Shamir and Schorr proposed an attack to Rabin's digital signature.The attack is called Shamir attack.In this paper

we will present a new and efficient bit perturbing method to avoid the Shamir attack.Harn and Kiesler proposed a public-key cryptosystem

digital signature and authentic encryption scheme based on Rabin's cryptosystem.In this paper

we will point out these cryptosystem are insecure and we also propose a secure digital signature scheme.