Optical Tempest

[Markus Kuhn]

Richard Clayton wrote on 2002-03-12 15:24 UTC:
> ... burning a few large votive candles might also work reasonably well :=)

No idea, where the Reuters journalist got the "candles" from in her
article ...

> The attack works well when the room is dark - and not otherwise

To be more precise, the attack works well when the room is sufficiently
dark when looked at with a good blue filter (filter number 713 from Lee
Filters seems to be a good match for the spectrum of common blue
phosphors).

If the eavesdropper looks primarily at the light from the blue phosphor,
which gives the brightest video signal, you'd better get special
to-be-invented TEMPEST-certified military candles with a wax mixture
that emits significant amounts of light in the 450 nm range. :)

More practical alternatives would be most light sources with a colour
temperature of at least 4000 K, in particular a fluorescent lamp with a
nearby emission line (the one in Richard's office seems to qualify, at
least for the above filter), or best of all sunlight (~ 6000 K).

Roland Perry wrote on 2002-03-12 19:21 UTC:
> (How far away from the TV does *your* remote control work?)

If I can put a £700 photomultiplier, a filter and a telescope carefully
targeted at the transmitter LED into the TV set, a hundred meters should
not be a problem.

http://www.cl.cam.ac.uk/~mgk25/emsec/optical-faq.html

Markus

-- 
Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK
Email: mkuhn at acm.org,  WWW: <http://www.cl.cam.ac.uk/~mgk25/>