Chapter 1 Introduction
1.3 Overview of Proposed Methods
In this study, we propose two methods for creating secret-fragment-visible mosaic images. One is for use when the source image is a full-color one, and the other is for use when the source image is a grayscale one. At the beginning, a scheme for creation of full-color secret-fragment-visible mosaic images is presented. Next, a data hiding method for covert communication is proposed utilizing the characteristics of the proposed secret-fragment-visible mosaic images creation process. Then, a method of image steganography implemented in the process of creating grayscale secret-fragment-visible mosaic images is proposed with the input image transformed from a full-color secret document. Finally, we achieve the goal of secret sharing through the use of multiple secret-fragment-visible mosaic images. Brief descriptions of these methods are given as follows.
1.3.1 Definitions of Terms
Before describing the proposed methods, some definitions of terms used in this study are introduced first as follows.
1.
Secret image: a secret image is one which is chosen as the source image to
produce a secret-fragment-visible mosaic image.2. Target image: a target image is an image selected from a database according
to a secret image, which is the picture stored in the database most similar to8
the secret image.
3. Target image database: a target image database is a database which store a
large number of target images.4.
Tile image: a tile image is one of the square pieces resulting from crumbling
a secret image.5.
Target block: a target block is the place that a tile image of the same size
should be fitted into.6.
Secret-fragment-visible mosaic image: a secret-fragment-visible mosaic
image is obtained by rearranging the fragments of a secret image in a certain way, yielding a visual effect like the target image.7.
Creation process: a creation process produces a secret-fragment-visible
mosaic image from a secret image.8.
Recovery process: a recovery process reconstructs the secret source image
from a secret-fragment-visible mosaic image.9.
Recovery sequence: a recovery sequence is a sequence which records the
corresponding labels of the tile images and the target blocks, based on which the recovery process can be conducted to retrieve the secret image.10. Image steganography: image steganography is a scheme to embed data into images for covert communication, which, though obvious, transmits the secret imperceptibly.
11. Embedding process: an embedding process hides secret data into an image.
12. Extraction process: an extraction process retrieves secret data from an image.
1.3.2 Brief Description of Proposed Method for
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Creation of Secret-fragment-visible Mosaic Images
The main steps of the proposed creation process of secret-fragment-visible mosaic images are described in Figure 1.5. First, we construct a target image database which records the color histogram of the stored images. Second, based on a given secret image, a similarity measure proposed in this study is computed and used to choose a target image from the database. Then, the secret image is divided into tile images, which then are rearranged to fit a target image to create a secret-fragment-visible mosaic image. The detailed creation process will be introduced in Chapter 3.
Figure 1.5 Creation process of secret-fragment-visible mosaic image.
1.3.3 Brief Description of Proposed Method for
Covert Communication via
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covert communication scheme via full-color secret-fragment-visible mosaic images by embedding a source-image recovery sequence and switching the orders of target blocks. A modified version of the secret-fragment-visible mosaic image creation process for use as the secret message embedding process for cover communication is shown in Figure 1.6. First of all, we calculate the 1-dimensional color histograms of a given secret image and a selected target image. Second, the secret messages which are transmitted by users are transformed into a bit string. With the histogram values of a secret image and a target image, we switch the corresponding target blocks of the tile images which are in the same histogram according to each bit to be embedded. After these steps, a secret-fragment-visible mosaic image into which the secret messages are hidden is created by the use of the modified fitting target blocks and a secret key.
Finally, a source-image recovery sequence is embedded into the resulting image by a scheme of lossless least significant bit (LSB) modification. With the recovery sequence, the secret image can be retrieved quickly and easily. The detailed algorithm is presented in Chapter 4.
Figure 1.6 Embedding process of data hiding by switching the orders of tiles.
Secret
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1.3.4 Brief Description of Proposed Method for
Image Steganography via Secret-fragment-visible Mosaic Images
The proposed method for image steganography is achieved in the process of creating grayscale secret-fragment-visible mosaic images. Grayscale images are still being used frequently in modern days. People can utilize software packages (many available on the Internet) to transform secret documents, such as e-mail, PDF and Microsoft WORD, into grayscale images. We use this type of image to create a secret-fragment-visible mosaic image in order to disguise the transformed secret image as a grayscale art image. With this technique, while we deliver the document disguised as a grayscale mosaic image to other people, hackers will hardly notice that the file is carrying a secret image. And because the transformed image keeps the readability of the original documents by image fragments, a receiver can understand what the document content is after he/she regains the secret image from a secret image recovery process. The detailed algorithms will be given in Chapter 5.
Figure 1.7 Image steganography process by creating grayscale secret-fragment-visible mosaics.
Secret
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1.3.5 Brief Description of Proposed Method for Secret Sharing via Secret-fragment-visible Mosaic Images
A method for secret sharing is proposed in this study. It is based on partitioning secret tile images into a number, say n, of sections and using them separately to create secret-fragment-visible mosaic images. As shown in the process illustrated by Figure 1.8, first, we select n target images from a database based on a secret image, and then divide the secret image into tile images of appropriate sizes. After that, we use a key to decide which target image can select tile images first, and then fit the selected tile image to the corresponding target block. The target images will take turns in this way of picking appropriate tile images.
According to the above operations, n secret-fragment-visible mosaic images are created from a secret image. Then, the recovery sequence of every mosaic image is divided into n
1 parts and hidden in the resulting share images. In this way, the
secret image will be retrieved only if all of the share images are collected together.The detailed operations are described in Chapter 6.
Figure 1.8 Process of secret sharing through secret-fragment-visible mosaic images.
Secret
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