User authentication is an integral part of our lives; we authenticate ourselves to personal computers and a variety of other things several times a day. Authentication is burdensome. When we wish to access to a computer or a resource, it is an additional task that we need to perform~-- an interruption in our workflow. In this dissertation, we study people\u27s authentication behavior and attempt to make authentication to desktops and smartphones less burdensome for users. First, we present the findings of a user study we conducted to understand people\u27s authentication behavior: things they authenticate to, how and when they authenticate, authentication errors they encounter and why, and their opinions about authentication. In our study, participants performed about 39 authentications per day on average; the majority of these authentications were to personal computers (desktop, laptop, smartphone, tablet) and with passwords, but the number of authentications to other things (e.g., car, door) was not insignificant. We saw a high failure rate for desktop and laptop authentication among our participants, affirming the need for a more usable authentication method. Overall, we found that authentication was a noticeable part of all our participants\u27 lives and burdensome for many participants, but they accepted it as cost of security, devising their own ways to cope with it. Second, we propose a new approach to authentication, called bilateral authentication, that leverages wrist-wearable technology to enable seamless authentication for things that people use with their hands, while wearing a smart wristband. In bilateral authentication two entities (e.g., user\u27s wristband and the user\u27s phone) share their knowledge (e.g., about user\u27s interaction with the phone) to verify the user\u27s identity. Using this approach, we developed a seamless authentication method for desktops and smartphones. Our authentication method offers quick and effortless authentication, continuous user verification while the desktop (or smartphone) is in use, and automatic deauthentication after use. We evaluated our authentication method through four in-lab user studies, evaluating the method\u27s usability and security from the system and the user\u27s perspective. Based on the evaluation, our authentication method shows promise for reducing users\u27 authentication burden for desktops and smartphones
