Re: Preserving the ability to have both SHA1 and SHA256 signatures

From: Christian Couder <christian.couder@gmail.com>
To: dwh@linuxprogrammer.org
Cc: git <git@vger.kernel.org>
Subject: Re: Preserving the ability to have both SHA1 and SHA256 signatures
Date: Sat, 8 May 2021 08:39:28 +0200	[thread overview]
Message-ID: <CAP8UFD0vp-zZv=Q1+KWv8PHnxTuspTw2aSCUp8QUic0HOSyq4w@mail.gmail.com> (raw)
In-Reply-To: <20210508022225.GH3986@localhost>

Hi,

(Not sure why, but, when using "Reply to all" in Gmail, it doesn't
actually reply to you (or Cc you), only to the mailing list. I had to
manually add your email back.)

On Sat, May 8, 2021 at 4:25 AM <dwh@linuxprogrammer.org> wrote:
>
> Hi Everybody,
>
> I was reading through the
> Documentation/technical/hash-function-transition.txt doc and realized
> that the plan is to support allowing BOTH SHA1 and SHA256 signatures to
> exist in a single object:
>
> > Signed Commits
> > 1. using SHA-1 only, as in existing signed commit objects
> > 2. using both SHA-1 and SHA-256, by using both gpgsig-sha256 and gpgsig
> >   fields.
> > 3. using only SHA-256, by only using the gpgsig-sha256 field.
> >
> > Signed Tags
> > 1. using SHA-1 only, as in existing signed tag objects
> > 2. using both SHA-1 and SHA-256, by using gpgsig-sha256 and an in-body
> >   signature.
> > 3. using only SHA-256, by only using the gpgsig-sha256 field.
>
> The design that I'm working on only supports a single signature that
> uses a combination of fields: one 'signtype', zero or more 'signoption'
> and one 'sign' in objects.

Here I understand that your design doesn't support both a SHA1 and a
SHA256 signature.

> I am thinking that the best thing to do is
> replace the gpgsig-sha256 fields in objects and allow old gpgsig (commits)
> and in-body (tags) signatures to co-exist along side to give the same
> functionality.

Is this part of your design, or a, maybe temporary, alternative to it?

> That not only paves the way forward but preserves the full backward
> compatibility that is one of my top requirements.

There has been patches and discussions quite recently about this, that
have been reported on in our Git Rev News newsletter:

https://git.github.io/rev_news/2021/02/27/edition-72/

You can see that, with the latest patches (not sure the documentation
is up-to-date though), signing both commits and tags
 can now be round-tripped through both SHA-1 and SHA-256 conversions.
How isn't that fully backward compatible?

Best,
Christian.