Blockchain-Based Certificateless Bidirectional Authenticated Searchable Encryption Scheme in Cloud Email System

Traditional email systems can only achieve one-way communication, which means only the receiver is allowed to search for emails on the email server. In this paper, we propose a blockchain-based certificateless bidirectional authenticated searchable encryption model for a cloud email system named certificateless authenticated bidirectional searchable encryption (CL-BSE) by combining the storage function of cloud server with the communication function of email server. In the new model, not only can the data receiver search for the relevant content by generating its own trapdoor, but the data owner also can retrieve the content in the same way. Meanwhile, there are dual authentication functions in our model. First, during encryption, the data owner uses the private key to authenticate their identity, ensuring that only legal owner can generate the keyword ciphertext. Second, the blockchain verifies the data owner's identity by the received ciphertext, allowing only authorized members to store their data in the server and avoiding unnecessary storage space consumption. We obtain a formal definition of CL-BSE and formulate a specific scheme from the new system model. Then the security of the scheme is analyzed based on the formalized security model. The results demonstrate that the scheme achieves multikeyword ciphertext indistinguishability and multi -keyword trapdoor privacy against any adversary simultaneously. In addition, performance evaluation shows that the new scheme has higher computational and communication efficiency by comparing it with some existing ones.