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

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Date:   Fri, 15 May 2020 10:32:22 +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 1/2] hwrng: iproc-rng200 - Set the quality value

Am Freitag, 15. Mai 2020, 00:18:41 CEST schrieb Lukasz Stelmach:

Hi Lukasz,
> 
> I am running tests using SP800-90B tools and the first issue I can see
> is the warning that samples contain less than 1e6 bytes of data. I know
> little about maths behind random number generators, but I have noticed
> that the bigger chunk of data from an RNG I feed into either ent or ea_iid
> the higher entropy they report. That is why I divided the data into 1024
> bit chunks in the first place. To get worse results. With ea_iid they
> get even worse (128 bytes of random data)

I read that you seem to just take the output data from the RNG. If this is 
correct, I think we can stop right here. The output of an RNG is usually after 
post-processing commonly provided by a cryptographic function.

Thus, when processing the output of the RNG all what we measure here is the 
quality of the cryptographic post-processing and not the entropy that may be 
present in the data.

What we need is to access the noise source and analyze this with the given 
tool set. And yes, the analysis may require adjusting the data to a format 
that can be consumed and analyzed by the statistical tests.

Ciao
Stephan