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 multi-agent 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 hiding what was performed by a certain specific agent, onymity disclosing who performed a certain specific action, and identity 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 relationships between our definitions and existing standard terminology, in particular Pfitzmann and Hansen's consolidated proposal.