Accountable Anonymous Group Messaging

TL;DR

This paper introduces a novel anonymous group messaging protocol that ensures accountability, preserves user anonymity, and efficiently manages unbalanced loads, addressing vulnerabilities in existing protocols like Mix-nets and DC-nets.

Contribution

It presents the first general messaging protocol combining provable anonymity with accountability for moderate-sized groups, using a cooperative shuffling and multiple DC-nets runs.

Findings

Prototype demonstrates practicality for groups of 40+ members.

Protocol effectively traces misbehavior and resists DoS and Sybil attacks.

Handles large, unbalanced message loads efficiently.

Abstract

Users often wish to participate in online groups anonymously, but misbehaving users may abuse this anonymity to spam or disrupt the group. Messaging protocols such as Mix-nets and DC-nets leave online groups vulnerable to denial-of-service and Sybil attacks, while accountable voting protocols are unusable or inefficient for general anonymous messaging. We present the first general messaging protocol that offers provable anonymity with accountability for moderate-size groups, and efficiently handles unbalanced loads where few members have much data to transmit in a given round. The N group members first cooperatively shuffle an NxN matrix of pseudorandom seeds, then use these seeds in N "pre-planned" DC-nets protocol runs. Each DC-nets run transmits the variable-length bulk data comprising one member's message, using the minimum number of bits required for anonymity under our attack model. The protocol preserves message integrity and one-to-one correspondence between members and messages, makes denial-of-service attacks by members traceable to the culprit, and efficiently handles large and unbalanced message loads. A working prototype demonstrates the protocol's practicality for anonymous messaging in groups of 40+ member nodes.