International Association
for Cryptologic Research

<p> Transitioning from classically to quantum secure key agreement protocols may require to exchange fundamental components, for example, exchanging Diffie-Hellman-like key exchange with a key encapsulation mechanism (KEM). Accordingly, the corresponding security proof can no longer rely on the Diffie-Hellman assumption, thus invalidating the security guarantees. As a consequence, the security properties have to be re-proven under a KEM-based security notion.</p><p>We initiate the study of the LDACS key agreement protocol (Edition 01.01.00 from 25.04.2023), which is soon-to-be-standardized by the International Civil Aviation Organization. The protocol's cipher suite features Diffie-Hellman as well as a KEM-based key agreement protocol to provide post-quantum security. While the former results in an instantiation of an ISO key agreement inheriting all security properties, the security achieved by the latter is ambiguous. We formalize the computational security using the systematic notions of de Saint Guilhem, Fischlin and Warinshi (CSF '20), and prove the exact security that the KEM-based variant achieves in this model; primarily entity authentication, key secrecy and key authentication. To further strengthen our “pen-and-paper” findings, we model the protocol and its security guarantees using Tamarin, providing an automated proof of the security against a Dolev-Yao attacker. </p>