From: Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Date: Tue, 10 Feb 1998 01:14:13 +0800
To: cypherpunks@ssz.com (Cypherpunks Distributed Remailer)
Subject: Re: Laptop TEMPEST
In-Reply-To: <199802090657.AAA19180@einstein.ssz.com>
Message-ID: <E0y1wgF-0007kz-00@heaton.cl.cam.ac.uk>
MIME-Version: 1.0
Content-Type: text/plain



Jim Choate wrote on 1998-02-09 06:57 UTC:
> Doesn't the FCC have to test the RF emissions of all laptops as well as
> monitors for sale as Class A and Class B in the US? Shouldn't that material
> be available? I searched the main site, www.fcc.gov, but didn't find
> anything regarding this.

I suspect that FCC material is not helpful with regard to Tempest.
Results of such EMI tests only measure the general power spectrum
emitted by a device. Of interest for Tempest purposes however is
not the power spectrum, but the spectrum of the cross-correlation
between an internal signal-of-interest in the device and the
emanated radiation. There are special cross-correlation meters
available to measure this, but this is not the equipment used in
EMI tests. I have here two patents filed by the German equivalent
of the NSA, that describes cross-correlation techniques for the
"detection of highly distorted digital signals". Measurement equipment
along those lines are what you need in order to estimate the
information carrying components of the emanated spectrum.

You can do cross-correlation not only between the VGA cable output
and the signal picked up by an antenna or a power/ground line tap,
but also between internal bus lines, the keyboard power/data lines,
the read amplifier signal in your harddisk, etc. Cross-correlation
tests are also useful to measure cross-talk effects between neighbor
cables in your building. As mentioned in our paper, secret data on
your LAN can easily leave your building via phone cables that ran
parallel to your network cable for only a few meters.

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/>