#Level: Difficult
The solution can be designed as follows:
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From the actual cover-image and stego-image extract the red, green and blue components.
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If you check the size of the components, all are 256 X 256. Take the difference between two corresponding component (you may use XOR operation). Line, the red plane of cover-image will be compared with red plane of stego-image.
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It shows a number of insertion positions (you can check the difference matrix for 1s).
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If you analyse the distances between the insertion positions, you can find that the first character (7-bits) inserted into first pixel. But that 7-bit value determines the next position of the pixel in a row-major way into 2-D matrix for image. The next 7-bits will be inserted at that pixel. Again this 7-bit value determines the next pixel and like that.
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If you look at the 3 different planes, you can find out 3 bits are inserted in red-plane, 2 bits are inserted into green-plane and 2 bits are inserted into blue-plane.
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Using this construct 7-bit ASCII value, hence the hidden character.
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As it is given 61 characters, we need to repeat it 61 times.