Re: [PATCH 1/2] hwrng: iproc-rng200 - Set the quality value

From: Lukasz Stelmach <l.stelmach@samsung.com>
To: Stephan Mueller <smueller@chronox.de>
Cc: Matt Mackall <mpm@selenic.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ray Jui <rjui@broadcom.com>,
	Scott Branden <sbranden@broadcom.com>,
	bcm-kernel-feedback-list@broadcom.com,
	Kukjin Kim <kgene@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Markus Elfring <elfring@users.sourceforge.net>,
	Matthias Brugger <mbrugger@suse.com>,
	Stefan Wahren <wahrenst@gmx.net>,
	linux-crypto@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-samsung-soc@vger.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [PATCH 1/2] hwrng: iproc-rng200 - Set the quality value
Date: Fri, 15 May 2020 00:18:41 +0200	[thread overview]
Message-ID: <dleftjtv0i88ji.fsf%l.stelmach@samsung.com> (raw)
In-Reply-To: <4493123.C11H8YMYNy@tauon.chronox.de> (Stephan Mueller's message of "Thu, 14 May 2020 22:20:26 +0200")

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It was <2020-05-14 czw 22:20>, when Stephan Mueller wrote:
> Am Donnerstag, 14. Mai 2020, 21:07:33 CEST schrieb Łukasz Stelmach:
>
> Hi Łukasz,
>
>> The value has been estimaded by obtainig 1024 chunks of data 128 bytes
>> (1024 bits) each from the generator and finding chunk with minimal
>> entropy using the ent(1) tool. The value was 6.327820 bits of entropy
>> in each 8 bits of data.
>
> I am not sure we should use the ent tool to define the entropy
> level. Ent seems to use a very coarse entropy estimation.
>
> I would feel more comfortable when using other measures like SP800-90B
> which even provides a tool for the analysis.
>
> I understand that entropy estimates, well, are estimates. But the ent
> data is commonly not very conservative.
>
> [1] https://github.com/usnistgov/SP800-90B_EntropyAssessment

Thank you for pointing this out.

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)

    Calculating baseline statistics...
    H_original: 4.107376
    H_bitstring: 0.795122
    min(H_original, 8 X H_bitstring): 4.107376

but I don't know how much I can trust it, when I get such warning

    *** Warning: data contains less than 1000000 samples ***

ea_non_iid refuses to run tests with less than 4096 bytes of input.

I may suspect that lack of any warnings from ent doesn't make its
results any more reliable.

Anyway. I collected 1024 files 1024 bits each once again and ran the
following tests

    for f in exynos-trng/random*; do ./ea_iid "$f" | grep ^min; done |  sort | head -1
    for f in rng200/random*; do ./ea_iid "$f" | grep ^min; done | sort | head -1

For both RNGs I got the same

    min(H_original, 8 X H_bitstring): 3.393082

which, if I understand correctly, means I should set quality to no more
than 434. Do you think 400 is OK?

Kind regards,
-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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