Anonymity, Privacy, Onymity, and Identity: A Modal Logic Approach

In this paper, we propose a taxonomy of privacy-related information-hiding/disclosure properties in terms of the modal logic of knowledge for multiagent systems. The properties considered here are anonymity, privacy, onymity, and identity. Intuitively, anonymity means the property of hiding who performed a certain specific action, privacy involves hiding what was performed by a certain specific agent, onymity refers to disclosing who performed a certain specific action, and identity relates to disclosing what was performed by a certain specific agent. Building on Halpern and O'Neill's work, we provide formal definitions of these properties and study the logical structure underlying them. In particular, we show that some weak forms of anonymity and privacy are compatible with some weak forms of onymity and identity, respectively. We also discuss the relationships between our definitions and existing standard terminology, in particular Pfitzmann and Hansen's consolidated proposal.