Reducing Third Parties in the Network through Client-Side Intelligence

The end-to-end argument describes the communication between a client and server using functionality that is located at the end points of a distributed system. From a security and privacy perspective, clients only need to trust the server they are trying to reach instead of intermediate system nodes and other third-party entities. Clients accessing the Internet today and more specifically the World Wide Web have to interact with a plethora of network entities for name resolution, traffic routing and content delivery. While individual communications with those entities may some times be end to end, from the user's perspective they are intermediaries the user has to trust in order to access the website behind a domain name. This complex interaction lacks transparency and control and expands the attack surface beyond the server clients are trying to reach directly. In this dissertation, we develop a set of novel design principles and architectures to reduce the number of third-party services and networks a client's traffic is exposed to when browsing the web. Our proposals bring additional intelligence to the client and can be adopted without changes to the third parties. Websites can include content, such as images and iframes, located on third-party servers. Browsers loading an HTML page will contact these additional servers to satisfy external content dependencies. Such interaction has privacy implications because it includes context related to the user's browsing history. For example, the widespread adoption of "social plugins" enables the respective social networking services to track a growing part of its members' online activity. These plugins are commonly implemented as HTML iframes originating from the domain of the respective social network. They are embedded in sites users might visit, for instance to read the news or do shopping. Facebook's Like button is an example of a social plugin. While one could prevent the browser from connecting to third-party servers, it would break existing functionality and thus be unlikely to be widely adopted. We propose a novel design for privacy-preserving social plugins that decouples the retrieval of user-specific content from the loading of third-party content. Our approach can be adopted by web browsers without the need for server-side changes. Our design has the benefit of avoiding the transmission of user-identifying information to the third-party server while preserving the original functionality of the plugins. In addition, we propose an architecture which reduces the networks involved when routing traffic to a website. Users then have to trust fewer organizations with their traffic. Such trust is necessary today because for example we observe that only 30% of popular web servers offer HTTPS. At the same time there is evidence that network adversaries carry out active and passive attacks against users. We argue that if end-to-end security with a server is not available the next best thing is a secure link to a network that is close to the server and will act as a gateway. Our approach identifies network vantage points in the cloud, enables a client to establish secure tunnels to them and intelligently routes traffic based on its destination. The proliferation of infrastructure-as-a-service platforms makes it practical for users to benefit from the cloud. We determine that our architecture is practical because our proposed use of the cloud aligns with existing ways end-user devices leverage it today. Users control both endpoints of the tunnel and do not depend on the cooperation of individual websites. We are thus able to eliminate third-party networks for 20% of popular web servers, reduce network paths to 1 hop for an additional 20% and shorten the rest. We hypothesize that user privacy on the web can be improved in terms of transparency and control by reducing the systems and services that are indirectly and automatically involved. We also hypothesize that such reduction can be achieved unilaterally through client-side initiatives and without affecting the operation of individual websites

Amplification and Oscillations in the Curvature Power Spectrum from Features in the Inflaton Potential

