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

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Date:	Thu, 7 Nov 2013 02:03:57 +0100
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	Theodore Ts'o <tytso@....edu>, Pavel Machek <pavel@....cz>,
	sandy harris <sandyinchina@...il.com>,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and
	/dev/random

On Wed, 06 Nov 2013, Stephan Mueller wrote:

> Am Mittwoch, 6. November 2013, 07:43:54 schrieb Theodore Ts'o:
> 
> Hi Theodore,
> 
> >On Wed, Nov 06, 2013 at 12:49:45PM +0100, Stephan Mueller wrote:
> >> Here is a quote from his answer to my question whether he was able to
> >> identify the root cause:
> >> 
> >> "its inherent in the microtiming of Hardware and there is nothing you
> >> can do about it if you want the root cause is quantum physics"
> >
> >That's unfortunate, since it leaves open the question of whether this
> >jitter is something that could be at least somewhat predictable if you
> >had a lot more information about the internal works of the CPU or
> >not....
> 
> I do not understand that answer: I thought we are talking about the 
> search of non-predictable noise sources. If you cannot predict the 
> sequence even if you have the state of the CPU, that is what we are 
> looking for, is it not?

unpredictabilitty of the individual event does not imply that you do not have
the ability to "guess" with more than 50% - that is just because it is not 
predictable
does not mean itt is bias-free (and more importantly here that the bias is not 
influenced by controllable factors like load). Attached is a simple 
demonstration of this problem (gauss.c) that runs in user-space and harvestts 
entropy by race occurrence, while the distribution is a nice normal 
distribution (on multticore it is an overlay of multtiple normal 
distributions - one per core-pairing possible) it is not load independent. 
Never the less the individual event (race/no-race) is not predictable.

> 
> Besides, how on earth shall an attacker even gain knowledge about the 
> state of the CPU or disable CPU mechanisms? Oh, I forgot, your NSA guy. 
> But if he is able to do that, all discussions are moot because he simply 
> disables any noise sources by flipping a bit, reads the memory that is 
> used to hold the state of the RNG or just overwrites the memory 
> locations where data is collected, because the general protection 
> mechanisms offered by the kernel and the underlying hardware are broken.

No need to gain knowledge of the internal CPU state itt would be
sufficient to be able to put the CPU in a sub-state-space in which
the distribution is shifted. it may be enough to reduce the truely random
bits of some key only by a few bits to make it suceptible to brute force
attacks.

> 
> Also, does your answer mean you would disregard radioactive decay that 
> is not predictable due to quantum physics and Heisenberg?
>
maybe I missed something - but what does radioactive decay induced 
emission have to do with Heissenberg ? 

thx!
hofrat

View attachment "gauss.c" of type "text/x-csrc" (4361 bytes)