On Dec 14, 2003, at 8:33 PM, Thomas Shaddack wrote:
TEMPEST shielding is fairly esoteric (at least for non-EM-specialists) field. But potentially could be made easier by simplifying the problem.
If we won't want to shield the user interface (eg. we want just a cryptographic processor), we may put the device into a solid metal case without holes, battery-powered, with the seams in the case covered with eg. adhesive copper tape. The input and output can be mediated by fibers, whose ports can be the only holes, fraction of millimeter in diameter, carefully shielded, in the otherwise seamless well-grounded box. There are potential cooling problems, as there are no ventilation holes in the enclosure; this can be alleviated by using one side of the box as a large passive cooler, eventually with an externally mounted fan with separate power supply. If magnetic shielding is required as well, the box could be made of permalloy or other material with similar magnetic properties.
I am not sure how to shield a display. Maybe taking an LCD, bolting it on the shielded box, and cover it with a fine wire mesh and possibly metalized glass? Using LCD with high response time of the individual pixels also dramatically reduces the value of eventual optical emissions.
I worked inside a Faraday cage in a physic lab for several months. And, later, I did experiments in and around Faraday cages. Shielding is fairly easy to measure. (Using portable radios and televisions, or even using the Software-Defined Radio as a low-cost spectrum analyzer.) My advice? Skip all of the nonsense about building special laptops or computers and special displays with mesh grids over the displays. Those who are _casually_ interested will not replace their existing Mac Powerbooks or Dell laptops with this metal box monster. Instead, devise a metal mesh bag that one climbs into to use whichever laptop is of interest. To reduce costs, most of the bag can be metallized fabric that is not mesh, with only part of it being mesh, for breathability. (Perhaps the head region, to minimize claustrophobia and to allow audio and visual communication with others nearby.) I would imagine a durable-enough metallized fabric bag could be constructed for under a few hundred dollars, which is surely cheaper for most to use than designing a custom laptop or desktop. Or consider heads-up LCD glasses. These have been available for PCs and gamers for a few years (longer in more experimental forms, of course, dating back to the VR days of the late 80s). Sony has had a couple of models, and so have others. Some have video resolutions (PAL, NTSC), some have VGA resolutions. Perfectly adequate for displaying crypto results and requesting input. These very probably radiate little. But of course a lightweight hood, a la the above mesh bag, would drop the emissions by some other goodly amount of dB. Experiments necessary, of course. Interface to a laptop or PC could be as you described it, with shielded cables. Or just use a small PC (Poqet, etc.) and move the keyboard and CPU under the draped hood. Leakage out the bottom, hence the earlier proposal for a full bag, like a sleeping bag. --Tim May