Optical Tempest? I have my doubts...

Major Variola (ret) mv at cdc.gov
Fri Jul 18 05:13:15 PDT 2003

At 01:33 PM 7/17/03 -0700, Meyer Wolfsheim wrote:
>For what it's worth, a "secure viewer" that displayed text in red on a
>black background should make an optical tempest attack much more

On a black background you have higher contrast, which you don't want

The eye is most sensitive to greenish, so if you are trying to reduce
the signal, use barely visible green.

On a nearly-same-luminance green background.
Green on green or gray on gray is *low contrast*.
That's what you want.

(You may as well use gray on gray, assume the adversary has color
vision, and might even
have the CIE chart on your monitor phosphor.  The different RGB
phosphors also have different decay times, which smears the signal
if the adversary has no color vision)

Possibly dither the text.  You might also have brighter
lines or areas on the screen to obscure the signal from the less-bright
e-beamed text areas.  Actual distribution should depend on the decay
over time
of the phosphor (you want the bright "distractors" to be as bright
as the text-pixel even though the distractor is no longer illuminated
by the e beam).

You also want some incandescants and fluorescant lights, the latter
running off batteries (with switching converters) so they run out of
phase and frequency with the ones on wall current.  Optical jamming.

Or just close the windowshade and put a towel under the door gap.
(Others may think you're [sm|t]oking, however.. and with the
multiple, multitinted fluorescants, they'll think you're growing too)

Or use a box.
(I once did med-imaging related vision experiments... the setup was a
Sun with
a calibrated greyscale monitor, in a medical office, behind the
receptionists until
we got more space... we had a giant cardboard box over the monitor &
subject to
control ambient light.. which stayed while I was programming it too.. it
amused the patients
who came in to see their cardiologist to see a guy working with his head
in a big box..
probably would have been more disconcerting if the MD was a psychiatrist

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