- import: Trust subkeys included in signature
With gpg sub keys one can rotate signing keys while having a stable
trust anchor. So far one still had to ship the sub key out of band but
a newer gpg has the option to include the sub key in the signature and
import it automatically. This is safe if we only allow importing a sub
key signed by the top key we already have in the key ring.
Add the --auto-key-import argument to gpg to import subkeys but also
set --import-options=merge-only,import-clean to restrict what we import
to only be sub keys signed by the top key we have in the keyring and
discard any irrelevant parts.
- import: Support env var to override gpg keyring
By default there is a fixed keyring in /usr or /etc. But when running
systemd-pull unprivileged in the user context or with a custom transfer
definition as in systemd-sysupdate --definitions=./... (e.g., for local
ParticleOS updates) it is limiting to require that all keys have to be
part of the OS keyring or otherwise no verification can be used. Also,
for testing it is valuable to point it at a different keyring.
Add a SYSTEMD_OPENPGP_KEYRING env var where the omission or empty
assignment sticks to the current behavior of the global OS keyrings but
a keyring path given will take precedence. While an env var can leak
down the process tree and is more difficult to secure for being the
trust anchor the advantage is that one can directly specify it in the
service unit as drop-in instead of having to patch the command
invocation. Anyway it's a niche use case and thus not part of the man
page.
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