International Journal of Electrical and Computer Engineering (IJECE) Vol.
No.
June 2014, pp.
ISSN: 2088-8708
A New Cryptographic Strategy for Digital Images Nidhi Sethi*.
Sandip Vijay** * Departement of Computer Science Engineering.
DIT University ** Departement of Electronicsad Communication Engineering.
DIT University Article Info
ABSTRACT
Article history:
In this paper, a new image block cipher encryption strategy for gray scale images using a different set of secret key and sizes is proposed.
Initially, the swapping and dispersion is done without keys and in second stage the image is mixed with the chirikov map involving first secret key.
'N' rounds are taken to complete this process.
The blended image is divided into blocks of block size 8X8.
These blocks are also swapped to achieve good confusion.
For making the encryption scheme more sturdy in each block the transmutation of pixels is done with the modified logistic map having three more secret keys.
The proposed scheme is simple, rapid and sensitive to the secret key.
Due to the high order of substitution, common attacks such as linear and differential cryptanalysis are unattainable.
The experimental results show that the proposed encryption technique is effective and has high security features.
Received Mar 1, 2014 Revised Apr 22, 2014 Accepted May 15, 2014 Keyword:
Symmetric encryption modified logistic map chirikov map differential cryptanalysis image encryption Copyright A 2014 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Nidhi Sethi.
Department of Computer Science and Engineering.
DIT University.
Makkawala.
Dehradun-248001.
India.
Email: nidhipankaj.
sethi102@gmail.
INTRODUCTION
T Recently, with the high demand in digital signal transmission and big losses due to illegal data access, data security has become a critical and imperative issue.
Encryption is being used to secure data and prevent them from unauthorized access.
Due to certain characteristics of digital images- redundancy of data, strong correlation among adjacent pixel, less sensitive as compare to the text data, especially the bulk quantity of data and the requirement of real-time processing, traditional ciphers such as DES.
AES.
RSA etc.
are not suitable for image encryption.
In order to protect digital images from unauthorized users doing illegal reproduction and modifications, a variety of image encryption schemes have been proposed.
The various ideas used in the existing image encryption techniques can be classified into three major types: pixel shuffling .
, 10, 18, .
, pixel transmutation .
, 2, 7, .
and the combination form .
, 8, 12, 13.
The shuffling algorithms interchange the position of pixels within the image itself and usually have low While the transmutation process transforms the original values of image into transmuted values.
This process has low hardware expense and estimating complications.
In the hybrid form the combination of both transmutation and shuffling is used and has good potential for security.
In the last decade many encryption schemes have been proposed to improve over security constraints in case of images.
In the following paragraph some recent image encryption schemes are discussed in crisp.
Chengqing Li et al.
, have reviewed four chaos based image encryption schemes.
He concluded that all lie under one umbrella and is composed of two basic techniques: permutation and combination of pixel value.
But in general all methods have security problems like insensitivity to change of plain-image, insensitivity to change of secret key, insecure diffusion function and the schemes can be broken with no more than .
ogl(MN) .
chosen images when iteration number is equal to one, where MN is dimension of Chong Fu et.
have used.
Chirikov standard map, to decor relate the strong relationship among Journal homepage: http://iaesjournal.
com/online/index.
php/IJECE A ISSN: 2088-8708 adjacent pixels hence employed to shuffle the pixel positions of the plain image.
After the decor relating the pixels, the pixel values are modified sequentially to confuse the relationship between cipher image and plain Blackedge et al.
have proposed a multilevel blocks scrambling scheme which is employed to scramble the blocks of coefficients which requires high computation.
The control parameters of the scrambling are randomly generated from the secret key dependent.
The key stream used to encrypt the scrambled image is extracted from the chaotic map and plain image.
W Puech et al.
have studied the various combination of chaotic maps based symmetric key cryptosystems like Logistic.
Henon.
Tent.
Cubic and Cheyshev.
He explained and reviewed the security, performance and reliability issues, of mapping.
RESEARCH METHOD
The proposed work comprises of image encryption algorithm which is broadly divided into two The first phase of the algorithm consists of swapping and dispersion.
Theses two processes do not involve any key.
These processes are only integrated to increase the confusion, diffusion and non linearity, but they themselves do not provide any security because of the absence of the key.
The second phase consists of the shuffling by Chirikov Standard map and mixing by modified logistic map.
The control parameters of Chirikov map and modified logistic map are the control parameters of diffusion and confusion respectively.
These control parameters and number of iterations is treated as secret keys.
The choice of chirikov map is made because after AonAo iterations the pixel at the corner most position or origin remain unchanged whereas in other maps the origin and some other pixel like (N.
N) or (N-1.
N-.
also remains same.
The modified logistic map is chosen because inspite of its simple equation it provides complex dynamic chaos.
1 Steps of Proposed Algorithm
(I)
Selection of keys :
Key Description Key 1-Chirikov map iteration Key2-Chirikov map control parameter .
Key 3 Key 4 Key 5 Key Value (II) First Phase (Without ke.
Step 1: Consider an image I (W x H) such that W and H are the width and height of I.
Split the image I to a set of N vectors of length L (L=64 in this wor.
Step 2: Calculate the value of O1 and O2 1 I i, j I i, j Step 3: Set x = O1 and y = O2 For i = 0A.
N-1, set the following information for each vector Vi from the set of N vectors .
Swapping Index= x.
Swapping Iteration =V.
Dispersing Index=y.
Dispersing Iteration=V.
x = x 1, y = y 1 Condition applied: If .
>= L, set them treat them as zero Step 4: Set the swapping index of the Vector Vi as a new start value from 0 to L-1.
