Cryptography Reference
In-Depth Information
Fig. 13.9.
Quads configuration in an image.
The retrieval process starts by identifying all the changeable quads in
the embedded image using the conditions in Eq. (13.22). The LSBs of the
difference values of all the changeable quads are collected to form a bitstream
B. The JBIG algorithm is then used to decompress bitstream B to retrieve the
location map. By use of the location map, all expandable quads are separated
from the changeable quads. The original image can be restored by dividing
each difference in the expandable quads by 2, and replacing the LSBs of
each difference in the changeable quads with the retrieved original bits. The
retrieved authentication hash is compared to the hash function of the restored
image. If they match exactly, the image content is authentic and the restored
image will be exactly the same as the original image.
Alatter tested the quad-based algorithm on several test image. These were
Fruits, Lena and Baboon. The experimental results indicate that the achiev-
able embedding capacity depends on the nature of the image. The algorithm
performs much better with Fruits and Lena than with Baboon. It performs
slightly better with Fruits than with Lena. With Fruits, the algorithm is able
to embed 982 kB (3.74 bits/pixel) with a image quality of 28.42 dB. It is also
able to embed 296 kB (0.77 bits/pixel) with the high image quality of 39.05
dB. With Baboon the algorithm is able to embed 808 kB (3.08 bits/pixel) at
20.18 dB and 130 kB (0.50 bits/pixel) at 32.62 dB.
Alatter respectively compared the performance of quad-based algorithm [12]
with that of Tians method described in [11] using grayscale Lena and Barbara
images. The results indicate that quad-based algorithm outperforms Tians at
