### Related Works
#### 1.2.2 Policy Preservation in Attribute-Based Encryption  
Because encrypted data on cloud storage servers remained in plaintext form when outsourced, access policies were shared with different users in all of the above schemes. This may cause potential exposure of sensitive information about the data owner as well as the consumers of the data. Several works ([PERSON] et al., 2019; [PERSON] et al., 2019) introduced CP-ABE schemes with partially hidden access control policies to protect the disclosure of this sensitive information. The access policies in these schemes divide attributes into two parts, i.e., attribute name and attribute value. In a partially hidden access policy, the attribute value that reveals sensitive information is made hidden, e.g., ([(Profession = \"*\" and Specialization = \"*\" and Affiliation = \"*\") or (Profession = \"*\" and Specialization = \"*\" and Research Associate = \"*\" or (Relation = \"*\"))). [PERSON] et al. (2019) used the anonymous key issuing protocol used in CC-MA-ABE ([PERSON] et al., 2019) and proposed an accountable multi-authority CP-ABE. [PERSON] et al. (2019) developed a decentralized CP-ABE (PPDCP-ABE) scheme to eliminate dependency on and trust in central authorities while maintaining the privacy of users. In their scheme, multiple authorities may operate autonomously. The [PERSON] commitment protocol ([PERSON], 2019) and zero-knowledge-proof protocol ([PERSON] et al., 2019) were used to protect the attributes' privacy. In ([PERSON] et al., 2019), a policy-hiding CP-ABE scheme was proposed to improve decryption efficiency. The authors pioneered the \"match-then-decrypt\" method, in which ciphertext components were routed to the decryption test. Without performing the actual decryption, it checked the satisfiability of the hidden attributes policy for the attribute private key. [PERSON] et al. (2019) designed a privacy-preserving decentralized CP-ABE where the secret key was taken out with privacy. Their proposal required no central AA or multi-authority collaboration. They used the scheme [31] proposed by [PERSON] and oblivious attribute certificates [34]. The users receive secret keys for legitimate identity attributes, but AAs cannot find any useful information. [PERSON] et al. [35] proposed an access control scheme with a hidden policy on multi-authority architecture. To hide policy, attributes were obfuscated using a one-way anonymous key agreement protocol. [PERSON] et al. [36] proposed a method for controlling Big Data access. Instead of hiding the attribute values, they hid the whole attribute for privacy purposes. They utilized an attribute bloom filter, which detects an attribute and its precise location in the access policy. [PERSON] et al. [37] presented a lightweight policy-preserving CP-ABE scheme for EHR sharing on the cloud. The access policy was fully hidden by use of the attribute cuckoo filter (ACF). PASH, proposed in [38], was designed to provide access control for smart health. The CP-ABE scheme supported a large universe and used partially hidden policies. It also handled decryption tests efficiently and provided full security. [PERSON] et al. [39] introduced a multi-authority ABE with privacy preservation and dynamic policy updating. This scheme was suggested for a multi-authority scenario, but it could not prevent malicious users from sharing their private keys. [PERSON] et al. [40] proposed PHOABE for cloud-assisted IoT, where multiple authorities were considered and a fully hidden policy was maintained by obfuscating the attributes. Their scheme was based on the scheme in [16]. It introduced a semi-trusted cloud server for outsourcing the heavy decryption process. This minimized computation overhead on resource-constrained devices. [PERSON] et al. [41] developed a hidden ciphertext-policy scheme for a large universe and proposed an efficient decryption procedure. [PERSON] et al. [42] proposed a policy-hidden CP-ABE scheme for providing access control in an IoT environment. The SHA1 hashing algorithm was used for policy anonymization.