small logo PRACTICAL ARCHITECTURAL APPROACH FOR COMPOSING EGOCENTRIC TRUST

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The PACE Project

Decentralized Trust Management


What is trust?

The concept of trust is not new to us nor is it limited only to electronic entities. In fact trust is an integral part of our social existence. Our interactions in society are influenced by the perceived trust worthiness of other entities. It thus plays an equally important role in our daily lives. Naturally, in addition to computer scientists, researchers from other fields such as sociology, history, economics, and philosophy too have devoted significant attention to the issue of trust. Given the fact that trust is a multi-disciplinary concept, there exist in the research literature several definitions of trust and discussions about the factors that determine trust.

We adopt the definition put forward by Grandison and Sloman who define trust as - the firm belief in the competence of an entity to act dependably, securely, and reliably within a specified context.

While there are several aspects to trust, we next outline a few that we believe facilitate a better understanding of the concept of trust. Trust is conditionally transitive. This means that if A trusts B and B trusts C, A trusts C only if certain possibly application-specific conditions are met. Trust can be multi-dimensional and depends upon the context. For example, A may trust B completely when it comes to repairing electronic devices but may not trust B when it comes to repairing cars. Trust can also be expressed in different ways such as a set of continuous values between 0 and 1, or binary values, or a set of discrete values. Trust values may also have an optional time-to-live attribute that specifies the duration for which the trust value is applicable, and beyond which the trust value does not correctly represent the trust relationship.


What is reputation?

Related to trust is the concept of reputation. Abdul-Rehman and Hailes define reputation as an expectation about an individual’s behavior based on information about or observations of its past behavior. In online communities, where an individual may have very less information to determine the trustworthiness of others, their reputation information is typically used to determine the extent to which they can be trusted. An individual who is more reputed is generally considered to be more trust worthy.

Reputation can be determined in several ways. For example, a person may either rely on his direct experiences, or rely on the experiences of other people, or a combination of both to determine the reputation of another person.


Classification of decentralized trust management systems

We classify trust management into three categories: credential and policy-based trust management, reputation-based trust management, and social network-based trust management. This categorization is based upon the approach adopted to establish and evaluate trust relationships between peers.

In credential and policy-based trust management systems such as in PolicyMaker, Keynote, REFEREE, peers use credential verification to establish a trust relationship with other peers. The primary goal of such systems is to enable access control. Therefore their concept of trust management is limited to verifying credentials and restricting access to resources according to application-defined policies. A resource-owner provides a requesting peer access to a restricted resource only if it can verify the credentials of the requesting peer either directly or through a web of trust. This is useful by itself only for those applications that assume implicit trust in the resource owner. Since these policy-based access control trust mechanisms do not incorporate the need of the requesting peer to establish trust in the resource-owner, they by themselves do not provide a complete generic trust management solution for all decentralized applications.

Reputation-based trust management systems on the other hand provide a mechanism by which a peer requesting a resource may evaluate its trust in the reliability of the resource and the peer providing the resource. Examples of such systems include SPORAS, HISTOS, XREP, NICE, DCRC/CORC, Beta, EigenTrust, etc. Peers in such systems establish trust relationships with other peers and assign trust values to these relationships. Trust value assigned to a trust relationship is a function of the combination of the peer’s global reputation and the evaluating peer’s perception of that peer.

The third kind of trust management systems, social network-based trust management systems, in addition, utilize social relationships between peers when computing trust and reputation values. In particular, they analyze a social network which represents the relationships existing within a community and form conclusions about peers’ reputations based on different aspects of the social network. Examples of such trust management systems include Regret that identifies groups using the social network, and NodeRanking that identifies experts using the social network.


Other definitions of trust

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This material is based upon work supported by the National Science Foundation under Grant No. 0205724, 0438996, and 0524033. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.