## 2 Related Work  
Numerous works of research such as revocable CP-ABE [25], lightweight CP-ABE [26], multi-authority CP-ABE [4; 27], and large universe CP-ABE [28] have been developed in response to upgrading the competency of the Ciphertext Policy Attribute-based Encryption (CP-ABE) scheme. However, most of the schemes exposed the access policy in plaintext, which incurred privacy leakage. As a result, there are other works that focus on encrypting the data while hiding the access policy during data sharing. [PERSON] et al. [29] proposed the hiding access policy by splitting attributes into attribute names and multiple attribute values, where they then hid the attribute values. Meanwhile, [PERSON] et al. [30] proposed the partially hidden access algorithm, whereby the hidden access policies are implemented with wildcards regardless of the number of attributes. Although the data construction effectively secures the shared information, it fails to offer total data security. It is due to the wildcard attributes not yet being in readable form and might cause privacy leakage. The authors of [23] supported fast decryption while hiding the access policy by sending the access matrix and the defined function along with ciphertext to the Cloud environment. However, it is unable to preserve privacy in the access policy. In [31], the authors eliminated all redundant attributes in the access policy, and their approach significantly reduced computation overhead. With the same intention, the author of [32] proposed the 'test-decrypt-verify' approach to reduce the computation cost in their CP-ABE scheme. In their scheme, the testing phase is added before the encryption phase, and the new component, Outsourcing Cloud Server, is adopted as an outsource agent to reduce the decryption calculation. However, the proposed scheme only employed a partially hidden access  
Figure 1: Sample Privacy Leakage State.  
policy. The authors of [33] also provided a partially hidden access policy by removing the attribute name from the access structure of the ciphertext. However, the probability of data exploitation by dishonest entities or unauthorized users is still high because only a part of the access policy is hidden.  
Other researchers expressed access policies using Linear Secret Sharing Schemes (LSSS) in the CP-ABE scheme for better data access control. In [PERSON] et al. [11], they proposed partial hiding access structures with LSSS that are able to accommodate any access structure. They proved that their scheme was suitable for outsourcing data with attribute values of the data that had been hidden. Other studies, such as [34; 35; 36] also constructed the LSSS-based access policy schemes with CP-ABE, which hid attribute values to secure the information. Besides the partially hidden access policy, [PERSON]'s scheme [34] supported attribute revocation and verifiable outsourced decryption. However, the attribute values carry more intricate information in comparison to generic attribute names, which leads to high computing overhead during the decryption process. Meanwhile, the authors in [35] proposed the control access scheme to provide privacy protection via partial concealment of access policy. They have utilized CP-ABE in the intelligent healthcare system known as the privacy-aware-health access control system (PASH). PASH hides the attribute value of the access policy in the encrypted Smart Health Record and only specifies the attribute name, which destroys the system-user privacy. Therefore, the existing CP-ABE scheme needs improvement to prevent information leakage from access policies and ensure data sharing on the Cloud is secured. Although various approaches have been proposed in the existing literature on the policy hiding scheme, the study on privacy-preserving with a fully hidden access policy is still inadequate. It could not solve the privacy leakage issues completely. Thus, the privacy-preserving of data users cannot be guaranteed.