Are we collaborative yet? A Usability Perspective on Mixnet Latency for Real-Time Applications

TL;DR

This paper tackles the problem of enabling usable real-time communication over mixnets while maintaining anonymity. It advances a practical methodology by simulating real-time collaboration with a simulated partner, using Automerge CRDTs, and modeling mixnet-like delays via an exponential framework in a within-subjects study across six delay levels. The key finding is that average delays of $1 ext{s}$ and $4 ext{s}$ preserve usability, while $7 ext{s}$ impairs performance and $10 ext{s}$ is detrimental, providing concrete guidance for latency settings in mixnet deployments. The work also documents how user frustration grows with delay, how participants adopt coping strategies, and how these results can inform mixnet configuration, including centralised and multi-hop scenarios. Overall, the study offers a first modern, context-rich assessment of latency tolerance for mixnets in real-time tasks, with clear implications for balancing usability and anonymity in future deployments.

Abstract

Mixnet networks deliberately induce additional latency to communications to provide anonymity. Recent developments have allowed mixnets to reduce their latency from hours to seconds while maintaining the same level of anonymity. As a result, real-time communications are now possible on mixnets. There has been limited research on how users tolerate different levels of delay, and it is unclear what latency levels mixnet operators should choose. Previous studies about latency do not apply to these 'mid-latency' mixnet scenarios. Our paper contributes the first measurement of users' tolerance to real-time applications under mixnet delay. We design a text-based collaborative quiz system to test user response to latency where participants complete a set of question tasks in collaboration with a simulated second user. Different levels of latency are added, analogous to a modern mixnet system. We show that average delay parameters of 1s and 4s maintain usability, a mean delay of 7s shows some difficulty and a mean delay of 10s is detrimental to user experience. Using these delay parameters, mixnet operators can ensure that most types of real-time communication applications are usable. Mixnets thus can balance usability and anonymity without compromising either.

Figures (4)

• Figure 1: A screenshot from the study, about half of the questions have appeared so far in this case.
• Figure 2: Number of participants reporting each frustration level depending on the mean delay. An increasing level of frustration can be seen as the delay increases.
• Figure 3: Participant reported completion time effect from the simulated user under different mean delay levels. As the delay increased, more participants believed the second user slowed their progress.
• Figure 4: Participant reported strategy deviation: Slightly more participants had to change strategy in some way as the delay increased.