A common log-in process at computers involves the entry of username and password; log out depends on the user to remember to log out, or a timeout to expire the user session. Once logged in, user sessions may be vulnerable to imposter attacks in which an impostor steps up to the user’s unattended computer and inherits the user’s access privilege. We propose a ring-based authentication system called “AuthoRing”, which restricts the imposter attackers from generating new inputs at the computer’s mouse and keyboard. During the log-in process, an eligible AuthoRing user wears a digital ring with accelerometers and wireless communication capability. When input is detected at the mouse or keyboard, the computer’s AuthoRing system correlates hand-motion data received from the ring with the input data from the computer’s window manager, and detects imposter attacks when these data are insufficiently correlated. We implemented the AuthoRing system and evaluated its security, efficiency, and usability; we found that imposter attacks can be effectively detected and the required operations happen quickly with negligible delays experienced by the user.