Paper 2018/791

Practical Fully Secure Unrestricted Inner Product Functional Encryption modulo $p$

Guilhem Castagnos, Fabien Laguillaumie, and Ida Tucker

Abstract

Functional encryption is a modern public-key cryptographic primitive allowing an encryptor to finely control the information revealed to recipients from a given ciphertext. Abdalla, Bourse, De Caro, and Pointcheval (PKC 2015) were the first to consider functional encryption restricted to the class of linear functions, i.e. inner products. Though their schemes are only secure in the selective model, Agrawal, Libert, and Stehlé (CRYPTO 16) soon provided adaptively secure schemes for the same functionality. These constructions, which rely on standard assumptions such as the Decision Diffie-Hellman (DDH), the Learning-with-Errors (LWE), and Paillier's Decision Composite Residuosity (DCR) problems, do however suffer of various practical drawbacks. Namely, the DCR based scheme only computes inner products modulo an RSA integer which is oversized for many practical applications, while the computation of inner products modulo a prime $p$ either requires, for their (DDH) based scheme, that the inner product be contained in a sufficiently small interval for decryption to be efficient, or, as in the LWE based scheme, suffers of poor efficiency due to impractical parameters. In this paper, we provide adaptively secure functional encryption schemes for the inner product functionality which are both efficient and allow for the evaluation of unbounded inner products modulo a prime $p$. Our constructions rely on new natural cryptographic assumptions in a cyclic group containing a subgroup where the discrete logarithm (DL) problem is easy which extend Castagnos and Laguillaumie's assumption (RSA 2015) of a DDH group with an easy DL subgroup. Instantiating our generic construction using class groups of imaginary quadratic fields gives rise to the most efficient functional encryption for inner products modulo an arbitrary large prime $p$. One of our schemes outperforms the DCR variant of Agrawal et al.'s protocols in terms of size of keys and ciphertexts by factors varying between 2 and 20 for a 112-bit security.