Optical Tempest FAQ

[Steve Thompson]

 --- Sunder <sunder at sunder.net> wrote: 
> IMHO, if you light up two or more other identical CRT's and have them 
> display random junk it should throw enough noise to make it worthless - 
> (and would put out enough similar RF to mess with RF tempest) there
> might 
> be ways to filter the photons from the other monitors out, but, it would
> 
> be difficult.

Every resistor/cap is different, as is just about every other electronic
component that you might find in either analogue or digital circuits --
including clock crystals.  Even VLSI chips cut from the same die will
exhibit (very) subtlely different analogue electrical properties as a
consequence of their initial physical location on the wafer.  Therefore,
it is reasonable to expect that otherwise "identical" CRT monitors will
exhibit subtle differences in signal timing, amplitude, and phase when
their primary and intermediate outputs are examined in detail. 

If I'm not entirely off the mark, this means that RF tempest gear can in
principle differentiate same make-and-model CRT devices.  This does not
mean that the suggestion is without merit, as it is likely that `jamming'
your primary monitor with another will make things somewhat more difficult
for an eavesdropper.  But that just means that he may have to hook up his
antenna directly to the water mains instead of sitting in his van down the
street.

As to the validity of this strategy to combat optical tempest, I am not
sure.  I would look up the state-of-the-art in audio-signal filtering to
see what can be done today to differentiate similar mixed signals.  

All in all I would suspect that to get halfway decent jamming, you would
have to synchronise your two monitors so that the video frames on each
were being started at the same instant, while also using different sources
and signals.  That is likely beyond the capability of anyone who is not
fairly good with electronics; off the top of my head I cannot suggest how
I would thing about doing it.

Caveat emptor: IANA electrical engineer by any stretch of the imagination,
so I may be entirely full of crap without knowing it.


Regards,

Steve




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