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

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Date:   Fri, 16 Dec 2016 14:15:57 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     George Spelvin <linux@...encehorizons.net>
Cc:     "Ted Ts'o" <tytso@....edu>, Andi Kleen <ak@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        David Laight <David.Laight@...lab.com>,
        "D. J. Bernstein" <djb@...yp.to>,
        Eric Biggers <ebiggers3@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vegard Nossum <vegard.nossum@...il.com>
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

On Fri, Dec 16, 2016 at 2:13 PM, George Spelvin
<linux@...encehorizons.net> wrote:
>> What should we do with get_random_int() and get_random_long()?  In
>> some cases it's being used in performance sensitive areas, and where
>> anti-DoS protection might be enough.  In others, maybe not so much.
>
> This is tricky.  The entire get_random_int() structure is an abuse of
> the hash function and will need to be thoroughly rethought to convert
> it to SipHash.  Remember, SipHash's security goals are very different
> from MD5, so there's no obvious way to do the conversion.
>
> (It's *documented* as "not cryptographically secure", but we know
> where that goes.)
>
>> If we rekeyed the secret used by get_random_int() and
>> get_random_long() frequently (say, every minute or every 5 minutes),
>> would that be sufficient for current and future users of these
>> interfaces?
>
> Remembering that on "real" machines it's full SipHash, then I'd say that
> 64-bit security + rekeying seems reasonable.
>
> The question is, the idea has recently been floated to make hsiphash =
> SipHash-1-3 on 64-bit machines.  Is *that* okay?
>
>
> The annoying thing about the currently proposed patch is that the *only*
> chaining is the returned value.  What I'd *like* to do is the same
> pattern as we do with md5, and remember v[0..3] between invocations.
> But there's no partial SipHash primitive; we only get one word back.
>
> Even
>         *chaining += ret = siphash_3u64(...)
>
> would be an improvement.

This is almost exactly what I suggested in my email on the other
thread from a few seconds ago :)

--Andy