International Association
for Cryptologic Research

In this work, we provide new, tighter proofs for the $T_{RH}$-transformation by Jiang {et al.} (ASIACRYPT 2023), which converts OW-CPA secure PKEs into KEMs with IND-1CCA security, a variant of typical IND-CCA security where only a single decapsulation query is allowed. Such KEMs are efficient and have been shown sufficient for real-world applications by Huguenin-Dumittan and Vaudenay at EUROCRYPT 2022. We reprove Jiang {et al.}'s $T_{RH}$-transformation in both the random oracle model (ROM) and the quantum random oracle model (QROM), for the case where the underlying PKE is rigid deterministic. In both ROM and QROM models, our reductions achieve security loss factors of $\bigO{1}$, significantly improving Jiang {et al.}'s results which have security loss factors of $\bigO{q}$ in the ROM and $\bigO{q^2}$ in the QROM respectively. Notably, central to our tight QROM reduction is a new tool called ``reprogram-after-measure'', which overcomes the reduction loss posed by oracle reprogramming in QROM proofs. This technique may be of independent interest and useful for achieving tight QROM proofs for other post-quantum cryptographic schemes. We remark that our results also improve the reduction tightness of the $T_{H}$-transformation (which also converts PKEs to KEMs) by Huguenin-Dumittan and Vaudenay (EUROCRYPT 2022), as Jiang {et al.} provided a tight reduction from $T_H$-transformation to $T_{RH}$-transformation (ASIACRYPT 2023).