Abstract: A method and apparatus for conducting secured multicast in an asynchronous transfer mode based passive optical network communication (“APON”) system having a central network device, such as an optical line terminal (“OLT”), and multiple network devices, such as optical network terminals (“ONT”), whereby the central network device communicates with the network devices using multicast encryption key that is selected or generated from among the churning keys belonging to the network devices, and whereby the central network device periodically updates the multicast encryption key by delivering to the network devices the difference values between the updated multicast encryption key and the respective churning key of each network device.