We open the new academic year with Haifeng Yu’s article on overcoming sybil attacks using social networks. In a sybil attack, a malicious user assumes multiple identities, and uses them to pose as multiple users. Sybil attacks are a threat of the new millennium – they arise in Internet-based distributed systems with a dynamic user population. Indeed, such attacks were not a concern in traditional distributed systems, where the set of participating processes was statically pre-defined. Sybil attacks are inherently difficult to deal with in systems where users do not wish to disclose binding private information, like credit card numbers. A recent popular approach for overcoming sybil attacks is using social networks. Intuitively, even if a malicious user can create many identities, he will have a hard time getting many honest users to befriend all of them in a social network. Thus, the graph structure of a social network can assist in revealing sybil nodes. In this column, Haifeng Yu presents a tutorial on how social networks can be leveraged to defend against sybil attacks, and a survey of recent suggestions employing this approach. Though Haifeng tackles the problem from a theoretical standpoint, (proving formal bounds etc.), this direction has garnered more attention from the systems community, perhaps because sybil attacks are perceived as a real threat for which social networks can provide a viable solution. Yet it appears that much theory for sybil defense using social networks

This paper proposes a fundamentally different approach to addressing the challenge posed by colluding nodes to the sustainability of cooperative services. Departing from previous work that tries to address the threat by disincentivizing collusion or by modeling colluding nodes as faulty, this paper describes two new notions of equilibrium, k-indistinguishability and k-stability, that allow coalitions to leverage their associations without harming the stability of the service. 1