Digital watermarks provide the capability to insert additional information onto various media such as still images, movies, and audios, by utilizing features of the media content. Several techniques that use content features such as text or images have already been proposed for printed documents. The authors propose two new techniques using a single dot pattern and an Artificial Fiber (AF) pattern in order to address the disadvantages of conventional information hiding technologies for paper media. In this chapter, the authors describe each scheme’s characteristics, and how to improve its robustness. As a result, they have attained greater than 80% extraction rate with an information hiding capacity of 91 Kbits in the case of the single dot pattern, and a 100% extraction rate with color characters as the foreground in the case of using artificial fiber patterns.
TopIntroduction
In 2010, 69% of the incidents in which information was leaked in Japan, involved paper media, and the percentage has been increasing each year (NPO Japan Network Security Association, 2010). In addition, paper media are the major source of information that is leaked over the Internet because these are more accessible than electronic data. For this reason, information hiding technologies for paper media have become increasingly important.
There are two general techniques for hiding information in paper media:
Both techniques have its advantages and disadvantages, and there is currently no technique that meets all of the requirements of robustness, adequate information hiding capacity, fast processing speed, accuracy, and low cost.
We propose two new techniques, one using a single dot pattern (Kaneda, Nagai, Iwamura, & Hangai, 2008) and the other using an artificial fiber pattern (Kaneda, Hirano, Iwamura, & Hangai, 2008), to address the disadvantages of conventional information-hiding technologies for paper media. The single dot pattern information-hiding scheme for paper media provides both good visual quality and good information hiding capacity, which is commonly problematic with conventional techniques. The artificial fiber pattern is a texture pattern for embedding information, and owing to the random nature of the paper fiber, it is expected that there will not be visually discernible incongruities. Figure 1 illustrates the position of our proposed techniques with respect to visual quality and information hiding capacity. In this figure, (1) and (2) correspond to techniques (1) and (2) mentioned above. In this chapter, we introduce the characteristics of our schemes and explain how to improve their robustness.
Figure 1. Position of our proposed techniques with respect to visual quality and information hiding capacity
TopBackground
As mentioned in the introduction, information-hiding technologies for paper media can be divided roughly into techniques that embed digital watermarks directly into the foreground and techniques that embed information in the background using dot or line patterns.
Bender et al. proposed a technique for embedding information into a text image by shifting a word interval (Bender, Gruhl, Morimoto, & Lu, 1996). Fujii et al. proposed changing character forms to embed information (Fujii, Nakano, Echizen, & Yoshino, 2005). These have the drawback of degrading the document’s visual quality, and the information hiding capacity is influenced by the number of characters that are available.
On the other hand, techniques for embedding information in the background are not directly influenced by the foreground. Tow proposed embedding information as a difference in the printing direction of a predetermined pattern (Figure 2a) (Palo Alto Research Center Incorporated, 2012; Tow, 1994). Ito et al. proposed hiding information using a combination of two different types of line screen on a halftone dot background, which produces a predetermined gradation for a binary image (Ito, Soda, Ihara, Kimura, & Fuse, 2005).
Figure 2. Existing information-hiding patterns