  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 the
encryption algorithm and finds that the position permutation-only part and the
substitution part can be separately broken with only $\lceil (\log_2(3MN))/8
\rceil$ and 2 chosen plain-images, respectively, where $MN$ is the size of the
plain-image. Concise theoretical analyses are provided to support the
chosen-plaintext attack, which are verified by experimental results also.
