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Searching for a black hole in arbitrary networks
 Distributed Computing
, 2002
"... Consider a networked environment, supporting mobile agents, where there is a black hole: a harmful host that disposes of visiting agents upon their arrival, leaving no observable trace of such a destruction. The black hole search problem is the one of assembling a team of asynchronous mobile agents, ..."
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Cited by 45 (25 self)
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Consider a networked environment, supporting mobile agents, where there is a black hole: a harmful host that disposes of visiting agents upon their arrival, leaving no observable trace of such a destruction. The black hole search problem is the one of assembling a team of asynchronous mobile agents, executing the same protocol and communicating by means of whiteboards, to successfully identify the location of the black hole; we are concerned with solutions that are generic (i.e., topologyindependent). We establish tight bounds on the size of the team (i.e., the number of agents), and the cost (i.e., the number of moves) of a sizeoptimal solution protocol. These bounds depend on the a priori knowledge the agents have about the network, and on the consistency of the local labellings. In particular, we prove that: with topological ignorance ∆ + 1 agents are needed and suffice, and the cost is Θ(n 2), where ∆ is the maximal degree of a node and n is the number of nodes in the network; with topological ignorance but in presence of sense of direction only two agents suffice and the cost is Θ(n 2); and with complete topological knowledge only two agents suffice and the cost is Θ(n log n). All the upperbound proofs are constructive.
Hardness and approximation results for black hole search in arbitrary graphs
 In Proc. 12th Coll. on Structural Information and Communication complexity (SIROCCO’05
, 2005
"... Abstract. A black hole is a highly harmful stationary process residing in a node of a network and destroying all mobile agents visiting the node, without leaving any trace. We consider the task of locating a black hole in a (partially) synchronous arbitrary network, assuming an upper bound on the ti ..."
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Cited by 28 (11 self)
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Abstract. A black hole is a highly harmful stationary process residing in a node of a network and destroying all mobile agents visiting the node, without leaving any trace. We consider the task of locating a black hole in a (partially) synchronous arbitrary network, assuming an upper bound on the time of any edge traversal by an agent. For a given graph and a given starting node we are interested in finding the fastest possible Black Hole Search by two agents (the minimum number of agents capable to identify a black hole). We prove that this problem is NPhard in arbitrary graphs, thus solving an open problem stated in [2]. We also give a 7/2approximation algorithm, thus improving on the 4approximation scheme observed in [2]. Our approach is to explore the given input graph via some spanning tree. Even if it represents a very natural technique, we prove that this approach cannot achieve an approximation ratio better than 3/2.
Searching for BlackHole Faults in a Network Using Multiple Agents
 In Proc. 10th International Conference on Principles of Distributed Systems (OPODIS
, 2006
"... Abstract. We consider a fixed communication network where (software) agents can move freely from node to node along the edges. A black hole is a faulty or malicious node in the network such that if an agent enters this node, then it immediately “dies. ” We are interested in designing an efficient co ..."
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Cited by 19 (1 self)
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Abstract. We consider a fixed communication network where (software) agents can move freely from node to node along the edges. A black hole is a faulty or malicious node in the network such that if an agent enters this node, then it immediately “dies. ” We are interested in designing an efficient communication algorithm for the agents to identify all black holes. We assume that we have k agents starting from the same node s and knowing the topology of the whole network. The agents move through the network in synchronous steps and can communicate only when they meet in a node. At the end of the exploration of the network, at least one agent must survive and must know the exact locations of the black holes. If the network has n nodes and b black holes, then any exploration algorithm needs Ω(n/k + Db) steps in the worstcase, where Db is the worst case diameter of the network with at most b nodes deleted. We give a general algorithm which completes exploration in O((n/k)logn / log log n+bDb) steps for arbitrary networks, if b ≤ k/2. In the case when b ≤ k/2, bDb = O ( √ n)andk = O ( √ n), we give a refined algorithm which completes exploration in asymptotically optimal O(n/k) steps.
Searching for a black hole in tree networks
"... A black hole is a highly harmful stationary process residing in a node of a network and destroying all mobile agents visiting the node, without leaving any trace. We consider the task of locating a black hole in a (partially) synchronous tree network, assuming an upper bound on the time of any edge ..."
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Cited by 13 (5 self)
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A black hole is a highly harmful stationary process residing in a node of a network and destroying all mobile agents visiting the node, without leaving any trace. We consider the task of locating a black hole in a (partially) synchronous tree network, assuming an upper bound on the time of any edge traversal by an agent. The minimum number of agents capable to identify a black hole is two. For a given tree and given starting node we are interested in the fastest possible black hole search by two agents. For arbitrary trees we give a 5/3approximation algorithm for this problem. We give optimal black hole search algorithms for two “extreme” classes of trees: the class of lines and the class of trees in which any internal node (including the root which is the starting node) has at least 2 children.
Black hole search in common interconnection networks
, 2006
"... Mobile agents operating in networked environments face threats from other agents as well as from the hosts (i.e., network sites) they visit. A black hole is a harmful host that destroys incoming agents without leaving any trace. To determine the location of such a harmful host is a dangerous but cru ..."
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Cited by 11 (3 self)
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Mobile agents operating in networked environments face threats from other agents as well as from the hosts (i.e., network sites) they visit. A black hole is a harmful host that destroys incoming agents without leaving any trace. To determine the location of such a harmful host is a dangerous but crucial task, called black hole search. The most important parameter for a solution strategy is the number of agents it requires (the size); the other parameter of interest is the total number of moves performed by the agents (the cost). It is known that at least two agents are needed; furthermore, with full topological knowledge, �(n log n) moves are required in arbitrary networks. The natural question is whether, in specific networks, it is possible to obtain (topologydependent but) more cost efficient solutions. It is known that this is not the case for rings. In this article, we show that this negative result does not generalizes. In fact, we present a general strategy that allows two agents to locate the black hole with O(n) moves in common interconnection networks: hypercubes, cubeconnected cycles, star graphs, wrapped butterflies, chordal rings, as well as in multidimensional meshes and tori of restricted diameter. These results hold even if the networks are anonymous.
Pathbased security for mobile agents
 Proceedings of the First International Workshop onthe Security of Mobile MultiAgent Systems (SEMAS2001) at the Fifth International Conference on Autonomous Agents (Agents 2001
, 2001
"... As mobile agents are increasingly adopted in intranets, on the Internet, and on computational grids, new security concerns become increasingly important. Unlike other kinds of mobile code, such as applets, which are pulled a single time to remote systems (singlehop), mobile agents may move using th ..."
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Cited by 9 (6 self)
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As mobile agents are increasingly adopted in intranets, on the Internet, and on computational grids, new security concerns become increasingly important. Unlike other kinds of mobile code, such as applets, which are pulled a single time to remote systems (singlehop), mobile agents may move using their own
Semantic Encryption Transformation Scheme
 in Proc. of 2004 International Workshop on Security in Parallel and Distributed Systems
, 2004
"... We present a scheme to protect mobile code from malicious hosts. We propose a transformation scheme that changes the semantics of a program using pseudorandom I/O scrambling, conditional elimination, and encryption using numeric variables for changing programs into encrypted but executable form that ..."
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Cited by 6 (3 self)
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We present a scheme to protect mobile code from malicious hosts. We propose a transformation scheme that changes the semantics of a program using pseudorandom I/O scrambling, conditional elimination, and encryption using numeric variables for changing programs into encrypted but executable form that yields a recoverable result. The goal of our transformation process is to prevent an attacker from knowing the purpose of a program in order to reduce tampering.