Re: Problem with random.c and PPC

[Oliver Xymoron <oxymoron@waste.org>]

On Sat, Aug 17, 2002 at 03:09:50AM -0600, Andreas Dilger wrote:
> > 
> > Not sure this is an ideal fix. We might instead have an entropy
> > low-water mark (say 1/2 pool size), below which /dev/urandom will not
> > deplete the pool. This way when we have ample entropy, both devices
> > will behave like TRNGs, with /dev/urandom falling back to PRNG when a
> > shortage is threatened.
> 
> Well, I can think of a few mechanisms that would work better than a
> simple on/off method that you are proposing.  The current code will
> basically "fill" the urandom pool each time it is depleted, and then
> when the entropy is gone it will just go on dumping out data.  You
> could make urandom only get more entropy each N times through its pool,
> or make N a function of the "fullness" of the available entropy.  Then
> if a system has lots of entropy sources urandom is TRNG, but if not
> it will gracefully degrade from TRNG to PRNG without wiping out all
> the entropy in the process.
> 
> As an alternative, instead of taking poolsize bytes of entropy each
> N uses, you could take some small amount of entropy to mix into the
> pool slowly.

Actually, that gives us exactly the scenario catastrophic reseeding
intends to avoid. Presume for a moment that the internal state of the
PRNG is somehow known to an attacker - the PRNG has been broken. If we
take eight bits out of the entropy pool, mix them in, then generate a
new random number, an attacker needs to only test 256 possible pool
states before he knows the entire state again. If he keeps up this
state extension attack, /dev/urandom will never return to an unknown
state. If, on the other hand, /dev/urandom waits until there are, say,
64 entropy bits to mix in, it can make a leap to a state that the
attacker will have a very hard time guessing and the PRNG has
recovered.

We merely need to avoid starvation of /dev/random, and I think a
straightforward low watermark approach will do that nicely. Starving
/dev/urandom is no worse than feeding it a trickle. 

-- 
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