Post on 07-May-2018
Evaluation of Image Stegnography using RSA and MRSA
S. Satheeshkumar1
and S. Sivakumar2
1Research Scholar, Department of Computer Science, C.P.A. College, Bodinayakanur,
satheeshvasan007@gmail.com 2Associate Professor and Head, Department of Computer Science, C.P.A. College, Bodinayakanur.
sivaku2002@yahoo.com
Abstract Steganography is one of the most powerful technique to conceal the existence of hidden secret data inside a cover
object. In stegnography, the Least Significant Bit (LSB) is one such technique in which least significant bit of the
image is replaced with data bit. As this method is vulnerable to steganalysis, so as to make it more secure, encrypt
the raw data before embedding it in the image. Though the encryption process increases execution time, but at the
same time it provides higher security. In this paper RSA and Modified RSA (MRSA) algorithm used for encryption
and the results evident that MRSA outperform RSA in terms of execution time. Keywords: Stegnography, Cryptography, LSB, RSA, MRSA
1. INTRODUCTION
Among the recent advances in computing technology and its intrusion in our day to day life, the
need for private and personal communication has increased. Privacy in digital communication is
desired when confidential information is being shared between two entities using computer
communication. To provide secrecy in communication we use various techniques. One such
technique is steganography that is the art of hiding the fact that communication is taking place,
by hiding information in other information[1-2].
Steganography techniques are not enough to give more security in cyber world [3]. In addition to
steganography, cryptography technique needed to make stronger the level of security. It is
always a good practice to use Cryptography and Steganography together for adding multiple
layers of security [4]. Because Cryptography (protecting information) scrambles a message so
it cannot be understood; the Steganography (hiding information) hides the message so it cannot
be seen[5]. The data encryption can be done by a software and then embed the cipher text in
image or any other media. The combination of these two methods will enhance the security of
the data embedded.
In cryptography, Public key cryptosystem avoid sending secret keys over communication
channels, i.e. reducing the risk of key compromise. Many algorithms are introduced based on the
public key cipher for privacy and authentication; the most widely known and used of them is the
Ron Rivest, Adi Shamir, and Leonard Adleman, who first publicly described the algorithm in
1977 RSA crypto-algorithm [6-8].
RSA crypto-algorithm is based on performing exponentiations in modular arithmetic both for
encryption and decryption [6]. RSA makes use of the modular exponent (ME) or modular
multiplication (MM) algorithm. Many research works in order to speed up the performance of
MM or ME algorithm [9-10]. Encrypt assistant multi-power RSA was proposed to enhance RSA
decryption performance [11].
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ME technique has made RSA method very attractive for data security and authentication.
However, the challenge of the RSA cryptosystem lies in the slow computation of encryption /
decryption and high time complexity [12]. The strength of RSA algorithm and a survey of fast
exponentiation method are explored [13]. The efficiency of RSA encryption and decryption is
primarily depends on the efficiency of the ME algorithm [14-15].
Modified modular exponent based RSA algorithm reduces execution time for encryption and
decryption process[16]. In Stegnography LSB is a method to embed secret message in cover
image. [17-19]
This research work analyse the performance of LSB method by incorporating encrypted data in
place of plain textual data in cover image. For encryption / decryption both RSA and MRSA
algorithms are used. To evaluate the performance of image stegnagraphy using RSA and MRSA
with execution time in seconds and quality of image in MSE and PSNR.
2. MATERIALS AND METHODS
For implementation of proposed method, four MATLAB standard images were taken. The cover
image details are tabulated in Tab. 1. Size of 94120 bits text message is used for encryption and
decryption in RSA and MRSA algorithms.
Table 1: Details of Cover images
Image Name Dimension Type
Cameraman 256 x 256 JPEG
Football 320x256 JPEG
Onion and pepper 198x135 JPEG
Coins 300x246 JPEG
To provide higher security the secret information is encrypted first, the encrypted ASCII is
converted to binary. In LSB technique the image pixels are also converted into binary form. The
image is now used as a cover to embed the encrypted text. This process is done by LSB encoder
which replaces the least significant bit of pixel value with the encrypted text. The cover image
with encrypted text is now termed as Stegno image. The whole process is illustrated with the
block diagram shown in Fig. 1.
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Figure 1: Proposed steganography mechanism for sender
Upon reception of Stegno image the receiver first converts the pixels into their corresponding
binary values. The LSB decoder then detaches the encrypted text from stegno image pixels. The
encrypted text is decrypted using decryption algorithm. Fig. 2 shows the entire working process
at the receiver end.
Figure 2: Proposed steganography mechanism for receiver
3. LEAST SIGNIFICANT BIT (LSB) A simple approach for embedding information in cover image is using Least Significant Bits
(LSB). The simplest steganography techniques embed the bits of the message directly into least
significant bit plane of the cover image in a deterministic sequence. 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.
Because this method uses bits of each pixel in the image, it is necessary to use a lossless
compression format, 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.
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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 needed to be changed to insert the character successfully. On average,
only half of the bits in an image will need to be modified to hide a secret message using the
maximal cover size. The result changes that are made to the least significant bits are too small to
be recognized by the human visual system (HVS), so the message is effectively hidden.
Respectively, The LSB encoder and decoder steps are shown in Pseudo code 1 and 2.
Pseudo code 1: LSB encoder steps Step 1: Extract the pixels of the cover image.
Step 2: Extract the characters of the text file.
Step 3: Insert characters of text file in each LSB component of pixels by replacing it.
Step 4: Repeat step 3 till all the characters has been embedded.
Step 5: Place some terminating symbol to indicate end of data.
