Chengrui Zhang

Permutation is a commonly used operation in image encryption schemes. Many schemes adopt a separate row and column permutation to increase the encryption efficiency while ignoring the security. This paper proposes an efficient known-plaintext attack (KPA) for row-column-based permutation. A special ID method based on the Pearson correlation coefficient is proposed to label each column or row. Only one pair of known plaintext-ciphertext images is enough to determine the permutation rules completely. With this attack method for permutation, a classic permutation-then-XOR image encryption scheme is broken with only two known plaintext-ciphertext image pairs. A numerical example of the proposed attack is provided to show the process intuitively and clearly. In addition, the cryptanalyses of two related image ciphers are introduced. Detailed theoretical derivations and extensive experiments confirmed the feasibility and efficiency of the proposed attacks. The related MATLAB codes are publicly available and can be found online.

Recently, an image encryption scheme with new chaotic signals has been proposed. Some limitations of this scheme will be pointed out. It adopts a “diffusion–permutation” architecture. The diffusion encryption is linear and can be simplified. Bit-level permutation significantly decreases the encryption efficiency. To solve the above problems, an improved scheme is proposed. The new scheme employs a Feistel-like network with four rounds of encryption. The dynamic substitution box (S-box) generated by Fisher–Yates permutation increases the nonlinearity of encryption. The optimized diffusion process is applied to obtain the avalanche effect. The experimental results show that the improved scheme performs well. In addition, the resistance against various attacks is analyzed from the perspective of a cryptanalyst.

In recent years, a number of image encryption schemes based on DNA coding and nonlinear dynamics have been proposed. Generally, these DNA-based schemes first encode plaintext images into DNA sequences and then encrypt them with pseudorandom elements produced by chaotic systems or other nonlinear dynamics. Although ciphertexts can pass some security tests, many image encryption schemes are being shown to have intrinsic flaws and that they cannot guarantee a high level of security. In this paper, we cryptanalyze a family of image encryption schemes for which the encryption kernel is DNA coding or its variant. The complex DNA operation can be simplified as a substitution box (S-box). The whole cryptosystem's security level is thus significantly decreased and is vulnerable to the chosen-plaintext attack. Applications of this concept to break five ciphers are theoretically presented and experimentally verified. In addition, some suggestions for resisting similar attacks are also given in this paper.