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

From: Matt Mackall <mpm@selenic.com>
To: Marc Haber <mh+linux-kernel@zugschlus.de>
Cc: Eric Dumazet <dada1@cosmosbay.com>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Adrian Bunk <bunk@kernel.org>,
	linux-kernel@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?
Date: Mon, 10 Dec 2007 17:35:25 -0600	[thread overview]
Message-ID: <20071210233525.GK19691@waste.org> (raw)
In-Reply-To: <20071210230643.GC32133@torres.zugschlus.de>

On Tue, Dec 11, 2007 at 12:06:43AM +0100, Marc Haber wrote:
> On Sun, Dec 09, 2007 at 10:16:05AM -0600, Matt Mackall wrote:
> > On Sun, Dec 09, 2007 at 01:42:00PM +0100, Marc Haber wrote:
> > > On Wed, Dec 05, 2007 at 03:26:47PM -0600, Matt Mackall wrote:
> > > > The distinction between /dev/random and /dev/urandom boils down to one
> > > > word: paranoia. If you are not paranoid enough to mistrust your
> > > > network, then /dev/random IS NOT FOR YOU. Use /dev/urandom.
> > > 
> > > But currently, people who use /dev/urandom to obtain low-quality
> > > entropy do a DoS for the paranoid people.
> > 
> > Not true, as I've already pointed out in this thread.
> 
> I must have missed this. Can you please explain again? For a layman it
> looks like a paranoid application cannot read 500 Bytes from
> /dev/random without blocking if some other application has previously
> read 10 Kilobytes from /dev/urandom.

/dev/urandom always leaves enough entropy in the input pool for
/dev/random to reseed. Thus, as long as entropy is coming in, it is
not possible for /dev/urandom readers to starve /dev/random readers.
But /dev/random readers may still block temporarily and they should
damn well expect to block if they read 500 bytes out of a 512 byte
pool.

-- 
Mathematics is the supreme nostalgia of our time.