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

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "Hannes Frederic Sowa" <hannes@stressinduktion.org>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"open list:BPF JIT for MIPS (32-BIT AND 64-BIT)"
	<netdev@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Geert Uytterhoeven" <geert@linux-m68k.org>,
	"Herbert Xu" <herbert@gondor.apana.org.au>,
	"Jean-Philippe Aumasson" <jeanphilippe.aumasson@gmail.com>,
	"Linux Crypto Mailing List" <linux-crypto@vger.kernel.org>,
	"Erik Kline" <ek@google.com>,
	"Fernando Gont" <fgont@si6networks.com>,
	"Lorenzo Colitti" <lorenzo@google.com>,
	hideaki.yoshifuji@miraclelinux.com
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation
Date: Thu, 13 Jan 2022 13:22:14 +0100	[thread overview]
Message-ID: <CAHmME9paa0Z+wBza4gDT3xPzKqhGk9AoL433sOu9H=NHxiZA_Q@mail.gmail.com> (raw)
In-Reply-To: <CAMj1kXGz7_98B_b=SJER6-Q2g-nOT5X3cfN=nfhYoH0eHep5bw@mail.gmail.com>

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.