COMPARISON OF DATA HIDING USING LSB AND DCT FOR IMAGE ... · Steganography is the art and science of hiding information by embedding data into cover media. There are two main domains

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COMPARISON OF DATA HIDING USING LSB AND DCT FOR IMAGE.

ZAMZAM HOHAMED AHMED

A project report submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Computer Science (Information Security)

Faculty of Computer Science and Information Technology

Universiti Tun Hussein Onn Malaysia

DECEMBER 2014

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ABSTRACT

Steganography is the art and science of hiding information by embedding data into cover

media. There are two main domains namely spatial and frequency domain. In this

project, a comparison is made between hiding in spatial domain using LSB technique

and hiding in frequency domain using DCT technique. Various BMP cover images with

256x256 and 512x512 resolutions for LSB and DCT techniques are used in the

experimentations. For DCT technique, various BMP image is converted to JPEG image.

Then data is hidden into the JPEG cover image using DCT technique. After that, the

JPEG stego image is converted back to BMP stego image. These BMP stego images

from LSB and DCT are compared using PSNR. Results from these experiments show

that 75% of the stego images hidden using LSB has shown higher PSNR values than

stego images hidden using DCT. This means that the stego image hidden using LSB has

shown a much closer similarity to the cover image than stego image hidden using DCT,

thus much harder to detect hidden data in the stego image by LSB.

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ABSTRAK

Steganografi adalah seni dan sains menyembunyikan maklumat dengan memasukkan

data ke dalam cover media. Terdapat dua domain utama iaitu domain spatial dan

frequency. Dalam projek ini, perbandingan yang dibuat antara menyembunyikan data di

dalam domain spatial menggunakan teknik LSB dan menyembunyikan data di dalam

domain frekuensi menggunakan teknik DCT. Pelbagai cover image BMP dengan

resolusi 256x256 dan 512x512 untuk teknik LSB dan teknik DCT digunakan dalam

eksperimen. Untuk teknik DCT, imej BMP ditukar kepada imej JPEG. Kemudian data

disembunyikan ke dalam JPEG menggunakan teknik DCT. Selepas itu, imej stego JPEG

ditukar kembali ke imej stego BMP. Imej stego BMP dari LSB dan DCT dibanding

menggunakan PSNR. Hasil daripada eksperimen ini menunjukkan bahawa 75% daripada

imej stego yang disembunyikan data menggunakan LSB telah menunjukkan nilai PSNR

yang lebih tinggi daripada imej stego yang disembunyikan data menggunakan DCT. Ini

bermakna bahawa imej stego menggunakan LSB telah menunjukkan persamaan yang

lebih dekat dengan cover image daripada imej stego yang disembunyi menggunakan

DCT, oleh itu jauh lebih sukar untuk mengesan data yang tersembunyi dalam imej stego

dengan LSB.

