# Separable Data-Hiding Scheme for Encrypted Image to Protect Privacy of User in Cloud
## 4 Experiments
### Performance Analysis of the Proposed Scheme  
Figure 7 shows the stego-images and recovered images in this experiment when the compression ratio was 0.25 and the block size was 8. The relevant experimental data are provided in Table 1. In Figure 7, the fingerprint image with the size of 256 \(\times\) 256 was treated as the image that was to be hidden by embedding it into the encrypted image, and then the stego-images were obtained as (a3),  
Figure 6: Eight original protected images in this experiment: (**a**) Lena, (**b**) Goldhill, (**c**) Airplane, (**d**) Boat, (**e**) Peppers, (**f**) Tiffany, (**g**) Man, and (**h**) Sailboat. (b3), (c3), (d3), (e3), (f3), (g3), and (h3). Obviously, the image that is to be hidden and the stego-image are the same size. In cloud computing, this small size makes it easier to store and transmit the data, and this is especially important when storage and transmission resources are limited. When the receiver only has the encryption key, he/she can directly recover the original protected image from the stego-image. Moreover, the quality of the recovered image, which is evaluated by PSNR and SSIM in Table 1, is acceptable when the compression ratio is low. When the receiver only has the data-hiding key, he/she can extract the hidden data completely without seeing the content of the image as long as the stego-images are not distorted. Similarly, when the receiver has both the encryption key and the data-hiding key, he/she can extract the hidden data and recover the protected image. Figure 7: Demonstration of stego-images and recovered images in this experiment when the compression ratio was 0.25: (**a1,b1,c1,d1,e1,f1,g1,h1**) are the original protected images; (**a2,b2,c2,d2,e2,f2,g2,h2**) are the images that are to be hidden; (**a3,b3,c3,d3,e3,f3,g3,h3**) are the stego-images; (**a4,b4,c4,d4,e4,f4,g4,h4**) are the recovered images. Table 2 gives the PSNR, SSIM, and EC with different compression ratios for Lena when the block size was 8. Table 2 indicates that the change of the compression ratio gave our scheme great flexibility. The larger the compression ratio is, the better the quality of the recovered images will be and the higher the embedding capacity will be, but the increase in the size of image also will consume more storage space in cloud storage. Therefore, users can make their own decisions concerning processing based on their own needs. Figure 8 shows an example of stego-image and the recovered image for Lena when the compression ratio was 0.9. Because of the larger compression ratio, more bits can be embedded into the encrypted images. So, size of the image that was to be hidden was chosen as 512 \(\times\) 464, as shown in Figure 8b. In addition, we also conducted additional experiments with different block sizes and compression ratios. Table 3 gives the PSNR, SSIM, and EC with different parameters for Lena. Table 3 indicates that the block size had little effect on either the embedding capacity or the quality of the image, but compression ratio affected their performance significantly. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
Figure 8: Example of stego-images and recovered images in this experiment, when the compression ratio was 0.9: **(a)** Original protected images; **(b)** the 512 \(\times\) 464 image that was to be hidden; **(c)** Stego-image with the size of 512 \(\times\) 464; **(d)** recovered images with PSNR = 36.56 dB and SSIM = 0.9757. To further illustrate the flexibility and effectiveness of our proposed scheme, some experimental comparisons of relevant schemes, including [PERSON] et al. 's scheme [24] and [PERSON] et al. 's scheme [25], are given in Table 4. At the same compression ratio, the embedding capacity of our scheme absolutely was superior to those of the other schemes. Although the quality of the recovered image before extracting the hidden data in our scheme was slightly lower than that of [PERSON] et al. 's scheme [25], it obviously was better than that of [PERSON] et al. 's scheme [24].