1. A method useful for judging authenticity of an object by reference to features encoded therein, 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: processing data corresponding to said image, said processing including computing intra-image gradient information for plural pixel locations in the image, said computing including comparing, for each of said pixel locations, a value of a pixel at said location with values of one or more neighboring pixels, said computing yielding a value that can be represented with five bits as the intra-image gradient information for said pixel location, said processing being performed by a processor in the consumer device configured to perform such act; sending the computed intra-image gradient information for N pixel locations in the image to the remote system, where N is less than all the pixel locations in the image, and the sent intra-image gradient information consists of one value for each of said N locations; 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; wherein the consumer device learns whether the object depicted in the captured image includes the encoded features—while not having key data necessary for such determination, thereby enhancing security; and wherein the remote system determines whether the object depicted in the captured image includes the encoded features—while not having access to the complete captured image, thereby enhancing privacy.