Date: Wed, 14 Dec 2016 12:21:15 +0100 From: Hannes Frederic Sowa <hannes@...essinduktion.org> To: "Jason A. Donenfeld" <Jason@...c4.com>, Netdev <netdev@...r.kernel.org>, kernel-hardening@...ts.openwall.com, LKML <linux-kernel@...r.kernel.org>, linux-crypto@...r.kernel.org Cc: Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>, "Daniel J . Bernstein" <djb@...yp.to>, Linus Torvalds <torvalds@...ux-foundation.org>, Eric Biggers <ebiggers3@...il.com> Subject: Re: [PATCH v2 1/4] siphash: add cryptographically secure hashtable function Hello, On 14.12.2016 04:59, Jason A. Donenfeld wrote: > SipHash is a 64-bit keyed hash function that is actually a > cryptographically secure PRF, like HMAC. Except SipHash is super fast, > and is meant to be used as a hashtable keyed lookup function. Can you show or cite benchmarks in comparison with jhash? Last time I looked, especially for short inputs, siphash didn't beat jhash (also on all the 32 bit devices etc.). > SipHash isn't just some new trendy hash function. It's been around for a > while, and there really isn't anything that comes remotely close to > being useful in the way SipHash is. With that said, why do we need this? > > There are a variety of attacks known as "hashtable poisoning" in which an > attacker forms some data such that the hash of that data will be the > same, and then preceeds to fill up all entries of a hashbucket. This is > a realistic and well-known denial-of-service vector. This pretty much depends on the linearity of the hash function? I don't think a crypto secure hash function is needed for a hash table. Albeit I agree that siphash certainly looks good to be used here. > Linux developers already seem to be aware that this is an issue, and > various places that use hash tables in, say, a network context, use a > non-cryptographically secure function (usually jhash) and then try to > twiddle with the key on a time basis (or in many cases just do nothing > and hope that nobody notices). While this is an admirable attempt at > solving the problem, it doesn't actually fix it. SipHash fixes it. I am pretty sure that SipHash still needs a random key per hash table also. So far it was only the choice of hash function you are questioning. > (It fixes it in such a sound way that you could even build a stream > cipher out of SipHash that would resist the modern cryptanalysis.) > > There are a modicum of places in the kernel that are vulnerable to > hashtable poisoning attacks, either via userspace vectors or network > vectors, and there's not a reliable mechanism inside the kernel at the > moment to fix it. The first step toward fixing these issues is actually > getting a secure primitive into the kernel for developers to use. Then > we can, bit by bit, port things over to it as deemed appropriate. Hmm, I tried to follow up with all the HashDoS work and so far didn't see any HashDoS attacks against the Jenkins/SpookyHash family. If this is an issue we might need to also put those changes into stable. > Dozens of languages are already using this internally for their hash > tables. Some of the BSDs already use this in their kernels. SipHash is > a widely known high-speed solution to a widely known problem, and it's > time we catch-up. Bye, Hannes
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