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Date:   Fri, 29 Jun 2018 14:07:32 +0530
From:   Vinod <vkoul@kernel.org>
To:     Timur Tabi <timur@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Stanimir Varbanov <stanimir.varbanov@linaro.org>,
        linux-crypto@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>,
        Matt Mackall <mpm@selenic.com>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-arm-msm@vger.kernel.org
Subject: Re: [PATCH 3/3] hwrng: msm - Add support for prng v2
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On 28-06-18, 17:04, Timur Tabi wrote:
> On Thu, Jun 21, 2018 at 6:53 AM, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> > On Thu, Jun 21, 2018 at 02:27:10PM +0300, Stanimir Varbanov wrote:
> 
> > So does it generate one bit of output for each bit of hardware-
> > generated entropy like /dev/random? Or does it use a hardware-
> > generated seed to power a PRNG?
> 
> I have some information to answer this question, although I'm not sure
> I can give a strict "yes/no" answer.
> 
> There are a couple relevant documents:
> 
> https://www.qualcomm.com/news/onq/2014/11/07/cryptographic-module-snapdragon-805-fips-140-2-certified
> https://csrc.nist.gov/CSRC/media/projects/cryptographic-module-validation-program/documents/security-policies/140sp2944.pdf
> 
> I also got response from a Qualcomm employee:
> 
> "The Qualcomm random number generator used in Snapdragon chips
> consists of an entropy source coupled with the HASH-DRBG deterministic
> random bit generator from NIST Special Publication 800-90A, using
> SHA-256 as the hash function.
> 
> The entropy source is based on sampled ring oscillators.  Four ring
> oscillators are used to provide high assurance of adequate entropy.
> The entropy from the ring oscillators is conditioned using the
> 'derivation function' specified by NIST Special Publication 800-90A.
> The conditioned entropy is essentially perfect fully entropic data.
> It is used both to seed and to periodically reseed the DRGB."
> 
> My understanding is that the PRNG is a real entropy source with some
> logic used to normalize the values.  To quote: "No RNG uses data
> directly from the entropy source; bits in the output are likely
> correlated and unlikely to occur with 50% probability. The entropy
> post-processing is designed to turn dirty data in clean data."
> 
> Based on the above, it seems to me that the Qualcomm PRNG qualifies as
> a real hardware RNG and porting to algif_rng is not the correct path.

I think Stan did bring this point earlier that PRNG is compliant to
FIPS-140-2. So it can be used by rng clients for various purposes but
should not be fed to dev/random as the hw_random does.

Herbert, can you please confirm..

-- 
~Vinod