Testing the limits of SPDM: Authentication of intermittently connected devices

The Security Protocol and Data Model (SPDM) is an open standard for authentication, attestation, and key exchange among hardware units, such as CPUs and peripheral components. In principle, SPDM was designed to operate over a somewhat stable communication channel, meaning that connection losses usually require the re-execution of the entire protocol. This puts into question SPDM’s suitability for battery-powered devices, which may keep only intermittent communications aiming to save energy. To address this question, we evaluate different authentication approaches that build upon and extend SPDM’s native key bootstrapping capabilities to handle intermittent authentication. In particular, we show that the combination of SPDM and a Time-based One-Time Password (TOTP) protocol is a promising solution for this scenario. We analyze the performance of the proposed authentication schemes using a proof-of-concept virtual device. The TOTP-based scheme was shown to be the fastest, the reconnection step being at least twice and up to $900\times$ faster than possible straightforward applications of SPDM. Also, our scheme requires less memory to operate. Finally, we discuss the possibility of integrating intermittent authentication capabilities into the SPDM standard itself.