Paper 2026/569

Hybrid KEM Constructions from Classical PKEs and Post-Quantum KEMs

Yukai Zhang, Fudan University

Haodong Jiang, Henan Key Laboratory of Network Cryptography Technology, Zhengzhou, 450001, Henan, China

Yunlei Zhao, Fudan University

Abstract

The rapid progress of quantum computing threatens widely deployed public-key cryptosystems such as RSA and Diffie–Hellman, accelerating the transition toward post-quantum cryptography (PQC). During this migration, hybrid key encapsulation mechanisms (KEMs) that combine classical and post-quantum primitives are strongly recommended by standardization bodies and cybersecurity agencies. However, existing hybrid designs mainly focus on combining post-quantum KEMs with Diffie–Hellman–style constructions, while the systematic integration of standardized classical public-key encryption (PKE) schemes with post-quantum KEMs remains largely unexplored. In this work, we introduce two generic hybrid constructions, $\mathsf{HybKEM}$ and $\mathsf{HybKEM}^{*}$, that combine a classical PKE scheme with a post-quantum KEM satisfying ciphertext second-preimage resistance (C2PRI). We prove that both constructions achieve IND-CCA security in the standard model. The refined construction $\mathsf{HybKEM}^{*}$ additionally relies on a new security notion of the classical PKE scheme, termed partial ciphertext second-preimage resistance (PC2PRI), which captures second-preimage resistance when a designated ciphertext component is fixed. This new property enables shared-key derivation from only a designated PKE ciphertext component in $\mathsf{HybKEM}^{*}$, leading to improved efficiency. Finally, we provide a systematic analysis of the PC2PRI property for several standardized classical encryption schemes, including ECIES, PSEC, and SM2.

@misc{cryptoeprint:2026/569,
      author = {Biming Zhou and Yukai Zhang and Haodong Jiang and Yunlei Zhao},
      title = {Hybrid {KEM} Constructions from Classical {PKEs} and Post-Quantum {KEMs}},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/569},
      year = {2026},
      url = {https://eprint.iacr.org/2026/569}
}