An Approach to Enhance Image Encryption Using Block Based

i

AN APPROACH TO ENHANCE IMAGE ENCRYPTION USING BLOCK-BASED TRANSFORMATION ALGORITHM

by

MOHAMMAD ALI MOH'D BANI YOUNES

Thesis submitted in fulfillment of the requirements for the degree of

Doctor of Philosophy

UNIVERSITI SAINS MALAYSIA 2009

ii

ACKNOWLEDGMENTS

"All praises and thanks to ALLAH"

I would like to express my sincere appreciation and heartfelt thanks to my

supervisor, Dr. Aman Jantan, for his creative guidance, intellectual support,

stimulating discussions and inspiring words. I am grateful for his excellent

hospitality and wonderful attitude.

Also, I would like to thank Dr. Omar Abu Shqeer, Dr. Ahmed Abdel Raouf Younis,

Dr. Samer Al-Dhalli, Dr. Ahmed Manasrah, and Dr. Ahmad Alzoubi for their

constructive contribution, invaluable advice, critical comments and patience.

A great thanks from my heart to my parents for their prayers, my wife for her

patience and support, my sons and daughter for their patience too, and my father in

law, mother in law, brothers, sisters, friends, and colleagues for their

encouragements.

Finally, I would like to thank all lecturers, administration, and staff of Universiti

Sains Malaysia and all academic and non academic staff of the School of Computer

Science for Graduate Studies for their help and support.

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TABLE OF CONTENTS

Page

TITLE i

ACKNOWLEDGEMENTS ii

TABLE OF CONTENTS iii

LIST OF TABLES vii

LIST OF FIGURES ix

LIST OF ALGORITHMS xi

LIST OF APPENDICES xii

ABSTRAK xiii

ABSTRACT xv

CHAPTER ONE: INTRODUCTION

1

1.1 Introduction 1

1.2 Research Motivations 3

1.3 Symmetric Key Algorithms 5

1.4 Image Encryption Using Block-Based Transformation Algorithm 6

1.5 Hiding Technique 8

1.6 Goal, Scope, and Objectives of the Research 8

1.7 Thesis Structure 9

CHAPTER TWO: LITERATURE REVIEW

12

2.1 Introduction 12

2.2 Digital Images 13

2.2.1 Digital Image Formats 15

2.3 Cryptographic Systems 21

2.3.1 Block and Stream Ciphers 27

2.4 Encryption Algorithms 30

2.4.1 Data Encryption Standard (DES) Algorithm 31

2.4.2 Blowfish Algorithm 33

2.4.3 Advanced Encryption Standard (AES) Algorithm 36

2.4.4 Serpent Algorithm 38

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2.4.5 Twofish Algorithm 39

2.4.6 RC6 Algorithm 39

2.4.7 MARS Algorithm 40

2.4.8 Triple-DES Algorithm 42

2.4.9 IDEA Algorithm 42

2.5 Current Research in Image Encryption 43

2.5.1 A new Image Encryption Approach Using Combinational Permutation Techniques 44

2.5.2 A General Cryptanalysis of Permutation-Only Multimedia Encryption Algorithms 45

2.5.3 Techniques for a Selective Encryption of Uncompressed and Compressed Images 46

2.5.4 Chaos-Based Image Encryption Algorithm 47

2.5.5 Analysis and Comparison of Image Encryption Algorithms 48

2.5.6 Image Encryption Using Fractional Fourier Transform and 3d Jigsaw Transform 48

2.5.7 Image Encryption for Secure Internet Multimedia Applications 49

2.5.8 Image and Video Encryption Using Scan Patterns 50

2.5.9 A New Chaotic Image Encryption Algorithm 52

2.5.10 Image Encryption and Decryption with Residual Intelligibility Measurements 52

2.5.11 Lossless Image Compression and Encryption Using SCAN 53

2.6 Steganography Technique for Digital Images 55

2.6.1 Cryptography and Steganography of Video Information in Modern Communication 57

2.6.2 A Technique for Image Encryption Using Digital Signature 57

2.7 Image Security Measurements 58

2.7.1 Image Correlation 59

2.7.2 Image Histogram 60

2.7.3 Image Entropy 62

2.7.4 Image Similarity 64

2.8 Summary 65

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CHAPTER THREE: A MODEL TO ENHANCE IMAGE ENCRYPTION USING BLOCK-BASED TRANSFORMATION TECHNIQUE

67

3.1 Introduction 67

3.2 Model Description 67

3.2.1 Transformation Technique 71

3.2.2 Encryption Process 77

3.2.3 Key 78

3.3 Security Measures 78

3.3.1 Image Correlation 79

3.3.2 Image histogram 88

3.3.3 Image Entropy 90

3.3.4 Image Similarity 92

3.4 The Technique of Hiding Information in the Image Data 94

3.4.1 Data Mixing (Sender Side) 94

3.4.2 Data Extracting (Receiver Side) 96

3.5 Decryption Process 99

3.6 Summary 100

CHAPTER FOUR: IMPLEMENTATION, TESTING AND RESULTS ANALYSIS

101

4.1 Introduction 101

4.2 Implementation Architecture 102

4.3 Experimental Design 103

4.4 Testing and Results 104

4.4.1 Correlation Test 106

4.4.2 Entropy Test 116

4.4.3 Histogram Test 119

4.4.4 Similarity Test 123

4.4.5 Hiding Efficiency 125

4.4.6 Retrieving the Original Image 129

4.5 Discussion 130

4.5.1 Shortcomings of the Proposed Technique 133

4.6 Summary 134

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CHAPTER FIVE: DISCUSSION AND FUTURE WORK 136