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TABLE OF CONTENTS

TITLE i

DECLARATION ii

DEDICATION iv

ACKNOWLEDGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

ABBREVIATIONS xiii

CHAPTER 1 INTRODUCTION 1

1.1 Background 1

1.1 History of steganography 2

1.2 Problem statement 2

1.3 Objectives 3

1.4 Scope 3

1.5 Dissertation outline 4

CHAPTER 2 LITERATURE REVIEW 5

2.1 The basic framework of steganography 5

2.2 The purpose of steganography 7

2.3 Types of steganography 7

2.3.1 Text steganography 8

2.3.2 Audio steganography 9

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2.3.3 Image steganography 9

2.4 Image format 9

2.4.1 Tiff file 10

2.4.2 Gif file 10

2.4.3 Bmf file 10

2.4.4 Jpeg file 11

2.5 Image compression 11

2.5.1 Jpeg compression 12

2.6 Steganography domain and technique 12

2.6.1 Spacial domain 13

2.6.1.1 Least significant bit Technique 13

2.6.1.1.1 Lsb algorithm 15

2.6.2 Frequency domain 16

2.6.2.1 Discrete cosine transform technique 16

2.6.2.2 Discrete wavelet transform 18

2.7 Peak signal to noise ratio (PSNR) 18

2.8 Summary 19

CHAPTER 3 METHODOLOGY 20

3.1 The proposed method 20

3.1.1 Proposed framework 21

3.2 Least significant bit technique 22

3.3 Discrete cosine transform technique 24

3.4 Comparison and measurement 25

2.5 Summary 26

CHAPTER 4 IMPLEMENTATION 27

4.1 Introduction 27

4.2 Least significant bit technique 27

4.2.1 Lsb embedding algorithm in matlab 28

4.2.2 Lsb extracting algorithm in matlab 29

4.3 Discrete cosine transform technique 29

4.3.1 Dct embedding algorithm in matlab 30

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4.3.2 Dct extracting algorithm in matlab 33

4.4 The peak signal to noise ratio for Dct and lsb 35

4.5 Summary 36

CHAPTER 5 RESULT AND DISCUSSION 37

5.1 Introduction 37

5.2 Least significant bit (LSB) technique 37

5.2.1 Lsb embedding algorithm result 38

5.2.2 Lsb extracting algorithm result 39

5.3 Discrete cosine transform (DCT) technique 40

5.3.1 Dct embedding algorithm result 40

5.3.2 Dct Extracting algorithm result 41

5.4 Comparison and measurement 42

5.4.1 Analysis of test result for lsb 47

5.4.2 Analysis of test result for dct 48

5.4.3 Analysis of test result for lsb and dct 49

5.5 Summary 51

CHAPTER 6 CONCLUSION 52

6.1 Conclusion 52

6.2 Future work 53

REFERENCES 54

APPENDIX 57

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LIST OF TABLES

5.1 BMP images using LSB 43

5.2 BMPimages using DCT 44

5.3 LSB stego images 256x256 45

5.4 LSB stego images 512x512 45

5.5 DCT stego images 256x256 46

5.6 DCT stego images 512x512 46

5.7 Summary of the results 50

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LIST OF FIGURES

2.1 The different embodiment disciplines of information hiding 6

2.2 General model of Steganography 8

2.3 Algorithm of Least Significant Bit 15

2.4 Block diagram of JPEG encoder and decoder 17

2.5 One Dimensional DCT 17

2.6 Two Dimensional DCT 18

3.1 Proposed Framework 21

3.2 LSB embedding algorithm 23

3.3 LSB extraction algorithm 23

3.4 DCT based embedding algorithm 24

3.5 DCT based extraction algorithm 25

4.1 LSB based embedding implementation code in Matlab 28

4.2 LSB based extracting implementation code in Matlab 29

4.3 DCT based embedding implementation code in Matlab 31

4.4 DCT based extracting implementation code in Matlab 34

4.5 Matlab code for PSNR of the LSB stego image 35

4.6 Matlab code for PSNR of the DCT stego image 36

5.1 The result obtained by running LSB embedding algorithm 38

5.2 The result obtained by running LSB extraction algorithm 39

5.3 The result obtained by running DCT embedding algorithm 40

5.4 The result obtained by running DCT extracting algorithm 41

5.5 LSB stego images 512x512 and 256x256 47

5.6 DCT stego images 512x512 and 256x256 48

5.7 Stego images 512x512 resolutions for LSB and DCT 49

5.8 Stego images 256x256 resolutions for LSB and DCT 50

xiii

LIST OF ABBREVIATIONS

LSB Least Significant Bit

MSB Most Significant Bit

DCT Discrete Cosine Transform

DWT Discrete Wavelet Transform

DFT Discrete Fourier Transform

PSNR Peak Signal-to-Noise Ratio

MSE Mean Squared Error

HVS Human Visibility System

TIFF Tagged Image Format File

GIF Graphic Interchange Format

BMP Bitmap

JPEG Joint Photographic Experts Group

ASCII American Standard Code for Information

Interchange

2D-DCT Two-Dimensional Discrete Cosine Transform

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CHAPTER 1

INTRODUCTION

1.1 Background

Steganography is the art and science of communicating in a way that the presence of a

secret message apart from the identity of the sender and intended recipient cannot be

detected by unauthorized users. Steganography is a technique which is used to hide a

secret message within a cover media in such a way that others cannot detect the presence

of the hidden message. Steganography is made from the Greek words steganos meaning

