# Image Encryption Algorithms: A Survey of Design and Evaluation Metrics
## 2 Image Encryption Algorithms
### Frequency Domain  
Using an image frequency domain in an image encryption algorithm can improve an algorithm's security. In frequency domain image encryption, instead of performing the encryption on the plaintext image directly in the spatial domain, the image is first transformed into the frequency domain, using Fourier transform (FT), wavelet transform (WT), discrete cosine transform (DCT), etc., and the confusion and diffusion are performed in the frequency domain. The cipher image is then produced by inverse-transforming the encrypted frequency domain into the spatial domain.  
[PERSON] et al. [39] proposed an image encryption algorithm based on a fractional-order Henon map, a 2D discrete wavelet transform (DWT), and a 4D hyperchaotic system. In the proposed algorithm, the plaintext image is transformed using DWT and then permuted using a chaotic sequence generated by the fractional-order Henon map. The 4D hyperchaotic system generates a chaotic sequence that is XORed with the permuted image to produce the cipher image.  
In [40], [PERSON] et al. introduced a hybrid domain image encryption algorithm for gray-scale images based on a 2-dimensional improved Henon map (2D-ICHM), integer wavelet transform (IWT), bit plane decomposition, and DNA operations. The algorithm starts by generating a SHA-512 hash from the plaintext image, which is used to derive the control parameters for the improved Henon map (2D-ICHM), which generates two chaotic sequences, namely X,Y. Then, the plaintext image is decomposed into eight binary bit planes, and the high bit planes are XORed with chaotic sequence X. Next, the IWT is applied to the XORed bit plane and then scrambled using chaotic sequence X. After that, inverse IWT is performed followed by DNA encoding, the DNA XOR operation, and DNA decoding. Finally, bidirectional diffusion is performed using chaotic sequences X and Y to produce the cipher image.  
[PERSON] et al [41] presented a chaotic image encryption algorithm using frequency domain DNA encoding. The proposed algorithm starts by transforming the plaintext image using a fast Fourier transform (FFT) and obtaining the amplitude and phase, which are then reconstructed as 1D sequences. Next, four chaotic sequences are generated by a 4D hyperchaotic map and the DNA encoding, decoding, and operation rules are generated using these chaotic sequences. After that, the amplitude and the phase sequences are DNA-encoded, DNA operations are performed, and then DNA is decoded to diffuse and confuse the amplitude and phase. Finally, the cipher image is produced by performing an inverse fast Fourier transform (IFFT).  
In [42], [PERSON] et al. introduced an efficient and secure color image cryptosystem based on chaotic logistics in fractional Fourier transform (FrFT). The proposed algorithm starts by splitting the plaintext image into red, green, and blue channels. Next, fractional Fourier transform (FrFT) is applied on the red, green, and blue color plaintext image channels. Afterward, a 2D logistic map is employed to scramble each of the color image pixel positions in the FrFT. Subsequently, the inverse fractional Fourier transform (iFrFT) is applied to the resulting encrypted channels. Finally, the red, green, and blue channels are combined to produce the cipher image.