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

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Message-ID:  <loom.20071204T184742-19@post.gmane.org>
Date:	Tue, 4 Dec 2007 18:49:40 +0000 (UTC)
From:	Russ Dill <Russ.Dill@....edu>
To:	linux-kernel@...r.kernel.org
Subject:  Re: Why does reading from /dev/urandom deplete entropy so much?

Marc Haber <mh+linux-kernel <at> zugschlus.de> writes:

> 
> 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.
> 
> 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.

An application either needs to be cryptographically secure, or it doesn't. If it
doesn't, then just use /dev/urandom to seed a PRNG.

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