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

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Message-ID: <CALx6S349hOFhnMgM_TgKXC1O7bmOvR87Nm=5B7_sNLEWiZU8Zg@mail.gmail.com>
Date:   Wed, 14 Dec 2016 15:14:01 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     "Jason A. Donenfeld" <Jason@...c4.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

On Wed, Dec 14, 2016 at 2:56 PM, Jason A. Donenfeld <Jason@...c4.com> wrote:
> 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.
>
Yeah, that's a "bug" with jhash function names.

> 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.
>
I'm confused, doesn't 2dword == 1qword? Anyway, I think the qword
functions are good enough. If someone needs to hash over some odd
length they can either put them in a structure padded to 64 bits or
call the hash function that takes a byte length.

>
>> 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.

I'd still drop the "24" unless you really think we're going to have
multiple variants coming into the kernel.

Tom