Cryptography Reference
In-Depth Information
noise-like. BPCS can extract embedded information by using the simple
thresholding in the complexity because secret data is replaced with only com-
plex regions. To compensate the extraction, the logical operation called con-
jugate operation is used for the binary patterns look like informative regions.
Therefore, we need to keep, for each block, the flag called conjugation flag
representing whether the conjugate operation had been applied.
In the case where we build a secure system to exchange secret messages
using BPCS, which is explained latter in detail, the conjugation flags must be
embedded within a fixed area of cover images. When the conjugate flags are
embedded in the manner, we can easily see unnatural patterns by observing
the image visually. It can be used as a signature or a distinguishing mark
between natural images and images with information embedded by BPCS,
and it is regarded as a visual attack [10, 11, 12] to BPCS.
We propose a countermeasure to the visual attack of BPCS. The reason
why BPCS is weak against the visual attack is to embed conjugation flags
within a fixed area. The proposed method can embed the flags into each
block by the thresholding in the complexity, as a result of the embedding, the
distinguishing mark would be removed.
8.4.1 Finding Signature of Information Embedding by Visual
Observation
In the secret key cryptosystem consisting of a sender and a receiver connected
with an insecure channel, the important messages between them is encoded
with a key at the sender and the encoded one is transmitted to the receiver
over the insecure channel. At the receiver side, the received message is decoded
with the key. It is possible to encode and decode with only one key. If we
exchange somehow the key by using secure channel once, after that, we can
keep exchanging encoded messages without exchanging the key again. If we
require the convenience of the cryptosystem to BPCS, it is desirable to choose
information which does not depend on image data and secret data as the key.
In order to extract the embedded information from stego-images with se-
cret data embedded by BPCS, we need to know the information about the
threshold and the block size used in embedding, and conjugation flags. Con-
jugation flags are not candidate for the key because those depend on image
data, block size, secret data and the threshold. Therefore, conjugation flags
must be embedded within cover images in the system.
The conjugation flags, however, are not embedded by the complexity
thresholding because the additional conjugation flag is needed for the blocks
in which the conjugation flags are embedded. Therefore the conjugation flags
must be embedded into a fixed area of cover images.
There are several ways to embed conjugation flags within a fixed area on
cover images. The simplest way to do that is to embed those information
into a fixed area on LSB (Least Significant Bit) plane. We can embed it
without degrading the quality of cover images because it is not noticeable
