Polynomial-Based Image Sharing - Visual Cryptography and Secret Image Sharing - page 402

Moreover, some papers applied various strategies to improving the quality
of stego-images generated by LSB-based approaches. In 2008, Yang [11] pro-
posed an Inverted-Pattern-LSB information hiding algorithm (IPLA for short)
to further highlight the quality of the stego-image. His method combines the
idea of processing secret messages before embedded and processing the stego-
image after embedding. Before embedding, the secret is transformed into a
suitable format so as to benefit the next embedding step. Moreover, the bits
that are used to record the transformation are the critical key in the course of
the secret revelation. The IPLA algorithm is shown below, which was applied
in Wang et al.'s scheme.

Inverted-Pattern-LSB Algorithm (H, S, k, n)
Input: A cover image H and a secret string S for k-bit LSB substitution, z is
the number of sections.
Output: A stego-image and an inverted pattern P.

Step 1. Partition both H and S into z sections evenly. Let H =
H 0 ;H 1 ; ;H z1 , S = S 0 ;S1 ; ;S z1 and R = R 0 ;R 1 ; ;R z1 ,
where R is the replaced string of H. Also, let the inverted pattern P =
p 0 ;p 1 ; ;p z1 , where p i is a bit, for i = 0; 1; ;z 1.

Step 2. For i = 0 to z 1
If MSE s i R i MSE

s i R i

Then p i = 0
Else pi i = 1

Step 3. For i = 0 to z 1
If p i = 0
Then Embed S i into Hi i
Else embed

S i into Hi i

Step 4. Return the inverted pattern P.

15.3.5 Scalable Secret Image Sharing
The conventional secret image sharing (SIS) scheme, such as the Thien-Lin
scheme [8], only has the threshold property that recovers either the entire
image or nothing. For the (t;n)-SIS scheme, the target secret image could only
be reconstructed by only t or more than t qualified participants. This limits
its possible applications. Recently, Wang and Shyu recommended adding the
scalable decoding capability (the scalability) into the threshold scheme [9]. The
so-called scalability is that the amount of secret information is proportional
to the number of shadows used in reconstruction. Wang and Shyu constructed
a polynomial based (2, n) scalable SIS (SSIS) scheme, which not only had the
threshold property but also the scalability. The clarity of an image in [9] is
measured in terms of three modes: the multisecret mode (spatially partitioning