 Open Access
 Total Downloads : 438
 Authors : M. Bala Kumar, P. Karthikka, N. Dhivya, T. Gopalakrishnan
 Paper ID : IJERTV3IS21325
 Volume & Issue : Volume 03, Issue 02 (February 2014)
 Published (First Online): 04032014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A Performance Comparison of Encryption Algorithms for Digital Images
M. Bala Kumara, P. Karthikkab, N. Dhivyac, T. Gopalakrishnand
a,b,c PG Scholar, Department of Electrical and Electronics Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi.
dDepartment of Electrical and Electronics Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi.
Abstract In recent years, image encryption plays an important role in the field of information security and multimedia applications. This paper presents the comparative study of three different encryption algorithms in terms of security and performance. From cryptographic viewpoint, the analysis parameters focussed here are information entropy, correlation coefficients, encryption quality, NPCR (Number of Pixels Changing Rate), UACI (Unified Average Changing Intensity) and processing time. The comparison result shows that each algorithm has its own advantages that suits for different applications.
Keywords Nonchaos, Chaos, Hyperchaos, Correlation coefficient, Differential measures, Encryption entropy.

INTRODUCTION
With the development of network and multimedia technology, digital images are widely used. The image data has some special characteristics like high capacity, redundancy and high correlation among pixels. In some cases, image applications require to satisfy their own needs like real time transmission and processing. One of the main goals that must be achieved during the transmission of information over the network is security. Cryptography is the technique that can be used for secure transmission. This technique will make the information to be transmitted into an unreadable form by encryption so that only authorized persons can correctly recover the information. The security of image can be achieved by various types of encryption schemes. Different chaos based and nonchaos based algorithms have been proposed. The general structure of encryption involves two processes namely, permutation and diffusion. The permutation scheme just shuffles the pixel values based on the algorithm whereas substitution changes the pixel values. Mohammad Ali [1] proposed a block transformation based encryption algorithm. Here the image is divided into a random number of blocks, the blocks are shuffled and the permuted image is given to the Blowfish encryption algorithm. The dividing of image into as much as small blocks reveals higher entropy and less correlation between the pixels. In [2], different types of permutation techniques like bit permutation, pixel permutation and block
permutation were analyzed and the combinational permutation technique was proposed. To generate the pseudo random index, LFSR (linear feedback shift registers) were used. In the combinational method, bit or pixel or block permutation techniques were used to break the correlation among the pixels in the image.
Wang et al [3] proposed a chaos based encryption algorithm in which both permutation and diffusion are considered as two separate stages. Pixel values are first obtained by image scanning to accelerate the encryption process. Image is first divided into blocks and tent map is used to generate the pseudorandom sequences. In [4], coupling of chaotic function and XOR operation is done to produce large key space to resist brute force attacks. The chaotic function is based on linearity. And this function is used to generate pseudorandom numbers so that rowwise and columnwise shuffling can be done. The algorithm in [5], uses two phases to shuffle the image. In the first phase scrambling of plainimage is done and in second stage mixing operation of scrambled image using discrete states variables of chaotic maps is done. Discrete Cosine Transform (DCT) is used for compression. The proposed algorithm is strong in providing security and is also very fast. Since the key space is large, the attacker cannot decrypt an encrypted image without the correct key.
Vikram Jagannathan et al [6], combined compression and encryption simultaneously employing Number theory paradigm. To achieve this they applied Congruence theory and Chinese Remainder Theorem. Both the lossless and lossy compression methods using Number theory are discussed. Compression ratio achieved on Lena image is 1.85 which is better than Huffman and LZW (LempelZivWelch). The proposed method has equivalent performance to JPEG2000
Rounds
Plain image
Permutation process
Diffusion process
Cipher image
Symmetric encryption key
Figure 1. Encryption architecture
which achieve 1.86 compression ratios. But the performance is not better than JPEGLS, CALIC and SPIHT. However the level of encryption offer is high.
The rest of this paper is organized as follows. Section II provides a description of the selected image encryption algorithms. The efficient measuring methods, security analysis and experimental results are presented in section III. Finally, the concluding notes are introduced in section IV.

ENCRYPTION TECHNIQUES
Image encryption algorithms can become an integral part of the image delivery process if they aim towards efficiency and at the same time preserve the highest security level. These algorithms can be classified into three types: non chaotic, chaotic and hyperchaotic. The following subsections describe briefly three algorithms of the above mentioned types. The general architecture of the encryption technique is shown in Figure 1.

Nonchaotic approach
Nonchaos based method uses different types of scanning to permute the image pixels. The scanning techniques used are zigzag, rectangular, horizontal, vertical, etc. The Non chaotic cryptosystem proposed by N. K. Pareek et al (2013) has three phases. They are mixing phase, permutation phase and diffusion phase. Key space of this technique is 128 bit that avoids bruteforce attack. Mixing process slightly modify the pixel intensity value. Here the image is divided into nonoverlapping blocks. Using zigzag scanning the image block pixels is shuffled. For the zigzag scanning process the selection of starting pixel location based on the secret key given to the encryption algorithm. In the diffusion process the current pixel is masked with its surrounding pixel. The selection of surrounding pixel also is based on the secret key. The dividing of image into non overlapping blocks varies in each round of the encryption algorithm. The block dimension variation gives the correlation break among the image pixels.

