Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> The Revocation Problem <s> A cloud storage service allows data owner to outsource their data to the cloud and through which provide the data access to the users. Because the cloud server and the data owner are not in the same trust domain, the semi-trusted cloud server cannot be relied to enforce the access policy. To address this challenge, traditional methods usually require the data owner to encrypt the data and deliver decryption keys to authorized users. These methods, however, normally involve complicated key management and high overhead on data owner. In this paper, we design an access control framework for cloud storage systems that achieves fine-grained access control based on an adapted Ciphertext-Policy Attribute-based Encryption (CP-ABE) approach. In the proposed scheme, an efficient attribute revocation method is proposed to cope with the dynamic changes of users' access privileges in large-scale systems. The analysis shows that the proposed access control scheme is provably secure in the random oracle model and efficient to be applied into practice. <s> BIB001 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> The Revocation Problem <s> In this work, we aim to make attribute-based encryption ABE more suitable for access control to data stored in the cloud. For this purpose, we concentrate on giving to the encryptor full control over the access rights, providing feasible key management even in case of multiple independent authorities, and enabling viable user revocation, which is essential in practice. Our main result is an extension of the decentralized CP-ABE scheme of Lewko and Waters [6] with identity-based user revocation. Our revocation system is made feasible by removing the computational burden of a revocation event from the cloud service provider, at the expense of some permanent, yet acceptable overhead of the encryption and decryption algorithms run by the users. Thus, the computation overhead is distributed over a potentially large number of users, instead of putting it on a single party e.g., a proxy server, which would easily lead to a performance bottleneck. The formal security proof of our scheme is given in the generic bilinear group and random oracle models. <s> BIB002
Revocation is a property to change the access rights of users when unexpected events occur, such as malicious behavior from a user or an expired service that a user had purchased BIB002 . There are two scenarios where the revocation can be conducted. The first one is called attribute revocation that happens when some of a user's attributes are removed from the current set due to being degraded in the system. For example, degrading a manager of an organization to a normal employee role leads to losing some of its possessed attributes and hence access rights. The second scenario occurs when a user leaves the system; its access rights have to be revoked so that the user is no longer able to access the system and decrypt any stored data on it, which is called user revocation BIB001 . Based on these considerations, designing a mechanism to revoke the user's certain access rights must be embedded in the system from the beginning. Otherwise, the whole system has to be rebuilt with the advent of each revocation event. In attribute-based encryption schemes, the attribute revocation is a severe problem and very costly to apply for two reasons. The first one is the same attribute set may be associated with different users' secret keys, causing significant computational overhead throughout the revocation process. This happens due to the need for updating all relevant keys for the non-revoked users and re-encrypting the related ciphertexts BIB002 . The second reason is most of the existing proposed attribute revocation methods are based on a fully trusted server, but this is an unrealistic assumption BIB001 . This is because the server could breach the trust and even be compromised, resulting in permitting unauthorized users to access the data stored in the cloud for gaining profits, e.g., when the cloud illegitimately permits a company to access the data of its competitors. Therefore, the following crucial requirements are needed to handle the revocation problem. • Permit instantaneous banning of a malicious user.