Gregory Gutin |

Gregory Gutin studied for PhD under Professor Noga Alon at Tel Aviv University and received his PhD (with distinction) in 1993. Since 2000 he’s been professor of Computer Science at Royal Holloway, University of London. His research interests include graphs and combinatorics (theory, algorithms and applications), parameterized algorithmics, access control, and combinatorial optimization. He’s published more than 200 papers and co-authored two editions of a monograph on directed graphs (Digraphs: Theory, Algorithms and Applications), both translated into Chinese. In 2014 he received Royal Society Wolfson Research Merit Award. Together with Jason Crampton, Gregory received best paper awards at SACMAT 2015 and SACMAT 2016.

In recent years, several combinatorial problems were introduced in the area of access control. Typically, such problems deal with an authorization policy, seen as a relation $UR \subseteq Users \times Resources$, where $(u, r) \in UR$ means that user u is authorized to access resource r.
Li, Tripunitara and Wang (2009) introduced the Resiliency Checking Problem (RCP), in which we are given an authorization policy, a subset of resources $P$, as well as integers $s \ge 0$, $d \ge 1$ and $t \geq 1$.

It asks whether upon removal of any set of at most $s$ users, there still exist $d$ pairwise disjoint sets of at most $t$ users such that each set has collectively access to all resources in $P$. This problem possesses several parameters which appear to take small values in practice. We thus analyze the parameterized complexity of RCP with respect to these parameters, by considering all possible combinations of $|P|, s, d, t$. In all cases, we are able to settle whether the problem is in FPT, XP, W[2]-hard, para-NP-hard or para-coNP-hard. The main theorem to prove FPT can be used for other applications.

It asks whether upon removal of any set of at most $s$ users, there still exist $d$ pairwise disjoint sets of at most $t$ users such that each set has collectively access to all resources in $P$. This problem possesses several parameters which appear to take small values in practice. We thus analyze the parameterized complexity of RCP with respect to these parameters, by considering all possible combinations of $|P|, s, d, t$. In all cases, we are able to settle whether the problem is in FPT, XP, W[2]-hard, para-NP-hard or para-coNP-hard. The main theorem to prove FPT can be used for other applications.