Previous Studies on Data Hiding Techniques Using Special Character

Character Codes

With prosperity of the computer network, the Internet has become a very popular medium for information communication. A lot of important information is interchanged on the Internet all the time. Especially, on text-typed Internet applications like the BBS, blog, and email, people communicate messages, discuss private matters, publish articles, and even do business. But article created or sent in these activities might be tampered with illegally by hackers on the line. Therefore, it is necessary to protect these articles. In this study, we design data hiding techniques to achieve this purpose by covert communication and authentication on the Internet applications.

Articles on the Internet applications belong to soft-copy texts [3]. In recent years, some methods of data hiding via text documents have been proposed, like using file headers [4], file structures, and file features [5]. However, the Internet applications are not conventional media, so these controllable items are not found in them. Hence, we implement data hiding techniques on them by embedding special character codes. In the past, some data hiding methods about using character codes have been proposed.

Wayner [6] proposed a method to use the context-free grammar to create secret text

messages in cover files for covert communication; the secret message is not embedded in the cover file directly. And a receiver extracts the hidden message by parsing. A constraint is that the cover text should be a meaningful message; otherwise, a reader will doubt it. Bender et al. [3] proposed the use of infrequent additional spaces to form secret data and transmitted them in soft-copy texts, including inter-sentence spacing, end-of-line spacing, and inter-word spacing in texts. For example, one space between words is taken to represent a “0” and two spaces a “1.”

An illustration of the method is shown in Figure 2.1.

Data hiding techniques via special character codes are also used for some popular text documents and Internet applications. A survey of them is conducted in this chapter.

(a) (b)

Figure 2.1 Example of data hidden using white space [3]. (a) Normal text. (b) White space encoded text.

2.1.1 Review of Data Hiding Techniques via Text Documents

Every day, numerous text documents are interchanged on the Internet. It is hard to prevent malicious users from intercepting and tampering with them, so developing data hiding techniques to protect important information on them is necessary.

For the XML which is a set of rules for encoding documents in machine-readable form, Inoue et al. [7] proposed a technique to embed secret data by inserting white

spaces in tags. Representation of a tag is accomplished by including either some white spaces before the close bracket, or no white space [8]. By inserting or deleting spaces, they can embed the data preserving all meanings of original documents. For the PDF which is a popular file format with independency of different computer platforms, data hiding techniques can also be attained by using equivalent white space codes or invisible ASCII codes, as proposed by Lai and Tsai [9] and Lee and Tsai [10] An experimental result found in Lee and Tsai [10] is shown in Figure 2.2. And even for software programs like the Visual C⁺⁺ and C⁺⁺ Builders, three ways to hide data using invisible ASCII control codes were proposed by Lee and Tsai [11], including 1) alternative space coding, 2) line-end space coding, and 3) null space coding.

(a) (b)

Figure 2.2 An experimental result found in Lee and Tsai [10]. (a) Cover file seen in Adobe Reader 8.1.2 window. (b) Stego-file seen in Adobe Reader 8.1.2 window with message “This is a covert communication method”

embedded.

2.1.2 Review of Data Hiding Techniques for Internet Applications

Although many information hiding techniques have been proposed in recent

decades, methods about hiding data in Internet applications directly are very few. Lee and Tsai [12] and Huang and Tsai [13] proposed some techniques for data hiding by embedding special codes in HTML files to substitute for the original white spaces in the files, and an experimental result found in Lee and Tsai [12] is shown in Figure 2.3.

In these cases, message data were hidden in HTML files so that these files became stego-media for secret communication or secret sharing when the HTML files are displayed on the Internet. However, these methods are indirect data hiding techniques for Internet applications. In another paper published by the same authors, Lee and Tsai [1], a direct data hiding technique was proposed to embed secret data into email text line ends using special ASCII control codes. These special ASCII control codes are invisible when displayed on the screen and so will not affect a user’s reading of the resulting email.

(a)

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Figure 2.3 An experimental result found in Lee and Tsai [12]. (a) Cover text seen in the window of the IE. (b) Stego-text (with message about “Cartesian coordinates” embedded) seen in the window of the IE.

2.1.3 Review of Other Techniques and Summary

For some text documents, data hiding methods using not only special character codes but also special file syntax or file features have been proposed. Chang and Tsai [14] used pseudo-spaces, the specific string “&nbsp,” to encode copyright data into the text of an HTML file; duplicated the copyright data to enhance the robustness against HTML manipulations; and combined the blank character code and the HTML special syntax to hide data. Zhong, et al. [15] proposed a data hiding method for PDF documents by adjusting the positions of the text characters slightly to embed the secret data. They also hid data by combining character codes and certain special file features.

In conclusion, the text-typed Internet applications and text documents are good choice as a covert channel for data hiding because they are commonly used for information exchanges in daily works and for communication on the Internet. Some data hiding techniques applied on different kinds of text document formats have been proposed over the past decade. However, studies on data hiding via the Internet applications like the BBS, blog, and email are very few and even not found yet, so we will propose new data hiding techniques and related applications for them in this study.

2.2 Review of Related Character