How to digitally watermark

[Dave.Birch@eworld.com]

>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...


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