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

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Date:	Tue, 4 Dec 2007 08:54:52 -0800
From:	"Ray Lee" <ray@...rabbit.org>
To:	"Adrian Bunk" <bunk@...nel.org>, "Matt Mackall" <mpm@...enic.com>
Cc:	"Marc Haber" <mh+linux-kernel@...schlus.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

(Why hasn't anyone been cc:ing Matt on this?)

On Dec 4, 2007 8:18 AM, Adrian Bunk <bunk@...nel.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).

Ray
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