The present invention relates generally to documents, e.g., such as legal tender, currency and identification documents, and apparatus for encoding documents. One claim recites: a physical document including a machine-detectable hidden signal. Presence of the hidden signal is determined by collecting visible light image data associated with the document and transforming the image data to a complementary domain to generate transform data. Presence of a telltale pattern in the transform data indicates a presence of the hidden signal. The hidden signal is associated with a type or status of the physical document. One type is legal tender. Another type is an identification document. An example of status is "do not copy". Of course, other combinations are claimed as well.