On Privacy Notions in Anonymous Communication

Abstract

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.

SoK on Performance Bounds in Anonymous Communication

“…A FORMAL PRIVACY DEFINITIONS A.1 Basic Goal Formalization from [18] In game-based security definitions, the game adversary chooses two batches, i.e. sets of communications that start in a random order (or simultaneously).…”

“…To sort the bounds on anonymity, we need a better understanding of what "anonymity" actually means in each case. Following the formal definitions of [18], we will distinguish the following forms of anonymity in this work:…”

“…We refer the interested reader to Appendix A and [18] for a formal definition and note that all our comparisons and improvements work similarly on the underlying formal model.…”

“…It is sufficient for all the attacks to compromise or influence outgoing links or the attached relays at the sender, as well as the receiver (or its incoming links/attached relays), as this enables to correlate events at the terminals of the 9 ≥ 1 2 for the unsynchronized and ≥ 1 for the synchronized setting (see Appendix D.3) 10 It is called "sender anonymity" in [17] and is shown to map directly to¯in [18]. communication.…”

“…For reasons of compatibility with the analyzed papers, we extend [18] by introducing an 1 for each notion . It expresses that every sender sends exactly once in each batch for a sender notion (¯, ( )¯), each receiver receives exactly once for a receiver notion (¯), and each 27 ( )¯is called Pair-Sender-Message Unlinkability, ( )¯: Pair-Sender-Receiver Unlinkability in [18]. Figure 4: Batches in illustrated as in [19] sender and receiver send/receive exactly once in each batch for an impartial notion (¯, ( )¯).…”