You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Review of Text Watermarking: Theory, Methods, and Applications 
Chapter Title:  A. STRUCTURAL-BASED APPROACH 
1. Abstract of Cited Paper (BIB001):  J. Brassil, S. Low, N. F. Maxemchuk, L. O’GormanAT&T Bell LaboratoriesMurray Hill, NJConsider 2 distinct document pages which we labelAandB. The pages are sufﬁciently similar that they areindistinguishable upon visual inspection, even whencompared "side-by-side." However in one of the pagesthe locations of certain text words have been shiftedhorizontally by an imperceptible amount. Suppose thateither page is selected at random and is reproduced,possibly recursively, using either an ordinary plain papercopier or facsimile device. Can one determine if such acopy was derived from A or B?In this paper we show how to use spatial informationto reliably hide data in document images. Experimentalresults reveal that this information survives thedistortions introduced by noisy image reproductiondevices. We also describe the implementation of aprototype document marking and distribution system.The system can be used to discourage unauthorizedreproduction and dissemination of either copyrighteddocuments distributed electronically, or paper copies ofconﬁdential executive memoranda. 
2. Abstract of Cited Paper (BIB002):  Each copy of a text document can be made different in a nearly invisible way by repositioning or modifying the appearance of different elements of text, i.e., lines, words, or characters. A unique copy can be registered with its recipient, so that subsequent unauthorized copies that are retrieved can be traced back to the original owner. In this paper we describe and compare several mechanisms for marking documents and several other mechanisms for decoding the marks after documents have been subjected to common types of distortion. The marks are intended to protect documents of limited value that are owned by individuals who would rather possess a legal than an illegal copy if they can be distinguished. We will describe attacks that remove the marks and countermeasures to those attacks. An architecture is described for distributing a large number of copies without burdening the publisher with creating and transmitting the unique documents. The architecture also allows the publisher to determine the identity of a recipient who has illegally redistributed the document, without compromising the privacy of individuals who are not operating illegally. Two experimental systems are described. One was used to distribute an issue of the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, and the second was used to mark copies of company private memoranda. 
3. Abstract of Cited Paper (BIB003):  Digital watermarking is widely believed to be a valid means to discourage illicit distribution of information content. Digital watermarking methods for text documents are limited because of the binary nature of text documents. A distinct feature of a text document is its space patterning. We propose a new approach in text watermarking in which interword spaces of different text lines are slightly modified. After the modification, the average spaces of various lines have the characteristics of a sine wave and the wave constitutes a mark. Both nonblind and blind watermarking algorithms are discussed. Preliminary experiments have shown promising results. Our experiments suggest that space patterning of text documents can be a useful tool in digital watermarking. 
4. Abstract of Cited Paper (BIB004):  This paper proposes a novel watermarking algorithm for grayscale text document images. The algorithm inserts the watermark signals through edge direction histograms. The concept of sub-image consistency is developed. The concept means the sub-images have similar-shaped edge direction histograms and it is shown to be valid over a wide range of document images. Algorithms to insert and detect watermark signals are proposed. The experiments performed with various document images produced plausible results in terms of robustness. 
5. Abstract of Cited Paper (BIB005):  In this paper, we propose a new method for watermarking electronic text documents that contain justified paragraphs and irregular line spacing. The proposed method uses a spread-spectrum technique to combat the effects of irregular word or line spacing. It also uses a BCH (Bose-Chaudhuri-Hocquenghem) error coding technique to protect the payload from the noise resulting from the printing and scanning process. Watermark embedding in a justified paragraph is achieved by slightly increasing or decreasing the spaces between words according to the value of the corresponding watermark bit. Similarly, watermark embedding in a text document with variable line spacing is achieved by slightly increasing or decreasing the distance between any two adjacent lines according to the value of the watermark bit. Detecting the watermark is achieved by measuring the spaces between the words or the lines and correlating them with the spreading sequence. In this paper, we present an implementation of the proposed algorithm and discuss its simulation results. 
6. Abstract of Cited Paper (BIB006):  Chinese text documents can be watermarked by analyzing and patterning the characters? structure. This paper proposes a new natural text watermarking algorithm, which makes full use of the pictographic characters of the two-dimensional and the people?s visual redundancy. In the algorithm, some Chinese characters can be split into two characters according the watermarking content, thereby increasing the text watermarking capacity, and by adjusting the characters? inter-space and displaying, we can enhance the text watermark?s imperceptibility. For performance evaluation, we implement the algorithm to make experiments on different kinds of data sets. The results show that our watermarking method owns the following advantages: stronger watermarking robustness, greater watermarking capacity and better watermarking imperceptibility. 
7. Abstract of Cited Paper (BIB007):  Abstract- Copyright protection and authentication of digital contents has become a significant issue in the current digital epoch with efficient communication mediums such as internet. Plain text is the rampantly used medium used over the internet for information exchange and it is very crucial to verify the authenticity of information. There are very limited techniques available for plain text watermarking and authentication. This paper presents a novel zero-watermarking algorithm for authentication of plain text. The algorithm generates a watermark based on the text contents and this watermark can later be extracted using extraction algorithm to prove the authenticity of text document. Experimental results demonstrate the effectiveness of the algorithm against tampering attacks identifying watermark accuracy and distortion rate on 10 different text samples of varying length and attacks. Keywords-watermarking; copyright protection; authentication; security; algorithm 
8. Abstract of Cited Paper (BIB008):  In order to promote the Chinese digital text's copyright protection technology, this paper proposes a Chinese Text Zero-Watermark scenario based on Sentence's Entropy. This scenario calculates the entropies of sentences based on wordfrequency and makes crucial sentence selection based on entropy. After that, watermark is constructed with the order of crucial sentences. Furthermore, to prove its effectiveness, the scenario is validated by simulation and it has shown fine robust and ability of anti-attack. 