"covered or protected" and graphei meaning "writing". While classical cryptography is

about concealing the content of messages, steganography is about concealing their

existence.

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1.1.1 History of Steganography

Steganography goes back to ancient times and used by different cultures such as:

Greeks, Chinese, and medieval Europe. A famous which case dates back to 1586, when

Mary Queen of Scots was conspiring to have Queen Elizabeth of England assassinated,

with a view to taking over the English throne [1]. Also during the 1980’s, Margaret

Thatcher became so irritated at press leaks of cabinet documents that she had the word

processors programmed to encode their identity in the word spacing, so that disloyal

ministers could be traced [2]. Similar techniques are now undergoing trials in an

electronic publishing project, with a view to hiding copyright messages and serial

numbers in documents. In some applications, it is enough to hide the identity of either

the sender or the recipient of the message, rather than its very existence [3]. Modern

steganography entered the world in 1985 with the advent of the personal computer being

applied to classical steganography problems. Also in modern Steganography practice the

larger the cover message is relative to the hidden message, the easier it is to hide the

latter [4].

1.2 Problem Statement

Today the growth in the information technology, especially in computer networks such

as Internet, Mobile communication, and Digital Multimedia applications has opened

new opportunities in scientific and commercial applications. But this progress has also

led to many serious problems such as hacking, duplications and malicious usage of

digital information. Usually message such as pictures can be altered by unauthorized

user between the sender and the receiver of the message so that the receiver of the

message cannot verify the integrity of the message which compromises one of the goals

of information security. With the help of steganography messages can be hidden inside

the digital media /cover object for transmission, because steganography can be applied

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differently in digital image, audio and video file, it’s difficult to detect, only receiver can

detect it and it can be done faster with the large number of softwares.

1.3 Objectives

The Objectives of this project is to:

1. To implement steganography images based on Least Significant Bit (LSB) and

Discrete Cosine Transform (DCT) techniques in Matlab

2. To test these techniques using cover images with 256x256 and 512x512 resolutions

3. To compare and analyze using Peak Signal to Noise Ratio (PSNR)

1.4 Scope

The scope of this research is:

1. Hiding data in image steganography

2. Using Least Significant Bit (LSB) and Discrete Cosine Transform (DCT)

3. Do testing on cover images with 256x256 and 512x512 resolutions only

4. BMP file format is chosen to be used in this dissertation due to their higher

resolution compared to other images. The BMP files are uncompressed, hence they

are large. The advantage of using BMP files is simplicity and wide acceptance of

BMP files in windows programs

5. To make apple-to-apple comparison of LSB and DCT, BMP is converted to JPEG

inorder to use DCT technique. the JPEG stego is then converted back to BMP stego.

These BMP tego from LSB and DCT are then compared using PSNR

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1.5 Dissertation Outline

The organization of this dissertation is as follows: Chapter 2 describes the different types

of steganography and reviews of the existing researches related to steganography

techniques. Chapter 3 describes the proposed framework to embed the secret message

inside the steganography images and extract the hiding data back from the stego images.

Chapter 4 describes the implementation of the proposed algorithm. Chapter 5 discusses

and analyze the experimental result of the approaches and its performance comparison.

Finally, Chapter 6 describes conclusion and future work of the dissertation.

