5176809
DATALOG with Constraints: A Foundation for Trust Management Languages
Trust management (TM) is a promising approach for authorization and access control in distributed systems, based on signed distributed policy statements expressed in a policy language. Although several TM languages are semantically equivalent to subsets of Datalog, Datalog is not sufficiently expressive for fine-grained control of structured resources. We define the class of linearly decomposable unary constraint domains, prove that DATALOG extended with constraints in any combination of such constraint domains is tractable, and show that permissions associated with structured resources fall into this class. We also present a concrete declarative TM language, RT1C, based on constraint DATALOG, and use constraint DATALOG to analyze another TM system, KeyNote, which turns out to be less expressive than RT1C in significant respects, yet less tractable in the worst case. Although constraint DATALOG has been studied in the context of constraint databases, TM applications involve different kinds of constraint domains and have different computational complexity requirements.
215952437
Beyond proof-of-compliance: security analysis in trust management
Trust management is a form of distributed access control that allows one principal to delegate some access decisions to other principals. While the use of delegation greatly enhances flexibility an...
9031501
Design of a role-based trust-management framework
We introduce the RT framework, a family of role-based trust management languages for representing policies and credentials in distributed authorization. RT combines the strengths of role-based access control and trust-management systems and is especially suitable for attribute-based access control. Using a few simple credential forms, RT provides localized authority over roles, delegation in role definition, linked roles, and parameterized roles. RT also introduces manifold roles, which can be used to express threshold and separation-of-duty policies, and delegation of role activations. We formally define the semantics of credentials in the RT framework by presenting a translation from credentials to Datalog rules. This translation also shows that this semantics is algorithmically tractable.
3062362
Managing Digital Rights using Linear Logic
Digital music players protect songs by enforcing licenses that convey specific rights for individual songs or groups of songs. For licenses specified in industry, we show that deciding whether a license authorizes a sequence of actions is NP-complete, with a restricted version of the problem solvable efficiently using a reduction to maximum network flow. The authorization algorithm used in industry is online, deciding which rights to exercise as actions occur, but we show that all online algorithms are necessarily non-monotonic: each allows actions under one license that it does not allow under a more flexible license. In one approach to achieving monotonicity, we exhibit the unique maximal set of licenses on which there exists a monotonic online algorithm. This set of well-behaved licenses induces an approximation algorithm by replacing each license with a well-behaved license. In a second approach, we consider allowing the player to revise its past decisions about which rights to exercise while still ensuring compliance with the license. We propose an efficient algorithm based on linear logic, with linear negation used to revise past decisions. We prove our algorithm monotonic, live, and sound with respect to the semantics of licenses
18708753
Unification and ML-Type Reconstruction
We study the complexity of type reconstruction for a core fragment of ML with lambda abstraction, function application, and the polymorphic let declaration. We derive exponential upper and lower bounds on recognizing the typable core ML expressions. Our primary technical tool is uniication of succinctly represented type expressions. After observing that core ML expressions, of size n, can be typed in DTIME(2 n), we exhibit two diierent families of programs whose principal types grow exponentially. We show how to exploit the expressiveness of the let-polymorphism in these constructions to derive lower bounds on deciding typability: one leads naturally to NP-hardness and the other to DTIME(2 n k)-hardness for each integer k 1. Our generic simulation of any exponential time Turing Machine by ML type reconstruction may be viewed as a nonstandard way of computing with types. Our worst-case lower bounds stand in contrast to practical experience, which suggests that commonly used algorithms for type reconstruction do not slow compilation substantially.
16397953
Parametricity and Variants of Girard's J Operator
Abstract The Girard–Reynolds polymorphic λ -calculus is generally regarded as a calculus of parametric polymorphism in which all well-formed terms are strongly normalizing with respect to β -reductions. Girard demonstrated that the addition of a simple “non-parametric” operation, J , to the calculus allows the definition of a non-normalizing term. Since the type of J is not inhabited by any closed term, one might suspect that this may play a role in defining a non-normalizing term using it. We demonstrate that this is not the case by giving a simple variant, J′ , of J whose type is otherwise inhabited and which causes normalization to fail. It appears that impredictivity is essential to the argument; predictive variants of the polymorphic λ -calculus admit non-parametric operations without sacrificing normalization.
