[s-t] bright lights, big computers digest #1

Eugen Leitl eugen at leitl.org
Fri Feb 4 13:52:54 PST 2005

[from somelist]

> Subject: Re: [s-t] The return of Das Blinkenlight
> Date: Mon, 31 Jan 2005 19:00:49 -0500
> >In the early 90's I was a product manager for a (now-defunct) company
> >that made LAN hubs-- this was when a 10Base-T port would cost you a couple
> This reminded me of a story from a few years ago.
> Apparently a lot of modem manufacturers tied the activity light on
> the modem directly to the circuit which modulated the sound.
> Then someone realized that with a telescope, and and optical
> transister, one could read that datastream as if hooked to the modem
> directly.
> And astonishing numbers of businesses had their modem pools facing
> windows, because the blinkenlights looked impressive.


Not just modems.  Some Cisco routers, even at megabit rates.  2002
publication, although the research was over the previous couple of

And (for instance) the Paradyne Infolock 2811-11 DES encryptor, which
has an LED on the plaintext data.

How we laughed.

The paper also covers using LEDs (such as keyboard LEDs) as covert
data channels.  And yes, it cites Cryptonomicon.

I'm not sure whether this was more or less cool than Marcus Kuhn's
work on reconstructing CRT displays from reflected light, by reverse
convolution with the impulse-response curves of the various phosphors.
Both papers are fantastic reads, very accessible, very stimulating.


Nick B

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