DUO{\textendash}Onions and Hydra{\textendash}Onions {\textendash} Failure and Adversary Resistant Onion Protocols

Abstract

A serious weakness of the onion protocol, one of the major tools for anonymous communication, is its vulnerability to network failures and/or an adversary trying to break the communication. This is facilitated by the fact that each message is sent through a path of a certain length and a failure in a single point of this path prohibits message delivery. Since the path cannot be too short in order to offer anonymity protection (at least logarithmic in the number of nodes), the failure probability might be quite substantial. The simplest solution to this problem would be to send many onions with the same message. We show that this approach can be optimized with respect to communication overhead and resilience to failures and/or adversary attacks. We propose two protocols: the first one mimics K independent onions with a single onion. The second protocol is designed for the case where an adaptive adversary may destroy communication going out of servers chosen according to the traffic observed by him. In this case a single message flows in a stream of K onions {\textemdash} the main point is that even when the adversary kills some of these onions, the stream quickly recovers to the original bandwidth {\textemdash} again K onions with this message would flow through the network.