5.1 Introduction 136

5.2 Technical Contribution 136

5.2.1 Strength and Efficiency of the Technique 137

5.3 Economical Contribution of the Research 141

5.3.1 Knowledge Gain 142

5.3.2 Security Enhancement 143

5.4 Future Work 144

5.4.1 Random Factor 145

CHAPTER SIX: CONCLUSION

148

REFERENCES 150

APPENDICES 160

LIST OF PUBLICATIONS 184

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LIST OF TABLES

2.1 Image color space versus bit depth (bpp) 14

2.2 BMP file header 17

2.3 Comparison of the algorithms used for image encryption

54

2.4 Value of r (or rs) interpretation 60

2.5 An example to illustrates the histogram data points 61

3.1 Transformation table example 76

3.2 The color value of pixel representation 81

3.3 The horizontal correlation between adjacent elements of the image 81

3.4 The vertical correlation between adjacent elements of the image 83

3.5 The diagonal correlation between adjacent elements of the image 85

3.6 The opposite diagonal correlation between adjacent elements of the image

86

3.7 The probability of gray level of Figure 3.12, page 91 92

4.1 Different number of blocks used in this research 103

4.2 Results of measurement values of the original images

105

4.3 Correlation results of the transformed image 1 for all cases 108

4.4 Correlation results against key length for image 1 of all cases 110

4.5 Correlation results of case 1 111

4.6 Correlation results of the encrypted image 1 for all cases 113

4.7 Results comparison between the Blowfish algorithm alone and the combination technique based on correlation test

114

4.8 Correlation results against key length of the encrypted image 1 for all cases

115

4.9 Entropy results of the encrypted image 1 against number of blocks for all cases with different key lengths

117

4.10 Results of entropy of the encrypted image1 versus key length for all cases

118

4.11 Standard deviation values against number of blocks with different key lengths

120

4.12 Results of standard deviation versus key length 122

4.13 The percentage of similarity between the original image and retrieved one

124

4.14 Two cases to test the impact of the insertion data of the encrypted images

125

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4.15 Results of correlation and entropy values with and without insertion data 126

4.16 Results of insertion data of case 1 127

4.17 Results of insertion data of case 2 128

4.18 Comparison results of correlation and entropy values of the original images and recovered ones

130

5.1 Correlation and entropy values of image1, encrypted by commercial algorithms

138

5.2 Correlation and entropy values of image1, encrypted by commercial available algorithms preceded by the proposed algorithm

138

5.3 Correlation and entropy values of commonly used encryption algorithms in Literature when applied to the image of Figure 5.3

140

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LIST OF FIGURES

1.1 General block diagram of the proposed technique 4

1.2 Symmetric key algorithms 5

1.3 A block diagram of the proposed technique versus Blowfish algorithm 7

2.1 Image pixels 16

2.2 Public key encryption and decryption model 24

2.3 A simple model of symmetric key encryption 25

2.4 Model of symmetric key cryptosystem 26

2.5 Electronic Code Book (ECB) mode encryption 28

2.6 Cipher Block Chaining (CBC) mode encryption 28

2.7 Cipher Feedback (CFB) mode encryption 29

2.8 Output Feedback (OFB) mode encryption 29

2.9 An illustration of DES algorithm 32

2.10 Rounds structure of the Blowfish encryption 34

2.11 An illustration of AES_Rijndael algorithm 38

2.12 Structure of MARS algorithm 41

2.13 An example of an image histogram 62

3.1 General block diagram of the transformation and encryption 68

3.2 An overview model of the proposed technique at the sender side 70

3.3 An overview model of the proposed technique at the receiver side 70

3.4 The transformation process 72

3.5 Encryption process using the Blowfish with and without transformation 77

3.6 The horizontal correlation of image data 80

3.7 The vertical correlation of image data 82

3.8 The diagonal correlation of image data 84

3.9 The opposite diagonal correlation of image data 85

3.10 The correlation between variables for all directions 87

3.11 Histogram of original and encrypted images 90

3.12 The color value of pixel representation 91

3.13 Mixing data at the sender side 95

3.14 Extracting data at the receiver side 96

3.15 Part of a ciphered image data without insertion 98

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3.16 Part of a ciphered image data with insertion 98

3.17 Illustration of the decryption and retransformation processes 99

4.1 The parts of the proposed model 101

4.2 The original images (image 1 and image 2) used in the experiment 104

4.3 Results of the transformed image 1 for all cases by using 8 bytes key, 16 bytes key and 32 bytes key

107

4.4 Correlation values against number of blocks of the transformed image 1 109

4.5 Correlation values against key length of image 1 for all cases 110

4.6 Results of encryption by using 30 × 30 blocks 111

4.7 Correlation value of case 1 112

4.8 Correlation values against number of blocks with different key lengths 113

4.9 Correlation values against key length of the encrypted image1 for all cases

115

4.10 Entropy values of the encrypted image 1 against number of blocks for all cases with different key lengths

117

4.11 Entropy values of the encrypted image 1 versus key length for all cases 119

4.12 Standard deviation values against number of blocks with different key lengths, 8 bytes key, 16 bytes key, 32 bytes key

121

4.13 Results of encryption and histograms of the original image ( image1, case 1) by using 30×30 blocks

121

4.14 Standard deviation values versus key length with different number of blocks

123

4.15 The similarity between the original image and retrieved one

124

4.16 Results of encrypted images with and without insertion of data 126

4.17 Color level before and after insertion of case 1 127

4.18 Color level before and after insertion of case 2 129

5.1 Correlation values of the algorithms shown in Table 5.1 138

5.2 Entropy values of the algorithms shown in Table 5.1 139

5.3 Image used to compare the proposed algorithm with the commonly used ones in literature

139

5.4 Correlation values of the algorithms shown in Table 5.3 140

5.5 Entropy values of the algorithms shown in Table 5.3 141

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LIST OF ALGORITHMS

Algorithm 1 Creation of the transformation table process 73

Algorithm 2 Transformation process 76

Algorithm 3 Image horizontal correlation 80 Algorithm 4 Image vertical correlation 82