For j from 0 to swapping iteration of vector Vi, swap the values from 0 to L-1, depending upon the conditions mentioned in the IJECE Vol.
No.
June 2014 : 456 Ae 462
IJECE
ISSN: 2088-8708
Step 5: Set Dispersion index of vectors Vi as a new start value of random number generation algorithm, which would be treated as initial condition parameter.
For j from 0 to dispersion iteration of vector Vi, generate random number N1with the values between .
to L-.
, then perform Vi(N.
= Vi(N.
mod Vi(Ni,.
Second Phase (With ke.
Step 5: Apply the Chirikov Mapping for AonAo iterations where n=15 over the whole image .
The initial parameter is K=512 used is key.
Step 6: Divide the whole image I1.
1,y.
into 8x8 size blocks.
B1.
B2, .
Bnob where nob= I1.
1,y.
/8X8.
Perform shuffling among the blocks.
Step 7: Create a matrix LM.
Convert each decimal gray value to its binary equivalent of the shuffled image LM.
Also create another matrix DMj is the 8- bit binary number obtained from 1D modified Logistic map.
Step 8: Perform exclusive-OR between DMj and SM.
to obtain the encrypted image EN.
2 Decryption: By using all the processes in reverse order, the original image can be retrieved with satisfactory security level, less computational complexity and hence fast, which proves to be a good candidate for real-time secure image transmission RESULTS AND ANALYSIS The proposed encryption algorithm is implemented in MATLAB 7 for computer simulations.
The standard a gray-scale AuLenaAy image of 128x128 in size and AuBabaAy image of 128 X 128 in size is taken for experimental purposes.
The original Lena image, intermediate image and its histogram are shown in figure 2.
The initial conditions and system parameters are: n=15.
K= 512, = 0.
39898, z.
= 0.
Figure 1.
Lena image original image, swapped & dispersed image, encrypted image .
Baba image original image, swapped & dispersed image, encrypted image A New Cryptographic Strategy for Digital Images (Nidhi Seth.
A ISSN: 2088-8708 Key Analysis:
Sensitivity of the keys: Almost all chaotic maps are sensitive to secret keys which means the initial The proposed encryption algorithm is receptive to any small difference to initial parameter.
Any change to the power of 10-14 in one of these parameter will result into entirely different.
Key Space: It is said in security that more is number of locks better is the safety .
In the proposed algorithm there are a total five initial parameters: two of Chirikov map and three of modified logistics Statistical Analysis:
Many attacks can be done which are based on the statistical analysis .
Statistical analysis has been performed on the test images to demonstrate the bad correlation among the pixels of the encrypted The results shown below shows that there is negligible correlation between pixels of the encrypted image in comparison to original image.
Correlation Coefficient Analysis: To estimate the encryption quality of the proposed encryption algorithm, the correlation is used .
For highly correlated image the correlation coefficients are almost 1 and for encrypted images the correlation coefficients is almost 0.
Entropy: Entropy is defined as the degree of randomness in the system.
It is known that the entropy H.
of a message source s can be calculated as:
Here the p.
is the probability of si.
We have calculated and found entropy 7.
9880, whuch is very close to the ideal value and hence the message leaking is imperceptible.
Differntial Attack Differential attack /cryptanalysis is a common name of attacks/cryptanalysis which is generally done to block ciphers which are working on binary sequences.
In this type of attack the dependency of cipher image and input image is analyzed.
NPCR: NPCR is Number of pixel change rate.
NPCR concentrates on the absolute number of pixels which changes value in differential attacks.
The formula and the respective condition is given in eq.
& .
0 C1 .
=C2.
1 if C1 .
=C2.
D i, j
NPCR
D .
= Here symbol T denotes the total number pixels in the cipher image and C1 and C2 are two ciphered images whose one pixel value is changed.
Table I shows the values of NPCR during experimentation .
If the value of NPCR is near 0.
99, it is treated as good.
Table 1.
Results of Encryption Scheme Baba Lena Entropy (Encryptio.
Correlation Coef.
NPCR
UACI
Histogram Analysis The histograms of enciphered images were analyzed and it was found that the histograms are usually uniform.
This property makes statistical attacks difficult in images .
The test on lena and baba image is shown below:
IJECE Vol.
No.
June 2014 : 456 Ae 462
IJECE
ISSN: 2088-8708
Figure 2.
Histogram of original and encrypted image (Len.
Figure 3.
Histogram of original and encrypted image (Bab.
The security analysis of the proposed work is compared with the other existing encryption schemes and comparision is shown in table 2 below:
Table 2.
Comparative Analysis of the proposed encryption scheme Image Author Correlation Entropy NPCR(%) UACI(%) Shubo Liu Debashish Choung Year of Pub.
Lena256X256 ----7.
Md.
Ali
Soheil
-----
----49%
----42%
Pareek
-----
Pareek
Lian
---------
---------
Over 99% ----1.
Proposed CONCLUSION In the history the Chaotic theory have proven to be a very good candidate for encryption .
The symmetric encryption schemes based on chaos theory have qualities like fast processing speed, simple, high sensitivity to keys and secure.
In the proposed algorithm two chaotic maps are used Chirikov map and A New Cryptographic Strategy for Digital Images (Nidhi Seth.
A ISSN: 2088-8708 modified logistic map.
To make the algorithm robust the image is scrambled and permuted without using Both security analysis and key analysis shows that the algorithm is resistant to many attacks like brute force attack, man in middle attack, plain text attack, entropy attack and chosen cipher attack.
There are tradeoffs between issues such as speed, cost, and complexity.
REFERENCES