An optical line terminal (OLT) including a processor coupled to a transmitter. The processor is configured to send a first encrypted fiber to coax unit (FCU) message containing an optical domain multicast key to an FCU via an optical network. The optical domain multicast key is associated with encryption in an optical domain associated with the optical network. The processor is also configured to send a second encrypted FCU message containing an electrical domain multicast key to the FCU, and to send an encrypted coax network unit (CNU) message containing the electrical domain multicast key to a CNU via the FCU and a coaxial network. The electrical domain multicast key is associated with encryption in an electrical domain associated with the coaxial network. The first and second encrypted FCU messages and the encrypted CNU message may be operations, administration and maintenance (OAM) messages.