The user a node runs as.

A secret macula reads from disk has to belong to this user, so that another user of the host cannot give the node a key file of their own. On a host with user ids it is the process's effective user id, the owner of every file the node creates, read with the crypto NIF. When that NIF did not load, the call raises nif_not_loaded instead of skipping the check. On a host without user ids it is none, and owner checks are skipped.