On Delegatability of a Certificateless Strong Designated Verifier Signature Scheme

A strong designated verifier signature (SDVS) is a special variant of digital signatures, since it only allows a designated recipient to verify the signer’s signature. The transcript simulation property of such signatures also prohibits a designated verifier from arbitrarily transferring his/her conviction to any third party. When implemented in certificateless cryptosystems, a certificateless SDVS is unnecessary to manage public key certificates and deal with the key-escrow problem of conventional identity-based systems. In 2014, Shim pointed out a crucial security property called non-delegatability for SDVS schemes. This property states that anyone should not be able to generate a valid SDVS without obtaining either the signer’s or the verifier’s private key. In other worlds, a non-delegatable SDVS scheme must ensure that any malicious adversary cannot forge a valid signature even if he/she has gotten the shared secret value between a signer and an intended verifier. In this paper, we first demonstrate that a previously proposed efficient certificateless SDVS scheme is vulnerable to the delegatability attack and then further propose an improved variant.