Mobile computing is the new norm. As people feel increasingly comfortable computing in public places such as coffee shops and transportation hubs, the threat of exposing sensitive information increases. While solutions exist to guard the communication channels used by mobile devices, the visual channel remains largely open. Shoulder surfing is becoming a viable threat in a world where users are often surrounded by high-power cameras, and sensitive information can be extracted from images using

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... nly modest computing power. In response, we present Cashtags: a system to defend against attacks on mobile devices based on visual observations. The system allows users to safely access pieces of sensitive information in public by intercepting and replacing sensitive data elements with non-sensitive data elements before they are displayed on the screen. In addition, the system provides a means of computing with sensitive data in a non-observable way, while maintaining full functionality and legacy compatibility across applications.