People make a range of everyday decisions about how and whether to share content with different people, across different platforms and services, during a variety of tasks. These sharing decisions can encompass complex preferences and a variety of access-control dimensions. In this thesis I examine potential methods for improving sharing mechanisms by better understanding the everyday online sharing environment and evaluating a potential sharing tool. I first present two studies that explore how current sharing mechanisms may fall short on social networking sites, leading to suboptimal outcomes such as regret or self censorship. I discuss the implications of these suboptimal outcomes for the design of behavioral nudging tools and the potential for improving the design of selective-sharing mechanisms. I then draw on a third study to explore the broader “ecosystem” of available channels created by the services and platforms people move between and combine to share content in everyday contexts. I examine the role of selective-sharing features in the broader audience-driven and task-driven dynamics that drive sharing decisions in this environment. I discuss the implications of channel choice and dynamics for the design of selective-sharing mechanisms. Using insights from current shortfalls and ecosystem-level dynamics I then present a fourth study examining the potential for adding topic-driven sharing mechanisms to Facebook. I use design mockups and a lab-based interview to explore participants’ hypothetical use cases for such mechanisms. I find that these mechanisms could potentially be useful in a variety of situations, but successful implementation would require accounting for privacy requirements and users’ sharing strategies.

by Yuen Ching Wah. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 61-[63]). / Abstract also in Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Why use cryptography? --- p.1 / Chapter 1.2 --- Why is authentication important ？ --- p.2 / Chapter 1.3 --- What is the relationship between authentication and digital sig- nature? --- p.3 / Chapter 1.4 --- Why is random number important? --- p.3 / Chapter 2 --- Background --- p.5 / Chapter 2.1 --- Cryptography --- p.5 / Chapter 2.1.1 --- Symmetric key cryptography --- p.5 / Chapter 2.1.2 --- Asymmetric key cryptography --- p.7 / Chapter 2.1.3 --- Authentication --- p.8 / Chapter 2.2 --- Elliptic curve cryptography --- p.9 / Chapter 2.2.1 --- Mathematical background for Elliptic curve cryptography --- p.10 / Chapter 2.3 --- Pseudorandom number generator --- p.12 / Chapter 2.3.1 --- Linear Congruential Generator --- p.13 / Chapter 2.3.2 --- Inversive Congruential Generator --- p.13 / Chapter 2.3.3 --- PN-sequence generator --- p.14 / Chapter 2.4 --- Digital Signature Scheme --- p.14 / Chapter 2.5 --- Babai's lattice vector algorithm --- p.16 / Chapter 2.5.1 --- First Algorithm: Rounding Off --- p.17 / Chapter 2.5.2 --- Second Algorithm: Nearest Plane --- p.17 / Chapter 3 --- Several Digital Signature Schemes --- p.18 / Chapter 3.1 --- DSA --- p.19 / Chapter 3.2 --- Nyberg-Rueppel Digital Signature --- p.21 / Chapter 3.3 --- EC.DSA --- p.23 / Chapter 3.4 --- EC-Nyberg-Rueppel Digital Signature Scheme --- p.26 / Chapter 4 --- Miscellaneous Digital Signature Schemes and their PRNG --- p.29 / Chapter 4.1 --- DSA with LCG --- p.30 / Chapter 4.2 --- DSA with PN-sequence --- p.33 / Chapter 4.2.1 --- Solution --- p.35 / Chapter 4.3 --- DSA with ICG --- p.39 / Chapter 4.3.1 --- Solution --- p.40 / Chapter 4.4 --- EC_DSA with PN-sequence --- p.43 / Chapter 4.4.1 --- Solution --- p.44 / Chapter 4.5 --- EC一DSA with LCG --- p.45 / Chapter 4.5.1 --- Solution --- p.46 / Chapter 4.6 --- EC-DSA with ICG --- p.46 / Chapter 4.6.1 --- Solution --- p.47 / Chapter 4.7 --- Nyberg-Rueppel Digital Signature with PN-sequence --- p.48 / Chapter 4.7.1 --- Solution --- p.49 / Chapter 4.8 --- Nyberg-Rueppel Digital Signature with LCG --- p.50 / Chapter 4.8.1 --- Solution --- p.50 / Chapter 4.9 --- Nyberg-Rueppel Digital Signature with ICG --- p.51 / Chapter 4.9.1 --- Solution --- p.52 / Chapter 4.10 --- EC- Nyberg-Rueppel Digital Signature with LCG --- p.53 / Chapter 4.10.1 --- Solution --- p.54 / Chapter 4.11 --- EC- Nyberg-Rueppel Digital Signature with PN-sequence --- p.55 / Chapter 4.11.1 --- Solution --- p.56 / Chapter 4.12 --- EC-Nyberg-Rueppel Digital Signature with ICG --- p.56 / Chapter 4.12.1 --- Solution --- p.57 / Chapter 5 --- Conclusion --- p.59 / Bibliography --- p.61

