A reading device reads ambiguous target elements of an authentication target printed on a medium. Each ambiguous target element has a number of different manners by which the ambiguous target element can be interpreted by the reading device. The authentication target as read by the reading device is compared against an authentication signature of the reading device. The authentication signature specifies for each ambiguous target element an expected manner by which the reading device interprets the ambiguous target element as one of the different manners by which the ambiguous target element can be interpreted. Where the authentication target as read by the reading device matches the authentication signature of the reading device, the reading device is signaled as having passed authentication.