From: tcmay@netcom.com (Timothy C. May)
Date: Thu, 17 Dec 92 16:23:45 PST
To: cypherpunks@toad.com
Subject: Faraday Cages
In-Reply-To: <44511.drzaphod@ncselxsi>
Message-ID: <9212180022.AA21357@netcom.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain



> Oh.. and what kind of prices are we talking about to, say, TEMPEST a small
> room.. maybe enuf for a workstation and misc. accessories.  |-]
> 
> TTFN!
> 
> DrZaphod

The usual, and cheaper, approach is to shield a computer or
workstation. This is the approach discussed in the last several days
here.

Shielding a room is common is certain applications, such as at NSA
(where the entire windowless building is so shielded), in component
testing labs, and so on. Such rooms are called Faraday cages.

It so happens that in 1972 I worked inside such a Faraday cage (in the
labs at UCSB of then-young Paul Hansma, now famous in the nanotech
community for his AFM imaging of cells and other biological
materials). The shield consisted of copper screen mesh
(fine-pitch...perhaps .5 mm gaps) mounted on a wooden frame, with 2
layers and an air gap between them. 

Attenuation depends on frequency, of course, and a wire mesh screen
lets through some frequencies (light, for example!). How many dbs of
attenuation at "frequencies of interest" depends on a lot of factors.
Extremely high frequency signals, becoming more common in computers as
processor speeds exceed 50 MHz, are very hard to shield, due to the
short wavelengths.

I suspect if the boys in the antenna-equipped vans are parked outside
your home, nearly any leakage is too much. But there are cheaper ways
for your secrets to be learned. In other words, there are more
pressing concerns.

I would estimate that shielding an entire room would be expensive,
perhaps costing $5K or more ot do a good job. Building a smaller
room-within-a-room might cost a little less.

Listening to a radio is one very crude way of monitoring your own
RF emissions, if you don't have access to specialized gear.

Using a laptop helps. (I have an old GRiD Compass, with plasma
display, bubble memory, and a black magnesium case...it is definitely
less RF-noisy than my PowerBook, and a lot less noisy than my IIci.)

(Perhaps Phil Karn can comment on strategies)

BTW, we've talked about setting up our own "van Eyck radiation"
systems to see just how easy it is to monitor computers. There is a
legend, possibly urban, that the NSA did indeed try to block
publication of papers on the "van Eyck" effect. This fits part of what
"Treason" was saying, though not all of it. (And there is no law
saying you can't try to shield your computers, or speak in a low
voice, for that matter.)

--Tim

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