Users today share a great deal of private information on the Web. While HTTPS protects this data during transmission, it does not protect data at rest, nor does it protect user data from the websites which store or transmit that data. These issues can be addressed with user-to-user encryption, an approach where data is encrypted and decrypted at the user's computer and is opaque to websites. In this paper we present MessageGuard, the first system that retrofits the Web with user-to-user encryption and is designed to work with all websites, in all browsers, on all platforms. We demonstrate that MessageGuard operates out-of-the-box on 47 of the Alexa top 50 sites, has minimal performance overhead, and is rated as highly usable by study participants.