Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: Computer aided veri cation techniques, such as model checking, can be used to improve the reliability of software. Model checking is an algorithmic approach to illustrate the correctness of temporal logic speci cations in the formal description of hardware and software systems. In contrast to traditional testing tools, model checking relies on an exhaustive search of all the possible con gurations that these systems may exhibit. Traditionally model checking is applied to abstract or high level designs of software. However, often interpreting or translating these abstract designs to implementations introduce subtle errors. In recent years one trend in model checking has been to apply the model checking algorithm directly to the implementations instead. This thesis is concerned with building an e cient model checker for a small concurrent langauge developed at the University of Stellenbosch. This special purpose langauge, LF, is aimed at developement of small embedded systems. The design of the language was carefully considered to promote safe programming practices. Furthermore, the language and its runtime support system was designed to allow directly model checking LF programs. To achieve this, the model checker extends the existing runtime support infrastructure to generate the state space of an executing LF program. / AFRIKAANSE OPSOMMING: Rekenaar gebaseerde program toetsing, soos modeltoetsing, kan gebruik word om die betroubaarheid van sagteware te verbeter. Model toetsing is 'n algoritmiese benadering om die korrektheid van temporale logika spesi kasies in die beskrywing van harde- of sagteware te bewys. Anders as met tradisionlee program toetsing, benodig modeltoetsing 'n volledige ondersoek van al die moontlike toestande waarin so 'n beskrywing homself kan bevind. Model toetsing word meestal op abstrakte modelle van sagteware of die ontwerp toegepas. Indien die ontwerp of model aan al die spesi kasies voldoen word die abstrakte model gewoontlik vertaal na 'n implementasie. Die vertalings proses word gewoontlik met die hand gedoen en laat ruimte om nuwe foute, en selfs foute wat uitgeskakel in die model of ontwerp is te veroorsaak. Deesdae, is 'n gewilde benadering tot modeltoetsing om di e tegnieke direk op die implementasie toe te pas, en sodoende die ekstra moeite van model konstruksie en vertaling uit te skakel. Hierdie tesis handel oor die ontwerp, implementasie en toetsing van 'n e ektiewe modeltoetser vir 'n klein gelyklopende taal, LF, wat by die Universiteit van Stellenbosch ontwikkel is. Die enkeldoelige taal, LF, is gemik op die veilige ontwikkeling van ingebedde sagteware. Die taal is ontwerp om veilige programmerings praktyke aan te moedig. Verder is die taal en die onderliggende bedryfstelsel so ontwerp om 'n model toetser te akkomodeer. Om die LF programme direk te kan toets, is die model toetser 'n integrale deel van die bedryfstelsel sodat dit die program kan aandryf om alle moontlike toestande te besoek.

This thesis looks at three aspects related to the design of E-commerce systems, online auctions and distributed grid computing systems. We show how formal verification techniques from computer science can be applied to ensure correctness of system design and implementation at the code level. Through e-ticket sales example, we demonstrate that model checking can locate subtle but critical flaws that traditional control and auditing methods (e.g., penetration testing, analytical procedure) most likely miss. Auditors should understand formal verification methods, enforce engineering to use them to create designs with less of a chance of failure, and even practice formal verification themselves in order to offer credible control and assistance for critical e-systems. Next, we study why many online auctions offer fixed buy prices to understand why sellers and auctioneers voluntarily limit the surplus they can get from an auction. We show when either the seller of the dibbers are risk-averse, a properly chosen fixed permanent buy-price can increase the social surplus and does not decrease the expected utility of the sellers and bidders, and we characterize the unique equilibrium strategies of uniformly risk-averse buyers in a buy-price auction. In the final chapter we look at the design of a distributed grid-computing system. We show how code-instrumentation can be used to generate a witness of program execution, and show how this witness can be used to audit the work of self-interested grid agents. Using a trusted intermediary between grid providers and customers, the audit allows payment to be contingent on the successful audit results, and it creates a verified reputation history of grid providers. We show that enabling the free trade of reputations provides economic incentives to agents to perform the computations assigned, and it induces increasing effort levels as the agents' reputation increases. We show that in such a reputation market only high-type agents would have incentive to purchase a high reputation, and only low-type agents would use low reputations, thus a market works as a natural signaling mechanism about the agents' type. / text

As software and hardware systems grow more complex and we begin to rely more on their correctness and reliability, it becomes exceedingly important to formally verify certain properties of these systems. If done naïvely, verifying a system can easily require exponentially more work than running it, in order to account for all possible executions. However, there are often symmetries or other properties of a system that can be exploited to reduce the amount of necessary work. In this thesis, we present a number of approaches that do this in the context of the CSP model checker FDR. CSP is named for Communicating Sequential Processes, or parallel combinations of state machines with synchronised communications. In the FDR model, the component processes are typically converted to explicit state machines while their parallel combination is evaluated lazily during model checking. Our contributions are motivated by this model but applicable to other models as well. We first address the scalability of the component machines by proposing a lazy compiler for a subset of CSP_M selected to model parameterised state machines. This is a typical case where the state space explosion can make model checking impractical, since the size of the state space is exponential in the number and size of the parameters. A lazy approach to evaluating these systems allows only the reachable subset of the state space to be explored. As an example, in studying security protocols, it is common to model an intruder parameterised by knowledge of each of a list of facts; even a relatively small 100 facts results in an intractable 2¹⁰⁰ states, but the rest of the system can ensure that only a small number of these states are reachable. Next, we address the scalability of the overall combination by presenting novel algorithms for bisimulation reduction with respect to strong bisimulation, divergence- respecting delay bisimulation, and divergence-respecting weak bisimulation. Since a parallel composition is related to the Cartesian product of its components, performing a relatively time-consuming bisimulation reduction on the components can reduce its size significantly; an efficient bisimulation algorithm is therefore very desirable. This thesis is motivated by practical implementations, and we discuss an implementation of each of the proposed algorithms in FDR. We thoroughly evaluate their performance and demonstrate their effectiveness.