Adam Shostack <adam@homeport.org> wrote.. Creating watermarks that can't be removed without degrading image quality is not especially difficult. The two tricky bits are durability and collusion protection.
I don't know how the scheme in question works, but the general way in which these kind of watermarks work is the use of spread spectrum coding. So long as you choose a long enough spreading code, you can survive most of the information being destroyed in a transform and still recover the information. If a particular transformation (e.g. JPEG coding) stops you from doing this, then you pick multiple orthogonal codes (interested readers who want to hit the text books could try the key words "Gold Codes") and encode after each transform. So, you encode your original TIFF then covert it to a JPEG and encode again with an orthogonal code from the same family. Since the two codes don't interfere with each other, you can recover the watermark however you are viewing the picture. One particular transformation that a long enough code can withstand is scanning a magazine picture. So, you could watermark your picture of the Eiffel Tower before selling it to a magazine. Then, when it pops up on the Net because someone's scanned it in you can prove that it was your picture. What use this is I don't know, since I can't see how you can prove who scanned it! Incidentally, spread spectrum means that you can put several signals into the same bandwidth and recover each of them (if you have all of the codes). Short codes are used to protect against interference (the NCR wireless LAN range uses an 11 bit code, for example) while very long codes are used to recover data in situations where the signal falls below the noise level (I think I remember reading that the Voyager spacecraft used 252,000 bit codes). I hope someone, somewhere finds this interesting... ----------------------------------------------------------------- David G.W. Birch 8 Frederick Sanger Road Director, Hyperion Surrey Research Park Tel: +44 (0)1483 301793 Guildford Fax: +44 (0)1483 561657 Surrey GU2 5YD, UK Where people, networks and money intersect.......Consult Hyperion http://www.hyperion.co.uk daveb@hyperion.co.uk