Protocols for Watermark Veriication

Digital watermarks have recently been proposed for the purpose of copy deterrence of multimedia content. Copy deterrence using digital watermarks is achieved by inserting a unique watermark into each copy of the image sold which could be used to trace unauthorized copies to the erring buyer. However, all the schemes proposed in the literature suuer from the following major limitation which is generally glossed over. The owner of the image reveals the embedded watermark to the buyer or to a \trusted" third party in the process of proving that a pirated copy originated from a speciic buyer. Needless to say that the watermark could subsequently be removed from the watermarked image by the buyer or the \trusted" third party. This leads to the same fundamental problem viz., the buyer or the \trusted" third party could now sell illegal copies with complete impunity. While this limitation appears to be inherent, it can actually be eliminated by using appropriate cryptographic protocols. In this paper, we present a novel way of demonstrating the presence of a watermark in an image without revealing knowledge about the watermark to the other party, thereby eliminating the possibility of subsequent watermark removal.