Re: Laptop TEMPEST (fwd)

Forwarded message:

> Subject: Re: Laptop TEMPEST
> From: Ryan Lackey <rdl@mit.edu>
> Date: 09 Feb 1998 02:34:15 -0500

> FCC RF/EMC testing is well nigh useless for TEMPEST protection.

I disagree, it would give you a gross baseline on the total emissions
between monitors and laptops. That field strenght measurement would at least
allow you to calculate radiuses of equal strength to calculate approximately
how far the emissions are from each class of device for equal probabilities
of detection. One of the specific goals is to measure how effective the
device is at effecting other co-located devices (such as seeing ghost images
on other monitors or causing static in paging equipment).

I suspect one could do it with a spectrum analyzer or a grid dip meter.

> I imagine there may be some other "dual use" technologies through for
> testing TEMPEST equipment.  Perhaps some medical equipment has stringent
> stray emanation specs?

We're not trying to bring the individual vertical retraces out of the
chaff...we're trying to calculate the total comparitive emission strengths.
If that isn't high enough then trying to get individual componants of that
field will be useless.


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At 08:30 AM 2/9/98 -0600, Jim Choate wrote:
>> FCC RF/EMC testing is well nigh useless for TEMPEST protection.
>
>I disagree, it would give you a gross baseline on the total emissions
>between monitors and laptops. That field strenght measurement would at least
>allow you to calculate radiuses of equal strength to calculate approximately
>how far the emissions are from each class of device for equal probabilities
>of detection. One of the specific goals is to measure how effective the
>device is at effecting other co-located devices (such as seeing ghost images
>on other monitors or causing static in paging equipment).

FCC specs aren't real tight.

My old laptop*, which passed FCC specs (at least when it was new,
before being hauled around on airplanes and trains, dropped a few times,
having cooked parts replaced, and generally getting treated like a laptop),
broadcast enough emissions that its video would show up on my parents' TV
when used 2-4 meters away.  The video sync wasn't right, so there were
several hard-to-read images of the screen on the TV, but it was obviously
emitting enough that a properly tuned receiver could read it.
On the other hand, it doesn't bother my TV from 8-10 meters away,
but I've got cable, while my parents use Real Rabbit-Ear Antennas and 
some kind of antenna-booster widget.

I have heard that passive-matrix emits less, but I don't know
if that's true from a TEMPEST standpoint or only noise output.

It is a bit easier to put a laptop into an RF-shielded enclosure,
but enclosures that worked fine for harmonics of 4.77 MHz machines
don't necessarily block harmonics of 477 MHz machines :-)
especially critical are the penetrations used to get air and
fiber-optic connections through, which are basically waveguides -
once the wavelength of the signal is short enough, they lose.
___
* AT&T Globalyst 250P, which is a NEC Versa Pentium-75 with a Death-Star
painted on it.  It has the expensive active-matrix 24-bit-color 640x480
screen (sigh - I'd rather have had the cheaper 800x600 8-bit-color :-).
A couple years old, and the case was a bit dented so perhaps it had been 
more radio-tight when it was new.
				Thanks! 
					Bill
Bill Stewart, bill.stewart@pobox.com
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