CUI- University of Geneva
Multimedia Security
TP 9 – Robust watermarking: embedding and extraction
Tasks:
1. Watermark generation, encryption and encoding.

The secret message is an ASCII string - student’s name and surname, i.e. “FedericoRossi”
of the length of 8-16 characters (if longer, shrink it to 16 characters). Convert it to a binary
string of the length of 128 bits (8 bits per character). Encrypt the binary string using
permutation based on an input decimal key wmKey (see TP2, task 1) and display it.

Perform encoding of the given binary sequence using Turbo code routine (the coding rate is
equal to 1/3). Repeat the encoded sequence and modulate it as 0  1 and 1  1 .

Allocate the obtained encoded sequence to a block of size 3232, and visualize the obtained
watermark block.
2. Watermark embedding (Figure 1).

Read a grayscale image “gbear.bmp” (host image x ). Perform the tiling of the watermark
blocks along the input image size (the tiled watermark w ).

Calculate Noise Visibility Function (NVF) of the host image based on non-stationary
Gaussian (nG) model and using the following formula:
1
,
nvf i , j  
2
1   l i , j   Th
where nvf i , j  is the normalized NVF in the range of (0-1);  l2 i , j  denotes the local
variance of the image centered at i, j  (use window size 33), and Th is the contrast
100
sensitivity equal to Th  2 , where  l2max is the maximal value of the local variance for
 l max
the whole image. (The NVF is used for the watermark masking in the image.)

Perform watermark embedding based on the NVF as follows:
y  x  1  nvf   S edge  nvf  S flat  w ,
where S edge  10  12 is the watermark strength in edge and texture regions of the image;
and S flat  1.5  3 is the watermark strength in flat regions of the image. Save the
watermarked image y in BMP format (i.e. “wm_gbear.bmp”) and display it.
message
WM
encryption
WM encoding
and repetition
WM block
formation
rep
WM block
tiling
w
y
wmKey
Host
Image
x
NVF
calculation
Figure 1. Block diagram of watermark embedding
3. Watermark extraction (Figure 2).
nvf
CUI- University of Geneva

Multimedia Security
Extract the watermark from the watermarked image y using the following formula:
ˆ blind  y  xˆ W iener ,
w
where ŵ blind is the estimated watermark, and x̂W iener is the result of Wiener filter applied to
the watermarked image (use window size 33). Visualize the extracted watermark.

Perform the untiling of the estimated watermark (inverse operation to the tiling based on
data averaging) to the watermark block of size 3232 and convert to a vector. Shrink the
vector twice (inverse operation to repetition) by data averaging and normalize it with
respect to its standard deviation.

Perform the decoding of the obtained sequence using Turbo code routine (use the same
coding rate). Decrypt the decoded binary string using inverse permutation and retrieve the
secret message (convert to ASCII string). Compare the obtained decoded binary string and
secrete message with the corresponding input ones.

Apply several attacks to the watermarked image y (i.e. filtering, JPEG compression,
cropping, scaling) and try to retrieve the hidden message. Explain why the different
operations and data manipulations (such as encoding/decoding, tiling/untiling,
encryption/decryption, sequence repetition/shrinking, etc.) were used for given robust
watermarking technique. Make the conclusions.
Wiener
filter
x̂W iener

WM
image
y
ŵ
WM block
untiling
WM sequence
formation
WM
decoding
WM
decryption
retrieved
message
wmKey
Figure 2. Block diagram of watermark retrieval
Useful information:
 The necessary files for data encoding/decoding using Turbo code routines are placed at
http://cui.unige.ch/~ms/TP/Software/TurboCode.
 The images for testing are placed at http://cui.unige.ch/~ms/TP/Images/.