The disclosure describes an authentication system and related methods for authenticating printed objects. The system uses an information-based metric along with one or more print quality metrics to provide accurate detection or classification of a counterfeit printed object. The print quality metric evaluates attributes of a subject image associated with the original printer, ink or paper to detect degradation of those operations due to copying operations like an image scanning and halftone printing subsequent to the original printing of the object. The information-based metric measures message symbol errors in an optically readable code, such as a digital watermark.