The Saturday 2005-02-19 at 09:43 -0800, Randall R Schulz wrote:
(*) I still don't know what you mean by an "ordinary" watermark or what varieties of watermarks you know of. As I mentioned earlier, the original meaning of watermark is a mark embedded in the paper itself, not part of the image printed upon it, as is the meaning of the term as appropriated by the computer industry.
It must be a special watermark designed to interfere with the scanner matrix. The document can be scanned, but the quality is bad, and OCR does not work. The design is most probably proprietary. There are also some special fonts designed so that the human eye can read it, but they can't be read by scanning the radio waves emitted by the CRT monitor - believe me, there are businesses very paranoid about such things. For example, PGP for windows support it: Secure Viewer. Select this option to protect the data from TEMPEST attacks upon decryption. If you select this option, the decrypted data is displayed in a special TEMPEST attack prevention font that is unreadable to radiation capturing equipment, and cannot be saved in decrypted format. For more information about TEMPEST attacks, see the section on vulnerabilities in An Introduction to Cryptography. -- Cheers, Carlos Robinson