Echomix is a practical mix network framework and a suite of associated protocols providing strong metadata privacy against realistic modern adversaries. It is distinguished from other anonymity systems by a resistance to traffic analysis by global adversaries, compromised contacts and network infrastructure, quantum decryption algorithms, and statistical and confirmation attacks typical for multi-client messaging setting. It is implemented as Katzenpost, a robust software project, and used in multiple deployed systems, and features relatively low latency and bandwidth overhead. The contributions of this paper are: (1) Improvements on leading mix network designs, supported by rigorous analysis. These include solutions to crucial vulnerabilities to traffic analysis, malicious servers and active attacks. (2) A cryptographic group messaging protocol with strong metadata protection guarantees and reliability. (3) Hybrid post-quantum nested packet encryption.
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