Demand response (DR) is a Smart Grid application that aims at managing consumption of electricity in response to supply-side signals. The exchange of sensitive information (i.e., electricity consumption data) between the consumers and the DR provider is necessary for the functioning of DR, but at the same time it is an obstacle to its wide-spread adoption, due to the related privacy concerns. This paper proposes the use of homomorphic encrypted user aggregation and reputation-based incentive distribution to address the trade-off between enhancing user privacy and correctly assessing the contribution of each user to the demand reduction.