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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 13 Jan 2022 13:29:39 +0100
Message-ID: <CAMj1kXHHvm6jeoWZVcRuRtxos3MJajMkuFj4-Hu6ZADjxu=y3A@mail.gmail.com>
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Hannes Frederic Sowa <hannes@stressinduktion.org>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
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        Herbert Xu <herbert@gondor.apana.org.au>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
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        Lorenzo Colitti <lorenzo@google.com>,
        hideaki.yoshifuji@miraclelinux.com
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On Thu, 13 Jan 2022 at 13:22, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> 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.

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.