Cryptanalysis of a spatiotemporal chaotic image/video cryptosystem and its improved version

Recently, a spatiotemporal chaotic image/video cryptosystem was proposed by Lian. Shortly after its publication, Rhouma et al. proposed two attacks on the cryptosystem. They as well introduced an improved cryptosystem which is more secured under attacks (R. Rhouma, S. Belghith, Phys. Lett. A 372 (2008) 5790) [29]. This Letter re-examines securities of Lian's cryptosystem and its improved version, by showing that not all details of the ciphered image of Lian's cryptosystem can be recovered by Rhouma et al.'s attacks due to the incorrectly recovered part of the sign-bits of the AC coefficients with an inappropriately chosen image. As a result, modifications of Rhouma et al.'s attacks are proposed in order to recover the ciphered image of Lian's cryptosystem completely; then based on the modifications, two new attacks are proposed to break the improved version of Lian's cryptosystem. Finally, experimental results illustrate the validity of our analysis.