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This work presents an exhaustive analysis of QSF, the KEM combiner used by X-Wing (Communications in Cryptology 1(1), 2024). While the X-Wing paper focuses on the applicability of QSF for combining ML-KEM-768 with X25519, we discuss its applicability for combining other post-quantum KEM with other instantiations of ECDH.
To this end, we establish simple conditions that allow one to check whether a KEM is compatible with QSF by proving ciphertext second‑preimage resistance C2PRI for several variants of the Fujisaki–Okamoto (FO) transform. Applying these results to post-quantum KEMs that are either standardized or under consideration for standardization, we show that QSF can also be used with all of these, including ML-KEM-1024, (e)FrodoKEM, HQC, Classic McEliece, and sntrup.
We also present QSI, a variation of QSF and show that any two KEM can be combined by hashing their concatenated keys. The result is a hybrid KEM which is IND-CCA-secure as long as one of the KEM is IND-CCA- and the other C2PRI-secure.
Finally, we also analyze QSF and QSI regarding their preservation of the recently introduced family of binding properties for KEM.
