netdev - Re: ipv4: Simplify ARP hash function.

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Message-Id: <20110708.104739.169518036069870432.davem@davemloft.net>
Date:	Fri, 08 Jul 2011 10:47:39 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mj@....cz
Cc:	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

From: Martin Mares <mj@....cz>
Date: Fri, 8 Jul 2011 19:40:55 +0200

> The hash function is linear, so it can be reduced to:
> 
> 	a = key ^ dev->ifindex
> 	return (a >> 8) ^ (a >> 16) ^ (a >> 24)				// (1)
> 	     ^ (hash_rnd >> 8) ^ (hash_rnd >> 16) ^ (hash_rnd >> 24)	// (2)

Is this really the same?  The inclusion of a full 32-bit xor
with hash_rnd before folding was intentional, so that the
final folding occurs on a completely "random" value.

> Where (1) is under control of the attacker and while (2) is not, the
> only effect of (2) is a random permutation on the hash buckets.
> 
> I.e., the attacker can generate arbitrarily long collision chains,
> although he cannot pick the bucket where the collisions happen :)
> 
> Am I right?

Please give an example :-)
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