14458363
Guillotine subdivisions approximate polygonal subdivisions: a simple new method for the geometric k-MST problem
A method of mixing poly(neopentylglycol)azelate (NPGA) is disclosed which enhances its solubility in polyurethane binders for composite solid propellants. The method comprises withholding the addition of NPGA from the propellant submix and from the total propellant formulation mixing procedure until after the addition of the curing agent has taken place and reaction has taken place for a predetermined period of time with the bonding agent until a "break" in viscosity occurs. The "break" in viscosity has taken place when the other binder ingredients have reacted with the curing agent HDI (hexamethylenediisocyanate) so that immiscibility between the binder ingredients is reduced. The late NPGA addition method resulted not only in apparently complete solubilization of the NPGA but also yielded a propellant with dramatically lower end of mix viscosity (reduced by a factor of 10), superior flow characteristics, and better physical properties. Propellants made by the disclosed method of mixing have desirable lower modulus and higher tensile strength. Thus, the propellant mixed in accordance with the late NPGA addition method yields a propellant with a high tensile strength to modulus ratio. The high tensile strength to modulus ratio at low temperature is an indicator of a good propellant.
17335559
Finding Optimal Bipartitions of Points and Polygons
We give efficient algorithms to compute an optimal bipartition of a set of points or a set of simple polygons in the plane. We examine various criteria involving the perimeter and the area of the convex hulls of the two subsets.
11907078
Optimal decomposition of polygonal models into triangle strips
Motivated by applications in computer graphics, we study the problem of computing an optimal encoding in "triangle strips" of a triangulation of a polygonal surface model. The goal is to facilitate the transmission and rendering of a polygonal model by decomposing its triangulation into a minimum number of "tristrips," each of which has its connectivity stored implicitly in the ordering of the data points. While this optimization problem has been conjectured to be hard, its complexity status has been open. We prove that the tristrip decomposition problem is, in fact, NP-complete. We also propose two methods for solving the problem to optimality, one based on an integer programming formulation, one based on a branch-and-bound scheme that relies on lower bounding techniques for its efficiency. We perform an extensive set of experiments to test the efficiencies of these methods and some of their variants. These methods have been integrated also with the practical system FTSG (Fast Triangle Strip Generator), in order to utilize optimization methods on small subproblems to improve the quality of the heuristic solutions obtained by FTSG. We use experimentation to judge the quality of the improvements.
2767172
Fast rendering of irregular grids
We propose a fast algorithm for rendering general irregular grids. Our method uses a sweep-plane approach to accelerate ray casting, and can handle disconnected and nonconvex (even with holes) unstructured irregular grids with a rendering cost that decreases as the "disconnectedness" decreases. The algorithm is carefully tailored to exploit spatial coherence even if the image resolution differs substantially from the object space resolution. In this paper, we establish the practicality of our method through experimental results based on our implementation, and we also provide theoretical results, both lower and upper bounds, on the complexity of ray casting of irregular grids.
1174606
Optimal covering tours with turn costs
We give the first algorithmic study of a class of “covering tour” problems related to the geometric Traveling Salesman Problem: Find a polygonal tour for a cutter so that it sweeps out a specified region (“pocket”), in order to minimize a cost that depends not only on the length of the tour but also on the number of turns. These problems arise naturally in manufacturing applications of computational geometry to automatic tool path generation and automatic inspection systems, as well as arc routing (“postman”) problems with turn penalties. We prove lower bounds (NP-completeness of minimum-turn milling) and give efficient approximation algorithms for several natural versions of the problem, including a polynomial-time approximation scheme based on a novel adaptation of the m-guillotine method.