StarFortress: Hybrid KEMs with Diffie-Hellman Inlining

Deirdre Connolly; Paul Grubbs

Paper 2026/125

StarFortress: Hybrid KEMs with Diffie-Hellman Inlining

Deirdre Connolly

, Oracle, Selkie Cryptography

Paul Grubbs

, University of Michigan–Ann Arbor

Abstract

This short paper formally specifies and analyzes the UG hybrid KEM construction from the IRTF CFRG’s recent draft on hybrid (post-quantum/traditional) KEMs. The UG construction is an optimized hybrid of a Diffie-Hellman (DH)-based KEM in a nominal group and a generic IND-CCA KEM. The main optimization is that the group elements derived in the DH-based KEM are “inlined” in the key derivation, saving unnecessary hashing. We perform two security analyses of the UG construction: one shows UG is IND-CCA even if the generic IND-CCA KEM is broken; the other complementary analysis shows UG is IND-CCA even if the DH assumptions in the nominal group are broken (by, e.g., a cryptographically-relevant quantum computer).

Metadata

Available format(s): PDF
Category: Public-key cryptography
Publication info: Preprint.
Keywords: Hybrid KEM Post-Quantum Cryptography Public-Key Cryptography KEM KEM combiner IND-CCA
Contact author(s): durumcrustulum @ gmail com
paulgrubbs12 @ gmail com
History: 2026-01-30: revised; 2026-01-26: received; See all versions
Short URL: https://ia.cr/2026/125
License: CC BY

BibTeX

@misc{cryptoeprint:2026/125,
      author = {Deirdre Connolly and Paul Grubbs},
      title = {{StarFortress}: Hybrid {KEMs} with Diffie-Hellman Inlining},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/125},
      year = {2026},
      url = {https://eprint.iacr.org/2026/125}
}