From: "George Spelvin" Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF Date: 16 Dec 2016 15:41:28 -0500 Message-ID: <20161216204128.25034.qmail@ns.sciencehorizons.net> References: Cc: ak@linux.intel.com, davem@davemloft.net, David.Laight@aculab.com, djb@cr.yp.to, ebiggers3@gmail.com, hannes@stressinduktion.org, jeanphilippe.aumasson@gmail.com, kernel-hardening@lists.openwall.com, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, linux@sciencehorizons.net, luto@amacapital.net, netdev@vger.kernel.org, torvalds@linux-foundation.org, tytso@mit.edu, vegard.nossum@gmail.com To: Jason@zx2c4.com, tom@herbertland.com Return-path: Received: from ns.sciencehorizons.net ([71.41.210.147]:33522 "HELO ns.sciencehorizons.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1757532AbcLPUld (ORCPT ); Fri, 16 Dec 2016 15:41:33 -0500 In-Reply-To: Sender: linux-crypto-owner@vger.kernel.org List-ID: Tom Herbert wrote: > Tested this. Distribution and avalanche effect are still good. Speed > wise I see about a 33% improvement over siphash (20 nsecs/op versus 32 > nsecs). That's about 3x of jhash speed (7 nsecs). So that might closer > to a more palatable replacement for jhash. Do we lose any security > advantages with halfsiphash? What are you testing on? And what input size? And does "33% improvement" mean 4/3 the rate and 3/4 the time? Or 2/3 the time and 3/2 the rate? These are very odd results. On a 64-bit machine, SipHash should be the same speed per round, and faster because it hashes more data per round. (Unless you're hitting some unexpected cache/decode effect due to REX prefixes.) On a 32-bit machine (other than ARM, where your results might make sense, or maybe if you're hashing large amounts of data), the difference should be larger. And yes, there is a *significant* security loss. SipHash is 128 bits ("don't worry about it"). hsiphash is 64 bits, which is known breakable ("worry about it"), so we have to do a careful analysis of the cost of a successful attack. As mentioned in the e-mails that just flew by, hsiphash is intended *only* for 32-bit machines which bog down on full SipHash. On all 64-bit machines, it will be implemented as an alias for SipHash and the security concerns will Just Go Away. The place where hsiphash is expected to make a big difference is 32-bit x86. If you only see 33% difference with "gcc -m32", I'm going to be very confused.