On the Security of a Provably Secure Certificate Based Ring Signature Without Pairing

Featured with anonymity and spontaneity, ring signature has been widely adopted in various environments to offer anonymous authentication. To simplify the certificate management in traditional public key infrastructure (PKI) and solve the inherent key escrow problem in the Identity-based cryptography, Qin et al. propose a pairingfree ring signature scheme in the certificate-based cryptosystem recently. Unfortunately, we demonstrate that their scheme is not secure against the malicious certificate authority (CA) and key replacement attacks by giving concrete attack. Concretely, a malicious certificate authority (CA) can forge a signature on arbitrary message in name of any user’s identity and a uncertified user is also able to forge a message.