Metrics for Traffic Analysis Prevention

This paper considers systems for Traffic Analysis Prevention (TAP) in a theoretical model. It considers TAP based on padding and rerouting of messages and describes the effects each has on the difference between the actual and the observed traffic matrix (TM). The paper introduces an entropy-based approach to the amount of uncertainty a global passive adversary has in determining the actual TM, or alternatively, the probability that the actual TM has a property of interest. Unlike previous work, the focus is on determining the overall amount of anonymity a TAP system can provide, or the amount it can provide for a given cost in padding and rerouting, rather than on the amount of protection afforded particular communications.