Trust management systems are frameworks for authorization in modern distributed systems, allowing remotely accessible resources to be protected by providers. By allowing providers to specify policy, and access requesters to possess certain access rights, trust management automates the process of determining whether access should be allowed on the basis of policy, rights, and an authorization semantics. In this paper we survey modern state-of-the-art in trust management authorization, focusing on features of policy and rights languages that provide the necessary expressiveness for modern practice. We characterize systems in light of a generic structure that takes into account components of practical implementations. We emphasize systems that have a formal foundation, since security properties of them can be rigorously guaranteed. Underlying formalisms are reviewed to provide necessary background.

Distributed authorization takes into account several elements, including certificates that may be provided by non-local actors. While most trust management systems treat all assertions as equally valid up to certificate authentication, realistic considerations may associate risk with some of these elements, for example some actors may be less trusted than others. Furthermore, practical online authorization may require certain levels of risk to be tolerated. In this paper, we introduce a trust management logic based on the system RT that incorporates formal risk assessment. This formalization allows risk levels to be associated with authorization, and authorization risk thresholds to be precisely specified and enforced. We also develop an algorithm for automatic authorization in a distributed environment, that is directed by risk considerations. A variety of practical applications are discussed.

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Through web service technology, distributed applications can be built in a exible manner, bringing tremendous power to applications on the web. However, this exibility poses signicant challenges to security. In particular, an end user (be it human or machine) may access a web service directly, or through a number of intermediaries, while these intermediaries may be formed on the y for a particular task. Traditional access control for distributed systems is not exible and ecient enough in such an environment. Indeed, it may be impossible for a web service to anticipate all possible access patterns, hence to dene an appropriate access control list beforehand. Novel solutions are needed.This paper introduces a trust-but-verify framework for web services authorization, and provides an implementation example. In the trust-but-verify framework, each web service maintains authorization policies. In addition, there is a global set of trust transformation rules, each of which has an associated transformation condition. These trust transformation rules convert complicated access patterns into simpler ones, and the transformation is done by a requester (the original requester or an intermediary) with the assumption that the requester can be trusted to correctly omit certain details. To verify authorization, the requester is required to document evidence that the associated transformation conditions are satised. Such evidence and support information can either be checked before access is granted, or can be veried after the fact in an oine mode, possibly by an independent third party.

We consider the problem of efficiently computing distributed geographical k-NN queries in an unstructured peer-to-peer (P2P) system, in which each peer is managed by an individual organization and can only communicate with its logical neighboring peers. Such queries are based on local filter query statistics, and require as less communication cost as possible, which makes it more difficult than the existing distributed k-NN queries. Especially, we hope to reduce candidate peers and degrade communication cost. In this paper, we propose an efficient pruning technique to minimize the number of candidate peers to be processed to answer the k-NN queries. Our approach is especially suitable for continuous k-NN queries when updating peers, including changing ranges of peers, dynamically leaving or joining peers, and updating data in a peer. In addition, simulation results show that the proposed approach outperforms the existing Minimum Bounding Rectangle (MBR)-based query approaches, especially for continuous queries.