A variety of systems responsive to watermarked documents are detailed. In one, watermarking is employed to facilitate secure online transactions. In another, a system for exchanging data is provided. One system includes a user terminal and a central site. The user terminal includes a watermark reader, and a capture device to capture an image of a watermarked document. The central site includes a database of image hashes. The user terminal communicates with the central site. The watermark reader reads a watermark and computes a hash of a captured image, and passes the hash to the central site for comparison with the database of image hashes.