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In the contemporary digital landscape, ensuring secure communication amid widespread data exchange is imperative. This dissertation focuses on enhancing the security and privacy of end-to-end encryption (E2EE) applications while maintaining or improving usability. The dissertation first investigates and proposes improvements in two areas of existing E2EE applications: countering man-in-the-middle and impersonation attacks through automated key verification and studying user perceptions of cryptographic deniability. Insights from privacy-conscious users reveal concerns about the lack of E2EE support, app siloing, and data accessibility by client apps. To address these issues, we propose an innovative user-controlled encryption system, enabling encryption before data reaches the client app. Finally, the dissertation evaluates local threats in the FIDO2 protocol and devises defenses against these risks. Additionally, it explores streamlining FIDO2 authentication management across multiple websites for user convenience and security.

Dynamic spectrum access (DSA) technique has been widely accepted as a crucial solution to mitigate the potential spectrum scarcity problem. Spectrum sharing between the government incumbents and commercial wireless broadband operators/users is one of the key forms of DSA. Two categories of spectrum management methods for shared use between incumbent users (IUs) and secondary users (SUs) have been proposed, i.e., the server-driven method and the sensing-based method. The server-driven method employs a central server to allocate spectrum resources while considering incumbent protection. The central server has access to the detailed IU operating information, and based on some accurate radio propagation model, it is able to allocate spectrum following a particular access enforcement method. Two types of access enforcement methods -- exclusion zone and protection zone -- have been adopted for server-driven DSA systems in the current literature. The sensing-based method is based on recent advances in cognitive radio (CR) technology. A CR can dynamically identify white spaces through various incumbent detection techniques and reconfigure its radio parameters in response to changes of spectrum availability. The focus of this dissertation is to address critical privacy and security issues in the existing DSA systems that may severely hinder the progress of DSA's deployment in the real world. Firstly, we identify serious threats to users' privacy in existing server-driven DSA designs and propose a privacy-preserving design named P²-SAS to address the issue. P²-SAS realizes the complex spectrum allocation process of protection-zone-based DSA in a privacy-preserving way through Homomorphic Encryption (HE), so that none of the IU or SU operation data would be exposed to any snooping party, including the central server itself. Secondly, we develop a privacy-preserving design named IP-SAS for the exclusion-zone- based server-driven DSA system. We extend the basic design that only considers semi- honest adversaries to include malicious adversaries in order to defend the more practical and complex attack scenarios that can happen in the real world. Thirdly, we redesign our privacy-preserving SAS systems entirely to remove the somewhat- trusted third party (TTP) named Key Distributor, which in essence provides a weak proxy re-encryption online service in P²-SAS and IP-SAS. Instead, in this new system, RE-SAS, we leverage a new crypto system that supports both a strong proxy re-encryption notion and MPC to realize privacy-preserving spectrum allocation. The advantages of RE-SAS are that it can prevent single point of vulnerability due to TTP and also increase SAS's service performance dramatically. Finally, we identify the potentially crucial threat of compromised CR devices to the ambient wireless infrastructures and propose a scalable and accurate zero-day malware detection system called GuardCR to enhance CR network security at the device level. GuardCR leverages a host-based anomaly detection technique driven by machine learning, which makes it autonomous in malicious behavior recognition. We boost the performance of GuardCR in terms of accuracy and efficiency by integrating proper domain knowledge of CR software. / Ph. D. / With the rapid development of wireless technologies in recent years, wireless spectrum which all the wireless communication signals travel over is becoming the bottleneck of the fast growing wireless market. The spectrum scarcity problem is largely due to the current spectrum allocation scheme. Some spectrum bands, like the cellular bands, are overly crowded, while some government-held spectrum bands are used inadequately. By allowing users from the crowded spectrum bands to dynamically access to those less frequently used spectrum bands, the spectrum scarcity problem can be significantly alleviated. However, there are two critical issues that hinder the application of dynamic spectrum access in the real world: privacy and security. For privacy, in order to determine when, where, and how the spectrum can be reused, users need to bear the risk of sharing their sensitive operation data. This is especially frustrating for governmental and military parties whose operation data is highly classified. We solve the privacy problem by designing a privacy-preserving dynamic spectrum access system. The system is based on secure multi-party computation, which keeps users’ input operation data private when performing spectrum allocation computation over those inputs. The system achieves 128-bit industry-level security strength, and it is also computation and memory efficient for real-world deployment. For security, dynamic spectrum access requires radio devices to contain many software components so that the radio devices can be dynamically programmed to access different spectrum bands. However, the software also exposes the radio devices to the risk of malware infection. We develop a malware detection system to capture the anomalous behaviors in radio software executions. By adopting advanced machine learning techniques, our system is even able to detect first-seen malware.

