linux-kernel - Re: [PATCH 0/3] enable CAAM's HWRNG as default

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Date:   Tue, 29 Oct 2019 12:58:24 -0700
From:   Andrey Smirnov <andrew.smirnov@...il.com>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, Chris Healy <cphealy@...il.com>,
        Horia Geantă <horia.geanta@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Iuliana Prodan <iuliana.prodan@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] enable CAAM's HWRNG as default

On Tue, Oct 29, 2019 at 9:43 AM Lucas Stach <l.stach@...gutronix.de> wrote:
>
> On Di, 2019-10-29 at 09:29 -0700, Andrey Smirnov wrote:
> > Everyone:
> >
> > This series is a continuation of original [discussion]. I don't know
> > if what's in the series is enough to use CAAMs HWRNG system wide, but
> > I am hoping that with enough iterations and feedback it will be.
> >
> > Feedback is welcome!
>
> I'm not sure if we can ever use the job based RNG interface to hook it
> up to the Linux HWRNG interface. After all the job based RNG interface
> is always a DRNG, which only gets seeded by the TRNG. The reseed
> interval is given in number of clock cycles, so there is no clear
> correlation between really true random input bits and the number of
> DRNG output bits.
>

Doesn't enabling prediction resistance gives us that correlation? E.g.
that every time new random data is generated, DRNG is reseeded? I am
assuming even if this is true we'd have to significantly limit
generated data length (< seed length?), so maybe what you propose
below is still simpler.

> I've hacked up some proof of concept code which uses the TRNG access in
> the control interface to get the raw TRNG random bits. This seems to
> yield about 6400 bit/s of true entropy. It may be better to use this
> interface to hook up to the Linux HWRNG framework.
>

OK, I'll take a look into that and send out a v2 with results.

Thanks,
Andrey Smirnov