Due to the attractive advantages of wireless communication technologies, wireless networking and mobile computing has developed expeditiously and gained ample prevalence. Thereby, many practical applications are being designed for the use of wireless ad hoc networks in both military and civilian scenarios. However, some security concerns have arisen from such networks, especially in that misbehaving nodes pose a major threat during the construction of a trusted network. Therefore, security is one of the key challenges in wireless ad hoc networks, requiring significant attention due to their own features and concerns. This thesis presents several computational models and security strategies for the design of secure, trustworthy networks, which are able to make rational decisions when encountering potential threats. In this thesis, we first propose a distributed network management model for secure group communication. Our approach simplifies the complexity of traditional group management and supports the inclusion of other security mechanisms for the purpose of secure communications. As a decentralized management method, trust can perform well in a dynamic and agile environment. Our proposed trust system defines the concept of trust, establishes the trust relationship between distributed nodes, involves the novel and effective computational model, and specifies a set of trust-based rules in this system for wireless nodes. We also propose a hybrid cryptosystem through the application of both symmetric and asymmetric key algorithms to provide reliable and secure protection of data confidentiality. With the design of selective encryption, uncertainty is incorporated into data encryption and the overhead spent on the data protection is significantly reduced. Thus, the communicating parties not only obtain reliable security protection, but also improve the efficiency of data communication. Through security analysis and simulation experiments, we have shown how decentralized management is useful in wireless and ad hoc scenarios, how trust provides feasible solutions for misbehavior detection, and how our proposed strategies offer security properties.

More and more effort is being spent on security improvements in today's computer environments, with the aim to achieve an appropriate level of security. However, for small computing devices it might be necessary to reduce the computational cost imposed by security in order to gain reasonable performance and/or energy consumption. To accomplish this selective encryption can be used, which provides confidentiality by only encrypting chosen parts of the information. Previous work on selective encryption has chiefly focused on how to reduce the computational cost while still making the information perceptually secure, but not on how computationally secure the selectively encrypted information is.  Despite the efforts made and due to the harsh nature of computer security, good quantitative assessment methods for computer security are still lacking. Inventing new ways of measuring security are therefore needed in order to better understand, assess, and improve the security of computer environments. Two proposed probabilistic quantitative security measures are entropy and guesswork. Entropy gives the average number of guesses in an optimal binary search attack, and guesswork gives the average number of guesses in an optimal linear search attack. In information theory, a considerable amount of research has been carried out on entropy and on entropy-based metrics. However, the same does not hold for guesswork. In this thesis, we evaluate the performance improvement when using the proposed generic selective encryption scheme. We also examine the confidentiality strength of selectively encrypted information by using and adopting entropy and guesswork. Moreover, since guesswork has been less theoretical investigated compared to entropy, we extend guesswork in several ways and investigate some of its behaviors.

Due to the attractive advantages of wireless communication technologies, wireless networking and mobile computing has developed expeditiously and gained ample prevalence. Thereby, many practical applications are being designed for the use of wireless ad hoc networks in both military and civilian scenarios. However, some security concerns have arisen from such networks, especially in that misbehaving nodes pose a major threat during the construction of a trusted network. Therefore, security is one of the key challenges in wireless ad hoc networks, requiring significant attention due to their own features and concerns. This thesis presents several computational models and security strategies for the design of secure, trustworthy networks, which are able to make rational decisions when encountering potential threats. In this thesis, we first propose a distributed network management model for secure group communication. Our approach simplifies the complexity of traditional group management and supports the inclusion of other security mechanisms for the purpose of secure communications. As a decentralized management method, trust can perform well in a dynamic and agile environment. Our proposed trust system defines the concept of trust, establishes the trust relationship between distributed nodes, involves the novel and effective computational model, and specifies a set of trust-based rules in this system for wireless nodes. We also propose a hybrid cryptosystem through the application of both symmetric and asymmetric key algorithms to provide reliable and secure protection of data confidentiality. With the design of selective encryption, uncertainty is incorporated into data encryption and the overhead spent on the data protection is significantly reduced. Thus, the communicating parties not only obtain reliable security protection, but also improve the efficiency of data communication. Through security analysis and simulation experiments, we have shown how decentralized management is useful in wireless and ad hoc scenarios, how trust provides feasible solutions for misbehavior detection, and how our proposed strategies offer security properties.

