Cryptography Reference
In-Depth Information
Table 8.1.
Meaning of the control pixel and its value.
Control Pixel
β(S
CP=0
)
Adjacent pixel patterns
Meaning
Value
BB
CAB
1
⌈β
TH
⌉−2or⌈β
TH
⌉−1
WW or BW
CF
1
BB or WW
SD
b
n
⌈β
TH
⌉ or⌈β
TH
⌉+1
SD
0(ifb
n
=0)
BW
CF
1(ifb
n
=1)
BB
CAB
0
⌊β
∗
TH
⌋−1or ⌊β
∗
TH
⌋
WW or BW
SD
b
n
BB or WW
CF
0
⌊β
∗
TH
⌋+1 or⌊β
∗
TH
⌋+2
CF
0(ifb
n
=0)
BW
SD
1(ifb
n
=1)
is less than α
TH
. If we make the value of the control pixel of S change to 1,
then the maximum change in the border length is 2, thus the complexity of
S
CP=1
may become⌈β
TH
⌉. The control pixel of the S
CP=1
can be extracted as
the secret data because, after the conjugate operation, α(S
CP=1
) falls within
a range of α
TH
and 1−α
TH
. Thus, it would be impossible for this example
to recover embedded information because the meaning of the control pixel is
determined as not conjugation flag, but one bit of secret data.
To avoid this miss determination, we assign another function to adjust
the complexity to the control pixel. Suppose that
CF
,
SD
and
CAB
represent
conjugation flag, secret data and the complexity-adjustment bit, respectively.
The control pixel holds one of CF, SD or CAB, and takes on one of 0 and
1 value. Those meaning and values depend on the complexity of S, one bit
of the secret data which is embedded into the control pixel of S and the
adjacent-pixel pattern of the control pixel.
The proposed method, which is robust against the visual attack, is given
as the following.
Step 1) Choose a pixel from four corners of S as the control pixel, and ini-
tialize it as 0.
Step 2) Map the bit sequences of secret data to the pixels of S except for the
control pixel.
Step 3) Calculate the complexity of S
CP=0
.
Step 4) By the calculated complexity, define the meaning and the value of
the control pixel.
• If β(S
CP=0
) <β
TH
−2, then the meaning of the control pixel of S
is CF and its value is 1. Then the conjugate operation is applied
to S.
• If β
TH
+2≤β(S
CP=0
)≤β
∗
TH
−2, then the meaning of the control
pixel of S is SD and its value is one bit of the secret data.
