Verifiable Dynamic Symmetric Searchable Encryption: Optimality and Forward Security

Symmetric Searchable Encryption (SSE) is a very efficient and practical way for data owners to outsource storage of a database to a server while providing privacy guarantees. Such SSE schemes enable clients to encrypt their database while still performing queries for retrieving documents matching some keyword. This functionality is interesting to secure cloud storage, and efficient schemes have been designed in the past. However, security against malicious servers has been overlooked in most previous constructions and these only addressed security against honest-but-curious servers. In this paper, we study and design the first efficient SSE schemes provably secure against malicious servers. First, we give lower bounds on the complexity of such verifiable SSE schemes. Then, we construct generic solutions matching these bounds using efficient verifiable data structures. Finally, we modify an existing SSE scheme that also provides forward secrecy of search queries, and make it provably secure against active adversaries, without increasing the computational complexity of the original scheme.