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CHAPTER 2

LITERATURE REVIEW

2.1 The Basic Framework of Steganography

An Example of steganography can be given in terms of communication between two

people, Alice and Bob, where Alice and Bob are two inmates who wish to communicate

in order to exchange some secret information. However, all communication between

them is examined by the eavesdropper, Wendy, the third party who will try hardly to

disclose, alter and/or destruct their secret message.

Specifically, in the general model for steganography, illustrated in Figure 2.1, Alice

wishes to send a secret message m to Bob. In order to do so, she "embeds" m into a

cover-object c, and obtains a stego-object s. The stego-object s is then sent through the

public channel. Thus we have the following definitions:

i. Cover-object: is the object used as the carrier to embed messages into many

different objects have been employed to embed messages into for example

images, audio, and video as well as file structures, and html pages to name a few.

ii. Stego-key: is the code that the sender of the secret message is going to use to

embed the message into the cover-object. This same stego-key will be used by

the recipient to extract the secret message.

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iii. Stego-object: is the combination of the cover object, the stego-key and the secret

message.

In a pure steganography framework, the technique for embedding the message is

unknown to Wendy and shared as a secret between Alice and Bob. However, it is

generally considered that the algorithm in use is not secret but only the key used by the

algorithm is kept as a secret between the two parties, this assumption is also known as

Kerchoff's principle in the field of cryptography. The secret key, for example, can be a

password used to seed a pseudo-random number generator to select pixel locations in an

image cover-object for embedding the secret message (possibly encrypted). Hence, there

is a need for a cover media, stego function, stego- key and the secret message to be

hidden. The cover media can be a plaintext, still image, video and audio. Performing

data hiding in Image was studied in a wide variety of literatures.

Figure 2.1: General model of Steganography [5]

Figure 2.1: General model of Steganography [5].

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2.2 The Purpose of Steganography

According to the major objective of steganography is to prevent some unintended

observer from stealing or destroying the confidential information. There are some

factors to be considered when designing a steganography system: [7]

i. Invisibility: Invisibility is the ability to be unnoticed by the human.

ii. Security: Even if an attacker realizes the existence of the information in the stego

object it should be impossible for the attacker to detect the information. The

closer the stego image to the cover image, the higher the security. It is measured

in terms of Peak Signal to Noise Ratio (PSNR).

2.3 Types of Steganography

The four main types of Steganography are digital image steganography, audio

steganography, text steganography and video steganography. Figure 2.2 shows the

overall structure of System Security which basically consists of Cryptography and

Information Hiding which has been divided also into another two categories which are

Watermarking and Steganography. This study will focus on information hiding using

digital image steganography.

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A cover object is the object designated to carry the embedded bits or secret information.

The cover objects can be a text file, image file, audio file or video file. These different

type of steganography with various cover objects are discussed further in the following

sections.

2.3.1 Text Steganography

Historically hiding information in the text was a simple and the most important method

of steganography but Due to the beginning of the Internet and due to the different type

of digital file formats it has decreased in importance. Text steganography using digital

files is not used very often because the text files have a very small amount of redundant

data [8].

Figure 2.2: The different embodiment disciplines of information hiding [6].

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2.3.2 Audio Steganography

Audio steganography is masking, which exploits the properties of the human ear to hide

information unnoticeably. An audible, sound becomes inaudible in the presence of

another louder audible sound. This property allows the selection of the channel in which

the information will be hidden. Although it is similar to images in steganographic

potential, the larger size of meaningful audio files makes them less likely to use than

images [8].

2.3.3 Image Steganography

Image Steganography will be used as cover object or host image for this project because

Images are considered as the most popular file formats used in steganography. They are

known for constituting a non-causal medium, due to the possibility to access any pixel of

the image at random. In addition, the hidden information could remain invisible to the

eye. However, the image steganography techniques will exploit "holes" in the Human

Visual System (HVS) [8] [9].

2.4 Image Format

this research fucusses on some specific image formats. The followings are the formats

that this research fucuss on.