Algorithm 5 Image diagonal correlation 84

Algorithm 6 Image opposite diagonal correlation 86

Algorithm 7 Image average correlation algorithm 88

Algorithm 8 Image histogram algorithm 89

Algorithm 9 Image entropy algorithm 91

Algorithm 10 Compute image similarity 93

Algorithm 11 Mixing process 95

Algorithm 12 Extracting process 97

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LIST OF APPENDICES

Appendix A The Encryption Results of Original Image (Image 1) 160

Appendix B The Transformation Results of the Original Image (Image 2) 165

Appendix C Correlation Test of Transformed Image 2 166

Appendix D The Encryption Results of the Original Image (Image 2) 168

Appendix E Correlation Test of Encrypted Image 2 171

Appendix F Entropy Test of the Encrypted Image 2 174

Appendix G Histogram Test of the Encrypted Image 2 176

Appendix H Similarity Test 182

xiii

SUATU PENDEKATAN UNTUK MENINGKATKAN PENYULITAN IMEJ MENGGUNAKAN ALGORITMA TRANSFORMASI BERASASKAN BLOK

ABSTRAK

Penyulitan data (data encryption) telah digunakan secara meluas untuk menjamin

keselamatan dalam rangkaian terbuka (open network), contohnya internet. Setiap

jenis data mempunyai cirinya yang tersendiri. Oleh itu, teknik yang berbeza

sepatutnya digunakan untuk melindungi data imej rahsia (confidential image data)

daripada dicapai oleh pihak yang tidak berkenaan. Kebanyakan algoritma penyulitan

digunakan untuk data teks. Walau bagaimanapun, disebabkan saiz data yang besar

dan kekangan masa sebenar (real time constrains), algoritma yang sesuai untuk data

teks mungkin tidak sesuai bagi data multimedia.

Dalam penyelidikan ini, satu algoritma transformasi berasaskan blok dicadangkan

untuk keselamatan imej iaitu dengan menggunakan gabungan transformasi imej dan

teknik penyulitan. Algoritma ini akan digunakan sebagai transform prapenyulitan

(pre-encryption transform) untuk mengelirukan hubungan di antara imej biasa

(original images) dan imej yang dijana (generated images). Imej yang dijana

(ditransformasi) kemudiannya disesuaikan pada algoritma penyulitan Blowfish.

Korelasi, histogram dan entropi digunakan untuk menentukan tahap keselamatan

imej.

Hasil daripada eksperimen yang dijalankan menunjukkan bahawa teknik gabungan

menghasilkan korelasi yang rendah, nilai entropi yang tinggi dan histogram yang

seragam. Sedangkan keputusan daripada penggunaan algorithma Blowfish

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menunjukkan peningkatan dalam tahap keselamatan imej yang disulitkan (encrypted

images). Kedua-dua teknik (gabungan transformasi imej dan teknik penyulitan)

menunjukkan kesamaan yang tinggi (high similarity) serta kualiti yang baik daripada

imej yang dipulihkan, jika dibandingkan dengan teknik asal. Ciri lain bagi teknik

gabungan adalah keteritlakkan (generality). Keputusan eksperimen menunjukkan

bahawa penggunaan teknik gabungan bersama dengan algoritma lain memberikan

prestasi yang lebih baik, jika dibandingkan dengan penggunaan algoritma lain secara

bersendirian.

Dalam usaha meningkatkan kekuatan penyulitan imej yang ditransformasikan, suatu

pendekatan steganografi baru bagi penyembunyian data (data hiding) juga

dicadangkan. Keputusan eksperimen menunjukkan bahawa korelasi dan nilai entropi

daripada imej yang disulitkan sebelum sisipan (insertion) adalah sama dengan nilai

korelasi dan entropi selepas sisipan. Oleh kerana korelasi dan entropi tidak berubah

semasa penyembunyian maklumat yang perlu, maka keadah ini memberikan

perlindungan (concealment) data yang baik dalam imej yang disulitkan serta

mengurangkan peluang ia dikesan. Data yang disembunyikan akan digunakan untuk

membolehkan penerima membangunkan semula (reconstruct) jadual transformasi

rahsia yang sama selepas mengekstraknya, dan imej asal boleh dihasilkan semula

dengan menyongsangkan (inverse) proses transformasi dan proses penyulitan.

xv

AN APPROACH TO ENHANCE IMAGE ENCRYPTION USING BLOCK-BASED TRANSFORMATION ALGORITHM

ABSTRACT

Data encryption is widely used to ensure security in open networks such as the

internet. Each type of data has its own features, therefore, different techniques should

be used to protect confidential image data from unauthorized access. Most of the

available encryption algorithms are used for text data. However, due to large data

size and real time constrains, algorithms that are good for textual data may not be

suitable for multimedia data.

In this research, a block-based transformation algorithm is proposed for image

security using a combination of image transformation and encryption techniques.

This algorithm will be used as a pre-encryption transform to confuse the relationship

between the original images and the generated ones. The generated (transformed)

images are then fed to the Blowfish encryption algorithm. Correlation, histogram,

and entropy have been used to measure the security level of the images.

The experimental results have shown that the combination technique resulted in a

lower correlation, a higher entropy value, and a more uniform histogram, compared

to using the Blowfish algorithm alone; resulting in an enhancement to the security

level of the encrypted images. This implies a high similarity and a good quality of

the retrieved image compared to the original one. Another feature of the combination

technique is its generality; it can be applied with any other traditional algorithm to

enhance its performance. Experimental results have shown that using the

xvi

combination technique along with the other algorithms resulted in a better

performance compared to using the other algorithms alone.

