Recently an efficient multicasting protocol has been reported which uses a concept known as pseudo diameter to reduce the number of duplicate packets generated during multicasting in wide area networks (WANs). This work is superior to the Distance Vector Multicast Routing protocols (DVMRP) from the viewpoint of better bandwidth utilization. In this thesis security aspect of the above mentioned multicast protocol has been considered. Asymmetric key cryptography concept has been used effectively to design the protocol.