Abstract
The collusion of different users to get access permissions more than each of those users can get by himself is a serious problem in the ciphertext-policy attribute-based encryption. The trust notation can provide security against the users' collusion attack. In this paper, we have introduced the attributes' trust semi-formal definition. Also, a new Multi-Authority Trust Access Control (MA-TAC) model for cloud storage multi-authority systems has been proposed which is based on user's attributes for trust calculation. Our MA-TAC model ensures granting access for trusted users, while protecting the users' identity. Also, the users' permissions are dynamically changing based on the current trust level of the user. MA-TAC ensures both security and stability for user's collusion attack. Finally, the experimental results have indicated that the user's permission set is dynamically changing based on the user's trust level, which is decaying over time.