In order to further strengthen the encryption of the transformed image, a

steganography approach for data hiding is also proposed. Experimental results have

shown that the correlation and entropy values of the encrypted image before the

insertion are similar to the values of correlation and entropy after the insertion. Since

the correlation and entropy have not changed while hiding necessary information, the

method offers a good concealment of the data in the encrypted image, thus reduces

the chance of the encrypted image being detected. The hidden data will be used to

enable the receiver to reconstruct the transformation table after extracting it and

hence the original image can be reproduced by the inverse of the transformation and

encryption processes.

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Many digital services require reliable security in storage and transmission of digital

images. Due to the rapid growth of the internet in the digital world today, the security

of digital images has become more important and attracted much attention. The

prevalence of multimedia technology in our society has promoted digital images to

play a more significant role than the traditional texts, which demand serious

protection of users' privacy for all applications. Encryption and steganography

techniques of digital images are very important and should be used to frustrate

opponent attacks from unauthorized access (Mitra et al, 2006), (Shujun et al, 2002),

(Lee et al, 2003).

Digital images are exchanged over various types of networks. It is often true that a

large part of this information is either confidential or private. Encryption is the

preferred technique for protecting the transmitted data (Hossam El-din et al, 2006).

There are various encryption systems to encrypt and decrypt image data, however, it

can be argued that there is no single encryption algorithm which satisfies the

different image types (Shujun and Zheng, 2002), (Mohammed Husainy, 2006).

In general, most of the available encryption algorithms are used for text data.

However, due to large data size and real time constrains, algorithms that are good for

textual data may not be suitable for multimedia data (Yas A. Alsultanny, 2008),

(Droogenbroech and Benedett, 2002), (Fong and Singh, 2002). According to Xun

(2001) and Wang (2005), even though triple-data encryption standard (T-DES) and

2

international data encryption algorithm (IDEA) can achieve high security, they may

not be suitable for multimedia applications (Xun et al, 2001), (Wang et al, 2005).

Therefore, encryption algorithms such as data encryption standard (DES), advanced

encryption standard (AES), and international data encryption algorithm (IDEA) were

built for textual data (Lee et al, 2003), (Syed, 2002), (Xun et al, 2001).

Although we can use the traditional encryption algorithms to encrypt images directly,

this may not be a good idea for two reasons. First, the image size is often larger than

text. Consequently, the traditional encryption algorithms need a longer time to

directly encrypt the image data. Second, the decrypted text must be equal to the

original text but this requirement is not necessary for image data. According to

Chang (2001), due to the characteristic of human perception, a decrypted image

containing small distortion is usually acceptable (Chang et al, 2001), (Jiri Jan, 2005),

(David Salomon, 2005). The intelligible information present in an image is due to the

correlation among the image elements in a given arrangement. According to Mitra

(2006), this perceivable information can be reduced by decreasing the correlation

among image elements using certain transformation techniques (Mitra et al, 2006).

In addition to cryptography, steganography techniques are getting significantly more

sophisticated and have been widely used. The steganography techniques are the

perfect supplement for encryption that allows a user to hide large amounts of

information within an image. Thus, it is often used in conjunction with cryptography

so that the information is doubly protected, that is, first it is encrypted, and then it is

hidden so that an adversary has to find the hidden information before the decryption

takes place (Kisik Chang et al, 2004), (Kessler, 2001), (Kathryn Hempstalk, 2005).

3

1.2 Research Motivations

Most of the algorithms specifically designed to encrypt digital images were proposed

in the mid-1990s. According to the Maniccam and Bourbakis (2004), there are two

major groups of image encryption algorithms: (a) Non-chaos selective methods, and

(b) Chaos-based selective or non-selective methods (Maniccam and Bourbakis,

2004). However, most of these algorithms are designed for a specific image format,

either compressed or uncompressed. There are methods that offer light encryption

(degradation), while others offer strong form of encryption. Some of the algorithms

are scalable and have different modes ranging from degradation to strong encryption.

According to Borko (2005), the user is expected to choose a method based on its

properties, which will be best for image security (Borko Furht et al, 2005).

Image encryption has applications in internet communication, multimedia systems,

medical and military imaging systems. Each type of multimedia data has its own

characteristics such as high correlation among pixels and high redundancy. Thus,

different techniques should be used to protect confidential image data from

unauthorized access (Hossam El-din et al, 2006), (Ozturk and Sogukpinar, 2004).

The motivation behind this research is the ever-increasing need for harder-to-break

encryption and decryption algorithms as the computer and network technologies

evolve. We believe that by proposing the block-based encryption and decryption

algorithm, it will help to reduce the relationship among image elements by increasing

the entropy value of the encrypted images as well as lowering the correlation.

4

However, in order to increase the robustness of the proposed encryption technique, a

steganography method using the least significant bit insertion will be applied without

impacting the quality of the image.

Considering the above points, we will divide this research into three parts: a new

transformation algorithm, a combination technique (transformation and encryption),

and a steganography approach that will be used to hide a secret information (that is,

the number of horizontal and vertical blocks of the transformed image) in the

encrypted image data before transmission to the receiver. A general block diagram of

the transformation and encryption techniques is shown in Figure 1.1.

Figure 1.1 General block diagram of the proposed technique

The next section will explain more about the adopted algorithm and the methodology

used.

