This book constitutes the thoroughly refereed post-proceedings of the Fourth International Workshop on Formal Aspects in Security and Trust, FAST 2006, held in Hamilton, Ontario, Canada, August 26-27, 2006. The 18 revised papers presented were carefully reviewed and selected from 47 submissions. The papers include focus of formal aspects in security and trust policy models, security protocol design and analysis, and formal models of trust and reputation.

Strategic Games on Defense Trees.- Timed Calculus of Cryptographic Communication.- A Semantic Paradigm for Component-Based Specification Integrating a Notion of Security Risk.- Game-Based Criterion Partition Applied to Computational Soundness of Adaptive Security.- Measuring Anonymity with Relative Entropy.- Formalizing and Analyzing Sender Invariance.- From Simulations to Theorems: A Position Paper on Research in the Field of Computational Trust.- A Tool for the Synthesis of Controller Programs.- Where Can an Insider Attack?.- Maintaining Information Flow Security Under Refinement and Transformation.- A Classification of Delegation Schemes for Attribute Authority.- Program Partitioning Using Dynamic Trust Models.- Locality-Based Security Policies.- A Theorem-Proving Approach to Verification of Fair Non-repudiation Protocols.- A Formal Specification of the MIDP 2.0 Security Model.- A Comparison of Semantic Models for Noninterference.- Hiding Information in Multi Level Security Systems.- A New Trust Model Based on Advanced D-S Evidence Theory for P2P Networks.