Konrad-Felix Krentz (Uppsala University), Thiemo Voigt (Uppsala University, RISE Computer Science)

Object Security for Constrained RESTful Environments (OSCORE) is an end-to-end security solution for the Constrained Application Protocol (CoAP), which, in turn, is a lightweight application layer protocol for the Internet of things (IoT). The recently standardized Echo option allows OSCORE servers to check if a request was created recently. Previously, OSCORE only offered a counter-based replay protection, which is why delayed OSCORE requests were accepted as fresh. However, the Echo-based replay protection entails an additional round trip, thereby prolonging delays, increasing communication overhead, and deteriorating reliability. Moreover, OSCORE remains vulnerable to a denial-of-sleep attack. In this paper, we propose a version of OSCORE with a revised replay protection, namely OSCORE next-generation (OSCORE-NG). OSCORENG fixes OSCORE’s denial-of-sleep vulnerability and provides freshness guarantees that surpass those of the Echo-based replay protection, while dispensing with an additional round trip. Furthermore, in long-running sessions, OSCORE-NG incurs even less communication overhead than OSCORE’s counter-based replay protection. OSCORE-NG’s approach is to entangle timestamps in nonces. Except during synchronization, CoAP nodes truncate these timestamps in outgoing OSCORE-NG messages. Receivers fail to restore a timestamp if and only if an OSCORE-NG message is delayed by more than 7.848s in our implementation by default. In effect, older OSCORE-NG messages get rejected.