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Date:   Thu, 13 Jan 2022 13:29:39 +0100
From:   Ard Biesheuvel <>
To:     "Jason A. Donenfeld" <>
Cc:     Hannes Frederic Sowa <>,
        Toke Høiland-Jørgensen <>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        Linux Kernel Mailing List <>,
        Geert Uytterhoeven <>,
        Herbert Xu <>,
        Jean-Philippe Aumasson <>,
        Linux Crypto Mailing List <>,
        Erik Kline <>,
        Fernando Gont <>,
        Lorenzo Colitti <>,
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation

On Thu, 13 Jan 2022 at 13:22, Jason A. Donenfeld <> wrote:
> On 1/13/22, Ard Biesheuvel <> wrote:
> >
> > The question is not whether but when we can/will change this.
> >
> > SHA-1 is broken and should be removed at *some* point, so unless the
> > feature itself is going to be obsolete, its implementation will need
> > to switch to a PRF that fulfils the requirements in RFC7217 once SHA-1
> > ceases to do so.
> >
> > And I should also point out that the current implementation does not
> > even use SHA-1 correctly, as it omits the finalization step. This may
> > or may not matter in practice, but it deviates from crypto best
> > practices, as well as from RFC7217
> >
> > I already pointed out to Jason (in private) that the PRF does not need
> > to be based on a cryptographic hash, so as far as I can tell, siphash
> > would be a suitable candidate here as well, and I already switched the
> > TCP fastopen code to that in the past. But SHA-1 definitely has to go.
> >
> Correction: this should be a cryptographically secure.

Of course. I said it does not need to be based on a cryptographic *hash*.

> That's part of
> the point of moving away from SHA-1 of course. But fortunately,
> siphash *is*
> considered to be cryptographically secure. Whether you want blake2s's
> keyed mode or siphash doesn't really matter to me. I thought the
> former's API mapped a bit neater here.

Fair enough. This is not on a hot path anyway, so it doesn't really
matter performance wise.

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