From: Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Date: Wed, 11 Feb 1998 02:40:58 +0800
To: cypherpunks@ssz.com
Subject: Re: TEMPEST
In-Reply-To: <199802100340.VAA27056@einstein.ssz.com>
Message-ID: <E0y2KTu-0000y4-00@heaton.cl.cam.ac.uk>
MIME-Version: 1.0
Content-Type: text/plain



Jim Choate wrote on 1998-02-10 03:40 UTC:
> 11MHz base frequency won't produce a clean ring at 125MHz or 210MHz, they're
> not a whole number multiple of the base. How is this accounted for? What did
> he calculate the Q at? At what range was he picking up these harmonics as
> well as the base? The lower harmonic implies a 4MHz beat and the higher a
> 1MHz beat, how did he account for these signals? Personaly I would be more
> inclined to suspect the information was riding on the beats. He didn't
> happen to measure the frequencies to 3 decimal places did he?

You might want to consider to read van Ecks paper yourself:

  Wim van Eck: Electromagnetic Radiation from Video Display Units: An
  Eavesdropping Risk? Computers & Security 4 (1985) 269--286

If you don't have C&S in your library, you might also find a
scan on <http://www.eskimo.com/~joelm/tempest.html>

> > If you have only a van Eck style receiver, yes. But as soon as you record
> > the reception over some time and observe the images phases to drift only
> > slightly against each other, you might be able to separate them using
> > similar processing techniques as used in computer tomography.
> 
> As I alluded to with the comment I made about integrating the received
> signals. The easiest way to do this sort of stuff with a small budget is
> with a flying capacitor integrator.

Please explain! I was more thinking in terms of digitizing everything
and solving large per-pixel systems of equations to separate the drifting
images, which does not sound much like something equivalent to a
single hardware integrator, but more like something that keeps a
workstation very busy for a few minutes.

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/>