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From: "George Spelvin" <linux@sciencehorizons.net>
To: Jason@zx2c4.com, tom@herbertland.com
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
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF
Date: 16 Dec 2016 15:41:28 -0500	[thread overview]
Message-ID: <20161216204128.25034.qmail@ns.sciencehorizons.net> (raw)
In-Reply-To: <CALx6S351VFRZmEQphRQy6YtmZYPnOtTN7=XiNrJmhWJGv4HUBg@mail.gmail.com>

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.

  reply	other threads:[~2016-12-16 20:41 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAGiyFdfmiCMyHvAg=5sGh8KjBBrF0Wb4Qf=JLzJqUAx4yFSS3Q@mail.gmail.com>
2016-12-15 23:28 ` [PATCH v5 1/4] siphash: add cryptographically secure PRF George Spelvin
2016-12-16 17:06   ` David Laight
2016-12-16 17:09     ` Jason A. Donenfeld
2016-12-16  3:46 ` George Spelvin
     [not found]   ` <CAGiyFdd6_LVzUUfFcaqMyub1c2WPvWUzAQDCH+Aza-_t6mvmXg@mail.gmail.com>
2016-12-16 12:39     ` Jason A. Donenfeld
2016-12-16 19:47       ` Tom Herbert
2016-12-16 20:41         ` George Spelvin [this message]
2016-12-16 20:57           ` Tom Herbert
2016-12-16 20:44         ` [kernel-hardening] " Daniel Micay
2016-12-16 21:09           ` Jason A. Donenfeld
2016-12-17 15:21         ` George Spelvin
2016-12-19 14:14           ` David Laight
2016-12-19 18:10             ` George Spelvin
     [not found]       ` <CAGiyFddB_HT3H2yhYQ5rprYZ487rJ4iCaH9uPJQD57hiPbn9ng@mail.gmail.com>
2016-12-16 15:51         ` Jason A. Donenfeld
2016-12-16 17:36           ` George Spelvin
2016-12-16 18:00             ` Jason A. Donenfeld
2016-12-16 20:17               ` George Spelvin
2016-12-16 20:43                 ` Theodore Ts'o
2016-12-16 22:13                   ` George Spelvin
2016-12-16 22:15                     ` Andy Lutomirski
2016-12-16 22:18                     ` Jason A. Donenfeld
2016-12-16 23:44                       ` George Spelvin
2016-12-17  1:39                         ` Jason A. Donenfeld
2016-12-17  2:15                           ` George Spelvin
2016-12-17 15:41                             ` [kernel-hardening] " Theodore Ts'o
2016-12-17 16:14                               ` Jeffrey Walton
2016-12-19 17:21                               ` Jason A. Donenfeld
2016-12-17 12:42           ` George Spelvin
2016-12-16 20:39         ` Jason A. Donenfeld
2016-12-16 20:49 Jason A. Donenfeld
2016-12-16 21:25 ` George Spelvin
  -- strict thread matches above, loose matches on Subject: below --
2016-12-16 20:43 Jason A. Donenfeld
2016-12-15 20:29 [PATCH v5 0/4] The SipHash Patchset Jason A. Donenfeld
2016-12-15 20:30 ` [PATCH v5 1/4] siphash: add cryptographically secure PRF Jason A. Donenfeld
2016-12-15 22:42   ` George Spelvin
2016-12-16  2:14   ` kbuild test robot
2016-12-17 14:55   ` Jeffrey Walton
2016-12-19 17:08     ` Jason A. Donenfeld

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