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I'm trying to remember a technique based on listening to static or whitenoise and taking the least significant bit. You then take that stream of bits and postprocess it by looking for bit changes and don't use them directly.

That's a vague and I'm sure inaccurate description, but it isn't enough for me to google it ... anyone know what I'm referring to?

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That sounds a lot like what https://www.random.org/ does to create random numbers.

Perhaps -- I can't find their description behind the process and I'm now curious about the math behind it.

I think they would take a series of 0's and 1's..

0010001110100100011

...and map that into 0's and 1's based on the changes, not the numbers themselves. Maybe like 00 or 11 mapped to 0 and 10 or 01 mapped to 1? I can't recall.

This sounds like the Von Neumann Extractor.

https://en.wikipedia.org/wiki/Randomness_extractor

It's purpose is to remove bias: the propensity of physical random processes to produce more 1s than 0s (or v.v.). Unlike some whiteners (AES?) it can be implemented with a handful of gates.

Yes, that's it. And it's used to eliminate bias.

I found a good explanation at https://www.youtube.com/watch?v=xquB4rDbsvc at about 14min.

random.org definitely uses that process.

See: https://www.random.org/statistics/source-purity/

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