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From:   Stephan Mueller <smueller@chronox.de>
To:     Lukasz Stelmach <l.stelmach@samsung.com>
Cc:     Matt Mackall <mpm@selenic.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Arnd Bergmann <arnd@arndb.de>,
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        Kukjin Kim <kgene@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Markus Elfring <elfring@users.sourceforge.net>,
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        linux-samsung-soc@vger.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [PATCH v2 1/2] hwrng: iproc-rng200 - Set the quality value
Date:   Wed, 20 May 2020 11:18:32 +0200
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Am Mittwoch, 20. Mai 2020, 11:10:32 CEST schrieb Lukasz Stelmach:

Hi Lukasz,

> It was <2020-05-20 =C5=9Bro 08:23>, when Stephan Mueller wrote:
> > Am Dienstag, 19. Mai 2020, 23:25:51 CEST schrieb =C5=81ukasz Stelmach:
> >> The value was estimaded with ea_iid[1] using on 10485760 bytes read fr=
om
> >> the RNG via /dev/hwrng. The min-entropy value calculated using the most
> >> common value estimate (NIST SP 800-90P[2], section 6.3.1) was 7.964464.
> >=20
> > I am sorry, but I think I did not make myself clear: testing random
> > numbers
> > post-processing with the statistical tools does NOT give any idea about
> > the
> > entropy rate. Thus, all that was calculated is the proper implementation
> > of
> > the post-processing operation and not the actual noise source.
> >=20
> > What needs to happen is that we need access to raw, unconditioned data
> > from
> > the noise source that is analyzed with the statistical methods.
>=20
> I did understand you and I assure you the data I tested were obtained
> directly from RNGs. As I pointed before[1], that is how /dev/hwrng
> works[2].

I understand that /dev/hwrng pulls the data straight from the hardware. But=
=20
the data from the hardware usually is not obtained straight from the noise=
=20
source.

Typically you have a noise source (e.g. a ring oscillator) whose data is=20
digitized then fed into a compression function like an LFSR or a hash. Then=
 a=20
cryptographic operation like a CBC-MAC, hash or even a DRBG is applied to t=
hat=20
data when the caller wants to have random numbers.

In order to estimate entropy, we need the raw unconditioned data from the,=
=20
say, ring oscillator and not from the (cryptographic) output operation.

That said, the illustrated example is typical for hardware RNGs. Yet it is=
=20
never guaranteed to work that way. Thus, if you can point to architecture=20
documentation of your specific hardware RNGs showing that the data read fro=
m=20
the hardware is pure unconditioned noise data, then I have no objections to=
=20
the patch.
>=20
> If I am wrong, do show me the code that processes the data from a HW RNG
> before copying them to user provided buffer[3].

I am not talking about any software post-processing. I am talking about pos=
t-
processing within the hardware.
>=20
> [1] https://lkml.org/lkml/2020/5/15/252
> [2]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/D=
oc
> umentation/admin-guide/hw_random.rst?h=3Dv5.6 [3]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/d=
ri
> vers/char/hw_random/core.c?h=3Dv5.6#n251
>=20
> Kind regards,


Ciao
Stephan