On Fri, Mar 07, 2003 at 10:46:06PM -0800, Bill Frantz wrote: > > The next more complex version sends the same random screen over and over in > sync with the monitor. Even more complex versions change the random screen > every-so-often to try to frustrate recovering the differences between > screens of data on the monitor. >
Five or six years ago I floated the suggestion that one could do worse than phase lock all the video dot clock oscillators in a computer room or office to the same master timing source. This would make it significantly harder to recover one specific monitor's image by averaging techniques as the interference from nearby monitors would have exactly the same timing and would not average out as it does in the more typical case where each monitor is driven from a video board with a slightly different frequency dot clock (due to aging and manufacturing tolerances). Modifying existing video boards to support such master timing references is possible, but not completely trivial - but would cost manufacturers very little if it was designed in in the first place. And of course one could "improve" the shielding on the monitor with the dummy unimportant data so it radiated 10 or 20 db more energy than the sensitive information monitor next to it. In many cases this might involve little more than scraping off some conductive paint or removing the ground on a cable shield. I am sure that it would take little effort with a spectrum analyzer and some hand tools to defeat most of the EMI suppression in many monitors and whilst this would not be entirely legal under FCC rules (at least for a manufacturer or dealer) it probably would be closer to legal than deliberately creating rf interference with an intentionally radiating jammer. I imagine, however, that the usefulness of the RF radiated by a modern TFT flat panel display fed with DVI digital video is already much less as there is no serial stream of analog pixel by pixel video energy at any point in such an environment. Most TFTs do one entire row or column of the display at a time in parallel which does not yield an easily separated stream of individual pixel energy. Thus extracting anything resembling an image would seem very difficult. So perhaps the era of the simplest to exploit TEMPEST threats is ending as both optical and rf TEMPEST is much easier with raster scan pixel at a time CRT displays than it is with modern more parallel flat panel display designs. -- Dave Emery N1PRE, [EMAIL PROTECTED] DIE Consulting, Weston, Mass. PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2 5D 27 BD B0 24 88 C3 18 --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]