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2.4.1 TIFF File

Tagged Image Format File (TIFF) is an image format file for high quality graphics. TIFF

files were created in the 1986 as a file format for scanned imaged in an attempt to get all

companies to use one standard file format instead of multiple. Though TIF files

originally only supported black and white, the update in 1988 added a color palette [10].

2.4.2 GIF File

Graphics Interchange Format is used for the purpose of storing multiple bitmap images

in a single file for exchange between platforms and images. It is often used for storing

multibit graphics and image data. GIF is not associated with a particular software

application but was designed “to allow the easy interchange and viewing of image data

stored on local or remote computer systems” [10] [11].

2.4.3 BMP File

The letters “BMP” stand for “bitmap”, Bitmap images were introduced by Microsoft to

be a standard image file format between users of their Windows operating system. The

file format is now supported across multiple file systems and operating systems, but is

being used less and less often. A key reason for this is the large file size, resulting from

poor compression and verbose file format. This is, however, an advantage for hiding

data without raising suspicion. To understand how bitmap images can be used to conceal

data, the file format must first be explained. A bitmap file can be broken into two main

blocks, the header and the data. The header, which consists of 54 bytes, can be broken

into two sub-blocks. These are identified as the Bitmap Header, and the Bitmap

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Information. Images which are less than 16 bit have an additional sub-block within the

header labeled the Color Palette [12] [13].

2.4.4 JPEG File

Joint Photographic Experts Group (JPEG) format is one of the Transform Domain

Techniques which has an advantage over LSB techniques because they hide information

in areas of the image that are less exposed to compression, cropping, and image

processing [9]. Also JPEG is most common image file format on the internet owing to

the small size of resultant images obtained by using it, and it is efficient for appearing

the stage image to something similar to the original image [14].

2.5 Image Compression

When working with larger images of greater bit depth, the images tend to become too

large to be transmitted over a standard Internet connection. In order to display an image

in a reasonable amount of time, techniques must be incorporated to reduce the image’s

file size. These techniques make use of mathematical formulas to analyze and condense

image data, resulting in smaller file sizes. This process is called compression. In images

there are two types of compression: lossy and lossless [9].

i. Lossless compression is known for being preferable when the original data

should stay in its entirety. In this manner, the original image information will

never be removed, and this makes it possible the reconstruction of the original

data from the compressed data. This is typical of images in GIF and BMP.

ii. Lossy compression saves storage space by discarding the points the human eyes

find difficult to identify. In this case the resulting image is expected to be

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something similar to the original image, but not the same as the original. JPEG

compression uses this technique.

2.5.1 JPEG Compression

The process of embedding information during JPEG compression results in a stego

image with a high level of invisibility, since the embedding takes place in the transform

domain. Originally it was thought that steganography would not be possible to use with

JPEG images, since they use lossy compression which results in parts of the image data

being altered. JPEG images are the products of digital cameras, scanners, and other

photographic image capture devices. This is simply why concealing secret information

in JPEG images might provide a better disguise [9] [15].

For JPEG, the Discrete Cosine Transform (DCT) is used. It is important to recognize

that the JPEG compression algorithm is actually divided into lossy and lossless stages.

The DCT and the quantization phase form part of the lossy stage, while the Huffman

encoding used to further compress the data is lossless. Steganography can take place

between these two stages. Using the same principles of LSB insertion the message can

be embedded into the least significant bits of the coefficients before applying the

Huffman encoding. By embedding the information at this stage, in the transform

domain, it is extremely difficult to detect, since it is not in the visual domain [16].

