Cryptanalysis of Li-Tzeng-Hwang's improved signature schemes based on factoring and discrete logarithms

Laih and Kuo proposed two efficient signature schemes based on discrete logarithms and factorization. Recently, Li et al. improve one of their schemes in order to use fewer keys for a signing document. In this paper, we shall prove that their improvement of Laih and Kuo's signature scheme is insecure. Moreover the improved signature scheme in fact is not based on two cryptographic assumptions simultaneously, and forging a signature on any message would not need to solve any difficult problems.