Reputation systems are a major feature of every modern e-commerce website, helping buyers carefully choose their service providers and products. However, most websites use centralized reputation systems, where the security of the system rests entirely upon a single Trusted Third Party. Moreover, they often disclose the identities of the raters, which may discourage honest users from posting frank reviews due to the fear of retaliation from the ratees. We present a reputation system that is decentralized yet secure and efficient, and could therefore be applied in a practical context. In fact, users are able to retrieve the reputation score of a service provider directly from it in constant time, with assurance regarding the correctness of the information obtained. Additionally, the reputation system is anonymity-preserving, which ensures that users can submit feedback without their identities being associated to it. Despite this anonymity, the system still offers robustness against attacks such as ballot-stuffing and Sybil attacks.