Provably-Correct Task Planning for Autonomous Outdoor Robots

Autonomous outdoor robots should be able to accomplish complex tasks safely and reliably while considering constraints that arise from both the environment and the physical platform. Such tasks extend basic navigation capabilities to specify a sequence of events over time. For example, an autonomous aerial vehicle can be given a surveillance task with contingency plans while complying with rules in regulated airspace, or an autonomous ground robot may need to guarantee a given probability of success while searching for the quickest way to complete the mission. A promising approach for the automatic synthesis of trusted controllers for complex tasks is to employ techniques from formal methods. In formal methods, tasks are formally specified symbolically with temporal logic. The robot then synthesises a controller automatically to execute trusted behaviour that guarantees the satisfaction of specified tasks and regulations. However, a difficulty arises from the lack of expressivity, which means the constraints affecting outdoor robots cannot be specified naturally with temporal logic. The goal of this thesis is to extend the capabilities of formal methods to express the constraints that arise from outdoor applications and synthesise provably-correct controllers with trusted behaviours over time. This thesis focuses on two important types of constraints, resource and safety constraints, and presents three novel algorithms that express tasks with these constraints and synthesise controllers that satisfy the specification. Firstly, this thesis proposes an extension to probabilistic computation tree logic (PCTL) called resource threshold PCTL (RT-PCTL) that naturally defines the mission specification with continuous resource threshold constraints; furthermore, it synthesises an optimal control policy with respect to the probability of success. With RT-PCTL, a state with accumulated resource out of the specified bound is considered to be failed or saturated depending on the specification. The requirements on resource bounds are naturally encoded in the symbolic specification, followed by the automatic synthesis of an optimal controller with respect to the probability of success. Secondly, the thesis proposes an online algorithm called greedy Buchi algorithm (GBA) that reduces the synthesis problem size to avoid the scalability problem. A framework is then presented with realistic control dynamics and physical assumptions in the environment such as wind estimation and fuel constraints. The time and space complexity for the framework is polynomial in the size of the system state, which is efficient for online synthesis. Lastly, the thesis proposes a synthesis algorithm for an optimal controller with respect to completion time given the minimum safety constraints. The algorithm naturally balances between completion time and safety. This work proves an analytical relationship between the probability of success and the conditional completion time given the mission specification. The theoretical contributions in this thesis are validated through realistic simulation examples. This thesis identifies and solves two core problems that contribute to the overall vision of developing a theoretical basis for trusted behaviour in outdoor robots. These contributions serve as a foundation for further research in multi-constrained task planning where a number of different constraints are considered simultaneously within a single framework

Cyber Security of Critical Infrastructures

Critical infrastructures are vital assets for public safety, economic welfare, and the national security of countries. The vulnerabilities of critical infrastructures have increased with the widespread use of information technologies. As Critical National Infrastructures are becoming more vulnerable to cyber-attacks, their protection becomes a significant issue for organizations as well as nations. The risks to continued operations, from failing to upgrade aging infrastructure or not meeting mandated regulatory regimes, are considered highly significant, given the demonstrable impact of such circumstances. Due to the rapid increase of sophisticated cyber threats targeting critical infrastructures with significant destructive effects, the cybersecurity of critical infrastructures has become an agenda item for academics, practitioners, and policy makers. A holistic view which covers technical, policy, human, and behavioural aspects is essential to handle cyber security of critical infrastructures effectively. Moreover, the ability to attribute crimes to criminals is a vital element of avoiding impunity in cyberspace. In this book, both research and practical aspects of cyber security considerations in critical infrastructures are presented. Aligned with the interdisciplinary nature of cyber security, authors from academia, government, and industry have contributed 13 chapters. The issues that are discussed and analysed include cybersecurity training, maturity assessment frameworks, malware analysis techniques, ransomware attacks, security solutions for industrial control systems, and privacy preservation methods

Simulation and statistical model-checking of logic-based multi-agent system models