Transformation process

Encryption Process

Retrieved image

Receiver sideSender side

Decryption process

Ciphered image

Original image

Ciphered image Retransformation process

Transformed image

Transformed image

Mixing process

Extracting process

5

1.3 Symmetric Key Algorithms

In general, symmetric key algorithms use a single, shared secret key. The same key is

used for both encrypting and decrypting the data. There are two primary types of

symmetric algorithms: block and stream ciphers. A block cipher is used to encrypt a

text to produce a ciphertext, which transforms a fixed length of block data size into

same length block of ciphertext in which a secret key and algorithm are applied to

the block of data. For example, a block cipher might take a 64-bit block of plaintext

as input, and output a corresponding 64-bit block of ciphertext. This transformation

process should be conducted by a user providing a secret key and the decryption

process is the inverse transformation to the ciphertext using the same key (April,

2005). Blowfish, Data Encryption Standard (DES), Triple-DES, IDEA, Rijndael and

RC2 are examples of symmetric block cipher. The symmetric key algorithms use a

single key for encryption and decryption processes as shown in Figure 1.2.

Figure 1.2 Symmetric key algorithms

The Blowfish algorithm is one of the symmetric block cipher algorithms that was

designed in 1993 by Bruce Schneier as a fast alternative of the existing encryption

algorithms, whereby it can be used as a replacement for the Data Encryption

Standard (DES) or the International Data Encryption Algorithm (IDEA). The

Blowfish encryption algorithm has been analyzed considerably, and is gaining

acceptance as a strong encryption algorithm. Its source code is also available and it is

not subjected to any patent royalties (Bruce Schneier, 1993), (William Stallings,

Plaintext Ciphertext Plaintext

6

2003). Hence, this algorithm will be used mainly in this research as part of the new

combination encryption technique.

The Blowfish algorithm consists of two parts: a key-expansion part and a data-

encryption part. It encrypts the data by using the block cipher method, which breaks

the text into 64-bit blocks before encrypting them. It takes a variable-length key from

32 bits to 448 bits of length, which implies flexibility in its security strength (John

and James, 2005), (Bruce Schneier, 1993), (William Stallings, 2003).

1.4 Image Encryption Using Block-Based Transformation Algorithm

In this research, we propose a new transformation algorithm to be used as a pre-

encryption transform, where the original image is divided into a random number of

blocks which are shuffled and placed randomly within the image to build a newly

transformed image. The generated transformed image is then fed to the Blowfish

encryption algorithm. Thus, we expect that the combination of the transformation

and encryption techniques will enhance the security level of the encrypted images.

This combination technique uses the original image to produce two output images:

a) a transformed image, using the proposed transformation algorithm.

b) a ciphered image of the transformed image, using the Blowfish algorithm.

A block diagram of the proposed technique versus Blowfish algorithm is shown in

Figure 1.3.

7

Key

Transformation process

Original image

Comparison Process

Encryption process

Statistical analysis

Blowfish algorithmProposed technique Comparison results

Ciphered image Ciphered image

Statistical analysis

Encryption process

Measurement results Measurement results

Figure 1.3 A block diagram of the proposed technique versus Blowfish algorithm

The measurements of correlation, entropy and histogram will be used to measure and

compare the security level of the original image, transformed and encrypted images

using the combination technique, and the encrypted image using the Blowfish

algorithm alone. Encrypted images produced by the combination technique are

expected to have lower correlation and higher entropy values, compared to those

produced by the Blowfish algorithm alone.

In addition to transformation technique, we also present a steganography approach to

be used for hiding secret information within encrypted image before being

transmitted to the receiver.

8

1.5 Hiding Technique

Steganography techniques can be used for hiding information within other

information. The least significant bit (LSB) insertion is one of the most widely used

methods for embedding a message in a digital image. Steganography involves hiding

information so it appears that no information is hidden at all. Therefore, it is

expected that the person will not be able to decrypt the information (Shujun and

Zheng, 2002), (Neil and Zoran, 2001), (Sushil, 2001). An alteration of the least

significant bit of the color value of some pixels in an image will not change the

quality of the image significantly. Therefore, a message can be sent within an image

using these bits (Stallings, 2003).

In this research, the number of horizontal and vertical blocks of the transformed

image, produced by the proposed algorithm, represents the secret information to be

mixed (hidden) with the encrypted image before being transmitted to the receiver.

This secret information will be needed at the receiver. Instead of sending the whole

transformation table, which is usually big, only the secret information is sent. At the

receiver side, the hidden information allows the receiver to regenerate the

transformation table. Thus, the original image can be retrieved by the

retransformation and decryption processes.

1.6 Goal, Scope, and Objectives of the Research

The goal of this research is to enhance the security level of the encrypted images

using the proposed transformation algorithm. The scope is limited to the image

encryption using the combination technique (block-based transformation algorithm

and Blowfish encryption algorithm) on Microsoft windows based machine. This

9

combination technique is applied to divide and shuffle the positions of the blocks of

the original image, encrypt the transformed image, and then embed secret

information (the number of horizontal and vertical blocks) in the encrypted image

data prior to transmission to the receiver. Furthermore, the focus of this research was

concerning a bit mapped (bmp) images using the standard Cipher Block Chaining

(CBC) mode of the Blowfish algorithm.

To achieve the above goal, the objectives of this research will be as follows:

1. To introduce a new algorithm for image transformation, and to test and evaluate it.

2. To compute and compare correlation, entropy and histogram of different images

with and without the proposed algorithm.

3. To compare the security levels of the encrypted images generated by the

combination technique and the Blowfish algorithm..

4. To introduce a steganography method to exchange the secret information between

the sender and the receiver that will be used for producing the transformation

table.

1.7 Thesis Structure

This thesis will be organized into six chapters. Chapter 1 provides an introduction to

the work, the motivations of this research, and explains important goal, scope, and

objectives of this study. Image encryption using block based transformation

algorithm is explained to provide a general aspects of symmetric encryption

algorithm, then the most important features of the blowfish encryption and

decryption algorithm are presented. A precise description of the proposed technique

10

and its diagram is presented. Furthermore, the important use of the steganography

technique is also presented.

