Cryptanalysis of new digital nominative proxy signature schemes for mobile communication

Recently, Park and Lee proposed a digital nominative proxy signature scheme for mobile communication. Seo and Sun pointed out that Park and Lee's scheme can't resist the forgery attack from the original signer. To provide the unforgeablity and nonrepudiation, Seo et al also proposed a new nominative proxy signature scheme for mobile communication. However, in the paper, we show that the new scheme can't satisfy the non-repudiation among their security requirement either. Based on Seo et al's scheme, an improved scheme is proposed. The improved scheme can resist not only the attack mentioned in Seo and Sun's schemes, but also the attack proposed by us to Seo et al's scheme. Also, the improved version retains the advantages of Seo et al's scheme. In addition, we discuss one of basic principles of design of proxy-protected signature schemes based on DLP (discrete logarithm problem).