1. A portable electronic device, comprising: an imaging sensor; a motion sensor; a processor; and a memory storing instructions that, when executed by the processor, cause the portable electronic device to authenticate a user, the instructions comprising: instructions to acquire multiple images of a face of the user by using the imaging sensor, instructions to obtain motion information by using the motion sensor, instructions to determine a first motion vector based on the multiple images, instructions to determine a second motion vector based on the motion information, and instructions to authenticate the user based on (1) determining a similarity measure between the first motion vector and the second motion vector, and (2) performing facial recognition on at least one of the multiple images.