Abstract: Arnold Cat transformation is a classical algorithm of image scrambling. But its periodicity restrains the times of image scrambling, so the number of the keys is not enough. This paper presented a modified Arnold method, which uses PN sequence and secure hash algorithm to generate a random parameter sequence, then divides an image into 4 pieces and carries out Arnold algorithm to the 4 pieces respectively. The Arnold algorithm effectively increases the number of the keys, and overcomes the attack via the exhaustive analysis, which enhances the security.

Key words: Image encryption, PN sequence, SHAT , Arnold Cat transformation, Matrix transformation