StarHunters— Secure Hybrid Post-Quantum KEMs From IND-CCA2 PKEs

Deirdre Connolly; Mike Ounsworth; Sophie Schmieg; Douglas Stebila

Paper 2026/427

StarHunters— Secure Hybrid Post-Quantum KEMs From IND-CCA2 PKEs

Deirdre Connolly

, Oracle, Selkie Cryptography

Mike Ounsworth

, Entrust (Canada)

Sophie Schmieg

, Google (United States)

Douglas Stebila

, University of Waterloo

Abstract

This paper formally specifies and analyzes the CK hybrid key encapsulation mechanism (KEM) construction from the IRTF CFRG’s recent draft on hybrid (post-quantum/traditional) KEMs CK combines two KEMs using a PRF to produce a hybrid KEM. Unlike the QSF framework of Barbosa et al., which combines an IND-CCA KEM with a nominal group (Diffie-Hellman-style), CK combines a C2PRI-secure post-quantum-secure KEM with an IND-CCA traditionlly-secure KEM constructed from an IND-CCA2 public key encryption (PKE) scheme, such as RSA-OAEP. We additionally show how to securely promote an IND-CCA2 PKE into an IND-CCA KEM. We perform two complementary security analyses of CK in the standard model: the first shows CK is IND-CCA assuming the traditional KEM is IND-CCA, the post-quantum KEM is C2PRI, and the KDF is a secure PRF; the second shows CK is IND-CCA assuming the post-quantum KEM is IND-CCA and the KDF is a secure PRF, even if the traditional KEM is completely broken. Neither proof requires the random oracle model.

Metadata

Available format(s): PDF
Category: Public-key cryptography
Publication info: Preprint.
Keywords: hybrid KEM KEM IND-CCA C2PRI Post-Quantum Cryptography PKE IND-CCA2 KEM Combiner
Contact author(s): durumcrustulum @ gmail com
mike @ ounsworth ca
sschmieg @ google com
dstebila @ uwaterloo ca
History: 2026-03-05: revised; 2026-03-02: received; See all versions
Short URL: https://ia.cr/2026/427
License: CC BY

BibTeX

@misc{cryptoeprint:2026/427,
      author = {Deirdre Connolly and Mike Ounsworth and Sophie Schmieg and Douglas Stebila},
      title = {{StarHunters}— Secure Hybrid Post-Quantum {KEMs} From {IND}-{CCA2} {PKEs}},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/427},
      year = {2026},
      url = {https://eprint.iacr.org/2026/427}
}