Juliusz Chroboczek wrote on 2003-03-03 22:56 UTC:
> MK> Putting an anti-tempest filter into freetype2 has been on my todo list
> MK> for a long time
>
> Could you guys be so kind as to tell us mere mortals what you're
> speaking about?
Compromising emanations of video display systems. Low-cost software-only
countermeasures. Modifying the font renderer so as to minimize
information-carrying radio frequency signals generated by the graphics
adapter, video cable and display. This involves primarily two
techniques:
- Lowpass filtering the glyphs in horizontal direction (removing everything
above 70% of the Nyquist limit (half pixel clock) is in practice a good
compromise between readability on the screen and unreadability with
with radio receiver).
- Replace the least few significant bits with pseudo-random bits for each
usage of a glyph on the screen. This causes frame-rate correlated
noise that eavesdroppers cannot eliminate by periodic averaging. This
technique is in particular useful for jamming emissions of VGA cards
and digital video cables such as NEC's FPD-Link (used in many laptops)
and DVI (used in recent desktop flat-panel displays).
Optionally you can also generate glyph variants by slightly offsetting
the Bezier control points (hinting is switched off in this anyway), and
then randomly pick one of the glyph variants to make radio character
recognition slightly more challenging.
Results, see pages 31-44 of
http://www.cl.cam.ac.uk/~mgk25/em-eaves.pdf
I have already written a new techreport with all the gory details on
this, which I hope to get around to finalise and publish in a few weeks.
[The UK patent office was even so kind to grant me GB2333883 on this
stuff. I'd be happy of course to grant XFree86 a free licence.]
> It's got something to do with deploying XFree86 in the American
> embassy in Moscow, right?
No, these folks put their computers inside 100 dB shielding instead. It
is more for people who can neither afford the �50000 for an NSA-94-106
certified shielded room (say, the embassy of Lower Slovbodia in Paris),
nor the �500 that an intitial psychiatric treatment course for mild
paranoia costs.
CRT and LCD eavesdropping is for much target equipment quite feasible
within around 10-30 meters, in rurals areas sometimes even in the 100
meter range. However, I don't think it is actually done frequently,
because there aren't many situations where you know in advance, which
screen is going to display something so interesting that it is worth
setting up all the fancy equipment necessary to do it. People who should
worry about such things seriously usually have already taken care of
voice eavesdropping, a in comparison far more significant threat.
And once you believe that you have taken care of RF shielding, you can
start to worry about optical VHF leaks from CRTs:
http://www.cl.cam.ac.uk/~mgk25/emsec/optical-faq.html
The fun never ends.
Markus
--
Markus Kuhn, Computer Lab, Univ of Cambridge, GB
http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__
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