1. A computer-implemented method comprising: receiving a plurality of images from a user of an online system; extracting from the plurality of images a characteristic of a camera used to capture the images; receiving a request from the user for access to the online system; responsive to the request from the user, sending a request to the user to authenticate the user by providing one or more additional images taken from the camera; receiving the requested one or more additional images from the user; extracting from the one or more additional images a characteristic of a camera used to capture the one or more additional images comparing the characteristic of the camera extracted from the one or more additional images with the characteristic of the camera extracted from the plurality of images; and responsive to the characteristic of the camera extracted from the one or more additional images matching the characteristic of the camera extracted from the plurality of images, providing the user with the requested access to the online system.