Step 6: Obtained stegno image.
Pseudo code 2 : LSB decoder steps
Step 1: Extract the pixels of the stegno image.
Step 2: Extract secret message characters from LSB component of pixels. Follow step 2 till up to terminating
symbol, otherwise follow step 3
Step 3: Extract secret message.
4. RSA
RSA is generally used as a public-key cryptosystem which is based on modular exponentiation.
In RSA algorithm there are two keys used to encryption (public key) and decryption (private
key). The public key is advertised to the receiver by sender and the private key should be kept
mystery. In this manner an unknown person can‟t decrypt the encrypted message without private
key. The working technique of RSA algorithm discussed in below:
i. Choose two random prime numbers(p, q)
ii. Calculate n p q
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iii. Calculate ( ) ( 1) ( 1)n p q
iv. Select an integer „e‟ such that 1 ( )e n and (e, ( )) 1GCD n .
v. Calculate 1
mod ( )d e n
vi. Encryption: Given a plain text m satisfying m < n, then the Cipher text c = me mod n.
vii. Decryption: The cipher text is decrypted by m = cd mod n.
The security of RSA based on the length of the key, longer the key-length are more safer
for the data. When the key length is longer, then the encryption and decryption takes long. RSA
includes Euclidean technique to compute „d‟ value and Modular Exponentiation (ME) technique
is used to perform encryption and decryption work, ME leads to consume much time while
execute RSA.
5. MODIFIED RSA
Basically RSA encryption and decryption based on ME algorithm, to improve the speed of RSA
algorithm a modification in ME component is essential. For reduce execution time of encryption
and decryption Modified Modular Exponentiation (MME) algorithm used in MRSA.
MME technique is shown in pseudo code 3.
Pseudo code 3 : MME technique
Read e,m,n
Initialize c=1, t=0, I=0 and A[] as array
For k=1 to e
Calculate c = (c*m)%n , A[k] = c, t = t+c
If (t%n) equal to zero then
Calculate I = ( e % k )
Print A[I]
Jump from condition and loop
Else if k equal to e then
Print c
End if
End of loop
MRSA algorithm will reduce execution time of encryption and decryption. The working
procedure of MRSA is shown in flow diagram Fig.3.
The MRSA algorithm provides more security and speed in encryption and decryption process
compare with RSA. Hence more security for text stegnography is possible with MRSA method.
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Figure 3: Flowchart for modified RSA Algorithm
6. RESULTS AND DISCUSSION
For experiments, four images are carried out to prove the efficiency of proposed method.
Figure 4: Process of stegnography work for cameraman cover image
Start
Input Prime Numbers p, q
Compute
Key generation
e, d
Input Message (M)
Compute C
C = Me
(mod n)
Compute M
M = Cd
(mod n)
End
Modified
Modular
Exponent
Algorithm
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For example, cameraman image taken as cover image and 94120 bits text message treated as
secret text message. The secret text message is encrypted by RSA and also MRSA. For
encryption and decryption, the following key values are carried out, p=13, q=19,n=247, public
key (41,247) and private key (137,247). Next encrypted text message hidden in cover image
using LSB encoder. Finally stegno image are obtained. Process of stegnography work for
cameraman cover image shown in Fig. 4. Similarly other three images are carried out. The cover
image and stegno image depicted in Fig. 5. The measurement of the quality between the cover
image and stegno image is done using PSNR (Peak Signal to Noise Ratio) and MSE (Mean
Square Error) value. The PSNR is defined as
𝑃𝑆𝑁𝑅 = 10𝑙𝑜𝑔 2552
𝑀𝑆𝐸
Where,
𝑀𝑆𝐸 =1
𝑀𝑁 𝑓 𝑖, 𝑗 − 𝑔(𝑖, 𝑗) 2
𝑁
𝑗=1
𝑀
𝑖=1
f(i,j) and g(i,j) means the intensity value of pixel at position (i,j) of the cover image and stegno
image respectively. Larger PSNR indicates the higher the image quality i.e. there is only small
difference between the cover-image and the stegno-image. A smaller PSNR means there is huge
distortion between the cover-image and the stegno image. Tab. 2 shows the PSNR and MSE
values between cover image and stegno image. From Tab.2 the image quality is good, based on
MSE and PSNR. Execution time based comparison of RSA and MRSA algorithm in LSB values
are tabulated in Tab. 3. From Tab. 3 MRSA give better performance compare with RSA and
results show that decryption is better than encryption based on execution time.
Figure 5: The different images of cover images and it‟s stegno
(a) Cameraman (b) Foot ball (c) onion and pepper (d) coins
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Table 2: MSE and PSNR values of our algorithms with different images
Image Name MSE PSNR
Cameraman 0.4144 51.9569
Football 0.5216 50.9574
Onion and pepper 0.5330 50.8636
Coins 0.4857 51.2668
Table 3: Comparative Results of RSA and MRSA
Image
Execution Time (sec)
RSA MRSA
Encryption Decryption Encryption Decryption
Cameraman 0.015464 0.003744 0.014778 0.002862
Football 0.012057 0.006985 0.010929 0.006833
Onion and pepper 0.014334 0.001570 0.012995 0.001221
Coins 0.019956 0.001156 0.018953 0.001030
7. CONCLUSION
An efficient steganography method for embedding encrypted secret messages into cover images
without producing any major changes has been accomplished using RSA and MRSA algorithm.
A new way of hiding information in an image have been developed, which makes our technique
additional secure level and more efficient than RSA. RSA and MRSA algorithm based LSB
technique for image steganography has been implemented. When compare MRSA with RSA,
MRSA gives better results based on their execution time.
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