There is no need to sign the picture itself: a secure time stamp would prove ownership of the oldest version of a picture: CryptoHashA(picture) --> H_i Then the timestamp service: CryptoHashB(H_i, H_j) --> H_receipt ; H_j is hash from another customer The H_receipt is published in a well-known place/time and is traceable to the picture. The actual scheme the Bellcore folks worked out is tree structured for efficiency reasons. An altered version of a photo would presumably have a different hash, but it would not have an earlier timestamp hash. Insofar as one could recognize that an image was incorporated into another altered image, misappropriated and stolen copyrights could be detected. Recognition is inherent in this process. This isn't very different from sound sampling: if the alteration completely obscures the orignal beyond recognition, then there isn't much of a case to prove theft. For a picture example, I could digitize a copyrighted image and use *only* it's color palette on one of my pictures (I haven't tried this yet, but it sounds like a good "empircal color theory"). BTW, Anyone have an email addr of the Bellcore folks who invented the secure timestamp system? I'd like to use the service if it is still operational. Paul E. Baclace peb@procase.com