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Dynamic and efficient key management for access hierarchies
 In Proceedings of the ACM Conference on Computer and Communications Security
, 2005
"... Hierarchies arise in the context of access control whenever the user population can be modeled as a set of partially ordered classes (represented as a directed graph). A user with access privileges for a class obtains access to objects stored at that class and all descendant classes in the hierarchy ..."
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Cited by 69 (8 self)
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Hierarchies arise in the context of access control whenever the user population can be modeled as a set of partially ordered classes (represented as a directed graph). A user with access privileges for a class obtains access to objects stored at that class and all descendant classes in the hierarchy. The problem of key management for such hierarchies then consists of assigning a key to each class in the hierarchy so that keys for descendant classes can be obtained via efficient key derivation. We propose a solution to this problem with the following properties: (1) the space complexity of the public information is the same as that of storing the hierarchy; (2) the private information at a class consists of a single key associated with that class; (3) updates (i.e., revocations and additions) are handled locally in the hierarchy; (4) the scheme is provably secure against collusion; and (5) each node can derive the key of any of its descendant with a number of symmetrickey operations bounded by the length of the path between the nodes. Whereas many previous schemes had some of these properties, ours is the first that satisfies all of them. The security of our scheme is based on pseudorandom functions, without reliance on the Random Oracle Model. 18 Portions of this work were supported by Grants IIS0325345 and CNS06274488 from the
The LCMlattice in monomial resolutions
, 1999
"... Describing the properties of the minimal free resolution of a monomial ideal I is a difficult problem posed in the early 1960’s. The main directions of progress on this problem were: ..."
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Cited by 38 (5 self)
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Describing the properties of the minimal free resolution of a monomial ideal I is a difficult problem posed in the early 1960’s. The main directions of progress on this problem were:
Convex drawings of Planar Graphs and the Order Dimension of 3Polytopes
 ORDER
, 2000
"... We define an analogue of Schnyder's tree decompositions for 3connected planar graphs. Based on this structure we obtain: Let G be a 3connected planar graph with f faces, then G has a convex drawing with its vertices embedded on the (f 1) (f 1) grid. Let G be a 3connected planar graph. ..."
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Cited by 32 (12 self)
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We define an analogue of Schnyder's tree decompositions for 3connected planar graphs. Based on this structure we obtain: Let G be a 3connected planar graph with f faces, then G has a convex drawing with its vertices embedded on the (f 1) (f 1) grid. Let G be a 3connected planar graph. The dimension of the incidence order of vertices, edges and bounded faces of G is at most 3. The second result is originally due to Brightwell and Trotter. Here we give a substantially simpler proof.
Trapezoid Graphs and Generalizations, Geometry and Algorithms
 DISCRETE APPLIED MATHEMATICS
, 1993
"... Trapezoid graphs are a class of cocomparability graphs containing interval graphs and permutation graphs as subclasses. They were introduced by Dagan, Golumbic and Pinter [DGP]. They propose an O(n²) algorithm for chromatic number and a less efficient algorithm for maximum clique on trapezoid graphs ..."
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Cited by 27 (0 self)
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Trapezoid graphs are a class of cocomparability graphs containing interval graphs and permutation graphs as subclasses. They were introduced by Dagan, Golumbic and Pinter [DGP]. They propose an O(n²) algorithm for chromatic number and a less efficient algorithm for maximum clique on trapezoid graphs. Based on a geometric representation of trapezoid graphs by boxes in the plane we design optimal, i.e., O(n log n), algorithms for chromatic number, weighted independent set, clique cover and maximum weighted clique on such graphs. We also propose generalizations of trapezoid graphs called ktrapezoidal graphs. The ideas behind the clique cover and weighted independent set algorithms for trapezoid graphs carry over to higher dimensions. This leads to O(n log k\Gamma1 n) algorithms for ktrapezoidal graphs. We also propose a new class of graphs called circle trapezoid graphs. This class contains trapezoid graphs, circle graphs and circulararc graphs as subclasses. We show that cli...
ResourceConstrained Project Scheduling: From a Lagrangian Relaxation to Competitive Solutions
, 2000
"... List scheduling belongs to the classical and widely used algorithms for scheduling problems, but for resourceconstrained project scheduling problems most standard priority lists do not capture enough of the problem structure, often resulting in poor performance. We use a wellknown Lagrangian relax ..."
