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Date: 16 Dec 2016 15:41:28 -0500
From: "George Spelvin" <linux@...encehorizons.net>
To: Jason@...c4.com, tom@...bertland.com
Cc: ak@...ux.intel.com, davem@...emloft.net, David.Laight@...lab.com,
djb@...yp.to, ebiggers3@...il.com, hannes@...essinduktion.org,
jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
linux@...encehorizons.net, luto@...capital.net, netdev@...r.kernel.org,
torvalds@...ux-foundation.org, tytso@....edu, vegard.nossum@...il.com
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF
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.
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