Digital watermarks have recently been proposed for authentication of both video data and still images and for integrity verification of visual multimedia. In such applications, the watermark 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 functions that return the same bit-string for a whole class of images derived from an original image using common processing operations. It is further required that two completely different images produce completely different bit-strings. In this paper, we discuss methods how such robust hash functions can be built. We describe an algorithm and evaluate its performance. We also show how the hash bits As another application, the robust image digest can be used as a search index for an efficient image database search