Polynomial-Based Image Sharing - Visual Cryptography and Secret Image Sharing - page 401

FIGURE 15.2
The format of stego-block

B ij with the size of 2 2 gray-level pixels.

FIGURE 15.3
Stego-block B ij used in Yang et al. which is revised from Figure 15.2, where
hash is used in the pixel of wˆi i = (w i1 ; ;w i5 ;h i ;y i3 ;y i4 ).

v i = (v i1 ;v i2 ; ;v i5 , y i6 ;y i7 ;y i8 ),
and u i = (u i1 ;u i2 ; ;u i5 ;y i6 ;y i7 ;y i8 ),
where b0 i is the parity bit, which is chosen to make w i including the cor-

B ij is shown in Figure

responding bit-string bi. i . The format of stego-block

15.2.

B ij generated in the Step 3 to create stego-image I j .

Step 4. Collect

15.3.3 Yang et al.'s Scheme
As observed from the Lin-Tsai scheme, the fake shares can be easily
made by the manner of fixing the parity bit b′i. 0 i . For example, if a i =
(w i1 ;w i2 ; ;w i5 ;b 0 i ;y i1 ,y i2 ) = (0; 0; 1; 1; 1; 0; 1; 1), the parity bit b 0 i is all the
same when the original wˆi i is changed to a new wˆi i as w i = (1; 0; 1; 1; 1; 0; 0; 1).
In such a way, the malicious attackers can easily modify w i 0 s in the stego-
image to pass the authentication detection. As a result, the revelation of the
target secret image is not a right one. Next, Yang et al. [12] proposed a revised
version to set a new check function, e.g., hash function, instead of parity bit
in [4]. The stego-block B ij in [4] is then revised as the format shown in Figure
15.3. As analyzed in Yang et al., the probability of passing the detection with
the fake share offers is ( 2 ) m 2 .

15.3.4 Inverted Pattern LSB Scheme
In order to conceal the existence of shadow data, steganographic methods
are used to keep the shadows imperceptible. The steganographic method is a
kind of data hiding technique that embeds secret data in preselected mean-
ingful images, called cover images, with people being visually unaware of the
secret's existence. One common technique to achieve the data hiding is the
LSB-based approach, in which secret data are embedded by directly replacing
the least significant bits (LSBs) with equal bits of the secret for each pixel.