Combinatorial-based Steganographic Scheme for PDF File Justified Texts

In this paper, we proposed a method that exploits justified texts of PDF files to embed secret data. The method is inspired by the work of Khosravi et al on "A new method for pdf steganography in justified texts" where they proposed a stegosystem allowing to embed in particular lines called a host line, 4 bits of secret message. We've improved it by exposing a combinatorics-based method able not only to embed 7 bits of secret message in these host lines but also b bits (b<14) in any line containing at least n spaces and m added spaces for (m, n > 0), thus allowing a sender to conceal a bigger message with very few changes made to the cover text.