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

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Date:	Sun, 9 Dec 2007 06:21:50 +0100
From:	Willy Tarreau <w@....eu>
To:	Theodore Tso <tytso@....edu>,
	Ismail Dönmez <ismail@...dus.org.tr>,
	Adrian Bunk <bunk@...nel.org>,
	Bill Davidsen <davidsen@....com>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

On Sat, Dec 08, 2007 at 06:46:12PM -0500, Theodore Tso wrote:
> On Sun, Dec 09, 2007 at 12:10:10AM +0200, Ismail Dönmez wrote:
> > > As long as /dev/random is readable for all users there's no reason to
> > > use /dev/urandom for a local DoS...
> > 
> > Draining entropy in /dev/urandom means that insecure and possibly not random 
> > data will be used and well thats a security bug if not a DoS bug.
> 
> Actually in modern 2.6 kernels there are two separate output entropy
> pools for /dev/random and /dev/urandom.  So assuming that the
> adversary doesn't know the contents of the current state of the
> entropy pool (i.e., the RNG is well seeded with entropy), you can read
> all you want from /dev/urandom and that won't give an adversary
> successful information to attack /dev/random.

Wouldn't it be possible to mix the data with the pid+uid of the reading
process so that even if another one tries to collect data from urandom,
he cannot predict what another process will get ? BTW, I think that the
tuple (pid,uid,timestamp of open) is unpredictable and uncontrollable
enough to provide one or even a few bits of entropy by itself.

Regards,
Willy

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