  Recently, a novel image encryption scheme based on improved hyperchaotic
sequences was proposed. A pseudo-random number sequence, generated by a
hyper-chaos system, is used to determine two involved encryption functions,
bitwise exclusive or (XOR) operation and modulo addition. It was reported that
the scheme can be broken with some pairs of chosen plain-images and the
corresponding cipherimages. This paper re-evaluates the security of the
encryption scheme and finds that the encryption scheme can be broken with only
one known plain-image. The performance of the known-plaintext attack, in terms
of success probability and computation load, become even much better when two
known plain-images are available. In addition, security defects on
insensitivity of the encryption result with respect to changes of secret key
and plain-image are also reported.
