In the Smart Grids context, all communications must be handled in a secure way, including multicast traffic. The Application Layer Multicast (ALM) algorithms provide better flexibility and can employ security mechanisms, however, causes overhead to all nodes to build the multicast tree. In this work is proposed another approach to provide a secure multicast focusing on filtering packets on nodes without need an overlay protocol. It uses the multihop property of Wireless Mesh Networks (WMN) usually employed to bring connectivity to smart meters. Also, there is the support to message authentication code (MAC) using symmetric cryptography and presents an algorithm to provide a secure key distribution system. The results show that this approach is lightweight, secure, and assures multicast message delivery, even on failures caused by attacks on the key distribution system. The key management protocol used to provide authentication and integrity are evaluated using an automated test tool.