Data sharing Privacy preservation allow any system user with Given either a search token or valid decryption rights of an keyword update token, a cloud encrypted data to share server does not know any his/her encrypted data to knowledge of the keyword(s) others efficiently and Given an original ciphertext or securely shared(reencrypted)ciphertext, a cloud server does not know any information of the underlying message as well as the keyword description tagged with the ciphertextPage . Data sharing ▪ allow any system user with valid decryption rights of an encrypted data to share his/her encrypted data to others efficiently and securely ▪ Given either a search token or keyword update token, a cloud server does not know any knowledge of the keyword(s) ▪ Given an original ciphertext or shared (reencrypted) ciphertext, a cloud server does not know any information of the underlying message as well as the keyword description tagged with the ciphertext. Privacy preservation 17