This work explains the fundamental definitions required to define and create Fuzzy Identity-Based Encryption schemes as an error-tolerant version of Identity-Based Encryption schemes, along with three different examples of such creations. These examples are Sahai-Waters' FIBE, Baek et al.'s EFIBE-I and EFIBE-II. The required Set-up, Key Generation, Encryption and Decryption algorithms for each scheme are formalized and the proofs of security using the Selective-ID model are introduced. Subtle differences between the three schemes are discussed, including their computational efficiency comparison. The writing is intended as a self-sufficient resource for readers, containing the schemes and background definitions.

This thesis brings a reader an overview about historical and modern world of cryptographic methods, as well evaluates actual state of cryptographic algorithm progressions, which are used in applications nowadays. The aim of the work describes common symmetric, asymmetric encryption methods, cryptographic hash functions and as well pseudorandom number generators, authentication protocols and protocols for building VPNs. This document also shows the basics of the successful modern cryptanalysis and reveals algorithms that shouldn't be used and which algorithms are vulnerable. The reader will be also recommended an overview of cryptographic algorithms that are expected to stay safe in the future.

The security protocols are widely used for providing safe communication. They are used for creating private communication channels in unsecured area. This thesis deals with the design of such protocols and their properties. The first part deals with properties and requirements of designed protocol as well as with the most common attacks on protocols. In the second part, the method of trace formula is described in more detail. This method is used for analytic design of security protocols. Later, the library of automated functions was created upon the principles of the method. The library support both the handling of protocol properties and protocol design. The thesis concludes with some examples of generated protocols.

