Cryptanalysis of Keyword Confidentiality in a Searchable Public-Key Encryption Scheme Against Malicious Server

Abstract

Public-key authenticated encryption with keyword search (PAEKS) is a novel cryptographic primitive to resist against keyword-guessing attacks (KGAs) and preserve the privacy of keywords in both ciphertexts and trapdoors. Recently, a designated-server PAEKS (dPAEKS) scheme was proposed to withstand KGAs. The scheme was claimed to satisfy both multi-ciphertext indistinguishability (MCI) and multi-trapdoor privacy (MTP). However, our cryptanalysis demonstrates that it is insecure against KGAs, where a malicious server (inside attacker) can obtain the information of the keywords embedded in the ciphertext and the trapdoor. As a result, both the MCI and MTP of the scheme are broken. In addition, the paper also shows that it is possible to break the security of MTP, even for an outside attacker. Finally, we also provide a method to fix these security flaws.