netdev - Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation

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Message-ID: <CAHmME9qy4-qkBAD9fJ6jqHxw2DYtscerZdriMYXw1T4iPD6Y-A@mail.gmail.com>
Date:   Fri, 14 Jan 2022 18:58:09 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ard Biesheuvel <ardb@...nel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Erik Kline <ek@...gle.com>,
        Fernando Gont <fgont@...networks.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Hideaki Yoshifuji <hideaki.yoshifuji@...aclelinux.com>
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation

Hi Hannes,

On Fri, Jan 14, 2022 at 6:44 PM Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> I don't think we can argue our way out of this by stating that there are
> no guarantees anyway, as much as I would like to change the hash
> function as well.

Shucks. Alright then.

> As much as I know about the problems with SHA1 and would like to see it
> removed from the kernel as well, I fear that in this case it seems hard
> to do. I would propose putting sha1 into a compilation unit and
> overwrite the compiler flags to optimize the function optimized for size
> and maybe add another mode or knob to switch the hashing algorithm if
> necessary.

Already on it! :)
https://lore.kernel.org/linux-crypto/20220114154247.99773-3-Jason@zx2c4.com/

> I haven't investigated recent research into breakage of SHA1, I mostly
> remember the chosen-image and collision attacks against it. Given the
> particular usage of SHA1 in this case, do you think switching the
> hashing function increases security?

Considering we're only using 64-bits of SHA-1 output, I don't think
the SHA-1 collision attacks give you that much here. And it seems like
there are other network-level security concerns with the whole scheme
anyway. So it might not be the largest of matters. However...

> I am asking because of the desire
> to decrease the instruction size of the kernel

Indeed this is what I was hoping for.

Jason