Hugues Mercier

Severity analysis and countermeasure for the wormhole attack in wireless ad hoc networks

In this paper, we analyze the effect of the wormhole attack on shortest-path routing protocols for wireless ad hoc networks. Using analytical and simulation results, we show that a strategic placement of the wormhole when the nodes are uniformly distributed can disrupt/control on average 32% of all communications across the network. We also analyze a scenario in which several attackers make wormholes between each other and a case where two malicious nodes attack a target node in the network. We show how to evaluate the maximum effect of the wormhole attack on a given network topology. Then, we compute the maximum effect of the wormhole attack on grid topology networks and show that the attackers can disrupt/control around 40% to 50% of all communications when the wormhole is strategically placed in the network. Finally, to defend against the wormhole attack, we propose a timing-based countermeasure that avoids the deficiencies of existing timing-based solutions. Using the proposed countermeasure, the nodes do not need synchronized clocks, nor are they required to predict the sending time or to be capable of fast switching between the receive and send modes. Moreover, the nodes do not need one-to-one communication with all their neighbors and do not require to compute a signature while having to timestamp the message with its transmission time.

A survey of error-correcting codes for channels with symbol synchronization errors

We present a comprehensive survey of error-correcting codes for channels corrupted by synchronization errors. We discuss potential applications as well as the obstacles that need to be overcome before such codes can be used in practical systems.

Some research issues in cognitive radio networks

The cognitive radio (CR) technology will allow a group of potential users to identify and access available spectrum resources provided that the interference to the users for whom the band has been licensed is kept below a prescribed level. However, this research area is at a very immature stage because various research challenges have to be addressed and solved. In this paper our objective is to present an overview of some research issues for CR networks. Specifically, we present some research and development in CR networks with focus on i) information- theoretic aspects, ii) spectrum sensing, iii) link adaptation, iv) advanced transceiver design, and v) admission control. We also discuss some important research problems related to these specific topics that needs to be addressed before deployment of CR systems in practice.

NIS02-1: Wormhole Attack in Wireless Ad Hoc Networks: Analysis and Countermeasure

The wormhole attack is one of the most severe security attacks in wireless ad hoc networks. In this paper, we analyze the effect of the wormhole attack in shortest path routing protocols. Using analytical and simulation results, we show that a strategic placement of the wormhole can disrupt on average 32% of all communications across the network. We also analyze a more severe attack in which several attackers make wormholes between each other and give an upper bound on the average number of communications that can be disrupted. Finally, we propose a new robust and secure on-demand distance vector routing protocol which is able to route packets as long as there is a non-faulty path between the source and the destination.

On the Number of Subsequences When Deleting Symbols From a String

We consider the problem of finding the number of subsequences when deleting symbols from a string. We present a framework to find closed-form expressions for the number of subsequences, and use it to prove the first explicit formulas for and the first formulas for nonbinary alphabets. We also present the first efficient algorithm to compute the number of subsequences for any string, number of deletions, and alphabet size.

Error-Correcting Codes for Dynamic Spectrum Allocation in Cognitive Radio Systems

Cognitive radio is an innovative technology proposed to increase spectrum usage by allowing dynamic allocation of the unused spectrum in changing environments. Cognitive users monitor the spectrum and are allowed to use it as long as it does not interfere with the primary users to whom it has been licensed. In this paper, we describe how some well established tools from the fields of error-correcting codes and information theory can be applied to the opportunistic spectrum access problem in cognitive radio. More precisely, we describe simple channel models based on information theoretic analysis and demonstrate that common error-correcting coding techniques, specifically low-density parity-check codes and insertion-deletion codes, can be used to solve this problem.

Reliability-bandwidth tradeoffs for distributed storage allocations

We consider the allocation of coded data over nodes in a distributed storage system under a budget constraint. A system with failed nodes can recover the original data of unit size if the amount of data in the active nodes is at least a unit. Building on the work of Leong et al. [1], we introduce the concepts of tight allocations and repair bandwidth in this distributed setting. For tight allocations, the amount of data in the failed nodes gives a lower bound to the repair bandwidth required to put the system back to its original state. Using this bound, we define the Minimum Expected Repair Bandwidth (MERB) to study the tradeoffs between reliability and repair bandwidth, both empirically and by proving bounds on MERB in terms of the reliability. We show that even computing MERB for a general allocation is #P-Hard and suggest a simpler objective function to optimize it approximately. Finally, we study the asymptotic behavior of MERB for large systems and show two distinct optimal allocation regimes depending on the failure probability of the storage nodes.

STEP-archival: Storage integrity and anti-tampering using data entanglement

We present STEP-archives, a model for censorship-resistant storage systems where an attacker cannot censor or tamper with data without causing a large amount of obvious collateral damage. MDS erasure codes are used to entangle unrelated data blocks, in addition to providing redundancy against storage failures. We show a tradeoff for the attacker between attack complexity, irrecoverability, and collateral damage. We also show that the system can efficiently recover from attacks with imperfect irrecoverability, making the problem asymmetric between attackers and defenders. Finally, we present sample heuristic attack algorithms that are efficient and irrecoverable (but not collateral-damage-optimal), and demonstrate how some strategies and parameter choices allow to resist these sample attacks.

Convolutional codes for channels with deletion errors

This correspondence studies convolutional codes for channels with deletion errors. We show that the Viterbi decoding algorithm can be modified to correct deletion errors and prove that it is able to find the closest codeword from the received sequence. The computational complexity of the algorithm is comparable to the complexity of the original Viterbi algorithm. We also prove that maximum-likelihood decoding of deletion-correcting trellis codes cannot be achieved using only a forward recursion in the trellis and a single survivor path per state.

Worst Case Nonzero-Error Interactive Communication

In the interactive communication model, two parties and possess respective private but correlated inputs and , and wants to learn from while minimizing the communication required for the worst possible input pair . Our contribution is the analysis of four nonzero-error models in this correlated data setting. In the private coin randomized model, both players are allowed to toss coins, and must learn with high probability for every input pair. The second and third models are similar to the first one, but the players are allowed to use a common source of randomness and to solve several independent instances of the same problem simultaneously, respectively. In the fourth model, is allowed to answer incorrectly for a small fraction of the inputs. We show that one round of communication is nearly optimal for the private coin randomized model. We also prove that the last three models are equivalent and can be arbitrarily better than the original worst case deterministic model when interaction is not allowed. Finally, we show that the deterministic model and all the nonzero-error models are equivalent for a class of symmetric problems arising from several practical applications, although nonzero-error and randomization allow efficient one-way protocols.