  Image steganography is art of hiding information onto the cover image. In
this proposal a transformed domain based gray scale image authentication/data
hiding technique using Z transform (ZT) termed as FDSZT, has been proposed.
ZTransform is applied on 2x2 masks of the source image in row major order to
transform original sub image (cover image) block to its corresponding frequency
domain. One bit of the hidden image is embedded in each mask of the source
image onto the fourth LSB of transformed coefficient based on median value of
the mask. A delicate handle has also been performed as post embedding operation
for proper decoding. Stego sub image is obtained through a reverse transform as
final step of embedding in a mask. During the process of embedding, dimension
of the hidden image followed by the content of the message/hidden image are
embedded. Reverse process is followed during decoding. High PSNR obtained for
various images conform the quality of invisible watermark of FDSZT.
