Dynamic Trust Models for Ubiquitous Computing Environments Colin English, Paddy Nixon, Sotirios Terzis, Andrew McGettrick, Helen Lowe Department of Computer and Information Sciences University of Strathclyde

Vision of Ubiquitous Computing Massively networked world with a diverse population of mobile entities Benefit from cooperation and interaction No specific security infrastructure New security challenges not addressed in existing security models Inherent risk of interaction where only partial information is available

Trust-Based Security Humans use trust everyday as a means of reasoning about and accepting risk Lacking a precise definition Certain aspects are widely acknowledged Trust is a subjective notion based on the evidence available Trust is also situation specific Trust is dynamic, evolving as new information becomes available

Trust Management Systems Current trend is to view trust implicitly through the delegation of privileges The lack of explicit trust evaluation precludes many of the aspects deemed necessary to reason about trust Problems for decentralised ubiquitous systems Reliance on complete information Reliance on a specific security infrastructure Dynamic aspects of trust are largely neglected

Position & Objectives The ability to form and evolve explicit values for trust in other principals in an interaction allows autonomous computational entities to make better decisions in situations where only partial information is available The aim is to help create a user-intuitive Information Society Objectives: Define a trust model to allow entities to reason about and compare the trustworthiness of other entities for security decisions Capture the dynamic aspects of trust with fine granularity Capture human intuitions about trust to ensure understanding

Characteristics of the Trust Model Range of explicit values representing trust provides a finer granularity of representation More information for security decisions Values stored in memory Three main sources of trust Personal Observations Recommendations Reputation

Dynamic Aspects of the Model Trust Formation evidence relevant to the current context carrying the most weight Trust Evolution Evaluation of the experience Certificate revocation insufficient Trust Exploitation Behaviour based on trust, risk and utility Risk is the probability of an outcome and the associated costs/benefits

Status Formal trust model Lattice of trust values Risk model Capture the interaction between trust and risk Entity recognition mechanisms Jean-Marc Seigneur Simulation trust based file sharing system trust based dynamic routing in ad-hoc networks

Open Issues The nature of context Recording of evidence Combining the three forms of evidence Issues of Byzantine behaviour Non-cooperative scenarios

Acknowledgements The work is this paper is supported by the EU project SECURE (IST ) funded by the FET Programme under the GCI.