linux-kernel - Re: [PATCH v2 1/2] hwrng: iproc-rng200

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Date:   Wed, 20 May 2020 11:18:32 +0200
From:   Stephan Mueller <smueller@...onox.de>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Markus Elfring <elfring@...rs.sourceforge.net>,
        Matthias Brugger <mbrugger@...e.com>,
        Stefan Wahren <wahrenst@....net>, linux-crypto@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v2 1/2] hwrng: iproc-rng200 - Set the quality value

Am Mittwoch, 20. Mai 2020, 11:10:32 CEST schrieb Lukasz Stelmach:

Hi Lukasz,

> It was <2020-05-20 śro 08:23>, when Stephan Mueller wrote:
> > Am Dienstag, 19. Mai 2020, 23:25:51 CEST schrieb Łukasz Stelmach:
> >> The value was estimaded with ea_iid[1] using on 10485760 bytes read from
> >> 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.
> > 
> > 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.
> > 
> > What needs to happen is that we need access to raw, unconditioned data
> > from
> > the noise source that is analyzed with the statistical methods.
> 
> 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 
the data from the hardware usually is not obtained straight from the noise 
source.

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

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

That said, the illustrated example is typical for hardware RNGs. Yet it is 
never guaranteed to work that way. Thus, if you can point to architecture 
documentation of your specific hardware RNGs showing that the data read from 
the hardware is pure unconditioned noise data, then I have no objections to 
the patch.
> 
> 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 post-
processing within the hardware.
> 
> [1] https://lkml.org/lkml/2020/5/15/252
> [2]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Doc
> umentation/admin-guide/hw_random.rst?h=v5.6 [3]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/dri
> vers/char/hw_random/core.c?h=v5.6#n251
> 
> Kind regards,


Ciao
Stephan