  Cloud storage is a new computing paradigm that allows users to store their
data in the cloud and access them anytime anywhere through the Internet. To
address the various security issues that may arise in the cloud storage
accessed by a large number of users, cryptographic encryption should be
considered. Currently, researches on revocable attribute-based encryption
(RABE) systems, which provide user revocation function and ciphertext update
function by extending attribute-based encryption (ABE) systems that provide
access control to ciphertexts, are actively being studied. Recently, Xu et al.
proposed a new RABE scheme that combines ABE and identity-based encryption
(IBE) schemes to efficiently handle ciphertext update and user revocation
functionality. In this paper, we show that there is a serious security problem
in Xu et al.'s RABE scheme such that a cloud server can obtain the plaintext
information of stored ciphertexts by gathering invalidated credentials of
revoked users. Additionally, we also show that the RABE scheme of Xu et al. can
be secure in a weaker security model where the cloud server cannot obtain any
invalidated credentials of revoked users.
