Re: TEMPEST laptops

[Tim May <tcmay@got.net>]

At 12:26 PM -0800 11/1/98, Dave Emery wrote:
>On Sun, Nov 01, 1998 at 09:53:37AM -0800, Tim May wrote:
>>
>> Anyway, were I to try to shield a laptop I'd start with microwave leakage
>> meters, a couple of t.v.s and radio (of different types and bands), and
>> then I'd start recording signal levels of various sorts as different
>> shielding layers and types were applied to the laptop(s). Simple lab stuff.
>>
>
>	As someone who has actually spent hellish weeks working to
>suppress the RF emissions of some commericial network gear I designed -
>to make it pass FCC B and VDE certification - let me say this is a black
>art and no fun.  There are sophisticated electromagnetics programs that
>can sometimes succeed in modeling the radiation from a computer system,
>but they are very expensive and inputting all the required information
>is painful or downright impossible (data just not available or in a
>usable format), so this nasty job usually gets done seat of the pants
>style using rules of thumb and educated guesses and hard won experiance
>and lots of trial and error.  The job consists of attaching lossy
>ferrite beads, copper tape and other RF and common mode current supressing
>devices, shielding plastic packages with spray metallic coatings, adding
...

All good points, but there's a big difference between trying to meet FCC
emissions requirements for a commercial product that has to meet cost,
weight, and cosmetic requirements (e.g., a plastic case!), and the scenario
of making a TEMPEST-like box for a laptop. Ferrite beads and copper tape
are a lot different from a sealed box made of 10-gauge copper sheet.

>	Nobody does this using ordinary radios and TVs, the standard
>tool is a broadband spectrum analyzer or special EMC receiver with
>quasipeak filters and special calibrated wideband dipole antennas that
>have known gain and pattern characteristics.  Isolating of radiating
>sites is often done with near field probes or sniffers attached to the
>spectrum analyzer that allow hot spots to be tracked to within a few cm.
>Often in order to get enough sensitivity one also needs special
>preamps, and a RF quiet site where signals from the DUT aren't drowned
>out by pagers and cellphones and emissions from nearby computers.   A
>good bit of this work is done way out in the country under non-metallic
>fiberglass buildings that don't create reflections that confuse the
>measurements.   And conducted as opposed to radiated noise is measured
>with special power line filters and cable filters...

Sure, but my point was that John Young should *at least* start with actual
measurements, as opposed to putting most of the onus on a FOIA request to
get TEMPEST docs declassified.

If he can get spectrum analyzers and all that stuff, so much the better.


--Tim May

Y2K: A good chance to reformat America's hard drive and empty the trash.
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Timothy C. May              | Crypto Anarchy: encryption, digital money,
ComSec 3DES:   831-728-0152 | anonymous networks, digital pseudonyms, zero
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