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

Matt Mackall
2007-12-04T18:02:44Z

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@kernel.org> 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/.

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.

--
Mathematics is the supreme nostalgia of our time.