City Research Online

Abstract

Due to the constantly evolving nature of cyber threats and attacks, organisations see an ever-growing requirement to develop more sophisticated defence systems to protect their networks and information. In an arms race such as this, employing as many techniques as possible is crucial for companies to stay ahead of would-be attackers.

Design diversity is a technique with a significant history behind it, which has become more widely used as the availability of off-the-shelf defence software has become more commonplace. The simple concept behind design diversity is the age-old saying that ”two minds think better than one”. When combining multiple tools for cyber defence, it’s reasonable to expect that when these tools use different techniques, or work under different assumptions and configurations, they would also likely detect different threats. Hence, the security events that one tool misses or misclassifies, the other could correctly handle, and vice-versa. We would expect design diversity to remain an important design paradigm for as long as building a completely foolproof security system stays within the realms of impossibility.

While design diversity is appealing, and it has been used to great success in the past, it is important to realise that any possible gains from using this technique are entirely dependent on how diverse the various tools are, and on the context in which it is applied. Applying design diversity will yield different results in different environments, so it is important that empirical results are provided in as many contexts as possible.

In this work, we have looked at the use of design diversity in two major contexts. The first context deals with the question of detecting malicious web scraping activity. We have analysed three separate datasets provided to us by Amadeus - a global provider for the travel and tourism industry - which contain the HTTP traffic they observed within their network, as well as the alerts raised by two of their web scraping detectors. We studied how the combined performance potential of the two tools compares to their individual performances, in 1-out-of-2 (1oo2) and 2-out-of-2 (2oo2) adjudication schemes, meaning that a combined system raises an alert if any one of the internal tools does so as well, or the combined system only raises an alert if both internal tools do so, respectively. We’ve also identified several aspects that highlight the different alert patterns of both tools, which we use to explain the inherent diversity between the two.

The second context in which we have studied the use of design diversity is in the use of machine learning models for the classification of malware and benign software samples. We’ve done this with the use of a dataset that looked at the performance of 37 different RNN machine learning models used to classify a pool of over 4000 software samples, which originated from a previously published paper whose authors we have collaborated with. With the higher number and degree of diversity of the detection tools (the machine learning models) in this study, we were able to expand our results with additional adjudication schemes, anywhere between 1oo10 and 10oo10, as well as more interesting schemes, such as simple majority schemes, e.g., 3oo5. Similarly to the first body of work, we studied and summarised the different aspects that led to diversity in the behaviour of machine learning models.

When utilising multiple diverse systems, each producing a result, a voting or adjudication system is needed to decide on the overall system output/decision. The use of conventional adjudications schemes (e.g., 1-out-of-2) provides a useful first point for the use of design diversity, but these schemes may be deficient in comparison with others such as those that use optimal adjudication. As opposed to conventional adjudication schemes, where the individual outputs of each internal tool are not taken into account - i.e., a 1oo2 scheme does not care which one of its two internal tools raised an alert, only that one of them did - this is not the case for optimal adjudication. In optimal adjudication, the combined outputs of all the internal tools are called syndromes, and the output of the overall system is going to be dependent on which unique syndrome was generated for any given classification sample. This affords us several benefits, which we will detail in depth, primarily that specific tools can be given higher confidence over others, and that the error cost of generating false positive or false negative outputs can be taken into account when deciding the output of the overall system, such that we can optimise for the lowest error cost overall.

We have looked at the use of optimal adjudication in particular with our second dataset concerning the use of machine learning models for the classification of software samples. We expand on the benefits afforded over using conventional adjudication schemes, and delve into the aspects that make the various machine learning models diverse from one another.

We expect the results from this thesis will provide insight into the use of different adjudication schemes in the contexts we highlight (contexts in which, to the best of our knowledge, previous research has not been published), as well as provide guidance on the creation of such combined systems for use in security deployments beyond the contexts we have studied.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QA Mathematics > QA75 Electronic computers. Computer science Q Science > QA Mathematics > QA76 Computer software
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Computer Science

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