2.2. Watermarking Algorithm
In this algorithm 9
, user identity is combined with biometric identification during the authentication process. The
user identity consists of name and an id. It is encoded using the SHA2 hash function which generates a unique hash
value. SHA2 is a secure one-way hash function, so it is not possible to obtain a user identification number based on
the hash value and it is infeasible to change a message without modifying its hash value. The hash value is then
converted into binary and then converted into image as a watermark of size 256x256, equal to the size of the
fingerprint image.
The Dual-Tree Complex Wavelet Transform (DTCWT) domain is used to embed the watermark data into
fingerprint images. DTCWT removes the directionality and shift variance problems present in the wavelet
transforms by using complex basis functions. For decomposition purposes, DTCWT uses directional filters. These
directional filters are able to extract the same information, such as minutia locations, even after the watermark has
been embedded into the image.
Multiplicative fusion is used to distribute the watermark evenly over the whole fingerprint image, including the
real and imaginary parts, without affecting the information present in the fingerprint image. Then use the
multiplicative fusion rule to combine the fingerprint image coefficients and the watermarked image coefficients after
decomposition. The multiplicative fusion rule does not greatly affect fingerprint image coefficients due to the fact
that the number of high value coefficients in the watermarked image is much less. In order to make the coefficients
values very low, perform normalizing for the coefficients after the decomposing process. In this step, divide the
DTCWT coefficients of that level to the average value of coefficients. Moreover, when decompose a fingerprint
image using DTCWT, the high value coefficients correspond to the minutia points. Other continuous lines
correspond to the low value DTCWT coefficients. By multiplying the watermarked image coefficients with the
fingerprint image coefficients, these high values remain high and the distribution of minutiae points is not changed.
In this way, the watermark is embedding into the fingerprint images. This algorithm relies on the information
fusion-based approach. After that apply the inverse wavelet transform to retrieve the image domain and finally
obtain the watermarked image.
Fig: 3 show the block diagram of the watermarking algorithm. The algorithm comprises the following steps:
x User information that identify the user, encode using hash function (SHA2)
x Convert hash value to binary image to construct water-marked image