Watermark embedding and extracting method for protecting documents

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, and 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 respectively, of the original document ( 10 );

minimizing an error probability corresponding to an interference between intra-word spaces and inter-word spaces in the watermarked document ( 20 ) by means of an optimization module ( 111 ), wherein the optimization module ( 111 ) determines a distance between the lengths of the intra-word and inter-word spaces, and wherein the optimization module ( 111 ) minimizes the error probability 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 )+ε if S Si >μ( i )+ε, and

S Si else;

where μ(i) is an average value of the length of the intra-word spaces S Si in the i-th line and ε is a host-rejection parameter which is an adjustable parameter to increase the distance between the lengths of the intra-word and inter-word spaces;

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 );

encoding ( 113 ) the message ( 30 ) into codewords and encoding ( 114 ) the codewords into the watermark;

embedding the watermark into the original document ( 10 ) to generate ( 115 ) the watermarked document ( 20 ) by modifying the intra-word spaces and inter-word spaces of the original document ( 10 ) according to the encoding ( 113 ) of the received message ( 30 ), 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; and

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 ).

2. The method according to claim 1 , wherein the image-based segmentation ( 403 ) applies clustering or optical character recognition if the received original document ( 10 ) has a non-homogeneous text background.

3. The method according to claim 1 , wherein the image-based segmentation ( 403 ) applies projections if the received original document ( 10 ) has a homogeneous text background.

4. The method according to claim 1 , 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).

5. The method according to claim 1 , wherein the original document ( 10 ) is selected from a digital document and a digitalized document.

6. A non-transitory computer-readable medium containing computer instructions stored therein for causing a computer processor to perform the method according to claim 1 .