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

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Message-ID: <20071204161811.GB15974@stusta.de>
Date:	Tue, 4 Dec 2007 17:18:11 +0100
From:	Adrian Bunk <bunk@...nel.org>
To:	Marc Haber <mh+linux-kernel@...schlus.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

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.

> Greetings
> Marc

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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