2. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 2, April 2012
frequency domain method usually used in image processing 2) Public Key Cryptography: Uses one key for
is the 2D discrete cosine transform (DCT) [10][11] and 2D encryption and another for decryption.
discrete wavelet transform[12]. In DCT steganography the 3) Hash Functions: Uses a mathematical transformation
image is divided into 8×8 blocks and DCT transformation on to irreversibly “encrypt” information.
each block is performed. The data bits are embedded in the A. RSA encryption algorithm
low frequency coefficients of DCT. SSB-4 & DCT RSA is a Public key cryptography named after its
steganography proposed by Nedal M. S. Kafri and Hani Y inventors: Ronald Rivest, Adi Shamir and Leonard Adleman.
Suleiman uses DCT approach with SSB-4 technique [11]. RSA can be used for encryption as well as for authentication
The DWT steganography uses both image‟s spatial as well as [3]. An example of Alice and Bob, who want to use
frequency characterstics. DWT divides the image in four sub asymmetric RSA algorithm for secure communication is
bands (LL, LH, HL and HH) and then data can be embedded shown in fig. 1. For encryption purpose, Alice would encrypt
in coefficients of one of the selected sub band. the message using Bob‟s Public key and send the cipher text
Steganography with cryptography can be combined so to Bob. Upon receiving the cipher text, Bob, who is owner of
that, even if an attacker does realize that a message is sent, he corresponding private key, can then decrypt the message with
would still have to decode it [13]. Piyush Marwaha and his private key. For authentication purposes, Alice would
Paresh Marwaha use DES encryption and LSB encrypt (or sign) the message using her own private key.
steganography for data security [14]. In this paper we Other people such as Bob can verify the authenticity of the
propose a method which uses RSA encryption and DWT message by using Alice‟s Public key, which is the only key
steganography for data security. that matches the signing private key.
Security and robustness are the main aspects affecting
steganography and its usefulness. Security relates to the
ability of an eavesdropper to figure the hidden information
easily. Robustness is concerned about the resist possibility of
modifying or destroying the unseen data.
C. PSNR (Peak Signal to Noise Ratio)
PSNR computes the peak signal to noise ratio, in decibels,
between two images. This ratio is used as quality
measurement between two images. To calculate PSNR; first
MSE is calculated as follows:
1 m1 n 1
MSE I (i, j) K (i, j)
mn i 0 j 0
2
(1)
Where MSE is the Mean Squared Error of Original image
(I) and stego image (K). Thereafter PSNR value is calculated
as follow:
MAX i
2
MAX i
PSNR 10. log 10
MSE 20. log 10
( 2) Fig. 1 RSA Encryption
MSE
The steps for RSA algorithm are:
Where, MAXi is the maximum pixel value of the image. In
other words MAXi = 2b − 1, where b is the bit depth of the 1) Select two prime numbers r, s.
original image. The larger PSNR indicates the higher the 2) Calculate n= r × s and φ(n)= (r-1)(s-1)
image quality i.e. there is only little difference between the 3) Select integer „e‟ such that e is relatively prime to
cover-image and the stego-image. On the other hand, a φ(n).
smaller PSNR means there is huge distortion between the gcd (φ (n),e)=1; 1<e < φ(n)
cover-image and the stegoimage. 4) Calculate d such that d × e=1mod(φ (n))
5) Now Public key (PU) for encryption is {e, n} and
II. BACKGROUND OF CRYPTOGRPHY Private Key (PR) for decryption is {d, n}.
6) At sender side, message (M) is converted into cipher
In cryptography, the message is scrambled to make it
text (C) as follows:
meaningless and unintelligible unless the decryption key is
C= Me mod n (3)
available. It makes no attempt to disguise or hide the encoded
7) At receiver side, cipher text is converted back to
message. Basically, cryptography offers the ability of
original message as follows:
transmitting information between persons in a way that
M= Cd mod n (4)
prevents a third party from reading it. Cryptography can also
provide authentication for verifying the identity of someone III. HAAR- DWT TRANSFORM
or something. There are several ways of classifying Wavelets are special functions which (in a form
cryptographic algorithms. The three types of algorithms are: analogous to sins and cosines in Fourier analysis) are used
1) Secret Key Cryptography: Uses a single key for both as basal functions for representing signals. In addition to
encryption and decryption. being an efficient, highly intuitive framework for the
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4. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 2, April 2012
1) Perform Haar-DWT transform on stego image to Table 1. Comparative analysis of PSNR values of different
decompose it into four sub bands (LL, LH, HL and steganography techniques
HH). PSNR Value
Image
2) Apply mod2 operation on coefficients (Pi) of selected LSB LSB-DCT RSA & DWT
sub band (LH) to extract data (mi) in following way: animal.jpg 52.62 54.66 55.87
Qi = mod2 (Pi)
human.jpg 53.31 55.24 57.36
Message bit mi = Qi
3) Concatenate the message bits to obtain cipher message. building.jpg 53.12 54.46 56.52
4) Decrypt the cipher message using RSA decryption keys flower.jpg 52.78 54.86 56.35
and display it on screen.
VI. CONCLUSION
V. EXPERIMENTAL RESULTS
In this paper we used a mixed approach cryptography and
Since the visual detection of stego images is depending on steganography is used for data security. By using RSA
the nature of the image [15] so, varieties of image categories encryption, ASCII codes corresponding to characters of plain
are utilized in the experiments. The experimental image data text are converted into 16 bits encrypted codes. Hence it
set consists of 100 JPEG images, which were taken by digital becomes difficult to get original text without knowing
camera. We focused on short messages with length of 3000 decryption keys. Then cipher data is hided into cover image.
bits because they are the most challenging to detect [15]. Average PSNR value of 56 is obtained for 100 images using
proposed method. The obtained experimental results indicate
that, the proposed method is a good and acceptable scheme
for data security. Furthermore, by embedding information in
the least significant bits of the DWT domain, the hidden
message resides in more robust areas, spread across the entire
stego image, and provides better resistance against statistical
attacks than other techniques. The future work may focus on
(a) Original animal.jpg (b) Stego animal.jpg the improvement and further development in this technique.
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