At 1:01 AM 8/16/96, Alan Horowitz wrote:

I repeat. Generations of sweating engineers have discovered and confirmed that there is not an easy, "Silver Bullet" cure of the canonical problem of shielding electromagnetic energy from reaching someone who knows how to interpret it.

Bill, you are welcome to look at a layer of tin foil and give a sigh of relief that you've shielded your gun or your crypto diskette or your private body parts feom someone who knows what they're doing. Go ahead, chant a mantra too, if it makes you feel better.+-

I've stayed out of this "tin foil" debate, but some basic physics is being missed, or misused. The invocation of TEMPEST and leakage of RF does not say much of anything about _imaging_. If a gun, for example, were _radiating_ RF energy of some particular sort, then it might indeed be true that tin foil/gold lame shielding would not stop _all_ of the emitted radiation, and that sensitive enough detectors might detect the characteristic signal 60 or 90 dB down. Maybe. But a gun is not a radiator, it is at most a _reflector_ of RF energy. Thus, the TEMPEST invocation is misleading. The signal would perhaps be down by 100-130 dB or more, as the "leakage" must first get around the shielding, be reflected, and then get back around the shielding. And what this does to "imaging" almost needs no explanation. What leaks around the periphery or through holes in a shielded container will provide essentially zero spatial information about the configuration of sources and reflectors inside the shielded container. (I spent much of 1972-73 working inside a Faraday cage on ultra-low-noise superconducting Josephson junctions. Believe me, what signals leaked in, or leaked out (by symmetry analysis) could not have been used to deduce the configuration of sources and reflectors inside the room.) I should say I'm skeptical that millimeter detectors will be widely deployed anytime soon. For detecting concealed weapons, "metal detectors" in specific locations are both cheaper and have fewer constitutional and "false positive" problems. --Tim May Boycott "Big Brother Inside" software! We got computers, we're tapping phone lines, we know that that ain't allowed. ---------:---------:---------:---------:---------:---------:---------:---- Timothy C. May | Crypto Anarchy: encryption, digital money, tcmay@got.net 408-728-0152 | anonymous networks, digital pseudonyms, zero W.A.S.T.E.: Corralitos, CA | knowledge, reputations, information markets, Licensed Ontologist | black markets, collapse of governments. "National borders aren't even speed bumps on the information superhighway."