Optical Tempest? I have my doubts...

Trei, Peter ptrei at rsasecurity.com
Thu Jul 17 12:44:03 PDT 2003

> Durden[SMTP:camera_lumina at hotmail.com] wrote:
> I dunno...I'm thinking that optical tempest is probably bullshit 99% of
> the 
> time, but what do I know? My Optical specialities are ultrafast and
> optical 
> networking.
> But I still don't believe that specular reflection of smallish type from a
> monitor will have anything that is recoverable. Of course, this is going
> to 
> be dependent on the quality of the wall material, but for most not-so-even
> plaster/drywall painted surfaces, I just can't believe the appopriate 
> spacial frequencies of the image are not scattered after that kind of 
> reflection.
> The conspiracy theorist is telling me there's some reason they floated the
> optical tempest story, though I can't quite figure out what that reason 
> is...
> -TD
> >
> >
> >BTW, m-o-o-t uses a randomised virtual keyboard with TEMPEST (both EM and
> >optical) resistant fonts. It's okay for inputting keys, but it's a hassle
> >for inputting text.
> >
> >Which means that your keys might be safe from keyloggers (both hardware
> and
> >software), but your plaintext isn't. Sigh. I'm trying to improve it by
> >putting the "senhorita" letters in one block and the rest elsewhere (not 
> >for
> >key input obviously), and you do learn where the keys are after a while, 
> >but
> >it's still a hassle.
	Please don't top-post.

	Optical TEMPEST of the type we've been discussing has
	nothing to do with specular reflection.

	As your CRT screen is raster-scanned, the point at which
	the electron beam is pointing at any given moment is much
	brighter than the rest of the screen. That the screen seems
	uniformly illuminated is a result of your persistance of vision.

	Optical tempest records the brightness of the light reflecting
	off the wall behind the user as a function of time. This can be used
	to reconstruct the brightness of each pixel on the screen, since
	they are refreshed sequentially in raster order.

	Peter Trei

More information about the cypherpunks-legacy mailing list