  Recently, a chaos-based image encryption algorithm using alternate structure
(IEAS) was proposed. This paper focuses on differential cryptanalysis of the
algorithm and finds that some properties of IEAS can support a differential
attack to recover equivalent secret key with a little small number of known
plain-images. Detailed approaches of the cryptanalysis for cryptanalyzing IEAS
of the lower round number are presented and the breaking method can be extended
to the case of higher round number. Both theoretical analysis and experiment
results are provided to support vulnerability of IEAS against differential
attack. In addition, some other security defects of IEAS, including
insensitivity with respect to changes of plain-images and insufficient size of
key space, are also reported.
