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To: "David G. Andersen" <danderse@cs.utah.edu>
From: Mark Murray <markm@freebsd.org>
In-Reply-To: Your message of "Thu, 18 Sep 2003 19:36:36 MDT."
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cc: freebsd-security@freebsd.org
Subject: Re: [Fwd: Re: FreeBSD Security Advisory FreeBSD-SA-03:12.openssh] 
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"David G. Andersen" writes:
>    You're mistaken.  /dev/random stops feeding you random bits
> when it doesn't have enough.  /dev/urandom depletes the entropy
> pool, but when it starts to run out, it falls back to hashing
> to generate pseudo-random sequences from the random bits that
> it can obtain.

Mostly correct :-).

/dev/urandom (in FreeBSD-4-*) always hashes the pool. It doesn't care
whether or not entropy has been harvested first, unlike /dev/random
which requires a positive entropy count before suppying output.
(This provides a doozy of a DoS, BTW, where "cat /dev/urandom > /dev/null"
renders /dev/random useless).

M
--
Mark Murray
iumop ap!sdn w,I idlaH