O. Garcia-Morchon Philips Research Philips Research Wireless IP-based systems will be increasingly used for building control systems in the future where wireless devices interconnect with each other, forming low-power and lossy networks (LLNs). The CoAP/6LoWPAN standards are emerging as the de-facto protocols in this area for resource-constrained devices. Both multicast and security are key needs in these networks. This draft presents a method for securing multicast communication in LLNs based on the DTLS security protocol which is already present in CoAP devices. This is achieved by using unicast DTLS-protected communication channel to distribute keying material and security parameters to group members. Group keys consisting of a Traffic Encryption Key (TEK) and a Traffic Authentication Key (TAK) are generated by group members based on the keying material received. A group member uses its DTLS record layer implementation to encrypt a multicast message and provide message authentication using the group keys before sending the message via IP multicast to the group.