Crytanalysis and Improvement on Chang et al.'s Signature Scheme

In 2004, Chang's et al. presented a digital signature scheme without using one-way hash function and message redundancy schemes. They claimed their scheme do not suffer from some forgery attacks such as Shieh's scheme. However, Fu (2005) and Zhang (http://eprint.iacr.org/2004/213.pdf) proposed several forgery attacks on it and showed that message redundancy schemes may be still needed in order to resist these attacks. In this paper, we present a new message recovery digital signature scheme without using one-way hash function, too and give another general forgery attack on Chang's signature scheme different from Fu's.