Dynamic Attributes Design in Attribute Based Encryption

In the past, sending encrypted messages to a group of users is not easy since it requires a shared group key in advance. Ciphertext-Policy Attribute Based Encryption (CPABE) achieves true that anyone who is expected to see the message content can decrypt messages in the absence of group keys. Say, a sender encrypts messages with an access control policy tree which is composed of attributes; a receiver is able to decrypt messages as long as his/her attributes satisfy the policy tree. CPABE saves the cost to distribute group keys; however, it does require users to keep their attributes up to date.