Hi, Carlos, On Sunday 20 February 2005 07:07, Carlos E. R. wrote:
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 paper money, you know, Euros, Dollars, etc. Well, euros I don't know, but the one we had before, pesetas, certainly.
Nowhere is the "arms race" aspect more stark than in the design of U.S. currency. The armamentarium now includes microprinting, watermarks (real watermarks), color fibers in the paper, color-shifting inks, embedded security thread. The one thing our currency does not use as an anti-counterfeiting technique is the variant half-tone pattern. That's because currency is not printed using half-tones. If this stuff intrigues you, check out <http://www.moneyfactory.com/newmoney/main.cfm/currency/new20> and <http://www.moneyfactory.com/newmoney/main.cfm/currency/new50>.
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 :-?
Probably not. The reason the technique can be applied to CRTs is that they are particle accelerators and the energy levels they employ are very much higher. They operate by modulating the electron beam at radio frequencies. This in turn produces radio frequency emissions which, if not shielded, are quite easy to detect at a distance and use to reconstruct the display. In comparison, LCD displays are solid state devices operating with low current and low voltage. The illumination source is the highest-power component in those devices and even it consumes far less power than a comparable sized CRT.
...
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.
That's called a Faraday cage.
C'es la vie. Let the better party win. Either that, or off with their heads...
:-)
-- Cheers, Carlos Robinson
Randall Schulz