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Rollercoaster: An Efficient Group-Multicast Scheme for Mix Networks

Daniel Hugenroth, Martin Kleppmann, and Alastair R. Beresford

30th USENIX Security Symposium, August 2021.


Mix network designs such as Loopix provide strong metadata anonymity guarantees that are crucial across many applications. However, because they limit the rate at which messages can be sent by each user, they incur high delays when sending many messages to multiple recipients – for instance, in decentralised collaborative apps.

In this paper we present an efficient multicast scheme named Rollercoaster that reduces the time for delivering a message to all members of a group of size m from O(m) to O(log m). Rollercoaster can be deployed without modifications to the underlying mix network, allowing it to benefit from the anonymity set provided by existing users. We further develop an extension that achieves the same asymptotic guarantees in the presence of unreliable group members.

While the scheme is applicable to many mix network designs, we evaluate it for the Loopix network, which is the most advanced and practical design to date. For this evaluation we developed a network simulator that allows fast, reproducible, and inspectable runs while eliminating external influences.