Searchable symmetric encryption over multiple servers

Searchable Symmetric Encryption (SSE) allows a user to store encrypted documents on server(s) and later efficiently searches these documents in a private manner. So far most existing works have focused on a single storage server. Therefore in this paper we consider the natural extension of SSE to multiple servers. We believe it is of practical interest, given that a user may choose to distribute documents to various cloud storage that are now readily available. The main benefit compared to a single server scheme is that a server can be set to hold only subset of encrypted documents/blocks. A server learns only content of documents/blocks that it stores in the event of successful leakage attack or ciphertext cryptanalysis, provided servers do not collude. We define formally an extension of single server SSE to multiserver and instantiate provably secure schemes that provide the above feature. Our main scheme hides total number of documents and document size even after retrieval, achieving less leakages compared to prior work, while maintaining sublinear search time for each server. We further study leakages under the new setting of non-colluding and colluding servers.