Hybrid Key Exchange and Signature Design for Quantum-safe C-ITS

Secure vehicular communication systems are critical for enabling technologies like Cooperative Intelligent Transportation Systems (C-ITS), which support the development of Connected Autonomous Vehicles. These critical communications rely on secure protocols that use public-key cryptographic algorithms for digital signatures and encryption within a publickey infrastructure (PKI). However, the emergence of large-scale, fault-tolerant quantum computers poses a significant security threat, as it will eventually compromise current asymmetric cryptographic schemes. In response to this challenge, our previous research [1] analyzed the performance of Post-Quantum Cryptography (PQC) algorithms under standardization and provided guidance for replacing vulnerable cryptographic schemes. This paper builds on those findings by introducing hybridization solutions to facilitate the transition of C-ITS to quantum-safe cryptography. Specifically, we present the design and specification of a hybrid PKI integrating different certificate types, including partial hybrid certificates for end-entities and full hybrid certificates for PKI authorities. Furthermore, we propose a novel hybrid key establishment scheme that combines Elliptic Curve Integrated Encryption Scheme (ECIES) with Module-Lattice-Based KeyEncapsulation Mechanism (ML-KEM), ensuring security even if one scheme fails.