BCMIX: A Blockchain-Based Dynamic Self-Reconfigurable Mixnet

The increasing awareness of privacy preservation has led to a strong focus on mix networks (mixnets) protecting anonymity. As an efficient mixnet, cMix greatly reduces the latency, but brings privacy leakage risks due to the use of presetting mix nodes controlled by service providers. Besides, cMix is susceptible to blocking attacks that paralyze the anonymous service. In this paper, we combine blockchain with cMix, and come up with a BlockChain-based dynamic self-reconfigurable MIXnet (BCMIX) approach for anonymous communication. In BCMIX, we design PoW voting, IP sharding and VRF voting algorithms to select mix nodes from blockchain miners. With these voting algorithms, BCMIX can mitigate blocking attacks and blockchain Sybil attacks. Then we present an additive homomorphic mixnet protocol altered from cMix. Furthermore, we design a transaction-based key-exchange protocol for users and mix nodes to negotiate keys with each other, which resists Man-in-the-Middle attacks that exist in cMix. We also demonstrate the security and anonymity of BCMIX under formal security models. To evaluate BCMIX, we leverage the distribution of mining pools in the real-world to test the system's performance and ability to resist the mentioned attacks. The results show that with the proper mining difficulty, the probability of an attacker launching collusion attacks and Sybil attacks is negligible. Compared with the existing anonymous systems, BCMIX provides better resilience to known attacks while enabling low-latency anonymous communication without requiring significant bandwidth or storage resources.