IoT devices are typically resource-limited in terms of network bandwidth and energy capacities. While, various efforts from the Internet Engineering Task Force (IETF) has designed various IoT-related standards such as 6LoWPAN, RPL and CoAP, integrating certificate-based Internet security protocols to IoT networks is yet a progressing task. We see the main cause of this being the size of the X.509-based Internet certificates. Typically, being 1-2 KBs, the large size of these certificates can cause IEEE 802.15.4-based IoT nodes to fragment the certificate into many smaller packet-size chunks, which causes many packet transmissions to occur in the network. This thesis presents LightCert, a lightweight scheme to compress the size of the security certificates using the similarity of contents in X.509 certificates. Specifically, LightCert identifies common fields in a certificate and suppresses the transmission of these contents within the IoT subnet scope. This allows LightCert nodes to minimize the packet transmission overhead for supporting certificate-based security mechanisms such as DTLS, by as much as ∼37%. The added overhead of exchanging certificates when using LightCert is kept low to as much as ∼5 mJ of energy and ∼0.48 sec of latency.