The Saturday 2005-02-19 at 18:11 -0800, Randall R Schulz wrote:
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.
Yes, I was wondering if that was the intent. It's common nowadays (at least here in the States) for financial institutions to issue checks that have include in their background some text (typically the words "VOID" or some such) delineated in varying halftone patterns that are virtually invisible to the human eye but that become quite stark when reproduced by a photocopier.
¡Ah! Here there has been cases with paer money, you know, Euros, Dollars, etc. Well, euros I don't know, but the one we had before, pesetas, certainly.
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:
Yup. I've heard of this as well. However, in this case the point is to make it difficult to "view" a CRT display by detecting its radio-frequency emissions.
Correct. I wonder if TFT displays are vulnerable :-?
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.
It's an arms race. Every attempt to obscure information is met with a counter-measure to assist its detection. And vice versa.
Offices can have a grounded wire mesh inside walls, and glass windows a thin metalissed film, to stop radiation going out - or in. I know they had such a thing, because mobile phones - I think you call them something different, what was it... ah, cellulars - did not work inside.
C'es la vie. Let the better party win. Either that, or off with their heads...
:-) -- Cheers, Carlos Robinson