This thesis presents SALMA (Simulation and Analysis of Logic-Based Multi- Agent Models), a new approach for simulation and statistical model checking of multi-agent system models. Statistical model checking is a relatively new branch of model-based approximative verification methods that help to overcome the well-known scalability problems of exact model checking. In contrast to existing solutions, SALMA specifies the mechanisms of the simulated system by means of logical axioms based upon the well-established situation calculus. Leveraging the resulting first-order logic structure of the system model, the simulation is coupled with a statistical model-checker that uses a first-order variant of time-bounded linear temporal logic (LTL) for describing properties. This is combined with a procedural and process-based language for describing agent behavior. Together, these parts create a very expressive framework for modeling and verification that allows direct fine-grained reasoning about the agents’ interaction with each other and with their (physical) environment. SALMA extends the classical situation calculus and linear temporal logic (LTL) with means to address the specific requirements of multi-agent simulation models. In particular, cyber-physical domains are considered where the agents interact with their physical environment. Among other things, the thesis describes a generic situation calculus axiomatization that encompasses sensing and information transfer in multi agent systems, for instance sensor measurements or inter-agent messages. The proposed model explicitly accounts for real-time constraints and stochastic effects that are inevitable in cyber-physical systems. In order to make SALMA’s statistical model checking facilities usable also for more complex problems, a mechanism for the efficient on-the-fly evaluation of first-order LTL properties was developed. In particular, the presented algorithm uses an interval-based representation of the formula evaluation state together with several other optimization techniques to avoid unnecessary computation. Altogether, the goal of this thesis was to create an approach for simulation and statistical model checking of multi-agent systems that builds upon well-proven logical and statistical foundations, but at the same time takes a pragmatic software engineering perspective that considers factors like usability, scalability, and extensibility. In fact, experience gained during several small to mid-sized experiments that are presented in this thesis suggest that the SALMA approach seems to be able to live up to these expectations.In dieser Dissertation wird SALMA (Simulation and Analysis of Logic-Based Multi-Agent Models) vorgestellt, ein im Rahmen dieser Arbeit entwickelter Ansatz für die Simulation und die statistische Modellprüfung (Model Checking) von Multiagentensystemen. Der Begriff „Statistisches Model Checking” beschreibt modellbasierte approximative Verifikationsmethoden, die insbesondere dazu eingesetzt werden können, um den unvermeidlichen Skalierbarkeitsproblemen von exakten Methoden zu entgehen. Im Gegensatz zu bisherigen Ansätzen werden in SALMA die Mechanismen des simulierten Systems mithilfe logischer Axiome beschrieben, die auf dem etablierten Situationskalkül aufbauen. Die dadurch entstehende prädikatenlogische Struktur des Systemmodells wird ausgenutzt um ein Model Checking Modul zu integrieren, das seinerseits eine prädikatenlogische Variante der linearen temporalen Logik (LTL) verwendet. In Kombination mit einer prozeduralen und prozessorientierten Sprache für die Beschreibung von Agentenverhalten entsteht eine ausdrucksstarke und flexible Plattform für die Modellierung und Verifikation von Multiagentensystemen. Sie ermöglicht eine direkte und feingranulare Beschreibung der Interaktionen sowohl zwischen Agenten als auch von Agenten mit ihrer (physischen) Umgebung. SALMA erweitert den klassischen Situationskalkül und die lineare temporale Logik (LTL) um Elemente und Konzepte, die auf die spezifischen Anforderungen bei der Simulation und Modellierung von Multiagentensystemen ausgelegt sind. Insbesondere werden cyber-physische Systeme (CPS) unterstützt, in denen Agenten mit ihrer physischen Umgebung interagieren. Unter anderem wird eine generische, auf dem Situationskalkül basierende, Axiomatisierung von Prozessen beschrieben, in denen Informationen innerhalb von Multiagentensystemen transferiert werden – beispielsweise in Form von Sensor- Messwerten oder Netzwerkpaketen. Dabei werden ausdrücklich die unvermeidbaren stochastischen Effekte und Echtzeitanforderungen in cyber-physischen Systemen berücksichtigt. Um statistisches Model Checking mit SALMA auch für komplexere Problemstellungen zu ermöglichen, wurde ein Mechanismus für die effiziente Auswertung von prädikatenlogischen LTL-Formeln entwickelt. Insbesondere beinhaltet der vorgestellte Algorithmus eine Intervall-basierte Repräsentation des Auswertungszustands, sowie einige andere Optimierungsansätze zur Vermeidung von unnötigen Berechnungsschritten. Insgesamt war es das Ziel dieser Dissertation, eine Lösung für Simulation und statistisches Model Checking zu schaffen, die einerseits auf fundierten logischen und statistischen Grundlagen aufbaut, auf der anderen Seite jedoch auch pragmatischen Gesichtspunkten wie Benutzbarkeit oder Erweiterbarkeit genügt. Tatsächlich legen erste Ergebnisse und Erfahrungen aus mehreren kleinen bis mittelgroßen Experimenten nahe, dass SALMA diesen Zielen gerecht wird

