Browsers warn users when the privacy of an SSL/TLS connection might be at risk. An ideal SSL warning would empower users to make informed decisions and, failing that, guide confused users to safety. Unfortunately, users struggle to understand and often disregard real SSL warnings. We report on the task of designing a new SSL warning, with the goal of improving comprehension and adherence.
We designed a new SSL warning based on recommendations from warning literature and tested our proposal with microsurveys and a field experiment. We ultimately failed at our goal of a well-understood warning. However, nearly 30% more total users chose to remain safe after seeing our warning. We attribute this success to opinionated design, which promotes safety with visual cues. Subsequently, our proposal was released as the new Google Chrome SSL warning. We raise questions about warning comprehension advice and recommend that other warning designers use opinionated design.