In chapter 2, we will provide an overview of the concepts of digital image

encryption. The general aspects of digital images and image file formats will also be

discussed. We will also provide the explanations on cryptographic systems in

general; block cipher with its modes of operation, stream cipher and some of the

most commonly used or well known encryption and decryption algorithms such as

DES, Blowfish, Rijndael-AES and IDEA. This chapter also highlights a background

of the current research in image encryption. Steganography technique for digital

images is also presented. At the end of this chapter, the image measurements; the

correlation among image elements, image entropy, and image histogram as well as

image similarity are discussed in certain degree of details.

In chapter 3, we will present and discuss in details the description of the model and

methodology that will be applied to enhance the security level of the encrypted

images by using the newly proposed approach. Hiding information in image using

steganography technique is also presented in this chapter.

In chapter 4, we will discuss the implementation, testing and results analysis of the

proposed technique. Hiding efficiency will also be presented in this chapter with the

focus on mixing and extracting data within the encrypted image. How the sender will

hide the secret information in the image data that enables the receiver to rebuild the

transformation table using the secret key will also be discussed.

11

In chapter 5, we will present the technical contribution; strength and efficiency of the

technique by comparing the combination technique with three commonly used

encryption algorithms; Blowfish, Twofish, or RijnDael. Economical contribution of

the research; knowledge gain and security enhancement will also be explained. Some

suggestions for future work will also be provided. Chapter 6 will summarize the

main points of the thesis and list the main conclusions of the work.

12

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter provides a detail description of the cryptographic systems used in this

research. Section 2.2 presents general aspects of the digital image encryption.

Section 2.3 defines fundamental concepts of cryptography systems and image

encryption. Categories of cryptography systems are also discussed; symmetric key

cryptography and public key cryptography. Some important symmetric key

algorithms such as block cipher and stream cipher algorithms are also introduced in

this section. Furthermore, we will discuss modes of operation; Electronic Code Book

Cipher (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), and Output

Feedback (OFB).

Section 2.4 presents some of the recently developed or well-known encryption and

decryption algorithms in cryptography, such as DES, Blowfish, Rijndael-AES and

IDEA. These algorithms define and cover the rules of encryption and decryption that

are used to provide security in image encryption. Section 2.5 presents an overview of

the research in the image encryption area. Section 2.6 provides an overview of the

steganography techniques for digital images.

Section 2.7 explains the image security measurements; the correlation among image

elements, image entropy, image histogram, and image similarity in a certain degree

of detail. These security measurements will be used in this research to compute and

evaluate the encrypted images produced by the combination technique.

13

Finally, section 2.8 summarizes the main points presented and discussed in this

chapter.

2.2 Digital Images

A digital image is defined by an array of individual pixels and each pixel has its own

value. The array, and thus the set of pixels, is called a bitmap. If we have an image of

512 pixels × 512 pixels, it means that the data for the image must contain

information about 262144 pixels (Steinmetz and Nahrstedt, 2002), (Kristian

Sandberg, 2000).

Digital images are produced through a process of two steps: sampling and

quantization. Sampling is the process of dividing the original image into small

regions called pixels, whereas quantization is the process of assigning an integer

value (i.e. color) to each pixel (David Salomon, 2007).

The number of colors (i.e. color space) that can be assigned to any picture element or

pixel is a function of the number of bits, which is sometimes referred to as the color

depth or bits resolution. This concept is also known as bits per pixel (bpp) that

represents the color for each value. The color space is computed using the following

equation:

b2ColorSpace = ….……………………….……………....Equation 1

where:

b: the bit depth

14

The color values used in each bitmap depend on the specific bitmap format. This

means that each pixel in a bitmap contains certain information, usually interpreted as

color information. The information content is always the same for all the pixels in a

particular bitmap. Thus, each color value in a bitmap is a binary number. A binary

number is a series of binary digits that can be either 0 or 1 and called bits. This

binary number in a given format will differ in length depending on the color depth of

the bitmap, where the color depth of a bitmap determines the range of possible color

values that can be used in each pixel. For example, each pixel in a 24-bit image can

be one of roughly 16.8 million colors. This means that each pixel in a bitmap has

three color values between 0 and 255 and then those colors are formed by mixing

together varying quantities of three primary colors: red, green and blue (Vaughan,

2004), (Rafael and Richard, 2002), (Sander, 2000). Table 2.1, illustrates the image

color space.

Table 2.1 Image color space versus bit depth (bpp)

Image properties Bits resolution Color space

Binary image (black and white) 1 2 colors

Gray scale (monochrome) 8 256 gray levels

Colored image 8 256 colors

Colored image 16 65536 colors

True color (RGB) 24 16,777,216 colors

As seen from Table 2.1, as the number of bits increases, the image quality is also

increased. However, storage requirements will increase, resulting in a direct

relationship between the image storage size and the bits resolution. Image storage

size for an uncompressed image is computed using the following equation:

15

BRIMGRIMGSS ×= …………….………………………..Equation 2

where:

IMGSS: Image storage size

IMGR: Image resolution (i.e. image width × image height)

BR: Bits resolution (bits depth)

For example, the storage size of a 640 pixels × 480 pixels, true colored image is

given as follows:

IMGSS = W × H × BR = 640 × 480 × 24 bits = (7372800/1024/8) = 900 KB.

2.2.1 Digital Image Formats

Basically, there are three types of image files: bitmap, vector, and metafiles. When

an image is stored as a bitmap file, its information is stored as a collection of pixels,

manifest as colored or black-and-white dots. When an image is stored as a vector

file, its information is stored as mathematical data. The metafile format can store

image information as pixels (bitmap), mathematical data (vector), or both (Betcher

and Gardner, 2006), (Sander, 2000). There is no single format that is appropriate for

all types of images. According to Glouglim (2001), larger files take longer to load,

require more disk space and can take longer to print, whereas small file sizes means

greater performance (McGlouglim, 2001). The most common file formats are

discussed below:

a) BMP Files

According to Bourke (1998), the BMP Bitmaps are defined as a regular rectangular

mesh of cells called pixels (Bourke, 1998). Each pixel contains a color value as

shown in Figure 2.1

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Figure 2.1 Image pixels

Bitmaps are characterized by only two parameters: the number of pixels, and the

information content (color depth) per pixel, and they are the most commonly used

type to represent images on the computer.