Conception Assistée des Logiciels Sécurisés pour les Systèmes Embarqués

A vast majority of distributed embedded systems is concerned by security risks. The fact that applications may result poorly protected is partially due to methodological lacks in the engineering development process. More specifically, methodologies targeting formal verification may lack support to certain phases of the development process. Particularly, system modeling frameworks may be complex-to-use or not address security at all. Along with that, testing is not usually addressed by verification methodologies since formal verification and testing are considered as exclusive stages. Nevertheless, we believe that platform testing can be applied to ensure that properties formally verified in a model are truly endowed to the real system. Our contribution is made in the scope of a model-driven based methodology that, in particular, targets secure-by-design embedded systems. The methodology is an iterative process that pursues coverage of several engineering development phases and that relies upon existing security analysis techniques. Still in evolution, the methodology is mainly defined via a high level SysML profile named Avatar. The contribution specifically consists on extending Avatar so as to model security concerns and in formally defining a model transformation towards a verification framework. This contribution allows to conduct proofs on authenticity and confidentiality. We illustrate how a cryptographic protocol is partially secured by applying several methodology stages. In addition, it is described how Security Testing was conducted on an embedded prototype platform within the scope of an automotive project.Une vaste majorité de systèmes embarqués distribués sont concernés par des risques de sécurité. Le fait que les applications peuvent être mal protégées est partiellement à cause des manques méthodologiques dans le processus d’ingénierie de développement. Particulièrement, les méthodologies qui ciblent la vérification formelle peuvent manquer de support pour certaines étapes du processus de développement SW. Notamment, les cadres de modélisation peuvent être complexes à utiliser ou ne pas adresser la sécurité du tout. Avec cela, l’étape de tests n’est pas normalement abordée par les méthodologies de vérification formelle. Néanmoins, nous croyons que faire des tests sur la plateforme peut aider à assurer que les propriétés vérifiées dans le modèle sont véritablement préservées par le système embarqué. Notre contribution est faite dans le cadre d’une méthodologie nommée Avatar qui est basée sur les modèles et vise la sécurité dès la conception du système. La méthodologie est un processus itératif qui poursuit la couverture de plusieurs étapes du développement SW et qui s’appuie sur plusieurs techniques d’analyse de sécurité. La méthodologie compte avec un cadre de modélisation SysML. Notre contribution consiste notamment à étendre le cadre de modélisation Avatar afin d’aborder les aspects de sécurité et aussi à définir une transformation du modèle Avatar vers un cadre de vérification formel. Cette contribution permet d’effectuer preuves d’authenticité et confidentialité. Nous montrons comment un protocole cryptographique est partiellement sécurisé. Aussi, il est décrit comment les tests de sécurité ont été menés sur un prototype dans le cadre d’un projet véhiculaire

Trajectory planning based on adaptive model predictive control: Study of the performance of an autonomous vehicle in critical highway scenarios

