In file transformation utilizing proxy re-signatures, once a user in the group is revoked; the data cloud server is able to re-sign the blocks, which were signed by the revoked user, with a resigning key. As a result, the effectiveness of user revocation can be significantly developed; computation and communication resources of existing users can be easily saved. Meantime, the data cloud server, who is not in the same trusted domain with each user, is only able to convert a signature of the revoked user into an existing user on the same block, but it cannot sign arbitrary blocks on side of either the revoked user or an existing user.