On the Security of a Privacy-Preserving Ranked Multi-Keyword Search Scheme
Ranked keyword search over encrypted cloud data is a hot research topic with huge practical implications. A complex variant of this problem is to allow searching among ciphertexts belonging to multiple data owners. Unfortunately, there exist only a few papers in the literature which address this important setting. Recently, Zhang et al. proposed a solution to this problem and claimed that their scheme provides two fundamental security requirements: ciphertext and trapdoor indistinguishability. We prove, however, that in their scheme neither the ciphertexts nor the trapdoors achieve indistinguishability. Our result shows that their scheme is insecure to be used in practice.