Authors: Michel Abdalla, Fabrice Benhamouda, Markulf Kohlweiss, Hendrik Waldner
Multi-client functional encryption (MCFE) is a more ﬂexible variant of Functional ENcryption whose functional decryption involves multiple ciphertexts from diﬀerent parties. Each party holds a diﬀerent secret key and can independently and adaptively be corrupted by the adversary. We present two compilers for MCFE schemes for the inner-product functionality, both of which support encryption labels. Our ﬁrst compiler transforms any scheme with a special key-derivation property into a decentralized scheme, as deﬁned by Chotard et al. (ASIACRYPT 2018), thus allowing for a simple distributed way of generating functional decryption keys without a trusted party. Our second compiler allows to lift an unnatural restriction present in existing (decentralized) MCFE schemes, which requires the adversary to ask for a ciphertext from each party. We apply our compilers to the works of Abdalla et al. (CRYPTO 2018) and Chotard et al. (ASIACRYPT 2018) to obtain schemes with hitherto unachieved properties. From Abdalla et al., we obtain instantiations of DMCFE schemes in the standard model (from DDH, Paillier, or LWE) but without labels. From Chotard et al., we obtain a DMCFE scheme with labels still in the random oracle model, but without pairings.