2.6 Steganography Domains and Techniques

Over the past years, many literatures discussed the technique of information hiding. Up

until now, there are two techniques developed in information hiding, spatial-domain

manner and frequency-domain manner. Owing to the fact that the media considered in

these literatures are image illustrations, we therefore will include images in our

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discussions. The so-called spatial-domain refers to the fact that the secret is mixed into

the distributed pixels (regions) directly. While in the frequency-domain, it is necessary

to transform the host-image first using a frequency-oriented mechanism, such as a

discrete cosine transformation based (DCT-based), wavelet-based, etc., after which the

secret is then combined with the relative coefficients in the frequency-form image. Let

us take another look at the spatial-domain manner. Generally speaking, it is simpler to

achieve the goal of information hiding in the course of secret embedding. The least

significant bit (LSB for short) secret embedding or LSB-like embedding is the most

commonly used method in the spatial-domain approach [17].

2.6.1 Spatial Domain

These techniques use the pixel gray levels and their color values directly for encoding

the message bits. These techniques are some of the simplest schemes in terms of

embedding and extraction complexity. The major drawback of these methods is amount

of additive noise that creeps in the image which directly affects the Peak Signal to Noise

Ratio and the statistical properties of the image. Moreover these embedding algorithms

are applicable mainly to lossless image-compression schemes like TIFF images. For

lossy compression schemes like JPEG, some of the message bits get lost during the

compression step. The most common algorithm belonging to this class of techniques is

the Least Significant Bit (LSB) replacement technique.

2.6.1.1 Least Significant Bit Technique

Popular steganographic tools based on LSB embedding vary on the existing approaches

for hiding information. Some algorithms change LSB of the pixels visited in a random

walk, others modify pixels in certain areas of images, or instead of just changing the last

bit they increment or decrement the pixel value. The concept of least significant bit

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(LSB) Embedding is simple. It exploits the fact that the level of precision in many image

formats is far greater than that perceivable by the standard of human vision. Therefore,

an altered image with slight variations in its colors will be indistinguishable from the

original by a human being, just by looking at it.

When using the least significant bit of the pixels' color data to store the hidden message,

the image itself is seemed unaltered. Modulating the least significant bit does not result

in human-perceptible difference because the amplitude of the change is small. To hide a

secret message inside an image, a proper cover image is needed. It is necessary to use a

lossless compression format, because this method uses bits of each pixel in the image,

otherwise the hidden information will get lost in the transformations of a lossy

compression algorithm. When using a 24-bit color image, a bit of each of the red, green

and blue color components can be used, so a total of 3 bits can be stored in each pixel.

For example, the following grid can be considered as 3 pixels of a 24-bit color image,

using 9 bytes of memory:

(00100111 11101001 11001000)

(00100111 11001000 11101001)

(11001000 00100111 11101001)

When the character A, which binary value equals 10000001, is inserted, the following

grid results:

(00100111 11101000 11001000)

(00100110 11001000 11101000)

(11001000 00100111 11101001)

In this case, only three bits are needed to be changed to insert the character successfully.

On average, only half of the bits in an image will be needed to be insider modified to

hide a secret message using the maximal cover size. The result changes that are made to

the least significant bit are too small to be recognized by the human visual system

(HVS), so the message is effectively hidden [18].

The least significant bit of the third color remains without any changes. It can be used

for checking the correctness of 8 bits which are embedded in these 3 pixels.

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2.6.1.1.1 LSB Algorithm

i. Select a cover image of size M*N as an input.

ii. The message to be hidden is embedded in RGB component only of an image.

iii. Use a pixel selection filter to obtain the best areas to hide information in the

cover image to obtain a better rate. The filter is applied to Least Significant Bit

(LSB) of every pixel to hide information, leaving most significant bits (MSB).

iv. After that Message is hidden using Bit Replacement method.

Figure 2.3: Algorithm of Least Significant Bit [19].

Message to be

Hidden

Carrier Image

RGB

Component

Pixel Filtering

Least Significant Bit

replacement method

Stego Image

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2.6.2 Frequency Domain

These techniques are applied in encoding message bits in the transform domain

coefficients of the image. Data embedding performed in the transform domain is widely

used for robust watermarking. Similar techniques can also realize large capacity

embedding for steganography. Candidate transforms include Discrete Cosine Transform

(DCT), Discrete Wavelet Transform (DWT), and Discrete Fourier Transform (DFT). By

being embedded in the transform domain, the hidden data resides in more robust areas,

spread across the entire image, and provides better resistance against signal processing.