L'activité de notre cerveau est intrinsèquement rythmique : des oscillations sont observées à tous les niveaux de son organisation. Cette rythmicité de l'activité cérébrale influence notre perception. En effet, au lieu de superviser continuellement notre environnement, notre cerveau effectue de brèves " clichés " du monde extérieur (entre 5 et 15 par seconde). Cela crée des cycles perpétuels : notre perception visuelle fluctue en fonction de la phase de l'oscillation sous- jacente. De nombreuses données témoignent du fait que les oscillations cérébrales à différentes fréquences sont fondamentales à la formation de notre perception visuelle. Lors de cette thèse, nous avons utilisé le Paradigme de Bruit Blanc comme outil pour comprendre l'influence des oscillations sur la perception visuelle et qui par extension pourra être utilisé pour contrôler cette perception. Le paradigme de bruit blanc visuel utilise des séquences de flashs dont la luminance varie aléatoirement (créant ainsi du " bruit blanc "), comme stimuli, qui contraignent l'activité cérébrale de manière prédictible. Les réponses impulsionnelles à ces séquences de bruit blanc sont caractérisées par une composante oscillatoire forte dans la bande alpha (~10Hz), similaire à un écho perceptuel. Puisque les réponses impulsionnelles sont un modèle de la réponse de notre cerveau à un flash dans la séquence de bruit blanc, elles peuvent être utilisées pour reconstruire (plutôt qu'enregistrer) l'activité cérébrale en réponse à de nouvelles séquences de stimulation. Par ailleurs, des cibles ont été introduites au sein des séquences de bruit blanc à un niveau proche du seuil de perception, et le décours temporel de cette activité reconstruite autour de la présentation des cibles a été extrait. Ainsi, l'EEG reconstruit peut être utilisé pour étudier l'influence de ces oscillations contraintes sur la perception visuelle, indépendamment des autres types de signaux généralement enregistrés dans l'EEG. Dans un premier temps, nous avons validé le paradigme de bruit blanc en montrant que : 1) les séquences de bruits blancs influencent bien la détection des cibles, 2) les échos perceptuels évoqués par les séquences de bruit blancs sont stables dans le temps, 3) ces échos sont un bon modèle de l'activité cérébrale enregistrée par EEG, et 4) leurs bases neuronales se situent dans les aires visuelles primaires. Dans un second temps, nous avons étudié la relation entre ces oscillations cérébrales contrôlées par la séquence de bruit blanc et la détection des cibles. Ici, nous montrons que l'activité EEG reconstruite nous aide à déterminer la véritable latence à laquelle la phase de l'oscillation (thêta) influence la perception. De plus, nous avons aussi montré que l'amplitude de l'oscillation (alpha) influence la détection des cibles et ce, indépendamment des fluctuations des facteurs endogènes (tel que l'attention). Enfin, tirant parti de ce lien entre oscillation et perception, nous construisons deux algorithmes qui permettent de contrôler la perception des sujets. Tout d'abord, nous mettons au point un modèle " universel " de la perception qui permet de prédire, pour n'importe quel observateur, si une cible dans une séquence de bruit blanc sera vue ou non. Ensuite, nous construisons un modèle individuel qui utilise l'écho perceptuel de chaque sujet comme clé de cryptage et nous permet de présenter des cibles à des moments où la cible sera détectée par un sujet seulement au détriment de tous les autres sujets, créant ainsi une sorte de système de cryptage neuronal (" Neuro-Encryption "). / Our brain activity is inherently rhythmic: oscillations can be found at all levels of organization. This rhythmicity in brain activity gives a rhythm to what we see: instead of continuously monitoring the environment, our brains take "snapshots" of the external world from 5 to 15 times a second. This creates perceptual cycles: depending on the phase of the underlying oscillation, our perceptual abilities fluctuate. Accumulating evidence shows that brains oscillations at various frequencies are instrumental in shaping visual perception. At the heart of this thesis lies the White Noise Paradigm, which we designed as a tool to better understand the influence of oscillations on visual perception and which ultimately could be used to control visual perception. The White Noise Paradigm uses streams of flashes with random luminance (i.e. white noise) as stimuli, which have been shown to constrain brain oscillations in a predictable manner. The impulse response to WN sequences has a strong (subject specific) oscillatory component at ~10Hz akin to a perceptual echo. Since the impulse response is a model of how our brains respond to one single flash in the sequence, they can be used to reconstruct (rather than record) the brain activity to new stimulation sequences. We then present near-perceptual threshold targets embedded within the WN sequences and extract the time course of these predicted/reconstructed background oscillations around target presentation. Thus, the reconstructed EEG can be used to study the influence of the oscillatory components on visual perception, independently of other types of signals usually recorded in the EEG. First, we validate the White Noise Paradigm by showing that: 1) the WN sequences do modulate behaviour, 2) the perceptual echoes evoked by these WN sequences are stable in time, 3) they are a (relatively) good model of the subject's recorded brain activity and 4) their neuronal basis can be found in the early visual areas. Second, we investigate the relationship between these constrained brain oscillations and visual perception. Specifically, we show that the reconstructed EEG can help us recover the true latency at which (theta) phase influences perception. Moreover, it can help us uncover a causal influence of (alpha) power on target detection, independently from any fluctuation in endogenous factors. Finally, capitalizing on the link between oscillations and perception, we build two algorithms used to control the perception of subjects. First, we build a "universal" forward model which can predict for any observer whether a particular target will be seen or not. Second, we build a subject-dependent model which can predict whether a particular subject (for whom EEG was recorded previously) will perceive a given target or not. Critically, this can be used to present targets optimized to be perceived by one subject only, to the detriment of all other subjects, creating a sort of "Neuro-Encryption" system.

