On the Decidability of Accessibility Problems (Extended Abstract)

Protection systems have provided the formal basis for the study of security and access mechanisms in computer systems for many years and, more recently, in the context of trust management. The main objective in the design and analysis of such systems is to express policies that prescribe how objects interact and share information with each other, and verify that undesirable actions cannot take place. The latter problem is referred to as the safety or accessibility problem, since it is often phrased in the form "Can object p gain (illegal) access to object q by a series of legal moves (as prescribed by a policy)?". Much work has gone into designing protection systems that have signi cant expressive power and e ective procedures for verifying accessibility. In this paper, we study one such general protection system, providing algorithms for deciding accessibility within the system. Our methods involve relating this problem to membership problems for certain classes of grammars, and reveal interesting connections between protection systems and the grammars that describe them. Of independent interest is the discovery of an elegantly characterised family of grammars that to the best of our knowledge has not been placed withi

On the Decidability of Accessibility Problems (Extended Abstract)

Protection systems have provided the formal basis for the study of security and access mechanisms in computer systems for many years and, more recently, in the context of trust management. The main objective in the design and analysis of such systems is to express policies that prescribe how objects interact and share information with each other, and verify that undesirable actions cannot take place. The latter problem is referred to as the safety or accessibility problem, since it is often phrased in the form "Can object p gain (illegal) access to object q by a series of legal moves (as prescribed by a policy)?". Much work has gone into designing protection systems that have significant expressive power and effective procedures for verifying accessibility. In this paper, we study one such general protectio..