Cryptography Reference
In-Depth Information
Step 4:
Generate a check bit b according to parity check policy to achieve
the authentication capability shown in Figure 16.2.
FIGURE 16.2
The block of the k-th stego-image in Lin-Tsai's scheme.
Unfortunately, in Lin-Tsai's scheme, the parity bit of the upper-right pixel
shown in Figure 16.2 is chosen to make this pixel an even or odd parity as
a binary parity sequence. Dishonest participants can derive the parity in-
formation from their own stego-images, and thus can easily and maliciously
counterfeit a stego-image. For instance, assume that the upper-right pixel V ij
= (11011 111). A dishonest participant can modify it to (11011 010), which
still meets the odd parity, but the 8-bit input of (k-1)-degree polynomial be-
comes changed. Thus, it successfully passes authentication but the (k-1)-degree
polynomial cannot be obtained. In addition, the dishonest participant can also
modify three other pixels, Y ij , W ij , and Z ij , to change the input and output
of (k-1)-degree polynomial without influencing the upper-right pixel V ij , thus
passing authentication. Therefore, their scheme has a weak authentication
process, which may allow a fake stego-image to pass the authentication check
quite easily.
16.2.2 Yang et al.'s Scheme
To overcome the weaknesses in Lin-Tsai's scheme, Yang et al. [28] proposed
an improved version in 2007. In Yang et al.'s scheme, the modulus value p is
 
 
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