On Sat, Jan 13, 2001 at 12:11:13PM -0800, Ray Dillinger wrote:

> We hear low-frequency sounds when we type.  But have we ever checked 
> for high-frequency sounds outside of human hearing range?  I'd bet 
> a keyboard has a number of squeaks and ticks and twangs up there.
> I'd also bet that most of the keys, after a keyboard's broken in, 
> don't sound exactly alike -- wear and tear, typing patterns, etc. 
> You might be able to resolve ambiguities of interferometry by using 
> the sounds of the keys themselves. 
>                               Bear

        For what very little it is worth, I have been told that this was
done quite sucessfully many many years ago with the old model 28
Teletype machines (anyone old enough to remember those ?) that made
quite characteristic noises as the typebox was positioned to print a
character (on a X, Y grid).   One can be sure that reading traffic from
the clatter of TTY machines was easier than a keyboard by interferometry
or key click sound signatures, but then the DSP required to do it was a lot
less readily available back then too...

        Model 28s were widely deployed by the US government by the way,
and often used to print crypto traffic.


        Dave Emery N1PRE,  [EMAIL PROTECTED]  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18

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