Re: Why does reading from /dev/urandom deplete entropy so much?

[Bill Davidsen]

Matt Mackall wrote:
> On Tue, Dec 04, 2007 at 08:54:52AM -0800, Ray Lee wrote:
> > (Why hasn't anyone been cc:ing Matt on this?)
> >
> > On Dec 4, 2007 8:18 AM, Adrian Bunk <bunk@xxxxxxxxxx> wrote:
> > > On Tue, Dec 04, 2007 at 12:41:25PM +0100, Marc Haber wrote:
> > >
> > > > While debugging Exim4's GnuTLS interface, I recently found out that
> > > > reading from /dev/urandom depletes entropy as much as reading from
> > > > /dev/random would. This has somehow surprised me since I have always
> > > > believed that /dev/urandom has lower quality entropy than /dev/random,
> > > > but lots of it.
> > > man 4 random
> > >
> > > > This also means that I can "sabotage" applications reading from
> > > > /dev/random just by continuously reading from /dev/urandom, even not
> > > > meaning to do any harm.
> > > >
> > > > Before I file a bug on bugzilla,
> > > > ...
> > > The bug would be closed as invalid.
> > >
> > > No matter what you consider as being better, changing a 12 years old and
> > > widely used userspace interface like /dev/urandom is simply not an
> > > option.
> > You seem to be confused. He's not talking about changing any userspace
> > interface, merely how the /dev/urandom data is generated.
> >
> > For Matt's benefit, part of the original posting:
> >
> > > Before I file a bug on bugzilla, can I ask why /dev/urandom wasn't
> > > implemented as a PRNG which is periodically (say, every 1024 bytes or
> > > even more) seeded from /dev/random? That way, /dev/random has a much
> > > higher chance of holding enough entropy for applications that really
> > > need "good" entropy.
> > A PRNG is clearly unacceptable. But roughly restated, why not have
> > /dev/urandom supply merely cryptographically strong random numbers,
> > rather than a mix between the 'true' random of /dev/random down to the
> > cryptographically strong stream it'll provide when /dev/random is
> > tapped? In principle, this'd leave more entropy available for
> > applications that really need it, especially on platforms that don't
> > generate a lot of entropy in the first place (servers).
>
> The original /dev/urandom behavior was to use all the entropy that was
> available, and then degrade into a pure PRNG when it was gone. The
> intent is for /dev/urandom to be precisely as strong as /dev/random
> when entropy is readily available.
>
> The current behavior is to deplete the pool when there is a large
> amount of entropy, but to always leave enough entropy for /dev/random
> to be read. This means we never completely starve the /dev/random
> side. The default amount is twice the read wakeup threshold (128
> bits), settable in /proc/sys/kernel/random/.
In another post I suggested having a minimum bound (use not entropy) and 
a maximum bound (grab some entropy) with the idea that between these 
values some limited entropy could be used. I have to wonder if the 
entropy available is at least as unpredictable as the entropy itself.

> But there's really not much point in changing this threshold. If
> you're reading the /dev/random side at the same rate or more often
> that entropy is appearing, you'll run out regardless of how big your
> buffer is.
Right, my thought is to throttle user + urandom use such that the total 
stays below the available entropy. I had forgotten that that was a lower 
bound, although it's kind of an on-off toggle rather than proportional. 
Clearly if you care about this a *lot* you will use a hardware RNG.

Thanks for the reminder on read_wakeup.

--
Bill Davidsen <davidsen@xxxxxxx>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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