The security of digital images attracts much attention recently, and many image encryption methods have been proposed. In IS-CAS2000, a new chaotic key-based algorithm (CKBA) for image encryption was proposed. This paper points out CKBA is very weak to the chosen/known-plaintext attack with only one plain-image, and its security to brute-force ciphertext-only attack is overestimated by the authors. That is to say, CKBA is not secure at all from cryptographic viewpoint. Some experiments are made to show the feasibility of the chosen/known-plaintext attack. We also discuss some remedies to the original scheme and their performance, and we find none of them can essentially improve the security of CKBA.
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