Message forwarding is a fundamental brick to spread information among users in opportunistic networks. In this paper, we consider the recently proposed interest-casting networking primitive for opportunistic networks, according to which a packet generated by a sender should be delivered to all users in the network — potentially unknown to the sender — sharing similar interests. However, the current implementation of interest-casting assume users exchange their interest profiles to take forwarding decisions, thus revealing very sensitive information to strangers. In this work, we approach for the first time the problem of designing an interest-casting protocol while not revealing sensitive information during the forwarding and message delivery process. In particular, we present a privacy-preserving mechanism based on the well-known Millionaires'' problem allowing users to discover whether they have similar interests without disclosing private information. Based on this mechanism, we propose four different privacy-preserving forwarding protocols to realise interest-casting in opportunistic networks, and we compare their performance on a real-world mobility trace.
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