20. The computer-readable medium of claim 17 , wherein the semantic information comprises data representing a photograph and the first machine-readable information comprises a digital watermark.
17. A non-transitory computer-readable medium having instructions stored thereon that, if executed by a computing device, cause the computing device to perform operations for authenticating an identification document, the identification document comprising: an image including steganographically encoded first machine-readable information including a first plural-bit message, a background including steganographically encoded second machine-readable information including a second plural-bit message, and semantic information including authentication information carried on or in the identification document, the operations comprising: determining the first plural-bit message based on the first machine-readable information; determining the second plural-bit message based on the second machine-readable information; determining the authentication information from the semantic information carried on or in the identification document; decrypting the first plural-bit message or the authentication information; and determining whether the identification document is authentic based at least in part on the first plural-bit message, the second plural-bit message, and the authentication information, wherein the second plural-bit message comprises information from the first plural-bit message and the authentication information.