Defeating Optical Tempest will be easy...

Peter Fairbrother zenadsl6186 at
Mon Jul 21 11:49:03 PDT 2003

Major Variola (ret) wrote:

> At 02:17 AM 7/21/03 +0200, Thomas Shaddack wrote:
>> On Sat, 19 Jul 2003, Tyler Durden wrote:
>> There is some minuscule proportion of X-rays produced by CRT displays.
> Produced by the ebeam decelerating on the shadow mask, but adsorbed
> by the glass.

a_b_sorbed. Absorb is a widely used word meaning 3to drink in, to soak up,2
both literally and figuratively. Adsorb is a specialized technical term,
meaning only 3to collect a condensed gas or liquid on a surface.2

The glass of CRT's absorbs so much of the X-rays that it might be hard to
detect a signal at all at any distance, but then the signal is not swamped
by noise from the not-immediately-illuminated areas, unlike the optical

"0.5 milliroentgens per hour at a distance of five (5) centimeters from any
point on the external surface of the receiver" is the US legal limit[*], and
low voltage (and thus very low x-ray emission) crt monitors are common now,
if not a de-facto standard.

However, I expect shot noise to be a limiting factor here. Unfortunately,
the Roentgen is such a wierd unit it's not that easy to convert it to
photons and do the math!

A light background on a CRT screen image will give out enough delayed light
to give problems in the s/n ratio of an optical TEMPEST attack. It's much
easier to "see" white text on a black background than black text on a white

I use 180:210:210[**] (r:g:b) text on a 255:255:255 window background at
present, with very light wallpaper, though I speckle both slightly. It's a
little hard to read, but much better than some other suggested combinations.

[*]< Probably far too high for safety! Originally for TV's, where the
viewing distance is much higher. But most modern monitors will emit much
less than that. I hope! >

[**]< I replaced the black in Marcus's anti-em-tempest fonts with
180:210:210, and varied the other colours in proportion. >

Peter Fairbrother

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