Idea: Simplified TEMPEST-shielded unit (speculative proposal)

[Tim May]

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