BMP is the native bitmap format of Windows. BMP is a general format that stores

images in different color depths without compression (Betcher and Gardner, 2006),

(Paul Bourke, 1998), (James and William, 1996).

The BMP advantages are that each pixel is independently available for any alteration

or modification, and that repeated use of them does not normally degrade the image

quality (Lancaster, 2003). The main disadvantage of this format is due to the size of

the files, which is usually larger compared to other formats or other lossy

compression schemes. In general, a BMP file consists of a header, descriptive

information about the image (such as width, height, etc), an optional Color Lookup

Table (CLUT) area which contains the actual colors of the image pixels, and a pixel

data area (Shalini, 2004). The parts of the BMP file are illustrated in Table 2.2.

Pixels

N Pixels horizontally

M P

ixel

s ver

tical

ly

17

Table 2.2 BMP file header

Header Stores general information about the BMP file.

Information header Stores detailed information about the bitmap image.

Optional palette Stores the definition of the colors being used for indexed color bitmaps.

Image data

Stores the actual image, pixel by pixel.

Color Lookup Tables (CLUTs) follow the header for those lower performance modes

in which they are used such as (1, 4, 8 or 16 lookup colors). These, in turn, are

followed by the actual pixel data. The main interest lies in the 24-bit uncompressed

RGB color mode. In this mode, there are no color lookup tables used. Each pixel

consists of an 8-bit red value, an 8-bit green value and an 8-bit blue value (Lancaster,

2003).

B) GIF Files

The Graphics Interchange Format (GIF) was originally developed by CompuServe in

1987. It is one of the most popular file formats for web graphics and exchanging

graphics files between computers. The GIF format supports 8 bits of color

information that is limited to 8 bits palette and 256 colors. Thus, only 256 different

colors are available to represent the picture. It can be viewed by all common

browsers. GIF also support animation, transparency and interlacing (Betcher and

Gardner, 2006), (Robert Fry, 2006).

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GIF images are automatically compressed when they are saved using a lossless

compression method known as LZW (Lempel-Ziv-Welch) that does not degrade the

image quality. GIF format provides four main features: interlacing, transparency, file

compression, and primitive animation. The interlacing feature allows the browser to

display portions of the image as it updates. The original image starts off with poor

quality but gets better as more of the interlacing parts are updated. Interlaced GIF

files allow users to view a portion of the image as the file is loading (Seeram and

Radiography, 2006).

One of GIF's weaknesses is that GIF images are limited to a maximum of 256 colors.

The quality of the image suffers if the color depth is reduced to less than the color

depth of the original image. GIF files can store any of the 16.8 million colors but

only a maximum 256 colors in each GIF file. Therefore, when converting an image

to GIF, the program compresses the file by reducing the number of colors in the

image from 24-bit (millions of colors) to 8-bit (256 colors). However, the GIF file

format has the ability to store multiple images in a single file and play the images in

a loop, thereby giving the appearance of animation (CIMC, 2006), (Sharon Wheeler,

2000).

c) JPEG Files

The Joint Photographic Experts Group, (JPEG) format, is one of the most popular

formats for web graphics. It supports 24 bits of color information. The JPEG file

format stores all of the color information in an RGB image, and then it compresses

the file size to save storage space, or it saves only the color information that is

essential to the image. Unlike GIF, JPEG does not support transparency.

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The compression method used in JPEG is usually lossy compression, meaning that

some visual quality is lost in the process. JPEGs can be saved in a variety of lossy

compression levels. This means more or less compression can be applied to the

image, depending upon which looks best. JPEG can be used by almost any browser.

Since JPEG is an image compressor, it is best used for photographic quality images

and detailed illustrations with many colors (Tom Lane, 2008).

The advantage of JPEG is that it is a highly compressed file format. Therefore, the

image can be compressed while the quality is maintained. JPEG weakness is that

lossy compression may result in low quality graphics. Another weakness noted in

JPEG formats is that there is no support for pixel transparency. JPEGs lose quality

every time they are opened, edited and saved. It is very important to minimize the

number of editing sessions between the initial and final version of a JPEG image

(Sharon Wheeler, 2000), (CIMC, 2006), (Graphics Academy, 1998).

d) PICT Files

The Picture File Format (PICT) is used primarily on the Macintosh platform. It is the

default format for Macintosh image files as its standard metafile format. The PICT

format is most commonly used for bitmap images, but can be used for vector images

as well. The PICT is a lossless format. Since the PICT format supports only limited

compression on Macintoshes with QuickTime installed, PICT files are usually large.

PICT is used for images in video editing, animations, desktop computer

presentations, and multimedia authoring (Chris Betcher and Margie Gardner, 2007).

20

e) EPS Files

The Encapsulated PostScript (EPS) file format is intended to make files usable as a

graphics file format. The EPS file format is a metafile format. It can be used for

vector images or bitmap images. It can also be used on a variety of platforms,

including Macintosh and Windows. If an EPS image is placed into a document, we

can scale it up or down without information loss (Chris Betcher and Margie Gardner,

2007).

f) PNG Files

The Portable Network Graphics (PNG) format is a bitmapped image format that

employs lossless data compression. It will likely be the successor to the GIF file

format. PNG is expected to become a mainstream format for web images and could

replace GIF entirely. It is platform independent and should be used for single images

only (not animations). Compared with GIF, PNG offers greater color support and

better compression, gamma correction for brightness control across platforms, better

support for transparency, and a better method for displaying progressive images

(Sharon Wheeler, 2000), (Fulton, 2005).

g) TIFF Files

The Tag Interchange File Format (TIFF) is a tag-based international standard for

storing and interchanging bitmaps between applications and hardware platforms. It is

compatible with a wide range of software applications and can be used across

platforms such as Macintosh, Windows, and UNIX. The TIFF format is complex,

thus TIFF files are generally larger than GIF or JPEG files. TIFF supports lossless

LZW compression. However, compressed TIFF takes longer to open. The format

21

consists of items called tags which are defined by the standard. Each tag is followed

by a tag dependent data structure (Graphics Academy, 1998).

