linux-kernel - Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random

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Date:	Mon, 14 Oct 2013 16:40:54 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Sandy Harris <sandyinchina@...il.com>
Cc:	Theodore Ts'o <tytso@....edu>, LKML <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random

Am Montag, 14. Oktober 2013, 10:14:00 schrieb Sandy Harris:

Hi Sandy,

>On Mon, Oct 14, 2013 at 9:38 AM, Sandy Harris <sandyinchina@...il.com> 
wrote:
>> Stephan Mueller <smueller@...onox.de> wrote:
>>> Can you please help me understand why you think that a whitening
>>> function (cryptographic or not) is needed in the case of the CPU
>>> Jitter RNG, provided that I can show that each individual bit
>>> coming from the folding operation has one bit of entropy?
>> 
>> Basically, sheer paranoia. I'd mix and whiten just on general
>> principles. Since efficiency is not a large concern, there is little
>> reason not to.
>> 
>> On the other hand, most RNGs use a hash because they need
>> to distill some large amount of low-entropy input into a smaller
>> high-entropy output. With high input entropy, you do not need
>> the hash and can choose some cheaper mixer.
>
>You could use strong mixing/whitening:
>
>Feed into random(4) and let it do the mixing.

That is exactly the goal with the patch found in patches/linux-3.9-
random.patch in the code distribution.

And that approach is exactly what I do in the linking code / patches for 
other crypto libs:

- kernel crypto API

- OpenSSL (implementation as an Engine that uses the internal DRNGs or a 
hook into RAND_poll that implements the seeding for the DRNGs)

- libgcrypt (hook into the seeding of the DRNGs)

>
>Use some early outputs from your RNG to key an AES
>instance. Then encrypt later outputs; this gives a 64 in 64
>out mixer that is cryptographically strong but perhaps a bit
>slow in the context.

That is exactly what the SP800-90A CTR DRBG or the X9.31 does. As these 
DRNGs are available for different crypto libs, I am simply reusing them 
with the crypto lib linking code.
>
>Alternately, quite a few plausible components for fast cheap
>mixing are readily available.

Thank you for the references. I have seen that in your maxwell(8) 
documentation. But again, I do not re-invent the wheel with the CPU 
Jitter RNG and therefore skipped the whitening step based on the reasons 
above.

Another thing: when you start adding whitening functions, other people 
are starting (and did -- thus I added section 4.3 to my documentation) 
to complain that you hide your weaknesses behind the whiteners. I simply 
want to counter that argument and show that RNG produces white noise 
without a whitener.

Ciao
Stephan
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