Secure conjunctive keyword searches for unstructured text

There are a number of searchable encryption schemes that allow secure conjunctive keyword searches over encrypted data, but all of them assume that the position of the keywords is known. This is a pity, since in unstructured text, e.g. the body of an e-mail, this position is unknown and one has to construct O(m
ⁿ
) search tokens for n keywords in a text of length m. In this paper we present a searchable encryption scheme that allows conjunctive keyword searches without specifying the position requiring only one search token with constant ciphertext length. We prove the security of our scheme using the external Diffie-Hellman assumption in the random oracle model.