Reputation systems measure the credibility of contributionsand contributors in collaborative web systems.Measuring the credibility is significant since a collaborative environment generally allows anyone withInternet access to provide contribution.Collaborative web systems are susceptible to malicious users who intentionally provide inaccuratecontents. With the help of reputation systems, the effect of such malicious activities can be reduced.Reputation systems allow Internet users to rate the contributions made by other users. However, there aremalicious users who will go beyond providing wrong contributions. They attempt to make reputationsystems useless by launching attacks such as slandering. Slandering happens when a malicious user orgroup of malicious users intentionally provide a negative rating to accurate contributions provided by goodusers. Such activity lowers the reputation of good users and in most cases it even helps improve thereputation of slandering users.This paper presents a reputation system called Tulungan that is designed to measure the contributor andrater reputation of users of a collaborative web system that is used for web filtering. User contributions arein the form of URL categorizations. It is the role of Tulungan to determine the correctness of thecategorizations. A simulation is presented to validate the resilience of Tulungan in the presence ofslandering users. The result of the simulation shows that Tulungan is not only resistant to slandering but itis still effective even if the number of good users is less than itsslandering counterpart. Even if there areonly 15% good users, the number of correct URL categorizations outnumbers incorrect contributions
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