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Cited by 24 (3 self)
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List scheduling belongs to the classical and widely used algorithms for scheduling problems, but for resourceconstrained project scheduling problems most standard priority lists do not capture enough of the problem structure, often resulting in poor performance. We use a wellknown Lagrangian relaxation to rst compute schedules which do not necessarily respect the resource constraints. We then apply list scheduling in the order of socalled completion times of jobs. Embedded into a standard subgradient optimization, our computational results show that the schedules compare to those obtained by stateoftheart local search algorithms. In contrast to purely primal heuristics, however, the Lagrangian relaxation also provides powerful lower bounds, thus the deviation between lower and upper bounds can be drastically reduced by this approach.
Inapproximability results for sparsest cut, optimal linear arrangement, and precedence constrained scheduling
 IN PROCEEDINGS OF 48TH ANNUAL SYMPOSIUM ON FOUNDATIONS OF COMPUTER SCIENCE (FOCS
, 2007
"... This paper is published in the proceedings of FOCS 2007 and is subject to some copyright restrictions. We consider (Uniform) Sparsest Cut, Optimal Linear Arrangement and the precedence constrained scheduling problem 1prec  � wjCj. So far, these three notorious NPhard problems have resisted all at ..."
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Cited by 19 (6 self)
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This paper is published in the proceedings of FOCS 2007 and is subject to some copyright restrictions. We consider (Uniform) Sparsest Cut, Optimal Linear Arrangement and the precedence constrained scheduling problem 1prec  � wjCj. So far, these three notorious NPhard problems have resisted all attempts to prove inapproximability results. We show that they have no Polynomial Time Approximation Scheme (PTAS), unless NPcomplete problems can be solved in randomized subexponential time. Furthermore, we prove that the scheduling problem is as hard to approximate as Vertex Cover when the socalled fixed cost, that is present in all feasible solutions, is subtracted from the objective function.
KEY MANAGEMENT FOR NONTREE ACCESS HIERARCHIES
, 2006
"... Access hierarchies are useful in many applications and are modeled as a set of access classes organized by a partial order. A user who obtains access to a class in such a hierarchy is entitled to access objects stored at that class, as well as objects stored at its descendant classes. Efficient sche ..."
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Cited by 16 (7 self)
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Access hierarchies are useful in many applications and are modeled as a set of access classes organized by a partial order. A user who obtains access to a class in such a hierarchy is entitled to access objects stored at that class, as well as objects stored at its descendant classes. Efficient schemes for this framework assign only one key to a class and use key derivation to permit access to descendant classes. Ideally, the key derivation uses simple primitives such as cryptographic hash computations and modular additions. A straightforward key derivation time is then linear in the length of the path between the user’s class and the class of the object that the user wants to access. Recently, work presented in [2] has given an efficient solution that significantly lowers this key derivation time, while
The monadic secondorder logic of graphs XV: On a Conjecture by D. Seese
 Journal of Applied Logic
, 2006
"... A conjecture by D. Seese states that if a set of graphs has a decidable monadic secondorder theory, then it is the image of a set of trees under a transformation defined by monadic secondorder formulas. We prove that the general case of this conjecture is equivalent to the particular cases of dire ..."
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Cited by 15 (6 self)
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A conjecture by D. Seese states that if a set of graphs has a decidable monadic secondorder theory, then it is the image of a set of trees under a transformation defined by monadic secondorder formulas. We prove that the general case of this conjecture is equivalent to the particular cases of directed graphs, partial orders and comparability graphs. We present some tools to prove the conjecture for classes of graphs with few cliques or few complete bipartite subgraphs, for line graphs and for interval graphs. We make an essential use of prime graphs, of comparability graphs and of characterizations of graph classes by forbidden induced subgraphs. Our treatment of infinite graphs uses a representation of countable linear orders by binary trees that can be constructed by monadic secondorder formulas. By using a counting argument, we show the intrinsic limits of the methods used so far to handle this conjecture.
Order dimension, strong Bruhat order and lattice properties for posets
 ORDER
, 2002
"... We determine the order dimension of the strong Bruhat order on finite Coxeter groups of types A, B and H. The order dimension is determined using a generalization of a theorem of Dilworth: dim(P) = width(Irr(P)), whenever P satisfies a simple ordertheoretic condition called here the dissective pro ..."
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Cited by 14 (6 self)
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We determine the order dimension of the strong Bruhat order on finite Coxeter groups of types A, B and H. The order dimension is determined using a generalization of a theorem of Dilworth: dim(P) = width(Irr(P)), whenever P satisfies a simple ordertheoretic condition called here the dissective property (or “clivage ” in [16, 21]). The result for dissective posets follows from an upper bound and lower bound on the dimension of any finite poset. The dissective property is related, via MacNeille completion, to the distributive property of lattices. We show a similar connection between quotients of the strong Bruhat order with respect to parabolic subgroups and lattice quotients.