Code review comment for lp:~xnox/ubuntu-archive-publishing/migrate-dist-upgrade-to-4k

> I'm not sure which is the more obvious place for us to enforce this.

I think the more transparent place to enforce it is in the code. Maybe the keys in the keyring should have better defaults, but if we want a specific digest, we should be explicit about that in the code, not implicitly hope that the keyring has it set how we want (even if/when it does).