1) The document proposes a secure data sharing scheme for sharing data within group members in the cloud. It aims to prevent collusion attacks and allow for secure sharing of data even as group membership changes dynamically. 2) The scheme uses encryption with private keys so that if a user's key is leaked, it will be difficult for others to access the shared data. A trusted authority authenticates users and stores encryption keys to determine responsibility if issues arise. 3) The scheme achieves secure key distribution without requiring secure communication channels by using public key verification. It also allows for fine-grained access control and secure revocation such that revoked users cannot access shared data even by colluding with the cloud.