I don't think signing digital images is especially useful right not. This is not to say we shouldn't discuss it, or that someone out there won't make some bucks on such a product. Some points: * signing at the digital camera level is the most unlikely, currently, as most digital images are currently made by _scanning_ conventional slides or prints. (Slide scanners are readily available, with >1200 dpi, and even higher, resolution.) * as others have noted, almost all digital images are further massaged in PhotoShop or the like. Even Nick Szabo's "picture distance" idea is of marginal use, as many common filter operations introduce very large picture distances and/or Hamming distance. Determining "alikeness" metrics of an image and a suspected forgery or copyright violation will be a challenge. (Could be a nice little niche business for a "expert witness" on such things, though.) * most ripped-off images are of course done by using scanners. All those "Playboy" and "Penthouse" images you see on the Net are done with scanners. (Both Playboy and Penthouse offer on-line services, so I suppose it's possible some of the bootleg images are in fact digital copies. In these cases, a signature might reveal the copyright violation. A simple Gaussian blur in PhotoShop would of course erase most conceivable signatures. There may be some way to introduce something that can't be easily blurred away, though...) * the most common means of detecting forgeries and copyright violations is of course the human recognition of such images, the recognition that an image came from a well-known source. --Tim May -- .......................................................................... Timothy C. May | Crypto Anarchy: encryption, digital money, tcmay@netcom.com | anonymous networks, digital pseudonyms, zero 408-688-5409 | knowledge, reputations, information markets, W.A.S.T.E.: Aptos, CA | black markets, collapse of governments. Higher Power: 2^756839 | Public Key: PGP and MailSafe available. Note: I put time and money into writing this posting. I hope you enjoy it.