For example, we can perform a block DCT and, depending on payload and robustness

requirements, choose one or more components in each block to form a new data group

that, in turn, is pseudo randomly scrambled and undergoes a second-layer

transformation. Modification is then carried out on the double transform domain

coefficients using various schemes. These techniques have high embedding and

extraction complexity. Because of the robustness properties of transform domain

embedding, these techniques are generally more applicable to the “Watermarking”

aspect of data hiding. Many steganographic techniques in these domain have been

inspired from their watermarking counterparts [20] [21].

2.6.2.1 Discrete Cosine Transform Technique

DCT is a method of hiding information in transform domain images. This method hides

messages in significant areas of the cover image. The DCT transforms a signal from an

image representation into a frequency representation, by grouping the pixels into 8 × 8

pixel blocks and transforming the pixel blocks into 64 DCT coefficients each A

modification of a single DCT coefficient will affect all 64 image pixels in that block.

The next step is the quantization phase of the compression. Here another biological

property of the human eye is exploited: The human eye is fairly good at spotting small

differences in brightness over a relatively large area, but not as good as to distinguish

17

between different strengths in high frequency brightness. This means that the strength of

higher frequencies can be diminished, without changing the appearance of the image

[15] [22].

Figure 2.4: Block diagram of JPEG encoder and decoder [15].

DCT Algorithm

a) The One-Dimensional DCT

n = size, p = pixel, G = coefficients

Figure 2.5: One Dimensional DCT [22].

1

0 2

)12(cos

2

1 n

t n

ftpCG tff

0,1

0,2

1

f

fCf

18

b) The Two-Dimensional DCT

n

ix

n

jypCC

nG

n

x

n

y

xyjiij

2

)12(cos

2

)12(cos

2

1 1

0

1

0

n = size, p = pixel, G = coefficients

Figure 2.6: Two Dimensional DCT [22].

2.6.2.2 Discrete Wavelet Transform

Wavelets are special functions which (in a form analogous to sins and cosines in Fourier

analysis) are used as basal functions for representing signals. The simplest DWT is Haar.

In Haar-DWT the low frequency wavelet coefficient are generated by averaging the two

pixel values and high frequency coefficients are generated by taking half of the

difference of the same two pixels. A signal is passed through a series of filters to

calculate DWT [23].

2.7 Peak Signal to Noise Ratio (PSNR)

The Peak Signal to Noise Ratio (PSNR) is an engineering term for the well-known

objective image quality metrics that uses for image measuring. Where PSNR formula is:

[7] [24]

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PSNR formula.

Where MAXI is the maximum possible pixel value of the image and MSE is Mean

Square Error.

The high PSNR value indicates high security because it indicates the minimum

difference between the original and stego values. So no one can suspect the hidden

information.

Capacity: The amount of information that can be hidden relative to the size of the

cover object without deteriorating the quality of the cover object.

Robustness: It is the ability of the stego to withstand manipulations such as

filtering, cropping, rotation and compression.

2.8 Summary

This chapter discusses basic frame work of steganography, the purpose os

steganography, types of steganography and different file formats of image

steganography. It also discusses in detail the steganography techniques, Least

Significant Bit (LSB) and Discrete Cosine Transform (DCT) and The Peak Signal to

Noise Ratio (PSNR) as a well-known objective image quality metrics that uses for image

measuring.

