Secure sharing of personal health records on cloud using key-aggregate cryptosystem

In modern health care environments, personal health record (PHR) owners store and share their PHR data via a cloud because of its on-demand resource access, measured service and rapid elasticity. To enable secure and flexible data sharing in the cloud, efficient management of encryption keys is required. Selective data sharing requires different documents to be encrypted with different keys, which implies data users like doctors to securely store the received keys and submit an equal number of keyword trapdoors (encrypted queries) to the cloud server to perform a keyword search over authorized encrypted files. The current work focuses on reducing key-size by generating a single aggregate key, but does not provide searchable encryption, which is required for flexible data sharing. Our proposed scheme addresses this issue by enabling a patient to distribute a single constant-size aggregate key to a data user for sharing a large number of documents and then user submits a single aggregate trapdoor to the cloud for searching over authorized encrypted documents. The novelty of this scheme lies in submitting a single trapdoor for keyword search over documents encrypted with different keys as opposed to traditional methods requiring submission of multiple trapdoors. Performance evaluation confirms that our proposed scheme is practically efficient and also reduces storage overhead by reducing both the number of keys and key-size without affecting security-level, which is highly desired in the resource constraint devices like smartphones.