  This paper describes MessageGuard, a browser-based platform for research into
usable content-based encryption. MessageGuard is designed to enable
collaboration between security and usability researchers on long-standing
research questions in this area. It significantly simplifies the effort
required to work in this space and provides a place for research results to be
shared, replicated, and compared with minimal confounding factors. MessageGuard
provides ubiquitous encryption and secure cryptographic operations, enabling
research on any existing web application, with realistic usability studies on a
secure platform. We validate MessageGuard's compatibility and performance, and
we illustrate its utility with case studies for Gmail and Facebook Chat.
