At 04:54 PM 2/9/98 +0000, Markus Kuhn wrote:
The horizontal and vertical deflection coils produce a lot of radiation at harmonics of the line and frame rate of your CRT, but this signal energy is not related to your screen content (only to your video mode), and therefore not of much concern for the eavesdropper. The low-radion monitor standards look only at those signal (<400 kHz). Therefore having a TCO92 monitor provides you absolutely no advantage with respect to eavesdropping.
The information carrying signals of VDUs are in much higher frequency ranges in the VHF/UHF bands. Laptops are pretty good broadcasters there, too.
One of the issues is that the fast rise-times on signals yields emissions all over the spectrum, not just at the base scanning rate. Thus, even though you're sending at e.g., 2400 bps, you've got an N-volt voltage/current swing accomplished in fractions of a microsecond. These higher harmonics radiate better than the lower ones. See Peter Smulders's paper on RS-232 interception, abstract included below. I have a local copy at http://rattler.otc.net/crypto/docs/rs232.pdf The Threat of Information Theft by Reception of Electromagnetic Radiation from RS-232 Cables Peter Smulders Eindhoven University of Technology, Department of Electrical Engineering, Eindhoven, The Netherlands Research into the possibility of picking up the electromagnetic radiation originating from video display units (VDUs) made clear that this type of information theft can be committed very easily [1]. It is not only this type of equipment which is vulnerable to interception at a distance; experiments on eavesdropping RS-232 cable signals prove that it is possible in some cases to intercept data signals running along an RS-232 cable, by picking up and decoding the electromagnetic radiation produced by the cable. This report gives the results of these Electromagnetic radiation arising from RS-232 cables may contain information which is related to the original RS-232 data signals. The seriousness of eavesdropping risks is shown by estimates of bit error rates feasible with a standard radio receiver as a function of the separation distance. In addition to this, results of experimental eavesdropping are presented. Keywords: RS-232 cable, Electromagnetic radiation, Eaves-dropping risks. Caution: Failure to detect intelligible emanations by the methods described in this paper do not mean an installation is secure against interception by sophisticated and resourceful opponents. ------------------------------------------------------------ David Honig Orbit Technology honig@otc.net Intaanetto Jigyoubu Lewinsky for President '2012