Increasing automation in automotive industry is an important contribution to overcome many of the major societal challenges. However, testing and validating a highly autonomous vehicle is one of the biggest obstacles to the deployment of such vehicles, since they rely on data-driven and real-time sensors, actuators, complex algorithms, machine learning systems, and powerful processors to execute software, and they must be proven to be reliable and safe. For this reason, the verification, validation and testing (VVT) of autonomous vehicles is gaining interest and attention among the scientific community and there has been a number of significant efforts in this field. VVT helps developers and testers to determine any hidden faults, increasing systems confidence in safety, security, functional analysis, and in the ability to integrate autonomous prototypes into existing road networks. Other stakeholders like higher-management, public authorities and the public are also crucial to complete the VTT process. As autonomous vehicles require hundreds of millions of kilometers of testing driven on public roads before vehicle certification, simulations are playing a key role as they allow the simulation tools to virtually test millions of real-life scenarios, increasing safety and reducing costs, time and the need for physical road tests. In this study, a literature review is conducted to classify approaches for the VVT and an existing simulation tool is used to implement an autonomous driving system. The system will be characterized from the point of view of its performance in some critical highway scenarios.O aumento da automação na indústria automotiva é uma importante contribuição para superar muitos dos principais desafios da sociedade. No entanto, testar e validar um veículo altamente autónomo é um dos maiores obstáculos para a implantação de tais veículos, uma vez que eles contam com sensores, atuadores, algoritmos complexos, sistemas de aprendizagem de máquina e processadores potentes para executar softwares em tempo real, e devem ser comprovadamente confiáveis e seguros. Por esta razão, a verificação, validação e teste (VVT) de veículos autónomos está a ganhar interesse e atenção entre a comunidade científica e tem havido uma série de esforços significativos neste campo. A VVT ajuda os desenvolvedores e testadores a determinar quaisquer falhas ocultas, aumentando a confiança dos sistemas na segurança, proteção, análise funcional e na capacidade de integrar protótipos autónomos em redes rodoviárias existentes. Outras partes interessadas, como a alta administração, autoridades públicas e o público também são cruciais para concluir o processo de VTT. Como os veículos autónomos exigem centenas de milhões de quilómetros de testes conduzidos em vias públicas antes da certificação do veículo, as simulações estão a desempenhar cada vez mais um papel fundamental, pois permitem que as ferramentas de simulação testem virtualmente milhões de cenários da vida real, aumentando a segurança e reduzindo custos, tempo e necessidade de testes físicos em estrada. Neste estudo, é realizada uma revisão da literatura para classificar abordagens para a VVT e uma ferramenta de simulação existente é usada para implementar um sistema de direção autónoma. O sistema é caracterizado do ponto de vista do seu desempenho em alguns cenários críticos de autoestrad

Abstractions and optimisations for model-checking software-defined networks

Software-Defined Networking introduces a new programmatic abstraction layer by shifting the distributed network functions (NFs) from silicon chips (ASICs) to a logically centralized (controller) program. And yet, controller programs are a common source of bugs that can cause performance degradation, security exploits and poor reliability in networks. Assuring that a controller program satisfies the specifications is thus most preferable, yet the size of the network and the complexity of the controller makes this a challenging effort. This thesis presents a highly expressive, optimised SDN model, (code-named MoCS), that can be reasoned about and verified formally in an acceptable timeframe. In it, we introduce reusable abstractions that (i) come with a rich semantics, for capturing subtle real-world bugs that are hard to track down, and (ii) which are formally proved correct. In addition, MoCS deals with timeouts of flow table entries, thus supporting automatic state refresh (soft state) in the network. The optimisations are achieved by (1) contextually analysing the model for possible partial order reductions in view of the concrete control program, network topology and specification property in question, (2) pre-computing packet equivalence classes and (3) indexing packets and rules that exist in the model and bit-packing (compressing) them. Each of these developments is demonstrated by a set of real-world controller programs that have been implemented in network topologies of varying size, and publicly released under an open-source license

