Cryptanalysis of a spatiotemporal chaotic image/video cryptosystem

This Letter proposes two different attacks on a recently proposed chaotic cryptosystem for images and videos in [S. Lian, Chaos Solitons Fractals (2007), doi: 10.1016/j.chaos.2007.10.054]. The cryptosystem under study displays weakness in the generation of the keystream. The encryption is made by generating a keystream mixed with blocks generated from the plaintext and the ciphertext in a CBC mode design. The so obtained keystream remains unchanged for every encryption procedure. Guessing the keystream leads to guessing the key. Two possible attacks are then able to break the whole cryptosystem based on this drawback in generating the keystream. We propose also to change the description of the cryptosystem to be robust against the described attacks by making it in a PCBC mode design.