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., topology-independent). 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 size-optimal 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 upper-bound proofs are constructive.

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 NP-hard in arbitrary graphs, thus solving an open problem stated in [2]. We also give a 7/2approximation algorithm, thus improving on the 4-approximation 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.

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. The Black Hole Search is the task of locating all black holes in a network, through the exploration of its nodes by a set of mobile agents. In this paper we consider the problem of designing the fastest Black Hole Search, given the map of the network, the starting node and, possibly, a subset of nodes of the network initially known to be safe. We study the version of this problem that assumes that there is at most one black hole in the network and there are two agents, which move in synchronized steps. We prove that this problem is not polynomial-time approximable within 389 (unless P=NP). We 388 give a 6-approximation algorithm, thus improving on the 9.3-approximation algorithm from [2]. We also prove APX-hardness for a restricted version of the problem, in which only the starting node is initially known to be safe.

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.

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). Any solution requires n log n) moves in general networks; the same lower bound holds for rings. In this paper we show that this lower bound does not hold for hypercubes and related networks. In fact, we present a general strategy which allows two agents to locate the black hole with O(n) moves in hypercubes, cube-connected cycles, star graphs, wrapped butterflies, chordal rings, as well as in multidimensional meshes and tori of restricted diameter.

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 (topology-dependent 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, cube-connected 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.

In this paper we consider a dangerous process located at a node of a network (called Black Hole) and a team of mobile agents deployed to locate that node. The nature of the danger is such that when an agent enters the dangerous node, it is trapped there leaving no trace of its destruction. The goal is to deploy as few agents as possible and to locate the black hole in as few moves as possible. We present a simple algorithm that works on any topology (a-priori known by the agents). Our algorithm, based on the pre-computation of an open vertex cover by cycles of the network, uses the optimal number of agents (two); its cost (number of moves) depends on the choice of the cover and it is optimal for several classes of networks.

Abstract — We introduce the notion of spy agents and describe how they can be deployed within diverse network protocol architectures in order to perform high fidelity trust assessments in remote environments. The spy agent framework developed here consists of: a spy agent structural architecture that instruments and instantiates spy agents with appropriate content; a spy agent routing framework that fabricates and deploys the overall spying scenario with specialised spying routing protocols; and an evaluation entity that implements all the necessary security analysis mechanisms. Keywords—Spy agents, trust evaluation, surveillance 1.

Abstract Mobile code suffers from the malicious host problem. When an adversary receives code he is able to effectively tamper with the code if he is able to relate the operations of the program with the appropriate context, that is, understanding the semantics of the program. In order to thwart an adversary from effectively tampering with a program he must be given an encrypted version such that he may be able to observe its operations but not understand why those operations are performed. Our notion of encrypting a program in such a way is to semantically alter it. In this paper, we add a White-box dimension called Cryptoprogramming to the Black-box notion of our Semantic Encryption Transformation Scheme. We construct an encrypted program by transforming the runtime logical pathways of the original program into a nonequivalent set of corresponding runtime logical pathways, and yet still allow for easy recoverability of the output of the program. 1

When using mobile agents, numerous security issues must be considered. In this note we propose two methods to improve the security and reliability of mobile agent based transactions in an environment which may contain some malicious hosts.