On secure data sharing in cloud environment

Over years, cloud computing has been rapidly changing the shape of modern computing environment. The problem of how to keep the confidentiality of user data against malicious entities including a cloud service provider has been recognized as a significant issue. This problem becomes even more complicated if a data is shared among multiple users. Recently, the idea of proxy reencryption has been introduced to support secure data sharing among group members in cloud environment. However, in this scheme, a malicious user can collude with the server to decrypt unauthorized messages. The conditional proxy re-encryption (CPRE) aims to fix this problem by introducing a condition value into message encryption process and re-encryption key generation. We observe that CPRE becomes significantly inefficient when the membership of the group changes very actively and the size of the group is large since a new condition value is selected and re-encryption keys have to be generated for each user whenever the group membership is changed. This paper introduces a new CPRE in which the condition value is not associated with re-encryption keys. Whenever a group membership is changed, only a new condition value is distributed to the users via cloud server. As a result, the overhead of each user becomes significantly reduced at each membership change. Copyright 2014 ACM