Stationery, or other printable media, is encoded with a digital watermark.
The watermark is not conspicuous to a human observer of the media, yet
conveys plural bits of auxiliary information when optically scanned and
digitally processed. The watermark can be formed by ink-jet printing, or
otherwise. The encoded information can be used for various purposes,
including authenticating the document as an original, linking to
associated on-line resources, and distinguishing seemingly-identical
versions of the same document (document serialization).