CS-TR-1333 May , 2012 Optimising the Release Order of Defensive Mechanisms

In the practical use of security mechanisms such as CAPTCHAs and spam filters, attackers and defenders exchange 'blows', each celebrating (temporary) success in breaking and defending. We are interested in the question of whether the order in which defensive algorithms are released has a significant impact on the time taken by attackers to break the combined set of algorithms. The rationale behind our approach is that attackers learn from their attempts, and that the release schedule of defensive mechanisms can be adjusted so as to impair that learning experience. This paper introduces this problem. We show that our hypothesis holds for an experiment using several simplified but representative spam filter algorithms—that is, the order in which spam filters are released has a statistically significant impact on the time attackers take to break all algorithms. We then model the problem as an optimization problem using a Markov Decision Process model. We present a tailored optimization algorithm to obtain efficiently the optimal release strategies for any given model. © 2012 Newcastle University. Printed and published by Newcastle University, Computing Science, Claremont Tower, Claremont Road, Newcastle upon Tyne, NE1 7RU, England. Bibliographical details ALSUHIBANY, S.A., ALONAIZI, A., SMITH, C., VAN MOORSEL, A. Optimising the Release Order of Defensive Mechanisms [By] S.A. Alsuhibany, A. Alonaizi, C. Smith, A. van Moorsel Newcastle upon Tyne: Newcastle University: Computing Science, 2012. (Newcastle University, Computing Science, Technical Report Series, No. CS-TR-1333)