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Date: Fri, 16 Dec 2016 14:15:57 -0800
From: Andy Lutomirski <>
To: George Spelvin <>
Cc: "Ted Ts'o" <>, Andi Kleen <>, 
	"David S. Miller" <>, David Laight <>, 
	"D. J. Bernstein" <>, Eric Biggers <>, 
	Hannes Frederic Sowa <>, "Jason A. Donenfeld" <>, 
	Jean-Philippe Aumasson <>, 
	"" <>, 
	Linux Crypto Mailing List <>, 
	"" <>, Network Development <>, 
	Tom Herbert <>, Linus Torvalds <>, 
	Vegard Nossum <>
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

On Fri, Dec 16, 2016 at 2:13 PM, George Spelvin
<> 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 :)


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