Due to the limited resources of Internet of Things (IoT) devices, Symmetric Key Cryptography (SKC) is typically favored over resource-intensive Public Key Cryptography (PKC) to secure communication between IoT devices. To utilize SKC, devices need to execute a key exchange protocol to establish a session key before initiating communication. However, existing SKC-based key exchange protocols assume that communication devices have a pre-shared secret or there are trusted intermediaries between them; neither is always realistic in IoT. We introduce a new SKC-based key exchange protocol for IoT devices. While also intermediary based, our protocol fundamentally departs from existing intermediary-based solutions in that intermediaries between two key exchange devices may be malicious, and moreover, our protocol can detect cheating behaviors and identify malicious intermediaries. We prove our protocol is secure under the universally composable model, and show that it can detect malicious intermediaries with probability 1.0. We implemented and evaluated our protocol on different IoT devices. We show that our protocol has significant improvements in computation time and energy cost. Compared to the PKC-based protocols ECDH, DH, and RSA, our protocol is 2.3 to 1,591 times faster on one of the two key exchange devices and 0.7 to 4.67 times faster on the other.

In order for resource-constrained Internet of Things (IoT) devices to set up secure communication channels to exchange confidential messages, Symmetric Key Cryptography (SKC) is usually preferred to resource-intensive Public Key Cryptography (PKC). At the core of setting up a secure channel is secure key exchange, the process of two IoT devices securely agreeing on a common session key before they communicate. While compared to using PKC, key exchange using SKC is more resource-aware for IoT environments, it requires either a pre-shared secret or trusted intermediaries between the two devices; neither assumption is realistic in IoT. In this paper, we relax the above assumptions and introduce a new intermediary-based secure key exchange protocol for IoT devices that do not support PKC. With a design that is lightweight and deployable in IoT, our protocol fundamentally departs from existing intermediary-based solutions in that (1) it leverages intermediary parties that can be malicious and (2) it can detect malicious intermediary parties. We provide a formal proof that our protocol is secure and conduct a theoretical analysis to show the failure probability of our protocol is easily negligible with a reasonable setup and its malicious helper detection probability can be 1.0 even when a malicious helper only tampers a small number of messages. We implemented our protocol and our experimental results show that our protocol significantly improves the computation time and energy cost. Dependent on the IoT device type (Raspberry Pi, Arduino Due, or Sam D21) and the PKC algorithms to compare against (ECDH, DH, or RSA), our protocol is 2.3 to 1591 times faster on one of the two devices and 0.7 to 4.67 times faster on the other.