Cryptanalysis of a Multiround Image Encryption Algorithm Based on 6D Self-Synchronizing Chaotic Stream Cipher.

Recently, a chaotic secure video communication system based on FPGA is proposed, which essentially encrypts the original image data of each frame in the video in multiple rounds by using the 6D self-synchronizing chaotic stream cipher. In this paper, the security performance of the multiround encryption algorithm is analyzed based on divide-and-conquer attack. In the case of one round encryption, the keys ψ i j (1 ≤ i ≤ 3 , 1 ≤ j ≤ 6) can be deciphered by setting 30 appropriate initial conditions according to known-plaintext attack. After that, according to chosen-ciphertext attack, the chaotic iterative equation of the decryption end is degenerated into a linear one if ciphertexts are all set to zero. Under this condition, the equations between the unknown keys and the plaintext-ciphertext pairs can be obtained by setting 24 appropriate initial conditions, and the keys ψ i j (4 ≤ i ≤ 6 , 4 ≤ j ≤ 6) can be deciphered by solving the equations in Mathematica. Secondly, in order to reduce the multiplication times of the unknown keys in the case of two-round encryption, the number of pixels in an image can be selected as little as possible. According to chosen-ciphertext attack, when the number of the pixels in an image is only one, the keys ψ i j (1 ≤ i ≤ 3 , 1 ≤ j ≤ 6) can be deciphered by setting 30 appropriate initial conditions. Then, when the number of pixels in an image is two, the keys ψ i j (4 ≤ i ≤ 6 , 4 ≤ j ≤ 6) can be deciphered by setting 24 appropriate initial conditions. However, in the case that the encryption round is three or more, the keys cannot be deciphered by the attack method proposed in this paper. The analysis results show that the multiround image encryption algorithm is not secure when the number of encryption rounds is one or two. [ABSTRACT FROM AUTHOR]