The use of collaborative network services is increasing, therefore, the protection of the resources and relations shared by network participants is becoming crucial. One of the main issues in such networks is the evaluation of participant reputation, since network resources access may or may not be granted on the basis of the reputation of the requesting node. Therefore, the calculation of the reputation of the nodes becomes a very important issue. There are several reputation models presented in the literature. Some of these models (e.g., Ebay or Sporas) are very simple and participants cannot express their preferences in the reputation computation process. On the contrary, there are other reputations models (e.g., Reget or Fire) too complex to be applied when privacy is a primary concern. In this paper, we propose a new reputation model based on OWA and WOWA operators. The key characteristics of our proposal are that reputation is computed in a private way using the homomorphic properties of elGamal cryptosystem and it is possible to introduce user preferences inside reputation computation. We present the feasibility of this new reputation model by considering a Web-based Social Network scenario.