Mix networks are a highly effective way to achieve anonymity, defending against a wide range of traffic-analysis attacks. However, mix networks are usually designed for infrastructure networks and cannot be directly applied in the context of mobile ad hoc networks (MANETs). The few existing solutions for MANETs require advance knowledge of the topology or a trusted central party. In this paper, we present TrustMix, a mix protocol for MANETs that operates without any central trusted party. In TrustMix, parties join groups and then messages are forwarded via multiple groups to provide anonymity. With TrustMix, users only need to find a party nearby that they consider trusted. They then forward the message to this party's group, and the party shuffles messages before forwarding to other groups, meaning that the original message and the forwarded message cannot be linked. Furthermore, even if the chosen party is adversarial, they can only break the anonymity if all parties in their group are adversarial as all of them contribute to the shuffling. In addition to anonymity, TrustMix also enforces rate limits on the number of messages through the use of linkable ring signatures, which allows detecting that parties send more messages that allowed without revealing identities. We prove the security of our protocol in the random oracle model. We evaluate its anonymity using an existing mix-network simulator and show that TrustMix significantly improves message anonymity. Finally, we present a proof-of-concept Android implementation and show that TrustMix achieves acceptable throughput with 5 mobile devices.

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