A Formal Framework for Trust Policy Negotiation in Autonomic Systems: Abduction with Soft Constraints

We show that soft constraints can be used to model logical reasoning, that is deduction and abduction (and induction). In particular, we focus on the abduction process and we show how it can be implemented with a (soft) constraint removal operator. As a running application example throughout the paper, we reason with access control policies and credentials. In this way, we can associate the level of preference defined by the "softness" of the constraint with a "level" of trust. The main benefit comes during the process of automated access authorization based on trust: soft constraint operations can be easily adopted to measure the level of trust required for each operation. Moreover, when the level is not sufficient, abduction can be used to compute the missing credentials and the levels that grant the access, making the request a (weighted) logical consequence. The proposed framework can be used to automate the deduction-abduction negotiation processes.