1. A method of confirming the presence of features encoded in an object, the method being performed by a camera-equipped consumer device that captured an image of the object, and also involving a remote system that has key data that is not available to the consumer device, the method comprising the acts: with a hardware processor in the consumer device, processing data corresponding to said image, said processing including computing intra-image gradient information corresponding to pixel locations in the image, said processing being performed by a processor in the consumer device configured to perform such act; sending computed intra-image gradient information for less than all pixel locations in the image to the remote system; and in response to said sending, receiving data at the consumer device indicating whether the remote system was able to confirm the presence of said features in said object from said sent intra-image gradient information.