From: Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
Date: Tue, 10 Feb 1998 05:26:56 +0800
To: cypherpunks@ssz.com
Subject: Re: TEMPEST
In-Reply-To: <199802091735.LAA21910@einstein.ssz.com>
Message-ID: <E0y20QG-00015B-00@heaton.cl.cam.ac.uk>
MIME-Version: 1.0
Content-Type: text/plain



Jim Choate wrote on 1998-02-09 17:35 UTC:
> This is the hardest since the actual modulation of the e-beam is done via a
> screen grid (who said tube theory was out of date?) and that signal is quite
> small and generaly has a cardoid emission pattern aligned axialy along the
> central axis of the CRT tube.

Things are somewhat more complicated and I am not convinced that
the e-beam is the primary source of radiation. Your claim that
the Tempest radiation is modulated by the screen grid does not agree with
my practical experience: All signals I get are close to harmonics of
the dot clock and not of the screen grid rate. In addition, the
Tempest monitor cannot distinguish between an all-black and an all-white
image, which it should in the case of a screen-grid caused modulation.
If there is indeed a screen-grid modulation, then it is *much* weaker
than any modulation that you get by software dithering.

Monitors are pretty strange antennas: For instance, my monitor still
radiates quite well (although noticeably weaker) if I switch its power
supply off. Just the passive resonance of the chassis gives a clear
signal in around a meter radius with a simple untuned dipole antenna.
Switching off a monitor alone does not protect you from eavesdropping
a VDU signal, especially if the signal is not just text but a pattern
optimized for reception.

After I unplug the VGA cable however, I can't pick up any signal with
our Tempest receiver unless I bring the antenna almost in contact with
the cable or connector. The closed PC chassis also appears to be no very
big source of VDU emanations, certainly much below the levels that
our receiver can detect.

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/>