Digital watermark methods for encoding auxiliary data into a host signal are used to authenticate physical and electronic objects. In one implementation, an identification document includes electronic circuitry. The electronic circuitry includes an image of an authorized bearer of the identification document. The image includes first digital watermarking having a plural-bit identifier. In some cases, the identification document includes a printed image of the authorized bearer of the identification document. The printed image includes second digital watermarking therein. The first digital watermarking and the second digital watermarking can be cross-correlated for authentication of the identification document.