linux-kernel - Re: [PATCH v2 0/3] add support of hardware random generator on MediaTek MT7622

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Date:   Wed, 21 Jun 2017 00:08:14 +0800
From:   Sean Wang <sean.wang@...iatek.com>
To:     Torsten Duwe <duwe@...e.de>
CC:     Herbert Xu <herbert@...dor.apana.org.au>, <mpm@...enic.com>,
        <robh+dt@...nel.org>, <mark.rutland@....com>,
        <clabbe.montjoie@...il.com>, <prasannatsmkumar@...il.com>,
        <romain.perier@...e-electrons.com>, <shannon.nelson@...cle.com>,
        <weiyongjun1@...wei.com>, <devicetree@...r.kernel.org>,
        <linux-crypto@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <keyhaede@...il.com>
Subject: Re: [PATCH v2 0/3] add support of hardware random generator on
 MediaTek MT7622

On Tue, 2017-06-20 at 16:59 +0200, Torsten Duwe wrote:
> On Tue, Jun 20, 2017 at 10:21:17PM +0800, Sean Wang wrote:
> > Hi Herbert,
> > 
> > thanks for effort reviewing on those patches.
> > 
> > By the way, also loop in Torsten
> > 
> > Could you kindly guide me how to determine appropriate 
> > rng->ops.quality value used by the driver?
> > 
> > I have tested with rngtest on mtk-cir and the result is got as
> > the below log shown.  If the rngtest always gives the result for
> > success rate over 99.8%, can I set the rng->ops.quality 998?
> > 
> > rngtest: starting FIPS tests...
> > rngtest: bits received from input: 20000032
> > rngtest: FIPS 140-2 successes: 998
> > rngtest: FIPS 140-2 failures: 2
> 
> No! You'd have to determine the failure threshold of the test and
> apply some math to find a lower boundary of your RNG's entropy.
> 
> What the quality is for: your RNG produces bits, but not all of them
> are completely independent of each other i.e. not completely random.
> So you simply lower the quality rating to express the net entropy
> contained in the data stream.
> 
> 	Torsten
> 


Hi Torsten,

Understood, appreciate your quick and clear explanation.
 
For the math, is there existing any well-known or recommended
open source software assisting identify the lower boundary of
RNG entropy? I think the logic should be common for all RNGs.

thanks again

	Sean


	Sean