[Buildroot] [PATCH v2 1/2] package/haveged: Change service file to run early

Alexander Dahl post at lespocky.de
Mon Jun 29 12:41:47 UTC 2020

Hello Norbert,

On Mon, Jun 29, 2020 at 11:55:26AM +0200, Norbert Lange wrote:
> Am Mo., 29. Juni 2020 um 11:30 Uhr schrieb Alexander Dahl <post at lespocky.de>:
> > On Mon, Jun 29, 2020 at 10:29:38AM +0200, Norbert Lange wrote:
> > > Haveged is not entropy, it's a substitute. I dont know how many times I
> > > need to point that out.
> >
> > As far as I understood the source for the entropy haveged collects is
> > random timing jitter from the CPU. Could you explain, why that is not
> > real entropy, although it passes the FIPS tests? Or point to an
> > explanation to learn from?
> This is already round 2 of the argumentation, see
> http://lists.busybox.net/pipermail/buildroot/2020-June/284465.html
> But yes. haveged is still not tapping any entropy the kernel has not available,
> it just blows up the low entropy available with a random number generator.

So, I read http://www.issihosts.com/haveged/history.html (again) and
as far as I understand it is a not a pseudo random number generator.
According to its own documentation
it is a random generator based on CPU timing jitter, measured by high
precision timers.  That randomness is fed to the kernel pools the same
way one would fed randomness from a true hwrng.  Right?

> I am not up to speed with FIPS tests, but from a really really long way back
> it wasn't a big issue to pass most tests with the Mersenne Twister and a few
> bits of true entropy.
> Basically it feeds PRNG back to kernel and lets it account as entropy source.

I could not find any backup for that claim.  If I understood
correctly, haveged does not read from the kernel PRNG.

> In short:
> Want better/faster entropy, use rng-tools, or choose to "trust" the
> CPU vendor if instructions are available.

rng-tools is useless without a hwrng or jitterentropy-rngd, which does
the same thing as haveged.  So for a system without a hwrng, how
should that recommendation help an user?

> A good PRNG is still not entropy.

I think we can agree on that one.


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