Formal Methods for Wireless Systems

I sistemi wireless sono costituiti da dispositivi che comunicano tra loro per mezzo di un canale radio. Questo paradigma di rete presenta molti vantaggi, ma la presenza del canale radio lo rende intrinsecamente vulnerabile. Di conseguenza, in tale ambito la sicurezza rappresenta un tema importante. I meccanismi di sicurezza messi a punto per i sistemi cablati presentano molti limiti quando vengono utilizzati in una rete wireless. I problemi principali derivano dal fatto che essi operano in modo centralizzato e sotto l'ipotesi di un “mondo chiuso”. Pertanto tecniche formali sono necessarie per stabilire una connessione matematicamente rigorosa tra la modellazione e gli obiettivi di sicurezza. Nella presente tesi si applica il formalismo ben noto del "process calculus" per modellare le principali caratteristiche della comunicazione wireless. Il contributo scientifico è essenzialmente teorico. Verrà proposto un primo process calculus per modellare il passaggio del tempo nei sistemi wireless. Verranno dimostrate alcune interessanti proprietà relative al tempo. Inoltre verrà presentata una rigorosa trattazione dei problemi di collisione. Verranno fornite anche “equivalenze comportamentali” (behavioural equivalence) e verranno dimostrate una serie di leggi algebriche. L'usabilità del calcolo verrà mostrata modellando il Carrier Sense Multiple Access, un diffuso protocollo di livello MAC in cui un dispositivo ascolta il canale prima di trasmettere. Verranno poi analizzati alcuni aspetti di sicurezza, in particolare verrà proposto un modello di trust per le reti ad hoc mobili. Tali reti sono costituite da nodi mobili che comunicano senza l’ausilio di altre infrastrutture. Le reti di tale calcolo verranno modellate come sistemi multilivello perché le relazioni di trust associano ai nodi livelli di sicurezza in base al loro comportamento. Tale modello di trust verrà incluso in un process calculus per reti ad hoc che sarà dotato di equivalenze comportamentali a partire dalle quali verrà sviluppata una "teoria osservazionale" (observational theory). Saranno garantiti sia alcune interessanti proprietà relative alla sicurezza, come la safety in presenza di nodi compromessi, sia risultati di non interferenza. Tale calcolo verrà utilizzato per analizzare una versione “sicura” di un algoritmo per il leader election nelle reti ad hoc. Verrà fornita anche una codifica del protocollo di routing per reti ad hoc chiamato endairA. Infine, il calcolo sul trust verrà esteso con aspetti legati al tempo, per spiegare la relazione tra tempo e trust. Infine quest’ultimo calcolo verrà applicato per dare una codifica del protocollo di routing per reti ad hoc chiamato ARAN.Wireless systems consist of wireless devices which communicate with each other by means of a radio frequency channel. This networking paradigm offers much convenience, but because of the use of the wireless medium it is inherently vulnerable to many threats. As a consequence, security represents an important issue. Security mechanisms developed for wired systems present many limitations when used in a wireless context. The main problems stem from the fact that they operate in a centralised manner and under the assumption of a \closed world". Formal techniques are therefore needed to establish a mathematically rigorous connection between modelling and security goals. In the present dissertation we apply the well-known formalism of process calculus to model the features of wireless communication. The scientic contributions are primarily theoretical.We propose a timed process calculus modelling the communication features of wireless systems and enjoying some desirable time properties. The presence of time allows us to reason about communication collisions. We also provide behavioural equivalences and we prove a number of algebraic laws. We illustrate the usability of the calculus to model the Carrier Sense Multiple Access scheme, a widely used MAC level protocol in which a device senses the channel before transmitting. We then focus on security aspects, in particular we propose a trust model for mobile ad hoc networks, composed only of mobile nodes that communicate each other without relying on any base station. We model our networks as multilevel systems because trust relations associate security levels to nodes depending on their behaviour. Then we embody this trust model in a process calculus modelling the features of ad hoc networks. Our calculus is equipped with behavioural equivalences allowing us to develop an observational theory. We ensure safety despite compromised nodes and non interference results. We then use this calculus to analyse a secure version of a leader election algorithm for ad hoc networks. We also provide an encoding of the endairA routing protocol for ad hoc networks. Finally, we extend the trust-based calculus with timing aspects to reason about the relationship between trust and time. We then apply our calculus to formalise the routing protocol ARAN for ad hoc networks

Simulation and statistical model-checking of logic-based multi-agent system models

