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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 13 Jan 2022 09:08:40 +0100
Message-ID: <CAMj1kXFhoyADsW9+79boCEV6zG7RZ3HquFStBUbGZoKEWVJAwQ@mail.gmail.com>
Subject: Re: [PATCH RFC v1 0/3] remove remaining users of SHA-1
To:     Sandy Harris <sandyinchina@gmail.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <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>,
        "Ted Ts'o" <tytso@mit.edu>
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On Thu, 13 Jan 2022 at 04:24, Sandy Harris <sandyinchina@gmail.com> wrote:
>
> Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> > There are currently two remaining users of SHA-1 left in the kernel: bpf
> > tag generation, and ipv6 address calculation.
>
> I think there are three, since drivers/char/random.c also uses it.
> Moreover, there's some inefficiency there (or was last time I
> looked) since it produces a 160-bit hash then folds it in half
> to give an 80-bit output.
>

That code was removed, hence the two /remaining/ users.

> A possible fix would be to use a more modern 512-bit hash.
> SHA3 would be the obvious one, but Blake2 would work,
> Blake3 might be faster & there are several other possibilities.
> Hash context size would then match ChaCha so you could
> update the whole CC context at once, maybe even use the
> same context for both.
>
> That approach has difficulties, Extracting 512 bits every
> time might drain the input pool too quickly & it is overkill
> for ChaCha which should be secure with smaller rekeyings.
>
> If you look at IPsec, SSL & other such protocols, many
> have now mostly replaced the hash-based HMAC
> constructions used in previous generations with things
> like Galois field calculations (e.g. AES-GCM) or other
> strange math (e,g. poly 1305). These have most of the
> desirable properties of hashes & are much faster. As
> far as I know, they all give 128-bit outputs.
>
> I think we should replace SHA-1 with GCM. Give
> ChaCha 128 bits somewhat more often than current
> code gives it 256.

You are conflating MACs with hashes. A MAC does is not suitable for
backtrack resistance, and GHASH in particular is really only suited to
be used in the context of GCM.