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Date: Fri, 8 Jul 2011 13:44:42 -0700 From: Roland Dreier <roland@...estorage.com> To: David Miller <davem@...emloft.net> Cc: johnwheffner@...il.com, mj@....cz, netdev@...r.kernel.org Subject: Re: ipv4: Simplify ARP hash function. >> Doesn't seem to matter much -- this is now equivalent to >> >> a = key ^ dev->ifindex >> return (a ^ (a >> 8) ^ (a >> 16) ^ (a >> 24)) // (1) >> ^ (rnd ^ (rnd >> 8) ^ (rnd >> 16) ^ (rnd >> 24)) // (2) >> >> where again the attacker controls (1), and (2) is a constant. > Right, but how can you attack it? Show me how you can grow > a hash chain of arbitrary length by modulating the key in > a deterministic way. Well, if two things hash to different buckets with the full hash function, then they already hashed to different buckets without the extra randomness. So why bother with hash_rnd? The answer is that you have to mix hash_rnd into the hash in a nonlinear way, so that an attacker can't know if two values end up in the same bucket or not. With your hash function, the attacker can just compute the hash (without hash_rnd) for all the values of key ^ ifindex and then use all the values that end up in the same bucket. - R. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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