A verifiable, centralized, coercion-free reputation system

Reputation systems are popular tools to evaluate the trustworthiness of an unknown party before a transaction, but the reputation score can greatly impact the rated subject, such that it might be inclined to suppress negative ratings. In order to elicit coercion-resistant, honest feedback, this paper proposes a reputation system that provides complete privacy of the ratings, i.e. neither the ratee nor the reputation system will learn the value of the rating. We take both, a cryptographic as well as a non-cryptographic approach, to the problem. Privacy of ratings may foster bad mouthing attacks where an attacker leaves intentionally bad feedback. We limit the possibility for this attack by providing a token system such that one can only leave feedback after a transaction, and provide a cryptographic proof of the privacy of our system. We consider the Virtual Organization formation problem and develop and evaluate a novel reputation aggregation algorithm for it.