Rollercoaster: An Efficient Group-MulticastScheme for Mix Networks

Daniel Hugenroth, Martin Kleppmann, Alastair R. BeresfordUniversity of Cambridge

USENIX Security 2021, Virtual Event

pro·blem state·ment

Mix networks*

*other brands of anonymity networks are available.

A. M. Piotrowska, J. Hayes, T. Elahi, S. Meiser, and G. Danezis,“The Loopix anonymity system,”in 26th USENIX Security Symposium, pp. 1199–1216, 2017.

Group multicast

Example: Decentralised Collaborative Tools

Many-to-many communication

Low latency

Attacker tries to observe:- Group membership- Communication

metadata

5s5s5s

Application

Cover traffic: hiding communication patterns

Payload Queue:

Cover Queue:

Application

Payload Queue:

Cover Queue:

Naïve sequential unicast

Payload queue overload causes high latency

5s5s5s

Naïve mix node multicast

The size of all messages need to be increased up to the maximum group size

Mix node learns size of active groups

ro·ll er·coa·ster

Rollercoaster

Source

Rollercoaster

Rollercoaster

Rollercoaster

ex·ten·sions I

Fault-toleranceOffline node breaks forwarding for entire sub-tree

Fault-tolerance

Fault-tolerance

Act as

appears offline!

ex·ten·sions II

p-restricted multicast

All messages increased to twice the size

Padded based on secret after split and delayed independently

No leakage about group size because of constant `p`

e·va·lu·a·tion

Our simulator

Deterministic simulator allows for reproducible results

Executes scenarios up to 500x faster than if done in real-time

Online-offline behaviour modeled based on real-world data

For online scenarios

For offline scenarios

Thanks for watching!

Daniel Hugenroth

University of Cambridgedh623@cam.ac.uk

Martin Kleppmann

University of Cambridgemk428@cam.ac.uk

Alastair R. Beresford

University of Cambridgearb33@cam.ac.uk

Rollercoaster: An Efficient Group-Multicast Scheme for Mix NetworksArtifact: https://github.com/lambdapioneer/rollercoaster