(I like the entropy estimator for a much more selfish reason: collecting randomness from some sources can be quite expensive, and we can use the entropy estimator as a high-water-mark to indicate that there's no point whatsoever providing more randomness because nobody's read much since we last shoved a bunch in. We could do much the same thing with a simple counter of "bytes read from /dev/*random by other users since last entropy fed in by a /dev/random writer", but that leaks information about those other users and might be considered ugly. The BSDs don't have a counter like this, and it makes the ekeyd ridiculously costly on those platforms because it has to pull entropy from the key and remix it for /dev/random all the time, even if the system is otherwise totally idle.)
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