QCCA Security of Fujisaki-Okamoto Transformation in the Quantum Random Oracle Model

We analyze the \(\mathsf {IND\text {-}qCCA}\) security of Fujisaki-Okamoto transformation in the QROM. Liu and Wang (PKC 2021) proved it for the implicit rejection variant at the cost of an additional hash; Ge, Shan and Xue (CRYPTO 2023) proved it for the explicit rejection variant if the underlying PKE satisfies \(\gamma \) -spreadness. In this work, we remove the additional hash for the implicit rejection \(\textsf{FO}\) . Besides, we show an example of PKE that is secure but is not \(\gamma \) -spread. And we remove the \(\gamma \) -spreadness requirement for the explicit rejection \(\textsf{FO}\) , however, at the cost of an additional hash.