BetterTimes: Privacy-assured Outsourced Multiplications for Additively Homomorphic Encryption on Finite Fields

We present a privacy-assured multiplication protocol using which an arbitrary arithmetic formula with inputs from two parties over a finite field F-p can be jointly computed on encrypted data using an additively homomorphic encryption scheme. Our protocol is secure against malicious adversaries. To motivate and illustrate applications of this technique, we demonstrate an attack on a class of known protocols showing how to compromise location privacy of honest users by manipulating messages in protocols with additively homomorphic encryption. We evaluate our approach using a prototypical implementation. The results show that the added overhead of our approach is small compared to insecure outsourced multiplication