Quote:
Originally Posted by ldardini
I think I have misunderstood the terms... while a DRM protected file has some encryption in it, a watermark is a clean plain file with some hidden text inside, like comments in JPG images or pattern in the spacing between words. Have I understood correclty?
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Well, I don't think there is a fixed definition of watermark (in this context), but yes, that is correct. Also, a watermark is anything the author/creator of the ebook may want it to be, and it can be obvious (as a "This book was borrowed by John Doe" text in the title page) or hidden (as the things you mention). But, since there is no standard protocol, format or specification, there's no way to know if a given file has a watermark or not, and any modification you do to a file might remove an existing watermark.
In sum, it is possible to add as many watermarks as you want to an ePUB file, but none is 100% safe, as it can be easily removed or invalidated (intentionally or not). Plain text is easy to find and remove. Whitespaces can be altered if someone decides to edit the file (to correct a typo, for example). Comments or watermarks in image files can disappear if a user wants to convert the images to a different format (to save space, or make it work better with his reader...).