14-03-2011, 10:00 PM
A Novel Technique for Image Steganography Based On Block-DCT and
Huffman Encoding
Arunima Kurup P& Poornima D Sreenagesh
S8, Department of Information Technology,
Mohandas College Of Engineering And Technology,Anad,Thiruvananthapuram
Huffman Encoding
Arunima Kurup P& Poornima D Sreenagesh
S8, Department of Information Technology,
Mohandas College Of Engineering And Technology,Anad,Thiruvananthapuram
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Abstract
Image steganography is the art of hiding information into a cover image. This paper presents a novel
technique for Image steganography based on Block-DCT, where DCT is used to transform original image
(cover image) blocks from spatial domain to frequency domain. Firstly a gray level image of size M× N is
divided into no joint 8 × 8 blocks and a two dimensional Discrete Cosine Transform(2-d DCT) is
performed on each of the P = MN / 64 blocks. Then Huffman encoding is also performed on the secret
messages/images before embedding and each bit of Huffman code of secret message/image is embedded in
the frequency domain by altering the least significant bit of each of the DCT coefficients of cover image
blocks. The experimental results show that the algorithm has a high capacity and a good invisibility.
Moreover PSNR of cover image with stego-image shows the better results in comparison with other
existing steganography approaches. Furthermore, satisfactory security is maintained since the secret
message/image cannot be extracted without knowing decoding rules and Huffman table
Introduction
With the development of Internet technologies,
digital media can be transmitted conveniently
over the Internet. However, message
transmissions over the Internet still have to face
all kinds of security problems. Therefore, how to
protect secret messages during transmission
becomes an essential issue for the Internet.
Encryption is a well-known procedure for secure
data transmission. The commonly used
encryption schemes include DES (Data
Encryption Standard) , AES (Advanced
Encryption Standard) and RSA. These methods
scramble the secret message so that it cannot be
understood. However, it makes the message
suspicious enough to attract eavesdropper’s
attention. Hence, a new scheme, called
“steganography”, arises to conceal the secret
messages within some other ordinary media (i.e.
images, music and video files) so that it cannot
be observed. Steganography differs from
cryptography in the sense that where
Cryptography focuses on concealing the contents
of a message, steganography focuses on
concealing the existence of a message.Two other
technologies that are closely related to
steganography are watermarking and
fingerprinting. Watermarking is a protecting
technique which protects (claims) the
owner’s property right for digital media (i.e.
images, music, video and software) by somehidden watermarks. Therefore, the goal of
steganography is the secret messages while the
goal of watermarking is the cover object itself.
Steganography is the art and science of hiding
information in a cover document such as digital
images in a way that conceals the existence of
hidden data. The word steganography in Greek
means “covered writing”(Greek words “stegos”
meaning “cover” and “grafia” meaning
“writing”)]. The main objective of
steganography is to communicate securely in
such a way that the true message is not visible to
the observer.
That is unwanted parties should not be able to
distinguish in any sense between cover-image
(image not containing any secret message) and
stego-image (modified cover-image that
containing secret message).
Thus the stego-image should not deviate much
from original cover-image. Today steganography
is mostly used on computers with digital data
being the carriers and networks being the high
speed delivery channels
Related Work
Steganography is a branch of information hiding
in which secret information is camouflaged
within other information. A simple way of
steganography is based on modifying the least
significant bit layer of images, known as the
LSB technique. The LSB technique directly
embed the secret data within the pixels of the
cover image. In some cases, LSB of pixels visited in random or in certain areas of image
and sometimes increment or decrement the pixel
value. Some of the recent research studied the
nature of the stego and suggested new
methodologies for increasing the capacity. Habes
proposed a new method (4 least Significant) for
hiding secret image inside carrier image. In this
method each of individual pixels in an image is
made up of a string of bits. He took the 4-least
significant bit of 8-bit true color image to hold 4-
bit of the secret message /image by simply
overwriting the data that was already there.The
schemes of the second kind embed the secret
data within the cover image that has been
transformedsuch as DCT (discrete cosine
transformation). The DCT transforms a cover
image from an image representation into a
frequency representation, by grouping the pixels
into non-overlapping blocks of 8 × 8 pixels and
transforming the pixel blocks into 64 DCT
coefficients each. A modification of a single
DCT coefficient will affect all 64 image pixels in
that block. The DCT coefficients of the
transformed cover image will be quantized, and
then modified according to the secret data. Tseng
and Chang in proposed a novel steganography
method based on JPEG. The DCT for each block
of 8×8 pixels was applied in order to improve the
capacity and control the compression ratio.
Capacity, security and robustness , are the three
main aspects affecting steganography and its
usefulness. Capacity refers to the amount of data
bits that can be hidden in the cover medium.
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
PSNR (Peak Signal to Noise Ratio)
The PSNR is expressed in dB’s. The larger
PSNR indicates the higher the image quality i.e.
there is only little difference between the cover-
image and the stego-image. On the other hand, a
smaller PSNR means there is huge distortion
between the cover-image and the stegoimage.
Conclusion
In this paper, we propose a steganography
process in frequency domain to improve security
and imagequality compared to the existing
algorithms which are normally done in spatial
domain. According to the simulation results the
stego-images of our method are almost identical
to other methods’ stego-images and it is difficult
to differentiate between them and the original
images. Our proposed algorithm also provides
additional three layers of security by means of
transformation (DCT and Inverse DCT) of cover
image and Huffman encoding of secret image.
The demand of robustness in image
steganography filed is not requested as strongly
as it is in watermarking filed. As a result, image
steganography method usually neglects the basic
demand of robustness. In our proposed method,
the embedding process is hidden under the
transformation i.e. DCT and inverse DCT. These
operations and Huffman encoding of secret
image keep the images away from stealing,
destroying from unintended users and hence the
proposed method may be more robust against
brute force attack.
References
[1] DES Encryption Standard (DES), National
Bureau of Standard (U.S.). Federal Information
Processing Standards Publication 46, National
Technical Information Service, Springfield, VA,
1997.
[2] Daemen,J., and Rijmen, V. “Rijndael: The
Advanced Encryption Standard”, Dr. Dobb’s
Journal, March 2001.
