Starfighters-On the General Applicability of X-Wing

In this work, we present a comprehensive analysis of QSF, the KEM combiner used by X-Wing (Communications in Cryptology 1(1), 2024). While the X-Wing paper focuses on the application of QSF to ML-KEM-768 and X25519, we discuss the combiner's applicability to other post-quantum KEMs and ECDH instantiations. In particular, we establish the compatibility of QSF to KEMs based on variants of the FujisakiOkamoto transform by proving ciphertext second-preimage resistance (C2PRI) for these variants. Building on these results, we show that QSF is compatible with, to the best of our knowledge, all post-quantum KEMs currently standardized or considered for standardization-including ML-KEM, (e)FrodoKEM, HQC, Classic McEliece, and various NTRU variants. Notably, this means these schemes can be used with QSF to construct PQ/T hybrid KEMs. In addition, we introduce QSI, a variant of QSF that combines two KEMs by hashing their shared keys, yielding a KEM that is IND-CCA-secure as long as one constituent KEM is IND-CCA-secure and the other is C2PRI-secure. We establish the same compatibility results for QSI as for QSF. Finally, we analyze both QSF and QSI regarding (their preservation of) the recently introduced family of binding properties for KEMs.