Chaotic approach
Chaos theory is a mathematical theory, and it is still in development. It enables the description of a series of phenomena from the field of dynamics. It is focussed on those strictly deterministic dynamic systems that present the peculiarity of being sensitive to initial conditions and when they have a property of recurrence, cannot be predicted over the long term. Since we are in need of huge data message, it needs a quick and secure algorithm for image encryption, so chaotic encryption is widely researched in these years. Discrete chaotic systems such as Logistic map, Tent map, Henon map are designed for image encryption and many researchers proposed an encryption model based on digital chaotic system.
XJ Tong [2013] proposed a novel chaotic map based on topological conjugacy and the chaotic characteristics are proved by Devaney definition. In order to produce a large key space, a new cat map named separated Cat map is also designed in this process for permutation. In the permutation phase, compared with other maps, the ey space of cat map is smaller. So to improve the security, the image is divided into four blocks. Each block is processed by cat map and the input parameters are obtained from the Henon map. Because of more control parameters the key space is much larger. With the help of newly designed chaotic map, a real chaotic random sequence is generated for the diffusion process. The experimental results prove that the proposed algorithm is simple and is of higher security.

Hyperchaotic approach
Recently, because hyperchaos has more than one positive Lyapunov exponent, and have more complex dynamical characteristics than chaos, so secure communication schemes based on hyperchaotic systems have been investigated, but at present, there is little work about the study of encryption algorithm based on hyperchaos. In general, as the prediction time of a chaotic system is longer than that of a hyperchaotic system, so it may be more valuable to study the application of hyperchaos in encryption algorithms. In recent years,
Figure 2. Histogram analysis of the different Encryption approaches. (a) plain image, (b) histogram of the plain image, (c) cipher image of nonchaotic approach, (d) histogram of (c), (e) cipher image of chaotic approach, (f) histogram of (e), (g) cipher image of hyperchaotic approach, (h) histogram of (g).
when multimedia security is concerned, the need to apply
both encryption and compression to digital image communication keeps rising. A number of image encryption schemes combined with compression have been proposed. Some methods divide the image encryption and image compression into two separate stages. Thus, the adversary is mainly on breaking the encryption without considering the compression process. Some methods overcome the defect mentioned by combining the image encryption and compression in a single process. However, these methods need insert additional operation into the procedure, and the procedure is complicated.
Hegui Zhu et al (2013) proposed a new image encryption algorithm integrated with compression using 2D hyperchaos discrete nonlinear dynamic system and Chinese remainder theorem is proposed. First, the 2D hyperchaos discrete nonlinear dynamic system is used to generate two hyper chaos sequences, and then use them to shuffle the position of pixels in the plain image. Next, the image is encrypted and compressed the shuffled image with a given compression ratio k by the famous Chinese remainder theorem. In the permutation process, a hyperchaotic orbit { , , = 0,1, } is obtained with initial value (0, 0), several
iterations are made to get two new sequences { , . } and

Comparison criteria and Experimental results
The performance of the encryption algorithms discussed in Section II is compared in terms of the following metrics: Histogram analysis, Correlation coefficient, Differential measures, Encryption entropy and Execution speed.

Histogram analysis
Histogram analysis is employed to illustrate the superior substitution and diffusion properties of the encryption algorithm. We have analysed the histograms of several encrypted images obtained from the above mentioned approaches and their corresponding plain images. One example of such histogram analysis is shown in Figure 2. Relative uniform distributions in encrypted image histograms point out good quality of method. Therefore, the encryption image does not provide any clue to employ any statistical attack on the discussed approaches, which makes statistical attacks difficult.

