Fifth European Workshop on Systems Security,
April 10, 2012, Bern, Switzerland
Anonymous communication capabilities are useful and desirable, but practical onion routing approaches are vulnerable to traffic analysis and DoS attacks—especially against a powerful adversary, such as a repressive government or compromised ISP. To fill this gap we introduce D3, the first practical anonymous group communication system offering anonymity and disruption resistance against strong traffic analysis and collusion attacks, with scalability to hundreds of group members and quick response to churn. D3 builds on a trust model we call anytrust. Anytrust is a decentralized client/server network model, in which each of many clients—representing group members—trust only that at least one of a smaller but diverse set of "servers" or "super-peers" behaves honestly, but clients need not know which server to trust. By combining and adapting verifiable shuffle and DC-nets techniques to anytrust, D3 achieves short shuffle latencies and efficient tree-based DC-nets ciphertext combining, while guaranteeing message anonymity and integrity, transmission proportionality among group members, and adaptability to both network/node failures and active disruption. Experiments with a working prototype demonstrate that D3 is practical at scales infea- sible in prior systems offering comparable security.