Watermark embedding and extracting method for protecting documents

Abstract

A method for watermarking documents comprising:identifying and locating spaces in a received original document (10) by a location analysis (110) module which distinguishes between intra-word spaces and inter-word spaces;minimizing an error probability of interference between intra-word spaces and inter-word spaces in the watermarked document (20) by an optimization module (111);encoding (113) the message (30) into codewords and encoding (114) the codewords into the watermark;embedding the watermark to generate (115) the watermarked document (20) by modifying the intra-word spaces and inter-word spaces of the original document (10).

1 . A method ( 11 ) for watermarking documents, receiving an original document ( 10 ) and a message ( 30 ) to obtain a watermarked document ( 20 ) embedding a watermark which encodes the received message ( 30 ), the method characterized by comprising:
 identifying and locating spaces in the original document ( 10 ) by a location analysis ( 110 ) module, the location analysis ( 110 ) distinguishing between intra-word spaces and inter-word spaces;   minimizing an error probability of interference between intra-word spaces and inter-word spaces in the watermarked document ( 20 ) by an optimization module ( 111 );   encoding ( 113 ) the message ( 30 ) into codewords and encoding ( 114 ) the codewords into the watermark;   embedding the watermark to generate ( 115 ) the watermarked document ( 20 ) by modifying the intra-word spaces and inter-word spaces of the original document ( 10 ).   
     
     
         2 . The method according to any preceding claim, wherein the location analysis ( 110 ) comprises:
 checking ( 401 ) whether the received original document ( 10 ) is a bitmap format document and if the original document ( 10 ) is in a non-bitmap format, converting ( 402 ) the original document ( 10 ) into a bitmap format document;   applying an image-based segmentation ( 403 ) to the bitmap format document to obtain a set of lengths and a set of locations of the spaces in the bitmap format document;   classifying ( 405 ) the identified and located spaces into intra-word spaces and inter-word spaces to obtain sets ( 406 ) of lengths of the intra-word and inter-word spaces, S S  and S B  respectively, and sets of locations of the intra-word and inter-word spaces, L S  and L B , of the original document ( 10 ).   
     
     
         3 . The method according to  claim 2 , wherein the image-based segmentation ( 403 ) applies clustering or optical character recognition if the received original document ( 10 ) has a non-homogeneous text background. 
     
     
         4 . The method according to  claim 2 , wherein the image-based segmentation ( 403 ) applies projections if the received original document ( 10 ) has a homogeneous text background. 
     
     
         5 . The method according to any any of  claims 2 - 4 , wherein the optimization module ( 111 ) minimizes the error probability of interference between intra-word spaces and inter-word spaces by:
 obtaining a modified set of intra-word spaces, S* S ={S* Si } where i denotes a line number of the original document ( 10 ) and S* Si  is equal to:
   μ( i )+εif  S   Si >μ( i )+ε,
 
   
       where ε is a host-rejection parameter; and
 S Si  else; 
 obtaining a modified set of inter-word spaces, S* B ={S* Bi } where i denotes a line number of the original document ( 10 ), where
 S* Bi ={S* Bi (k)}, S* Bi (k) being the modified length of the k-th inter-word space of the i-th line and S Bi (k) being the original length of the k-th inter-word space of the i-th line of the original document ( 10 ), 
 S* S ={S* Si (k)}, S* Si (k) being the modified length of the k-th intra-word space of the i-th line and S Si (k) being the original length of the k-th intra-word space of the i-th line of the original document ( 10 ), and
     S*   Bi ( k )= S   Bi ( k )+(Σ k   S   Si ( k )−Σ k   S*   Si ( k ))/ N   b ( i )
 
 
 N b (i) being the number of inter-word spaces in the i-th line of the original document ( 10 ). 
 
 
     
     
         6 . The method according to  claim 5 , wherein generating ( 115 ) the watermarked document ( 20 ) comprising:
 selecting a subset from the set of inter-word spaces, S* B , mapped to the set of inter-word spaces in the t-th line, S t   B , through a secret key;   selecting a codeword w t  from the encoded message ( 30 )   embedding the selected codeword w t  in the selected subset of inter-word spaces by calculating a modified inter-word space S BW  of the watermarked document ( 20 ) using the expression:
     S   t   BW ( k )= S   t   B ( k )+ w   t ( k )· c   t ( k )· S   t   B ( k )= S   t   B ( k )(1+ w   t ( k )· c   t ( k ))
 
   where w t (k) is the selected codeword, for the k-th inter-word space in the t-th line; S t   B (k) is the k-th inter-word space in the t-th line of the inter-word space S B  of the original document ( 10 ), S t   BW (k) is the k-th inter-word space in the t-th line of the inter-word space S BW  of the watermarked document ( 20 ) and c t (k) is a weighting factor;   modifying the intra-word spaces and inter-word spaces of the original document ( 10 ) replacing the intra-word space, S S , and the inter-word space, S B , respectively by the modified set of intra-word spaces, S* S , and the modified inter-word space, S BW , in the watermarked document ( 20 ).   
     
     
         7 . The method according to any preceding claim, wherein encoding ( 113 ) the message ( 30 ) of length K comprises adding a channel coding to the message ( 30 ) to obtain a modified message (m′) of length L>K, dividing the modified message (m′) into payload blocks ( 510 ,  510 ′) of length N and adding a synchronization block ( 520 ,  520 ′) of length T before each payload block ( 510 ,  510 ′) respectively to obtain a symbol sequence (m″) of length L·(1+T/N). 
     
     
         8 . The method according to any preceding claim, wherein the original document ( 10 ) is selected from a digital document and a digitalized document. 
     
     
         9 . A method ( 12 ) for extracting watermarks from a received document ( 60 ) resulting from a watermarked document ( 20 ) which has a watermark embedded encoding a message ( 30 ) of a number Q of symbols, the method characterized by comprising:
 identifying and locating spaces in the received document ( 60 ) by a location analysis ( 630 ) module, the location analysis ( 630 ) distinguishing between intra-word spaces and inter-word spaces and obtaining a vector S′ BW  corresponding to the inter-word spaces of the watermarked document ( 20 );   mapping ( 640 ) the inter-word spaces of the vector S′ BW  to symbols according to an alphabet of codewords w={w i }, i=1, 2, . . . Q; Q being the number of symbols of the message ( 30 ) and the symbols being univocally mapped to the codewords w;   de-packetizing ( 650 ) the mapped symbols to obtain synchronization blocks and payload blocks of symbols;   channel decoding ( 660 ) the payload blocks to extract the message ( 30 ).   
     
     
         10 . The method according to  claim 9 , wherein the received document ( 60 ) is a distorted copy of the watermarked document ( 20 ). 
     
     
         11 . The method according to any of  claims 9 - 10 , further comprising applying noise reduction ( 610 ) and geometric correction ( 620 ) to the received document ( 60 ) before the location analysis ( 630 ). 
     
     
         12 . A non-transitory computer-readable medium containing computer instructions stored therein for causing a computer processor to perform the method according to any of  claims 1 - 8 . 
     
     
         13 . A non-transitory computer-readable medium containing computer instructions stored therein for causing a computer processor to perform the method according to any of  claims 9 - 11 .