Στη διδακτορική διατριβή γίνεται μελέτη των χαρακτηριστικών του πληθωριστικού δυναμικού που προκαλούν σημαντική ενίσχυση, καθώς και ταλαντώσεις, στο φάσμα διαταραχών καμπυλότητας. Τα χαρακτηριστικά που παρουσιάζουν ιδιαίτερο ενδιαφέρον είναι οι απότομες μεταπτώσεις (σκαλοπάτια) στο δυναμικό και οι στροφές στον εσωτερικό χώρο πολλών πεδίων. Και στις δύο περιπτώσεις δίνεται ιδιαίτερη έμφαση στην αθροιστική επίδραση περισσότερων του ενός τέτοιων χαρακτηριστικών, ενώ η ανάλυση που διεξάγεται είναι τόσο αριθμητική όσο και αναλυτική. Στην περίπτωση του πληθωρισμού από ένα πεδίο γίνεται μελέτη συγκεκριμένων μοντέλων εμπνευσμένων από το πλαίσιο των α-attractors. Σε πολλές από τις μελετώμενες περιπτώσεις η αύξηση του φάσματος διαταραχών καμπυλότητας είναι αρκετή ώστε να οδηγεί σε σημαντική παραγωγή αρχέγονων μελανών οπών καθώς και στοχαστικών βαρυτικών κυμάτων, πιθανά ανιχνεύσιμων στο μέλλον. Οι ταλαντώσεις στο φάσμα διαταραχών καμπυλότητας αποτυπώνονται στο φάσμα των στοχαστικών βαρυτικών κυμάτων.In this thesis we study features of the inflaton potential that can lead to a strong enhancement of the power spectrum of curvature perturbations. In single-field inflation, which constitutes the biggest part of this work, we focus on models with steep step-like features in the potential that result in the temporary violation of the slow-roll conditions during the inflaton evolution. These features enhance the power spectrum of the curvature perturbations by several orders of magnitude at certain scales and also produce prominent oscillatory patterns. Our analysis regarding the inflationary dynamics is both analytical and numerical. We describe quantitatively the size of the enhancement, as well as the profile of the oscillations, which are shaped by the number and position of the features in the potential. The models that we use include some simplified potentials, as well as potentials that are constructed within the framework of $\alpha$-attractors in supergravity. We also demonstrate that the induced tensor power spectrum inherits the distinctive oscillatory profile of the curvature spectrum and is potentially detectable by near-future space interferometers. In addition, the enhancement of the power spectrum by several step-like features may trigger the production of a sizeable number of primordial black holes under suitable conditions. We also extend our work on the enhancement of the curvature spectrum during inflation to the two-field case. Our emphasis here is given on sharp turns in field space, which is a clearly multi-field phenomenon that occurs when the component of the slow-roll parameter perpendicular to the background trajectory grows large. Our focus is mainly on the additive eeffect of several turns, leading to the resonant growth of the curvature spectrum. Three or four features in the evolution of η are sufficient in order to induce an enhancement of the power spectrum by six or seven orders of magnitude, which can lead to the significant production of primordial black holes and stochastic gravitational waves, in analogy with the case of steps in the potential

