Abstract We introduce a new hybrid modal logic HSML for reasoning about sabotage-style graph games with edge deletions and provide a complete Hilbert-style axiomatization. We extend the completeness analysis to protocol models with restrictions on available edge deletions and clarify the connections between HSML-style logics of edge deletions and recent modal logics for stepwise point deletion from graphs.

Abstract We integrate Dung’s argumentation framework with a topological space to formalize Clark’s no false lemmas theory for solving the Gettier problem and study its logic. Our formalization shows that one of the two notions of knowledge proposed by Clark, justified belief with true grounds, satisfies Stalnaker’s axiom system of belief and knowledge except for the axiom of closure under conjunction. We propose a new notion of knowledge, justified belief with a well-founded chain of true grounds, which further improves on Clark’s two notions of knowledge. We pinpoint a seemingly reasonable condition which makes these three notions of knowledge collapse into the same one and explain why this result looks counter-intuitive. From a technical point of view, our formal analysis driven by the philosophical issues reveals the logical structure of the grounded semantics in Dung’s argumentation theory.

A key challenge in large-scale collaborative distributed systems is to properly incentivize the rational/selfish users so that they will properly collaborate. Within such a context, this paper focuses on designing incentive mechanisms for overlay multicast systems. A key limitation shared by existing proposals on the problem is that they are no longer able to provide proper incentives and thus will collapse when rational users collude or launch sybil attacks.

This paper focuses on designing incentive mechanisms for overlay multicast systems. Existing proposals on the problem are no longer able to provide proper incentives when rational users collude or launch sybil attacks. To overcome this key limitation, we propose a novel decentralized DCast multicast protocol and prove that it offers a novel concept of safety-net guarantee: A user running the protocol will always obtain at least a reasonably good utility despite the deviation of any number of rational users that potentially collude or launch sybil attacks.

Recommendation systems can be attacked in various ways, and the ultimate attack form is reached with a {\em sybil attack}, where the attacker creates a potentially unlimited number of {\em sybil identities} to vote. Defending against sybil attacks is often quite challenging, and the nature of recommendation systems makes it even harder. This paper presents {\em DSybil}, a novel defense for diminishing the influence of sybil identities in recommendation systems. DSybil provides strong provable guarantees that hold even under the worst-case attack and are optimal. DSybil can defend against an unlimited number of sybil identities over time. DSybil achieves its strong guarantees by i) exploiting the heavy-tail distribution of the typical voting behavior of the honest identities, and ii) carefully identifying whether the system is already getting ``enough help'' from the (weighted) voters already taken into account or whether more ``help'' is needed. Our evaluation shows that DSybil would continue to provide high-quality recommendations even when a million-node botnet uses an optimal strategy to launch a sybil attack.