  In this paper, we propose to improve the security performance of a recently
proposed color image encryption algorithm which is based on multi-chaotic
systems. The existing cryptosystem employed a pixel-chaotic-shuffle mechanism
to encrypt images, in which the generation of shuffling sequences are
independent to the plain-image/cipher-image. As a result, it fails to the
chosen-plaintext and known-plaintext attacks. Moreover, the statistical
features of the cryptosystem are not up to the standard. Therefore, the
security improvements are framed to make the above attacks infeasible and
enhance the statistical features as well. It is achieved by modifying the
pixel-chaotic-shuffle mechanism and adding a new pixel-chaotic-diffusion
mechanism to it. The keys for diffusion of pixels are extracted from the same
chaotic sequences generated in the previous stage. The simulation analyses and
studies are performed to demonstrate that the updated version of cryptosystem
has better statistical features and resistant to the chosen-plaintext and
known-plaintext attacks than the existing algorithm.
