Breaking a novel colour image encryption algorithm based on chaos

Recently, a colour image encryption algorithm based on chaos was proposed by cascading two position permutation operations and one substitution operation, which are all determined by some pseudo-random number sequences generated by iterating the logistic map. This paper evaluates the security level of this encryption algorithm and finds that the position permutation-only part and the substitution part can be separately broken with only ⌈(log2(3MN))/8⌉ and 2 chosen plain-images, respectively, where MN is the size of the plain-image. The effectiveness of the proposed chosen-plaintext attack is supported by concise theoretical analyses, and is verified by experimental results.