[117292] in Cypherpunks
Re: Project: Hardening Crypto Against Big Brother
daemon@ATHENA.MIT.EDU (Dave Emery)
Mon Aug 30 23:17:28 1999
Date: Mon, 30 Aug 1999 22:50:01 -0400
From: Dave Emery <die@die.com>
To: Sunder <sunder@brainlink.com>
Cc: cypherpunks@toad.com
Message-ID: <19990830225001.F21186@die.com>
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In-Reply-To: <37CAE455.93668506@brainlink.com>; from Sunder on Mon, Aug 30, 1999 at 04:06:45PM -0400
Reply-To: Dave Emery <die@die.com>
On Mon, Aug 30, 1999 at 04:06:45PM -0400, Sunder wrote:
> Hmmm, does it also help tempest shielding? If so where does one obtain leaded
> crystal glass, say suitable for a 14" LCD monitor again? :)
A lot of people confuse effectiveness against ionizing radiation
with effectiveness against electromagnetic radiation in the radio
spectrum range of energies. Leaded glass might be effective against
ionizing radiation, but is far less effective against electromagnetic
(rf) energy at radio wavelengths than copper mesh or copper coated
fabric.
Rf shielding by metal mesh works by creating a very effective
anti-antenna that winds up radiating a signal that almost perfectly
cancels the original incident electromagnetic wave in the forward
direction (the energy is reflected backwards toward the source). The
incident electromagnetic wave induces currents in the outside of the
shield that produce a magnetic field that cancels the original magnetic
field on the inside. Radio wavelengths are in the order of several
feet to inches in the range of most interest for TEMPEST shielding and
copper screen or copper foil or metalized fabric acts as a conductor for
such faraday cages with currents flowing in patterns that are measured
in inches to feet. The anti-antenna has dimensions in the order of a
wavelength measured broadside to the radiation and for highly
conductive materials like copper very tiny fractions of a wavelenth (the so
called skin depth) measured in the direction of the radiation. This is
why fine mesh works well for TEMPEST sheilding, the holes are much less
than a wavelength and the mesh acts as if it were a solid sheet.
And the copper in the mesh is plenty thick enough relative to the
skin depth in copper.
Sheilding ionizing gamma or X ray radiation works on a
completely different scale. Rather than wavelengths in inches or feet,
one is talking way less than nanometers, and rather than interaction
with the mesh of a copper fabric on a scale measured in inches, one is
talking about interaction with individual atoms of lead, more specificly
their inner electrons and nuclii. This is effectively a very different
process than the electric currents induced in a copper screen by radio
frequency electromagnetic energy which have scale on the order of
inches. The electromagnetic currents only involve moments of free outer
orbital electrons in the copper, whilst the absorption of gamma or X ray
energy involves inner orbital electrons, and at high enough energies the
nucleus itself.
And of course lead is also good for blocking particles as
well as gamma rays and X rays. This is due to the cross section of
the nuclii and the high density of lead makes it attractive. Radio
waves do not contain particles with mass, but many kinds of ionizing
radiation from radioactive sources do.
Lead is not a terribly good conductor of electricity, however,
and the anti-currents induced by incident electromagnetic waves in the
radio frequency range get converted to heat and do not radiate the anti
wave that cancels the incoming wave quite as effectively as copper or
other highly conductive metals. This makes lead significantly less
effective for TEMPEST shielding than copper (as well as a lot heavier
and more of a nuisance to deal with environmentally).
As for leaded glass, leaded glass isn't very conductive and
therefore is a pretty poor TEMPEST faraday cage shield material. It
attenuates rf somewhat more than unleaded glass, but orders of magnitude
less than a similar thickness of copper or other conductive metal or
even metal mesh or metalized fabic. What counts is surface conductivity
in ohms per square... and the effective skin depth in leaded glass is
much greater than it is in metal because of the lower conductivity.
There are metalic coatings available for glasses that are more
or less transparent to visible light, but conductive enough to act as
meaningful faraday shields for rf - such coatings would be much more
effective than leaded glass...
So in summary, lead is for ionizing (nuclear) radiation and X
rays, and copper mesh or foil is for radio waves in the range of
interest to TEMPEST and these are two different kinds of radiation in
respect to how they interact with matter and what is effective in
shielding them. Lots of poorly science educated, but otherwise
intelligent people assume that because TEMPEST involves radiation (of
long wavelength waves) lead must be the right stuff to shield it.
Of course the governments stealth aircraft projects have come up
with materials that interact with microwave (dimensions in inches)
radiation in more complex ways than I describe - such materials take
advantage of interactions between the magnetic field of the radio wave
and certain exotic compounds that have strong atomic scale magnetic
properties (the simplest and least classified being lossy ferrites)...
These materials create shielding that actually absorbs energy from the
rf field rather than just re-radiating it (reflecting it) back in the
opposite direction. Such materials are much more expensive and
hazardous than copper mesh, and for most TEMPEST applications are
overkill.
Sorry if I have oversimplified a bit for a purist's tastes...
--
Dave Emery N1PRE, die@die.com DIE Consulting, Weston, Mass.
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2 5D 27 BD B0 24 88 C3 18
