Dissent: Accountable Group Anonymity

Users often wish to participate in online groups anonymously, but misbehaving users may abuse this anonymity to 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 N x N 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.

Security Analysis of Accountable Anonymity in Dissent

Henry Nathaniel Corrigan-Gibbs, Bryan Alexander Ford

Users often wish to communicate anonymously on the Internet, for example in group discussion or instant messaging forums. Existing solutions are vulnerable to misbehaving users, however, who may abuse their anonymity to disrupt communication. Dining Cryptographers Networks (DC-nets) leave groups vulnerable to denial-of-service and Sybil attacks, mix networks are difficult to protect against traffic analysis, and accountable voting schemes are unsuited to general anonymous messaging. DISSENT is the first general protocol offering provable anonymity and accountability for moderate-size groups, while efficiently handling unbalanced communication demands among users. We present an improved and hardened DISSENT protocol, define its precise security properties, and offer rigorous proofs of these properties. The improved protocol systematically addresses the delicate balance between provably hiding the identities of well-behaved users, while provably revealing the identities of disruptive users, a challenging task because many forms of misbehavior are inherently undetectable. The new protocol also addresses several non-trivial attacks on the original DISSENT protocol stemming from subtle design flaws.

2014

On the Privacy of Anonymized Networks

Matthias Grossglauser, Pedram Pedarsani

The proliferation of online social networks, and the concomitant accumulation of user data, give rise to hotly debated issues of privacy, security, and control. One specific challenge is the sharing or public release of anonymized data without accidentally leaking personally identifiable information (PII). Unfortunately, it is often difficult to ascertain that sophisticated statistical techniques, potentially employing additional external data sources, are unable to break anonymity. In this paper, we consider an instance of this problem, where the object of interest is the structure of a social network, i.e., a graph describing users and their links. Recent work demonstrates that anonymizing node identities may not be sufficient to keep the network private: the availability of node and link data from another domain, which is correlated with the anonymized network, has been used to re-identify the anonymized nodes. This paper is about conditions under which such a de-anonymization process is possible. We attempt to shed light on the following question: can we assume that a sufficiently sparse network is inherently anonymous, in the sense that even with unlimited computational power, deanonymization is impossible? Our approach is to introduce a random graph model for a version of the de-anonymization problem, which is parameterized by the expected node degree and a similarity parameter that controls the correlation between two graphs over the same vertex set. We find simple conditions on these parameters delineating the boundary of privacy, and show that the mean node degree need only grow slightly faster than log n with network size n for nodes to be identifiable. Our results have policy implications for sharing of anonymized network information.

2011

Proactively Accountable Anonymous Messaging in Verdict

Henry Nathaniel Corrigan-Gibbs, Bryan Alexander Ford

Among anonymity systems, DC-nets have long held attraction for their resistance to traffic analysis attacks, but practical implementations remain vulnerable to internal disruption or “jamming” attacks, which require time-consuming detection procedures to resolve. We present Verdict, the first practical anonymous group communication system built using proactively verifiable DC-nets: participants use public-key cryptography to construct DC-net ciphertexts, and use zero-knowledge proofs of knowledge to detect and exclude misbehavior before disruption. We compare three alternative constructions for verifiable DC-nets: one using bilinear maps and two based on simpler ElGamal encryption. While verifiable DC-nets incur higher computational overheads due to the public-key cryptography involved, our experiments suggest that Verdict is practical for anonymous group messaging or microblogging applications, supporting groups of 100 clients at 1 second per round or 1000 clients at 10 seconds per round. Furthermore, we show how existing symmetric-key DC-nets can “fall back” to a verifiable DC-net to quickly identify misbehavior, speeding up previous detections schemes by two orders of magnitude.

2013