random: Allow pure entropy sources to provide a min-entropy estimate

[markj]

Summary

The current implementation of the NIST health tests assumes a
min-entropy estimate of one bit per sample, which is quite conservative.
For so-called "pure" sources (e.g., virtio-random, TPM) it might be nice
to support larger estimates so that the tests catch failed devices more
quickly.

Thus:

• let each pure random source provide an estimate, so that downstreams can override defaults if they want to;
• increase the default estimate for pure sources;
• for pure sources initialize the state machine at source registration time.

[cem]

8 bits per byte might be high for some sources... I'm not sure. It's probably fine for the relatively simplistic tests in the health test?

sys/dev/random/randomdev.h

106

This might also be interesting to Fortuna/etc for boot time min entropy estimation / initial seeding.

[markj]

In D52232#1193086, @cem wrote:

8 bits per byte might be high for some sources... I'm not sure. It's probably fine for the relatively simplistic tests in the health test?

Well, 8 bits per sample, but yes this might turn out to be excessive. Perhaps I should include a tunable to override that value.

[cem]

In D52232#1193211, @markj wrote:

Well, 8 bits per sample, but yes this might turn out to be excessive. Perhaps I should include a tunable to override that value.

I guess I wouldn't expect any source to provide only one byte per sample, so that's only ~2 bits per byte at word-size input, which is not super high.

[markj]

In D52232#1193347, @cem wrote:

In D52232#1193211, @markj wrote:

Well, 8 bits per sample, but yes this might turn out to be excessive. Perhaps I should include a tunable to override that value.

I guess I wouldn't expect any source to provide only one byte per sample, so that's only ~2 bits per byte at word-size input, which is not super high.

Yeah, I'm not aware of any devices that provide only the bare minimum, but since the API permits it I still need to be conservative.