Κοσμολογικός Πληθωρισμός σε τροποποιημένες θεωρίες βαρύτητας

Στην παρούσα εργασία μελετάται ο κοσμολογικός πληθωρισμός σε τροποποιημένες θεωρίες βαρύτητας O πληθωρισμός επιλύει ορισμένα από τα κοσμολογικά προβλήματα του προτύπου Big Bang και στην πιο απλή περιγραφή του περιλαμβάνει ένα βαθμωτό πεδίο. Ένα από τα μοντέλα που βρίσκεται σε εξαιρετική συμϕωνία με τα τελευταία παρατηρησιακά δεδομένα είναι το μοντέλο Starobinsky, το οποίο προκύπτει από μια f(R) θεωρία βαρύτητας με f(R) = R/2 + (k/2)R2 . Αυτό αποτελεί κίνητρο για την κοσμολογική μελέτη πιο γενικών f(R) θεωριών , αϕού αυτές είναι ισοδύναμες, στο σύστημα Einstein, με ένα βαθμωτό πεδίο με την παρουσία δυναμικού. Η γενίκευση αυτού με την προσθήκη επί πλέον όρων απαιτεί αυτοί να είναι εξαιρετικά μικροί για επιτυχή πληθωρισμό. Για τον λόγο αυτό μελετούμε θεωρίες βαρύτητας υψηλότερης τάξης που περιέχουν και παραγώγους του βαθμωτού Ricci R . Στην πιό απλή εκδοχή εστιαζόμαστε στις θεωρίες f(R) + R \Box R, οι οποίες είναι ισοδύναμες με μία θεωρία του Einstein με την παρουσία δύο βαθμωτών πεδίων. Σε αυτήν την κατηγορία η περίπτωση f(R) = R/2 + (k/2)R2 οδηγεί σε ασταθές δυναμικό οπότε θεωρούμε πιό γενικές μορϕές της συνάρτησης f με εγγενή χαρακτηριστικά. Τα αποτελέσματα δείχνουν ότι παρόλο που τα προκύπτοντα δυναμικά είναι ευσταθή, τα ελάχιστά τους είναι κενά τύπου anti de Sitter. Για επιτυχή πληθωρισμό ένα ανέβασμα (uplift) του δυναμικού είναι αναγκαίο, έτσι ώστε το Σύμπαν μετά από μια περίοδο εκθετικής διαστολής να καταλήγει σε ένα κενό Minkowski. Για τον σκοπό αυτό ίσως είναι αναγκαία η γενίκευση αυτών σε υπερβαρυτικά πρότυπα τύπου no-scale, που χαρακτηρίζονται από θετικά ορισμένα βαθμωτά δυναμικά με κενά τύπου Minkowski.In this thesis we study cosmological inflation in modified gravity theories. Inflation solves some of the fundamental problems arising in the standard Big Bang Cosmology and in the simplest models is driven by a single scalar field. A popular model, which is in excellent agreement with the latest cosmological observations, after the launch of the Planck satellite, is the Starobinsky model. This arises from an f(R) gravity theory, with f(R) = R/2 + (k/2)R2, which is equivalent to a standard Einstein theory and a scalar field moving under the influence of a potential, which exhibits an extended plateau and is characterized by one mass parameter. The success of this model motivates us to study more general f(R)-theories, which are known to be conformally equivalent to an Einstein theory including a scalar field moving in a scalar potential whose form is dictated by the choice of f(R). It is shown that generalizing the Starobinsky model by the addition of extra terms it demands that they are unnaturally small in order to continue to have successful inflation. For this reason we study higher order gravity theories which involve derivatives of the Ricci scalar R too. In the simplest version, we focus on f(R) + R \Box R -theories, which are equivalent to an Einstein theory containing two scalar fields. In this class of theories, the case f(R) = R/2 + (k/2)R2 leads to an unstable potential and thus we consider more general forms of f(R) having all desired properties curing the aforementioned pathologies. The results of this study show that although the arising potentials can be stable, their minima correspond to anti de Sitter vacua. In order to describe successful inflation, an uplift of the potential is necessary, so that the Universe, after a period of de-Sitter accelerated expansion, falls to a Minkowski vacuum. An option towards reaching this goal is to encompass these models within the so-called no-scale Supergravity theories, which feature positive defined potentials having stable Minkowski vacua

Ενίσχυση και ταλαντώσεις στο φάσμα διαταραχών καμπυλότητας από χαρακτηριστικά του πληθωριστικού δυναμικού

