Watermark embedding and extracting method for protecting documents

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, SS and SB respectively, and sets of locations of the intra-word and inter-word spaces, LS and LB 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 SSi>μ(i)+ε, and
• SSi else;
• where μ(i) is an average value of the length of the intra-word spaces SSi 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 SBi(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 SSi(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)=SBi(k)+(ΣkSSi(k)−ΣkS*Si(k))/Nb(i)
• Nb(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, StB, through a secret key;
• selecting a codeword wt from the encoded message (30)
• embedding the selected codeword wt in the selected subset of inter-word spaces by calculating a modified inter-word space SBW of the watermarked document (20) using the expression:
• StBW(k)=StB(k)+wt(k)·ct(k)·StB(k)=StB(k)(1+wt(k)·ct(k))
• where wt(k) is the selected codeword, for the k-th inter-word space in the t-th line;
• StB(k) is the k-th inter-word space in the t-th line of the inter-word space SB of the original document (10), StBW(k) is the k-th inter-word space in the t-th line of the inter-word space SBW of the watermarked document (20) and ct(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, SS, and the inter-word space, SB, respectively by the modified set of intra-word spaces, S*S, and the modified inter-word space, SBW, in the watermarked document (20).