nous avons constaté que très peu de travaux ciblaient les documents PDF
qui représentent cependant une part non anecdotique des données
échangées en ligne.
+Parmi ces travaux, \cite{PD2008} propose la modification du nombre
+d'espaces entre les mots ou entre les paragraphes.
+Similairement, les auteurs de~\cite{DBLP:journals/sigpro/LeeT10}
+ajoutent des caractères invisibles dans le document.
+En supprimant ces espaces ou caractères invisibles, la marque s'enlève
+facilement.
+Dans~\cite{PD2008}, les auteurs modifient de manière imperceptible
+le positionnements des caractères. D'autres éléments de postionnement
+sont intégrés dans~\cite{WT08}.
+Une attaque qui remodifierait aléatoirement de manière faible ces positions
+ détruirait la marque dans les deux cas.
-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.
-
Substitutive Quantization Index Modulation (QIM) methods were
introduced by Chen and Wornell~\cite{CW01}. The Spread Transform Dither
Modulation (STDM) is an implementation of this scheme and it has been
