Joint block and stream cipher based on a modified skew tent map

Image encryption is very different from that of texts due to the bulk data capacity and the high redundancy of images. Thus, traditional methods are difficult to use for image encryption as their pseudo-random sequences have small space. Chaotic cryptography use chaos theory in specific systems working such as computing algorithms to accomplish dissimilar cryptographic tasks in a cryptosystem with a fast throughput. For higher security, encryption is the approach to guard information and prevent its leakage. In this paper, a hybrid encryption scheme that combines both stream and block ciphering algorithms is proposed in order to achieve the required level of security with the minimum encryption time. This scheme is based on an improved mathematical model to cover the defects in the previous discredited model proposed by Masuda. The proposed chaos-based cryptosystem uses the improved Skew Tent Map (STM) RQ-FSTM as a substitution layer. This map is based on a lookup table to overcome various problems, such as the fixed point, the key space restrictions, and the limitation of mapping between plain text and cipher text. It uses the same map as a generator to change the byte position to achieve the required confusion and diffusion effects. This modification improves the security level of the original STM. The robustness of the proposed cryptosystem is proven by the performance and the security analysis, as well as the high encryption speed. Depending on the results of the security analysis the proposed system has a better dynamic key space than previous ones using STM, a double encryption quality and a better security analysis than others in the literature with speed convenience to real-time applications.

     

Cryptanalyzing an image encryption scheme using reverse 2-dimensional chaotic map and dependent diffusion

In the recent literature, many research studies have proven that Known and Chosen plaintext attacks are very efficient tools that are widely used to cryptanalyze partially or completely some chaos-based and non-chaos cryptosystems. In this paper, we addressed some weaknesses in the first Zhang et al., cryptosystem “An image encryption scheme using reverse 2-dimensional chaotic map and dependent diffusion”. First, we analyzed the encryption process of Zhang et al., and we found that the non-linear diffusion process can be removed because its argument is present in the ciphered image. Then, based on this observation we derived a partial cryptanalysis equation that removes the effect of the diffusion function and accordingly permits to recover the permuted version of the ciphered image. As a result of the previous operation, the brute-force attack became more suitable. In addition, we mounted a chosen plaintext attack based on a proposed chosen plain image. Consequently, the encryption key space is reduced or recovered for one round, also, the average values of NPCR and UCAI randomness parameters become small compared to the optimal values, and moreover, they are very low for specific pixel position attacks.