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Date: Thu, 22 Dec 2016 16:51:40 +0100
From: Hannes Frederic Sowa <>
To: "Jason A. Donenfeld" <>
Cc:, Theodore Ts'o <>, Andy
 Lutomirski <>, Netdev <>, LKML
 <>,  Linux Crypto Mailing List
 <>, David Laight <>,
 Eric Dumazet <>,  Linus Torvalds
 <>, Eric Biggers <>, Tom
 Herbert <>, Andi Kleen <>, "David S.
 Miller" <>, Jean-Philippe Aumasson
Subject: Re: Re: [PATCH v7 3/6] random: use SipHash in
 place of MD5

On Thu, 2016-12-22 at 16:41 +0100, Jason A. Donenfeld wrote:
> Hi Hannes,
> On Thu, Dec 22, 2016 at 4:33 PM, Hannes Frederic Sowa
> <> wrote:
> > IPv6 you cannot touch anymore. The hashing algorithm is part of uAPI.
> > You don't want to give people new IPv6 addresses with the same stable
> > secret (across reboots) after a kernel upgrade. Maybe they lose
> > connectivity then and it is extra work?
> Ahh, too bad. So it goes.

If no other users survive we can put it into the ipv6 module.

> > The bpf hash stuff can be changed during this merge window, as it is
> > not yet in a released kernel. Albeit I would probably have preferred
> > something like sha256 here, which can be easily replicated by user
> > space tools (minus the problem of patching out references to not
> > hashable data, which must be zeroed).
> Oh, interesting, so time is of the essence then. Do you want to handle
> changing the new eBPF code to something not-SHA1 before it's too late,
> as part of a new patchset that can fast track itself to David? And
> then I can preserve my large series for the next merge window.

This algorithm should be a non-seeded algorithm, because the hashes
should be stable and verifiable by user space tooling. Thus this would
need a hashing algorithm that is hardened against pre-image
attacks/collision resistance, which siphash is not. I would prefer some
higher order SHA algorithm for that actually.


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