Space-Efficient and Secure Substring Searchable Symmetric Encryption Using an Improved DAWG

A searchable symmetric encryption (SSE) scheme is a method which searches encrypted data without decrypting it. In this paper, we address the substring search problem such that for a set D of documents and a pattern p, we find all occurrences of p in D. Here a document and a pattern are defined as strings and are encrypted. A directed acyclic word graph (DAWG), which is a deterministic finite automaton, is known for solving a substring search problem on a plaintext. We improve a DAWG so that all transitions of a DAWG have distinct symbols and present a space-efficient and secure substring SSE scheme using an improved DAWG. The novel feature of an improved DAWG is that we can solve the substring search problem using only the labels of transitions. The proposed substring SSE scheme consists of an index with a simple structure and the size is O(n) for the total size n of documents.