Heekuck Oh

Over years, cloud computing has been rapidly chang-ing 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 prob-lem becomes even more complicated if a data is shared among multiple users. Recently, the idea of proxy re-encryption 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 con-dition 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 signifi-cantly reduced at each membership change