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A computerized workflow management system may enforce a security policy,specified in terms of authorized actions and constraints, thereby restrictingwhich users can perform particular steps in a workflow. The existence of asecurity policy may mean it is impossible to find a valid plan (an assignmentof steps to authorized users such that all constraints are satisfied). Work inthe literature focuses on the workflow satisfiability problem, a\emph{decision} problem that outputs a valid plan if the instance issatisfiable (and a negative result otherwise).  In this paper, we introduce the \textsc{Bi-Objective Workflow SatisfiabilityProblem} (\BOWSP), which enables us to solve \emph{optimization} problemsrelated to workflows and security policies. In particular, we are able tocompute a "least bad" plan when some components of the security policy may beviolated. In general, \BOWSP is intractable from both the classical andparameterized complexity point of view. We prove there exists anfixed-parameter tractable (FPT) algorithm to compute a Pareto front for \BOWSPif we restrict our attention to user-independent constraints.  We also present a second algorithm to compute a Pareto front which uses mixedinteger programming (MIP). We compare the performance of both our algorithms onsynthetic instances, and show that the FPT algorithm outperforms the MIP-basedone by several orders of magnitude on most of the instances.  Finally, we study the important question of workflow resiliency and prove newresults establishing that known decision problems are fixed-parameter tractablewhen restricted to user-independent constraints. We then propose a new way ofmodeling the availability of users and demonstrate that many questions relatedto resiliency in the context of this new model may be reduced to instances of\BOWSP.

The Bi-Objective Workflow Satisfiability Problem and Workflow Resiliency