9. Abstract of Cited Paper (BIB009):  Digital watermarking is the process of embedding information into a digital signal. This work targets web applications that need to have hidden secure data within their Arabic e-text files. Many related watermarking techniques have been proposed as for text watermarking. However, most of them are suitable for English and cannot be generalized for different other languages such as Arabic. Arabic e-text watermarking is found having unique characteristics features that can be considered interestingly. In this paper, we are utilizing the extension Arabic character ‘Kashida’ to propose an improved method for Arabic e-text watermarking. We utilize all the extendable characters possibly fitted in words to represent some watermark bits. We embed bits within 'Kashida' characters in the cover text based on a secret key similar to classical cryptography. Our study showed that this watermarking scheme made the task of an attack much harder compared to previous similar and related methods. It also showed possibility to hide more secret data bits without degrading the security, which is believed to be attractive for web e-text data application such as preserving intellectual properties or copyright features. 
10. Abstract of Cited Paper (BIB010):  Based on the principle of zero-watermarking, this paper adopts the pronunciation of polyphone characters and the features between two polyphones of Chinese characters to construct the zero-watermarking algorithm, and produces a Chinese text watermark algorithm based on polyphone. Simulation results show that the algorithm not only meets with better invisibility and robustness, but can identify the attacks of deleting, adding and replacing characters. 
11. Abstract of Cited Paper (BIB011):  The issue of protecting scientific and intellectual products and publications ownership constitutes an important deal within the domain of securing and protecting information. Digital watermarking is used to identify the ownership of the copyright and in authentication process. In this paper, an invisible blind watermarking method utilizing the word space in Arabic text is proposed. Since the pseudo-space is very small space used to separate two parts of the same word, it is inserted before normal space and after pointed letter to hold the binary bit 'one', and it is inserted before normal space and after un-pointed letter to hold binary bit 'zero'. The comparative results obtained by testing our proposed method with some of existing Arabic watermarking methods using four variable size text samples. The results show that our method has a higher capacity ratio and imperceptibility than other watermarking techniques.

Literature Review:
The structural-based approach alters the structure or feature of the text in order to embed the necessary bits. It involves the general formatting of the properties of the cover text, in which the text content is modified using its words or sentences to hide the watermark information. The locations of words and letters or the writing style can also be altered to hide watermark bits. This includes repeating some letters or altering the features of the text. The general properties of the text are studied, and certain physical properties in the text layout are utilized in order to hide the watermark bits. The method of shifting the words and sentences upwards or downwards in order to embed watermark bits was first proposed by Brassil BIB003 , BIB001 , BIB002 . It may consist of a line shift algorithm in which the sentence moves upwards or downwards, a word shift algorithm that moves the words horizontally or feature coding where the feature of a certain text is altered in order to embed the watermark bits. A number of slightly different methods were subsequently proposed in order to improve the methods initially developed by Brassil. Analyzing the average word distance in each line also constitutes one of the structural-based methods that are being applied in watermarking, where the distance for the embedded watermark is based on some formulas BIB004 . Other methods using an algorithm based on word classifications has also been developed BIB005 . It is classified according to its features and based on class labels of the word within a segment. Another algorithm that has been proposed exploits the justified paragraphs and irregular spacing contained in the text in order to embed the desired watermark information BIB006 . A number of other new and innovative approaches have been proposed such as the method proposed by Jalil et al. BIB008 (2010). Their algorithm utilizes the content of the text in order to embed the watermark. A keyword is chosen by the text's author, and the watermark is generated based on the length of the preceding and the following word; to and from the keyword occurrence in the text. This process is illustrated in Fig. 6 , where 'is' constitutes the keyword, and the watermark is generated based on the text content. New methods for text watermarking for languages other than English, for example Chinese and Arabic, have also FIGURE 6. Watermark generation using word context BIB008 . found their way into text watermarking. The first Chinese text watermarking structural-based approach was developed by BIB007 who proposed a method that utilized the features of the pictographic characters in Chinese script. In this algorithm, the Chinese pictographic characters are utilized by splitting the character, converting the selected character's coding, and then extending the watermark redundancy space BIB007 . Other Chinese text watermarking methods also make use of the properties of Chinese characters as they offer themselves as ideal objects for hiding information. Also, a watermarking algorithm can be constructed by adopting the pronunciation of polyphone characters and the features between two polyphones of Chinese characters BIB011 . Other method uses Chinese sentences entropy, where it is calculated based on word frequency and makes crucial sentence selection based on entropy BIB011 , BIB009 . In the case of watermarks to be embedded in Arabic text content, the majority of the currently developed methods utilize the special text feature of the Arabic script characters. The characters and words are extended, letters are replace with identical character of different Unicode or the diacritics are manipulated to embed watermark bits BIB012 - BIB010 . The aforementioned structural-based algorithms use the text structure and features to manipulate the watermark bits and add them to the text. Those methods are not resistant to formatting-based attacks including copying and pasting, OCR and retyping. Some methods are resistant to printing and font changing, which depends on the technique's capability. In the case of content-based attacks, it also depends on the robustness of the proposed technique.