Security Analysis and their Improvement of Two Signature Schemes

Recently, Wang et. al presented a short group signature scheme without random oracles, and Kang et al. gave an efficient ID-based strong designated verifier signature scheme. And security proof of the two signature schemes were provided. Unfortunately, in this work we show that Wang et al.´s group signature is universally forgeable. Namely, any one can forge a group signature on arbitrary message, then an improved vision is presented; And we also show that Kang et al.´s scheme is forgeable, namely, a third party can produce a signature on arbitrary message m by a given signature information. Finally, we give the corresponding attack on the two signature schemes.