netdev - Re: Flaw in "random32: update the net random state on interrupt and activity"

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Date:   Wed, 5 Aug 2020 18:53:04 +0200
From:   Willy Tarreau <w@....eu>
To:     tytso@....edu
Cc:     Marc Plumb <lkml.mplumb@...il.com>, netdev@...r.kernel.org,
        aksecurity@...il.com, torvalds@...ux-foundation.org,
        edumazet@...gle.com, Jason@...c4.com, luto@...nel.org,
        keescook@...omium.org, tglx@...utronix.de, peterz@...radead.org,
        stable@...r.kernel.org
Subject: Re: Flaw in "random32: update the net random state on interrupt and
 activity"

Hi Ted,

On Wed, Aug 05, 2020 at 11:34:32AM -0400, tytso@....edu wrote:
> That being said, it certainly is a certificational / theoretical
> weakness, and if the bright boys and girls at Fort Meade did figure
> out a way to exploit this, they are very much unlikely to share it at
> an open Crypto conference.  So replacing LFSR-based PRnG with
> something stronger which didn't release any bits from the fast_pool
> would certainly be desireable, and I look forward to seeing what Willy
> has in mind.

I'll post a proposal patch shortly about this, hopefully this week-end
(got diverted by work lately :-)). Just to give you a few pointers,
it's a small modification of MSWS. It passes the Practrand test suite
on 256 GB of data with zero warning (something that Tausworthe is
supposed to fail at).

By default, MSWS *does* leak its internal state, as Amit showed us (and
seeing that the paper on it suggests it's safe as-is for crypto use is
a bit shocking), but once slightly adjusted, it doesn't reveal its state
anymore and that would constitute a much more future-proof solution for
quite some time. Tausworthe was created something like 20 years ago or
so, hence it's not surprizing that it's a bit dated by now, but if we
can upgrade once every 2 decades I guess it's not that bad.

Cheers,
Willy