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CHAPTER 3

METHODOLOGY

3.1 The Proposed Method

Comparison of Spatial domain technique with Frequency domain technique has been

proposed to analyze which technique is more secure by having a high Peak Signal to

Noise Ratio (PSNR). As we know the goal of Steganography is to secure

communications from an eaves- dropper, steganographic techniques strive to hide the

very presence of the message itself from an observer. So in order to reach a high

security and advanced method the Least Significant Bit will be compared to the Discrete

Cosine Transform and the performance of the two techniques will be measured using the

Peak Signal to Noise Ratio (PSNR).

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3.11 Proposed Framework

In this study data hiding using Least Significant Bit (LSB) technique in Spatial Domain

will be compared to data hiding using Discrete Cosine Transform (DCT) technique in

Frequency Domain and the cover object that will be used to hide the data is cover image.

Figure 3.1 below is showing the proposed framework for this dissertation.

Figure 3.1: Proposed Framework

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As shown in Firgure 3.1. In order to exchange the secret message in a secure way that

prevent the eavesdropper from recognizing, hacking and/or altering the stego image and

the secret message the sender and the Receiver will use the Least Significant Bit (LSB)

technique and Discrete Cosine Transform (DCT) technique. The Sender wants to send a

secret message to the Receiver where the Attacker is the third part or the eavesdropper

who wants to attack the secret message in order to change according to his/her personal

proposes. To prevent the Attacker from succeeding with his/her mission and even

recognizing the stego image from the other images, the proposed framework has two

stages. In the first stage, the secret message is embedded in the BMP cover image by

using Least Significant Bit’s embedding algorithm. then, the stego image is sent by the

sender to the receiver where the receiver extracts the hiding data from the BMP stego

image by using Least Significant Bit’s extracting algorithm. In the second stage, the

BMP cover image is converted to JPEG and the secret message is embedded in the JPEG

cover image by using Discrete Cosine Transfrom’s embedding algorithm then, the JPEG

stego image is converted to BMP stego image and sent by the sender to the receiver

where the receiver extracts the hiding data from the JPEG stego image by coverting back

the BMP stego image into JPEG stego image and then he/she uses Discrete Cosine

Transfrom’s extracting algorithm. Both BMP stego images that produced from the two

technique’s embedding algorithms will be measured using Peak Signal to Noise

Ratio(PSNR). section 3.2 and section 3.3 will descuss the two stage’s embedding and

extracting algorithms in details.

3.2 Least Significant Bit Technique

The Least Significant Bit technique embedding algorithm embed the secret message into

the least significant bit of the image. The embedding process starts from the first

character to the end of the secret message. Each character of the secret message will

firstly be change into binary format according to the American Standard Code for

Information Interchange (ASCII) code, and then embed each bit of each character

starting from the least significant bit to the most significant bit (20

to 27). 1 byte of

23

image data can only hide 1 bit of message. Thus, 8 bytes of image data will be needed to

hide 1 character because each character is made up of 8 bits. As for extracting a loop

function is used until the end of the secret message is found. The Least Significant Bit’s

embedding algorithm is shown in Figure 3.2 where Figure 3.3 shows the Least

Significant Bit’s extracting algorithm.

Figure 3.2: LSB embedding algorithm

Figure 3.3: LSB extracting algorithm

BMP Stego

Extracting Algorithm Secret Message

BMP Cover

BMP Cover

Secret Message

Embedding Algorithm BMP Stego

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3.3 Discrete Cosine Transform Technique

The Discrete Cosine Transform (DCT) transforms a signal from an image representation

into a frequency representation, by grouping the pixels into 8 × 8 pixel blocks and

transforming the pixel blocks into 64 DCT. DCT is used in steganography as- Image is

broken into 8×8 blocks of pixels. Working from left to right, top to bottom, the DCT is

applied to each block. Each block is compressed through quantization table to scale the

DCT coefficients and message is embedded in DCT coefficients. Figure 3.4 shows the

main procedures for all encoding and message embedding processes based on the DCT

where Figure 3.5 shows the procedures for all decoding and message extracting

processes based on the DCT.

Figure 3.4: DCT based embedding algorithm

54

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