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

From: David Laight <David.Laight@ACULAB.COM>
To: "'Jason A. Donenfeld'" <Jason@zx2c4.com>,
	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 15:59:21 +0000	[thread overview]
Message-ID: <13d51088799746469d26a442fb3c6fd5@AcuMS.aculab.com> (raw)
In-Reply-To: <CAHmME9oa8dAeRQfgj-U00gUtVOJ_CTGwtyBxUB4=8+XO_fFjNQ@mail.gmail.com>

From: Jason A. Donenfeld
> Sent: 14 January 2022 15:21
> 
> 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...

Which probably means that SHA-1 is complete overkill and something
much simpler could have been used instead.
Is the buffer even big enough to have ever warranted the massive
unrolling of the sha-1 function.
(I suspect that just destroys the I-cache on most cpu.)

The IPv6 address case seems even more insane - how many bytes
are actually being hashed.
The unrolled loop is only likely to be sane for large (megabyte)
buffers.

	David

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