International Association
for Cryptologic Research

Functional Encryption is a powerful cryptographic primitive that allows for fine-grained access control over encrypted data. In the multi-user setting, especially Multi-Client and Multi-Input, a plethora of works have been proposed to study on concrete function classes, improving security, and more. However, the CCA-security for such schemes is still an open problem, where the only known works are on Public-Key Single-Client FE (Benhamouda, Bourse, and Lipmaa, PKC'17). This work provides the first generic construction of CCA-secure Multi-Client FE for Inner Products, and Multi-Input FE for Inner Products, with instantiations from $\mathsf{SXDH}$ and $\mathsf{DLIN}$ for the former, and $\mathsf{DDH}$ or $\mathsf{DCR}$ for the latter. Surprisingly, in the case of secret key MIFE we attain the same efficiency as in the public key single-client setting. In the MCFE setting, a toolkit of CCA-bootstrapping techniques is developed to achieve CCA-security in its $\mathit{secret~key}$ setting.