Correlation coefficient

Statistical analysis such as correlation coefficient is used to measure the relationship between neighbouring pixels in an
1 2 image. For a plain image, the correlation between two
1
2
{ , . }. These sequences are arranged in ascending order and two index order sequences are obtained correspondingly. These index order sequences are applied to permute the
adjacent pixels are always high either in horizontal, vertical or diagonal directions, this can be calculated by equation,
image pixel positions. Hence, the image is confused and a shuffled image is obtained. Then the shuffled image is
E(x) 1
N
xi
(1)
grouped into blocks containing four pixel values which are diffused into one by The Chinese Remainder Theorem and thus encryption and compression are done simultaneously.
N i 1
Figure 3. Correlation plots of two horizontaly adjacent pixels, (a) plain image, (b) correlation plot of plain image, (c) Cipher image of NonChaotic approach, (d) correlation plot of (c), (e) Cipher image of Chaotic approach, (f) correlation plot of (e), (g) Cipher image of HyperChaotic approach, (h) correlation plot of (g).
N i
i
D(x) 1 N (x E(x
N
i 1
))2
(2)
NPCR measures the difference in the pixel numbers and UACI is about the average intensity of differences between
N
cov(x, y) 1
( E(
))( y E( y ))
two images. Always NPCR values should be in the range of
r xy
xi
i 1
cov(x, y)
xi i i
(3)
(4)
99% and UACI be 33% and this indicates the sensitivity of algorithm.
D(x) D( y)
D(i, j) NPCR i, j
100%
(5)
where x and y are gray values of two adjacent pixels in the image and E(x) and D(x) are the Expectation and Variance
UACI
M N
1 D(i, j) 100%
(6)
intensity respectively. Correlation coefficient of original image is usually high (nearly one). Weaker the correlation
M N i , j
255
coefficient of the encrypted image better the algorithm. Table 1 shows the outcome of the three approaches discussed above using standard Lena gray image. From the table it can be seen that, the hyperchaotic approach induces more randomness than the other two approaches. Figure 3 shows the correlation plots of two horizontally adjacent pixels in all the three discussed approaches. Similar results are obtained for adjacent vertical and diagonal pixels.
TABLE 1 CORRELATION COEFFICIENT FACTOR
Algorithm 
Horizontal 
Vertical 
Diagonal 
Nonchaotic approach 
0.0174 
0.0109 
0.0063 
Chaotic approach 
0.0354 
0.0258 
0.0185 
Hyper Chaotic approach 
0.0113 
0.0150 
0.0035 

Differential measures
To resist differential attack, any change in plain image will cause a significant change in the cipher image. For this two common measures are used. i.e., NPCR and UACI. The
Differential attack would become inefficient if one minor change in the plain image can cause a significant change in the cipher image. Higher NPCR values are desired for ideal encryption schemes. The UACI values must be in range of 33%.
TABLE 2 NPCR
Algorithm
Lena (gray)
Nonchaotic approach
0.9962
Chaotic approach
0.9947
Hyper Chaotic approach
0.9967
TABLE 3 UACI
Algorithm
Lena (gray)
Nonchaotic approach
0.3351
Chaotic approach
0.3347
Hyper Chaotic approach
0.3362
Table 2 and 3 shows the comparison of these values. From these tables, it is observed that, all the compared techniques have achieved the standard criterion and hence they resist various differential attacks.

Encryption ntropy
The idea of information entropy was created by Shannon. It has applications in many areas, such as, lossless data compression, statistical inference, cryptography. In recent times it is used in other disciplines, such as, biology, physics, and machine learning. Information entropy is a measure of the uncertainty associated with a random event and it is used to tell how much information there is in an event. The more uncertain or random the event is, the more information entropy it will contain. Therefore, it is very useful for analysing the randomness of an encryption scheme and is estimated by using Equation (7). Here X is the information source with length L.
i l 1
algorithms used for comparison for a standard Lena image of size 512×512 and is shown in Table 5. It is seen that the hyperchaotic approach consumes less processing time.
TABLE 5 ENCRYPTION SPEED
Algorithm
Seconds
Nonchaotic approach
1.5024
Chaotic approach
1.8126
Hyper Chaotic approach
0.4023
III. CONCLUSION
In this brief, we have performed a performance comparison of the three different image encryption algorithms. From the experimental results, the performance measures like Correlation coefficient, Differential measures, Encryption entropy and Execution speed have been estimated using standard test images. Each of the techniques has its own
H (x) p(xi) log2 ( p(xi))
i 0
(7)
advantages. Identification of suitable algorithm for a particular application depends on the prerequisites of that
where ( ) represents the probability of occurrence of .
For true random information source X emits 28 symbols
with equal probability = 1 , i.e. = (0, 1, , 8 ).
application.
REFERENCES
28 2
After computing the information entropy using Equation (7), we get = 8. Actually, given that a practical information source seldom generates random messages, we know that its information entropy value is smaller than 8 in general. However, when the messages are encrypted, their information entropy should ideally be 8. Particularly, if an image encryption algorithm creates symbols with information entropy less than 8, there is a possibility of predictability, which is a threat to the cryptosystem security.
TABLE 4 ENTROPY
Algorithm
Lena(gray)
Cipher
Nonchaotic approach
7.4288
7.9971
Chaotic approach
7.4288
7.9974
Hyper Chaotic approach
7.4288
7.9970
Table 4 shows the entropy values of the discussed algorithms. For a gray scale image having 256 levels, the theoretical value of entropy is 8 bits. But practically achieving this ideal value is not possible. The encryption algorithms that reach the nearest value 8 are appreciable. From Table 4 it is inferred that chaotic approach gives better results compared to other approaches.

Encryption speed
Another important tool to evaluate the efficiency of algorithms is measuring the amount of time required to encrypt an image. In this investigation, actual time in CPU cycles will be used as a measure of execution time. In many realtime applications, it is important that the encryption and decryption algorithms are fast enough to meet the requirements. Designer should attempt to optimize a cryptosystem to make the execution time as lower as possible. We have calculated the execution time of the

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