Privacy-Enhanced Searches Using Encrypted Bloom Filters

It is often necessary for two or more or more parties that do not fully trust each other to share data selectively. For example, one intelligence agency might be willing to turn over certain documents to another such agency, but only if the second agency requests the specific documents. The problem, of course, is finding out that such documents exist when access to the database is restricted. We propose a search scheme based on Bloom filters and group ciphers such as Pohlig-Hellman encryption. A semi-trusted third party can transform one party's search queries to a form suitable for querying the other party's database, in such a way that neither the third party nor the database owner can see the original query. Furthermore, the encryption keys used to construct the Bloom filters are not shared with this third party. Multiple providers and queries are supported; provision can be made for third-party "warrant servers", as well as "censorship sets" that limit the data to be shared

Worm propagation strategies in an IPv6 Internet

In recent years, the Internet has been plagued by a number of worms. One popular mechanism that worms use to detect vulnerable targets is random IP address-space probing. This is feasible in the current Internet due to the use of 32-bit addresses, which allow fast-operating worms to scan the entire address space in a matter of a few hours. The question has arisen whether or not their spread will be affected by the deployment of IPv6. In particular, it has been suggested that the 128-bit IPv6 address space (relative to the current 32-bit IPv4 address space) will make life harder for the worm writers: assuming that the total number of hosts on the Internet does not suddenly increase by a similar factor, the work factor for finding a target in an IPv6 Internet will increase by approximately 296, rendering random scanning seemingly prohibitively expensive

Flow Cytometry: A tool for assessing drinking water quality and evaluating chlorine disinfection performance.

Chlorine disinfection is a process that has been in use for over a century for drinking water treatment; however rare detections of faecal indicator organisms in final treated water still occur. Assessing the performance of the disinfection process in-situ is challenging. Most often this is achieved by monitoring abiotic parameters such as chlorine, turbidity and pH, whereas microbiological sampling takes place daily. Typically, chlorine disinfection occurs within chlorine contact tanks which should be designed to achieve plug flow and minimise short circuiting. In reality, the design of contact tanks vary considerably, and water utilities have inherited many legacy assets that do not conform to modern day design standards. Furthermore, microbiological culture-based data is hard to evaluate when there are sporadic detections of culturable organisms. There is therefore a need to quantify deviation from optimal plug flow design of chlorine contact tanks and a requirement for an alternative microbiological approach to achieve this. This thesis explores the application of use of flow cytometry (FC), a novel culture independent technique for measuring bacterial cell viability, for disinfection applications. Firstly, an assessment of FC and its value as a monitoring tool for the water industry was carried out. This utilised the largest drinking water FC dataset in the world and concluded that there was no link between coliform detections and FC data, yet coliform detections were shown to be driven by the contact time (Ct) in disinfection, not just the sole parameter of chlorine residual. Secondly, the key process variables of chlorine disinfection were investigated and pilot scale studies demonstrated that hydraulic efficiency during chlorination impacted upon disinfection efficacy and FC provided insights of bacterial inactivation rates where traditional culture-based methods could not. The findings from this work culminated in an assessment of Ct across Scottish Water and the cost of investment required to bring high risk (large production volume) water treatment works (WTW) up to current standards was estimated. Finally, the implications of this thesis and the learning around chlorine disinfection and the application of FC for the water industry were discussed.EngD in Wate

When are bacteria dead? A step towards interpreting flow cytometry profiles after chlorine disinfection and membrane integrity staining

Flow cytometry is increasingly employed by drinking water providers. Its use with appropriate fluorescent stains allows the distinction between intact and membrane-damaged bacteria, which makes it ideally suited for assessment of disinfection efficiency. In contrast to plate counting, the technology allows the visualization of the gradual loss of membrane integrity. Although this sensitivity per se is very positive, it creates the problem of how this detailed viability information compares with binary plate counts where a colony is either formed or not. Guidelines are therefore needed to facilitate interpretation of flow cytometry results and to determine a degree of membrane damage where bacteria can be considered ‘dead’. In this study we subjected Escherichia coli and environmental microorganisms in real water to increasing chlorine concentrations. Resulting flow cytometric patterns after membrane integrity staining were compared with culturability and in part with redox activity. For laboratory-grown bacteria, culturability was lost at lower disinfectant concentrations than membrane integrity making the latter a conservative viability parameter. No recovery from chlorine was observed for four days. For real water, loss of membrane integrity had to be much more substantial to completely suppress colony formation, probably due to the heterogenic composition of the natural microbial community with different members having different susceptibilities to the disinfectant

