linux-kernel - Re: [PATCH v3 1/3] crypto: hw_random

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Message-ID: <2736911.EcZSki4vB1@positron.chronox.de>
Date:   Sun, 26 Mar 2017 19:11:28 +0200
From:   Stephan Müller <smueller@...onox.de>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Kukjin Kim <kgene@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-crypto@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v3 1/3] crypto: hw_random - Add new Exynos RNG driver

Am Samstag, 25. März 2017, 17:26:52 CEST schrieb Krzysztof Kozlowski:

Hi Krzysztof,

> +static int exynos_rng_set_seed(struct exynos_rng_dev *rng,
> +			       const u8 *seed, unsigned int slen)
> +{
> +	u32 val;
> +	int i;
> +
> +	dev_dbg(rng->dev, "Seeding with %u bytes\n", slen);
> +
> +	if (slen < EXYNOS_RNG_SEED_SIZE) {
> +		dev_warn(rng->dev, "Seed too short (only %u bytes)\n", slen);
> +		return -EINVAL;
> +	}
> +
> +	for (i = 0 ; i < EXYNOS_RNG_SEED_REGS ; i++) {
> +		val = seed[i * 4] << 24;
> +		val |= seed[i * 4 + 1] << 16;
> +		val |= seed[i * 4 + 2] << 8;
> +		val |= seed[i * 4 + 3] << 0;
> +
> +		exynos_rng_writel(rng, val, EXYNOS_RNG_SEED(i));
> +	}

Would it make sense to add another outer loop here to allow all of slen to be 
injected into the DRNG? Note, in some cases, a user wants to add more seed 
into the DRNG than the actual seed size. In this case, the DRNG acts as a 
compression operation of entropy. This is used when the entropy-to-data ratio 
is not 1:1. In a lot of cases, users have a seed which has less entropy in 
bits per data bit.

Ciao
Stephan