Attacks against Two Provably Secure Certificateless Signature Schemes

Certificateless cryptography, first introduced by Al-Riyami and Paterson in 2003, can solve the key escrow problem which is inherent in identity-based (ID-based) cryptography and eliminate the use of certificates as in the conventional Public Key Infrastructure (PKI). Recently, Xu et al. proposed a certificateless signature scheme and Yang et al. presented a mediated certificateless signature scheme .They all claimed that their schemes are secure and efficient and gave security proofs of the schemes. In this paper, we analyze the two certificateless signature schemes and show that the two schemes are insecure against key replacement attacks and malicious KGC(key generation centre) attacks.