Comparing flow cytometry with culture-based methods for microbial monitoring and as a diagnostic tool for assessing drinking water treatment processes

Flow cytometry (FCM) and the ability to measure both total and intact cell populations through DNA staining methodologies has rapidly gained attention and consideration across the water sector in the past decade. In this study, water quality monitoring was undertaken over three years across 213 drinking water treatment works (WTW) in the Scottish Water region (Total n = 39,340). Samples subject to routine regulatory microbial analysis using culture-based methods were also analysed using FCM. In addition to final treated water, the bacterial content in raw water was measured over a one-year period. Three WTW were studied in further detail using on-site inter-stage sampling and analysis with FCM. It was demonstrated that there was no clear link between FCM data and the coliform samples taken for regulatory monitoring. The disinfectant Ct value (Ct = mg·min/L) was the driving factor in determining final water cell viability and the proportion of intact cells (intact/total cells) and the frequency of coliform detections in the water leaving the WTW. However, the free chlorine residual, without consideration of treatment time, was shown to have little impact on coliform detections or cell counts. Amongst the three treatment trains monitored in detail, the membrane filtration WTW showed the greatest log removal and robustness in terms of final water intact cell counts. Flow cytometry was shown to provide insights into the bacteriological quality of water that adds significant value over and above that provided by traditional bacterial monitoring

Exploring the use of flow cytometry for understanding the efficacy of disinfection in chlorine contact tanks

A pilot scale chlorine contact tank (CCT) with flexible baffling was installed at an operational water treatment plant (WTP), taking a direct feed from the outlet of the rapid gravity filters (RGF). For the first time, disinfection efficacy was established by direct microbial monitoring in a continuous reactor using flow cytometry (FCM). Disinfection variables of dose, time, and hydraulic efficiency (short circuiting and dispersion) were explored following characterisation of the reactor's residence time distributions (RTD) by tracer testing. FCM enabled distinction to be made between changes in disinfection reactor design where standard culture-based methods could not. The product of chlorine concentration (C) and residence time (t) correlated well with inactivation of microbes, organisms, with the highest cell reductions (N/N0) reaching <0.025 at Ctx of 20 mg.min/L and above. The influence of reactor geometry on disinfection was best shown from the Ct10. This identified that the initial level of microbial inactivation was higher in unbaffled reactors for low Ct10 values, although the highest levels of inactivation of 0.015 could only be achieved in the baffled reactors, because these conditions enabled the highest Ct10 values to be achieved. Increased levels of disinfection were closely associated with increased formation of the trihalomethane disinfection by-products. The results highlight the importance of well-designed and operated CCT. The improved resolution afforded by FCM provides a tool that can dynamically quantify disinfection processes, enabling options for much better process control.Engineering and Physical Sciences Research Council (EPSRC): EP/G037094/1. and Scottish Water

Chlorine disinfection of drinking water assessed by flow cytometry: new insights

The efficacy of chlorine disinfection was assessed for the first time over a range of disinfection conditions using flow cytometry (FCM) to provide new insights into disinfection processes. Inactivation was assessed for pure culture bacteria (Escherichia coli) and micro-organisms in real treated water from operational water treatment works (WTWs). A dose dependent increase in inactivation rate (k) was observed for both test matrices, with values of 0.03 to 0.26 and 0.32 to 3.14 L/mg min for the WTW bacteria and E. coli, respectively. After 2 min, E. coli was reduced by 2 log for all chlorine doses (0.12 to 1.00 mg/L). In the case of the WTW filtrate bacteria, after 2 min log reductions were between 0.54 and 1.14 with increasing chlorine concentration, reaching between 1.32 and 2.33 after 30 min. A decrease in disinfection efficacy was observed as temperature decreased from 19 to 5 °C for both microbial populations. With respect to chlorination at different pH (pH 6, 7, 8), membrane damage was more pronounced at higher pH. This was not consistent with the higher disinfection efficacy seen at lower pH. when culture based methods are used to assess bacterial reductions. This provides evidence that more understanding into the fundamental mechanisms of chlorine disinfection are required and that methodological alterations may be required (e.g. pH standardisation) to fully utilise FCM over the entire range of chlorination conditions observed in operational environment