Ma Yu Tak. / Thesis submitted in: October 2006. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (leaves 70-73). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 2 --- IEEE 802.11 Standard --- p.4 / Chapter 3 --- Vulnerabilities of IEEE 802.11 --- p.8 / Chapter 3.1 --- Authentication Vulnerabilities --- p.8 / Chapter 3.2 --- Medium Access Control Vulnerabilities --- p.9 / Chapter 3.3 --- Proposed Counter-Measures --- p.10 / Chapter 4 --- Denial-of-Service Attacks by Exploiting the MAC protocol --- p.12 / Chapter 5 --- Simulation Results --- p.20 / Chapter 5.1 --- General DoS Attack Simulations --- p.21 / Chapter 5.1.1 --- Topology 1: A Simple Wireless Network --- p.21 / Chapter 5.1.2 --- Topology 2: Wireless Network in Ad-Hoc Mode --- p.24 / Chapter 5.1.3 --- Topology 3: Network with Hidden Node Problem --- p.29 / Chapter 5.2 --- Targeted DoS Attack Simulations --- p.32 / Chapter 5.2.1 --- Topology 4: A Simple Wireless Network --- p.32 / Chapter 5.2.2 --- Topology 4: A Simple Network with Reversed TCP Flows --- p.38 / Chapter 6 --- Detecting and Solving the Attacks --- p.41 / Chapter 6.1 --- Detection of Attacker --- p.41 / Chapter 6.1.1 --- Detecting General DoS Attackers --- p.41 / Chapter 6.1.2 --- Detecting Targeted DoS Attackers --- p.44 / Chapter 6.2 --- Possible Solutions to the DoS Attacks --- p.53 / Bibliography --- p.70 / Chapter A --- TCP Exponential Backoff with Non-Zero Throughput --- p.74 / Chapter A.1 --- TCP Exponential Backoff Background --- p.74 / Chapter A.2 --- Illustration by Simulation --- p.76 / Chapter A.3 --- Implication of the Finding --- p.77 / Chapter B --- Idle Sense in networks with Hidden Node Problem --- p.79 / Chapter B.1 --- Simulation findings --- p.79 / Chapter B.1.1 --- Four hidden nodes case --- p.79 / Chapter B.1.2 --- Analysis of the simulation results --- p.81 / Chapter B.1.3 --- Study of mixed node types --- p.82 / Chapter B.2 --- Possible approaches to use Idle Sense with Hidden Node Problem --- p.84 / Chapter B.2.1 --- Performance Evaluation --- p.88 / Chapter B.3 --- Conclusions --- p.91

This thesis investigates the human-factor problems in password authentication and proposes some usable solutions to these problems by focusing on both forms of knowledge based authentication: textual passwords and graphical passwords. It includes a range of empirical studies to examine users' password-related behaviour and practices in authentication, and helps users to adopt secure password behaviour. This thesis consists of two parts. The first part focuses on traditional text-based passwords. Design flaws and usability issues of existing text-password mechanisms used by many organisations cause employees to adopt insecure password practices. The first work in this thesis investigates the reasons for employees' lack of motivation regarding password protection against security failures. An empirical study is conducted to identify the factors causing employees' insecure behaviours in organisations, and several persuasion strategies are tested to persuade employees to use passwords more safely. The results of the study revealed that some persuasion strategies are effective in motivating users to adopt good password practices. The study also found that the failure of password policies and authentication schemes deployed by organisations is a common problem among the organisations. Considering the results of the first study, in the second work of this thesis, a password guideline/advice study is conducted to help users to create stronger and more memorable passwords. A password guideline including a number of password creation methods and a persuasive message is proposed, and its effectiveness in improving the strength of user-chosen passwords is evaluated. The results show that the users who received the proposed guideline produced stronger and more memorable passwords than the users followed the usual password restrictions while creating their passwords. The results also demonstrate that the given password creation methods can be helpful and inspirational for users to create their own encryption formula. All these works reveal the weaknesses of user-chosen textual passwords and inefficacy of existing text-based password mechanisms. Although these studies show that text-based password mechanisms can be strengthened, they are still problematic where usability is concerned. Thus, the second part of this thesis focuses on another form of knowledge-based authentication: graphical passwords. A novel hybrid authentication scheme integrating text and images is introduced to minimise the brute force and shoulder surfing attacks which text and graphical passwords suffer. In the last work of this thesis, the proposed hybrid scheme is implemented and evaluated. The evaluation shows that the proposed scheme provides security and usability at the same time, and it also makes the password creation process enjoyable for users. In summary, the thesis contributes to the analysis of some key security and usability problems which arise in knowledge-based authentication. A series of empirical studies has been conducted. Based on their results, usable solutions to the human-factor problems in password-based authentication are proposed and evaluated.

Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2007-09-27 21:48:20.594 / Wireless networks are less reliable than wired networks because channels are “exposed” to the surrounding environment that is susceptible to interference and noise. To minimize losses of data due to collisions, wireless networks need a mechanism to regulate the access on the transmission medium. Medium Access Control (MAC) protocols control access to the shared communication medium so that it can be used efficiently. In this thesis, we first describe the collision-controlled Dual Prohibition Multiple Access (DPMA) protocol [45]. The main mechanisms implemented in DPMA, such as binary dual prohibition, power control, interference control, and support for differentiated services (DiffServ), are presented in detail. We conducted a thorough simulation study on DPMA protocol from several aspects. First, we conduct simulations to observe the effects of binary competition number (BCN), unit slot length and safe margin on the performance of DPMA. Secondly, the DiffServ capability of DPMA is demonstrated through simulation results. Finally, we compare the DPMA protocol with the CSMA/CA protocol and find that DPMA with optimal configuration has better performance than CSMA/CA under both low and high network density. / Master

This thesis addresses the problem of usability in the context of administration of access control systems. We seek to relate the notion of declarative semantics, a recurring theme in research in access control, with usability. We adopt the concrete context of POSIX ACLs and the traditional interface for it that comprises two utilities getfacl and setfacl. POSIX ACLs are the de facto standard to which POSIX conformant systems such as Linux and OpenBSD adhere. The natural semantics of getfacl and setfacl is operational. By operational we mean that the semantics of these are speci ed procedurally. We have designed and implemented an alternate interface that we call askfacl whose natural semantics is declarative. Declarative semantics means "what you see is what it is." We also discuss our design of askfacl and articulate the following thesis that underlies our work: If the natural semantics of the interface for ACLs is declarative, then a user is able to more quickly, accurately and confidently, inspect and edit ACLs than if the semantics is operational. To validate our thesis we conducted a between participant human-subject usability study with 42 participants. The results of our study measurably demonstrate the goodness of declarative semantics in access control.

IEEE 802.11 wireless networks are plagued with problems of unauthorized access. Left undetected, unauthorized access is the precursor to additional mischief. Current approaches to detecting intruders are invasive or can be evaded by stealthy attackers. We propose the use of spectral analysis to identify the type of wireless network interface card (NIC). This mechanism can be applied to support the detection of unauthorized systems that use NICs which are different from that of a legitimate system. We focus on two functions, active scanning and dynamic rate switching, required by the 802.11 standard that are implemented in the hardware and software of the wireless NIC. We show that the implementation of these functions influence the transmission patterns of a wireless stream that are observable through traffic analysis. Furthermore, differences in the behavior of a wireless stream caused by differences in the implementation of these functions are exploited to establish the identity of a NIC. Our mechanism for NIC identification uses signal processing to analyze the periodicity embedded in the wireless traffic caused by active scanning and rate switching. A spectral profile is created from the periodic components of the traffic and used for the identity of the wireless NIC. We show that we can discern between NICs manufactured by different vendors and NICs within the same manufacturer using the spectral profile.

In this paper, we propose a novel information-theoretical fairness metric to evaluate the fairness of bandwidth allocation to distributed nodes in local area network. When the source traffic pattern is fixed, the proposed metric is a mapping from the set of all medium access control (MAC) protocols to the interval [0,1] in the real line such that a larger value corresponds to a MAC protocol that allocates bandwidth more fairly. The metric is applicable for a wide range of medium access control schemes including those in which the packet lengths are not identical and/or multipacket reception (MPR) capability is available. To verify the correctness of the novel metric, we use it to evaluate the fairness levels of an IEEE 802.11 wireless LAN composed of homogeneous or heterogeneous nodes. Our simulations indicate that for the IEEE 802.11 protocol, the short-term fairness level is low while the long-term fairness level is high. The information-theoretical fairness metric leads to a conclusion that is consistent to the previous finding in the lecture.

published_or_final_version / Computer Science and Information Systems / Master / Master of Philosophy

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