  Recently, an image block encryption algorithm was proposed based on some
well-known chaotic maps. The authors claim that the encryption algorithm
achieves enough security level and high encryption speed at the same time. In
this paper, we give a thorough security analysis on the algorithm from the
perspective of modern cryptology and report some critical security defects on
the algorithm. Given five chosen plain-images and the corresponding
cipher-images, the attacker can obtain an equivalent secret key to successfully
decrypt the other cipher-images encrypted with the same secret key. In
addition, each security metric adopted in the security evaluation on the
algorithm is questioned. The drawn lessons are generally applicable to many
other image encryption algorithms.
