linux-kernel - Re: [PATCH 0/3] enable CAAM's HWRNG as default

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Date:   Fri, 8 Nov 2019 23:19:48 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Andrey Smirnov <andrew.smirnov@...il.com>
Cc:     Lucas Stach <l.stach@...gutronix.de>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, Chris Healy <cphealy@...il.com>,
        Horia Geantă <horia.geanta@....com>,
        Iuliana Prodan <iuliana.prodan@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] enable CAAM's HWRNG as default

On Tue, Oct 29, 2019 at 12:58:24PM -0700, Andrey Smirnov wrote:
>
> > I'm not sure if we can ever use the job based RNG interface to hook it
> > up to the Linux HWRNG interface. After all the job based RNG interface
> > is always a DRNG, which only gets seeded by the TRNG. The reseed
> > interval is given in number of clock cycles, so there is no clear
> > correlation between really true random input bits and the number of
> > DRNG output bits.
> 
> Doesn't enabling prediction resistance gives us that correlation? E.g.
> that every time new random data is generated, DRNG is reseeded? I am
> assuming even if this is true we'd have to significantly limit
> generated data length (< seed length?), so maybe what you propose
> below is still simpler.

Prediction resistance should be sufficient in general.  However,
is the prediction resistance reseeding done in real time?

> > I've hacked up some proof of concept code which uses the TRNG access in
> > the control interface to get the raw TRNG random bits. This seems to
> > yield about 6400 bit/s of true entropy. It may be better to use this
> > interface to hook up to the Linux HWRNG framework.
> 
> OK, I'll take a look into that and send out a v2 with results.

I've backed out the patch-set for now but if we can clarify the
prediction resistance implementation details then I'm happy to
put it back in.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt