On the Security of a Certificateless Strong Designated Verifier Signature Scheme

Recently, Chen et al. proposed the first non-delegatable certificateless strong designated verifier signature scheme and claimed that their scheme achieves all security requirements. However, in this paper, we disprove their claim and present a concrete attack which shows that their proposed scheme is  forgeable. More precisely, we show that there exist adversaries who are able to forge any signer's signature for any designated verifier on any message of his choice.