The Internet of Things (IoT) devices exchange certificates and authorization tokens over the IEEE 802.15.4 radio medium that supports a maximum transmission unit (MTU) of 127 bytes. However, these credentials are significantly larger than the MTU and are, therefore, sent in a large number of fragments. As IoT devices are resource constrained and battery powered, there are considerable computations and communication overheads for fragment processing both on the sender and receiver devices, which limit their ability to serve real-time requests. Moreover, the fragment processing operations increase energy consumption by CPUs and radio transceivers, which results in shorter battery life. In this article, we propose CATComp—a compression-aware authorization protocol for constrained application protocol (CoAP) and datagram transport layer security (DTLS) that enables IoT devices to exchange small-sized certificates and capability tokens over the IEEE 802.15.4 media. CATComp introduces additional messages in the CoAP and DTLS handshakes that allow communicating devices to negotiate a compression method, which devices use to reduce the credentials' sizes before sending them over an IEEE 802.15.4 link. The decrease in the size of the security materials minimizes the total number of packet fragments, communication overheads for fragment delivery, fragment processing delays, and energy consumption. As such, devices can respond to requests faster and have longer battery life. We implement a prototype of CATComp on Contiki-enabled RE-Mote IoT devices and provide a performance analysis of CATComp. The experimental results show that communication latency and energy consumption are reduced when CATComp is integrated with CoAP and DTLS.