The present invention relates generally to steganography and data hiding. In one implementation we provide a method including: obtaining auxiliary data; providing a mask for a photo-reactive material, the mask corresponding at least in part to the auxiliary data; and exposing the material through the mask to steganographically impart the auxiliary data in the photo-reactive material. The auxiliary data is machine-readable from the exposed material. In another implementation the material is exposed through photolithographic illumination. Of course, other implementations are described and claimed as well.