Cross Reference Injection for Proof of Ownership Portable Document Format

Portable Document Format (PDF) is one of the most widely distributed file types via the internet. With the increasing use of PDF formats, copyright protection of PDFs is one of the issues that needs to be considered, including proof of ownership of documents. To overcome this problem, this study proposes the application of watermarking texts. PDF is a text document that has a special structure. The internal structure of the PDF consists of header, body, cross reference (xref) and trailer. Watermarking in PDF files is generally inserted in the body section. Inserting watermarks outside the body is one of the interesting challenges to be studied. Watermark that is inserted outside the body, especially in the Cross reference (xref) has a limited capacity space. In this research, a watermark in the form of a text file is inserted by overwriting 2-byte space characters and 5-byte generation number characters on each xref line. Several PDF files are selected with different file sizes and with varying numbers of xref lines. Experiments in this study have succeeded in inserting 7-byte characters in each xref PDF line. The watermark that is inserted in the xref section is not easily perceived and does not increase the size of the PDF file.