The next section will explain the cryptographic system and image encryption.

2.3 Cryptographic Systems

Cryptography is the conversion of data into a secret code for transmission over a

public network. Cryptography enables the sender to securely store sensitive

information or transmit it across insecure networks so that it cannot be read by

anyone except the intended recipient (Harris Chen, 2001), (Gary Kessler, 2007).

Encryption of sensitive data is necessary. Cryptography is used to render the

information unintelligible if transmission is intercepted by unauthorized individuals

(Jae Shim, 2000). The intelligible form (original data) of information is called

plaintext and the unintelligible form (protected data) is called ciphertext (Elbirt and

Paar, 2005), (Stallings, 2003). The process of converting the plaintext into ciphertext

is called encryption, while the reverse process of transforming ciphertext into the

corresponding plaintext is called decryption.

In general, most cryptographic algorithms use a secret value called a key. The

security of encrypted data is entirely dependent on two things: the strength of the

cryptographic algorithm and the secrecy of the key. The key is used for encryption

and decryption and must be kept secret, thereby requiring the sender and receiver to

agree on the same key before making any data transmissions. The key is independent

of the plaintext. Therefore, the same plaintext encrypts to different ciphertext with

22

different keys, and thus both processes are impossible without the use of the correct

key (Weber and Fahrny, 2003), (Natasa, 2005), (Schneier, 1996).

Cryptography can be strong or weak. Cryptographic strength is measured in the time

and resources it would require to recover the plaintext. The result of strong

cryptography is ciphertext that is very difficult to decipher without possession of the

appropriate decoding tool. The sender and the recipient must keep the key secret

because anyone who knows the key can use it to decrypt the plaintext. In addition,

the strength of the algorithm is important. An unauthorized entity can take encrypted

ciphertext and attempt to break the encryption by determining the key based on the

ciphertext (Zhong et al, 2005).

While cryptography is the science of securing data, cryptanalysis is the science of

analyzing and breaking secure communication, and therefore it is the process of

recovering the plaintext or key, usually by using the ciphertext and knowledge of the

algorithm (Albassal and Wahdan, 2004), (Bagnall et al, 1997), (Harris Chen, 2001).

According to Kessler (2007), there are two types of algorithms: symmetric that uses

a single key for both encryption and decryption, and asymmetric that uses one key

for encryption and another for decryption (Kessler, 2007).

Cryptography can also be used to ensure the security of the communication path

through the following: (a) data integrity which means ensuring that the data has not

been modified by unauthorized entities. Thus, the message received by the recipient

is the same as the message sent by the sender. (b) Non-repudiation ensures that the

sender of any message cannot deny his/her actions. This can be achieved with digital

23

signatures in conjunction with asymmetric key encryption. (c) Authentication is the

process of proving the identity, and (d) privacy/confidentiality is the process to

ensure that no one can read the message except the intended recipient (Alina Stan,

2007), (Solomon and Chapple, 2005).

The modern field of cryptography can be divided into several areas of study. In this

section, two related categories for cryptography systems: public key cryptography

and symmetric key cryptography will be discussed (Kaufman et al, 2002).

Public Key Cryptography (PKC) is also known as asymmetric cryptography. It uses

one key for encryption and another for decryption. The encryption key known as

public key is intelligible and can be distributed for all parities, while the decryption

key known as private key is intelligible only to the recipient. Each user creates a pair

of keys; if one is used for encryption then the other is used for decryption (Kaufman

et al, 2002).

The public and private keys are mathematically related so that data encrypted with

the public key can only be decrypted with the private key. This guarantees message

privacy during transit. An important characteristic of public key encryption

algorithms is that it should be computationally infeasible to determine the decryption

key given only knowledge of the algorithm and the encryption key. Public key

encryption can be used to exchange the secret key between the parties in a symmetric

key cryptosystem (Stallings, 2003), (Baek, 2004). Figure 2.2, shows the main

ingredients of public key cryptography system.

24

Figure 2.2 Public key encryption and decryption model (Stallings, 2003).

As illustrated in Figure 2.2, there are three basic steps to send a message by using

public key encryption:

• Sender and receiver exchange the public keys, while the private key is kept secret

by its owner.

• The sender uses the recipient's public key in encrypting a message for sending.

• The recipient's secret key is used to decrypt the received message.

In symmetric key cryptography, encryption and decryption are performed using the

same secret key. The key can only be known by the sender and receiver to maintain

integrity (Thomas Shinder, 2002). According to Whitman and Jason (2005), the

primary disadvantage of symmetric key algorithms is that the key must remain secret

at all times. For this reason, the key must be protected and secured requiring the

sender to transmit the key to the recipient in a secure fashion (Jason Isom, 2005),

(Whitman and Mattord HJ, 2005). Symmetric encryption is the most widely used

algorithm. It has five ingredients as shown in Figure 2.3 (Stallings, 2006).

Plaintext output

Transmitted ciphertext

Alice's private

key

Encryption algorithm (e.g., RAS)

Decryption algorithm (reverse of encryption algorithm)

Plaintext input

Bobs's public

key ring

Alice's public

key

Joy Mike

Ted Alice