Clarify info on commit email addresses

Brendan Lynchrequested to merge
584734-clarify-commit-email-info into master

What does this MR do?

Clarifies that SSH commit signature verification requires the committer email to match a verified email address on the user's GitLab account.

User feedback:

It is not clear at all how gitlab.com decides which commit email addresses are accepted for signed commits (similar to gpg.ssh.allowedSignersFile on a local machine). This page makes it seem as if adding the public key to gitlab.com is sufficient to verify all commits signed with the private key, irrespective of the commit email (and this would make sense, since everyone can set every email address for commits). However, only the Gitlab account email(s) seem(s) to be accepted.

The docs state that GitLab uses the SSH public keys associated with your GitLab account to cryptographically verify the commit signature, which implies that adding the key was sufficient. However, verification also requires the committer email in git config user.email to match a verified email on the account (the same requirement documented for GPG signing).

Verification logic

SSH verification (lib/gitlab/ssh/signature.rb):

def committer?
  # Lookup by email because users can push verified commits that were made
  # by someone else. For example: Doing a rebase.
  committer = User.find_by_any_email(committer_email)
  committer && signed_by_key.user == committer
end

def signed_by_user_email_verified?
  signed_by_key.user.verified_emails.include?(committer_email)
end

GPG verification (lib/gitlab/gpg/signature.rb):

if gpg_key.verified_and_belongs_to_email?(email)
  :verified
elsif gpg_key.user.all_emails.include?(email)
  :same_user_different_email
else
  :other_user
end

Both check if the committer email matches the signing user's verified emails. The key difference is that SSH requires the email to be verified (verified_emails), while GPG allows unverified emails but marks them as same_user_different_email instead of fully verified.

Related issues

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