LCE: Don't play dice with random numbers [LWN.net]

LCE: Don't play dice with random numbers

Posted Nov 25, 2012 7:38 UTC (Sun) by kleptog (subscriber, #1183)
In reply to: LCE: Don't play dice with random numbers by rgmoore
Parent article: LCE: Don't play dice with random numbers

You clearly know more about digital cameras than I. The idea is not new, see for example http://www.lavarnd.org/what/process.html . I've never seriously proposed it because I figured there was more too it. That site for example also doesn't consider the issues you raised, it just asserts that by throwing enough SHA-1 hashes and bit-swapping at the problem the result is random. I think he misses the point that the problem is not getting random data, but knowing how random your data is.

For example, if you have 50kB of data that you know has 50% entropy you can "distill" it. The trick is knowing the 50%.

You're right you don't need to cover the lens, I left that out for simplicity. I tried this out on a camera being used for another purpose. It had some light and some dark, and the "randomness" was clearly dependant on the light level at that point. But like you say the camera has internal processing (to handle e.g. varying light conditions) and it's hard to know what effect that is having on the output.

Raw output would be best, but I don't know if cheap webcams offer that :)

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LCE: Don't play dice with random numbers

Posted Nov 26, 2012 4:23 UTC (Mon) by rgmoore (✭ supporter ✭, #75) [Link]

Raw output would be best, but I don't know if cheap webcams offer that :)

I think that raw output is going to be pretty much mandatory if you hope to get anything like the theoretical entropy out of the sensor. But thinking about it, the biggest loss won't be from interpolation, it will be from using lossy compression. An essential trick to the lossy compression algorithms used to send images and video is that they keep only the most significant information in the picture. Since you're hoping to use the least significant information as a source of entropy, that's a killer.