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Date:   Wed, 5 Aug 2020 11:34:32 -0400
From:   tytso@....edu
To:     Willy Tarreau <w@....eu>
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"

On Wed, Aug 05, 2020 at 04:49:41AM +0200, Willy Tarreau wrote:
> 
> Not only was this obviously not the goal, but I'd be particularly
> interested in seeing this reality demonstrated, considering that
> the whole 128 bits of fast_pool together count as a single bit of
> entropy, and that as such, even if you were able to figure the
> value of the 32 bits leaked to net_rand_state, you'd still have to
> guess the 96 other bits for each single entropy bit :-/

Not only that, you'd have to figure out which 32-bits in the fast_pool
actually had gotten leaked to the net_rand_state.

I agree with Willy that I'd love to see an exploit since it would
probably give a lot of insights.  Maybe at a Crypto rump session once
it's safe to have those sorts of things again.  :-)

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.

Cheers,

					- Ted

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