lem, A. Ker (Ker, 2005) proposed the LSB match-
ing scheme, but this scheme is vulnerable to detec-
tion algorithms. In order to minimize the effect of
detectability Rehab H. (Alwan et al., 2006) proposed
a novel scheme of image embedding which detects
the edge of the image using Sobel mask filters. On
the LSB of each edge pixels, a gray level connectiv-
ity is applied using fuzzy logic and the ASCII code
information is embedded into the edge pixels. The
well known steganographic scheme is Least Signifi-
cant Bit (LSB) substitution scheme. This scheme di-
vides the secret message into n bit blocks and em-
beds each of these n bits by directly replacing the
n LSBs of a pixel of the cover image (Wang et al.,
2001; Thien and Lin, 2003). Using LSB substitution
schemes, more number of secret bits can be hidden
into cover image with low computational complex-
ity (Chan and Chen, 2004; Nagaraj V. Dharwadkar,
2010). Based on the ability of the steganographic
schemes to recover the cover images during extrac-
tion, the schemes are classified as reversible (Hon-
singer et al., 2001; Fridrich et al., 2002; Tian, 2003)
and irreversible(Chan and Cheng, 2004; Mielikainen,
2006). The reversible steganographic schemes are
able to recover the original cover image during ex-
traction of the secret message; where as in the irre-
versible steganographic scheme the secret message is
extracted from the stegoimages with no capability of
recovering the cover image into its original state.
The embedding capacity, visual quality and secu-
rity are three important issues concerned to a success-
ful steganographic schemes (Wang et al., 2001). The
crucial issue of the steganographic scheme is rigidity
of scheme to different types of attacks. To address is-
sues like embedding capacity, visual quality and secu-
rity of stegoimage, Chin-Feng et.al.(Lee et al., 2009).
In 2010, to address similar issues we have proposed
a scheme (Nagaraj V.Dharwadkar, 2010) which is an
improved reversible steganographic scheme based on
dual stegoimages. In Chin-Feng et.al. scheme, a max-
imum of two secret bits are embedded into a pair of
pixels which are originated from one original cover
image and its copy. This scheme achieves an embed-
ding capacity of 0.75 bpp. Where as the earlier pro-
posed our scheme embeds the three secret bits into
pair of pixels (Nagaraj V.Dharwadkar, 2010). This
scheme achieves an embedding capacity of 1.21 bpp.
This scheme is purely blind scheme and it will not use
any auxiliary array in extraction algorithm.
After analysing these schemes it was found that
the embedding capacity of these scheme can be pos-
sible to further increase using auxiliary array. To
achieve the high embedding capacity, in this paper
we propose an improved reversible steganographic
scheme using two cover images. In this scheme,
five bits of secret image are embedded into a pixel
pair which are alternatively selected from the orig-
inal cover image and its copy. We have used aux-
iliary array known as stego-key which increases the
embedding capacity and security of the scheme. This
scheme provides reversibility and high security with
less distortion in stegoimage. We have analyzed the
proposed scheme for its embedding capacity and its
robustness to different types of image processing at-
tacks.
The rest of the paper is organized as follows. The
proposed steganographic scheme is explained in Sec-
tion 2. Section 3 gives details of the experimental re-
sults. Section 4 gives the comparison of the proposed
scheme with the Chin-Feng Lee et. al and our own
earlier proposed scheme. The effect of image pro-
cessing attacks on dual stegoimages is discussed in
Section 5. Section 6 concludes the paper.
2 PROPOSED SCHEME
In the gray-scale image the intensity value of pixel
is represented by 8 bits value. The proposed scheme
relies on binary stream of intensity of pixel to de-
fine space for embedding the secret bits. We con-
sider two identical cover images P and Q each of
size m × n. In Figure 1, the cover images P and Q
are represented as a matrices (P
i, j
)
1≤i≤m;1≤ j≤n
and
(Q
i, j
)
1≤i≤m;1≤ j≤n
. For embedding data we choose
a pair of pixels (P
i, j
,Q
i, j
) each from P and Q. If
P
i, j
is used for embedding secret, then P
i, j
is re-
ferred as the Embed pixel and Q
i, j
is referred as the
Ref pixel. For the next pair (P
i+1, j
,Q
i+1, j
), P
i+1, j
is Ref pixel and Q
i+1, j
is Embed pixel. Likewise,
pair of pixels are chosen row-wise from each cover
image P and Q. The proposed scheme embeds five
bits into a pair of pixels by maintaining the negligi-
ble difference between the original pixel and modi-
fied pixel. To achieve negligible difference the mod-
ified pixel is scaled up or down. The scale factors
which are used to preserve the difference narrow is
encoded in a location-map which is known as stego-
key. The hidden message is encoded in two places
: the image and the location-map. The secret bits
S
k
are selected and embedded into Embed pix
k
for
0 ≤ k ≤ 4 using look-up table as shown in Figure
2 to generate resultant Res pix.Later, the difference
d = Embed pix− Res pix is computed. If the differ-
ence |d| > 3 and d > 0 and to make Res pix equal
to Embed pix 4 is added count number of times and
stored in Res pix. Otherwise if d < 0 to make Res pix
equal to Embed pix 4 is subtracted count number of
SECRYPT2012-InternationalConferenceonSecurityandCryptography
16