Visual hash for oblivious watermarking

Digital watermarks have recently been proposed for authentication and fingerprinting of both video data and still images and for integrity verification of visual multimedia. In such applications, the watermark must be oblivious and has to depend on a secret key and on the original image. It is important that the dependence on the key be sensitive, while the dependence on the image be continuous (robust). Both requirements can be satisfied using special image digest (hash) functions that return the same bit-string for a whole class of images derived from an original image using common processing operations including rotation and scaling. It is further required that two completely different images produce completely different bit-strings. In this paper, we extend our previous work on robust image digest functions describing ideas how to make the hash function independent of image orientation and size. The robust image digest can be clearly utilized for other applications, such as a search index for an efficient image database search.