netdev - Re: Extensible hashing and RCU

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Message-ID: <20070220155928.GA26600@2ka.mipt.ru>
Date:	Tue, 20 Feb 2007 18:59:28 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	"Michael K. Edwards" <medwards.linux@...il.com>
Cc:	David Miller <davem@...emloft.net>, dada1@...mosbay.com,
	akepner@....com, linux@...izon.com, netdev@...r.kernel.org,
	bcrl@...ck.org
Subject: Re: Extensible hashing and RCU

On Tue, Feb 20, 2007 at 07:49:11AM -0800, Michael K. Edwards (medwards.linux@...il.com) wrote:
> On 2/20/07, Evgeniy Polyakov <johnpol@....mipt.ru> wrote:
> >Jenkins _does_ have them, I showed tests half a year ago and in this
> >thread too. Actually _any_ hash has them it is just a matter of time
> >to find one.
> 
> I think you misunderstood me.  If you are trying to DoS me from
> outside with a hash collision attack, you are trying to feed me
> packets that fall into the same hash bucket.  The Jenkins hash does
> not have to be artifact-free, and does not have to be
> cryptographically strong.  It just has to do a passable job of mixing
> a random salt into the tuple, so you don't know which string of
> packets to feed me in order to fill one (or a few) of my buckets.
> XORing salt into a folded tuple doesn't help; it just permutes the
> buckets.

Adding XOR with constant value does not change distribution.
Variable salt will end up with differnet buckets for the same flow.
It is forbidden - it is not the situation created for passwd/des decades
ago.

> Cheers,
> - Michael

-- 
	Evgeniy Polyakov
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