Cryptanalysis of a Certificateless Ring Signcryption Scheme

Certificateless public key cryptography (CL-PKC) is an appealing paradigm with the advantages of both conventional public key cryptosystem and ID-based cryptosystem because it avoids using certificates and eliminates the key escrow problem. Ring signcryption is an attractive primitive which allows one user to send a message anonymously, confidentially and authentically. Recently, Qi et al. proposed a novel certificateless ring signcryption scheme with bilinear pairings, and claimed the scheme is provably-secure in the random oracle model. In this paper, we reveal that Qi et al.'s scheme is not secure against both a Type I adversary and a Type II adversary. By giving specific attacks, we found it is unable to meet the fundamental requirements of confidentiality and unforgeability.