Post-quantum hybrid key exchange: a systematic mapping study

A key exchange protocol (KEX) is a mechanism that allows two parties to secure their communications using encryption. Since it was apparent that large-scale quantum computers can comfortably break most commonly used cryptographic protocols, proposals of quantum-safe KEX began to appear. Some of these proposals combine well-known classical cryptographic key exchange protocols with novel post-quantum schemes. The main rationale behind this hybrid structure is to retain the time-tested trust on "pre-quantum KEXs" while facilitating a smoother transition toward a post-quantum world. In this paper we present a systematic mapping study of post-quantum hybrid KEXs, given insights into their characteristics, terminology, efficiency, security and open design challenges. Furthermore, we propose a hybrid KEX classification taking into account several relevant design considerations associated to these schemes. Our systematic mapping shows that primary research in this subject strongly suggests that hybrid KEXs achieve an acceptable performance for a number of important applications, and that they can fit into widely used network protocols. These features indicate that hybrid KEX will shortly become an important building block for secure network communications, even in a worst-case scenario where large-scale quantum computers are prevalent.