Research On Blockchain Based Multi-user Dynamic Searchable Symmetric Encryption Schemes

With the advent of the era of big data,data has become an important factor of production with rapidly increasing scale.The emergence of cloud storage has eased users' data storage pressure.However,cloud storage cannot guarantee the confidentiality of outsourced data,and cloud service providers whose quality of storage and query services will be affected by factors such as economic interests are not completely reliable.Searchable encryption technology ensures the confidentiality of data by realizing ciphertext storage and query.Blockchain can ensure correct execution of data query and enhance the credibility of cloud services.The combination of these two technology can effectively solve the security of cloud storage question.In practical applications,outsourced data needs to be shared to other users.The most typical and extensive sharing scenarios are represented by “single-writer/multi-reader”(medical data sharing)and “multi-writer/multi-reader”(government data sharing).Most of the existing blockchain-based dynamic searchable symmetric encryption schemes do not meet the needs of multi-user applications,and pay less attention to how to achieve forward/backward security in multi-user scenarios.This thesis proposes the following two schemes for the above two sharing modes.In the scenario of “single-writer/multi-reader”,this thesis proposes a dynamic searchable encryption scheme with verifiable and forward-backward security.The scheme constructs an inverted index based on a bit matrix and a hash table on the blockchain,introduces proxy server and key aggregation encryption technology to achieve file-level access control,and uses hash function to generate verification information for encrypted files and keyword query results.At the same time,the scheme introduces state parameters and random parameters,and realizes forward and backward security through parameter transformation in the query and update stages.This thesis analyzes and proves that the scheme has forward and backward security,and finally evaluates the performance of the scheme.In the scenario of “multi-writer/multi-reader”,this thesis constructs a dynamic searchable symmetric encryption scheme in which multiple data owners jointly maintain encrypted indexes.The scheme has a two-layer index structure.It queries which user contains keywords on the blockchain with a bit matrix to encrypt and store the corresponding relationship of “keyword-user”.Here blockchain undertakes the functions of index storage,retrieval,and access control.Data owner builds local file index.When receiving keyword query request,data owner returns the corresponding file.In addition,the scheme realizes forward and backward security by randomizing the retrieval,update and other requests.This thesis proves that the scheme is forward and backward secure,and evaluates the scheme performance.