HM-AKA+: Protecting against a semi-trusted third party with Hybrid Crypto

Abstract

Hybrid protocols that combine Post-Quantum (PQ) and traditional primitives, allowing flexible switching based on the threat level, provide optimal trade-offs between efficiency and security. The recently published HM-AKA scheme introduced a basic framework for this setup, but it fails to provide security against impersonation attacks by a semi-trusted TTP in some of its proposed modes. This vulnerability arises due to the asymmetric be havior of PQ primitives compared to traditional public-key primitives. We identify this threat and propose an enhanced version, HM-AKA+, that maintains the framework’s flexibility while ensuring security.