Date: Thu, 6 Sep 2012 11:28:16 -0700 From: David O'Brien <obrien@FreeBSD.org> To: Dag-Erling =?unknown-8bit?B?U23DuHJncmF2?= <des@des.no> Cc: Arthur Mesh <arthurmesh@gmail.com>, freebsd-security@freebsd.org, RW <rwmaillists@googlemail.com> Subject: Re: svn commit: r239569 - head/etc/rc.d Message-ID: <20120906182816.GE13179@dragon.NUXI.org> In-Reply-To: <86392vqc86.fsf@ds4.des.no> References: <201208221843.q7MIhLU4077951@svn.freebsd.org> <5043DBAF.40506@FreeBSD.org> <20120903005708.7082f230@gumby.homeunix.com> <20120906171824.GC14757@dragon.NUXI.org> <86392vqc86.fsf@ds4.des.no>
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On Thu, Sep 06, 2012 at 07:42:49PM +0200, Dag-Erling Smrgrav wrote:
> David O'Brien <obrien@FreeBSD.org> writes:
> > RW <rwmaillists@googlemail.com> writes:
> > > IMO the order should be reversed or the low-grade stuff should be
> > > piped through sha256.
> > We considered that. Arthur wanted to do it sooner, but I'm concerned
> > about impact of multiple sha256 invocations on a large amount of data
> > on low-end MIPS.
>
> Is there a reason to choose sha256 over a weaker, faster hash?
I see Arthur,
But still wanted to give my own longer responce.
Using a weaker hash could reduce the amount of entropy in the output
(due to collisions).
The Yarrow paper makes this argument (but willing to potentially loose
some entropy) in 5 'The Generic Yarrow Design an Yarrow-160'
The reason is if you take an 'm' bit random value and apply a hash
function that produces 'm' bits of output, the result has less than
'm' bits of entropy due to the collisions that occur. This is a very
minor effect, and overall results in the loss of at most a few bits
of entropy.
For a good entropy input I likely agree with you.
But for the poor-grade entropy input I don't think we want to
prematurely loose any of it.
But I have not fully thought thru potential loss of entropy in a hash
of a hash [where entropy was or wasn't lost].
--
-- David (obrien@FreeBSD.org)
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