Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

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Alphanumeric authentication, by means of a secret, is not only a powerful mechanism, in theory, but prevails over all its competitors in reality. Passwords, as they are more commonly known, have the potential to act as a fairly strong gateway. In practice, though, password usage is problematic. They are (1) easily shared, (2) trivial to observe and (3) maddeningly elusive when forgotten. Moreover, modern consumer devices only exacerbate the problems of passwords as users enter them in shared spaces, in plain view, on television screens, on smartphones and on tablets. Asterisks may obfuscate alphanumeric characters on entry but popular systems, e.g. Apple iPhone and Nintendo Wii, require the use of an on-screen keyboard for character input. ! ! A number of alternatives to passwords have been proposed but none, as yet, have been adopted widely. There seems to be a reluctance to switch from tried and tested passwords to novel alternatives, even if the most glaring flaws of passwords can be mitigated. One argument is that there has not been sufficient investigation into the feasibility of the password alternatives and thus no convincing evidence that they can indeed act as a viable alternative. ! ! Graphical authentication mechanisms, solutions that rely on images rather than characters, are a case in point. Pictures are more memorable than the words that name them, meaning that graphical authentication mitigates one of the major problems with passwords. This dissertation sets out to investigate the feasibility of one particular observation-resilient graphical authentication mechanism called Tetrad. The authentication mechanism attempted to address two of the core problems with passwords: improved memorability and resistance to observability (with on-screen entry).! ! Tetrad was tested in a controlled lab study, that delivered promising results and was well received by the evaluators. It was then deployed in a realistic context and its viability tested in three separate field tests. The unfortunate conclusion was that Tetrad, while novel and viable in a lab setting, failed to deliver a usable and acceptable experience to the end users. This thorough testing of an alternative authentication mechanism is unusual in this research field and the outcome is disappointing. Nevertheless, it acts to inform inventors of other authentication mechanisms of the problems that can manifest when a seemingly viable authentication mechanism is tested in the wild.

In the Bulk Synchronous Parallel (or BSP) model of parallel communication represented by BSPlib, the relaxed coupling of the global computation, communication and synchronisation, whilst providing a definite semantics, does not prescribe exactly when and where communication is to be carried out during the computation. It merely states that it cannot happen before requested by the application and that at certain points local computation cannot proceed unless updates have been applied from the other participating processors. The nature of the computation and this framework is open to exploitation by the implementation of the runtime system and can be made to suit particular physical environments without requiring application program changes. This bulk and global view of parallel computation can be used to implement protocols that both maintain and take into account global state for optimising performance. Such global protocols can provide performance improvements which are not easily achieved with local and greedy strategies and may in turn be locally sub-optimal. This global perspective and the exploitable nature of BSP computation is applied to congestion avoidance, transport layer protocols suitable for BSP computation, global stable check-pointing, and work process placement and migration, to achieve a better overall performance. An important consideration for the compositionality of parallel computer systems into larger systems is that in order for the composite to exhibit good performance, the individual components must also do so. However, it is not obvious how the individual components contribute to the global performance. Already mentioned is that non-locally optimal strategies might lead to globally optimal performance, but also of importance is that variance observed at the local level also influences performance. A number of decisions in the transport protocol design and implementations have been made in order that the observed variance in the protocol's behaviour is minimised. It is demonstrated why this is required using the BSP model. The analysis also suggests a regression technique which can be applied to sampled global performance data.