Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> The Multiauthority Attribute Based Access Control System <s> In previous privacy-preserving multiauthority attribute-based encryption (PPMA-ABE) schemes, a user can acquire secret keys from multiple authorities with them knowing his/her attributes and furthermore, a central authority is required. Notably, a user’s identity information can be extracted from his/her some sensitive attributes. Hence, existing PPMA-ABE schemes cannot fully protect users’ privacy as multiple authorities can collaborate to identify a user by collecting and analyzing his attributes. Moreover, ciphertext-policy ABE (CP-ABE) is a more efficient public-key encryption, where the encryptor can select flexible access structures to encrypt messages. Therefore, a challenging and important work is to construct a PPMA-ABE scheme where there is no necessity of having the central authority and furthermore, both the identifiers and the attributes can be protected to be known by the authorities. In this paper, a privacy-preserving decentralized CP-ABE (PPDCP-ABE) is proposed to reduce the trust on the central authority and protect users’ privacy. In our PPDCP-ABE scheme, each authority can work independently without any collaboration to initial the system and issue secret keys to users. Furthermore, a user can obtain secret keys from multiple authorities without them knowing anything about his global identifier and attributes. <s> BIB001 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> The Multiauthority Attribute Based Access Control System <s> Decentralizing multi-authority attribute-based encryption (ABE) has been adopted for solving problems arising from sharing confidential corporate data in cloud computing. For decentralizing multi-authority ABE systems that do not rely on a central authority, collusion resistance can be achieved using a global identifier. Therefore, identity needs to be managed globally, which results in the crucial problems of privacy and security. A scheme is developed that does not use a central authority to manage users and keys, and only simple trust relations need to be formed by sharing the public key between each attribute authority (AA). User identities are unique by combining a user’s identity with the identity of the AA where the user is located. Once a key request needs to be made to an authority outside the domain, the request needs to be performed by the authority in the current domain rather than by the users, so, user identities remain private to the AA outside the domain, which will enhance privacy and security. In addition, the key issuing protocol between AA is simple as the result of the trust relationship of AA. Moreover, extensibility for authorities is also supported by the scheme presented in this paper. The scheme is based on composite order bilinear groups. A proof of security is presented that uses the dual system encryption methodology. <s> BIB002
To tackle the single authority schemes' problems, an effective way is to minimize the trust level of the single authority, strengthen the privacy of user data, and enhance the system security and performance by replacing the single authority with multiple ones for disjoint attribute management, which becomes much harder for an adversary to compromise. Therefore, in this section, the type of scheme that allows securely storing data on a public cloud storage system and employs multiple authorities to manage sets of attributes, is presented. A critical challenge of current multiauthority access control systems (also all single authority schemes) is the inefficiency of the key generation process. This issue occurs in the single authority systems when one authority manages all attributes in a system and issues secret keys for all system users. Therefore, compromising or crashing this authority makes the whole system unavailable. The same issue happens in multiple authorities schemes, when each authority in the system administrates a disjoint attribute set (i.e., each authority administrates a different set of attributes), which presents a performance bottleneck. To mitigate the effects of this issue, all attribute sets ought to be managed by all system attribute authorities individually. Although many recent multiauthority CP-ABE schemes have been proposed  BIB002 , some limitations are still not considered. The existing multiauthority access control systems can be classified into three categories with their limitations summarized below. • The first type of scheme (e.g., a scheme by Han et al. BIB001 ) contains many authorities that have to work together, resulting in a high communication cost and lack of scalability since it is hard for authorities to join or leave freely. Furthermore, these authorities might collude with each other and combine their information to gain unauthorized data about the users.