This thesis presents SALMA (Simulation and Analysis of Logic-Based Multi- Agent Models), a new approach for simulation and statistical model checking of multi-agent system models. Statistical model checking is a relatively new branch of model-based approximative verification methods that help to overcome the well-known scalability problems of exact model checking. In contrast to existing solutions, SALMA specifies the mechanisms of the simulated system by means of logical axioms based upon the well-established situation calculus. Leveraging the resulting first-order logic structure of the system model, the simulation is coupled with a statistical model-checker that uses a first-order variant of time-bounded linear temporal logic (LTL) for describing properties. This is combined with a procedural and process-based language for describing agent behavior. Together, these parts create a very expressive framework for modeling and verification that allows direct fine-grained reasoning about the agents’ interaction with each other and with their (physical) environment. SALMA extends the classical situation calculus and linear temporal logic (LTL) with means to address the specific requirements of multi-agent simulation models. In particular, cyber-physical domains are considered where the agents interact with their physical environment. Among other things, the thesis describes a generic situation calculus axiomatization that encompasses sensing and information transfer in multi agent systems, for instance sensor measurements or inter-agent messages. The proposed model explicitly accounts for real-time constraints and stochastic effects that are inevitable in cyber-physical systems. In order to make SALMA’s statistical model checking facilities usable also for more complex problems, a mechanism for the efficient on-the-fly evaluation of first-order LTL properties was developed. In particular, the presented algorithm uses an interval-based representation of the formula evaluation state together with several other optimization techniques to avoid unnecessary computation. Altogether, the goal of this thesis was to create an approach for simulation and statistical model checking of multi-agent systems that builds upon well-proven logical and statistical foundations, but at the same time takes a pragmatic software engineering perspective that considers factors like usability, scalability, and extensibility. In fact, experience gained during several small to mid-sized experiments that are presented in this thesis suggest that the SALMA approach seems to be able to live up to these expectations.In dieser Dissertation wird SALMA (Simulation and Analysis of Logic-Based Multi-Agent Models) vorgestellt, ein im Rahmen dieser Arbeit entwickelter Ansatz für die Simulation und die statistische Modellprüfung (Model Checking) von Multiagentensystemen. Der Begriff „Statistisches Model Checking” beschreibt modellbasierte approximative Verifikationsmethoden, die insbesondere dazu eingesetzt werden können, um den unvermeidlichen Skalierbarkeitsproblemen von exakten Methoden zu entgehen. Im Gegensatz zu bisherigen Ansätzen werden in SALMA die Mechanismen des simulierten Systems mithilfe logischer Axiome beschrieben, die auf dem etablierten Situationskalkül aufbauen. Die dadurch entstehende prädikatenlogische Struktur des Systemmodells wird ausgenutzt um ein Model Checking Modul zu integrieren, das seinerseits eine prädikatenlogische Variante der linearen temporalen Logik (LTL) verwendet. In Kombination mit einer prozeduralen und prozessorientierten Sprache für die Beschreibung von Agentenverhalten entsteht eine ausdrucksstarke und flexible Plattform für die Modellierung und Verifikation von Multiagentensystemen. Sie ermöglicht eine direkte und feingranulare Beschreibung der Interaktionen sowohl zwischen Agenten als auch von Agenten mit ihrer (physischen) Umgebung. SALMA erweitert den klassischen Situationskalkül und die lineare temporale Logik (LTL) um Elemente und Konzepte, die auf die spezifischen Anforderungen bei der Simulation und Modellierung von Multiagentensystemen ausgelegt sind. Insbesondere werden cyber-physische Systeme (CPS) unterstützt, in denen Agenten mit ihrer physischen Umgebung interagieren. Unter anderem wird eine generische, auf dem Situationskalkül basierende, Axiomatisierung von Prozessen beschrieben, in denen Informationen innerhalb von Multiagentensystemen transferiert werden – beispielsweise in Form von Sensor- Messwerten oder Netzwerkpaketen. Dabei werden ausdrücklich die unvermeidbaren stochastischen Effekte und Echtzeitanforderungen in cyber-physischen Systemen berücksichtigt. Um statistisches Model Checking mit SALMA auch für komplexere Problemstellungen zu ermöglichen, wurde ein Mechanismus für die effiziente Auswertung von prädikatenlogischen LTL-Formeln entwickelt. Insbesondere beinhaltet der vorgestellte Algorithmus eine Intervall-basierte Repräsentation des Auswertungszustands, sowie einige andere Optimierungsansätze zur Vermeidung von unnötigen Berechnungsschritten. Insgesamt war es das Ziel dieser Dissertation, eine Lösung für Simulation und statistisches Model Checking zu schaffen, die einerseits auf fundierten logischen und statistischen Grundlagen aufbaut, auf der anderen Seite jedoch auch pragmatischen Gesichtspunkten wie Benutzbarkeit oder Erweiterbarkeit genügt. Tatsächlich legen erste Ergebnisse und Erfahrungen aus mehreren kleinen bis mittelgroßen Experimenten nahe, dass SALMA diesen Zielen gerecht wird

Advances in Information Security and Privacy

With the recent pandemic emergency, many people are spending their days in smart working and have increased their use of digital resources for both work and entertainment. The result is that the amount of digital information handled online is dramatically increased, and we can observe a significant increase in the number of attacks, breaches, and hacks. This Special Issue aims to establish the state of the art in protecting information by mitigating information risks. This objective is reached by presenting both surveys on specific topics and original approaches and solutions to specific problems. In total, 16 papers have been published in this Special Issue