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Date: Fri, 16 Dec 2016 17:06:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'George Spelvin' <linux@...encehorizons.net>, "ak@...ux.intel.com"
<ak@...ux.intel.com>, "davem@...emloft.net" <davem@...emloft.net>,
"ebiggers3@...il.com" <ebiggers3@...il.com>, "hannes@...essinduktion.org"
<hannes@...essinduktion.org>, "Jason@...c4.com" <Jason@...c4.com>,
"jeanphilippe.aumasson@...il.com" <jeanphilippe.aumasson@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"luto@...capital.net" <luto@...capital.net>, "netdev@...r.kernel.org"
<netdev@...r.kernel.org>, "tom@...bertland.com" <tom@...bertland.com>,
"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
"tytso@....edu" <tytso@....edu>, "vegard.nossum@...il.com"
<vegard.nossum@...il.com>
CC: "djb@...yp.to" <djb@...yp.to>
Subject: RE: [PATCH v5 1/4] siphash: add cryptographically secure PRF
From: George Spelvin
> Sent: 15 December 2016 23:29
> > If a halved version of SipHash can bring significant performance boost
> > (with 32b words instead of 64b words) with an acceptable security level
> > (64-bit enough?) then we may design such a version.
>
> I was thinking if the key could be pushed to 80 bits, that would be nice,
> but honestly 64 bits is fine. This is DoS protection, and while it's
> possible to brute-force a 64 bit secret, there are more effective (DDoS)
> attacks possible for the same cost.
A 32bit hash would also remove all the issues about the alignment
of IP addresses (etc) on 64bit systems.
> (I'd suggest a name of "HalfSipHash" to convey the reduced security
> effectively.)
>
> > Regarding output size, are 64 bits sufficient?
>
> As a replacement for jhash, 32 bits are sufficient. It's for
> indexing an in-memory hash table on a 32-bit machine.
It is also worth remembering that if the intent is to generate
a hash table index (not a unique fingerprint) you will always
get collisions on the final value.
Randomness could still give overlong hash chains - which might
still need rehashing with a different key.
David
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