Practical traceable multiauthority CPABE with outsourcing decryption and access policy updation Abstract   Multiauthority ciphertext policy attributebased encryption (CPABE) is one of the promising approaches where multiple authorities issue and manage various attributes to achieve finegrained access control on stored data in cloud platforms. However, the practical use of multiauthority CPABE (MCPABE) requires satisfaction of different features. Firstly, in MCPABE, users having the same set of attributes must share the same decryption privileges. This may potentially lead to exposure of decryption keys by some malicious users. Thus, there is a need of designing MCPABE with traceability to overcome this problem. Secondly, as the cost of decryption increases with the complexity of access policy, it is preferred to provide the functionality of outsourcing decryption to reduce the computational burden on the user. Finally, the data owners must have flexibility to change the access policy. Hence, policy updation must be an integral part of MCPABE scheme. In this paper, we construct a new multiauthority CPABE scheme over large attribute universe with decentralization of authorities that supports whitebox traceability along with policy updation and outsourcing decryption. Our proposed cryptosystem is built on prime order groups and supports monotonic access structures. These features improve efficiency and expressiveness of our system. We implemented our proposed cryptosystem in Charm cryptographic framework and analyzed its performance with a diverse set of test cases.

An Anonymous Attributebased Access Control System Supporting Access Structure Update It is quite common nowadays for clients to outsource their personal data to a cloud service provider. However, it causes some new challenges in the area of data confidentiality and access control. Attributebased encryption is a promising solution for providing confidentiality and finegrained access control in a cloudbased cryptographic system. Moreover, in some cases, to preserve the privacy of clients and data, applying hidden access structures is required. Also, a data owner should be able to update his defined access structure at any time when he is online or not. As in several realworld application scenarios like ehealth systems, the anonymity of recipients, and the possibility of updating access structures are two necessary requirements. In this paper, for the first time, we propose an attributebased access control scheme with hidden access structures enabling the cloud to update access structures on expiry dates defined by a data owner.