[texhax] watermark info

[Walt Burkhard]

Hi,

I am looking for a scheme to include an invisible watermark within a pdf file.  
The watermark would be visible only when viewed with an editor or an appropriate
viewer.

Currently, files are created using ps2pdf.   I would like this inclusion process to
be fairly quick.  

Do we have anything in CTAN or elsewhere (open source!) that could be used to do this? 

Many thanks in advance.  

Walt Burkhard