netdev - Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

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Message-ID: <CAHmME9rxCYfwyF6EADWqpAEt+yqCPgCLUVH0FPdAy7r-oPnrRg@mail.gmail.com>
Date:   Fri, 16 Dec 2016 16:51:35 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
Cc:     George Spelvin <linux@...encehorizons.net>,
        Andi Kleen <ak@...ux.intel.com>,
        David Miller <davem@...emloft.net>,
        David Laight <David.Laight@...lab.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        kernel-hardening@...ts.openwall.com,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Netdev <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Theodore Ts'o" <tytso@....edu>, vegard.nossum@...il.com,
        "Daniel J . Bernstein" <djb@...yp.to>
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

Hi JP & George,

My function names:
- SipHash -> siphash
- HalfSipHash -> hsiphash

It appears that hsiphash can produce either 32-bit output or 64-bit
output, with the output length parameter as part of the hash algorithm
in there. When I code this for my kernel patchset, I very likely will
only implement one output length size. Right now I'm leaning toward
32-bit. Questions:

- Is this a reasonable choice?
- When hsiphash is desired due to its faster speed, are there any
circumstances in which producing a 64-bit output would actually be
useful? Namely, are there any hashtables that could benefit from a
64-bit functions?
- Are there reasons why hsiphash with 64-bit output would be
reasonable? Or will we be fine sticking with 32-bit output only?

With both hsiphash and siphash, the division of usage will probably become:
- Use 64-bit output 128-bit key siphash for keyed RNG-like things,
such as syncookies and sequence numbers
- Use 64-bit output 128-bit key siphash for hashtables that must
absolutely be secure to an extremely high bandwidth attacker, such as
userspace directly DoSing a kernel hashtable
- Use 32-bit output 64-bit key hsiphash for quick hashtable functions
that still must be secure but do not require as large of a security
margin

Sound good?

Jason