Re: Stealth Buildings Was Re: "X-Ray Gun" for imperceptible searches

[tcmay@got.net (Timothy C. May)]

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


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