Private Constrained PRFs (and More) from LWE

In a constrained PRF, the owner of the PRF key K can generate constrained keys Kf that allow anyone to evaluate the PRF on inputs x that satisfy the predicate f (namely, where f(x) is “true”) but reveal no information about the PRF evaluation on the other inputs. A private constrained PRF goes further by requiring that the constrained key Kf hides the predicate f . Boneh, Kim and Montgomery (EUROCRYPT 2017) presented a construction of private constrained PRF for point function constraints, and Canetti and Chen (EUROCRYPT 2017) presented a completely different construction for NC constraints. In this work, we show two constructions of LWE-based constraint-hiding constrained PRFs for general predicates described by polynomial-size circuits. The two constructions are based on two distinct techniques that we show have further applicability by constructing weak attribute-hiding predicate encryption schemes. In a nutshell, the first construction imports the technique of modulus switching from the FHE world into the domain of trapdoor extension and homomorphism. The second construction shows how to use the duality between FHE secret-key/randomness and ABE randomness/secret-key to construct a scheme with dual use of the same values for both FHE and ABE purposes. ∗Weizmann Institute of Science, {zvika.brakerski,rotem.tsabary}@weizmann.ac.il. †MIT, [email protected]. ‡ENS, [email protected].