linux-kernel - Re: [PATCH v3 1/3] siphash: add cryptographically secure hashtable function

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Date:   Wed, 14 Dec 2016 23:56:41 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        "Daniel J . Bernstein" <djb@...yp.to>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers3@...il.com>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v3 1/3] siphash: add cryptographically secure hashtable function

Hey Tom,

On Wed, Dec 14, 2016 at 10:35 PM, Tom Herbert <tom@...bertland.com> wrote:
> Those look good, although I would probably just do 1,2,3 words and
> then have a function that takes n words like jhash. Might want to call
> these dword to distinguish from 32 bit words in jhash.

So actually jhash_Nwords makes no sense, since it takes dwords
(32-bits) not words (16-bits). The siphash analog should be called
siphash24_Nqwords.

I think what I'll do is change what I already have to:
siphash24_1qword
siphash24_2qword
siphash24_3qword
siphash24_4qword

And then add some static inline helpers to assist with smaller u32s
like ipv4 addresses called:

siphash24_2dword
siphash24_4dword
siphash24_6dword
siphash24_8dword

While we're having something new, might as well call it the right thing.

> Also, what is the significance of "24" in the function and constant
> names? Can we just drop that and call this siphash?

SipHash is actually a family of PRFs, differentiated by the number of
SIPROUNDs after each 64-bit input is processed and the number of
SIPROUNDs at the very end of the function. The best trade-off of speed
and security for kernel usage is 2 rounds after each 64-bit input and
4 rounds at the end of the function. This doesn't fall to any known
cryptanalysis and it's very fast.