Reputation mechanisms allow users to mutually evaluate their trust. This is achieved through the computation of a reputation score summarizing their past behaviors. Depending on these scores, users are free to accept or refuse to interact with each other. When users are virtual, volatile, or distant, an accurate evaluation of reputation scores is complex. Furthermore, users expect reputation mechanisms to preserve the privacy of their interactions and of their feedback. Existing solutions often rely on costly cryptographic tools that may lead to impractical solutions. In this article, we propose a usable privacy preserving reputation mechanism. This mechanism is distributed and handles non-monotonic ratings. Its implementation on cheap single board computers validates its adequacy to large-scale systems.