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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
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