A new image encryption technique
Steps in our method:
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Generate initial conditions and control parameters.
SHA-256 generates digest of 256 bits regardless the size of the input. If there is one-bit difference between two inputs, their message digest will be completely different. So, this is used to generate digest of the color image to which encryption is to be done. This digest is used to generate initial conditions for PWLCM, Lorenz system and Chen’s hyper-chaotic system.
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Encryption process
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Permutation step
The proposed diffusion of colour image is performed in two ways: First each pixel is permuted according a chaotic sequence generated by PWLCM (piece wise linear chaotic map) given as;
Then pixels of each channels is permuted separately by using Lorenz system of equations given as;
To perform the permutation these sequences are first sorted and then pixels are swapped accordingly.
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S-box substitution
In this step, we substitute the obtained permuted matrices using 16 S-boxes to enhance the nonlinearity of the proposed scheme. For S-boxes generation, we choose the set of all the degree 8 primitive irreducible polynomials over the field ℤ2.
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Pixel transposition step
By using the multiplicative operation of a group (ℤ_2 [𝑦])/⟨ℎ_1 (𝑦)⟩ ∖{0} we generate a three random sequence and it was permuted using Chen’s hyper-chaotic system which is given as:
Then the substituted blocks are transposed using bitwise XOR.
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Decryption process
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Pixel transposition step
The decryption process of the proposed scheme is the same as the encryption process, but it starts from the reverse side.
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Inverse S-box substitution
We generated the inverse S-box utilize degree 8 primitive irreducible polynomials and substitute it to get only the permuted image.
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Inverse permutation
We repeat the swapping operation in permutation step in reverse manner to get the original image.
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