Paper 2017/151 Original: eprint.iacr.org/2017/151 ↗

Practical Functional Encryption for Quadratic Functions with Applications to Predicate Encryption

Abstract

We present two practically efficient functional encryption schemes for a large class of quadratic functionalities. Specifically, our constructions enable the computation of so-called bilinear maps on encrypted vectors. This represents a practically relevant class of functions that includes, for instance, multivariate quadratic polynomials (over the integers). Our realizations work over asymmetric bilinear groups and are surprisingly efficient and easy to implement. For instance, in our most efficient scheme the public key and each ciphertext consist of \(2n + 1\) and \(4n + 2\) group elements respectively, where n is the dimension of the encrypted vectors, while secret keys are only two group elements. Our two schemes build on similar ideas, but develop them in a different way in order to achieve distinct goals. Our first scheme is proved (selectively) secure under standard assumptions, while our second construction is concretely more efficient and is proved (adaptively) secure in the generic group model. As a byproduct of our functional encryption schemes, we show new predicate encryption schemes for degree-two polynomial evaluation, where ciphertexts consist of only \(O(n)\) group elements. This significantly improves the \(O(n^2)\) bound one would get from inner product encryption-based constructions.