GnuPG stub keys

"Stub" keys are references telling to GnuPG that it can find given private keys on ?smart cards (including "dongles").

The following options exist to create a stub key exist:

Both will automatically create the stubs in the .gnupg/private-keys-v1.d/ directory associated with them.

Stubs can be seen in the output of --list-private-keys through the presence of # and > after sec and ssb: