Re: TEMPEST laptops (fwd)

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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At 1:11 AM -0500 11/5/98, Jim Choate wrote:
>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.

	And both Lasers and Masers scatter under certain conditions. If you
can see the laser, it is scattering a bit of [energy light photons]

	That is what Mr. May is talking about.

	I was under the (apparently false) impression that things like
animal bodies, and ordinary building materials (like wood, Lathe &
plaster/drywall) wouldn't stop or significantly bounce the beams back the
way you didn't want them to go.

	I was thinking more of controling the direction of the RF, rather
than trying to completely supress it.

	It doesn't appear that will work.

>> 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).

	We're not cooking Hotdogs here, and we aren't using (for the most
part) microwaves. Yes, you can heat a hotdog on a PII, but that is more
from heat radiation than RF.

	Microwave ovens also leak RF.
--
"To sum up: The entire structure of antitrust statutes in this country is a
jumble of economic irrationality and ignorance. It is a product: (a) of a
gross misinterpretation of history, and (b) of rather nave, and certainly
unrealistic, economic theories." Alan Greenspan, "Anti-trust"
http://www.ecosystems.net/mgering/antitrust.html

Petro::E-Commerce Adminstrator::Playboy Ent. Inc.::petro@playboy.com

On Thu, Nov 05, 1998 at 12:11:20AM -0600, Jim Choate wrote:

> 
> 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.

	Lasers are a technology for generating light, not kinds of
radio waves.  And masers are a fairly obscure technology for amplifiying
weak microwave signals in cryogenicly cooled devices.  Both deal with
electomagnetic radiation and obey Maxwells laws ....

> 
> > 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).

	Most microwave ovens incorperate a motorized metallic device
called a stirrer that sits directly in the beam of energy from the
magnetron and is designed to reflect microwave energy bouncing around
the cavity and change the standing wave pattern as it rotates, resulting
in much more even distribution of energy.   Without the stirrer hots
spots would be much worse...


> 
> > 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.
> 
	Difraction becomes very significant for openings near in scale to
the wavelength of the energy in question, thus the edges of the top will
act to scatter energy in all directions...

> 
> 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.

	This makes no sense whatsoever.  The EM radiation takes place
when changes in the current flowing happen.   Thus radiation occurs only
when the TTL signal changes state, not during either its high voltage or
low volage state.  The amplitude of the current step is determined by
the impedance of the circuit and how fast the logic switches and how
great the voltage or current swing is , and how effectively it gets
radiated is determined by the geometry of the conductor and its
sourounding ohjects.  EM radiation results only from changes in the
magnetic and electric fields, not from their steady state values.   Thus
it is entirely meaningless to talk of the change in steady state values
as only "3 db" when no radiation results from either the steady high or
steady low value.   And indeed near the conductor, the energy radiated
from the current step will be 100 db greater than the ambiant (decibels
are relative units, thus it is meaningless to talk of "radiating a rf
signal that is 100 db").


-- 
	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

At 10:11 PM -0800 11/4/98, Jim Choate wrote:
>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.

Oh come on, let's not get into sophistry. And lasers are not considered to
be radio frequency devices by anyone I know of...visible, IR, and UV lasers
are all treated as _photon_ devices, "light."  (Yes, yes, I know about
particles vs. waves.)

>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.

Nope, they'll still get out. The parallel mirror scenario.


>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).

We're talking about signal strength of the emitted RF  being knocked down
80 or 100 dB  by the shielding. This is a common way of talking about the
effectiveness of a Faraday cage.

--Tim May

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