Due to the attractive advantages of wireless communication technologies, wireless networking and mobile computing has developed expeditiously and gained ample prevalence. Thereby, many practical applications are being designed for the use of wireless ad hoc networks in both military and civilian scenarios. However, some security concerns have arisen from such networks, especially in that misbehaving nodes pose a major threat during the construction of a trusted network. Therefore, security is one of the key challenges in wireless ad hoc networks, requiring significant attention due to their own features and concerns. This thesis presents several computational models and security strategies for the design of secure, trustworthy networks, which are able to make rational decisions when encountering potential threats. In this thesis, we first propose a distributed network management model for secure group communication. Our approach simplifies the complexity of traditional group management and supports the inclusion of other security mechanisms for the purpose of secure communications. As a decentralized management method, trust can perform well in a dynamic and agile environment. Our proposed trust system defines the concept of trust, establishes the trust relationship between distributed nodes, involves the novel and effective computational model, and specifies a set of trust-based rules in this system for wireless nodes. We also propose a hybrid cryptosystem through the application of both symmetric and asymmetric key algorithms to provide reliable and secure protection of data confidentiality. With the design of selective encryption, uncertainty is incorporated into data encryption and the overhead spent on the data protection is significantly reduced. Thus, the communicating parties not only obtain reliable security protection, but also improve the efficiency of data communication. Through security analysis and simulation experiments, we have shown how decentralized management is useful in wireless and ad hoc scenarios, how trust provides feasible solutions for misbehavior detection, and how our proposed strategies offer security properties.

<p>Many security services today only provides one security configuration at run-time, and cannot then utilize the trade-off between performance and security. In order to make use of this trade-off, tunable security services providing several security configurations that can be selected at run-time are needed. To be able to make intelligent choices on which security configuration to use for different situations we need to know how good they are, i.e., we need to order the different security configurations with respect to each security attribute using measures for both security and performance.</p><p>However, a key issue with computer security is that it is due to its complex nature hard to measure.</p><p>As the title of this thesis indicates, it discusses both security measures and tunable security services. Thus, it can be seen to consist of two parts. In the first part, discussing security measures for tunable security services, an investigation on the security implications of selective encryption by using guesswork as a security measure is made. Built on this an investigation of the relationship between guesswork and entropy. The result shows that guesswork,</p><p>after a minor redefinition, is equal to the sum of the entropy and the relative entropy.</p><p>The second part contributes to the area of tunable security services, e.g., services that provides several security configurations at run-time. In particular, we present the mobile Crowds (mCrowds) system,</p><p>an anonymity technology for the mobile Internet developed at Karlstad University, and a tunable encryption service, that is based on a selective encryption paradigm and designed as a middleware. Finally, an investigation of the tunable features provided by Mix-Nets and Crowds are done, using a conceptual model for tunable security services.</p>

Many security services today only provides one security configuration at run-time, and cannot then utilize the trade-off between performance and security. In order to make use of this trade-off, tunable security services providing several security configurations that can be selected at run-time are needed. To be able to make intelligent choices on which security configuration to use for different situations we need to know how good they are, i.e., we need to order the different security configurations with respect to each security attribute using measures for both security and performance. However, a key issue with computer security is that it is due to its complex nature hard to measure. As the title of this thesis indicates, it discusses both security measures and tunable security services. Thus, it can be seen to consist of two parts. In the first part, discussing security measures for tunable security services, an investigation on the security implications of selective encryption by using guesswork as a security measure is made. Built on this an investigation of the relationship between guesswork and entropy. The result shows that guesswork, after a minor redefinition, is equal to the sum of the entropy and the relative entropy. The second part contributes to the area of tunable security services, e.g., services that provides several security configurations at run-time. In particular, we present the mobile Crowds (mCrowds) system, an anonymity technology for the mobile Internet developed at Karlstad University, and a tunable encryption service, that is based on a selective encryption paradigm and designed as a middleware. Finally, an investigation of the tunable features provided by Mix-Nets and Crowds are done, using a conceptual model for tunable security services.