Made available in DSpace on 2016-04-04T18:31:36Z (GMT). No. of bitstreams: 1 Rodrigo Frandsen da Silva.pdf: 3782928 bytes, checksum: cd013447a70916926978a2ddc8e42dbb (MD5) Previous issue date: 2012-12-19 / Pontif?cia Universidade Cat?lica de Campinas / In this dissertation we evaluate a new technique that performs optical encryption of signals travelling through transparent optical networks (TON). It is, thus, intended to prevent eavesdroppers to capture and retrieve optical signals. The technique consists in dividing an optical signal into several spectral slices and applying different attenuation and delays to each of them. After this process the signal is again multiplexed and transmitted through the considered TON. At this point the signal is ideally unintelligible to any receiver who does not know the encryption key, i.e. the set of utilized attenuations and delays. To evaluate the strength of such key, we measure the bit error rate (Bit Error Rate, BER) of the encoded signal, BERC. Generally speaking, the higher BERC, the lower is the chance of the encrypted signal being decoded by an eavesdropper. At the authorized receiver side, signal is again divided into spectral slices which are subjected to a set of attenuations and delays that are complementary to the ones utilized as the encryption key. All slices are again multiplexed and, as a result, at the output of the decoder the original encoder input signal is ideally reconstructed. The quality of the decoded signal is measured by evaluating the BER of the decoded signal, BERd, which should be low enough to allow proper reception of the signal transmitted by the TON. Simulations with the software VPITransmissionMaker, VPIPhotonics Company Inc., were used to investigate the performance of the technique for different gains and delays. For the operation of spectral slicing, the profiles of the utilized filters were adjusted to represent the ones of state-of-the-art off-the-shelf equipment. Results indicate that BERC may reach values of up to 42% and 32% for non-return to zero (NRZ) and differential quadrature phase shift keying (DQPSK) signals, respectively. In both of these cases it was possible to find results of BERd lower than 10-12, after properly adjusting the spacing between the filters; this was observed even for propagation over amplified standard fiber links with lengths as long as 400 km. To the best of our knowledge, this is the first time that such analyses are presented. / Neste trabalho avaliamos uma t?cnica para realizar a criptografia totalmente ?ptica em redes ?pticas transparentes. Dessa maneira, pretende-se impedir que um intruso consiga capturar e analisar o sinal ?ptico. A t?cnica consiste em dividir um sinal ?ptico em diversas fatias espectrais e aplicar diferentes atenua??es e atrasos a cada uma delas. Ap?s este processo o sinal ? novamente multiplexado e transmitido por uma rede ?ptica transparente. Neste ponto o sinal est? idealmente inintelig?vel para qualquer receptor que n?o conhe?a a chave criptogr?fica (conjunto de atenua??es e atrasos) utilizada. Para avaliar a for?a da chave criptogr?fica, mede-se a taxa de erros de bit (Bit Error Rate, BER) do sinal codificado, BERC. De forma geral, quanto maior BERC, menor ? a chance de o sinal ser decodificado por um receptor n?o autorizado. Na decodifica??o o sinal ? novamente dividido em fatias espectrais, e para todas as fatias s?o aplicados valores distintos de atenua??o e atraso de forma a reconstituir o sinal de entrada. Na sa?da do decodificador, avalia-se a BER do sinal decodificado BERD, que deve ser suficientemente baixa para permitir a recep??o do sinal transmitido pela TON. Simula??es com o software VPITransmissionMaker, da empresa VPIPhotonics Inc, foram utilizadas para investigar o desempenho da t?cnica para diferentes ganhos e atrasos. Para a opera??o de fatiamento espectral, os perfis dos filtros usados foram ajustados para representar equipamentos de mercado. Resultados indicam que BERC pode atingir valores de at? 42% e 32% para sinais com modula??o non return to zero - on-off keying (NRZ-OOK) e por deslocamento de fase diferencial em quadratura (differential quadrature phase shift keying, DQPSK), respectivamente. Em ambos os casos foi poss?vel encontrar resultados de BERd inferiores a 10-12, mediante ajuste adequado do espa?amento entre os filtros, mesmo ap?s 400 km de propaga??o por enlaces de fibra padr?o. No melhor de nosso conhecimento, esta ? a primeira vez que tais an?lises s?o apresentadas.

The Rijndael cipher was chosen as the Advanced Encryption Standard (AES) in August 1999. Its internal structure exhibits unusual properties such as a clean and simple algebraic description for the S-box. In this research, we construct a scalable family of ciphers which behave very much like the original Rijndael. This approach gives us the opportunity to use computational complexity theory. In the main result, we generate a candidate one-way function family from the scalable Rijndael family. We note that, although reduction to one-way functions is a common theme in the theory of public-key cryptography, it is rare to have such a defense of security in the private-key theatre. In this thesis a plan of attack is introduced at the circuit level whose aim is not break the cryptosystem in any practical way, but simply to break the very bold Rijndael security claim. To achieve this goal, we are led to a formal understanding of the Rijndael security claim, juxtaposing it with rigorous security treatments. Several of the questions that arise in this regard are as follows: ``Do invertible functions represented by circuits with very small numbers of gates have better than worst case implementations for their inverses?' ``How many plaintext/ciphertext pairs are needed to uniquely determine the Rijndael key?'

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