# Privacy-Preserved Image Protection Supporting Different Access Rights
## 4 Experimental Results
### Evaluation  
We predetermine the privacy blocks that need to be protected to create the access policy. Let us assume that each user is in one class and each class has one access right in the access policy. According to the image map \(IM\) and authentication information \(AI\), RGB layers of face region \(Plaintext{x}t_{j}\) are encrypted by AES with \(SK_{j}\). Figure 3 shows the encrypted result of the test portrait image. We can see from Figure 3 that the distribution of encrypted pixel values is relatively uniform and it has no relationship with the original image. The user sends a request to the cloud server to ask for seeing the encrypted image. The cloud server can determine if the user has rights to access this image or not according to access policy \(T\). When receiving the derivative key \(DK_{j}\) and corresponding public keys, the user can compute \(SK_{j}^{\prime}\) to decrypt the encrypted blocks. Taking \(DK_{2}\) and \(e_{2}\) as an example, computing the \(SK_{1}^{\prime}\) of \(C_{2}\) as \(DK_{2}^{e_{2}}\) mod \(n=203896588144105^{24590087}\) mod \(3602891806881971=1616955632228291\) to decrypt the block of \(C_{1}\). The decrypted results for different accessible classes \(C_{1}\), \(C_{2}\), and \(C_{3}\) are shown in Figure 4. Note that the users cannot decrypt other privacy-protected blocks while they do not have access rights on them in access policy. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & \(j=1\) & \(j=2\) & \(j=3\) \\ \hline \(e_{j}\) & 23185931 & 24590087 & 29512477 \\ \(d_{j}\) & 3078401025127763 & 3107156642851655 & 2843269558133509 \\ \(SK_{j}\) & 1616  
For adapting to different access policies, we try to encrypt three blocks for single class, and one block for different users in the same class, which is shown in Figure 5. The corresponding decryptions of multi-blocks are given in Figure 6. We can see that there is no difference between different users and a single user since encryption and decryption are dependent on authorized classes. Furthermore, we also notice that the quantity of blocks needed for a class does not lead to any encryption and decryption failure. Figure 3: The encrypted result. The above examples are considered as the hierarchical access control. We can see in Figure 7a that the class \(C_{3}\) can access every class since it has a very high level of permissions. The class \(C_{2}\) can access the class \(C_{1}\), whereas it cannot access the class \(C_{3}\). As a result, the hierarchical access control may have some restrictions on data access. To make the data access more flexible, the proposed method can provide a flexible access control as shown in Figure 7b. The class \(C_{2}\) and class \(C_{3}\) can access each other; however, only the class \(C_{2}\) can access the class \(C_{1}\). Figure 8 shows the result of the flexible access control. The parameters of \(C_{j}\) used for Figure 7b in the initialization phase are listed in Table 2. For  
Figure 5: The encrypted result of multi-blocks. Figure 6: Images of accessible classes \(C_{1}\), \(C_{2}\), and \(C_{3}\) decrypted by \(SK_{1}{}^{\prime}\), \(SK_{2}{}^{\prime}\), and \(SK_{3}{}^{\prime}\), respectively. (**a**) class \(C_{1}\); (**b**) class \(C_{2}\); (**c**) class \(C_{3}\). the class C\({}_{3}\), it can compute \(SK_{2}{}^{\prime}\) of C\({}_{3}\) as \(DK_{3}^{\circ}\) mod \(n=2822790476719118^{18714097}\) mod \(3602891806881971=2547499887988133\) to decrypt the block of C\({}_{2}\). However, it cannot access the class C\({}_{1}\) since it cannot get \(SK_{1}{}^{\prime}\) by \(DK_{3}^{\circ}\times c_{2}\) mod \(n=2822790476719118^{(18714097\times 33389011)}\) mod \(3602891806881971=29881307\
eq 2294561867680533\). Therefore, the proposed method can support flexible access control in practice. The \(k\) out of \(n\) visual cryptography [1; 2; 3; 4; 5; 6] can also be extended into a secret sharing scheme.