Abstract: In a file sharing system, a key manager unit realizes a correspondence between the first user identifier and the first public key in response to a registration request of the first user, generates a first key material for encrypting the first file into a first encrypted file, and generates a first credential according to the first user identifier, the first file identifier, the first public key and the first key material after receiving an access-right claim request to the first file from the first user. A file storage unit stores the first encrypted file and the first credential. The first user uses the first user identifier, the first file identifier and the first private key to retrieve the first key material out of the first credential, and uses the first key material to decrypt the first encrypted file into the first file.