From: Matt Mackall <mpm@selenic.com>
To: Ray Lee <ray@madrabbit.org>
Cc: Adrian Bunk <bunk@kernel.org>,
Marc Haber <mh+linux-kernel@zugschlus.de>,
linux-kernel@vger.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?
Date: Tue, 4 Dec 2007 12:01:59 -0600 [thread overview]
Message-ID: <20071204180158.GT19691@waste.org> (raw)
In-Reply-To: <2c0942db0712040854u17a830b9see663742b2716457@mail.gmail.com>
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.
next prev parent reply other threads:[~2007-12-04 18:02 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-04 11:41 Why does reading from /dev/urandom deplete entropy so much? Marc Haber
2007-12-04 14:16 ` Eric Dumazet
2007-12-04 16:18 ` Adrian Bunk
2007-12-04 16:47 ` Alan Cox
2007-12-04 18:17 ` Eric Dumazet
2007-12-05 21:26 ` Matt Mackall
2007-12-06 7:02 ` Eric Dumazet
2007-12-06 16:09 ` Matt Mackall
2007-12-09 12:42 ` Marc Haber
2007-12-09 16:16 ` Matt Mackall
2007-12-10 23:06 ` Marc Haber
2007-12-10 23:35 ` Matt Mackall
2007-12-11 1:34 ` Theodore Tso
2007-12-11 19:46 ` Phillip Susi
2007-12-11 20:02 ` Ray Lee
2007-12-12 5:34 ` David Schwartz
2007-12-04 16:54 ` Ray Lee
2007-12-04 16:55 ` Alan Cox
2007-12-04 18:02 ` Matt Mackall
2007-12-04 19:50 ` Theodore Tso
2007-12-04 20:36 ` Matt Mackall
2007-12-04 20:40 ` Alan Cox
2007-12-04 20:48 ` Mike McGrath
2007-12-04 21:54 ` Matt Mackall
2007-12-04 22:03 ` Theodore Tso
2007-12-04 22:12 ` Mike McGrath
2007-12-04 22:28 ` Matt Mackall
2007-12-04 21:08 ` Matt Mackall
2007-12-04 21:18 ` Mike McGrath
2007-12-04 22:15 ` Matt Mackall
2007-12-04 22:23 ` Mike McGrath
2007-12-04 22:33 ` Matt Mackall
2007-12-05 14:26 ` Mike McGrath
2007-12-05 14:49 ` Theodore Tso
2007-12-08 7:38 ` Jon Masters
2007-12-08 17:32 ` Theodore Tso
2007-12-08 17:33 ` Mike McGrath
2007-12-08 17:49 ` Theodore Tso
2007-12-08 17:54 ` Jon Masters
2007-12-08 18:15 ` Matt Mackall
2007-12-08 18:24 ` Theodore Tso
2007-12-08 19:36 ` entropy gathering (was Re: Why does reading from /dev/urandom deplete entropy so much?) Jeff Garzik
2007-12-08 19:53 ` Matt Mackall
2007-12-08 20:04 ` Jeff Garzik
2007-12-08 20:19 ` Matt Mackall
2007-12-08 21:07 ` Willy Tarreau
2007-12-08 20:31 ` Theodore Tso
2007-12-08 20:47 ` Jeff Garzik
2007-12-08 20:42 ` Willy Tarreau
2007-12-08 23:47 ` Theodore Tso
2007-12-09 1:07 ` Jon Masters
2007-12-08 18:31 ` Why does reading from /dev/urandom deplete entropy so much? Jeff Garzik
2007-12-08 20:26 ` David Schwartz
2007-12-08 17:43 ` Matt Mackall
2007-12-08 17:47 ` Jon Masters
2007-12-08 18:05 ` Theodore Tso
2007-12-08 17:45 ` Jon Masters
2007-12-10 16:37 ` Pavel Machek
2007-12-04 18:01 ` Matt Mackall [this message]
2007-12-06 20:08 ` Bill Davidsen
2007-12-05 12:23 ` Marc Haber
2007-12-05 12:29 ` Marc Haber
2007-12-05 13:33 ` Theodore Tso
2007-12-05 15:10 ` Marc Haber
2007-12-06 19:32 ` Bill Davidsen
2007-12-08 22:03 ` Adrian Bunk
2007-12-08 22:10 ` Ismail Dönmez
2007-12-08 23:46 ` Theodore Tso
2007-12-09 5:21 ` Willy Tarreau
2007-12-09 6:52 ` Jon Masters
2007-12-09 6:21 ` Ismail Dönmez
2007-12-09 12:31 ` Theodore Tso
2007-12-09 14:06 ` Ismail Dönmez
2007-12-11 15:42 ` Bill Davidsen
2007-12-20 22:27 ` Marc Haber
2007-12-26 18:27 ` Phillip Susi
2007-12-04 18:49 ` Russ Dill
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20071204180158.GT19691@waste.org \
--to=mpm@selenic.com \
--cc=bunk@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mh+linux-kernel@zugschlus.de \
--cc=ray@madrabbit.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).