Enhancing the Unlinkability of Circuit-Based Anonymous Communications with k-Funnels.

Anonymous communication systems are essential tools for preserving privacy and freedom of expression. However, traffic analysis attacks make it challenging to maintain unlinkability in circuit-based anonymity networks like Tor, enabling adversaries to deanonymize communications. To address this problem, we introduce k-funnel, a new security primitive that enhances the unlinkability of circuit-based anonymity networks, and we present BriK, a Tor pluggable transport that implements k-funnels. k-Funnels offer k-anonymity to a group of k clients by jointly tunneling their circuits' traffic through a bridge while ensuring that the client-generated flows are indistinguishable. BriK incorporates several defense mechanisms against traffic analysis attacks, including traffic shaping schemes, synchronization protocols, and approaches for monitoring exposure to statistical disclosure attacks. Our evaluation shows that BriK is able to support web browsing and video streaming while offering k-anonymity. We evaluate the security of BriK against traffic correlation attacks leveraging state-of-the-art deep learning classifiers without considering auxiliary information and find it highly resistant. Although k-funnels require the cooperation of mutually trusted clients, limiting their coordination, our work presents a new practical solution to strengthen unlinkability in circuit-based anonymity systems.