# Blockchain-Assisted Verifiable and Multi-User Fuzzy Search Encryption Scheme
###### Abstract
The case information of different patients will be first classified in a certain way, and then the medical data center will store the patient's case and other private information through encryption in the medical database or cloud server, which can only be accessed by authorized doctors. In practice, doctors in different departments have access rights to different files, e.g., doctors in the cardiology department can only access cases of cardiac patients. Although some fuzzy search schemes support multiple user scenarios, there is the problem of difficulty in sharing encryption and decryption keys, and it is unfriendly in the actual situation in smart healthcare to review and authorize the users applying for access online through administrators. In addition, some scheme [12; 13; 14; 15] models serve as honest but curious entities, which are not realistic in practice. Cloud servers have too much power and are not conducive to the data owner's control over the data. Some malicious cloud servers even seriously harm the data owner's interests, such as returning wrong or incomplete files. This greatly affects the practical application of fuzzy search encryption schemes. Therefore, a decentralized fuzzy search encryption scheme is necessary and feasible to solve the possible problems of malicious cloud servers. Aiming at the above problems, the Blockchain-Assisted Verifiable and Multi-User Fuzzy Search Encryption Scheme is proposed, which has more comprehensive functions and efficient performance. And it is more adaptable to the ciphertext search needs under new network services such as smart healthcare. The main innovations are as follows. * For the existence of malicious cloud servers in fuzzy search schemes, semi-decentralization is used to weaken the power of cloud servers, a smart contract is constructed to guarantee fairness between users and servers, and blockchain is introduced to assist in achieving integrity verification of results. * Aiming at the lack of fine-grained access control for users in one-to-many application scenarios, the is used to distribute the shared key with a tree structure, and only authorized users who meet the access results can decrypt the shared key. * By improving the index tree structure and bigram participle algorithm, the proposed scheme not only optimizes the search efficiency and search precision but also enables users to check the search results locally for a second time. * Through security analysis, it is proved that the scheme is secure under the adaptive selection of keyword attacks. Functional comparison and experimental analysis show that the search efficiency of the scheme has been improved under the premise of supporting multi-functionalities such as multiple keywords, fuzzy search, dynamic updating, and result validation. In particular, the search time of our scheme is more advantageous with the increase in search keywords and searched files. The rest of this paper is arranged as follows. Related work on fuzzy search and blockchain-based searchable encryption is in Section 2. Some preliminaries used in the scheme are given in Section 3. In Section 4, the definitions of algorithm and security model are provided. Details of the program will be presented in Section 5. Sections 6 and 7 provide the program analysis and efficiency analysis, respectively. Section 8 summarizes the work and gives conclusions.