Showing papers by "Stavros D. Nikolopoulos published in 2013"

The Longest Path Problem Is Polynomial on Cocomparability Graphs

Abstract: The longest path problem is the problem of finding a path of maximum length in a graph. As a generalization of the Hamiltonian path problem, it is NP-complete on general graphs and, in fact, on every class of graphs that the Hamiltonian path problem is NP-complete. Polynomial solutions for the longest path problem have recently been proposed for weighted trees, Ptolemaic graphs, bipartite permutation graphs, interval graphs, and some small classes of graphs. Although the Hamiltonian path problem on cocomparability graphs was proved to be polynomial almost two decades ago, the complexity status of the longest path problem on cocomparability graphs has remained open; actually, the complexity status of the problem has remained open even on the smaller class of permutation graphs. In this paper, we present a polynomial-time algorithm for solving the longest path problem on the class of cocomparability graphs. Our result resolves the open question for the complexity of the problem on such graphs, and since cocomparability graphs form a superclass of both interval and permutation graphs, extends the polynomial solution of the longest path problem on interval graphs and provides polynomial solution to the class of permutation graphs.

Risk stratification for Arrhythmic Sudden Cardiac Death in heart failure patients using machine learning techniques

Abstract: Arrhythmic Sudden Cardiac Death (SCD) is still a major clinical challenge even though much research has been done in the field. Machine learning techniques give a powerful tool for stratifying arrhythmic risk. We analyzed 40 Holter recordings from heart failure patients, 20 of which were characterized as high arrhythmia risk after 16 months follow up. The two groups (high and low risk) were not statistically different in basic clinical characteristics. We performed windowed analysis and computed 25 Heart Rate Variability (HRV) indices. We fed these indices as input to two classifiers: Support Vector Machines (SVM) and Random Forests (RF). The classification results showed that the automatic classification of the two groups of subjects is possible.

An O(n)-time algorithm for the paired domination problem on permutation graphs

Abstract: A vertex subset D of a graph G is a dominating set if every vertex of G is either in D or is adjacent to a vertex in D. The paired domination problem on G asks for a minimum-cardinality dominating set S of G such that the subgraph induced by S contains a perfect matching; motivation for this problem comes from the interest in finding a small number of locations to place pairs of mutually visible guards so that the entire set of guards monitors a given area. The paired domination problem on general graphs is known to be NP-complete. In this paper, we consider the paired domination problem on permutation graphs. We define an embedding of permutation graphs in the plane which enables us to obtain an equivalent version of the problem involving points in the plane, and we describe a sweeping algorithm for this problem; if the permutation over the set N"n={1,2,...,n} defining a permutation graph G on n vertices is given, our algorithm computes a paired dominating set of G in O(n) time, and is therefore optimal.

Watermarking Images in the Frequency Domain by Exploiting Self-Inverting Permutations

Abstract: In this work we propose efficient codec algorithms for watermarking images that are intended for uploading on the web under intellectual property protection. Headed to this direction, we recently suggested a way in which an integer number w which being transformed into a self-inverting permutation, can be represented in a two dimensional (2D) object and thus, since images are 2D structures, we have proposed a watermarking algorithm that embeds marks on them using the 2D representation of w in the spatial domain. Based on the idea behind this technique, we now expand the usage of this concept by marking the image in the frequency domain. In particular, we propose a watermarking technique that also uses the 2D representation of self-inverting permutations and utilizes marking at specific areas thanks to partial modifications of the image’s Discrete Fourier Transform (DFT). Those modifications are made on the magnitude of specific frequency bands and they are the least possible additive information ensuring robustness and imperceptiveness. We have experimentally evaluated our algorithms using various images of different characteristics under JPEG compression. The experimental results show an improvement in comparison to the previously obtained results and they also depict the validity of our proposed codec algorithms.

Watermarking Images in the Frequency Domain by Exploiting Self-inverting Permutations.

