linux-kernel - Re: issue with /dev/random? gets depleted very quick

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Message-ID: <20090614195854.GD7272@vanheusden.com>
Date:	Sun, 14 Jun 2009 21:58:54 +0200
From:	Folkert van Heusden <folkert@...heusden.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: issue with /dev/random? gets depleted very quick

[ /dev/random gets emptied very quickly ]
...
> > > Is this a problem? It really shouldn't be. Everyone should be
> > > using /dev/urandom anyway. And the anti-starvation threshold guarantees
> > 
> > Well, if I understood correctly how /dev/*random works, urandom is fed
> > by /dev/random. So if there's almost nothing left in the main pool and
> > urandom demands bits then we have an issue.
> > Also, if you frequently want to generate keys (thing gpg, ssl), I think
> > you want bits from /dev/random and not urandom.
> 
> There is really no difference.
> In an ideal world, we could accurately estimate input entropy and thus
> guarantee that we never output more than we took in. But it's pretty
> clear we don't have a solid theoretical basis for estimating the real
> entropy in most, if not all, of our input devices. In fact, I'm pretty
> sure they're all significantly more observable than we're giving them
> credit for. And without that basis, we can only make handwaving
> arguments about the relative strength of /dev/random vs /dev/urandom.
> So if you're running into /dev/random blocking, my advice is to delete
> the device and symlink it to /dev/urandom.

Two questions:
- if the device gets empty constantly, that means that filling
  applicaties (e.g. the ones that feed /dev/random from /dev/hwrng or
  from an audio-source or whatever)
- if we don't know if we're accounting correctly, why doing at all?
  especially if one should use urandom instead of random

> Also note that if something in the kernel is rapidly consuming entropy
> but not visibly leaking it to the world, it is effectively not consuming
> it.

Then the counter should not be decreased?

> In this case, if no one hears the tree fall, it hasn't actually fallen.
> There is exactly as much 'unknown' data in the entropy pool as before.
> If anything, the pool contents are now harder to guess because it's been
> mixed more.


Folkert van Heusden

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