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

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Date:   Sat, 17 Dec 2016 11:14:47 -0500
From:   Jeffrey Walton <noloader@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        kernel-hardening@...ts.openwall.com,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        George Spelvin <linux@...encehorizons.net>, ak@...ux.intel.com,
        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>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        luto@...capital.net, Netdev <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vegard Nossum <vegard.nossum@...il.com>
Subject: Re: [kernel-hardening] Re: [PATCH v5 1/4] siphash: add
 cryptographically secure PRF

> As far as half-siphash is concerned, it occurs to me that the main
> problem will be those users who need to guarantee that output can't be
> guessed over a long period of time.  For example, if you have a
> long-running process, then the output needs to remain unguessable over
> potentially months or years, or else you might be weakening the ASLR
> protections.  If on the other hand, the hash table or the process will
> be going away in a matter of seconds or minutes, the requirements with
> respect to cryptographic strength go down significantly.

Perhaps SipHash-4-8 should be used instead of SipHash-2-4. I believe
SipHash-4-8 is recommended for the security conscious who want to be
more conservative in their security estimates.

SipHash-4-8 does not add much more processing. If you are clocking
SipHash-2-4 at 2.0 or 2.5 cpb, then SipHash-4-8 will run at 3.0 to
4.0. Both are well below MD5 times. (At least with the data sets I've
tested).

> Now, maybe this doesn't matter that much if we can guarantee (or make
> assumptions) that the attacker doesn't have unlimited access the
> output stream of get_random_{long,int}(), or if it's being used in an
> anti-DOS use case where it ultimately only needs to be harder than
> alternate ways of attacking the system.
>
> Rekeying every five minutes doesn't necessarily help the with respect
> to ASLR, but it might reduce the amount of the output stream that
> would be available to the attacker in order to be able to attack the
> get_random_{long,int}() generator, and it also reduces the value of
> doing that attack to only compromising the ASLR for those processes
> started within that five minute window.

Forgive my ignorance... I did not find reading on using the primitive
in a PRNG. Does anyone know what Aumasson or Bernstein have to say?
Aumasson's site does not seem to discuss the use case:
https://www.google.com/search?q=siphash+rng+site%3A131002.net. (And
their paper only mentions random-number once in a different context).

Making the leap from internal hash tables and short-lived network
packets to the rng case may leave something to be desired, especially
if the bits get used in unanticipated ways, like creating long term
private keys.

Jeff