Re: ipv4: Simplify ARP hash function.

From: Martin Mares <mj@ucw.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 :-)