RE: Cross-signing commits

["Randall S. Becker" <rsbecker@nexbridge.com>]

On March 5, 2021 4:59 PM, brian m. carlson wrote:
> On 2021-03-05 at 21:53:14, Soni L. wrote:
> > On 2021-03-05 6:44 p.m., brian m. carlson wrote:
> > > Can you explain what you mean by "cross-signing"?  Are you proposing
> > > a situation where two parties sign the same commit?
> >
> > Yep. See, the repos enforce signing, but they can also be forks. If
> > someone wants to track upstream in one of their branches they just
> > can't. Would be cool if they could just say they trust the commits by
> > signing the relevant commits with their own key instead - on the
> > assumption that they actually reviewed said commits.
> 
> Git doesn't natively support having multiple signatures in a commit, although
> it is of course possible to do, since OpenPGP supports it.
> However, as you noted, changing the signature changes the object ID, so if
> you re-sign a commit for any reason, that changes the commit ID.
> There isn't any way around this at all; that's just how it works.
> 
> So you can either re-sign or have an unchanged commit ID, but not both at
> the same time.
> 
> You can use additional empty signed commits or signed tags, or you can use
> some sort of external system that keeps track of additional signatures or
> approvals if you want.

If your workflow requires multiple signatures on the same commit, you have options:

1. Use signed tags. So you can put multiple OpenPGP signed tags on a commit, representing each person's individual sign-off. Tags would be my preference as they show up explicitly in the git log --decorate output. Of course you will need a naming standard for this class of tags.
2. Create empty child commits with the desired commit as parent, and each person can sign their own commit - not really a great idea as history gets messy and potentially confusing.

Regards,
Randall

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