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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.

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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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