make chaos can be applied to the design of cryptosystem. Recently, a lot of chaos-based image encryption algorithms are proposed. Three of them are analyzed in this paper; moreover, their security flaws and the improvements are also given. The main contains are as follows:A novel image encryption based on a skew tent map is analyzed, and it has two flaws:(1) the first one of the keystream can be easily obtained, which is fixed regardless of the plaintext, once the key is unchanged. (2) The ciphertext reveals much information. Based on these two flaws, a chosen plaintext against this cryptosystem is presented:moreover, some remedial methods are given to enhance the security.An image blocks encryption scheme based on spatiotemporal chaos is analyzed, and there are there flaws:(1) The method of block limits the type of block. (2) The keystream generated from the spatiotemporal chaos system neither depends on the plaintext nor does it depends on the ciphertext. which makes it fixed and reused in encrypting every image. (3) The plaintext is low sensitivity. Based on these flaws, only 6 pairs of plaintext/ciphertext are used for revealing the equivalent keys. The experiment results demonstrate that the attack can successfully decrypt the cipher image. To avoiding the utilizing of the flaws, it needs some improvements.An image scrambling encryption of pixel bit based on chaos map is analyzed, which is pure scrambling algorithm and vulnerable to chosen plaintext attack. To overcome this drawback, some enhancement measures are proposed. The improved scheme transforms drastically the statistical characteristic of the original image and has high gray scrambling degree. Both theoretical analysis and numerical simulation indicate that the improved algorithm not only maintain the merits of the original one. but also can resist some attacks and enhance security.