Method for protecting content using watermarking

DETAILED DESCRIPTION It is the principal object of the invention to describe a digital watermarking scheme wherein the watermark is robust against collusion by multiple individuals who each possess a watermarked copy of the data.
It is another object to describe such a scheme wherein the watermark cannot be removed by an adversary who obtains multiple copies of the original work.
It is a more general object of the invention to describe a watermarking method that is secure against any form of attack including, without limitation, averaging attacks.
It is still a further object of the invention to describe a watermarking procedure wherein each of a set of copies of the work has a slightly-modified form of a "baseline" watermark that is placed within a critical region of the data.
The slight variations in the watermarks, however, are not perceptually visible and do not interfere with the works.
If multiple persons collude to attempt to create an "illicit" copy of the work (i.
e.
, a copy without a watermark), however, at least one of the modified watermarks is present in the copy, thereby identifying both the illicit copy and the copier.
It is still thus another object to describe a watermarking scheme of the type recited above wherein combining copies of the same data set does not destroy the watermark.
It is a further object of the invention to describe such a watermarking scheme that may be used to identify one or more of the parties who are colluding to destroy the watermark.
It is another more general object of the invention to describe a digital watermarking process that may be used as evidence in a Court because it is robust against collusion.
According to the preferred embodiment of the invention, the work to be protected is digitized into a data file or string of data.
A first digital watermark is then inserted in a first copy of the data file, preferably in a critical region of the data.
A "critical" region may consist of the entire document or alternatively will be some valuable portion of the work that will end up being significantly corrupted if the watermark is corrupted