Correcting design flaws: An improved and cloud assisted key agreement scheme in cyber physical systems Abstract   The on demand availability of resources in Cyber physical system (CPS) has emerged as a viable service providing platform to improve the resource usability and reducing the infrastructure costs. Nevertheless, the development recompenses can only be realized after avoiding security and privacy issues. A secure and reliable CPS can offer improved efficiency, usability and reliability along with autonomy. To secure such systems, in 2018 Challa etxc2xa0al. (2018) proposed a security system to extend an authenticated key agreement between a user and a cloud server via trusted authority; as an application, they also customized their system to work with autonomous smart meter and cloud server. Challa et al. then claimed the security of their proposed scheme through formal, informal and automated validations. However, this paper unveils the weaknesses of their scheme and shows that their scheme cannot facilitate in forming a session key between the usersmart meter and the cloud server. Precisely, in the presence of more than one registered userssmart meters, the latter in their scheme may never receive a response message because of a critical design error. Moreover, their scheme lacks the untraceable anonymity and the lack of request verification on cloud server side may also lead to replay andor denial of services attack. The article then introduces an improved and secure authentication system free of correctness issues, to facilitate a key agreement between user and cloud server via trusted authority. As an application, the proposed system also works for smart meter and cloud server to reach a key agreement. Based on the hardness assumption of Elliptic Curve Decisional DiffiHellman Problem (ECDDHP), the formal Random oracle model proves the security of the proposed scheme. Moreover, the robustness of the scheme is explained through informal analysis. The proposed system while providing all known security features has slightly increased the computation and communication costs as compared with the scheme of Challa et al. The proposed scheme completes a cycle of authentication by exchanging 2080xc2xa0bits in just 13.4066xc2xa0ms.

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.