International Association
for Cryptologic Research

Xagawa and Yamakawa (PQCrypto 2019) proved the transformation SXY can tightly turn DS secure PKEs into IND-qCCA secure KEMs in the quantum random oracle model (QROM). But transformations such as KC, TPunc that turn PKEs with standard security (OW-CPA or IND-CPA) into DS secure PKEs still suffer from quadratic security loss in the QROM. In this paper, we give a tighter security reduction for the transformation KC that turns OW-CPA secure deterministic PKEs into modified DS secure PKEs in the QROM. We use the Measure-Rewind-Measure One-Way to Hiding Lemma recently introduced by Kuchta et al. (EUROCRYPT 2020) to avoid the square-root advantage loss. Moreover, we extend it to the case that underlying PKEs are not perfectly correct. Combining with other transformations, we finally obtain a generic KEM from any IND-CPA secure PKE. Our security reduction has roughly the same tightness as the result of Kuchta et al. without any other assumptions and we achieve the stronger IND-qCCA security. We also give a similar result for another KEM transformation achieving the same security notion from any OW-CPA secure deterministic PKE.