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

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Message-ID: <20071204203636.GB19691@waste.org>
Date:	Tue, 4 Dec 2007 14:36:36 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Theodore Tso <tytso@....edu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ray Lee <ray@...rabbit.org>, Adrian Bunk <bunk@...nel.org>,
	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 Tue, Dec 04, 2007 at 02:50:21PM -0500, Theodore Tso wrote:
> On Tue, Dec 04, 2007 at 12:02:37PM -0600, Matt Mackall wrote:
> > On Tue, Dec 04, 2007 at 04:55:02PM +0000, Alan Cox wrote:
> > > > 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).
> > > 
> > > 
> > > As reported about a month ago, the evidence is that the /dev/random
> > > stream is not cryptographically strong. Collecting uuids generated from
> > > the kernel uuid random generator from the random generator in the kernel
> > > shows abnormal patterns of duplicates.
> > 
> > Pointer, please.
> 
> Alan, are you sure you're not talking about Helge Deller's attempt to
> push a Time-based UUID generator into the kernel because you can get
> duplicates from the current userspace library?
> 
> I've not heard of *any* claim where the kernel uuid random generator
> has been returning duplicates.

Before we added proper locking, it could theoretically happen on SMP
with readers in lockstep. That was early 2.6.

The only serious critique I know of is the Gutterman-Pinkas-Reinman
paper which was a year out of date before publication. Now that
another year has passed, perhaps I should respond to it..

-- 
Mathematics is the supreme nostalgia of our time.
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