A cheap USB camera would make a good source.
The cheaper the better, too. Pull a frame off,
hash it, and it's got entropy, even against a
white background. No lava lamp needed.

I sort of agree, but I feel cautious about recommending that people
use their holiday snaps.  And then post them on line...  if you see
where I am going :)

But it is a good suggestion.

That's not at all what I suggested. There are so many ways that one can creatively screw up reasonable cryptographic advice that I don't think it's worth bothering with.

The point is that if you take a cheap 640x480 (or 320x240) webcam and point it against a photographic grey card, there's going to be a lot of noise in it, and this noise is at its bottom quantum in nature. Thus, there's a lot of entropy in that noise. Photographic engineers work *hard* to remove that noise, and you pay for a lack of noise.

I'm willing to bet that if I give you hashes of frames, knowing this process, you can't get pre-images. I'll bet that you can't get pre- images even if I let you put a similar camera next to the one I'm using. In short, I'm willing to bet that a cheap camera is a decent random number source, even if you try to control the image source, to the tune of 128-256 bits of entropy per frame.

No lava lamps are needed, no weird hardware. Just use the noise in a CCD.

        Jon

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