The growing number of popular peer to peer applications during the last five years has implied for researchers to focus on how to build trust in such very large scale distributed systems. Reputation systems have shown to be a very good solution to build trust in presence of malicious nodes. We propose in this paper a new metric for reputation systems on top of a Distributed Hash Table that uses a notion of risk to make the applications aware of certain behaviours of malicious nodes. We show that our metric is able to significantly reduce the number of malicious transactions, and that it also provides very strong resistance to several traditional attacks of reputations systems. We also show that our solution can easily scale, and can be adapted to various Distributed Hash Tables.