A three-dimensional approach towards measuring sender anonymity

We first illustrate using examples that existing measures in literature are not sufficient to fully characterize the anonymity provided by an anonymous system. We then propose a new isolation measure, based upon presence of outliers in a distribution, and show that this measure is critical towards quantifying the overall anonymity provided by the system. We provide justification for three distinct aspects of anonymity, important from the perspectives of a user, a system designer, and an attacker, leading to a three-dimensional approach towards measuring sender anonymity. We further show how two anonymous systems can be compared in terms of the degree of anonymity provided, using the proposed 3-tuple metric and appropriate weights reflecting the attributes desired in the system. Finally, we apply the proposed metric to an existing anonymous system and discuss the insights gained.