Cloud computing is the biggest innovation in computing world. It provides great facilities of data sharing and data storing to its users. Here a main risk occurs as data security in aspects of data integrity, data privacy and data access by unauthorized users. TTA (Trusted Third Party) is used by cloud service providers to ensure data security and privacy. In cloud, data modification and data sharing among the group of users is very simple task .To maintain integrity of the shared data, group members needs to compute signatures on all shared data which are available in blocks.