Re: [RFC PATCH 0/5] CPU Jitter RNG

From: tytso
Date: Tue Feb 04 2014 - 14:23:42 EST

On Tue, Feb 04, 2014 at 11:06:04AM -0800, H. Peter Anvin wrote:
> The quantum noise sources there are in a system are generally two
> independent clocks running against each other. However, independent
> clocks are rare; instead, most clocks are in fact slaved against each
> other using PLLs and similar structures.

One of the things that would be useful for us to understand is in
general, where in a system we have independent clocks. For example, I
think (correct me if I'm wrong), a 2.5" or 3.5" HDD has its own clock
which is separate from the CPU/chipset. That is actually how and
where we get any entropy; I am not at all convinced that we are
getting any variation from "chaotic air turbulence in the HDD" ---
that paper was published in 1994, and hard drive technologies have
changed quite a bit since then, with extra layers of caching, track
bufers, etc.

However, where a decade ago the ethernet card probably had its own
independent clock crystal/oscillator, I'm going to guess that these
days with SOC's and even on laptops, with ethernet device part of the
chipset, it is probably being driven off the same master oscillator.

I wonder if there's anyway we can either figure out manually, or
preferably, automatically at boot time, which devices actually have
independent clock oscillators.

- Ted

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