In this thesis we study features of the inflaton potential that can lead to a strong enhancement of the power spectrum of curvature perturbations. In single-field inflation, which constitutes the biggest part of this work, we focus on models with steep step-like features in the potential that result in the temporary violation of the slow-roll conditions during the inflaton evolution. These features enhance the power spectrum of the curvature perturbations by several orders of magnitude at certain scales and also produce prominent oscillatory patterns. Our analysis regarding the inflationary dynamics is both analytical and numerical. We describe quantitatively the size of the enhancement, as well as the profile of the oscillations, which are shaped by the number and position of the features in the potential. The models that we use include some simplified potentials, as well as potentials that are constructed within the framework of $\alpha$-attractors in supergravity. We also demonstrate that the induced tensor power spectrum inherits the distinctive oscillatory profile of the curvature spectrum and is potentially detectable by near-future space interferometers. In addition, the enhancement of the power spectrum by several step-like features may trigger the production of a sizeable number of primordial black holes under suitable conditions.We also extend our work on the enhancement of the curvature spectrum during inflation to the two-fi eld case. Our emphasis here is given on sharp turns in field space, which is a clearly multi- field phenomenon that occurs when the component of the slow-roll parameter perpendicular to the background trajectory grows large. Our focus is mainly on the additive e effect of several turns, leading to the resonant growth of the curvature spectrum. Three or four features in the evolution of η are sufficient in order to induce an enhancement of the power spectrum by six or seven orders of magnitude, which can lead to the signifi cant production of primordial black holes and stochastic gravitational waves, in analogy with the case of steps in the potential.Στη διδακτορική διατριβή γίνεται μελέτη των χαρακτηριστικών του πληθωριστικού δυναμικού που προκαλούν σημαντική ενίσχυση, καθώς και ταλαντώσεις, στο φάσμα διαταραχών καμπυλότητας. Τα χαρακτηριστικά που παρουσιάζουν ιδιαίτερο ενδιαφέρον είναι οι απότομες μεταπτώσεις (σκαλοπάτια) στο δυναμικό και οι στροφές στον εσωτερικό χώρο πολλών πεδίων. Και στις δύο περιπτώσεις δίνεται ιδιαίτερη έμφαση στην αθροιστική επίδραση περισσότερων του ενός τέτοιων χαρακτηριστικών, ενώ η ανάλυση που διεξάγεται είναι τόσο αριθμητική όσο και αναλυτική. Στην περίπτωση του πληθωρισμού από ένα πεδίο γίνεται μελέτη συγκεκριμένων μοντέλων εμπνευσμένων από το πλαίσιο των α-attractors. Σε πολλές από τις μελετώμενες περιπτώσεις η αύξηση του φάσματος διαταραχών καμπυλότητας είναι αρκετή ώστε να οδηγεί σε σημαντική παραγωγή αρχέγονων μελανών οπών καθώς και στοχαστικών βαρυτικών κυμάτων, πιθανά ανιχνεύσιμων στο μέλλον. Οι ταλαντώσεις στο φάσμα διαταραχών καμπυλότητας αποτυπώνονται στο φάσμα των στοχαστικών βαρυτικών κυμάτων

Outsourcing Malicious Infrastructure to the Cloud

Abstract—Malicious activities, such as running botnets, phishing sites or keyloggers, require an underlying infrastructure for carrying out vital operations like hosting coordination mechanisms or storing stolen information. In the past, attackers have used their own resources or compromised machines. In this paper, we discuss the emerging practice of attackers outsourcing their malicious infrastructure to the Cloud. We present our findings from the study of the first major keylogger that has employed Pastebin for storing stolen information. Furthermore, we outline the traits and features of Cloud services in facilitating malicious activities. Finally, we discuss how the nature of the Cloud may shape future security monitoring and enhance defenses against such practices. I

SAuth: Protecting User Accounts from Password Database Leaks

Password-based authentication is the dominant form of access control in web services. Unfortunately, it proves to be more and more inadequate every year. Even if users choose long and complex passwords, vulnerabilities in the way they are managed by a service may leak them to an attacker. Recent incidents in popular services such as LinkedIn and Twitter demonstrate the impact that such an event could have. The use of one-way hash functions to mitigate the problem is countered by the evolution of hardware which enables powerful password-cracking platforms. In this paper we propose SAuth, a protocol which employs authenticationsynergyamongdifferentservices. Users wishing to access their account on service S will also have to authenticate for their account on service V, which acts as a vouching party. Both services S and V are regular sites visited by the user everyday (e.g., Twitter, Facebook, Gmail). Should an attacker acquire the password for service S he will be unable to log in unless he also compromises the password for service V and possibly more vouching services. SAuth is an extension and not a replacement of existing authentication methods. It operates one layer above without ties to a specific method, thus enabling different services to employ heterogeneous systems. Finally we employ password decoys to protect users that share a password across services