When PETs misbehave: A Contextual Integrity analysis

Privacy enhancing technologies, or PETs, have been hailed as a promising means to protect privacy without compromising on the functionality of digital services. At the same time, and partly because they may encode a narrow conceptualization of privacy as confidentiality that is popular among policymakers, engineers and the public, PETs risk being co-opted to promote privacy-invasive practices. In this paper, we resort to the theory of Contextual Integrity to explain how privacy technologies may be misused to erode privacy. To illustrate, we consider three PETs and scenarios: anonymous credentials for age verification, client-side scanning for illegal content detection, and homomorphic encryption for machine learning model training. Using the theory of Contextual Integrity, we reason about the notion of privacy that these PETs encode, and show that CI enables us to identify and reason about the limitations of PETs and their misuse, and which may ultimately lead to privacy violations.