Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for a biometric authentication system. In one aspect, a method includes, a vibrating component of a mobile device is vibrated; a vibration signal of the mobile device is detected as a user grips the mobile device; a dynamic grip signature is determined for the user based on the detected vibration signal; and the user is authenticated based on a comparison of the dynamic grip signature to an initial grip template.