[why-o-why]

Sample it with what? An infinite precision ADC?

This is how old temperature-noise based TRNGs can be attacked (modern ones use a different technique, usually a ring-oscillater with whitening... although i have heard noise-based is coming back but i've been out of the loop for a while)

[rcxdude]

Well, sampling is technically an analog operation that is separate from the conversion operation that makes the result digital. But then analog circuits don't ever actually hold a single real number, in practice there is always noise and that in practice limits the precision to less than what can be fairly easily achieved digitally.

[why-o-why]

Sure, but we are talking about generating a random number, not sampling noise: those are two different things, albeit the former can be derived from the latter but not directly and as simply as the parent post claimed. Just sampling analog noise does not generate a "true" random number that can satisfy a set of design parameters to configure the NIST 800-90b entropy assessment (well, one could pick shitty parameters for the probability tests, but let's assume experts at the helm). Hence the need for software whitening.

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

https://github.com/usnistgov/SP800-90B_EntropyAssessment

(^^^ this is a fun tool, I recommend playing with it to learn how challenging it is to generate "true" random numbers.)

An infinite precision ADC couldn't be subject to thermal attack because you could just sample more bits of precision. (Of course, then we'd be down to Planck level precision so obviously there are limits, but my point still stands, at least _I_ think it does. :))