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pdfdrm

embed serial number to PDF file?

To prevent the casual distribution of pdf document, is there any way such as embedding the serial number to the file?

My idea is to embed the id bound to user and enable to find who distribute the file. I know it's not preventing the distribution but may discourage the casual distribution by the certain level.

Any other solution is also welcome.

Thanks!

Solution

  • Common way is placing of meta data, but you can easily remove them. Let's search hideouts (most of them low-level)!

    • Non-mark text
    • Text under overlapping objects
    • Objects of older versions (doesn't noticed by reader, but there with redundant information)
    • Marks in streams between BX-EX (with weird information from readers point of view)
    • Information before %PDF-X
    • Information above %%EOF
    • Substitution of names for some elements (like font name)
    • Steganography
      • Manipulation from used fonts
      • Whitespacing
      • Images with setganograpy

    My favorite are steganography and BX-EX block within stream, with proper compression and/or encryption it is hard to find (if do not know, where it is). To make search harder wrap some normal blocks with BX-EX.

    Some of ways are easy to remove, some harder, but decided attacker will be able to find and sanitize them all. Think about copy-paste of text or print trough PDF-printer.