Zheng proposed two new authenticated encryption schemes, called signcryption, in 1997. But the studies of Petersen and Michels showed that these schemes have the problem “to achieve non-repudiation is to lose confidentiality”. They outlined a solution to solve the problem. Later, He and Wu pointed out that there are forged signatures in the scheme proposed by Petersen and Michels. They also proposed improvement to mend the leak in security. In this paper, we shall show that the scheme of He-Wu still violates the property of unforgeability. Also, a new scheme based on Nyperg-Rueppel digital signature is presented.
