RE: [PATCH v5 1/4] siphash: add cryptographically secure PRF

[David Laight <David.Laight@ACULAB.COM>]

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