Provably secure public key cryptosystem with limited number of encryptions for authorised sharing of outsourced data created by B.R. Purushothama and B.B. Amberker

Cloud computing paradigm can be used to outsource data and computation to the cloud infrastructure. Providing access control to the outsourced data in public cloud is a challenging task. In this paper, we address the problem of trust reduction on the untrusted cloud storage server to provide the authorised access to the outsourced data in public cloud environment. To enable the trust reduction, we coin the notion of public key cryptosystem with restriction on the number of encryptions. We address the problem of restricting/limiting the number of messages to be encrypted for a user (receiver) without changing the primary or long-term key pair of the user. We formalise the security model and prove that the proposed scheme is semantically secure under Decisional Diffie-Hellman Assumption. We show that the proposed scheme can be used for reducing the trust requirement in untrusted cloud storage service providers for providing authorised access to the outsourced data in public cloud environment.