Selective encryption (SE) methods for images often encrypt the sign bits, i.e., the most significant bits (MSBs) of the codewords corresponding to key syntax elements (in compressed images) or to pixel intensities (for uncompressed images). Our work is motivated by the key observation that the binary code (BC) used for these representations has an impact on the quality of the reconstruction at the eavesdropper (Eve), which can be regarded as a measure of the degree of security of the encryption (the lower the quality, the higher the level of security). Therefore, we address the design of BCs that enhance the security of MSB-based SE by increasing the degradation at Eve’s side when she uses a simple replacement attack (replacing all MSBs either by 0s or by 1s). We first consider the scenario of fixed-length BCs, i.e., where all the codewords have the same length. We formulate the design problem as an optimization problem with the objective of maximizing the distortion at Eve’s without any constraint or with a constraint on the entropy of the MSBs in order to shorten the size of the MSB stream to reduce the computational overhead of encryption. We show that the problem can be cast as a binary integer linear program equivalent to a weighted non-bipartite graph matching problem, for which polynomial-time solution algorithms exist. We empirically assess the performance of the optimized BCs on a Mixed Gaussian source, as well as on Gaussian and Laplacian sources, the latter two being commonly used to model the distribution of transform coefficients and prediction residuals. Our experiments lead to the conclusion that MSB-based SE schemes could benefit from the proposed BC designs. For the case of uncompressed images, we also propose a family of structured BCs for the pixel intensity values. These BCs are constructed such that intensity values that are close have reconstructions that are far apart. As a result, the reconstruction with the replacement attack significantly destroys the smooth areas and blurs the edges, therefore increasing the degree of security. Next, we focus on the construction of variable-length BCs for the non-zero quantized AC coefficients in JPEG compressed images. For this, we first formulate the BC design problem as the problem of maximizing the distortion at Eve’s side with a constraint on the entropy. This problem can also be cast as a weighted non-bipartite graph matching problem and, therefore, can be solved efficiently. Furthermore, by gaining insights from the optimization results, a simpler and faster method for BC design is devised, which consists of only swapping a few codewords in the original code used in JPEG. We assess the practical performance of the proposed BCs for the SE method of JPEG images that encrypts only the MSBs of the non-zero quantized AC coefficients, along with the full encryption of the DC coefficients. Our experimental results show that high visual security can be achieved with only a small sacrifice in compression efficiency. In addition, the proposed BCs can be tuned to achieve various levels of degradation at Eve’s side, a property that is convenient for certain applications. / Thesis / Doctor of Philosophy (PhD)

De nos jours, les vidéos et les images sont devenues un moyen de communication très important. L'acquisition, la transmission, l'archivage et la visualisation de ces données visuelles, que ce soit à titre professionnel ou privé, augmentent de manière exponentielle. En conséquence, la confidentialité de ces contenus est devenue un problème majeur. Pour répondre à ce problème, le chiffrement sélectif est une solution qui assure la confidentialité visuelle des données en ne chiffrant qu'une partie des données. Le chiffrement sélectif permet de conserver le débit initial et de rester conforme aux standards vidéo. Ces travaux de thèse proposent plusieurs méthodes de chiffrement sélectif pour le standard vidéo H.264/AVC. Des méthodes de réduction du chiffrement sélectif grâce à l'architecture du standard H.264/AVC sont étudiées afin de trouver le ratio de chiffrement minimum mais suffisant pour assurer la confidentialité visuelle des données. Les mesures de qualité objectives sont utilisées pour évaluer la confidentialité visuelle des vidéos chiffrées. De plus, une nouvelle mesure de qualité est proposée pour analyser le scintillement des vidéos au cours du temps. Enfin, une méthode de chiffrement sélectif réduit régulé par des mesures de qualité est étudiée afin d'adapter le chiffrement en fonction de la confidentialité visuelle fixée. / Nowadays, videos and images are major sources of communication for professional or personal purposes. Their number grow exponentially and the confidentiality of the content has become a major problem for their acquisition, transmission, storage, and display. In order to solve this problem, selective encryption is a solution which provides visual privacy by encrypting only a part of the data. Selective encryption preserves the initial bit-rate and maintains compliance with the syntax of the standard video. This Ph.D thesis offers several methods of selective encryption for H.264/AVC video standard. Reduced selective encryption methods, based on the H.264/AVC architecture, are studied in order to find the minimum ratio of encryption but sufficient to ensure visual privacy. Objective quality measures are used to assess the visual privacy of encrypted videos. In addition, a new quality measure is proposed to analyze the video flicker over time. Finally, a method for a reduced selective encryption regulated by quality measures is studied to adapt the encryption depending on the visual privacy fixed.