  Cloud computing can provide a flexible way to effectively share data among
multiple users since it can overcome the time and location constraints of
computing resource usage. However, the users of cloud computing are still
reluctant to share sensitive data to a cloud server since the cloud server
should be treated as an untrusted entity. In order to support secure and
efficient data sharing in cloud computing environment, Wei et al. recently
extended the concept of identity-based encryption (IBE) to support key
revocation and ciphertext update functionalities, and proposed a
revocable-storage identity-based encryption (RS-IBE) scheme. In this paper, we
show that the RS-IBE scheme of Wei et al. does not satisfy the correctness
property of RS-IBE. In addition, we propose a method to modify the existing
RS-IBE scheme to be a correct and secure scheme.
