AnoA: A Framework for Analyzing Anonymous Communication Protocols

Abstract

Abstract. Anonymous communication (AC) protocols such as the widely used Tor network have been designed to provide anonymity over the Internet to their participating users. While AC protocols have been the subject of several security and anonymity analyses in the last years, there still does not exist a framework for analyzing these complex systems and their different anonymity properties in a unified manner.In this work we present AnoA: a generic framework for defining, analyzing, and quantifying anonymity properties for AC protocols. In addition to quantifying the (additive) advantage of an adversary in an indistinguishability-based definition, AnoA uses a multiplicative factor, inspired from differential privacy. AnoA enables a unified quantitative analysis of well-established anonymity properties, such as sender anonymity, sender unlinkability, and relationship anonymity. AnoA modularly specifies adversarial capabilities by a simple wrapper-construction, called adversary classes. We examine the structure of these adversary classes and identify conditions under which it suffices to establish anonymity guarantees for single messages in order to derive guarantees for arbitrarily many messages. This then leads us to the definition of Plug'n'Play adversary classes (PAC), which are easy-to-use, expressive, and satisfy this condition. We prove that our framework is compatible with the universal composability (UC) framework and show how to apply AnoA to a simplified version of Tor against passive adversaries, leveraging a recent realization proof in the UC framework.

Oblivious DNS over HTTPS (ODoH): A Practical Privacy Enhancement to DNS

“…While ODoH provides strong cryptographic security properties for individual queries, it has limited defences against correlation attacks. Backes et al [4] formalise this as Sender Anonymity, i.e., whether an attacker can identify the sender of a message with some nonnegligible probability. As discussed in Das et al [24,25], resisting correlation attacks in the presence of an adversary that can observe both endpoints requires added latency or bandwidth overhead.…”

Comprehensive Anonymity Trilemma: User Coordination is not enough

“…In this work we present an impossibility result for sender anonymity of ACNs that allow messages to be sent, to mix, and to confuse a potential adversary with dummy messages. We measure sender anonymity based on the AnoA framework [25] as the inability of an adversary to distinguish between two different senders of their own choosing, say, Alice and Bob. We start by showing, with an intuitive counter-example, why the existing anonymity trilemma by Das et al does not sufficiently capture this space of protocols.…”

“…We define (sender) anonymity by a game between a challenger (controlling the protocol) and a global passive adversary, following the AnoA framework [25]. The challenger receives all protocol parameters and a description of how users want to send messages (the user distribution), as well as a challenge bit b that influences which of two adversarially chosen senders actually sends a particular challenge message.…”

“…Strong sender anonymity lets the adversary freely choose the pair of challenge senders and requires that the AC protocol's behavior is indistinguishable to the adversary. Hence, strong sender anonymity corresponds to the full anonymity set (see [25,Lemma 7]) that encompasses all users. Sender anonymity for (non-full) anonymity sets would result in restricting the adversary in Definition 1 to choose the pair of challenger senders from the same anonymity set.…”

A Survey on Measuring Anonymity in Anonymous Communication Systems

“…In 2013, Backes et al [102] proposed a framework for analyzing anonymous communication protocols, AnoA, in which a formal definition of anonymity was given based on a novel generalization of differential privacy. The security notion in the framework is based on interacting Turing Machines.…”