Reliable Re-Encryption in unreliable clouds

The organization stores encrypted data on cloud and issues decryption keys to authorized users. Data owner issues decryption keys to authorized users. When user leaves organization, then he/she becomes unauthorized user for data access. When a user is left the organization, then data owner will issue re-encryption commands to the cloud to re-encrypt the data. So we prevent the left user from decrypting the data by using the old decryption key. And to generate new decryption keys to valid users only. So only authorized users can continue to access the data. By considering cloud architecture, such commands may not be received properly due to unreliable network communications. So we proposing a time based re-encryption scheme, which enables the cloud servers to automatically re-encrypt data based on their internal clocks of system. Within given time period users can access data, after timestamp they cannot access data.