On Privacy Notions in Anonymous Communication

Many anonymous communication networks (ACNs) with different privacy goals have been developed. However, there are no accepted formal definitions of privacy and ACNs often define their goals and adversary models ad hoc. However, for the understanding and comparison of different flavors of privacy, a common foundation is needed. In this paper, we introduce an analysis framework for ACNs that captures the notions and assumptions known from different analysis frameworks. Therefore, we formalize privacy goals as notions and identify their building blocks. For any pair of notions we prove whether one is strictly stronger, and, if so, which. Hence, we are able to present a complete hierarchy. Further, we show how to add practical assumptions, e.g. regarding the protocol model or user corruption as options to our notions. This way, we capture the notions and assumptions of, to the best of our knowledge, all existing analytical frameworks for ACNs and are able to revise inconsistencies between them. Thus, our new framework builds a common ground and allows for sharper analysis, since new combinations of assumptions are possible and the relations between the notions are known.