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 K10 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: fenced(Ki mod 128)i = 410, which works very well when K10 is not too large; (4) when K10 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. © 2009 Elsevier B.V. All rights reserved.