Line art on a banknote or other security document is slightly changed to effect
the encoding of plural-bit digital data (i.e. the banknote is digitally watermarked).
When such a banknote is thereafter scanned, the resulting image data can be recognized
as corresponding to a banknote by detection of the encoded data. (Alternatively,
the image data can be recognized as corresponding to a banknote by machine detection
of other forms of watermarking, or by reference to visible structures characteristic
of banknotes.) In response to detection of a banknote, the detecting apparatus
(e.g., a scanner, photocopier, or printer) can intervene so as to prevent or deter
counterfeiting. For example, the apparatus can interrupt a copying operation, and
display a message reminding the operator that it is illegal to reproduce currency.
Another option is to dial a remote service and report the attempted reproduction
of a banknote. Yet another option is to permit the copying, but to insert forensic
tracer data (e.g. steganographically encoded binary data) in the resultant copy.
The tracer data can memorialize the serial number of the machine that made the
copy and/or the date and time the copy was made. To address privacy concerns, such
tracer data is not normally inserted in photocopied output, but is so inserted
only when the subject being photocopied is detected as being a banknote.
