{'fraction_non_alphanumeric': 0.02515498330948975, 'fraction_numerical': 0.006437768240343348, 'mean_word_length': 5.423430321592649, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Researcher’s Perspective of Substitution Method on Text Steganography

The linguistic steganography studies are still in the stage of development and empowerment practices. This paper will present several text steganography on substitution methods based on the researcher’s perspective, all scholar paper will analyse and compared. The objective of this paper is to give basic information in the substitution method of text domain steganography that has been applied by previous researchers. The typical ways of this method also will be identified in this paper to reveal the most effective method in text domain steganography. Finally, the advantage of the characteristic and drawback on these techniques in generally also presented in this paper.


Introduction
Steganography can be defined as associated knowledge of hiding the messages via medium of data to become invisible and undetectable for human sense. Secure privacy information is critical point of steganography in applying performance as a part of information hiding. The implementation of steganography itself, the methods can be divided into two categories; steganography in medium of image, audio, video and other digitally invisible code named technical steganography. Therefore, this paper is specifically focusing in linguistic steganography using substitution method. Generally, the function of steganography in text domain basically can be formulized using equation showed on Figure 1. Basically, this process analogically can be illustrated using Prisoner's Problem. The analogy is represented in Figure 2, Alice is sending an original text (M) along with a cover message (C) in order to process embedding known as stego text (S) containing a stego key (K).  discover the message as suspicious thing. Then, Bob will figure out e − l(s) = {M, C} in order to retrieve original text M and cover message C with a stego key K for decoding the process use function. The process of embedding and extracting the information of stego text S will be known by Wendy, so that, we can clarify the process steganography is successful.

Figure 2: A typical steganography processes
Based on that idea, the general objective is signifying the development of steganography technique about technique steganography in medium of text that obtain from past researchers' effort. The implementation of steganography in text domain can be classified into two categories; format based steganography and linguistic steganography. Therefore, the main concentration of this study is about linguistic steganography and also discussing about text steganography for comparison, with objective to signify the classification of technique. For linguistic steganography, it covers messages which modified the information that encoded the massage based on order of linguistically [1]. Whereas, format based steganography is used in covering messages which manipulated the component in text such as, feature in word, space, line and any other character in sentence of text.
Linguistic steganography is a set of methods and techniques that permit the hiding of any digital information within texts based on some linguistic knowledge [2] [3] that will provide more detailed definition. It not only requiring the stenographic cover be composed of natural language text or some sort, but the text itself is either generated to have a cohesive linguistic structure, or that the cover text is natural language text to begin with.
Furthermore, natural language steganography or linguistic steganography is a technique which aims to hide secret information in text document by manipulating the semantic and/or syntactic structures of the sentence [4]. From the definitions above, we can conclude that the main idea of linguistic steganography is to hide secret message using linguistic knowledge.

Previous Study of Substitution Method on Text Steganography
Linguistic steganography can cover the hidden message concerning language of word and order modification linguistically. The implementation of this technique typically uses substitution method to enhance the linguistic steganography. Table 1 below show the previous studies of researcher that have been applied. Based on Table 1, most researchers had studies many kinds of substitution method, but there are some substitution methods were not categorized as linguistic because of the substitution method had change the format of the language itself. So, it proves that, most of substitution methods are linguistic steganography.
There are some advantages in linguistic steganography. Firstly, unlike format based steganography, linguistic steganography especially in synonym substitution technique has own implementation performance; English text using LUNABEL function [5] and high invisibility in Malay linguistic technique [6], simple variant in Chinese text synonym [7] minimalized creating syntax error in English text or Arabic text using context-based [8] [9] [10] and in English text using mark-insertion can achieve maximum cumulative distortion [11]. Secondly, one of the techniques which is traditional synonym substitution enable to hide the hidden message in large capacity [12] [13]. Thirdly, linguistic steganography also has advantage in specific condition linguistic steganography which can be useful in printing text using synonym substitution [14]. Another technique that has this advantage is synonym replacement which is very useful in Spanish language [15]. Moreover, there are also using HTML as the ID attribute and Short Message Service (SMS) in their technique [16] [17] Then, they also identified the disadvantages in this technique. Firstly, low security can also be the issue in linguistic steganography [12] and synonym paraphrasing technique in Spanish language is easy to be attack by intruders [15]. Secondly, the limitation implementing of linguistic steganography is only capable in own language because this technique is based on linguistic. This technique has complex algorithm in English text using LUNABEL function [5] and using traditional synonym substitution [13]. In English text, using context-based has incomplete vocabulary [8]. Then, semantic transformation technique has the possibility generates a lot of semantic spam. Plus, in Malay linguistic, the issue is, it was consuming much in process embedding/extracting hidden message [6]. Most researcher also use synonym substitution as their method to hide secret text, but not secret message. The secret message still easily predictable.
For the format based, they use manipulation of substitution text to change the structure on secret message. As example, some of the use object like Persian/Arabic Unicode [18] [19] [20], Hindi text [21], Telugu text [22] [23], or emoticon symbol [24] on the text steganography to hide secret message. There is a lot of efforts which have been proposed by previous researcher. The number of the research on linguistic based is much higher than format based refer on Figure 1. Averagely, most widely researched by previous researcher especially in substitution method using synonym substitution.

Conclusion
The field studies in linguistic steganography were very interesting to researchers. This paper presented to explore several techniques in steganography methods of linguistic steganography in order to observe the development of these methods previously. In linguistic steganography, the techniques covered the hidden message concerning language of word and order modification linguistically. Moreover, format based and linguistic steganography almost has similar advantage in development, performance, and implement for hiding hidden message. Some techniques have recommended secure protection, high performance, or can embed large amount the hidden message. However, both techniques also have certain issues which seems to be the limitation of those techniques; low security, complex algorithm performance, or time consuming. As the majority of past researcher apply synonym on their studies, the synonym seem like very weak to protect the secret message in stego text, human can predict the secret message easily. In the future studies, researchers could apply antonym as replacement to the synonym substitution since it can hide real secret message compared to the synonym. At the future, there will be more investigation about on this field will be consider.