Watermarking is a form of steganography, which is the study of how to hide data
from other data. In watermarking, there is one method, namely Discrete Cosine
Transform (DCT) which can be used to hide watermarks so that they are not visible
to the eye. Watermarking DCT method is a method that can be used to increase
security because it is robust against manipulation of stego-objects. DCT is a long
algorithm and requires a lot of calculations. There are two inputs made to the DCT
method in the form of an image matrix which then converts it into a frequency matrix
of the same size. The data used are 100 sample images taken from Camera HP SONY
Xperia XZ with 23 MP resolution The variables used for testing are images with
bitmap formats (*. Bmp), PNG (*. Png), TIFF (*. Tiff). GIF (* .gif). Security testing
is done by adding attack techniques. From the test results, it is found that the best
format is using the BMP format.