Towards compactly encoded signed IoT messages

In this paper, we measure the overhead in message size and in firmware code-size for exchanging signed messages with constrained devices typically found in the Internet-of-Things (IoT) domain. The application of digital signatures on every important message in the IoT helps to increase its resilience against attacks and increases the data quality level. Other papers have shown that Elliptic Curve Cryptography (ECC) signatures can be applied on modern constrained devices. Our focus is on message size as we identified this to be the most prevailing problem as it relates to the storage required at each handling node and directly relates to the communication overhead, which in turn results in an increased energy consumption. We give analytical information for typical message sizes in two different protocols (MQTT and COAP), as well as real measurements on a Zolertia ReMOTE when it comes to firmware code-size overheads. Our measurements indicate that a suitable encoding of message level security can decrease the message overhead of signatures by at least 30 percent and still balance its usability by keeping it decodable into JSON, the prevailing exchange format of many IoT upper layer protocols. Our new HYBRID format is suitable to achieve end-to-end, i.e., device-to-application, integrity.