Abstract: In this work we propose efficient codec algorithms for watermarking images that are intended for uploading on the web under intellectual property protection. Headed to this direction, we recently suggested a way in which an integer number w which being transformed into a self-inverting permutation, can be represented in a two dimensional (2D) object and thus, since images are 2D structures, we have proposed a watermarking algorithm that embeds marks on them using the 2D representation of w in the spatial domain. Based on the idea behind this technique, we now expand the usage of this concept by marking the image in the frequency domain. In particular, we propose a watermarking technique that also uses the 2D representation of self-inverting permutations and utilizes marking at specific areas thanks to partial modifications of the image’s Discrete Fourier Transform (DFT). Those modifications are made on the magnitude of specific frequency bands and they are the least possible additive information ensuring robustness and imperceptiveness. We have experimentally evaluated our algorithms using various images of different characteristics under JPEG compression. The experimental results show an improvement in comparison to the previously obtained results and they also depict the validity of our proposed codec algorithms.

A dynamic watermarking model for embedding reducible permutation graphs into software

Abstract: Software watermarking involves embedding a unique identifier or, equivalently, a watermark value, within a software to discourage software theft; towards the embedding process, several graph theoretic watermarking algorithmic techniques encode the watermark values as graph structures and embed them in application programs. Recently, we presented an efficient codec system for encoding a watermark number w as a reducible permutation graph F[π∗] through the use of self-inverting permutations π∗. In this paper, we propose a dynamic watermarking model for embedding the watermark graph F[π∗] into an application program P. The main idea behind the proposed watermarking model is a systematic use of appropriate calls of specific functions of the program P. More precisely, our model uses the dynamic call-graph G(P, I key ) of the program P, taken by the specific input I key , and the graph F[π∗], and produces the watermarked program P∗ having the following key property: its dynamic call-graph G(P∗, I key ) and the reducible permutation graph F[π∗] are isomorphic graphs. Within this idea the program P∗ is produced by only altering appropriate real-calls of specific functions of the input program P. Moreover, the proposed watermarking model incorporates such properties which cause it resilient to attacks.

A Survey on Algorithmic Techniques for Malware Detection

Abstract: Malware is a specific type of software intended to breed damages ranging from computer systems fallout to deprivation of data integrity and confidentiality. Recently, along with the high usage of distributed systems and the increasing speed in telecommunications, the early detection of malware constitutes one of the major concerns in information society. A strong advantage that malware employs in order to elude detection is the ability of polymorphism (metamorphic or polymorphic engines). In this work we present efficient algorithmic techniques that, leveraging higher level abstractions of malware structure, perform an isomorphism check in malware’s produced graph structures, such as function call-graphs and control flowgraphs, in order to detect every possible polymorphic version of a malware. Moreover, we propose an algorithmic approach for malware detection which focuses on the use of behavioural graphs as a more flexible representation of malware’s functionality with respect to its interaction with the operating system. The main idea of our approach is mainly based on behavioural graph similarity issues.

Deceleration capacity alterations before Non-Sustained Ventricular Tachycardia episodes in post myocardial infarction patients

Abstract: While Non-Sustained Ventricular Tachycardia (NSVT) can be characterized as innocent in healthy persons, such arrhythmias in post-infarction patients can be associated with an increased risk for Arrhythmic Sudden Cardiac Death (SCD). The Autonomic Nervous System (ANS) may influence the electrical status before fatal arrhythmias initiations. In this work we study the differences between the behavior of Deceleration Capacity (DC) of heart rate before the onset of NSVT and in the rest of the signal. Twenty (20) patients having presented NSVT episodes are examined. Nine (9) of them have been classified as high risk for SCD after 16 months of follow up, while the rest (11) have been considered as low risk. For each NSVT episode the 30 min period before the episode and the 150 min period exactly before this period were compared. Windowed analysis was performed. Mean values showed that DC is reduced before NSVT episodes in both high and low risk patients. High risk patients presented lower mean values for DC compared to the low risk.

Design and Evaluation of a Graph Codec System for Software Watermarking.

