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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 13 Jan 2022 13:22:14 +0100
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Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Hannes Frederic Sowa <hannes@stressinduktion.org>,
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        Herbert Xu <herbert@gondor.apana.org.au>,
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        Lorenzo Colitti <lorenzo@google.com>,
        hideaki.yoshifuji@miraclelinux.com
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On 1/13/22, Ard Biesheuvel <ardb@kernel.org> 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. 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.