-Several methods of Steganography and Digital Watermarking in PDF and
-Text documents have been proposed. In~\cite{PD2008}, a steganographic approach
-is presented by hiding information using inter-word and
-inter-paragraph spacing in a text. The main disadvantage of this
-method is that the hidden message can be destroyed by simply deleting
-some spaces between the words in the stego text. In~\cite{PD2008},
-two
-different algorithms are proposed which are considered as an
-alternative for the original TJ operator method. The TJ operator
-displays the text string in a PDF document, allows individual
-character positioning and uses character and word spacing parameters
-from the text state. The alternative method has less embedding
-capacity than the original method. In~\cite{LLGC13} an encryption technique
-is proposed by combining the information hiding technique in PDF
-documents and the quadratic residue as basis and then apply it to
-copyright protection and digital learning. The main drawback of this
-method is that the hidden message can be easly removed. In~\cite{DBLP:journals/sigpro/LeeT10}, an
-embedding method in source programs using invisible $ASCII$ codes is
-proposed. This method is very easy to detect by simply extracting the
-modified text from the document, converting it to hexadecimal,
-extracting all the inserted invisible $ASCII$ characters, and then,
-decoding the embedded message. In~\cite{WT08}, a data hiding in PDF files and
-applications by imperceivable modifications of PDF object parameters
-is proposed. This method serves to hide data by slight modifications
-of the values of various PDF object parameters such as media box and
-text matrices. The method is considered to have sufficient
-transparency while its main drawback is its very low embedding
-capacity.
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