X-Git-Url: https://bilbo.iut-bm.univ-fcomte.fr/and/gitweb/hdrcouchot.git/blobdiff_plain/596713f36ad5e56e5860473b80cfcd4821a616c8..4eb1ee7d55b10be299e02f6374289a378ff478d7:/ahmad.tex?ds=inline

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+++ b/ahmad.tex
@@ -2,50 +2,70 @@ En Ã©tudiant le watermarking,
 nous avons constatÃ© que trÃ¨s peu de travaux ciblaient les documents PDF
 qui reprÃ©sentent cependant une part non anecdotique des donnÃ©es
 Ã©changÃ©es 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 caractÃ¨res invisibles dans le document.
+En supprimant ces espaces ou caractÃ¨res invisibles, la marque s'enlÃ¨ve
+facilement.
+Dans~\cite{PD2008}, les auteurs modifient de maniÃ¨re imperceptible
+le positionnements des caractÃ¨res. D'autres Ã©lÃ©ments de postionnement
+sont intÃ©grÃ©s dans~\cite{WT08}.
+Une attaque qui remodifierait  alÃ©atoirement de maniÃ¨re faible ces positions
+ dÃ©truirait la marque dans les deux cas.
+La quantification (au sens du traitemetn du signal) est une rÃ©ponse
+Ã  ces attaques: des positions modifiÃ©es de maniÃ¨re mal intentionnÃ©e  
+peuvent grÃ¢ce cette dÃ©marche Ãªtre rapprochÃ©es (abstraites) en des positions
+prÃ©Ã©tablies et conserver ainsi leur information et donc la marque.
+STDM~\cite{CW01} est une instance de ces schÃ©mas de marquage.
+
+Ce chapitre prÃ©sente une application de STDM au marquage de documents PDFs.
+\JFC{annonce du plan}
+
+\section{Rappels sur STDM}
+
+\section{Spread Transform Dither Modulation}
+\label{sec:STDM}
+Les paramÃ¨tres de ce schÃ©ma sont
+\begin{itemize}
+\item le facteur de quantification $\Delta$ est un rÃ©el positif; plus $\Delta$
+est grand, plus la distortion peut Ãªtre importante;
+\item le niveau d'indÃ©cision  $d_0$ qui est un rÃ©el dans
+$[-\dfrac{\Delta}{2},\dfrac{\Delta}{2}]$; plus ce nombre a une valeur absolue
+Ã©levÃ©e, plus les erreurs peuvent Ãªtre corrigÃ©es;
+On dÃ©finit $d_1$ par 
+$$d_1 = \begin{cases} 
+  d_0 + \Delta/2, & \textrm{ si }~~d_0<0 \\  
+  d_0 - \Delta/2, & \textrm{ sinon } 
+\end{cases}
+$$
+\item un nombre $L$ d'Ã©lÃ©ments dans lequel chaque bit est embarquÃ©;
+\item un vecteur $p$ de projection de taille $L$; 
+
+\end{itemize}
+
+Soit donc $x$ un vecteur de taille $L$ dans lequel on souhaite embarquer 
+le bit $m\in\{0,1\}$. 
+Ce vecteur est remplacÃ© par $x'$ dÃ©fini par 
+ 
+\begin{equation}\label{eq:stdm}
+x' = f(x,m) = x+ ((\lfloor(\frac{(x^T p) -d_m}{\Delta})\rfloor\Delta +d_m )~ - x^T p)p
+\end{equation}
+
+Avec les mÃªmes paramÃ¨tres $\Delta$, $d_0$ , $L$ et $p$ le message 
+$\hat{m}$ extrait de 
+$x'$ de taille $L$ est dÃ©fini par:
+\begin{equation}
+\hat{m} = arg \min_{ m \in \{0, 1\}} \mid x'^T p - f(x,m) \mid
+\end{equation}
+
+Les auteurs de~\cite{CW01} ont montrÃ© que la variance de l'erreur 
+est Ã©gale Ã  $\Delta^2/12L$ 
+lorsque chacun des $L$ Ã©lÃ©ments de $x$ suit une ditribution uniforme 
+$U(\Delta)$. 
+
+
+\section{Application au marquage de documents PDF}
+
 
 
-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
-considered  robust  under  different watermarking
-attacks~\cite{DM10,WLSYNW13,CW99}.
-
-In this  paper, the goal  is to  present a blind  digital watermarking
-scheme for PDF documents based on  a variant of the Quantization Index
-Modulation   method   called   Spread  Transform   Dither   Modulation
-(STDM). The main difficulty in PDF  documents is to find a significant
-watermarking  space in  order  to  embed the  secret  message under  a
-sufficient Transparency-Robustness tradeoff. Our contribution consists
-in using  the $x$-coordinates of a  group of characters to  embed each
-bit  of  the  secret  message  while  choosing  the  appropriate  mean
-distortion value which gives  the strong tradeoff between transparency
-and robustness.
\ No newline at end of file