Abstract: In this paper, we propose an efficient and easily implemented codec system for encoding watermark numbers as graph structures thought the use of self-inverting permutations. More precisely, based on the fact that a watermark number w can be efficiently encoded as self-inverting permutation π∗, we present an efficient encoding algorithm which encodes a self-inverting permutation π∗ as a reducible flow-graph F [π∗] and a decoding algorithm which extracts the permutation π∗ from the graph F [π∗]. Our codec algorithms are very simple, use elementary operations on sequences and linked structures, and the produced flow-graph F [π∗] does not differ from the graph data structures built by real programs. Moreover, our codec algorithms have very low time and space complexity and the flow-graph F [π∗] incorporates important structural properties which cause it resilient to attacks. We have evaluated several components of our codec system in a simulation environment in order to obtain a clear view of their practical behaviour; the experimental results show that we can decide with high probability whether the graph F [π∗] suffer an attack on its edges.

Evaluating the WaterRpg software watermarking model on Java application programs

Abstract: Recently, we have presented a dynamic watermarking model, which we named WaterRpg, for embedding a reducible permutation graph F[π*] into an application program P The main idea behind the proposed watermarking model is to modify the dynamic call-graph G(P, Ikey) of the program P, taken by the specific input Ikey, so that the dynamic call-graph G(P*, Ikey) of the resulting watermarked program P* and the the reducible permutation graph F[π*] are isomorphic; within this idea the program P* is produced by only altering appropriate calls of specific functions of the input application program P Our model belongs to execution trace watermarks category In this paper, we implement our WaterRpg watermarking model on several Java application programs and evaluate it under various criteria in order to gain information about its practical behavior More precisely, we selected a number of Java application programs and watermark them using two main watermarking approaches supported by our WaterRpg model, namely naive and stealthy approachs The experimental results show the stable functionality of all the Java programs P* watermarked under both the naive and stealthy cases The experiments also show that the watermarking approaches supported by our model can help develop efficient watermarked Java programs with respect to resilience, size, time, space, and other watermarking metrics

Details on the Application of Multiresolution Wavelet Analysis on Heartbeat Timeseries

Abstract: In this paper we investigate important details of the application of Multiresolution Wavelet Analysis (MWA) on heartbeat timeseries which improve the diagnostic capabilities of the computation of Heart Rate Variability (HRV) and can extract sensitive information from the examined signal. We present experimental results using different wavelet families showing that the Haar wavelet is the most appropriate wavelet to use for heartbeat timeseries. Next, we apply several metrics on the detail wavelet coefficients and show that the diagnostic capability of each one of those metrics may vary from scale to scale. The selection of a different metric for each scale of analysis leads to better categorization performance.

Watermarking Java application programs using the WaterRpg dynamic model

Abstract: We have recently presented an efficient codec system for encoding a watermark number w as a reducible permutation graph F [π*], through the use of self-inverting permutations π* and proposed a dynamic watermarking model, which we named WaterRpg, for embedding the watermark graph F [π*] into an application program P. In this paper, we implement our watermarking model WaterRpg in real application programs, taken from a game database, and evaluate its functionality under various watermarking issues supported by our WaterRpg model. More precisely, we selected a number of Java application programs and watermark them using two main approaches. First, we show in detail a straightforward or naive approach for watermarking a given program P which is based only on the well-defined call patterns of our model, and then we prove structural and programming properties of the call patterns based on which we can watermark the program P in a more stealthy way. The experimental results show the efficient functionality of all the programs P* watermarked under the naive-case and all the stealthy-cases. The size and the time overhead of the propose watermarking are very low.

Watermarking Digital Images in the Frequency Domain: Performance and Attack Issues

Abstract: In this work we propose an efficient model for watermarking images that are intended for uploading on the web under intellectual property protection. Headed to this direction, we recently suggested a way in which an integer number \(w\) which being transformed into a self-inverting permutation, can be represented in a two dimensional (2D) object and thus, since images are 2D structures, we propose a watermarking algorithm that embeds marks on them using the 2D representation of \(w\) in the frequency domain. In particular, we propose a watermarking technique that uses the 2D representation of self-inverting permutations and utilizes marking at specific areas thanks to partial modifications of the image’s Discrete Fourier Transform (DFT). Those modifications are made on the magnitude of specific frequency bands and they are the least possible additive information ensuring robustness and imperceptiveness. We have experimentally evaluated our algorithms using various images of different characteristics under JPEG compression, Gaussian noise addition, and geometric transformations. The experimental results show an improvement in comparison to the previously obtained results and they also depict the validity of our proposed codec algorithms.