Filter Design for Delay-Based Anonymous Communications

TL;DR

This paper explores the design of delay-based filters in anonymous communication systems, linking them to MIMO channel estimation, and provides theoretical solutions to enhance privacy against adversaries.

Contribution

It introduces a novel connection between delay-based anonymous communication and MIMO filter design, offering optimal filter solutions for different adversary models.

Findings

Optimal filters increase adversary's estimation error

Theoretical solutions for short-term and long-term adversaries

Properties of filters can improve timed mix implementations

Abstract

In this work, we address the problem of designing delay-based anonymous communication systems. We consider a timed mix where an eavesdropper wants to learn the communication pattern of the users, and study how the mix must delay the messages so as to increase the adversary's estimation error. We show the connection between this problem and a MIMO system where we want to design the coloring filter that worsens the adversary's estimation of the MIMO channel matrix. We obtain theoretical solutions for the optimal filter against short-term and long-term adversaries, evaluate them with experiments, and show how some properties of filters can be used in the implementation of timed mixes. This opens the door to the application of previously known filter design techniques to anonymous communication systems.