Cryptanalysis of correlation based watermark using single copy

In invisible digital watermarking, given an image I, a signal s/sub i/ is added, which produces a watermarked copy I/sup (i)/ = I + s/sup (i)/, such that I, I/sup (i)/ are visually indistinguishable. The addition means the element wise addition in the image matrix. This image I/sup (i)/ is given to the i-th buyer. In the detection stage, the available image (may be attacked using image processing. or cryptanalytic techniques) I/sup (#)/ is compared to the original image I and a signal s/sup (#)/ = I/sup (#)/ - I is recovered. The buyer i is suspected if s/sup (i)/ possesses significant correlation with s/sup (#)/. For successful cryptanalysis, one has to mount an attack to construct I/sup (#)/ from I/sup (i)/ (both visually indistinguishable) such that there is no significant correlation between s/sup (#)/ = I/sup (#)/ - I and s/sup (i)/. Thus, the buyer i will not be identified. To the attacker, only I/sup (i)/ is available, but I, s/sup (i)/ are not known. Thus there is no facility for the attacker to test the correlation between s/sup (i)/ and s/sup (#)/. However, the attacker needs to be convinced indirectly that the correlation between s/sup (i)/ and s/sup (#)/ has been removed. We use the CKLS scheme to demonstrate the success of the attack. The paradigm of attack needs only a single copy in contrast to the collusion attack, which needs more than one copy.