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Message-ID: <1590546.5d4hGCCWgT@positron.chronox.de>
Date: Tue, 18 Jul 2017 10:37:40 +0200
From: Stephan Müller <smueller@...onox.de>
To: Arnd Bergmann <arnd@...db.de>
Cc: "Jason A. Donenfeld" <jason@...c4.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-crypto@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v12 2/4] random: conditionally compile code depending on LRNG
Am Dienstag, 18. Juli 2017, 10:13:55 CEST schrieb Arnd Bergmann:
Hi Arnd,
> On Tue, Jul 18, 2017 at 9:58 AM, Stephan Müller <smueller@...onox.de> wrote:
> > When selecting the LRNG for compilation, disable add_disk_randomness and
> > its supporting function.
> >
> > CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > CC: Arnd Bergmann <arnd@...db.de>
> > CC: Jason A. Donenfeld <Jason@...c4.com>
> > Signed-off-by: Stephan Mueller <smueller@...onox.de>
>
> I think this needs a better explanation. Why do we ignore the extra
> entropy here?
I was not sure whether to add all the details about the reason into the patch
submission.
The reason is explained here in [1] page 3 and re-iterated in [2].
The gist is the following:
A HID or block device event providing entropy to the respective individual
noise sources processing generates an interrupt. These interrupts are also
processed by the interrupt noise source. The majority of entropy is delivered
by the high-resolution time stamp of the occurrence of such an event. Now,
that event is processed twice in the legacy /dev/random implementation: once
by the HID or block device noise source and once by the interrupt noise
source. Thus, the two time stamps of the one event (HID noise source and
interrupt noise source, or block device noise source and interrupt noise
source) used as a basis for entropy are highly correlated. Correlation or even
a possible reuse of the same random value diminishes entropy significantly.
The additional data provided via the block noise source (block device number)
has no real entropy.
Bottom line: for entropy, the HID and block device noise sources are just a
derivative of the interrupt noise source. Thus, discarding the block device
noise source will not lose any entropy. Regarding the HID noise source, only
the key/mouse event numbers are injected into the LRNG without attributing any
entropy to them.
[1] http://www.chronox.de/lrng/doc/lrng.pdf
[2] https://www.spinics.net/lists/linux-crypto/msg26316.html
Ciao
Stephan
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