Cryptanalysis and Improvement of Several Digital Signature Schemes

In 2007, Zhang proposed an efficient identity based ring signature scheme. Through cryptanalysis, we found that the ring signature scheme is insecure. There exists group changing attack. In 2007, Ming, et al. proposed a certificate less universal designated verifier signature scheme. In the same year, Yang, et al. designed a secure mediated certificate less signature scheme. We found that the two signature schemes couldn´t resist the public key substitution attack or the malicious KGC (Key Generation Center) attacks. In order to avoid these flaws, we improve the last two digital signature schemes. The improved schemes are provably secure.