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Date:   Fri, 14 Jan 2022 18:58:09 +0100
From:   "Jason A. Donenfeld" <>
To:     Hannes Frederic Sowa <>
Cc:     Toke Høiland-Jørgensen <>,
        Netdev <>,
        LKML <>,
        Geert Uytterhoeven <>,
        Herbert Xu <>,
        Ard Biesheuvel <>,
        Jean-Philippe Aumasson <>,
        Linux Crypto Mailing List <>,
        Erik Kline <>,
        Fernando Gont <>,
        Lorenzo Colitti <>,
        Hideaki Yoshifuji <>
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
<> 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! :)

> 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.


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