Efficient Forward Secure Searchable Symmetric Encryption for Multi-user

Most of the existing forward secure searchable encryption schemes are mainly for single-user settings and cannot be easily extended to multi-user settings since the keyword state is maintained locally; there are few forward secure schemes that support multi-user retrieval, and they require additional trusted proxy servers, which brings additional overhead and insufficient practicality. Aiming at the current lack of practical multi-user forward secure searchable encryption scheme, a searchable encryption scheme (EMFS) based on a two-chain index structure is designed, which meets forward security and supports multi-user retrieval without the need for a proxy server. The double-layer index consists of the main chain index and a slave chain index. The main chain index is generated by the one-way trapdoor function and the global state and does not depend on the separate state of each keyword, thus avoiding the synchronization problem of the keyword state among multiple users; the slave chain index is generated by stream cipher, which improves the search efficiency. Compared with three existing single-user forward security schemes (Sophos, FAST, BESTIE), the results show that the EMFS scheme has good scalability and practicability: (1) The EMFS scheme achieves the expansion of multi-user retrieval at a reasonable cost of search perfor-mance; (2) The EMFS scheme is especially suitable for matching datasets with a large number of files; (3) The EMFS scheme has the advantage of small client storage overhead.