On Security of a Secure Channel Free Public Key Encryption with Conjunctive Field Keyword Search Scheme

Public key encryption with keyword search is a practical cryptographic paradigm that enables one to search for the encrypted keyword without compromising the security of the original data. Recently, Hwang et al. proposed a secure channel free public key encryption with conjunctive field keyword search (SCF-PECKS) scheme and claimed that their scheme can withstand the keyword guessing attack and does not need the secure channel. In this paper, by presenting three concrete attacks, we demonstrate that Hwang et al.'s SCF-PECKS scheme fails to achieve the security against keyword guessing attacks by either outsider attackers or malicious insider servers. The presented attacks show that Hwang et al.'s scheme is vulnerable to the keyword guessing attack regardless of whether the trapdoors are sent via public channel or secure channel. Therefore, devising a secure SCF-PECKS scheme remains an unsolved problem until now.