Re: Sources of randomness

[tcmay@got.net (Timothy C. May)]

At 9:43 PM 11/2/95, Carl Ellison wrote:
>>Date: Thu, 2 Nov 1995 00:39:29 -0800
>>From: tcmay@got.net (Timothy C. May)
>
>>Digitized video input, especially from something like a noisy channel (t.v.
>>channel, for example), is quite likely to produce a lot more entropy bits
>>per second than nearly any of us ever need. (One's PGP key could be seeded
>>in a fraction of a millisecond, for example.)
>
>My cable feed into my Mac, for example, has surprisingly little noise --
>and it's only local noise (stuff only at my house, not available to the
>eavesdropper) which counts as entropy.

I meant a t.v. channel, as in broadcast t.v., such as is easily gotten with
rabbit ears, or nothing (since the idea is to get noise, from the ether, or
from the tuner itself). A snow-filled picture has pixels which are unlikely
in the extreme to be predicted/deduced by outside attackers, who cannot
know the details of antenna orientation, microscopic variations in
geometry, LRC, tuner sensitivity, etc. That is, snowy pictures are not
guessable.

(I'm not convinced that _cable_ signals are not random enough: my cable
picture is far from perfect, and my specific cable configuration and tuner
variations are certainly contributing to variability of the image pixels.
Granted, regularities in the image that are known to attackers--this is all
in theory, of course--will serve to reduce the entropy. But there's so much
bandwidth to burn that I suspect that even cable signals are OK to use. Of
course, removing the cable input is trivial and produces a spectacularly
snowy set of images....)

--Tim May

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