  This paper studies the security of a recently-proposed chaos-based image
encryption scheme, and points out the following problems: 1) there exist a
number of invalid keys and weak keys, and some keys are partially equivalent
for encryption/decryption; 2) given one chosen plain-image, a subkey $K_{10}$
can be guessed with a smaller computational complexity than that of the simple
brute-force attack; 3) given at most 128 chosen plain-images, a
chosen-plaintext attack can possibly break the following part of the secret
key: $\{K_i\bmod 128\}_{i=4}^{10}$, which works very well when $K_{10}$ is not
too large; 4) when $K_{10}$ is relatively small, a known-plaintext attack can
be carried out with only one known plain-image to recover some visual
information of any other plain-images encrypted by the same key.
