How to add an invisible watermark to PDF documents?

Question

I'd like to add an invisible watermark to each PDF document that I produce so that in case it leaks, I know where it leaked from. Anyone knows what would a good way for an invisible watermark be in a PDF document and how to add one?

Answer 1

Here a funny idea: Just place some watermark/copyright text on top or below of a dot or other punctuation mark. It is invisible for the naked eye if it uses the same color as the text but can be extracted using copy and paste. You can also give it a dark gray color, so you still have a chance to read it on screen.

Here some principal solution:

\documentclass{article}

\usepackage{graphicx}
\usepackage{xcolor}

\newcommand\watermark[1]{%
    #1%
    \sbox0{#1}%
    \llap{%
    \makebox[\wd0][c]{%  hor. centering
    \raisebox{.5\ht0}{%  approx. vert. centering
    \csname Gin@isotrue\endcsname% = "keepaspectratio"
    \resizebox*{.8\ht0}{.8\ht0}{% Scale down (the height is also used for the width to avoid the surrounding spaces)
    \parbox{10em}{% Allow line breaks
            \color{black!90}%  
            This PDF was created by John Doe for Jane Doe.
        }%
    }}}}%
}

\begin{document}
Text text text.

Text text text\watermark.

\end{document}

The dot will then look like this: (note the hard to see text)

Answer 2

How about embedding it in an image with steganography, most of the image manipulation libraries such as the GD library have routines available or you can run copies individually and use TikZ!

The message is encrypted in the yellow dots! Some color printers still do it see printer steganography

Answer 3

With the new version (v2.2 - still in CTAN upload process, but should be available soon) of pdfcomment you could use an 'invisible' PDF tooltip.

\documentclass{article}
\usepackage{marginnote}
\usepackage[author={Peteris Krumins}]{pdfcomment}
\begin{document}
This\marginnote{\pdftooltip{\rule{0pt}{0.5cm}\rule{0.5cm}{0pt}}{handed over to:\textCR Peteris Krumins}} is just a test sentence.
\end{document}

As soon as you hover over the 0.5cm/0.5cm rectancle in the margin the tooltip pops up. Of course you can put the tooltip in the header/footer or whereever you want.

Answer 4

Easy way to achieve this, is to intentionally put some carefully placed typos (or punctuation, any sort of slight modification), of no real consequence to the document, prior to issuing it to the respective party, ensuring that you keep a ledger of what changes were made. When the leaked document is identified, cross-reference it with your ledger, then (depending on the nature of your work) call your lawyer and instruct him/her to issue a C&D, or instruct your assets to act accordingly.

Answer 5

Here is a somewhat academic idea: encode the watermark in the spacing of words or even letters. I don't know a package to support this, though. The obvious benefit is that this encoding survives printing the document before passing it on.

Addendum: A variation of this is to individualize a document by tweaking the textwidths of some (or all) paragraphs ever so slightly. That will result in different line breaking and spacing decisions. This would not qualify as an encoding but as long as you know which version was given to whom you would know who leaked a document.

I did some experiments that suggest that very small variations to the text width are invisible to the eye but lead to the desired variations of paragraph layout.

Answer 6

If you're looking to track the document itself, it might not be a bad idea to include a serial number or something so that you can track it in printed and photocopied copies...

Answer 7

Any text can be copied & pasted into a text editor, thus losing all special markings. Random and unique changes in the choice and order of words would seem to be the only sure-fire way of tracing leaks.

Answer 8

@MartinScharrer's idea is very nice.

I'll add a few more ideas (which also should be rather easy to put into practice) without providing the real implementation:

1. Make the text invisible, by coloring it white on white background (or $anycolor on $anycolor background).

2. Make the text invisible by placing it on a black line (fontsize small enough to not overflow the line, or line thick enough to let the text disappear).

3. Use 'text rendering mode 3' (== "don't strike and don't fill the character shapes") to write the text.

4. Create your own very simple font (for example with the help of FontForge). It only needs to contain all the ASCII characters (or even less: only those characters you want to use -- maybe only numbers?), but each character's shape ("glyph"), including the 'blank'/'space' one, should be the same and look very harmless and innocent, for example a black square. You could load this font even into applications like LibreOffice: you type a text when this font is active, and what appears on screen, in print or in exported PDFs are only black squares. However, from the PDF the text can still be marked and copied or converted to text. If you adjust the font's character widths correctly, the black squares will even form long black lines.