Paper 2025/983

LP2+: a robust symmetric-key AKE protocol with perfect forward secrecy, and an advocacy for thorough security proofs

Abstract

Symmetric-key authenticated key establishment (AKE) protocols are particularly well suited in resource constraint environments such as internet of things (IoT) devices. Moreover, they often rely on better understood assumptions than asymmetric ones. In this paper, we review the security model for symmetric-key AKE protocols. We show why several existing models allow trivial attacks while they do not protect against some non-trivial ones. We fix these issues with our new security definitions. We show that the protocols $\textrm{LP2}$ and $\textrm{LP3}$ of Boyd et al. do not satisfy the claimed security properties. We propose a new 2-message protocol based on them, called $\textrm{LP2+}$. This protocol is proved to satisfy correctness, weak synchronization robustness, entity authentication, key indistinguishability and, as a consequence, it admits perfect forward secrecy. An instantiation of $\textrm{LP2+}$ is presented, whose security only relies on that of a pseudo-random function (PRF). Its total execution time in normal cases is dominated by only 14 evaluations of the PRF, making it a lightweight protocol that is particularly well suited for resource-constrained environments such as IoT devices. The flaws found in the security models as well as in the security arguments could have been avoided with precise and detailed proofs. We thus take this paper as an opportunity to advocate for thorough security proofs. Therefore, we have made the choice of rigor over concision.