From: Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Date: Tue, 10 Feb 1998 01:11:17 +0800
To: cypherpunks@toad.com
Subject: Re: Soft Tempest
In-Reply-To: <v03102800b1044ad31629@[207.167.93.63]>
Message-ID: <E0y1wTB-0000ZO-00@heaton.cl.cam.ac.uk>
MIME-Version: 1.0
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Tim May wrote on 1998-02-09 06:14 UTC:
> The physics suggests just the opposite: the RF emissions from laptops are
> expected to be lower from first principles, and, I have heard, are
> measurably much lower. (I say "have heard" because I don't have any access
> to RF measurement equipment...I once spent many hours a day working inside
> a Faraday cage, but that was many years ago.)
> 
> The first principles part is that the deflection yokes in a CRT are the
> largest radiated component of what got named "van Eck radiation." (I'd just
> call it RF, but whatever.)

You have to differentiate between information carrying emanations
and non-information carrying ones. The horizontal and vertical
deflection coils produce a lot of radiation at harmonics of the
line and frame rate of your CRT, but this signal energy is not
related to your screen content (only to your video mode), and therefore
not of much concern for the eavesdropper. The low-radion monitor
standards look only at those signal (<400 kHz). Therefore having
a TCO92 monitor provides you absolutely no advantage with respect
to eavesdropping.

The information carrying signals of VDUs are in much higher frequency
ranges in the VHF/UHF bands. Laptops are pretty good broadcasters
there, too.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>