Before the Qingming Dynasty, my classmates turned a link to me and tried to do it. One of the related topics was to give two pictures, the original one and the processed one. They asked to decrypt the information.
The title gave a hint to
Fourier, so it is easy to guess that digital watermarking technology is based on FFT. However, I didn’t find this prompt at first, so I made some attempts according to my experience. First, I considered whether it was a kind of image, that is, hiding the information at the end of the image file by using the characteristics of image coding format and compressed file coding format. After failure, I considered whether it was hiding the bit that did not affect the image display by using the characteristics of PNG. After failure, consider whether it is LSB or not. After failure, compare the two pictures with differ command, find that the different points are distributed in four corners of the image. Then compare the specific content of the two PNG format pictures with WinHex and so on. It is found that from the first IDAT, the hexadecimal value and CRC are different, excluding the simple embedded text. The possibility of a piece. Then I found the hint, and then it was easy.
Using MATLAB to do FFT on the image, and then subtraction can get a more noisy two-dimensional code, multiplied by the parameters can get a clear two-dimensional code.
I was not sensitive to the image because did not take the course of digital image processing. When I found the difference, I did not immediately respond that using frequency domain to hide the information. That’s take too many time.
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