1. A method comprising the acts: receiving a request to assign an identifier for use with a first content object represented by digital data; assigning an identifier for use with the first content object, and transmitting the assigned identifier to a remote device, the assignment of the identifier proceeding independently of the digital data representing the first content object; and checking to determine if an identifier has earlier been assigned to a former content object having a perceptual similarity with the first content object; wherein said request to assign the identifier occurs in connection with posting the first content object to a social networking service, or sending the first content object for printing.