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Date: Fri, 16 Dec 2016 12:57:44 -0800
From: Tom Herbert <>
To: George Spelvin <>
Cc: "Jason A. Donenfeld" <>, Andi Kleen <>, 
	"David S. Miller" <>, David Laight <>, 
	"Daniel J . Bernstein" <>, Eric Biggers <>, 
	Hannes Frederic Sowa <>, 
	Jean-Philippe Aumasson <>,, 
	Linux Crypto Mailing List <>, LKML <>, 
	Andy Lutomirski <>, 
	Linux Kernel Network Developers <>, Linus Torvalds <>, 
	"Theodore Ts'o" <>,
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

On Fri, Dec 16, 2016 at 12:41 PM, George Spelvin
<> wrote:
> 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?
Sorry, that is over an IPv4 tuple. Intel(R) Xeon(R) CPU E5-2660 0 @
2.20GHz. Recoded the function I was using to look like more like 64
bit version and yes it is indeed slower.

> 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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