In one embodiment, a stylus includes electrodes operable to transmit signals wirelessly to a device through a touch sensor of the device. The stylus also includes one or more computer-readable non-transitory storage media within the stylus and embodying logic that is operable when executed to access first data representing a gesture made with the stylus by a user, such as a sequence of manipulations of the stylus. The logic is further operable to access second data representing a pre-defined authentication sequence and compare the first data with the second data. This comparison may authenticate the user to the stylus or the device, authenticate the stylus to the device or the user, or authenticate the device to the stylus or the user.