Re: TEMPEST laptops (fwd)

[Jim Choate <ravage@EINSTEIN.ssz.com>]

Forwarded message:

> Date: Wed, 4 Nov 1998 21:45:29 -0800
> From: Tim May <tcmay@got.net>
> Subject: Re: TEMPEST laptops

> Radio waves scatter...they don't just travel in pure line of sight. And
> even if they travelled only in line of sight, the reflections from inside
> the box and then into the room and then off surfaces....

It depends on the frequency. Last time I checked a laser or a maser (both
are radio waves strictly speaking) travel LOS. The scattering comes from
beam divergence and incidental refractions and reflections from the
molecules in the air and supported detritus.

> Microwave ovens work by having the waves bounce around inside a box. Any
> significant hole or crack (up to roughly half the wavelength) would let the
> waves out.

Depends on the size of the hole and location. In most microwave ovens there
are definite dead-spots (corners and the exact center of the area are
notorius).

> An open top box will not work.

If the microwaves (for example) are transmitted parallel to the open side it
might very well work just fine. It's going to depend on a variety of
factors that will preclude such a blanket statement from being valid.

> Unlikely to prevent someone from figuring out what the real signal is. It's
> very difficult, generally, to hide a signal with another signal. Noise
> won't work, because noise can be filtered or autocorrelated out. A "spoof"
> signal can be corrected for.

For these to work there must be a time-correlated aspect to the signal that
doesn't appear in the noise. If you mask the signal with the same sort of
time correlated cover (eg phase shifting) it also might work.

> And we are talking about 100 dB sorts of suppression. Mere factors of a few
> with fake signals and noise are meaningless on this scale.

The absolute magnitude isn't really important.

Most of the signals that are emitted by a computer are not in the 100dB
dynamic range (@2x=3db that's a signal range of 1:33) , more likely 40-50db
if that. For a TTL (5V) signal it barely covers 3dB (LOW is <2.5v and a
high is >=4.75). There simply is no way in hell a signal with a 3dB range is
going to emit a rf signal that is 100dB. There are other logic families with
wider dynamic ranges (eg CMOS w/ 18V Vcc can be nearly 18V or approx. about
12dB).


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