Re: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag calculation

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Network Development" <netdev@vger.kernel.org>,
	LKML <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>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag calculation
Date: Fri, 14 Jan 2022 16:20:33 +0100	[thread overview]
Message-ID: <CAHmME9oa8dAeRQfgj-U00gUtVOJ_CTGwtyBxUB4=8+XO_fFjNQ@mail.gmail.com> (raw)
In-Reply-To: <CAMj1kXE3JtNjgF3FZjbL-GOQG41yODup4+XdEFP063F=-AWg8A@mail.gmail.com>

On Fri, Jan 14, 2022 at 4:08 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> Yeah, so the issue is that, at *some* point, SHA-1 is going to have to
> go. So it would be helpful if Alexei could clarify *why* he doesn't
> see this as a problem. The fact that it is broken means that it is no
> longer intractable to forge collisions, which likley means that SHA-1
> no longer fulfills the task that you wanted it to do in the first
> place.

I think the reason that Alexei doesn't think that the SHA-1 choice
really matters is because the result is being truncated to 64-bits, so
collisions are easy anyway, regardless of which hash function is
chosen (birthday bound and all). But from Geert's perspective, that
SHA-1 is still taking up precious bytes in m68k builds. And from my
perspective, it's poor form and clutters vmlinux, and plus, now I'm
curious about why this isn't using a more appropriately sized tag in
the first place.

On Fri, Jan 14, 2022 at 3:12 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> "checksum" -- the thing is only 64-bits, and as you told Andy Polyakov

Whoops, meant Lutomirski here. x86 Andy, not crypto Andy :)